energies Article Analysis of Changes in the Aggregate Exergy Efficiency of China’s Energy System from 2005 to 2015 Yuancheng Lin 1,2 , Chinhao Chong 1,2 , Linwei Ma 1,2,* , Zheng Li 1 and Weidou Ni 1 1 State Key Laboratory of Power Systems, Department of Energy and Power Engineering, Tsinghua-BP Clean Energy Research and Education Centre, Tsinghua University, Beijing 100084, China; [email protected] (Y.L.); [email protected] (C.C.); [email protected] (Z.L.); [email protected] (W.N.) 2 Tsinghua-Rio Tinto Joint Research Centre for Resources, Energy and Sustainable Development, Laboratory for Low Carbon Energy, Tsinghua University, Beijing 100084, China * Correspondence: [email protected] Abstract: Analysis of the change of overall energy efficiency performance of an energy system is a fundamental work for the energy-saving policymaking. However, previous studies seldom focus on energy stages from useful energy to final service, while most attention are paid on stages from energy source to useful energy. In this paper, we develop a high-resolution the Societal Exergy Analysis and Logarithmic Mean Divisia Index (SEA-LMDI) method to analysis changes and driving factors of the aggregate exergy efficiency, in which the boundary of the SEA is extended to passive systems and final services, and a LMDI decomposition method is developed to quantify contributions of efficiency factors and structure factors of all six stages on the aggregate exergy efficiency. A case study of China during 2005–2015 reveals that: (a) the aggregate exergy efficiency from energy source to final service is only from 3.7% to 4.8% during 2005–2015, showing a huge theoretical potential of efficiency improvement. (b) Large passive losses are identified in passive systems and nearly Citation: Lin, Y.; Chong, C.; Ma, L.; Li, Z.; Ni, W. Analysis of Changes in 2/3 of useful energy can be theoretically saved by improving passive systems. (c) Deep analysis of the Aggregate Exergy Efficiency of industrial coal-fired boilers indicates that the internal structural adjustments are also important for China’s Energy System from 2005 to the aggregate improvement. 2015. Energies 2021, 14, 2304. https:// doi.org/10.3390/en14082304 Keywords: aggregate exergy efficiency; driving factors; Societal Exergy Analysis; LMDI; Sankey diagram Academic Editor: Paula Fernández González Received: 6 March 2021 1. Introduction Accepted: 13 April 2021 Improving the energy efficiency at all stages of energy systems is considered as a Published: 19 April 2021 critical action to mitigate climate change while also meeting people’s increasing demand for final energy service [1]. The IEA (International Energy Agency, Paris, France) claims Publisher’s Note: MDPI stays neutral that the energy efficiency improvement from 2016 to 2018 lowered the energy demand by with regard to jurisdictional claims in 4% and avoided 3.5 billion tons of carbon dioxide emissions. It is expected that this energy published maps and institutional affil- ◦ iations. efficiency improvement will continue to contribute to achieving the 2 C goal of the Paris Agreement [2]. At the national level, policies related to energy efficiency have also been the main concern for policymakers, such as in China [3] and Germany [4]. As we know, the energy system is a complex system. To improve energy efficiency at different stages in energy systems, many technologies have emerged. For policymakers, Copyright: © 2021 by the authors. given the limitation of time and resources, priority should be given to key technologies Licensee MDPI, Basel, Switzerland. that have bigger potential and will deliver better gains [5]. To identify these key fields of This article is an open access article energy efficiency improvements and guide future policymaking, the first step is to estimate distributed under the terms and conditions of the Creative Commons the overall energy efficiency performance of the entire energy system. Attribution (CC BY) license (https:// In previous work, the assessment model of the overall energy efficiency performance creativecommons.org/licenses/by/ of energy systems can be divided into two categories: the top-down energy intensity 4.0/). model and the bottom-up thermodynamic efficiency model [6]. The energy intensity is Energies 2021, 14, 2304. https://doi.org/10.3390/en14082304 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 27 Energies 2021, 14, 2304 2 of 27 In previous work, the assessment model of the overall energy efficiency performance of energy systems can be divided into two categories: the top-down energy intensity definedmodel and as the the energy bottom-up consumption thermodynamic per unit effici of GDPency model (Gross [6]. Domestic The energy Product), intensity provid- is ingdefined a top-down as the energy approach consumption to connect per energy unit of consumption GDP (Gross Domestic with economic Product), development; providing however,a top-down the approach energy intensity to connect model energy cannot consumption fully reflect with technological economic development; progress in how- fields underlyingever, the energy energy intensity systems, model which cannot will leadfully toreflect the neglect technological of some progress key technologies in fields un- [7]. Inderlying contrast, energy the thermodynamic systems, which will efficiency lead to model the neglect can provide of some a key bottom-up technologies approach [7]. In to observecontrast, technological the thermodynamic progress efficiency underlying model energy can systems.provide a bottom-up approach to ob- serveBased technological on different progress thermodynamic underlying laws, energy the systems. thermodynamic efficiency model includes the first-lawBased on energy different efficiency thermodynamic and the second-law laws, the energythermodynamic efficiency efficiency (or exergy model efficiency). in- Includes the first-law the first-law energy energy efficiency efficiency model, and the energy second-law is conserved energy and efficiency cannot (or be exergy created ef- or destroyed,ficiency). In but the the first-law energy energy quality efficiency differences model, between energy the is inputsconserved and and the cannot useful be outputs cre- areated not or considered. destroyed, Thisbut the will energy lead to quality some unreasonabledifferences between results. the For inputs example, and thethe first-lawuseful energyoutputs efficiency are not considered. of a heat pump This will lead be greater to some than unreasonable 100%. On theresults. other For hand, example, the exergy the efficiencyfirst-law energy can distinguish efficiency theof a quality heat pump difference will be between greater than 1 kJ electricity100%. On the and other 1 kJ heathand, by theirthe exergy ability efficiency to perform can work, distinguish where the exergy quality represents difference the between maximum 1 kJ amountelectricity of and useful 1 workkJ heat can by be their obtained ability when to perform the system work, is broughtwhere exergy to equilibrium represents with the themaximum surroundings amount [8 ]. Asof auseful result, work exergy can be is consideredobtained when as athe more system scientific is brought metric to equilibrium and the exergy with efficiencythe sur- roundings [8]. As a result, exergy is considered as a more scientific metric and the exergy analysis has been widely used to evaluate the overall energy efficiency performance of efficiency analysis has been widely used to evaluate the overall energy efficiency perfor- energy systems, since exergy can consider both the energy quantity difference and the mance of energy systems, since exergy can consider both the energy quantity difference energy quality difference (see Section 2.2). and the energy quality difference (see Section 2.2). The exergy efficiency analysis of societal energy systems, also referred to as Societal The exergy efficiency analysis of societal energy systems, also referred to as Societal Exergy Analysis (SEA), first divides a societal energy system into several stages, including Exergy Analysis (SEA), first divides a societal energy system into several stages, including energy source, energy transformation, and energy end-use, then further analyzes the exergy energy source, energy transformation, and energy end-use, then further analyzes the ex- input and output of each stage, and finally aggregates these stages to evaluate the aggregate ergy input and output of each stage, and finally aggregates these stages to evaluate the exergy efficiency [9]. Most previous studies of SEA only focus on the energy stages from aggregate exergy efficiency [9]. Most previous studies of SEA only focus on the energy the energy source to useful energy, as shown in Figure1. However, what people need stages from the energy source to useful energy, as shown in Figure 1. However, what is not energy itself, but the “final services” it provides. For instance, what people really people need is not energy itself, but the “final services” it provides. For instance, what need is not heat energy itself, but the thermal comfort provided by heat energy. Loss not people really need is not heat energy itself, but the thermal comfort provided by heat en- only occurs in the process from the energy source to useful energy but also in providing ergy. Loss not only occurs in the process from the energy source to useful energy but also final services, such as heat losses due to poor insulation of a room [10]. Therefore, to in providing final services, such as heat losses due to poor insulation of a room [10]. There- systematically identify key fields of energy efficiency improvements, the analysis boundary fore, to systematically identify key fields of energy efficiency improvements, the analysis of the exergy efficiency analysis is supposed to be extended to final service. boundary of the exergy efficiency analysis is supposed to be extended to final service. Figure 1. The energy flow of a societal energy system in the Societal Exergy Analysis. Figure 1. The energy flow of a societal energy system in the Societal Exergy Analysis. Moreover,Moreover, limitedlimited attentionattention was paid to the driving driving factors factors analysis analysis in in the the SEA.