A Review on the Policy, Technology and Evaluation Method of Low-Carbon Buildings and Communities

Abstract

In order to address global warming, most countries in the world have established carbon neutral targets and are continuously taking action to achieve carbon neutrality. The building sector accounts for 36% of end-use energy consumption and 37% of energy-related CO₂ emissions globally, so carbon mitigation in building sector is considered to be the most critical step in completing the “last mile” of global carbon neutrality. Low-carbon buildings and communities are the foundation for achieving low-carbon cities and the key transition to reach the goal of carbon neutrality. Therefore, this review aimed to: (a) provide a comprehensive review of countries’ policies on low-carbon buildings and communities and a theoretical basis for improving the corresponding laws and regulations; (b) investigate low-carbon technologies related to building and community construction and operation, as well as identify the current technology gaps; (c) provide a comprehensive overview of low-carbon buildings and communities assessment systems to analyze and evaluate the economic, technical, environmental and social benefits of current building and community energy systems; and (d) identify enablers and barriers in low-carbon buildings and communities to provide direction for future research. The results of this paper can provide comprehensive insights in to further achieving low-carbon buildings and communities.

1. Introduction

As the global climate warms, countries continue to face increasing environmental challenges. In order to shorten the emissions gap between the national emissions reduction target and the global temperature control target of 1.5 °C, countries are constantly carrying out carbon neutrality actions [1]. As of October 2022, 139 countries have clarified their carbon neutrality goals through legislation, strategy documents, statements/commitments, proposals/discussions, and more than 90% of them have set their carbon neutrality goals for 2050 [2,3].

Table 1. Acts and objectives of major carbon neutral legislative countries.

Refs.	Country/Organization	Climate and Energy Act (Last Revision Year)	Content and Objectives
[4]	European Union	European Green Deal (2020)	Proposed a carbon reduction target for the EU to become carbon neutral by 2050.
[5]		European Climate Law (2020)	sets the target of the European Union becoming climate neutral by 2050.
[6]	United Kingdom	Climate Change Act (2022)	The document formally commits the UK to becoming carbon neutral by 2050, creating a zero-carbon emitting society.
[7]	Germany	German Federal Climate Protection Law (2019)	Germany should reduce its total greenhouse gas emissions by 65 per cent from 1990 levels by 2030 and become carbon neutral by 2045.
[8]		Climate Action Plan 2030 (2021)	Specify specific action measures for each industry sector.
[9]	France	National Low-carbon Strategy (2020)	France has established a carbon budget and set a target of achieving carbon neutrality by 2050.
[10]	Sweden	Climate Act (2017)	To achieve zero greenhouse gas emissions by 2045 and a 70% reduction in the transport sector by 2030.
[11]	New Zealand	Climate Change Response (Zero Carbon) Amendment Bill (2019)	By 2050, New Zealand will have zero greenhouse gas emissions, with the exception of biogenic methane from agriculture.
[12]	China	statements/commitments (2020)	strive to reach a peak before 2030 and strive to achieve carbon neutrality by 2060

shows the major carbon-neutral legislation in these countries.

“The Global Warming of 1.5 °C” report highlighted the increase in CO₂ as a major factor in rising temperatures. Building construction and operations collectively accounted for 36% (149 EJ) of global energy consumption in 2020, with an estimated 127 EJ for building operations and 22 EJ for building materials manufacturing [4]. Compared with other end-use sectors, building and construction accounts for 37% of energy-related CO₂ emissions [5]. Figure 1 shows the building and construction’s share of global final energy and energy-related CO₂ emissions in 2020. In order to achieve the ambitious goal of carbon neutrality, great efforts must be made by the building and construction-related sectors. Low-carbon buildings and communities can effectively reduce CO₂ emissions during the whole lifecycle of buildings, which will make them gradually become the mainstream trend in the international architecture industry. The selection of low-carbon building materials and the low-carbon operation of building energy systems will ensure that the building and the community reduce energy consumption and CO₂ emissions.

Despite the rapid growth of low-carbon buildings and communities, their proportion in the global construction industry is still small, mainly because of the insufficient promotion of low-carbon buildings and communities. Current studies on low-carbon buildings and communities cover many aspects, but lack a systematic review of relevant policies, low-carbon technologies, and evaluation methods in various countries. This paper provides a comprehensive overview of the relevant policies, applied technologies and evaluation systems of low-carbon buildings and communities in various countries, and determines the development motivations and obstacles at present, so as to facilitate the references and the induction of relevant scholars. The objective of this paper is to help governments of all countries to effectively adjust their policies and standards in the field of construction based on their own economic and technological levels, improve the existing low-carbon buildings and communities evaluation methods, and provide comprehensive technical guidance to technology companies in the field of low-carbon building, so as to improve the application of low-carbon technology in low-carbon buildings and communities in multiple dimensions, and help existing buildings and communities to complete the decarbonization process, and help relevant researchers to fully understand the development situation and technical level of low-carbon buildings and communities.

2. Materials and Methods

This paper provides an extensive overview of research on the policy, technology and evaluation of low-carbon buildings and communities. The structure and content of this paper are presented as shown in Figure 2. Firstly, the definitions and demonstration projects of low-carbon buildings and communities are introduced in Section 3, and then the relevant policy objectives, contents and incentives of different countries are compared and analyzed. To achieve the policy goals, relevant low-carbon technologies have been rapidly developed, which are summarized in Section 4. In this paper, the low-carbon technologies are analyzed from both construction and operation aspects. Construction technologies are focused on building materials and envelopes, where materials can reduce CO₂ emissions through the use of alternative and natural materials, and envelopes mainly involving the roof, wall, floor, shade and fenestration have different effectiveness. The operation is mainly focused on the low-carbon energy system (LCES), which uses renewable energy, energy storage and optimization technologies to decrease CO₂ emissions. In order to assess the benefits of low-carbon technologies, evaluation of low-carbon buildings and communities is necessary. This paper reviews evaluation systems on low-carbon buildings and communities, and summarizes the economic, environmental, technical, and comprehensive evaluation indicators and methods for LCES in Section 5. Finally, enablers and barriers in low-carbon buildings and communities are analyzed to provide direction for future research.

The research methodology consists of three steps in this paper, as shown in Figure 3. First, worldwide policies were searched using the relevant official websites of various countries and organizations, and literature related to technology and assessment were searched using Web of Science and Google Scholar, with the following main search formulas:

• ((ALL = (building material OR building envelope)) AND ALL = (low-carbon OR zero carbon OR zero energy))
• ((((ALL = (community OR building)) AND ALL = (energy system)) AND ALL = (low-carbon OR zero carbon OR zero energy)) AND ALL = (design OR operation))
• (((ALL = (community OR building)) AND ALL = (assessment OR evaluation OR analysis)) AND ALL = (low-carbon OR zero carbon OR zero energy))

1, 2 and 3 were used to search for the construction, systems, and evaluation of low-carbon buildings and communities, respectively, which covered the literature published from 1998–2022 and are detailed in Section 4 and Section 5 of this paper. Then, an initial screening and analysis of the searched literature is performed according to certain criteria, which mainly applies to the title and abstract. The selection criteria were as follows: (1) the objective of the study related to buildings or communities; and (2) the purpose is for energy conservation and carbon mitigation. Finally, all relevant literature papers were classified and critically analyzed in detail, and the key analytical results are summarized in Section 7.

3. Low-Carbon Building and Community Policies

Although the development of low-carbon undertakings is related to the long-term survival of mankind, from the perspective of capital benefits, the low-carbon industry itself will involve large investment efforts and slow recovery benefits if there are no mandatory regulations and financial support from the government [13]. Therefore, national policies in the field of low carbon are crucial for the common benefit of mankind. For the construction industry, this will require the global construction industry to make great efforts in the process of decarbonization to achieve the goal of “carbon neutrality” due to its large proportion of energy consumption. The construction industry’s achievement of ultra-low emissions or even zero emissions is an important starting point to achieve carbon neutrality. According to the national economic and technological situation, countries around the world have studied, discussed and formulated corresponding guiding and incentive policies for low-carbon building development, which have promoted the development of low-carbon buildings and communities to a large extent, and also provided a more specific implementation path for “carbon neutrality” in the field of the construction industry.

3.1. Definition and Projects

3.1.1. Low-Carbon Building

Due to the climate conditions and economic development situation of various countries all over the world, the definitions and evaluation standards for low-carbon buildings and communities are different, but they all meet the basic requirements of the domestic construction industry development [14]. The explanation given by the IPCC on low-carbon building is that a green building with low energy consumption and less pollution emissions, especially CO₂, during construction and operation [1]. The World Green Building Council (World GBC) has a similar definition for low-carbon building: A ‘green’ building is a building that, in its design, construction or operation, reduces or eliminates negative impacts, and can create positive impacts, on our climate and natural environment [14]. Low-carbon building can be interpreted as the building with less CO₂ emissions in the whole lifecycle of the building, or it can be interpreted as reducing the consumption of fossil energy, improving energy efficiency, and reducing CO₂ emissions in the whole lifecycle of buildings.

There are already many successful projects of low-carbon building worldwide, which have been recognized by professionals and organizations in this field. The Crystal commissioned by Siemens and designed by Wilkinson Eyre in London, England, covers more than 6300 square meters and is a model of high energy efficiency. It saves 50% of electricity and 65% of CO₂ emissions compared with a similar office building by introducing renewable sources to satisfy its heating and cooling needs. It also uses smart lighting technology, with electricity mainly provided by photovoltaic solar panels. The rainwater received by the roof is treated by the sewage disposal system to be purified or converted into drinking water [15]. The Pixel Building in Melbourne, Australia, meets 105 environmental requirements and is Australia’s first carbon neutral office building. Equipped with solar panels and rainwater harvesting, the building is self-sufficient in water and energy. It meets 102 requirements under the US LEED standards and gain the highest LEED score in the world. Over its 50-year lifecycle, the building will compensate for all the CO₂ emissions generated by its construction through the renewable energy it generates and feeds into the grid [16]. The Bullitt Center in Seattle uses cutting-edge sustainability technologies, such as collecting and filtering household wastewater in an underground reservoir and a green roof to filter rainwater. It also has toilets that can break down waste with aerobic devices, rooftop solar arrays that can power the entire building for a year, and large windows that provide natural lighting and ventilation. In addition, its concrete floors are equipped with solar-powered hot water circulation radiant heating systems, as well as 400 feet of underground heat exchange wells to help regulate office temperatures. Based on the first year of operation, the actual energy intensity of the building is 31.3 kWh/(m²·a), which is 79% lower than the Seattle Energy Ordinance’s building energy requirement of 139.8 kWh/(m²·a) [17]. The Eco-House in Kitakyushu, Japan, uses solar power directly to harness the sun’s heat. It also uses circulation fans to control the flow of hot air, making the indoor environment more comfortable. The heating system uses biomass energy and no CO₂ emissions are produced [18]. The Bahrain World Trade Center in Manama, Bahrain, is the first skyscraper in the world to use wind power as a source of electricity. Between its two towers are three wind turbines with a diameter of 29 m that are horizontally supported. Wind turbines are expected to support 11 to 15 percent of the building’s electricity needs. Three wind turbines provide about 1300 megawatt-hours of electricity a year, equivalent to two million tons of coal or six million barrels of oil, for 300 average homes, significantly reducing CO₂ emissions from buildings [19].

3.1.2. Low-Carbon Community

At present, the definition of a “low-carbon community” is mostly described from different research perspectives. From the perspective of la ow-carbon economy, the low-carbon community is the transformation of the urban community production mode, lifestyle and value concepts under the model of the low-carbon economy. From the perspective of carbon emissions, low-carbon community means that in addition to reducing the carbon emissions generated by all activities in the community, it also hopes to achieve the goal of zero carbon emissions through ecological greening and other measures [20]. From the perspective of architectural design, low-carbon community is a complete energy saving and environmental protection system. In the whole lifecycle of building materials and buildings, it can reduce the dependence on petrochemical energy, improve energy efficiency, and ultimately reduce CO₂ emissions [21]. Starting from the concept of sustainable development, the core of low-carbon community construction is zero energy consumption system. The design concept of zero energy is to maximize the use of natural energy, reduce environmental damage and pollution, achieve the goal of zero fossil energy use, and realize the basic recycling of energy demand and waste treatment living mode [22].

A number of low-carbon or carbon-neutral communities have emerged around the world, such as Beddington Zero Energy Development (Bed ZED) in the UK, Vauban in Germany and Wikscher in Sweden. All these areas have planned to change people’s behavior patterns with low-carbon or sustainable concepts to reduce energy consumption and reduce CO₂ emissions. Bed ZED in the UK is the first World Wide Fund for Nature (WWF) and UK Eco-Regional Development Group to advocate the construction of a “zero-energy” community. It is called the “future home” of mankind, also known as the “Beddington Zero Energy Development” plan. It integrates various environmental strategies to reduce the use of energy, water, and cars. Such as energy-saving buildings, using new and renewable energy, using environmentally friendly materials, optimizing community structure, and advocating green transportation. After years of exploration and development, Bed ZED has become a community with carbon balance value approaching zero. It uses 81% less energy for heating, 45% less electricity, 58% less water, and 64% less car miles per year than the average community [23]. The community of Vauban is known as the benchmark of sustainable community in Germany, with its multi-collective construction and low energy consumption, self-sufficiency of buildings. It takes the sun as the economic factor, develops the solar energy professionally and recycles the waste reasonably and efficiently to ensure the resource reuse. The Vauban community has the highest density of “passive energy buildings” in Europe and already has nearly 150 “very low energy” passive houses. By 2010, it had reduced CO₂ emissions from residential, commercial, industrial and transport energy by 25%. New homes are designed to be low-energy, and even though they cost 3% more to build, energy costs and CO₂ emissions are reduced by 30% [24]. Low-carbon community similar to the above have developed all over the world, but due to the climatic characteristics, geographical characteristics, development level, development mode and other factors in different regions, the development law and indicators have different focuses.

3.2. Policies for Low-Carbon Buildings and Communities

Different countries and regions attach different importance to the development of low-carbon buildings and communities, and have different requirements for building energy conservation, but they have formulated corresponding laws and regulations and energy consumption standards according to their own economic and technological conditions. As of November 2021, 80 countries worldwide have mandatory or voluntary building energy regulations at the national or local level, and 43 of these countries have mandatory regulations for residential and non-residential buildings at the national level [25]. Table 2 shows the latest policies and core goals of several countries and organizations in relation to low-carbon buildings and communities.

In 2015, the United States issued the “ Planning for Federal Sustainability in the Next Decade “, which requires that all new federal buildings with a total area of more than 5000 square feet be designed to achieve net zero energy beginning in 2020, water or waste net-zero by 2030 [26]. The U.S. Department of Energy’s “Net-Zero Energy Commercial Building Initiative” calls for all new commercial buildings to be net-zero by 2030 and 50% of existing commercial buildings meet net zero energy requirements by 2040. All commercial buildings will achieve net zero energy by 2050 [27].

The UK launched the Code for Sustainable Homes in 2006, which stipulated that all new homes after 2016 must meet zero carbon emissions standards. All new buildings must realize Zero CO₂ emissions by 2019. The UK promulgated the Heat and Buildings Strategy in 2021, which explicitly required to improve household energy standards and reduce the CO₂ emissions of heating. Over the next ten years, 38 percent of the UK’s carbon reduction comes from improving the thermal performance of existing homes, with another 24 percent involving such improvements in public buildings and new homes. The UK will completely stop selling gas-fired boilers by 2035, and switch to low-carbon alternatives. The decree requires the UK to put electric heat pumps (HPs) at the heart of its net-zero strategy, and to develop low-carbon or hydrogen-ready heating system [28].

The European Union issued the Energy Performance of Buildings Directive (EPBD) in 2010, which stipulated that all new public buildings owned or used by the government should meet the requirement of near-zero energy consumption, and all new buildings were required to meet near-zero energy requirements by 2020. The European Union revised the EPBD in 2021, clearly proposing that new public buildings should achieve net zero emissions by 2027, new buildings should achieve net zero emissions by 2030, and all buildings should achieve zero emissions by 2050. The directive set sustainability standards for new buildings and emphasizes decarbonization standards for existing buildings as well as the worst-performing ones. At the same time, the decarbonization of existing buildings should be accelerated by mainstreaming photovoltaic facilities and energy storage applications in building renovation and design. The EPBD defined zero energy building (ZEB) as a building with “very high energy efficiency” due to the economic imbalance of the EU member states and the large span of climate zones [29]. Countries may formulate their own implementation plans and requirements based on their actual conditions, taking full account of energy-saving technologies and cost-benefit ratios.

Germany enacted the German Building Energy Act (Gebäudeenergiegesetz, GEG) in 2020. The act provided a clear path to achieve the goals of the federal government’s energy policy, specifically to make existing buildings carbon neutral by 2045. Through efficiency measures in the building envelope and building technology, the final energy consumption was saved by approximately 40% compared to 2020. The Act regulated structural and heating system standards for buildings and regulated the energy efficiency of new buildings and renovations of existing buildings [30].

Estonia has achieved near-zero energy for all new buildings after 30 December 2020. The government requires that the annual energy consumption per unit area of single-family low-rise residential buildings should be controlled at 50 kWh/(m²·a), and that of high-rise residential buildings and public buildings should be controlled at 100 kWh/(m²·a) [31,32].

Based on the EU strategy, Denmark requires that the annual energy consumption per unit area of residential apartments and other buildings should be controlled at 20 kWh/(m²·a) after 2020. The annual energy consumption per unit area of public buildings, such as offices and schools, shall be controlled at 25 kWh/(m²·a) [33].

Japan is conducting active research in the field of zero energy buildings. The Japanese government requires new public buildings to meet energy consumption targets on average by 2020. All new buildings should achieve the target of ZEB by 2030 [34]. The specific requirement is that the ZEB ready should reduce the primary energy consumption by 50%. Compared with the existing buildings, the primary energy consumption of nearly ZEB is reduced by 75%, and the annual energy consumption per unit area is about 55.5 kWh/(m²·a). ZEB reduces primary energy consumption by 100% with zero or negative energy expenditure [35].

The Chinese government issued the general code for energy efficiency and renewable energy application in buildings in 2021.The code proposes new standards for the energy consumption of new residential buildings and public buildings. It specifies that residential buildings in cold regions should have an average energy efficiency of 75%, while they are 65% and 72% for residential buildings in other climate zones and public buildings, respectively. The code mandates carbon emissions calculations for all new, expanded and remodeled buildings, as well as for energy-efficient modifications to existing buildings. It also emphasizes that buildings should give priority to passive energy conservation measures, while specifying the application of renewable energy. At the same time, it proposes to comprehensively improve the efficiency of HVAC system and lighting, so as to ensure that the carbon emissions intensity of new residential and public buildings is reduced by more than 7 kg/(m²·a) on average [36].

Table 2. Latest policies and core goals for low-carbon buildings and communities in some countries.

Refs.	Country/Union	Policy (Year)	Core Objectives
[27]	United States of America	Net-Zero Energy Commercial Building Initiative (2021)	Net zero energy for all public buildings by 2050.
[28]	United Kingdom	Heat and Buildings Strategy (2021)	Reduce greenhouse gas emissions from public places by 75% from 2017 levels by 2037.
[29]	European Union	Energy Performance of Buildings Directive (2021)	Zero emissions for all buildings by 2050.
[30]	Germany	The German Buildings Energy Act (2020)	All existing buildings will be carbon neutral by 2045.
[31]	Estonia	EPBD Implementation in Estonia	Low-rise residential building: 50 kWh/(m2·a) High-rise residential buildings and public buildings: 100 kWh/(m2·a)
[33]	Denmark	EPBD Implementation in Denmark	Residential building: 20 kWh/(m2·a) Public building: 25 kWh/(m2·a)
[34]	Japan	Plan for Global Warming Countermeasures (2021)	All new buildings will consume zero energy on average by 2030.
[36]	China	General code for energy efficiency and renewable energy application in buildings (2021)	The carbon emissions intensity of new buildings should be reduced by more than 7 kg/(m2·a) on average.

Table 2. Latest policies and core goals for low-carbon buildings and communities in some countries.

Refs.	Country/Union	Policy (Year)	Core Objectives
[27]	United States of America	Net-Zero Energy Commercial Building Initiative (2021)	Net zero energy for all public buildings by 2050.
[28]	United Kingdom	Heat and Buildings Strategy (2021)	Reduce greenhouse gas emissions from public places by 75% from 2017 levels by 2037.
[29]	European Union	Energy Performance of Buildings Directive (2021)	Zero emissions for all buildings by 2050.
[30]	Germany	The German Buildings Energy Act (2020)	All existing buildings will be carbon neutral by 2045.
[31]	Estonia	EPBD Implementation in Estonia	Low-rise residential building: 50 kWh/(m2·a) High-rise residential buildings and public buildings: 100 kWh/(m2·a)
[33]	Denmark	EPBD Implementation in Denmark	Residential building: 20 kWh/(m2·a) Public building: 25 kWh/(m2·a)
[34]	Japan	Plan for Global Warming Countermeasures (2021)	All new buildings will consume zero energy on average by 2030.
[36]	China	General code for energy efficiency and renewable energy application in buildings (2021)	The carbon emissions intensity of new buildings should be reduced by more than 7 kg/(m2·a) on average.

To sum up, lots of countries have issued building energy efficiency standards and codes in the field of low-carbon buildings and communities. The object of low-carbon building policies in various countries starts from public buildings and gradually covers residential buildings. In public buildings, the government more frequently promotes mandatory high energy efficiency standards, while in residential buildings, energy-saving guidance is carried out step by step based on building area and operational energy consumption. The effectiveness of mandatory standards for residential buildings is insufficient. Countries are also increasing the energy saving transformation of existing buildings, and then formulate relevant energy consumption standards and codes for existing buildings, while the energy consumption standards for new buildings are more stringent. Efforts are also being made to reduce carbon emissions from building power, with buildings that rely on fossil fuels gradually switching to electrification. Developed countries have higher requirements for building electrification levels than developing countries, and some countries have even introduced mandatory electrification codes.

3.3. Incentive Policies

In the development process of low-carbon buildings and communities, in addition to the promotion of mandatory standards and evaluation system, incentive policies are also of great significance. In general, economic incentives are mainly divided into tax, tax credit, grant or subsidy, energy tax, CO₂ tax and other ways [37]. The government can effectively constrain the carbon emissions behavior of low-carbon buildings and communities by rewarding low CO₂ emissions and punishing high CO₂ emissions.

The UK government uses the public financial support to encourage low-carbon building, stimulate energy saving, constrain CO₂ emissions and normalize the fiscal and taxation policies, including energy tax, tax breaks, energy-saving subsidies, etc. [37]. It fosters a relatively mature low-carbon building market [38,39]. It also promotes a “green home” program and offers discounted or interest-free loans for energy saving trust products to introduce low-carbon building into the daily lives of British households [40].

In the United States, incentive policies and tools are based on certification of low-carbon buildings and communities evaluation systems, and LEED certified buildings can reduce the tax base [41]. Take Connecticut’s tax system for green buildings as an example, the new or extensively renovated commercial buildings that apply for LEED gold certification can receive an 8% tax exemption, and LEED platinum certification can receive a 10.5% tax exemption. In addition, projects that apply for LEED certification can get waivers on fees charged by the government [42,43]. In Longmont, Colorado, the new or expanded commercial buildings that adopt higher building standards than the city or apply for LEED certification can claim up to 30% of various tax deductions. In addition, individual states also set up LEED-related funds to ensure that new buildings and community meet low-carbon targets through deposit refunds. In Washington, the mayor established a Green Building Fund for low-carbon building technology support, monitoring, education, and incentives for individual buildings [44].

China’s incentive policies for low-carbon buildings include financial subsidies, preferential evaluation and awards, credit and financial support, and reduction and exemption of supporting urban fees, among which financial subsidies are the most popular incentive policies [45]. According to China’s Green Building Evaluation Standards, green buildings can be divided into three levels: one-star, two-star and three-star [46]. Various provinces in China have introduced a series of incentive policies for low-carbon building. Take the incentive fund management method of Beijing low-carbon Building and Ecological Demonstration Zone as an example. The project with two-star green building operation mark will be awarded 50 RMB/m², while the three-star building is 80 RMB/m². The maximum reward for a single project is no more than 8 million RMB [47,48]. In addition, most of the energy-saving technology implemented in China since 2012, such as energy-saving renovation of building windows and doors, shading system, roof, and exterior wall insulation, are subsidized by the state. The subsidy funds are calculated comprehensively by considering the economic development level of different regions, transformation content and implementation progress, energy conservation and improvement on thermal comfort, etc. The regional subsidies are divided according to the eastern, central and western regions, which is 15 RMB/m² in the eastern region, 20 RMB/m² in the central region and 25 RMB/m² in the western region [49]. Incentives vary from places around the world, but they are all effective for the construction and development of low-carbon buildings and communities.

In general, most developing countries follow the policy ideas of developed countries in the formulation of low-carbon buildings and communities incentive policies, but there is a gap in the level of subsidies. Fiscal incentives in most countries focus more on energy efficiency renovation of existing buildings than on subsidies for new construction. Incentives in developing countries tend to be financial subsidies, while developed countries have more comprehensive low-carbon financial policies, attracting enterprises and individuals to invest in low-carbon buildings and communities development through government investment. In addition to financial subsidies, most governments have developed graded reward and punishment mechanisms based on the evaluation system of low-carbon buildings and communities. For low-carbon buildings with exemplary role models, the government has set up awards similar to the Low-carbon Building Innovation Award, and increased fines for new buildings that do not meet energy saving codes and energy consumption standards. This has certain significance for the scientific research, publicity and promotion of low-carbon buildings and communities.

4. Technology Analysis of Buildings and Communities

To meet the policy requirements, low-carbon technologies are rapidly developed for building and community to reduce CO₂ emissions. The whole lifecycle CO₂ emissions of buildings are divided into three components: direct emissions, indirect emissions, and implicit emissions. The direct and indirect emissions refer to the operational CO₂ emissions of buildings, while the implicit emissions mean the CO₂ emissions for building construction and maintenance, including building materials and construction CO₂ emissions. The lifecycle CO₂ emissions of buildings account for more than half of the total CO₂ emissions in China, while building operation, materials and construction accounting for 42.6%, 55.4% and 2.0%, respectively. Therefore, low-carbon technologies for building materials, envelope design and energy systems are critical to achieve low-carbon buildings and communities. These techniques have been developed by a number of researchers and are summarized and analyzed in Section 4.1 and Section 4.2.

4.1. Low-Carbon Building Materials and Design

The materials and structural designs of the building envelope play a decisive role in the implicit CO₂ emissions of buildings, and the related low-carbon technologies are widely studied, which are sorted out and summarized in this section.

4.1.1. Low-Carbon Building Materials

The production of traditional building materials results in massive CO₂ emissions. For example, it is estimated that every ton of cement production releases about 900 kg of CO₂ [50]. In order to reduce the CO₂ emissions, it is critical to use building materials with low energy consumption, low CO₂ emissions, low pollution and potentially recyclable [51]. Orsini and others [52] has proposed eight methods to reduce CO₂ emissions from building materials, including using alternative materials, using reused, recycled and waste materials, using natural materials, using local materials, innovating the production process, using renewable energy in production, increasing performance and correct applications. In order to properly choose low-carbon materials, Chan and others [51] analyzed the applications and characteristics of commonly used low-carbon building materials, and they are summarized in

Table 3. Commonly used low-carbon building materials.

Refs.	Low-Carbon Building Materials	Introduction	Characteristics	Application
[51]	Precast Hollow Core Slabs	Commonly used in concrete flooring systems in innovative buildings.	(1) Can be constructed quickly at the site and form a self-supporting system with an excellent lower surface finish. (2) Have a factory-assured quality that offers benefits for value-added construction.	Can be widely used in education and office buildings, warehouses and factories, apartments and shopping complexes, and also in wall panels.
[51]	Precast Half Slabs	A pre-stressed slab system applied with slab topping concrete.	Have high structural performance in terms of cracks and deflection control compared with standard concrete.	-
[51]	Steel Framework System	It consists of panels fabricated out of thin steel plates.	(1) The panels can be fabricated in any shape or size. (2) It has a high initial and handling cost.	It is usually used in high-rise building construction and heavy concrete work.
[51]	Prefabricated Timber Frame System	-	The material can form a skeletal structure which can transfer the loads through large spans.	It can be used for building warehouses, bridges, and industrial buildings.
[51]	Glued Laminated Timber	It is a composite of individual solid laminated woods.	(1) Support for the environment. (2) Flexible in size and shape. (3) Custom-made to requirements. (4) Unique appearance, comfortable and warm feel. (5) Excellent strength, up to one-and-a-half to two times that of steel.	-
[53]	Ground Granulated Blastfurnace Slag	-	(1) Can prevent wastage by substituting for material used in concrete mixtures. (2) Has higher workability and can be used in ready-mixed concrete. (3) Has the high defiance to chloride ingress and conferring sustainability. (4) A composition of 50% GGBS and 50% OPC can confer more strength to concrete.	-
[51]	Pulverized Fuel Ash	It is a by-product of coal-powered power stations.	(1) Highly economical. (2) Environmentally friendly. (3) Can make the concrete highly dense and reduce its permeability. (4) Can add greater strength to buildings.	-
[54]	Unfired Brick	-	(1) With the help of unfired brick, wall thickness can be reduced therefore the structural loading will be minimized and the buildings’ available space will be increased. (2) Has an ability of hygroscopic environmental regulation. (3) Low-carbon material.	Can be used in the development of walls.
[55]	Ethylene Tetrafluoroethylene	It is a fluorine-based plastic and more robust than polytetrafluoroethylene.	Has 95% light transmission and flexibility.	Can be used as cladding for buildings.
[56]	Geopolymer Concrete	It is made from GGBS and fly ash.	Can reduce carbon emissions and stock wastage.	-

. It can be seen that most of these materials have specific application scenarios and focus on the use of alternative materials and processes, which are analyzed in the following sections.

Alternative Materials and Processes

Many low-carbon technologies and alternative low-carbon materials have been adopted to reduce CO₂ emissions from traditional building materials. Blended cements can reduce CO₂ emissions by replacing the amount of clinker used in production process with one or more supplementary cementitious materials such as granulated slag, fly ash, activated rice husk ash and silica fume [57]. Among them, geopolymer concrete (GPC) is produced from industrial wastes such as fly ash, slag, and basalt as an alternative to ordinary Portland cement (OPC) concrete. The production of one ton GPC releases only 0.184 tons of CO₂, compared to about one ton of OPC concrete [58], but it is less durable than OPC [59]. Fired clay bricks can be replaced by stabilized mud bricks and compacted fly ash blocks, which is more low-carbon and energy conservation due to the elimination of high temperature firing process [57]. In addition, the correct application of low-carbon materials is critical, which includes the adoption of prefabrication and pre-assembly processes [60,61] and the application of building information model and augmented reality tools [62,63].

Natural Materials

Since natural materials are natural existing, common, and reusable, they have the potential to become low-carbon materials. Natural materials mainly refer to stone, earth and biomass, where biomass can be wood, bamboo and hemp, etc. Crishna and others [64] used the lifecycle assessment method for dimension stone production and calculated the carbon footprint of sandstone, granite and slate as 77 kg CO₂e/ton, 107 kg CO₂e/ton and 251 kgCO₂e/ton, respectively. Rammed earth is a low-carbon technique for compacting soil to form solid walls and is divided into stabilized and un-stabilized rammed earth. Unstabilized rammed earth, made mainly of soil, sand and gravel, is almost zero carbon, but is susceptible to erosion. Stabilized rammed earth also contains additives such as cement or lime, which is stable and has been used successfully in many parts of the world [57]. Woods have a wide range of applications for reducing construction carbon emission. Wood-based buildings have lower CO₂ emissions compared with buildings made of concrete, steel, brick and aluminum [65]. By comparing mid-rise buildings with conventional concrete and steel, woods in buildings could reduce construction emissions by 69%, with an average reduction of 216 kgCO₂e/m² of floor area [66]. However, due to the long growth cycle and slow recovery after harvesting, the extensive use of wood for construction will cause damage to the environment. As the renewable biomass, bamboo with a short lifecycle of 4–5 years and high mechanical properties may replace existing building materials such as steel and concrete in the future [67]. However, bamboo is an anisotropic material with many factors affecting its material properties and lack of reliable design criteria, so it is not used as a primary resource for construction projects [7]. Currently, it is mainly used in temporary buildings and small civil buildings [68,69]. Some other fast-growing biomass resources, such as straw and flax, can be used as insulation alternatives in construction or retrofitting [51]. In addition, some studies have used mushrooms, wool and junk food as raw materials for construction [52]. However, natural materials have impediments in terms of quality and strength, and their preparation and transportation processes consume a lot of energy, which leads to carbon emissions [51].

4.1.2. Envelope Design

Variety of Low-Carbon Envelope Designs

The design of building envelope can contribute to the reduction of energy consumption and CO₂ emissions during building operation, by decreasing building load demand. Suresh and others [70] made an exhaustive technical review of energy-efficient building envelope involving wall, floor, roof and others. Based on these, Figure 4 summarized the common low-carbon envelope design.

The building envelope includes opaque envelopes such as walls, roofs and floors and other transparent envelopes, which differ in their low-carbon design. For opaque envelopes, insulation materials, phase change materials, highly reflective paints and building envelope integrated green plants (BIGP) have been researched in recent decades. Insulation materials can reduce heat loss by increasing the thermal resistance of the envelope to achieve energy savings. Different from them, phase change materials change their state to store or release thermal energy according to the environment temperature, which can reduce the building load. Cunha and others [71] reviewed its characteristics and application in buildings, and presented the connections between the phase change materials, energy efficiency and energy poverty. For exterior walls and roofs, the highly reflective paints with high reflectivity and emissivity, can reduce heat entry from outside. Moreover, BIGP as one of the most promising building envelope designs, can improve energy efficiency for the plant shading and transpiration [72]. The energy saving potential of BIGP in hot summer and cold winter regions of China is verified and analyzed. It showed that the energy saving rate with BIGP is about 18% in winter, and about 25% in summer [73]. Fenestration as transparent envelopes is one of the least energy efficient parts in buildings. To solve this problem, low-e glasses have been widely used in recent decades. However, its solar transmittance is so low that natural lighting and passive heating in winter is poor, which also increases the energy consumption to some extent. It has certain safety problems with a self- exploding rate of 3%, which is three times of the ordinary flat glass. Therefore, smart windows that can dynamically adjust light transmission are developed. Based on different mechanisms, they are classified into electro-, thermo-, mechano-, photochromic and beyond. For each kind of smart window, Ke and others [74] provided an overview, and discussed emerging technologies such as integrated devices toward multifunctionality and dual stimuli triggered smart windows.

Table 4. Summary of smart windows.

Refs.	Categories	Principle	Illustration	Advantage	Disadvantage
[74,75,76,77,78]	Electrochromic	It can reversibly modulate the optical properties upon application of potential.		It can reduce energy consumption, offer privacy and comfort for occupants	It requires the multilayered structure, electric control system and one-time low energy input.
[74,79,80,81,82]	Thermochromic	Its transparency can be changed according to the temperature, which allows it passively modulate the light adaptively in respond to the dynamic ambient temperature.		(1) It can reduce energy consumption by automatically transmitting more indoor solar irradiation in cold days than in hot days without extra energy input. (2) it is purely driven by materials and does not require additional control systems.	Its properties depend on the climatic conditions.
[74]	Mechanochromic	Either the internal structures or surface morphologies of composed mechanoresponsive optical materials can be deformed and reconfigured by mechanical strain, which alters the optical transmittance through scattering or diffraction of visible light.		It has simple construction, low cost and fast response time.	It requires mechanical control systems and additional control.
[74]	Photochromic	It can reversibly change its color when exposed to certain wavelengths of light.		It allows simple fabrication without extra energy input, similar to the thermochromic windows	It is only suitable for tropic countries

summarizes the windows in this paper. As can be seen, thermochromic and photochromic smart windows can automatically adjust the transparency without control systems, which have a more low-carbon potential. However, they are worse in terms of stability, compared with electrochromic and mechanochromic smart windows. In addition, photochromic smart windows have limitations in application.

Efficiency of Various Low-Carbon Envelope Designs

The potential for energy saving and carbon reduction of envelope design is influenced by many factors, and it has been studied by researchers to provide scientific advices to architects in making low-carbon strategies during envelope design. Xia and Li [83] compared the carbon reduction potential of seven design strategies including photovoltaic, wind energy, passive heating, natural ventilation, lighting, shading and green planting for an office building in Shanghai and concluded that photovoltaics had the highest potential and natural ventilation was the best passive design option, while passive heating and wind power had the least impacts. Li, et al. [84] obtained the optimal window-to-wall ratio for prefabricated residential buildings under the low-carbon target and investigated the effects of window height and aspect ratio on CO₂ emissions. The results revealed that round windows were the worst in terms of energy saving and emissions reductions, while rectangular windows with an aspect ratio of 3:4 were found to be the most beneficial for reducing building CO₂ emissions. A summary analysis of the effectiveness of passive building envelope measures in the United Arab Emirates was conducted by researchers [73], and the key factors of natural ventilation could realize energy conservation from 30% to 79%, excessive light levels and glare, glazing type and orientation with up to 55% energy savings potential, and thermal insulation with a more than 20% energy savings potential. Arumugam and others [85] investigated passive technologies, such as insulation, phase change material (PCM), natural and/or night ventilation and recommended appropriate energy saving strategies for buildings in various climate based on the Koppen–Geiger climate classification.

4.2. LCES

Low-carbon technologies for building and community operations mainly refer to their LCESs. Currently, there is no clear definition of a LCES in building or community. The energy system that uses renewable energy technologies, diverse energy storage technologies and system optimization technologies to save energy and reduce CO₂ emissions is considered, involving multiple energy system types, such as integrated energy system (IES), multi-energy system (MES), distributed energy system (DES), hybrid energy system (HES) and renewable energy system (RES), microgrid, etc. The scope of the study focused on building-level and small-scale community-level energy systems, which differs little in terms of system composition and technology, so they are described without distinction.

4.2.1. Energy System Composition

The LCES consists of supply side, energy conversion side and demand side, as shown in Figure 5. Renewable energy, natural gas and electrical energy on the supply side are used to satisfy the heating, cooling, and electricity requirement on the user side through technical equipment on the energy conversion side.

The LCES generally includes the heating system, the cooling system, the power system and the energy storage system. Heating systems include solar thermal collectors (STCs), air source heat pumps (ASHPs), ground source heat pumps (GSHPs), water source heat pumps (WSHPs), combined heat and power generation (CHP). The main equipment of the cooling systems are absorption chillers (AC), electric chillers (EC), ASHPs, GSHPs and WSHPs. Electricity is mainly supplied by the grid, photovoltaic (PV), wind turbines (WT), hydroelectricity (HDE), CHP and combined cooling, heating, and power generation (CCHP). Energy storage systems include heat storage (HS) equipment, cold storage (CS) equipment and electricity storage (ES) equipment. However, LCESs proposed by various studies differ, as the system equipment configuration options are influenced by many factors such as geographical location, climatic conditions, building characteristics, living habits and economic levels, etc. To investigate the effect of climate condition and building energy performance, which influence the energy demand of the building, on the configuration of off-grid hybrid RESs, Mokhtara and others [86] optimized the system configuration of low and high efficient buildings in seven zones according to the renewable energy potential map of Algeria. PV, WT, diesel generators (DG) and a battery was the optimal configuration in Adrar and Tindouf for low efficient buildings, while PV, DG and battery was obtained in the other locations. However, better system configuration for high-performance buildings was acquired in Biskra and Tamenrast with a PV battery that can achieve 100% renewable energy. Considering the variation of future climate change and energy price, Sobhani and others [87] suggested that there would be an increase in cooling demand as well as a decrease in heating and electricity requirement by 14.6% and 2.29% for a nearly zero energy building after 20 years, which resulted in an oversized system to satisfy heating and power needs and lack sufficient capacity to meet cooling demand in the future. Moreover, the energy system configuration of offices, hospitals and hotels in different climate zones of Finland was discussed, and the system construction varied for these buildings [88]. For the HES of a building integrating commercial, office and residential functions, Rikkas [89] optimized its configuration, sizing and operation and found that cooling and HS was better options than electricity storage, district heating and HPs could work together, but the options of district cooling and HPs were mutually exclusive.

Table 5. Types and configurations of LCESs.

Refs.	System Type	Application	Heating Equipment	Cooling Equipment	Electricity Equipment	Energy Storage Equipment
[92]	DES + IES + MES	Comprehensive community	STC + GB + WHRU	ASHP + AC	PV + ICE + Grid	HWT + LB
[93]	IES	University	WHB + GB	AC + EC	PV + WT + GT + Grid	AA-CAES
[94]	Microgrid	Commercial buildings	-	-	PV + Grid	Micro PHS
[90]	Microgrid	-	-	-	PV	HDS + LB
[95]	DES + MES	Office building	FC-CHP + solar heating system	AC	PV + FC-CHP + Grid	HWT + CWT + HDS
[96]	IES	Commercial building	WHRU + GB + EB	AC + EC	GT + PV + WT + Grid	-
[86]	HES + RES	Residential buildings	-	-	PV + WT + DG	LB
[87]	RES	Residential building	STC + HP + GB	AC + EC	PV + WT+ Grid	HWT
[89]	HES	Mixed-type building	district heating + HP	district cooling + HP	PV + Grid	HS + CS + ES
[88]	HES	Hospital/hotel/office	STC + WHRU + GB + HE	AC + EC	PV + ICE + Grid	HWT
[97]	IES	Building	WHRU + EB +GSHP	AC + GSHP	PV + WT + GT + Grid	HWT + CWT + ES
[98]	MES	University	Hydrogen-fired boiler + ASHP + GSHP + biogas CHP	ASHP + GSHP	PV + WT + HDE + biogas CHP + Grid	HWT + CWT + HDS
[99]	IES	Residential communities	WHRU + EB + GB + HE	EC + AC	PV + WT + GT	ES

summarized the types and configurations of LCESs in different studies. It can be seen that most LCESs include CHP or CCHP system with the stepped use of different qualities of energy, which improve energy efficiency and reduce CO₂ emissions. However, due to the use of gas boilers (GB), Gas turbines (GT) and DG, there is still a certain amount of CO₂ emissions. In addition, the LCESs purchases from the grid when electricity demand exceeds supply, so 100% renewable energy is not guaranteed [90].

Figure 5. The basic framework of LCES. (Modified from ref. [91]).

Figure 5. The basic framework of LCES. (Modified from ref. [91]).

4.2.2. Renewable Energy Technology

Renewable energy does not produce CO₂ emissions during its operation, so it is also called zero-carbon energy. The application of renewable energy is very important to achieve low-carbon community. The renewable energy sources commonly used in low-carbon community include solar energy, wind energy, geothermal energy, water energy, biomass energy and air energy, as presented in Figure 5. The main technology that uses geothermal, water and air energy is HP, which is an efficient heating and cooling device. Since its energy efficiency is influenced by various factors, its type needs to be selected according to the application scenarios. Due to space constraints in buildings and community, the general technology for harnessing wind energy is small and micro wind turbines on high rise buildings are commonly used. Biomass technologies are generally divided into biogas technologies and biomass thermal conversion technologies. However, they are mainly applied in rural areas and few used in cities due to collection and storage, transportation and environmental problems. The most commonly utilized renewable energy source is solar energy, which is utilized with technologies such as photovoltaic panels, solar collectors and series connection between photovoltaic thermal module and solar thermal collector (PVT-ST) [100]. Considering the limitations of installation space, many research studies focused on rooftop solar photovoltaics [67], building-integrated photovoltaics (BIPV) and bifacial PV modules. In summary, many renewable energy utilization technologies have been applied to building and community energy systems. However, renewable energy is intermittent, stochastic, and volatile, which can cause system instability. Therefore, energy storage technology or an auxiliary energy system is essential to enhance the stability of the renewable energy system.

4.2.3. Energy Storage Technology

Energy storage technology can solve the intermittency problem of renewable energy and improve the consumption capacity of renewable energy. Energy storage in community energy systems includes HS, CS and ES. Cooling and heat storage equipment, which is inexpensive, safe and can be used in large scale, plays an important role in system operation and flexible control. Compared to them, the cost of electricity storage is high. The commonly used storage battery is lithium-ion battery (LB), which is very promising in the construction field due to its high energy density, but its cost is higher in large-scale applications [101]. Underground compressed air energy storage (CAES) and pumped hydro energy storage (PHS) are technologies that can be applied on a large scale, discharging for tens of hours with capacities of up to 1 GW. CAES stores electrical energy through compressing air and drives the turbine to generate electricity by releasing the compressed air. Advanced adiabatic compressed air energy storage (AA-CAES) can achieve multi-energy storage and supply, and be used in integrated energy systems to consume renewable energy and reduce the CO₂ emissions [93]. The application of micro-pumped storage in commercial buildings is more economical compared to battery storage, but it is limited by its geographical location [94]. Oak Ridge National Laboratory has developed a modular pumped storage technology called ground-level integrated diverse energy storage (GLIDES), consisting of a pre-pressure pressure vessel, a storage tank, a hydraulic turbine/generator and a pump/motor. It is seen as a combination of CAES and PHS, which not only improves the efficiency of compressed air storage but also eliminates the geographical constraints of conventional pumped storage [102]. As the cost of PV and wind power generation decreases, the preparation of hydrogen by electrolysis of water for fuel cell (FC) power generation may become a low-cost option in places where renewable energy resources are abundant [103]. Reference [90] used a combination of hydrogen and LB energy storage, in which the hydrogen storage (HDS) system could solve the power imbalance problem of photovoltaic day and night power generation, and the LB could stabilize the short-term fluctuation of power and prevent the electrolyzer and FC from starting and stopping frequently, achieving a zero-emissions energy supply by replacing the carbon-based energy system with a hydrogen-based energy system. Reference [95] used a combination of HDS, cold water tank (CWT) and hot water tank (HWT) for energy storage to establish a building hydrogen-based MES to reduce CO₂ emissions. In summary, various energy storage has been applied to LCES, and

Table 6. Principles, pros and cons of different electricity storage technologies.

Refs.	Electricity Storage	Principle	Advantages	Disadvantages
[90]	LB	-	High energy density, high carrying capacity, high operating voltage, low self-discharge rate, no memory effect, high adaptability to high and low temperatures, environmental protection, long service life	Expensive for large scale applications
[93]	CAES	Use of electrical energy for compressed air storage during low load demand and release compressed air to drive the turbine for power generation during peak load demand	Can be applied in a large scale, and can be discharged for tens of hours and at a low cost	Technological barriers in safe and stable operation
[94]	PHS	When water is pumped from the lower pool to the upper pool, the electrical energy is converted into potential energy of the water and stored, and the potential energy is converted into electrical energy when the water flows from the upper pool to the lower pool through the turbine.	Can be applied in a large scale, with capacities up to 1 GW, and can be discharged for tens of hours and at a low cost	Limited by geographical location
[102]	GLIDES	By pumping liquid from the reservoir into the pressure vessel to compress condensable gases such as air and CO2 in the pressure vessel for energy storage, the high-pressure gas then pushes the water from the high-pressure head through the turbine to flow back into the reservoir for discharge	There are no restrictions on the installation location, a variety of low-grade heat sources can be integrated and waste heat can be used to improve discharge efficiency	-
[90]	HDS	Hydrogen production by electrolysis of water for FC power generation	High energy density per unit mass, clean and pollution-free, etc.	-

summarizes the principles, advantages, and disadvantages of different electricity storage technologies.

4.2.4. System Optimization Technology

Energy system optimization technologies, which can reduce the CO₂ emissions, improve the energy efficiency and economic benefits of the system, are important for low-carbon buildings and communities. The main research contents and methods of low-carbon energy system optimization are presented in Figure 6. The optimization problem consists of objective functions, decision variables and constraints. Methods for solving energy system optimization problems include traditional and meta-heuristic approaches. The traditional methods include nonlinear programming (NLP), linear programming (LP), and mixed integer programming (MIP). With the increasing diversity and complexity of energy systems, traditional methods are gradually being replaced by meta-heuristics due to the inability to provide optimal solutions in limited time. Meta-heuristic methods can quickly and efficiently obtain the global optimum for continuous and nonlinear problems, including physics-based, biology-based, mathematics-based, and sociology-based methods. Genetic algorithms (GA), particle swarm optimization (PSO) algorithms, simulated annealing (SA) and evolutionary algorithms are the most commonly used heuristics for energy system optimization [104]. At present, the optimization content of energy system mainly includes system design optimization, operation optimization, design, and operation collaboration optimization.

Design Optimization

As the first step of energy system decarbonization, design optimization refers to the selection of system equipment type and capacity size configuration to obtain the optimal objective value. Many research studies have been carried out to optimize the design of energy system with different methods, as presented in

Table 7. Design optimization of LCESs.

Refs.	Application	Single- Objective	Multi-Objective	Methods	Solvers	Considerations
[86]	Off-grid hybrid RESs		√	PSO + ε-constraint	MATLAB	Effects of climate change and building energy performance
[87]	A RES for nearly zero energy buildings	√		GA	MATLAB	climate change and energy price variations in the future
[88]	A solar hybrid CCHP system	√		PSO	MATLAB	Influence of building type and climate condition
[98]	Carbon-neutral energy systems of Cornell University campus	√		Global optimization algorithm	GAMS	Climate neutrality and 100% renewables
[103]	A hybrid microgrid-hydrogen storage facility in Saudi Arabia		√	Repeated algorithm	HOMER	Five configurations and hydrogen production
[105]	A solar heating system	√		Hooke-Jeeves	TRNSYS + GenOpt	Combination of phase change energy storage tank and electromagnetic heating unit
[106]	Stand-alone photovoltaic/wind-generator systems	√		GA	-	System design characteristics such as the PV modules tilt angle, the WT installation height and the number of battery chargers
[107]	A low temperature local hybrid energy system		√	GA	TRNSYS + MATLAB + MOBO	Cross-seasonal storage of solar energy by a bore thermal energy storage

Note: √ refers to the adoption of single-objective or multi-objective.

. Five microgrid system configurations were simulated and optimized by Hybrid Optimization of Multiple Energy Resources (HOMER) software in Saudi Arabia’s Yanbu city, and the results showed that Generator-PV-wind-battery-hydrogen electrolyzer-FC microgrid configuration had the best economic and environmental benefits [103]. The Hooke-Jeeves algorithm was used to iteratively optimize the configuration of solar collector inclination and area, induction heater power and storage tank volume for a solar heating system with coupled phase change storage tank and induction heater using TRNSYS simulation software and GenOpt optimization program [105]. The energy combination, optimal configuration, seasonal operation, and corresponding capacity of each technology for a carbon-neutral energy system on the Cornell campus was calculated with a multi-period mixed-integer nonlinear optimization model [98]. Since the GA can obtain the global optimal solution with relatively simple computation, reference [106] used GA to optimize the device capacity size for a stand-alone photovoltaic/wind-generator system. A hybrid renewable energy system for a local residential community was modeled in TRNSYS, and the optimal capacity configuration of the system was obtained by Multi-Objective Building Optimizer (MOBO) software using the GA algorithm, with MATLAB as the intermediary between TRNSYS and MOBO [107].

Operation Optimization

Operation optimization refers to the energy supply and storage devices operating strategy to minimize the objective function within a defined operating range [95]. Its objective functions generally include the operating cost and CO₂ emissions. Just like the AA-CAES storage model, a stair-step carbon trading mechanism wasintroduced to optimize the system dispatch with the objective of minimizing the operating cost and carbon trading cost of the IES. The results showed that both the operating cost and carbon emissions were reduced compared with the traditional IES [93]. In addition, an operation optimization on the balance between energy costs and CO₂ emissions of hydrogen-based building energy systems was conducted, and the results revealed that the integrated operation of the system could improve the efficiency of renewable energy supply and benefited on economic and environmental [95]. However, the solution of the LCES operation optimization is difficult and time-consuming for the complex scenarios and numerous variables, so many methods have been proposed to obtain the optimal operation strategies, as shown in

Table 8. Operation optimization of LCESs.

Refs.	Application	Single- Objective	Multi-Objective	Methods	Solvers	Considerations
[93]	IES	√		-	-	The AA-CAES storage model, a stair-step carbon trading mechanism and several scenarios
[94]	The building-integrated microgrid	√		-	-	Photovoltaic and pumped storage
[96]	Low-carbon oriented IES in commercial building		√	MIP	CPLEX	Electric vehicle and DR
[97]	IESs a building in Northeast China		√	Improved ALO	-	Comparison with standard PSO algorithm and standard ALO algorithm
[95]	Distributed Hydrogen-based MES	√		MIP	Gurobi	Emissions trading schemes, carbon emissions reductions formulated by economic benefit
[109]	Zero-carbon multi-energy system	√		ROSO + MILP	Gurobi	Incorporating Electric Vehicle (EV) multi-flexible approach, virtual powerplant
[110]	Multi carrier energy systems	√		MILP	CPLEX	The thermal energy market and DR
[112]	Solar hybrid CCHP systems of an office building in Beijing		√	PSO	-	Redundant design
[113]	IESs of a hotel in northern China		√	GA + SDP	MTALAB	Energy storage and DR

Note: √ refers to the adoption of single-objective or multi-objective.

. The common method is to formulate it as a MILP problem [108] and then solve it by commercial solvers such as CPLEX and Gurobi [96] to reduce the calculation time. For example, to achieve the optimal scheduling of zero-carbon multi-energy systems and electric vehicles with multi-flexible potential, a day-ahead optimal scheduling with Robust-stochastic optimization (ROSO) method was proposed, which modified the problem as mixed integer linear programming (MILP) and solved it by Gurobi solver [109]. The CPLEX solver of GAMS optimization software was employed to solve the stochastic optimization of energy hub operation considering the thermal energy market and demand response (DR) [110]. Nevertheless, with the increase of LCES variety and complexity, this method could not provide optimal solutions within finite time [111]. Meta-heuristic algorithms and their enhancements have been used to address the shortcomings of the conventional approach. For instance, improved ant lion optimization (ALO) algorithm can be used for the optimal operation strategy of building energy systems due to its advantages in strong global search ability and convergence [97]. Based on the PSO algorithm, a two-layer operation optimization method has been applied to optimize the solar hybrid CCHP system by considering redundant design [112], which was considered to be a more accuracy optimization method. Single meta-heuristic algorithms can quickly converge to a local optimal solution for the complex non-linear optimization model, so a two-layer method combining the GA and stochastic dynamic programming (SDP) were proposed, which used GA for the outer demand-side optimization and SDP for the inner supply-side optimization to optimize the IES by taking energy storage and DR into account [113].

Co-Optimization of Design and Operation

Since system design and operation interact with each other, the overall structure, equipment capacity configuration and operation strategy of the system need to be optimized synergistically to maximize the benefits of the system. A two-layer multi-objective co-optimization model took into account the coupling relationship between equipment capacity and operation patterns was proposed. Generally, the device capacity configuration is optimized with objectives of maximizing the total economic, environmental and energy benefits in the upper layer and the operating parameters are optimized with objectives of maximizing the daily economic, environmental, and energy benefits in the lower layer. Its framework is presented in Figure 7.

As can be seen, the upper layer makes decision to guide the lower optimization. Meanwhile, the lower layer utilized the upper layer judgements as constraints or inputs, so that it can make decisions independently within its own defined range. The information interchange between the upper and lower levels has a timely and feedback impact. Consequently, the solution to the two-layer optimization is an NP-hard problem [91]. There are two common solutions for this two-layer model. The first approach uses intelligent algorithms for upper-level optimization and MILP for lower-level optimization [90]. For example, to minimize the total net present value cost and CO₂ emissions over the lifecycle, Guo and others [114] applied a multi-objective GA based on non-dominated ranking genetic algorithm II (NSGA-II) to solve the optimal design problem and a MILP to solve the optimal dispatch problem. A typical CCHP microgrid system of a hospital was used to verify the effectiveness of the method. However, such a solution is highly random in scheduling and prone to local optimal solution. To address this problem, Zhao and others [90] proposed an optimal scheduling strategy with the minimum operating cost of energy storage equipment to substitute the MILP in the lower layer, and employed PSO algorithm to obtain the optimal configuration of the PV-hydrogen zero carbon emissions microgrid. Another popular method is to combine NSGA-II and the technique of ranking by similarity to ideal solutions (TOPSIS) to solve this two-layer model. This approach has been used to optimize the configuration and operation of the DES combining multiple energy storage in a near zero energy community [115]. This method can also be used to solve the two-stage co-optimization model by optimizing the main equipment capacities in the first stage and the capacities of energy storage devices in the second stage for the energy system of near-zero community [116]. Different from the two-layer model, the two-stage model that ignores the interaction between the two stages and the results obtained from the second stage do not feedback to the first stage, so it is less used than the two-layer model. In addition to two-layer and two-stage optimization, other optimization methods based on mathematical programming have also been used for the collaborative optimization of LCESs for different problems, such as the combination of fmincon functions and the sequential quadratic programming (SQP) algorithm for different energy vector [117] as well as integration of the MILP, the ε-constraint and the Linear Programming Techniques for Multi-dimensional Analysis of Preference (LINMAP) to solve precision and scale difference [118].

Table 9. Co-optimization of LCESs.

Refs.	Application	Single-Objective	Multi-Objective	Methods	Solvers	Considerations
[90]	PV-Hydrogen Zero Carbon Emissions Microgrid		√	PSO	-	A scheduling strategy considering the minimum operational cost of energy storage equipment
[89]	HES for a mixed-type building in Finland	√		Dynamic LP/MILP	-	A mixed-type building with commercial, office, and residential parts
[92]	A novel distributed energy system integrated with hybrid energy storage in different nearly zero energy community scenarios		√	NSGA-II + TOPSIS	-	12 nearly zero energy community scenarios considering different community types and scales with electric vehicles as a new-type load
[117]	DES		√	Fmincon functions and SQP algorithm	MATLAB	The time-varying operation of the energy conversion units in response to electricity and hydrogen demands as well as the seasonal behavior of the storage system
[118]	IES		√	MILP + the ε-constraint + LINMAP	Python	Precision and scale difference
[116]	A new DES that combines multi-energy storage for a nearly zero energy community		√	NSGA-II + TOPSIS	-	Three modes of EV charging
[115]	A DES combining multiple energy storages		√	NSGA-II + TOPSIS	-	Comparation of the two-layer co-optimization method proposed, multi-parameter co-optimization method, two-stage co-optimization method and two- layer nested co-optimization method
[114]	CCHP microgrid system for a hospital in the southern China		√	NSGA-II + MILP	-	The total net present cost and CO2 emission

Note: √ refers to the adoption of single-objective or multi-objective.

summarized those methods for co-optimization of LCESs.

5. Evaluation of Low-Carbon Buildings and Communities

5.1. Low-Carbon Building Evaluation System

The low-carbon building evaluation system classifies low-carbon building through the quantitative index and evaluation level [119]. In order to regulate the development of low-carbon buildings and improve the living environment of human beings, countries around the world have formulated relevant certification standards. All the countries have reached the consensus that guide the low-carbon building from the content rather than from the form of the new characteristics of low-carbon energy savings is essential.

The Leadership in Energy & Environmental Design (LEED^TM) building rating system is established and promoted by the Green Building Council of the United States. It is considered to be the most perfect and influential evaluation standard in all kinds of building environmental protection assessment, low carbon building assessments and building sustainability assessment standards in the world. LEED evaluation factors include Sustainable Sites, Water Efficiency, Energy & Atmosphere, Materials & Resources, Indoor Environmental Quality, Innovation & Design Process, etc. The rating standard is to assign different scores to the effects of the project by each index content, thus classifying the construction project as Certified, Silver, Gold and Platinum [120]. Due to its geographical characteristics, such as relatively small land and resources, surrounded by sea, and early economic development, the UK pays close attention to environmental issues and is one of the countries that started green and low-carbon building [121]. BREEAM is the full name of Building Research Establishment Environmental Assessment Method, commonly known as the British Institute of Building Green Building Assessment System. Founded in 1990, BREEAM is the first and most widely used low-carbon building assessment method in the world. BREEAM includes index system evaluation of 10 aspects, including Management, Health & Wellbeing, Energy, Transport, Water, Material, Waste, Land Use & Ecology, and Pollution. BREEAM evaluates buildings by assigning points to the specific contents of each index. According to the points, the certification levels of building projects are classified into Unclassified, Pass, Good, Very good, Excellent, and Outstanding [122]. The German Sustainable Building Evaluation Standard (DGNB evaluation system) is developed based on the high-quality construction industry standards in Germany with the strong support of the German government. With LEED^TM and BREEAM compared to adopt international standards, such as DGNB belongs to the second generation of the green building assessment system, the system overcomes the first generation of the green building standards and focuses on ecological technology factors such as limitations, emphasize from the sustainability of ecological, economic and social dimension of three basic in emphasized to reduce the pressure for the environment and resources at the same time. To develop a customer-oriented index system so that “sustainable building standards” can help guide better planning and design of construction projects and create a better living environment. The DGNB evaluation system is the only certification system that focuses on both the economic quality and ecological quality of green buildings. DGNB system includes six core building qualities, namely Environmental Quality, Economic Quality, Sociocultural and Functional Quality, Technical Quality, Process Quality, Site Quality. According to the scores of construction projects, buildings are divided into four levels: Platinum, Gold, Silver and Bronze [123,124]. China has also issued relevant government documents and standards for low-carbon building. The Ministry of Construction of China issued the Assessment Standard for Green Building (CASGB) in 2006, which was updated and reconstructed in 2019 [125]. It is clear that the low-carbon building evaluation index system should be composed of five indicators: Safety and Durability, Health and Comfort, Convenience of Life, Resource Saving and Livable Environment. Based on the scores of various buildings, the grade of buildings is divided into four levels: Basic level, One star, Two-star and Three-star. At the same time, standard for building carbon emissions calculation was issued in 2014, and the updated calculation standard of building carbon emissions was issued in 2019, which clarified the definition, calculation boundary, emissionsfactor and calculation method of building emissions [126]. In addition, other countries also have the corresponding evaluation criteria. For example, Japan’s Comprehensive Assessment System for Building Environmental Efficiency (CASBEE) certification, Green Building Tool (GBTool) from Canada, Green star certification from Australia and High Environmental Quality (HQE) certification from France. Figure 8 shows the evaluation system of low-carbon building in different countries and the detailed evaluation standards are listed in

Table 10. Evaluation standards for low-carbon building in countries around the world.

Refs.	Countries	Evaluation Standard	Evaluation Content
[127]	The United States	LEEDTM	Sustainable Sites, Water Efficiency, Energy & Atmosphere, Materials & Resources, Indoor Environmental Quality, Innovation & Design Process.
[123]	Germany	DNGB	Environmental Quality, Economic Quality, Sociocultural and Functional Quality, Technical Quality, Process Quality, Site Quality.
[128]	Britain	BREEAM	Management, Health & Wellbeing, Energy, Transport, Water, Material, Waste, Land Use & Ecology.
[129]	Canada	GBTool	Environmental Sustainability Indicators, Resource Consumption, Environmental Load, Indoor Air Quality, Maintainability, Economy, Operation Management.
[130]	Australia	Green Star	Management, Indoor Environment, Energy, Transportation, Water, Material Saving, Land Use and Ecology, Emissions, Innovation.
[126]	China	CASGB	Safety and Durability, Health and Comfort, Convenience of Life, Resource Saving and Livable Environment.
[131]	France	HQE	Eco-construction target, Eco-management target, Comfort target, Health target.

.

5.2. Low-Carbon Community Evaluation System

The evaluation system of low-carbon community and low-carbon building complements each other but are different. The evaluation systems with a wide range of influence and application scope include LEED for Neighborhood Development (LEED-ND) in the United States, BREEAM-Communities in the United Kingdom, CASBEE for Urban development (CASBEE-UD) in Japan, and Guidelines for Pilot Construction of Low-carbon community in China and other evaluation system.

The LEED-ND system scores and certificates communities based on six aspects: community site selection, energy utilization, water environment, transportation, ecological environment, community planning and design [132]. Among them, the score of community energy utilization item accounts for 26%, occupying a high proportion. The LEED-ND system encourages community energy systems to reduce the environmental and economic impact of conventional energy through the use of active and passive solar technologies and on-site renewable energy. It also encourages communities to develop efficient district heating and cooling systems and minimize building energy consumption through optimized energy system design and operation [133].The British BREEAM-Communities system points out that low-carbon community should realize energy savings through reasonable energy system design and operation management, and promote the consumption of renewable energy through solar energy utilization technology to reduce the consumption of conventional energy and carbon emissions intensity [134]. Japan’s CASBEE-UD system proposed the evaluation index of BEE for low-carbon community assessment, which aimed to achieve the highest building environmental quality through the least building environmental load [135]. It pointed out that communities should achieve energy savings and emissions reductions through the conversion and utilization of solar energy and other renewable energy. At the same time, the system equipment should be rationally carried out itemized measurement and operation scheduling to realize the efficient operation of the system equipment [136,137]. China issued “Handbook of Technical Assessment of Ecological Settlements in China”, “low-carbon community pilot construction guideline” and other guidelines. The Handbook of Technical Assessment of Ecological Settlements in China points out that community energy systems should be optimized, including the design and operation optimization of cooling and heating supply system, the energy distribution system, domestic hot water system and lighting system. In addition, renewable energy should be made full use to realize community energy conservation and emissions reduction, and reduce the impact of conventional energy on community environment [138]. The guidelines for pilot construction of low-carbon community provides quantitative reference of binding evaluation indexes and guiding evaluation indexes respectively for new urban communities, existing urban communities and rural communities, among which the least limits of CO₂ emissions reduction rates for the above three communities are 20%, 10% and 8%, respectively [139]. The low-carbon community evaluation system is also constantly improving and developing, which is essential for the construction of low-carbon community.

5.3. LCES Evaluation

LCES evaluation, as a key part of low-carbon building and community evaluations, has been conducted to help decision-maker or engineer to identify the most suitable LCES. It reflects the strengths and weaknesses of low-carbon energy systems compared with reference systems, and potential improvement can be obtained [140]. The evaluation indexes include economic, environmental, technical, and comprehensive aspects, among which comprehensive evaluation uses weights to combine economic, environmental and technical indicators to address the trade-offs between each other.

Table 11. LCES performance analysis.

Refs.	Economy	Environment	Technology	Comprehension
[117]	√	√	√
[141]	√
[109]	√
[142]	√	√	√	√
[92]	√	√	√	√
[143]	√	√		√
[93]	√	√		√
[90]	√	√	√	√
[95]	√	√		√
[144]	√	√	√	√
[145]	√	√	√	√
[146]		√	√
[147]	√	√	√	√
[148]		√	√
[149]	√	√	√	√
[150]	√	√	√	√

Note: √ refers to the adoption of economy, environment, technology or comprehensive analysis.

summarizes the evaluation content of some current studies.

5.3.1. Economic Evaluation

As can be seen in Table 11, most of the current studies on low-carbon energy systems involve economic evaluation, which focuses on assessing the economic impacts of low-carbon technologies. Its evaluation indexes contain cost saving, levelized cost of energy, net present value. payback period and so on, as shown in

Table 12. The typical economic evaluation criteria of LCESs.

Refs.	Criteria	Formula	Parameters Description
[141]	Investment cost	$C^{\mathrm{IV}}={\left(x^{\mathrm{IV}}\right)}^{\mathrm{T}}·IC$	Where $x^{\mathrm{IV}}$ are the vector of equipment capacities, IC is the vector of unit investment cost for each.
[151]	Operation and maintenance cost	$O\&M_F={\displaystyle \sum _k^K}O\&M_k;$ $O\&M_k=f\left(C_k\right)$	Where $O\&M_k$ is considered as a percentage of the annual investment cost of the equipment; $C_k$ is the capital cost.
[103]	Net present value	$NPV={\displaystyle \sum _{n=1}^t}{i}_d\left(C_{Com}+C_{O\&M}+C_{Fuel}+C_{Rep}+C_{sal}\right)$	Where $n$ is the system lifetime in years; $i_d$ is the discount rate; $C_{Com}$ is the capital cost of a system component; $C_{O\&M}$ is the operation and maintenance cost; $C_{Fuel}$ represents the fuel cost; $C_{Rep}$ is the replacement cost and $C_{sal}$ is the salvage value.
[103]	Annual total cost	$C_{ann}=CRF\times NPV$	Where $CRF$ is the capital recovery factor.
[103]	Levelized cost of energy	$LCOE=\frac{C_{ann}-c_{boiler}{H}_{served}}{E_{served}}$	Where $c_{boiler}$ is the marginal cost of boilers; $H_{served}$ is the total thermal load served and $E_{served}$ is the total electrical load served.
[92]	Annual cost per unit supply area	$ACunit=\frac{AC}{Agross}$; $AC=C_{inv}+C_{ope}+C_{ng}+C_{grid}+C_{pun}$	Where $A_{gross}$ is gross floor area, AC is the annual cost (AC) including initial investment cost ($C_{inv}$), annual operation cost ($C_{ope}$), natural gas consumption cost ($C_{ng}$), power punishment cost ($C_{pun}$), and power purchase cost from the state grid ($C_{grid}$).
[144]	Simple payback period	$SPP=\frac{CAPE{X}^{cchp}-CAPE{X}^{con}}{OPE{X}^{con}-OPE{X}^{cchp}}$	Where $CAPE{X}^{con},CAPE{X}^{cchp}$ are the capital cost of conventional energy system and LCESs respectively. The OPEX is the total operation cost of g years.

. The economic evaluation was conducted on a near-zero carbon district energy system integrating waste-to-biogas and found that the system could realize the initial cost reduction by 77.07% under the carbon emissions constraint and lowered the levelized cost of energy (LCE) [141]. Fuzzy economic cost was introduced to evaluate the economy in complex scenario, just like considering the electric vehicle flexibility to evaluate the operational economics of a zero-carbon multi-energy system and demonstrate an 8.5% reduction in total cost at a high electric vehicle flexibility ratio of 0.54 [109]. However, the economic performance of low-carbon technologies is usually poor when the initial investment is taken into account, and even negative as the scale of low-carbon equipment reaches a certain level. The economy performance of GSHP and CCHP coupled system was lower than other performance index compared to the SPS, which was 15.13% for annual cost savings [145]. It was also quite low for the solar-hybrid CCHP systems, which was only 4.16% [147]. The economy performance of the solid-oxide FC-based CCHP system was also poor with the levelized cost of energy having and it had little difference from the average commercial electricity price in China [144]. Economy analysis is also helpful to improve the design of low-carbon energy system, such as the cost of a distributed energy system with 5000 m² of PV panels is 13% higher compared to the centralized scenario, and the cost can reach three times higher than that of power from the grid when the area of PV panels increases to 10,000 m² [117]. It indicates that decreases the cost of key low-carbon equipment can improve the economy of low-carbon energy systems [151]. Although the application of low-carbon technologies causes an increase in the initial investment, the operating costs can be reduced and it is proved to reach 23.9% for the multi-storage devices of AA-CAES compared with the conventional IES [93].

5.3.2. Environmental Evaluation

The environment evaluation is to assess the system’s environmental impact, including emissions of NO_x, SO₂, CO and CO₂, etc. For LCESs, it is mainly focused on the evaluation of CO₂ emissions, so the CO₂ emissions accounting is necessary. There are four approaches to calculate CO₂ emissions and they are influence factor decomposition method [152], measurement method [153], material balance method [154] and IPCC list method [154] with their characteristic shown in Table 13 [91]. The IPCC list approach is the most widely applied. This approach is straightforward and practical with a complete source of different energy emissions factors and established calculation formulas. Its CO₂ accounting formula is as follows:

$$E_{C{O}_2}={\displaystyle \sum }_{i=1}^M{\alpha }_i{Q}_{C{O}_2·i}$$

(1)

where $E_{C{O}_2}$ is the total CO₂ emissions, ${\alpha }_i$ is the CO₂ emissions factor for type i resource, $Q_{C{O}_2·i}$ is the consumption of type i resource and M is the is the number of resource species.

The CO₂ emissions of LCESs is influenced by various factors such as electricity prices, carbon taxes and the low-carbon technologies. With the increase of carbon tax and electricity price as well as the decrease of PV panel price, the CO₂ emissions of the distributed energy system decreased [151]. In addition, energy systems with various low-carbon technologies have different CO₂ emissions reduction effects. For instance, the solid-oxide FC-based CCHP system has a higher CO₂ emissions reduction ratio (CERR) than the microturbine based CCHP system and the internal combustion engine based CCHP system [144]. The IES with the multi-storage devices of AA-CAES reduced CO₂ emissions by 14.5% compared with conventional IES [93]. The CCHP system with coupled GSHP has a CERR of 35.02% compared to the SPS [145]. The CERR of the DES with solar hybrid clean fuel was 51.43% [146]. Moreover, a CCHP system powered by a combination of solar and biomass energy has a CERR of about 95.7% under design conditions [148] and a microgrid system combining hydrogen energy and electrical energy storage could achieve zero carbon operation [90].

Table 13. Four common methods for CO₂ emissions accounting.

Refs.	Method		Input Parameters	Application Scope	Characteristics
[91,152]	Influence factor decomposition method	IPAT model	Population size, per capita GDP, CO2 intensity	Fossil fuel combustion	Linear analysis
		STIRPAT model	Population size, per capita GDP, CO2 intensity	Fossil fuel combustion	Nonlinear analysis
		LMDI model	Emissions intensity, energy structure, energy intensity and economic development	Fossil fuel combustion	Dynamic variation analysis
		Kara model	Emissions intensity, energy intensity, per capita income, population size	Fossil fuel combustion	Dynamic variation analysis
		Lespeyres decomposition method	Changes in total output, industrial structure and carbon emissionsintensity in various industrial sectors	Fossil fuel combustion	Dynamic variation analysis
[91,153]	Measurement method	-	Air flow, CO2 concentration, conversion factor	Land use change and forests	Precision, high measurement requirements
[91,154]	Material balance method	-	Total input materials	Industrial production	Complete basic data records are required with reliable results
[91,154]	IPCC list method	-	Fuel consumption, emissions factors	Fossil fuel combustion	Simple and practical calculation

Table 13. Four common methods for CO₂ emissions accounting.

Refs.	Method		Input Parameters	Application Scope	Characteristics
[91,152]	Influence factor decomposition method	IPAT model	Population size, per capita GDP, CO2 intensity	Fossil fuel combustion	Linear analysis
		STIRPAT model	Population size, per capita GDP, CO2 intensity	Fossil fuel combustion	Nonlinear analysis
		LMDI model	Emissions intensity, energy structure, energy intensity and economic development	Fossil fuel combustion	Dynamic variation analysis
		Kara model	Emissions intensity, energy intensity, per capita income, population size	Fossil fuel combustion	Dynamic variation analysis
		Lespeyres decomposition method	Changes in total output, industrial structure and carbon emissionsintensity in various industrial sectors	Fossil fuel combustion	Dynamic variation analysis
[91,153]	Measurement method	-	Air flow, CO2 concentration, conversion factor	Land use change and forests	Precision, high measurement requirements
[91,154]	Material balance method	-	Total input materials	Industrial production	Complete basic data records are required with reliable results
[91,154]	IPCC list method	-	Fuel consumption, emissions factors	Fossil fuel combustion	Simple and practical calculation

5.3.3. Technology Evaluation

The technical evaluation of LCES is mainly focused on the assessment of system performance, including system efficiency, thermodynamic performance and reliability with evaluation indexes shown in

Table 14. The typical technology evaluation criteria of LCESs.

Refs.	Criteria	Formula	Parameters Description
[148]	Energy efficiency	${\eta }_{ee}=\frac{E+Q_c+Q_h}{Q_{biomass}+Q_{solar}}$	$E\mathrm{is}\mathrm{the}\mathrm{electrical}\mathrm{output},Q_c$$\mathrm{is}\mathrm{the}\mathrm{total}\mathrm{cooling}\mathrm{output},Q_h$$\mathrm{is}\mathrm{the}\mathrm{hot}\mathrm{water}\mathrm{output},Q_{biomass}$$\mathrm{is}\mathrm{the}\mathrm{biomass}\mathrm{energy}\mathrm{input},\mathrm{and}{Q}_{solar}$ is the solar energy input.
[148]	Exergy efficiency/Second-law efficiency/rational efficiency	${\eta }_{ex}=\frac{E+\left(\frac{T_0}{\overline{T_{rw}}}-1\right)Q_c+\left(1-\frac{T_0}{\overline{T_h}}\right)Q_h}{E{X}_{biomass}+\left(1-\frac{T_0}{\overline{T_{sol}}}\right)Q_{solar}}$	$T_0$$\mathrm{is}\mathrm{the}\mathrm{reference}\mathrm{ambient}\mathrm{temperature},E{X}_{biomass}$$\mathrm{is}\mathrm{the}\mathrm{exergy}\mathrm{of}\mathrm{the}\mathrm{biomass}\mathrm{input},\mathrm{and}$ $\overline{T_{rw}},\overline{T_h}$$\mathrm{and}\overline{T_{sol}}$ are the mean temperatures of the refrigerated water, domestic hot water and solar collector, respectively.
[148]	Primary energy ratio	The solar subsystems: ${\eta }_{e,sol}=\frac{Q_{c,sol}}{Q_{solar}}$ The biomass subsystems: ${\eta }_{e,bio}=\frac{E+Q_{c,bio}+Q_h}{Q_{biomass}}$	$E\mathrm{is}\mathrm{the}\mathrm{electrical}\mathrm{output},\mathrm{and}{Q}_{c,sol}$$\mathrm{and}{Q}_{c,bio}$$\mathrm{are}\mathrm{the}\mathrm{two}\mathrm{parts}\mathrm{of}\mathrm{the}\mathrm{total}\mathrm{cooling}\mathrm{output}\mathrm{from}\mathrm{the}\mathrm{solar}\mathrm{subsystem}\mathrm{and}\mathrm{biomass}\mathrm{subsystem},\mathrm{respectively}.Q_h$$\mathrm{is}\mathrm{the}\mathrm{hot}\mathrm{water}\mathrm{output},Q_{biomass}$$\mathrm{is}\mathrm{the}\mathrm{biomass}\mathrm{energy}\mathrm{input},\mathrm{and}{Q}_{solar}$ is the solar energy input.
[145]	Primary energy saving ratio	$PESR=\frac{F_{sp}-F_{lces}}{F_{sp}}=1-\frac{F_{lces}}{F_{sp}}$	$F_{LCES}$$\mathrm{represents}\mathrm{the}\mathrm{total}\mathrm{energy}\mathrm{consumption}\mathrm{of}\mathrm{LCES}\mathrm{all}\mathrm{year}\mathrm{round},F_{sp}$ represents all primary energy consumption of SP system.
[151]	Net interaction level	$NIL=$ $\frac{{{\displaystyle \sum }}_{t=1}^{8760}{E}_{grid,buy\left(t\right)}+{{\displaystyle \sum }}_{t=1}^{8760}{E}_{grid,sell\left(t\right)}}{{{\displaystyle \sum }}_{t=1}^{8760}{E}_{load\left(t\right)}}$	$E_{load\left(t\right)}$$\mathrm{is}\mathrm{the}\mathrm{hourly}\mathrm{electricity}\mathrm{load},E_{grid,buy\left(t\right)}$$\mathrm{is}\mathrm{the}\mathrm{electricity}\mathrm{purchased}\mathrm{from}\mathrm{the}\mathrm{grid}\mathrm{at}\mathrm{time}\mathrm{t}.E_{grid,sell\left(t\right)}$ is the electricity sold to the grid at time t.
[90]	Load loss probability	$L_{LP}=\frac{{{\displaystyle \sum }}_{t=1}^T{P}_{loss}\left(t\right)∆t}{{{\displaystyle \sum }}_{t=1}^T{P}_L\left(t\right)∆t}$	$P_{loss}\left(t\right)$$\mathrm{is}\mathrm{the}\mathrm{load}\mathrm{loss}\mathrm{power}\mathrm{and}\mathrm{renewable}\mathrm{energy}\mathrm{waste}\mathrm{power},\mathrm{respectively},P_L\left(t\right)$ is the equivalent load of the whole LCES.

. Many LCESs with various technologies have been evaluated with these indexes. The thermodynamic performance of the CCHP system driven by combined solar and biomass energy was assessed by the energy efficiency and exergy efficiency with results of 57.9% and 16.1%, and the contribution of the solar and biomass subsystems was evaluated by primary energy ratio (PER) with result that the PER of biomass subsystem was higher than that of solar subsystem [148]. The thermodynamic and energy performance of the solar hybrid clean fuel DES were also evaluated with the net solar-to-electric efficiency of 24.66%, the exergy efficiency of 38.31%, and PER of 83.86% [146]. Solid-oxide FCs as a low-carbon technology can improve the efficiency of LCESs. The in-use overall efficiency based on the low heating value of a solid-oxide FC-based CCHP system was higher than both the microturbine based and internal combustion engine based CCHP systems [144]. Renewable energy technologies are commonly used in LCESs to save primary energy. The CCHP system coupled with GSHP had a primary energy saving rate (PESR) of 26.10% compared to the SPS [145] and the PESR of a solar hybrid CCHP system with an optimal operation strategy was 36.15% [147]. Moreover, the DES combining solar energy with hybrid energy storage technologies in near-zero energy communities could achieve a PESR of about 50% [92]. The reliability of LCESs is evaluated by indexes of load-loss probability, self-sufficiency, and net interaction level. Using load-loss probability as the assessment criterion, the reliability of a microgrid system combining hydrogen energy and electrical energy storage was assessed with the results showing that the system could provide electricity steadily [90]. The dependability of LCESs could be strengthened by taking some measures. With the increase of area of PV panels, the self-sufficiency improved by 41% [117]. The net interaction level can also be enhanced by raising the price of electricity and lowering the cost of essential equipment [151].

5.3.4. Comprehensive Evaluation

Most previous studies have focused on the single-index evaluation in terms of system cost, but are unsatisfactory in terms of the energy conservation and environmental issues that are receiving more and more attention [155]. This is due to the frequent conflict between minimizing costs and environmental impacts, as the utilization of LCESs is often expensive [156]. Considering these facts, a comprehensive evaluation has been proposed to addresses the trade-offs between various indicators by considering economic, environmental, technical, and integrative indicators simultaneously [140]. In order to comprehensively assess the performance of a LCES, two or more criteria are usually considered to evaluate the system. The economic and environmental indicators are introduced to analyze the trade-off optimization of distributed hydrogen-based multi-energy system, and the optimized system can significantly reduce the CO₂ emissions and operating costs as well as improve the efficiency of renewable energy system [95]. The trade-off between economy, environment and robustness was proven to be effective in coping with the inherent uncertainty of wind power under different carbon reduction requirements [142].

Among these, the crucial aspect of a comprehensive evaluation is weight determination and decision-making, and many researchers have adopted various methods to solve them, as shown in Figure 9. For example, Zeng and others [145] used the equal-weighting method to comprehensively assess the economic, environmental and energy performance of the coupled system of GSHP and CCHP, with a comprehensive savings ratio of 25.42%. As a widely used method, the equal-weighting method with the advantages of simplicity and convenience can often produce results almost as good as those optimal weighting methods, but it does not take subjective and objective factors into consideration. Jing and others [144] employed the gray relational analysis (GRA) approach and the entropy information method to perform a comprehensive economic, environmental and technical evaluation of a solid-oxide FC-based CCHP system. However, the weight could not reflect the preferences of decisionmakers since it was only established using the entropy information approach based on objective data [150]. Conversely, based on the subjective extraction of weights, Ren and others [156] compared two multi-criteria decision-making methods of the Preference Ranking Organization Method for Enrichment Evaluation (PROMETHEE) and the analytic hierarchy process (AHP) for the comprehensive evaluation of economic, environmental and energetic aspects of distributed energy systems with results almost the same. To consider both the subjectivity of decisionmakers and the objectivity of numerical data, Wang and others [157] presented an optimal weighting method which combined the analytic hierarchy process and entropy information method together with the linear combination weighting and then adopted the GRA approach to obtain the comprehensive evaluation results of economic, environmental, technical, and social performance of several distributed CCHP systems. Yang and others [150] also combined the improved GRA and optimal combination weighting methods for the evaluation of community-optimized CCHP systems, CCHP systems, renewable energy systems and conventional SPSs considering technical, economic, environmental, and social aspects, with the result that the optimized CCHP was the best solution for the given application. The optimal combination weighting methods employed the rank correlation analysis and entropy information method to acquire subjective and objective weights, respectively, and utilized the maximum entropy principle and minimized weighed generalized distance to obtain optimal weighting coefficients. Perera and others [149] used eight criteria to evaluate the economics, environment, energy efficiency, and reliability of distributed electrical hub design by the first performing Pareto analysis with a two-dimensional Pareto front to reduce the dimensionality of the optimization problem, performing Pareto multi-objective optimization, and then using fuzzy TOPSIS and level diagram methods for multi-criteria decision-making.

6. Enablers and Barriers

With the introduction of low-carbon targets in various countries, energy saving and emissions reduction in the building sector is facing considerable challenges. Low-carbon buildings and communities are developing rapidly under a variety of incentives and government efforts, but they also face some challenges. The development level of low-carbon buildings and communities around the world is still unbalanced, which is closely related to the importance that governments and people of various countries attach to low-carbon buildings and communities, as well as the policies, economy, low-carbon technology level, and low-carbon evaluation standards of each region.

6.1. Government Policies and Public Attitude

For countries’ policies on low-carbon buildings and communities, incentive policies and mandatory measures come first [37]. The governments need to issue more mandatory regulations to ensure the development of low-carbon buildings and communities and make great efforts to guide the public to improve the existing buildings into low-carbon buildings, and increase the proportion of incentive in low-carbon buildings and communities development [158]. Whether it is tax reduction, fiscal subsidies, or the launch of “low-carbon” financial products, they all play a key role in promoting the scale and marketization of low-carbon buildings and communities. At the same time, the government should establish a responsibility system for low-carbon buildings and communities to ensure the effective implementation of policies [159].

In addition, talent development in low-carbon industries cannot be ignored. The development of low-carbon buildings and communities is often tailored to local conditions, there is a lack of professional guidance in the planning stage and construction process. Therefore, professional designers and operators of low-carbon buildings and communities are of great significance to its development.

The development of low-carbon economy and marketization also plays a key role [13]. The development of low-carbon buildings and communities in many countries has long relied on administrative power and financial funding. The projects are mostly government-funded, pushed from top to bottom by mandatory requirements and government financial incentives. The market mechanism of low-carbon buildings and communities has not yet been formed, which also has a certain impact on its development [50].

Public concern and awareness are also crucial. With global warming, environmental and climate issues gradually become the focus of people’s attention. Public awareness and attention to the environment is increasing. However, the concept of low-carbon buildings and communities is not widely popularized, and people’s understanding is still inadequate [50].

6.2. Technology

As global warming intensifies, low-carbon technologies are entering a boom in development. For low-carbon buildings and communities, technologies in construction and operation have been reviewed above and their development barriers and future innovations will be discussed in this section.

In construction, there are many low-carbon material solutions on alternative materials and processes as well as natural materials, mainly focusing on the application of wood and prefabricated construction method [50]. However, the selection of low-carbon materials is critical, and different solutions are used in various scenarios. The use of low-carbon materials is low due to high cost, limited technical knowledge and lack of client knowledge. For natural materials, there are impediments in quality and strength. Therefore, more information on material performance, design training in alternative materials, cost reductions for low-carbon materials, more demonstration projects and case studies are needed to overcome these barriers. Moreover, many low-carbon envelope designs have been applied in the recent decades and they are developing towards green and intelligent, but also face barriers in lacking standard, expensive investment and a dearth of knowledge [160].

In terms of operation, there are many energy systems using renewable energy technologies, energy storage technologies and optimization technologies to reduce CO₂ emissions in building operations, but natural gas and the power grid are often used as auxiliary energy, which still has large carbon emissions. Energy storage technology is the key to achieve LCES, but it is immature and costly. With the development and maturity of technologies, hydrogen production to store renewable energy for power generation is promising for zero-carbon energy systems in low-carbon buildings and communities. Moreover, the LCES can reach better benefits of economy, environment and energy by the optimization technology, but the optimization methods, application scenarios and new-type loads need to be further improved [92], by, for example, improving multi-objective two layer co-optimization methods, integrating various types of buildings, communities and climatic conditions, as well as taking electric vehicle loads into consideration. In addition, due to the inconsistent granularity of models and methods, synergistic planning with different sectors faces challenges in terms of modeling complexity, optimization accuracy and computational cost [118]. Therefore, it is necessary to establish a comprehensive and universal optimization system, including optimization methods, algorithms, and a platform.

6.3. Evaluation

The evaluation system of low-carbon buildings and communities is relatively mature, but countries have different levels of low-carbon economic development that bring about different priorities in evaluation systems. However, there is still a lack of effective evaluation systems in many regions. From the current overall situation, some countries have serious limitations and one-sidedness in specific evaluation systems and lack comprehensive evaluation systems for different types of low-carbon buildings and communities. At the same time, the contents of some evaluation systems are limited to environmental protection, and those related to social, economic, health and other aspects are not systematic, which restricts the development of low-carbon buildings and communities to a certain extent. In addition, it is worth noting that the grade evaluation of low-carbon buildings and communities in most countries is voluntary. As a result, not all new or existing buildings are required to carry out low-carbon evaluations, which has a negative impact on the promotion of the evaluation system.

7. Conclusions

Low-carbon buildings and communities are important parts of low-carbon cities and are significant for countries around the world to achieve their carbon peaking and carbon-neutralization goals. This paper summarized the research on low-carbon buildings and communities with regard to policy, technology and evaluation methods, and briefly analyzed the current enablers and barriers to the development of low-carbon buildings and communities. The following conclusions can be drawn.

• This paper introduced some mature examples of low-carbon buildings and communities that effectively utilize new energy and storage technologies to optimize energy systems, save resources, and reduce carbon emissions. The review also summarized building energy efficiency and incentive policies in most countries. Developed countries such as the United States and the European Union promulgated and implemented low-carbon buildings and communities policies earlier, and their policies were more systematic and comprehensive. At present, the policy focus is on the application of electrification and intelligentization in buildings, which enables buildings to better reduce carbon emissions and facilitates the monitoring of building energy consumption by the government and individuals. The incentive policies for low-carbon buildings and communities in developed countries focus on giving incentives and subsidies according to the grading system of their own low-carbon buildings and communities evaluation systems, while the relevant incentive policies in China and other developing countries pay more attention to the energy-saving transformation of existing buildings. At the same time, due to the different levels of economic and technological development of provinces in China, there are differences in financial subsidies among different regions. This paper also reveals that in developing countries, the economic concept of low-carbon buildings and communities lacks strong support from the government in the financial market, resulting in a small proportion of individual capital investment, which has resulted in the vitality of low-carbon buildings and community markets not being stimulated to the maximum extent. This has implications for other countries in the development of low-carbon buildings and communities in the future;
• In terms of technology, CO₂ emissions are reduced mainly from the construction and operation of buildings and communities. For construction, most research currently uses wood instead of traditional reinforced concrete, prefabricated instead of traditional construction methods, and reasonable envelope structures to reduce building loads. For operation, renewable energy technologies and energy storage technologies are needed to establish a low-carbon energy system. The economic, environmental and energy efficiency of energy systems can be improved through optimization techniques;
• The rating system of low-carbon buildings and communities is relatively mature at present, but the development level of low-carbon economy varies from country, and the application of evaluation system has its own emphasis. The existing low-carbon buildings and communities evaluation systems generally assess and assign points to different aspects, such as factors affecting building or community living and energy consumption, and then evaluate the grade through the score. Among these, the evaluation of LCESs has focused on economic, environmental, and technological aspects, and many evaluation indicators and methods have been applied;
• The development of low-carbon buildings and communities depends on the mandatory norms and incentive policies of the government. Meanwhile, the cultivation of low-carbon building talents, the improvement of financial markets and the public’s attitude towards environmental protection also have a profound impact on the development of low-carbon buildings and communities. In terms of technology, the development of low-carbon building materials and designs is limited due to high cost, the lack of knowledge, and standards. For LCESs, natural gas and the power grid are often used as auxiliary energy, which still has large CO₂ emissions. Hydrogen production to store renewable energy for power generation is promising for zero-carbon energy systems in low-carbon buildings and communities. In addition, it is necessary to establish a comprehensive and universal optimization system, including optimization methods, algorithms, and platforms. In terms of evaluation, some countries have serious limitations and one-sidedness in specific evaluation systems, and lack comprehensive evaluation systems for different types of low-carbon buildings and communities.