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Exergy analysis of solar thermal systems: A necessary complement to conventional energy analysis?

: Torio, Herena; Schmidt, Dietrich

International Solar Energy Society -ISES-:
Rapid transition to a renewable energy world. ISES Solar World Congress 2011. DVD-ROM : 28 Aug - 2 Sep 2011, Kassel, Germany; conference proceedings
Freiburg: ISES, 2011
ISBN: 978-3-9814659-0-7
Solar World Congress (SWC) <30, 2011, Kassel>
Conference Paper
Fraunhofer IBP ()

Current analysis and optimization methods of energy use in buildings are based on the assessment of primary energy flows, where all energy conversion steps from the extraction of energy sources (e.g. fuels) to the final demands to be supplied are assessed. The primary energy approach aims at limiting the use of fossil fuels for providing a given demand, leading to reduce energy demands and maximize the use of renewable energy sources. It is based, thus, on a distinction between renewable and fossil energy sources, being renewable energy flows often disregarded in the final balance. Thereby, an assessment of the efficiency of renewable energy use cannot be obtained from such analyses.
Exergy is a thermodynamic magnitude defined as the maximum theoretical work obtainable from the interaction of a system with its environment as the equilibrium state is reached between both. Exergy analysis allows for the detection and quantification of the improving potential of complex energy systems and has been widely used for the optimization of thermodynamic systems since the middle of the last century. Exergy analysis shows the true thermodynamic efficiency of an energy system. All energy flows, renewable or not, are regarded in the exergy balances, thereby allowing the assessment of the efficiency of renewable energy use in a given energy system.
Low temperature energy demands in buildings such as space heating and cooling have low quality, i.e. low exergy content. Low temperature renewable energy systems (e.g. solar thermal systems) are able to lower significantly the quality of the supplied energy in buildings, thereby increasing the exergy efficiency of energy supply.
In the present paper results from exergy analysis of a solar thermal unit for supplying part of the domestic hot water and space heating demands of a multi-family dwelling are presented and compared to conventional primary and final energy assessments. Differences and added value of exergy analysis as compared to conventional primary energy assessment for solar thermal systems is investigated by comparing the conclusions and insight gained from both methods.
The quality of solar thermal heat varies greatly depending on outdoor conditions and on the dynamic behavior of the solar thermal unit, i.e. on inlet and outlet collector temperatures. Therefore, dynamic energy and exergy simulations are required for an accurate and meaningful comparison of such analyses. Furthermore, different configurations and operation modes for the solar thermal system are investigated, allowing to achieve deeper understanding about the differences and information provided by each of the two analyses methods investigated.