Natural future of energy utilization

International Journal of Energy ResearchVolume 41, Issue 6 p. 757-760 PerspectiveFree Access Natural future of energy utilization Xin-Rong Zhang, Corresponding Author Xin-Rong Zhang xrzhang@pku.edu.cn orcid.org/0000-0002-6689-5754 Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing, 100871 China Correspondence Xin-Rong Zhang, Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, China. E-mail: xrzhang@pku.edu.cnSearch for more papers by this author Xin-Rong Zhang, Corresponding Author Xin-Rong Zhang xrzhang@pku.edu.cn orcid.org/0000-0002-6689-5754 Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing, 100871 China Correspondence Xin-Rong Zhang, Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, China. E-mail: xrzhang@pku.edu.cnSearch for more papers by this author First published: 21 March 2017 https://doi.org/10.1002/er.3740Citations: 7AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Summary To build a harmonious world where human development is compatible with environmental sustainability, energy utilization tends to be more natural. This perspective article reveals the natural essence of energy utilization and discusses the natural future in different energy domains by representative examples and applications. To clarify the natural essence of energy utilization, it also presents some characteristics with reference to stability, safety, feasibility, efficiency and eco-sustainability. Copyright © 2017 John Wiley & Sons, Ltd. 1 Introduction Energy is always the significant factor drawing much attention by researchers, governments and even all mankind. It is because that energy not only drives the development of the human society by its power, but also degrades the sustainability of the world by its environmental problems. Hence, active efforts should be made to build a harmonious world where human development is compatible with environmental sustainability. Greenization has been the top priority in different stages of energy utilization including source, system and service 1. Clean energy sources have been widely studied and come into use with more efficient and environment-benign systems bridging the cleaner sources and better service. Based on the environment friendly characteristics, energy utilization tends to be more natural in various energy domains, such as energy resources, power generation, heating and cooling, energy storage and transport. It is not simply use of natural resources for energy applications. Actually, although natural resources have already been used as fuels, refrigerants and working fluids for a long time, some science and technology problems are still to be solved. The natural essence of energy utilization is minimizing adverse influences on both nature and human society. Thus, some characteristics in consideration of practical designs and operations are necessary. This perspective article reveals the natural essence of energy utilization and discusses the natural future in different energy domains by representative examples and applications. To clarify the natural essence of energy utilization, it also presents some characteristics with reference to stability, safety, feasibility, efficiency and eco-sustainability. 2 Natural Future of Energy Utilization in Different Domains 2. 1 Energy resources Fossil fuels are conventional energy resources used to provide power and heat through the combustion processes. Whereas, pollution and greenhouse effect of combustion products are the main problems. Additionally, fossil fuels are non-renewable energy resources and have limited sources in nature. Combustion is accompanied with large amount of irreversibility that decreases both the quantity and quality of usable energy. Therefore, it is better to use renewable energy resources in nature from the viewpoint of natural essence of energy utilization. Various kinds of renewable energy resources, including wind power (both onshore and offshore), solar photovoltaic, hydropower, solar thermal, geothermal and air source energy, should be widely exploited for electricity generation to substitute the use of fossil fuels. Owing to the rapid technology progress and market expansion, the cost-effectiveness of renewable electricity generation has been dramatically improved. An increasing number of countries intend to increase the share of renewable electricity generation with positive policies. Renewable electricity generation is expected to grow by over 30% between 2014 and 2020, reaching 7300 TWh 2. Accordingly, electric vehicles are also encouraged for substitute of fossil fuels in the transportation sector. In fact, electric car sales grew by 70% in 2015 2, and sustained rapid growth is achievable with proper infrastructure deployment, advantageous policies and funding support. 2.2 Power generation In spite of direct electricity generation by some renewable energy resources, a majority of electricity generates from thermal energy by power cycles. Thus, the thermal efficiency and working fluid of a power cycle are main concerns for the natural essence of energy utilization. Air-based Brayton cycle has high turbine inlet temperature, but high compressibility of air requires large amount of compression work which decreases the thermal efficiency. Liquid water is nearly incompressible, so much less pump work is needed in water-based Rankine cycle, and similar thermal efficiency can be achieved with lower turbine inlet temperature. To improve the thermal efficiency, water-based Rankine cycle operates under ultra-supercritical conditions with higher turbine inlet temperature. However, corrosion of conventional structure material is caused by the high temperature and pressure. Hence, a new type of natural working fluid with better characteristics and the corresponding cycle with better performance are required for higher efficiency and stability considering the natural essence of energy utilization. Supercritical carbon dioxide (CO2) cycle is one of the most promising power cycles tackling with the above problems. Carbon dioxide has moderate critical temperature (30.98 °C) and pressure (7.38 MPa) which make supercritical operation feasible with existing technology. Carbon dioxide near the critical point has little compressibility, so the compression work can be substantially reduced. Moreover, supercritical CO2 has less corrosivity compared with water at the same temperature; thus, high turbine inlet temperature is viable without degradation of plant stability. The high density of CO2 allows using components with small dimensions, including the pipe lines with small diameters, reciprocating compressors with better performance and compact heat exchangers, which results in significant cost savings. Carbon dioxide has excellent performances on flow and heat transfer under near critical conditions, so the losses from heat transfer processes and the heat transfer surfaces can be both reduced. Due to smaller pressure ratio, higher turbine outlet temperature makes it possible to recuperate thermal energy in the turbine exhaust, which enhances the thermal efficiency. Thus, according to Ref. 3, supercritical CO2 power plant has higher thermal efficiency and requires less land area compared with water-based and air-based power plants. 2. 3 Heating and cooling Heating and cooling systems have extensive applications in domestic, commercial and industrial sectors. As the core system components, refrigerants are closely related with system performance and environmental problems. Calm 4 reviews the progression of refrigerants and breaks the history into four generations under different backgrounds. Constricted by the less developed technology, volatility within the operation range was the only consideration for the early refrigerants regardless of their flammability, toxicity and stability. It should be noted that natural refrigerants had already been used, although some may yield low performances. Synthetic fluorochemicals were selected for the second generation of refrigerants owing to the development of chemical engineering. Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) dominated this generation for their safety and durability. However, due to the harmful effects of ozone depletion, the Vienna Convention was signed in 1985 followed by Montreal Protocol to phase out CFCs. Consequently, for the third generation of refrigerants, HCFCs with lower ODPs were transitional CFC replacements, and hydrofluorocarbons (HFCs) with no harm to the ozone layer were eventually the dominate refrigerants. Whereas, HFCs have high GWPs and are controlled by the Kyoto Protocol. An amendment to the Montreal Protocol was made in 2016 5, in which negotiators from over 170 nations reached a legally binding accord to phase down HFCs. Consequently, people should turn to natural refrigerants with novel designs taking into account the natural essence of energy utilization. Owing to the stable performance and absolute safety, air has been used commercially in aircraft cooling regardless of the low theoretical efficiency. Water absorbs much heat during the phase change from liquid to gas; thus, it is an ideal refrigerant for applications above 0 °C (lower temperature may be achieved with proper additives). Ammonia remains the major refrigerant for large-scale industrial applications, in particular the systems in food and beverage processing and cold storage. In spite of its excellent performance and environment-benign features, ammonia has low flammability, toxicity and corrosivity with safety categorized B2, which limit use in occupied spaces and populated areas. With strict restrictions of charge amounts considering their high flammability, several of hydrocarbons are also used as refrigerants for their much smaller GWP and better thermodynamic properties than HFC, but the use in large capacity industrial systems is limited. In a word, safety and efficiency are the vital factors related with the natural essence of energy utilization. Carbon dioxide is one of the air components with no flammability, toxicity, ODP and negligible GWP. It is well suited as a low temperature refrigerant, resulting in broad applications of commercial and industrial refrigeration, such as warehouse, supermarket, ice rink and food processing. Moreover, it offers opportunities to safely use ammonia in occupied spaces by cascade systems with CO2 on the low stage distributed to the evaporators and ammonia on the high stage contained in a specific machine room (or outdoors). In addition, CO2-based heating and hot water apply to domestic, commercial and industrial uses. Although CO2 typically operates at a higher pressure than other refrigerants, specific components have been invented and produced by many manufacturers with rapid development of machinery technology. And the risk of high pressure operation is insignificant compared with flammability and toxicity of some other refrigerants. In summary, the main applications and limitations of the mentioned natural refrigerants are shown in Table 1. Table 1. Main applications and limitations of some natural refrigerants. Refrigerants Main applications for heating and cooling Limitations Domestic Commercial Industrial Air √ Low theoretical efficiency Water √ √ √ High freezing point Ammonia √ √ Low flammability, toxicity and corrosivity Hydrocarbons √ √ High flammability Carbon dioxide √ √ √ High operating pressure (solved) 2.4 Energy storage and transport Energy storage technology has been rapidly developing with the growing renewable energy capacity and need of smart energy management. Chemical storage methods have broad applications and a long history because of their high round trip efficiency. However, they are not suitable for large-scale systems on account of the safety and cost concerns. They are also against the natural essence of energy utilization for the use of synthetic chemical substances. Hence, physical storage methods are extensively researched for large-scale applications with increasing number of pilot and commercial projects coming into operation. Pumped hydro storage and compressed air energy storage are mature physical methods for large-scale energy storage. Pumped hydro storage shares 99% of the total installed energy storage, and compressed air energy storage has lower capital cost in comparison to other technologies 6. On one hand, both of them take advantage of natural conditions for energy storage process or space. On the other hand, their applications are restricted by dependence on these natural conditions. To realize universal feasibility independent on additional conditions, CO2 thermodynamic cycles, including transcritical heat pump cycle, Rankine cycle and Brayton cycle, are studied for potential energy storage applications. Thermal energy storage is another technology for storage of heating and cooling energy. In spite of advanced PCM, the natural mediums are the prior candidates considering the natural essence of energy utilization. The most common medium for hot storage is hot water, and pressurized water is required for higher temperature than the boiling point. Ice slurry is usually chosen to be the cold storage medium with additives for below zero operations. Moreover, both hot water and ice slurry are used as energy transport mediums. Based on the above discussion, domains and characteristics of natural future of energy utilization are summarized in Figure 1. Figure 1Open in figure viewerPowerPoint Domains and characteristics of natural future of energy utilization. [Colour figure can be viewed at wileyonlinelibrary.com] 3 Closing Remarks Minimizing adverse effects on nature and human is the natural essence of energy utilization, which covers power generation, heating and cooling, energy storage, transport, etc. The proposed characteristics including stability, safety, feasibility, efficiency and eco-sustainability are the main concerns for future applications. Examples and applications in several energy domains are discussed with emphasis on CO2, which is prospective for extensive use with ignorable limitations. Acknowledgements The support of National Key Research and Development Program (2016YFD0400106) and the support from Beijing Engineering Research Center of City Heat are gratefully acknowledged. References 1Dincer I. Greenization. International Journal of Energy Research 2016; 40(15): 2035– 2037. doi:10.1002/er.3619. 2 IEA. In Track Clean Energy Progress 2016. IEA Publications: Paris, 2016 https://www.iea.org/publications/freepublications/publication/tracking-clean-energy-progress-2016.html. 3Ahn Y, Bae SJ, Kim M, et al. Review of supercritical CO2 power cycle technology and current status of research and development. Nuclear Engineering and Technology 2015; 47(6): 647– 661. doi:10.1016/j.net.2015.06.009. 4Calm JM. The next generation of refrigerants—historical review, considerations, and outlook. International Journal of Refrigeration 2008; 31(7): 1123– 1133. doi:10.1016/j.ijrefrig.2008.01.013. 5 UNEP. Report of the twenty-eighth meeting of the parties to the Montreal protocol on substances that deplete the ozone layer. Kigali, 2016. http://conf.montreal-protocol.org/meeting/mop/mop-28/final-report/SitePages/Home.aspx 6Hameer S, Niekerk JL. A review of large-scale electrical energy storage. International Journal of Energy Research 2015; 39(9): 1179– 1195. doi:10.1002/er.3294. Citing Literature Volume41, Issue6May 2017Pages 757-760 FiguresReferencesRelatedInformation