Evaluation Model and Strategy for Selecting Carbon Reduction Technology for Campus Buildings in Primary and Middle Schools in the Yangtze River Delta Region, China

Abstract

Cutting down global warming and reducing greenhouse gas emissions such as carbon dioxide are important global targets. Accounting for a third of global energy consumption, the building construction industry is an important target for carbon reduction. Campus buildings, of which there are a large number in China, differ from other building types, as they have noteworthy energy-use characteristics and technology selection requirements. This study identifies the carbon reduction technologies in Chinese primary and middle schools commonly used for energy and water conservation, and then evaluates their performance according to degrees of carbon reduction, maturity and economic suitability. Based on these three indicators, the study creates a three-dimensional evaluation model for the different technologies examined in order to obtain a selection ranking. The study offers guidance for project practice in the construction of primary and middle schools and helps to promote the development of the low-carbon campus.

1. Introduction

With increased urbanization, greenhouse gas emissions, such as carbon dioxide, have increased substantially on a global scale, bringing about climate warming and rising sea levels. The United Nations Framework Convention on Climate Change (UNFCCC), adopted in 1992 in response to the climate crisis [1], was the first international convention to attempt to take control of greenhouse gas emissions. With the Paris Agreement of 2015, 166 countries agreed, starting in 2020, to limit increases in global average temperatures to within 2 °C in this century but with the aspiration to further limit global temperature rise to within 1.5 °C above temperatures in the pre-industrial period [2].

In 2018, the total carbon emissions of the United States and the European Union were 8.9 billion tons, while those of China were 10 billion [3], exceeding the total sum of United States and European Union emissions. To speed up the pace of carbon reduction, at the 2009 United Nations Climate Change Conference in Copenhagen, China proposed that the GDP per unit of carbon dioxide emissions would fall by 40–45% by 2020 compared with 2005, that carbon emissions would peak by around 2030 and that China would strive to reach this peak sooner if possible. According to data from the Intergovernmental Panel on Climate Change (IPCC), the construction industry accounted for 32% of global terminal energy consumption in 2010 [4]. Reducing the carbon emissions of buildings is, therefore, an urgent requirement. It is estimated that, by the end of 2020, China will have added a further 30 billion m² of construction, including 13 billion m² of urban construction. If these buildings could save 50% on current energy consumption, this would mean saving about 160 million tons of standard coal per year [5].

With the popularization of compulsory education in China and the liberalization of the second-child policy, the demand for campus buildings has also increased. In 2018, there were 213,800 schools for compulsory education in China. School building areas increased by 5.5% compared with 2017, and primary schools increased by 4.7% [6]. Campus buildings have different energy-use characteristics compared with ordinary buildings because there is no energy use during the winter and summer vacations. In addition, campus buildings play an important role in developing low-carbon concepts for students. Therefore, based on the rapid growth of campus areas and their particular energy-use characteristics, it is crucial to establish appropriate energy saving and carbon reduction technology systems, as these may have a significant impact on future carbon reduction.

The operational use of buildings has been identified as a primary carbon source, making up around 80% [7,8,9] of the building’s emissions, and thus, for the existing studies regarding low-carbon buildings, the operation stages and the corresponding technologies employed were the focus. The construction of the enclosure and the applied materials represent the most important considerations. Dowd and Mourshed pointed out that high carbon emissions are largely caused by heat loss through the external surfaces of the structure [10]. Gan et al. used computer simulations to estimate the reduction in carbon emissions associated with different materials and window standards. Windows and wall insulation were found to be a key issue for super-tall buildings [11]. Kim, Tae and Roh applied a combination of different wall thicknesses, window to wall ratios and materials under specific carbon reduction rates for apartment buildings, and found the largest carbon reduction was obtained with a W/B value of 35%, among 36 types of low-carbon durability design [12]. Saroglou studied a high rise building with a ventilated double skin façade (DSF), and compared three parameters, including solar radiation, total heat transfer and cooling energy loads to judge three scenarios with various DSFs, which was used to develop strategies to measure DSF at various heights [13]. In an analysis of construction materials, Rodrigues, Martins and Nunes compared the proportion of metal, stone and concrete in actual buildings, and calculated their corresponding carbon emissions, which can be used in materials selection to achieve a particular carbon reduction rate [14]. Xia and Li compared the carbon reduction benefits of shading, internal space separation and natural ventilation in several typical plans of existing office buildings in operation [15]. Zhang and Zhou used existing research to identify 18 possible factors that may affect the development of low-carbon buildings in China’s urban areas, and they conducted a survey and analyzed the data to obtain the final six proposals, providing a reference for the effective development of urban low-carbon buildings [16].

For different types of school building, meeting the relevant low-carbon reduction criteria and the demands associated with their high-quality indoor environment was found to be somewhat difficult [17]. Pereira found that, based on the average energy use of schools in the United States, 47% was used for space heating and 14% was used for lighting [18]. Hence, the technologies associated with lighting, air-conditioning and enclosures having lower rates of energy consumption were a primary focus. Stazi reported that open windows are important for thermal comfort and energy consumption. Consequently, he examined a classroom in an Italian primary school as a typical case to explore the influencing factors that affect the opening of windows, which mainly included indoor and outdoor temperature, as well as carbon concentrations in the indoor air. Among these factors, the indoor temperature was found to be the biggest factor that had the most significant influence [19]. Zhang and Bokel considered the schoolyard and the effects of passive designs (e.g., greening and roof design) on the thermal and wind environment to identify the relevant outdoor parameters that can also influence the basic data associated with energy consumption used for indoor cooling. Based on the above, the integrated design recommendations for outdoor and indoor environments can be developed [20].

In addition to the technologies related to carbon emissions, the investment and efficiency of low carbon technologies are also noteworthy. On one hand, the various economic strengths of investors influence the selection of technology applications, especially larger investments such as the installation of renewable energy resources onsite (e.g., photovoltaic (PV) solar panels and heat pumps). On the other hand, selecting the most cost-effective technology is a public concern. Wang and Chen established a carbon reduction building model combined with income and economic development. The study organized high-income areas, medium-income areas, and low-income areas under three scenarios of different temperature reduction targets that included applicable low-carbon technologies and forecasts [21]. Lindberg built a technology evaluation model based on zero-energy buildings, mainly including PV panels, electric boilers, pellet boilers, HP and other technologies that can generate electricity. With different percentages of energy generated by buildings, the contribution of various technologies and the investment of each technology in various stages can be obtained [22]. Koo also considered eight scenarios that included the application of work time, lighting, energy resources and corresponding investment, and the scenarios all related to primary, middle and high schools in the center of Seoul, South Korea. With different NPV (Net Present Value) and SIR (Saving–to-investment ratio) targets, the corresponding scenarios could be used [23]. Suk, Lee and Jeong reviewed the investment and return cycle statistics associated with low-carbon technologies from the perspective of the investor [24]. They also conducted surveys of acceptable technology recovery cycles and the willingness of 35 Korean companies to make certain technology investments, which provided a reference for the formulation of relevant policies.

Stated thus, previous studies regarding carbon reduction usually employed various fixed scenarios, which had corresponding levels of carbon reduction efficiency and investment. However, the acceptance and difficulties in application, which also play a vital role in practical engineering, were usually ignored. Moreover, carbon reduction in the studies was achieved by the electricity savings that occur in a building’s operational stages, while water savings can also cut down the carbon emissions for supply systems that also require energy. Previous studies also mainly obtained results based on comparisons of several scenarios simulated by only one building, which is often difficult to apply to other buildings in practice. Therefore, a comprehensive evaluation model that can generate corresponding results under various conditions and scenarios is needed.

In this study, several cases located in the Yangtze River Delta, China, were considered, along with potentially applicable low-carbon emissions technologies. The Yangtze River Delta region is located in the south-eastern coastal area of China, and it is one of six world-class urban agglomerations that accounts for nearly 25% of the country’s total economic output [25]. In this region, the number of students in primary and middle schools exceeds 13% of the total number across the country, making it the leading region in China [26]. However, the major industries in the region include electronics and other manufacturing industries, which are dominated by primary energy sources such as coal-fired power plants. As a result, the energy structure in the region will not change in the short term, making carbon reduction more difficult to achieve. Therefore, the region is typical in terms of its educational demands and high carbon emissions as a result of economic development.

Based on the investigation of seven school buildings, this study established a multi-dimensional evaluation model of carbon reduction technologies. Considering the carbon reduction associated with electricity and water resource savings, the model uses the carbon reduction effect, economic investment, and maturity of application in practice as the evaluation parameters. The average situation of each technology applied in the seven cases provided more representative conditions, and the considered technologies included air conditioning, enclosure construction, renewable resource use and lighting that suit the characteristics of the school building type and meet the school indoor environmental demands. From this, an evaluation of each technology can be conducted, and the carbon reduction technology selection strategy can be obtained. Accordingly, this research can provide a valuable input to make more effective technology choices in future practice.

2. Method

2.1. Technology List

At present, building technologies related to savings in water and energy use are the main driver of carbon reduction. According to China’s ‘Green Building Evaluation Standards GB/T50378-2014 [27]’ and the ‘Assessment Standard for Green Campus GB/T 51356-2019 [28]’, high-efficiency equipment, the improvement of existing appliances and the use of renewable energy are the main methods of saving water and electrical energy. The technologies commonly used in campus buildings from this perspective are shown in

Table 1. Application of main carbon reduction technologies.

Carbon Reduction Types	Technology Type	Technologies	Practice
Water Resource Saving	High-performance appliances	Campus water saving irrigation	• Drip irrigation, sprinkler irrigation, filtration irrigation • Devices such as soil moisture sensors
	Renewable energy use	Rainwater recycling	• Recycling rainwater from roads and playgrounds for green irrigation and road flushing
	High-performance appliances	Secondary water saving appliances	• The indicators meet the specific requirements of the national water saving domestic water appliances standards for each appliance. For example, the flow rate of the nozzle should not be less than 0.125 L per second and the flushing valve of the toilet should not be less than 5 L. For urinal flushing, the single flush water volume of the valve should not be less than 3 L, etc.
Electricity Saving	High-performance equipment	Energy saving lamps	• Use of energy saving lamps (such as LED lamps) to improve power efficiency
	High-performance equipment	High-quality enclosure (HQE)	• The use of materials such as thermal insulation layers ensures that the maintenance structure of each part of the building meets the specified indicators in the ‘Design Standard for the Energy Efficiency of Public Buildings (GB50189-2015) [29]’
	Renewable energy use	Air-heat pumps	• Use of air-source heat pumps to provide a certain amount of electricity or domestic hot water heating
	Renewable energy use	Solar water heating systems	• Solar photovoltaic panels, collectors, etc.
	High-performance equipment	High efficiency air conditioning equipment (HEAE)	• Equipment with high-energy efficiencyair conditioning units

, which are summarized through cases provided by Zhejiang Green Building and Building Energy Reduction Association. There are eight of these technologies, comprising water saving irrigation systems, rainwater recycling, water saving appliances, energy saving lamps, high-quality enclosure (HQE), which offers thermal insulation etc., air-heat pumps, solar water heating systems and high-efficiency air conditioning equipment (HEAE). Among these technologies, water saving irrigation systems mainly adopt drip irrigation to reduce water waste. Water saving appliances aim to improve manufacturing standards—for example, the use of nozzles to reduce water in sanitary ware but with the same cleaning effect. Rainwater recycling uses rainwater for greening and road flushing. Solar water heating systems and air-heat pumps provide heat for domestic hot water by replacing traditional electric heating with renewable energy. Energy saving lamps, HQE and HEAE maintain the same indoor environment with lower energy consumption by raising efficiency.

2.2. Evaluation Model

2.2.1. Carbon Reduction Degree

A building has energy consumption and carbon emissions throughout its life cycle and this study focuses on the operational period. The carbon reduction degree indicates the technology’s impact on reducing CO₂ emissions across the building. The average carbon reduction rate of the same technology used in different cases represents the real-life degree of carbon reduction. Here, the carbon reduction rate refers to the ratio of the annual carbon emission reduction of the building after using the technology to the annual carbon emissions of the building when the technology is not used. The higher the carbon reduction rate, the more low-carbon benefits the corresponding technology offers. Annual water saving and energy saving can be converted into the annual carbon reduction according to the following formula [30,31]:

C_water = Q_supply × W₁ + Q_drain × (W₂ + C₂)

where C_water refers to the reduced CO₂ emissions due to annual water savings; Q_supply refers to the annual water supply (the unit is m³/a); Q_drain refers to the annual displacement (the unit is m³/a); W₁ is the CO₂ emission generated by the power consumption of the water supply system (take 0.3 kg/m³); W₂ is the CO₂ emission generated by the power consumption of the sewage system (take 0.25 kg/m³); and C₂ is the CO₂ emission from the conversion of the carbon source of the sewage system (0.7 kg/m³).

C_electric = Q × 0.7035

where C_electric refers to the reduced CO₂ emissions due to annual energy saving (kg/a) and Q refers to the annual electricity saving as a result of adopting the technology (kWh/a).

According to the formula, the amount of carbon reduction and the carbon reduction rate achieved by each technology can be calculated for each case. The average carbon reduction rate of the technology is used as the basis to assess the carbon reduction effect. In order to facilitate a comprehensive analysis with other index values, the average carbon reduction rate is normalised to obtain the final carbon reduction degree, as follows:

E_n = e_n × γ

where E_n indicates the degree of carbon reduction of the technology; n is 1, 2, 3...8, representing the 8 technologies to be studied; e_n is the average carbon reduction rate of the corresponding technology; γ is the coefficient normalized for the data (the value takes 1/max e_n); and the max e_n is the highest average carbon reduction rate in the studied technologies.

2.2.2. Maturity Degree

The aturity degree is another index in the technology evaluation model. Its level reflects the current acceptance of a technology by designers and the market. A technology with a high maturity degree indicates that the current practical application is appropriate for promotion. The research is based on the maturity data of the cases studied and the maturity degree is calculated as shown in the following formula. When other indices are similar, the higher the maturity degree, the more the technology is recommended.

I_n = ρ_n/A

where I_n indicates the maturity degree of the technology; n is 1, 2, 3...8, representing the 8 technologies to be studied; ρ_n is the number of cases using the corresponding technology; and A is the total number of research cases.

When assessing the maturity degree, the various examples of six technologies (rainwater recycling, water saving irrigation, secondary water saving appliances, air heat pumps, solar water heating systems and energy saving lamps) have relatively consistent energy saving effects in the market. Therefore, assessing the number of cases in which the technology is used can help assess the final maturity degree. However, the use of HQE and HEAE is required to meet certain levels in order to be assessed as ‘high performing’. For example, a 10% reduction in air-conditioning equipment energy consumption is the minimum according to the ‘Green Building Evaluation Standards GB/T50378-2014′, and the heat transfer coefficient of every part of the building must meet the ‘Design Standard for Energy Efficiency of Public Buildings GB50189-2015 [29]’.

2.2.3. Economy Degree

Some carbon reduction technologies, such as renewable energy, involve the application of auxiliary equipment, which leads to greater investment [32]. We should also note the phenomenon in which the benefit of carbon reduction is not necessarily proportional to economic input. Therefore, it is necessary to make an evaluation that identifies the best carbon reduction benefits for the particular technology investments. The incremental cost of each technology component (yuan per m²) can be obtained by synthesizing the current market price. The data can then be used as the economic degree of the technology through normalization. Normalization, here, is used to treat the other incremental cost data equally after the highest incremental cost (yuan per m²) in the technique is recorded as 1. The economy degree is calculated as follows. The closer the value is to 1, the higher the incremental cost is. When other index values are similar, the higher the cost, the lower the recommendation.

T_n = t_n × ε

where T_n refers to the economy degree; n is 1, 2, 3...8, representing the 8 technologies to be studied; t_n refers to the incremental cost per floor area (yuan per m²) in market; ε is the coefficient of data normalization, which is calculated as 1/max t_n; and max t_n here is the maximum cost per floor area among the technologies.

2.2.4. Multi-Index Evaluation System

In this study, a comprehensive three-dimensional evaluation of the selected technologies was designed for campus buildings. This took into account the degree of carbon reduction, maturity and economy. First, a two-dimensional evaluation panel was constructed with evaluation panels (Figure 1, Figure 2 and Figure 3) for carbon reduction–maturity, carbon reduction–economy and maturity–economy. In each two-dimensional panel, the study ranked the selected technology under the corresponding index according to the data for each index. As for the degree of carbon reduction and maturity, this was considered excellent where the data value was in the [2/3,1] range, good where the data value was in the [1/3,2/3] range and ordinary where the data value was in the [0,1/3] range. However, the economy degree was the opposite. Here, a data value of [2/3,1] was ordinary, [1/3,2/3] was good and [0,1/3] was excellent.

The two-dimensional evaluation panel was divided into nine grids according to the data for each index. This evaluation enabled us to rank the recommended level of each technology. As shown in Figure 1, Figure 2 and Figure 3, the green part (I) represents a high recommendation, the light grey part (II) represents a moderate recommendation and the dark grey part (III) represents the poorest recommendation. Each technology has an exact point in the two-dimensional panels according to its own index value, and the area where the point is located represents the technical recommendation level.

The technology recommendation level in the three two-dimensional panels may not be the same. The final recommendation level under the comprehensive three-dimensional evaluation is obtained according to the two-dimensional evaluation recommendation, as shown in

Table 2. Three-dimensional evaluation method.

Recommendation Level in Carbon reduction–Maturity Evaluation	Recommendation Level in Maturity–Economy Evaluation	Recommendation Level in Carbon Reduction–Economy Evaluation	Final Recommendation Level
High	High	High	High
High	High	Moderate
High	High	Poor
High	Moderate	High
High	Poor	High
Moderate	High	High
Poor	High	High
High	Moderate	Poor	Moderate
High	Poor	Moderate
Moderate	High	Poor
Moderate	Poor	High
Poor	Moderate	High
Poor	High	Moderate
High	Moderate	Moderate
Moderate	High	Moderate
Moderate	Moderate	High
Moderate	Moderate	Moderate
Moderate	Moderate	Poor
Moderate	Poor	Moderate
Poor	Moderate	Moderate
Poor	Poor	High	Poor
Poor	Poor	Moderate
Poor	Poor	Poor
Poor	High	Poor
Poor	Moderate	Poor
High	Poor	Poor
Moderate	Poor	Poor

below.

3. Case Study

3.1. Case and Technology Selected

The study focused on primary and middle school cases in the Yangtze River Delta region. For the selected cases, some carbon emission technologies were applied so that we can analyze their performance. Seven cases are shown in

Table 3. Selected cases in the Yangtze River Delta region.

Number	Case Names	Location	Volume Rate	Number of Teachers and Students
1	Haining Maqiao Central Primary School Reconstruction Project	Jiaxing	0.69	1100
2	Hangzhou Olive Tree Foreign Language School	Hangzhou	1.00	1578
3	Huzhou Binhu Primary School	Huzhou	0.57	2300
4	New construction at Xiangyang Primary School North Campus	Jiaxing	0.75	1600
5	Yueqing Baishi Primary School Expansion Project	Wenzhou	0.62	1600
6	Jiashan Experimental School affiliated to Shanghai Normal University	Jiaxing	0.8	1350
7	Taizhou Yuehu Primary School Project I	Taizhou	1.6	1080

, which were provided by the Zhejiang Green Building and Building Energy Reduction Association. These cases are all real projects, which have met certain degrees of the Green Building Evaluation Standards GB/T50378-2014 [27], and are posted on the public website [33]. Based on this, the evaluation system is of practical significance and is suitable for other campus constructions which are of a similar scale, the floor areas of which are, on average, 10,000 m².

3.2. Carbon Reduction Degree

The degree of carbon reduction reflects the low-carbon effect of technologies applied in primary and middle schools. The degree of carbon reduction, energy and water saving achieved by the application of the eight technologies is obtained from the case data according to the definition and calculation method for each technology in each case. The study calculated the average carbon reduction rates in seven cases for each technology, and the normalized result is the final carbon-reduction degree, as shown in

Table 4. Carbon reduction rate of technologies in the selected cases.

Technology		Cases Name							Average
		Haining Maqiao	Hangzhou Olive Tree	Huzhou Binhu	Xiang Yang	Yueqing Baishi	Jiashan	Taizhou Yuehu
Water saving irrigation	Carbon reduction rate	0.03%	0.20%	0	0.08%	0.07%	0.22%	0	0.12%
Rainwater recycling	Carbon reduction rate	0.82%	1.70%	0.26%	0.11%	0.15%	0.52%	0.71%	0.59%
Water saving appliances	Carbon reduction rate	0.20%	1.26%	0.13%	0.27%	0.22%	1.27%	0.51%	0.56%
Solar water heating systems	Carbon reduction rate	5.97%	0	2.19%	4.26%	0	8.62%	5.33%	5.26%
Energy saving lamps	Carbon reduction rate	0.62%	1.63%	1.27%	1.20%	0.74%	3.36%	1.42%	1.47%
Air-heat pumps	Carbon reduction rate	0	24.27%	0	12.00%	9.16%	0	15.14%	15.14%
High quality enclosure (HQE)	Carbon reduction rate	0.91%	0.26%	0.15%	0.03%	1.66%	0.43%	0.88%	0.62%
High efficiency air conditioning equipment (HEAE)	Carbon reduction rate	7.34%	17.50%	16.88%	12.40%	11.16%	17.20%	9.56%	13.15%

and

Table 5. Carbon reduction degree of the technologies.

Technology	Water Saving Irrigation	Rainwater Recycling	Water Saving Appliance	Solar water Heating System	Energy Saving Lamp	Air-Heat Pump	HQE	HEAE
Average of carbon reduction rates	0.12%	0.59%	0.56%	5.26%	1.47%	15.14%	0.62%	13.15%
Carbon reduction degree	0.79%	3.90%	3.70%	34.74%	9.71%	100%	4.10%	86.86%

.

Based on actual application status, it is clear that air-heat pumps and HEAE are excellent at reducing carbon emissions. The reason for this is that the air-heat pumps can reduce electric energy consumption by converting air heat into energy at a high efficiency. Although there is no energy use during the summer and winter vacations, schools have higher requirements for thermal environments at other times. In addition, air conditioning is often used throughout school hours on working days, leading to a large amount of electricity consumption. Therefore, improving the efficiency of air conditioners leads to a significant carbon reduction effect for the whole building. Solar water heating systems have a moderate influence on carbon reduction because the total consumption of hot water for handwashing is not high. As a result, this technology has fewer low-carbon effects for the whole building compared with its high efficiency in solar energy collection.

Energy saving lamps, water saving appliances and irrigation, rainwater recycling and high-quality insulation have the lowest influence on carbon reduction among the eight technologies. As for the degree of carbon reduction in these five technologies, energy saving lamps and HEAE behave better than the other three. Substantial requirements for light and for a thermally insulated environment lead to high electricity consumption. As a result, the performance of lamps for lighting and walls providing thermal insulation can directly reduce energy use. In general, technologies applied in the water saving area have less effect than those in electricity saving. This is because the coefficient of conversion from water reduction to carbon reduction is lower, and the practical application of water resource saving in this market has technical limits at present.

3.3. Maturity Degree

The results in

Table 6. Maturity degree of technologies.

Technology	Application in Cases
	Haining Maqiao	Hangzhou Olive Tree	Huzhou Binhu	Xiang Yang	Yueqing Baishi	Jiashan	Taizhou Yuehu	Maturity Degree
Water saving irrigation	√	√		√	√	√		71%
Rainwater recycling	√	√	√	√	√	√	√	100%
Water saving appliances	√	√	√	√	√	√	√	100%
Solar water heating systems	√		√	√		√	√	71%
Energy saving lamps	√	√	√	√	√	√	√	100%
Air-heat pumps		√		√	√		√	57%
HQE	√	√					√	43%
HEAE		√	√			√		43%

assess how the technologies are applied in the seven selected cases, according to the method used to calculate the maturity degree above.

The eight technologies all have an acceptable basis. It appears that rainwater recycling, water saving appliances, and energy saving lamps are widely accepted. This is related to the attributes of campus buildings. Specifically, rooves and squares in campuses are often used to recycle rainwater for greening, which is cleaner and does not need further filtration. Water saving appliances are easy to install and have direct effects when used in lavatories. Energy saving lamps are necessary in classrooms for teaching, drawing and reading, which consume large amounts of energy. So, these three technologies are particularly appropriate for campus buildings and are the most used.

Air-heat pumps, HEAE and HQE are used less in real-life situations, mainly because of high costs and complicated construction processes.

3.4. Economy Degree

After summing up all the cost data from several companies, checked by other experts, the incremental costs of the eight technologies are shown in

Table 7. Economy degree of technologies.

Technology	Market Price		Incremental Cost (Yuan/m2 of Floor Area)	Economy Degree
	Unit Price (Yuan)	Unit
Water saving irrigation	35	Yuan/m2 of green area	3	5%
Rainwater recycling	100,000	Yuan/Set	6.7	11%
Water saving appliances	50	Yuan/m2 of lavatory	12.5	19.5%
Solar water heating systems	1000	Yuan/m2 of solar photovoltaic panels	25	39%
Energy saving lamps	8	Yuan/m2 of floor area	8	12.5%
Air-heat pumps	300,000	Yuan/Set	20	31%
HQE	10,000	Yuan/m2 of enclosure area	61	95%
HEAE	1000	Yuan/Set	64	100%

, which calculates the economy degree of each corresponding technology.

From the data above, we can see that the investment required for HQE and HEAE is much higher and has a longer recovery cycle than for other technologies, whereas water saving irrigation, rainwater recycling and energy saving lamps are more affordable. Solar water heating systems and air-heat pumps are cost effective because they provide a good balance between cost and carbon reduction, and they are more appropriate for future promotion in campus projects.

4. Results and Discussion

4.1. Results of Three-Dimensional Evaluation

According to the three indices, each technology has an exact point in the two-dimensional panels. These are shown in Figure 4, Figure 5 and Figure 6 and the area in which the point is located represents the recommended level.

In the carbon reduction–maturity degree panel, air-heat pumps, HEAE and solar water heating systems all have a high recommendation. Of these, air-heat pumps and HEAE are much better than solar water heating systems at carbon reduction. This is shown by the large amount of existing energy consumption. However, solar water heating systems are more convenient to install and so are more widely accepted in real life situations.

In the carbon reduction–economy degree panel, only the air-heat pump has a high recommendation. Although the incremental cost is high per set, one or two sets can meet the requirements of a campus at the average floor area, which is affordable for the whole project. Therefore, air-heat pumps are good both for carbon reduction and from the economy perspective.

In the maturity–economy degree panel, HEAE and HQE have a poor recommendation because of their high cost, which is one of the reasons for low utilization.

4.2. Optimisation Strategy of Eight Technologies

As shown in

Table 8. Three-dimensional evaluation method.

Technology	Recommendation Level in Carbon Reduction–Maturity Evaluation	Recommendation Level in Carbon Reduction–Economy Evaluation	Recommendation Level in Maturity–Economy Evaluation	Final Recommendation Level
Air-heat pumps	High	High	High	High
Solar water heating systems	High	Moderate	High	High
HEAE	High	Moderate	Poor	Moderate
Energy saving lamps	Moderate	Moderate	High	Moderate
Rainwater recycling	Moderate	Moderate	High	Moderate
Water saving appliances	Moderate	Moderate	High	Moderate
Water saving irrigation	Moderate	Moderate	High	Moderate
HQE	Poor	Poor	Poor	Poor

, the results of the two-dimensional evaluations were combined to obtain the final recommendation level for each technology.

Air-heat pumps and solar water heating systems are given a high recommendation in the three-dimensional evaluation. The air-heat pump, in particular, is the most suitable for campus buildings compared with the others, because of its high-efficiency energy conversion, low economic investment and existing market acceptance. Solar water heating systems are not as good as air-heat pumps. This is mostly because of limited hot water consumption when determining the maximum carbon reduction. In addition, as the cost of photovoltaic panel installation is high, air-heat pumps are highly recommended.

HEAE, energy saving lamps, rainwater recycling, water saving appliances and water saving irrigation have a moderate recommendation level. In terms of selection strategy, HEAE has a significant advantage in carbon reduction for large existing electricity consumption, which is more in line with the overall goal. With the quick pace of technical development, market costs will fall in the future. As a result, this technology is the most highly recommended in the moderate band.

Although the maturity of energy saving lamps and rainwater recycling is the same as for water saving appliances, the economy and carbon reduction degree of water saving appliances are lower than the other two. Although they need greater investment in the early stages, compared with water saving irrigation, the extra cost is not too high. In particular, energy saving lamps play a key role in the indoor environment and are among the top two technologies in terms of the carbon reduction degree, which makes them appropriate for campus buildings. As for the application of water saving appliances and irrigation, water saving appliances have superior acceptance levels and better low-carbon effects, whereas irrigation systems have the lowest cost among all the technologies studied. Considering that the leading economic advantage is limited, water saving appliances are more highly recommended than irrigation systems. Technologies at the moderate recommendation level are ranked as follows: HEAE, energy saving lamps, rainwater recycling, water saving appliances and water saving irrigation systems.

Only HQE falls into the poor recommendation category, with the three indicators all judged as ordinary. The main reason for this is that the window ratio of campus buildings is high, and the materials used, like glass and aluminum, lead to poor thermal insulation performance compared with uninterrupted walls. High requirements for thermal comfort lead to higher requirements for thermal enclosure, meaning large investments for complicated structural practices. Meanwhile, direct carbon reduction effects cannot be promised.

The applied strategy for the eight technologies was obtained from the three-dimensional evaluation. Air-heat pumps and solar water heating systems are highly recommended. HEAE, energy saving lamps, rainwater recycling, water saving appliances and water saving irrigation have a moderate recommendation, whereas HQE has a poor recommendation. It is clear that technologies for renewable energy perform better under the more comprehensive evaluation, especially those for converting renewable energy to electricity. Renewable energy uses clean energy and significantly reduces carbon emissions. In terms of application efficiency, the results are not as expected because there is currently a technical market limit in deciding the total amount of energy saved. However, low application costs make the corresponding technologies widely acceptable.

In a study of low-carbon technologies for public buildings, Xiao analyzed the application of green technology to offices and shopping malls, paying most attention to HEAE and lighting systems, and less attention to renewable energy such as solar water heating systems and air-heat pumps [34]. The similarities and differences between the conclusions of the two studies arise from the differences in the type of architecture examined. Both campus and office buildings rely on high-quality indoor environments, so air conditioning and lighting are necessities. Office buildings are not suitable for renewable energy technologies partly because available space is limited compared to campus sites. The related equipment (such as rainwater recycling cisterns, sufficient roof area for laying photovoltaic panels, etc.) is difficult to construct. Meanwhile, Asdrubali pointed out, the payback times for technologies in a primary school, including envelope insulation, heat pumps, PV panels, solar collectors, and LEDs. Comparing this result with the economy degree, it is the same, in that the lighting application is the most cost-effective, then the PV and heat pump, and the enclosure insulation is the last [35]. Therefore, technology products must be applicable for the energy use and the architectural characteristics of particular building types.

5. Conclusions

The construction industry offers a target for carbon reduction, which is a major global goal. With the high pace in the construction of primary and middle schools in China, choosing appropriate technologies is important to assist with the construction of the low-carbon campus. This study is based on actual cases in the Yangtze River Delta region and focuses on eight common carbon reduction technologies. It makes a comprehensive analysis based on the three indices of carbon reduction degree, economy degree and maturity degree.

First, the paper defines the measurement method for these three indicators and carries out two-dimensional analysis. Air-heat pumps, HEAE and solar water heating systems are judged as the top recommendations in terms of the carbon reduction–maturity degree. However, in terms of the carbon reduction–economy degree, only air-heat pump systems are recommended as a priority technology that is cost-effective at meeting carbon reduction goals. Rainwater recycling, energy saving lamps, water saving irrigation, water saving appliances, air-heat pumps and solar water heating systems are also recommended as priority technologies that can be widely used in campus buildings.

After the two-dimensional analysis, three-dimensional analysis of the selection strategies of the eight technologies was obtained. Here, air-heat pumps and solar water heating systems have a high recommendation, whereas HEAE, energy saving lamps, rainwater recycling, water saving appliances and water saving irrigation have a moderate recommendation. HQE is assessed as a low recommendation. This selection ranking takes into account the benefits of carbon reduction, the convenience of promotion, and the economic investment. Future projects could be based on the results of the three-dimensional analysis of technology selection but should also be based on each project’s particular characteristics using two-dimensions or a single indicator of the technology recommendation order.

A limitation in this study is the number of cases. More cases and data would improve the selection ranking and strategy. This evaluation method is of practical significance as it is based on technology selected from existing projects. The study’s use of campus buildings is of educational significance for the whole of society in order to promote carbon reduction and environmental protection.