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Life Cycle Exergy Analysis of Solar Energy Systems

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Life Cycle Exergy Analysis of Solar Energy Systems enewa f R bl o e ls E a n t e n r e g Journal of y m a a n d d n u A Gong and Wall, J Fundam Renewable Energy Appl 2014, 5:1 F p f p Fundamentals of Renewable Energy o l i l ISSN: 2090-4541c a a n t r i DOI: 10.4172/2090-4541.1000146 o u n o s J and Applications Research Article Open Access Life Cycle Exergy Analysis of Solar Energy Systems Mei Gong1 and Göran Wall2* 1School of Business and Engineering, Halmstad University, PO Box 823, SE-30118 Halmstad, Sweden 2Oxbo gard, SE-43892 Härryda, Sweden Abstract Exergy concepts and exergy based methods are applied to energy systems to evaluate their level of sustainability. Life Cycle Exergy Analysis (LCEA) is a method that combines LCA with exergy, and it is applied to solar energy systems. It offers an excellent visualization of the exergy flows involved over the complete life cycle of a product or service. The energy and exergy used in production, operation and destruction must be paid back during life time in order to be sustainable. The exergy of the material that is being engaged by the system will turn up as a product and available for recycling in the destruction stage. LCEA shows that solar thermal plants have much longer exergy payback time than energy payback time, 15.4 and 3.5 years respectively. Energy based analysis may lead to false assumptions in the evaluation of the sustainability of renewable energy systems. This concludes that LCEA is an effective tool for the design and evaluation of solar energy systems in order to be more sustainable. Keywords: Exergy analysis; Sustainable development; Life cycle of exergy analysis. Some of the papers [11,13,18] estimated material analysis recycling with present technology and future developed technology as well as using recycled material that will reduce the need for energy in Introduction the construction stage. With a dependence on finite natural resources, increasing Two different methods combining LCA and exergy analyses have energy demands and increasing environmental problems, the use of been proposed, e.g. Exergetic Life Cycle Analysis (ELCA) [19] or Life renewable resources becomes even more important. This is one part Cycle Exergy Analysis (LCEA) [20]. Cumulative Exergy Consumption of a sustainable development. The ultimate source of most of our (CExC) was introduced by Szargut et al. [21] to calculate the sum of all renewable energy supplies is the sun. The world’s primary energy use exergy input in all steps of a production process. Ayres with co-authors is easily exceeded by the solar energy by a factor of about 10000 [1]. [22] stated the advantage of using exergy in the context of LCA, and The renewable energies except geothermal and tidal energy depend on concluded that exergy is appropriate for general statistical use, both solar radiation. Thus, it is essential for future energy systems to rely as a measure of resource stocks and flows and as a measure of waste on energy from the sun and presently, the use of direct solar energy is emissions and potential for causing environmental harm, which was rapidly increasing. Solar energy systems can be classified into direct- also indicated by Wall in 1977 [3]. However, no detailed comparison solar systems and indirect-solar systems [2]. Here solar energy systems has been made with existing methods, like the LCA. ELCA [19,23] refer to direct-solar systems. introduced by Cornelissen is based on the framework of LCA with Exergy is a useful concept in the work towards sustainable exergy applied to the inventory analysis and the impact assessment. development [3]. Exergy accounting of the use of energy and material Finnveden and Östlund [24] used exergy consumption as an indicator in resources provides important knowledge on how effective and balanced LCA. Several metal ores and other natural resources were analyzed with a society is regarding conserving nature’s capital. This knowledge can system boundaries compatible with LCA. Since the cumulative exergy identify areas in which technical and other improvements should be consumption index is just the sum of the chemical exergy contents of undertaken, and indicate the priorities, which should be assigned to all original input flows, Valero [25] has introduced “exergetic cost for conservation measures, efficiency improvements and optimizations. replacement of material” into the analysis. Lombardi (2001) performed Hepbasli [4] offers a careful review of exergy analysis applied to an ELCA and a classical environmental LCA for a carbon dioxide low renewable energy resources for a sustainable future. Thus, the exergy emission power cycle in which exergy was considered to be an indicator concept and exergy tools are essential to the creation of a new of resource depletion. engineering toolbox or paradigm towards sustainable development. Life cycle exergy analysis (LCEA) includes sustainability aspects In the literature, a number of studies applies energy and exergy [20,26]. LCEA uses the same framework as LCA, but makes an analyses to the whole or part of solar energy systems, e.g. solar important distinction between renewable and non-renewable resources. heater [5,6], solar power plant [7], solar photovoltaic (PV) system [8] In LCEA, renewable resources as solar energy are excluded in the cost combined solar photovoltaic and thermal (PV/T) plants [9,10]. Exergy of solar radiation is often regarded as input into the energy systems of these studies, and the overall energy efficiency is about 25% for solar *Corresponding author: Göran Wall, Oxbo gard, SE-43892 Härryda, Sweden, Tel: power plant, 14-15% for PV/T system and less than 12% for PV system. 46 35 16 71 00; E-mail: [email protected] Received November 10, 2014; Accepted December 09, 2014; Published Life cycle analysis/assessment (LCA) is a tool used to evaluate the December 16, 2014 total environmental impact and total energy resource use of a product or service during its complete lifetime or from cradle to grave. It covers Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi: 10.4172/20904541.1000146 three steps – construction, operation and destruction. In previous studies LCA has been applied to solar energy system, e.g. PV system Copyright: © 2014 Gong M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted [11-14], solar thermal power system [15], solar heating system [16] use, distribution, and reproduction in any medium, provided the original author and and PV/T system [17,18]. However, none of these studies made use source are credited. J Fundam Renewable Energy Appl ISSN: 2090-4541 JFRA, an open access journal Volume 5 • Issue 1 • 1000146 Citation: Gong M, Wall G (2014) Life Cycle Exergy Analysis of Solar Energy Systems. J Fundam Renewable Energy Appl 5: 146. doi: 10.4172/20904541.1000146 Page 2 of 8 of calculation due to being free of charge, and/or otherwise wasted. tot− tot =∆=tot − 〉 Ein E out TS0 ∑(Ein E out ) j 0 (5) LCEA has also been applied to industrial processes [27,28] and to wind j power systems [29]. where ΔStot is the total entropy increased, tot Thus, LCEA is a powerful tool in the design of sustainable systems, Ein is the total input energy especially in the design of renewable energy systems. The application of Etot LCEA to different solar energy systems offers an excellent visualization out is the total output energy and (Ein-Eout)j is the exergy destruction of the exergy flows involved during the life time of the system. The in process j. analyzed plants are net producers of exergy, since the exergy consumed The exergy loss, i.e. exergy destruction and exergy waste, indicates can be paid back during their life time, however, in a varying degree. possible process improvements. In general “tackle the biggest loss The exergy of material that is part of the system in various components first” approach is not always appropriate since every part of the system during its operation will turn up as a product in the destruction phase depends on each other, so that an improvement in one part may cause and is depicted in the LCEA diagram. The recycling of this material will increased losses in other parts. As such, the total losses in the modified considerably reduce the payback time for future energy systems. process may in fact be equal or even larger, than in the original process In this paper we present the first application of LCEA to solar configuration. Also, the use of renewable and non-renewable resources, systems together with a proposal of how to evaluate the recycled as well as recycled resources must be considered. Therefore, the problem material in the LCEA method. The aim is to: (1) increase the awareness needs a more complete and careful approach. of and encourage the use of LCEA, (2) show the advantages of exergy Exergy factor and the reference state instead of energy in systems analysis, (3) apply LCEA to solar energy systems, and (4) introduce a new way to evaluate recycled material in Exergy factor is defined as the ratio of exergy to energy, and is LCEA. sometimes referred to as quality factor, exergy coefficient and exergy quality factor. Method The exergy factor for electricity and solar radiation is 1 and 0.93 Concept of Exergy respectively according to Eq. 4 with the temperature of the sun and the earth 6000K and 300K respectively, more detailed calculation can be The exergy concept originates from works of Carnot [30], Gibbs found in [38].
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