**1. Introduction**

Energy is a fundamental social need and plays an essential role in driving economic growth. The emerging economy requires the support of a reliable, affordable, and energy system with low carbon emissions and air pollutants. Energy transition [1], process optimisation [2], efficiency enhancement, [3] and waste heat recovery [4] are the keys to support a sustainable energy system, especially when increasing energy consumption is unavoidable. There is no straightforward and absolute answer to sustainable design as various temporal [5], and spatial [6] factors have to be considered. The trade-off between the economic, environmental, and social factors remains an active research subject. The recently reported share of global renewable electricity generation by International Energy Agency (IEA) [7] is 26%, dominated by solar photovoltaic (PV) followed by onshore wind and hydropower. Table 1 shows the greenhouse gas (GHG) and the water footprint of different energy sources. Some of the sources of renewable energy having a lower GHG footprint but a higher water footprint. The environmental performance of renewable energy cannot be concluded merely by the GHG or water footprint. The priority—GHG or water footprint reduction—and the selection of renewable energy is highly dependent on local conditions (e.g., resources). Involving the water–energy nexus complicates the issue further [8] because the water footprint consists of embodied energy. A more comprehensive sustainability assessment of different renewable energy

sources is still needed to facilitate appropriate decision making. Non-renewable energy generally has a higher GHG and water footprint than that of renewable energy. However, it should also be noted that this footprint can be varied according to the technology, operation, and even the assessed life cycle boundary. Some of the most frequently implemented examples are biomass, where a large variation is reported [9], and biogenic carbon, which can significantly affect the estimated footprint [10].

The development of sustainable systems remains a challenge in reality due to the extensive range of economic, environmental, and social factors that have to be included during the system life cycle. It is important to ensure that sustainable system design is not transforming one problem into another problem. Comprehensive tools, methodologies, and assessment frameworks remain an on-going topic of research. Figure 1 shows the GHG and air pollutant performance related to the increased share of renewable energy in the European Union (EU). Although the GHG, NOx and SO2, have decreased in line with reduced fossil fuel consumption, particulate matter (PM) and volatile organic compound (VOC) emissions have increased. More effort is required for a sustainable system design rather than focusing on solely questions of mitigating climate change or air pollution. Table 2 shows the levelized energy cost with and without subsidies and the changes in cost. The price of renewable energy has fallen significantly, particularly wind and solar energy, which are cheaper than non-renewable sources even without financial assistance. However, wind and solar are intermittent energy sources, which are not continuously available for conversion and available at every location. Lithium-ion batteries are the most competitive option with the highest potential for lifetime cost reduction [11], with the exception of long discharge applications. Based on a report by Lazard [12] considering the levelized cost of storage, solar PV with a storage system is economically attractive for short-duration wholesale and commercial use (102–139 USD/MWh) but remains a challenge for residential and longer-duration wholesale use (457–663 USD/MWh). Direct displacement of dispatchable energy sources by variable renewable energy can lead to cascade failures in the grid (blackouts) without consideration of flexible energy system design [13].



PV = photovoltaic; CSP = concentrated solar power. The GHG footprint is based on the reported study in [14] except for CSP <sup>a</sup> and geothermal b. <sup>c</sup> The share of water footprint reported in [16] for solar power is not specified by the type of technology.

**Figure 1.** The changes in GHG and air pollutant performance with an increasing share of renewable energy (comparison between 2005 and 2018). Data extracted from [19].



Fikse et al. [23] stated that traditional system engineering practices attempt to anticipate disruptions; however, they may be susceptible to unforeseen factors. This is particularly reflected in the unexpected outbreak and impact of COVID-19. The disease threatens human life while also serving to highlight existing or potential vulnerabilities of emergency responses and various system designs (i.e., capacity, allocation, and flexibility). A sustainable system design with inherent resilience would likely be valuable in future research. One of the apparent crises during COVID-19 is the shortage of personal protective equipment, particularly in countries reliant on international production. Global value chains may be reconsidered after the COVID-19 outbreak, as suggested by Kambhampati [24], due to the profound risk they pose. COVID-19 has also had an influence on the energy system. Figure 2 shows the supplies of minerals that support energy production. In addition to Cu, Li, Co, and Ni, renewable energy used Si, Zn, Mo, and rare earth minerals, which are non-renewables. It has been reported that electric cars use five times more minerals than a conventional car, and onshore wind plants require eight times more minerals compared to gas-fired plants [25]. As shown in Figure 2, the production

of Si, Zn, Mo, and rare earth minerals is dominated by China. The reliability, security, and price fluctuations of mineral supplies is an under-analysed global challenge in the promotion of a 100% renewable energy future.

**Figure 2.** Supplies of minerals that support energy production. Information collected from [25], except for zinc [26] and silicon [27].

Figure 3 shows the structural changes and impacts of energy demand during movement restriction. Domestic electricity demand has generally increased, and there has been a shift in the timing of peak demand during the middle of the day. The reduction of electricity demand in selected countries has also been reported by IEA [28]. The fall in overall electricity consumption is due mainly to the shutdown of industry, and the share of renewable energy has been reported to have increased. The temporary impacts are generally favourable where the consumption and environmental footprint is reduced. However, IEA [29] highlighted that the energy industry that emerges from COVID-19 would change significantly, particularly given an expectation of a reduction in investment. This could inhibit sustainable development because energy is likely to be affected first. In Germany, the pandemic has led to a decrease in power demand and negative electricity prices. Amelang [30] highlighted that negative rates have no benefit to consumers as the difference between negative power prices and the feed-in tariffs ultimately have to be paid. Oversupply reflects inefficiency and highlights a need for a better demand response and flexible renewables design. Positively, however, this crisis has highlighted a weakness and represents an opportunity to steer system design research onto a more resilient, secure, and sustainable path.

**Figure 3.** The structural changes and impacts of energy demand during movement restriction of COVID-19 in New York City [31], the European Union [32] <sup>a</sup> [33] <sup>b</sup> [34] <sup>c</sup> [30] d, and the UK [35] e.

The circular economy approach has undergone substantial development and increasingly used as a framework for system energy design, assessment, and implementation at various levels, beginning with production plants, through municipal and governmental strategic plans. Korhonen et al. [36] is one contributor who highlighted the need for scientific research into the quantification of actual environmental impacts. This is because a highly implemented circular economy progresses toward sustainability features. However, some of the circularity features (e.g., reprocessing waste) can be energy-intensive. Another work by Kirchherr et al. [37] emphasised that a potential variety of the circular economy concept which is complemented with a lack of quantification, if not fully based on a footprint strategy, can result in a conceptual deadlock. Table 3 summarises the six circular strategies consisting of the 9Rs (Refuse, Rethink, Reduce, Reuse, Refurbish, Remanufacture, Repurpose, Recycle, Recovery) practices with quantitative/qualitative indicators. A set of indicators comprising scale indicators and the circularity rate (%), and covering socioeconomic cycling, ecological cycling potential, and non-circularity, has also been proposed by Mayer et al. [38] for the EU. The proposed indicators can be considered to be relatively comprehensive. However, they serve mainly as a monitoring framework (system assessment tools) rather than a system engineering model.



eDiM = Ease of disassembly metric, TRP = Total restored product, MCI = Material circularity metric, EVR = Eco-cost value ratio, PLCM = Product-level circularity metric, SCI = Sustainable circular index, CR = Old scrap collection rate, RR = Recycling process efficiency rate, EOL-RR = End of life recycling rate, RIR = Recycling input rate, OSR = Old scrap ratio, NTUM = Number of times use of a materials, CIRC = Material circularity indicator, LMA = Lifetime of materials in anthroposphere, GRI = Global resource indicator, CEI = Circular economy index, CPI = Circular economy performance indicator, VRE = Value-based resource efficiency.

System design is important in facilitating sustainable development. Various works that aspire to promote sustainable system design by addressing the issues of enhancing energy and environmental performance were presented in the Special Issue (SI) of the 22nd Conference on Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction (PRES'19). The aim of this

study is to review the emerging tools for sustainable system design, including the recent developments reported in the Special Issue (SI) of PRES 2019. The energy system design tools that are considered in this study are summarised in Figure 4, including modelling, simulation, optimisation, and analysis or assessment.

**Figure 4.** The research direction of sustainable system design. Modified based on Loucks [40]. The considered tools reviewed in this study include those used for modelling and simulation, optimisation, and analysis or assessment, that can contribute to energy system design. Red arrows represent modelling and simulation. Orange arrows represent optimisation. The grey arrow represents analysis or assessment. For example, modelling and simulation utilise system inputs and the available system design and operating policy to identify the system outputs. Analysis or assessment is conducted to determine the performance of an existing system.

The papers are categorised into four topics in Section 3, focused on modelling, simulation, optimisation, and assessment studies on the topics of:

