Exergy and Sustainability

Abstract

A major challenge in striving for a more sustainable society is the selection of technological systems. Given the capital intensity of industrial production plants, power generation systems and infrastructure, investment decisions create path dependencies for decades to come. It is difficult to know which technological system is preferable when considering the multiple objective of environmental, economic and social sustainability. E.g., a system that is preferred from the environmental point of view is not necessarily the system that is preferred from the economic and/or social point of view. Furthermore, the results of the assessments change over time because of new insights into environmental, economic and social sustainability and because they are prone to changing needs, economic conditions and societal preferences. Because of these uncertainties, it is hard to decide which technological system or systems should be chosen, e.g. to meet national and international targets with regard to climate change. Another way of assessing technological systems is the use of exergy analysis, a thermodynamic assessment method. Exergy analysis makes visible where work potential is lost. This work potential is needed for all the things we would like to do, i.e. nothing happens without the consumption of some work potential. Work potential that is lost, is lost forever. The only way to replenish the amount of work potential available on earth is by capturing new work potential from solar and/or tidal energy. Researchers active in the field of exergy and sustainability claim that the loss of work potential, also known as exergy loss, and sustainability are related. However, the loss of work potential is no part of the regular sustainability assessment methods. The objective of this research is to provide insight into the value of exergy analysis in sustainability assessment of technological systems. A literature research into the relationship between exergy and sustainability resulted in a theoretically founded relationship between exergy losses and the environmental impact of technological systems. A problem with investigating the relationship between exergy and sustainability is that there is no single measure of sustainability. Combining the results of the environmental, economic and social sustainability assessments into one sustainability indicator leads to a loss of information and necessitates the use of weighting factors. Another difficulty is that a commonly accepted operationalization of the term 'sustainability' does not exist. Accordingly, a list of requirements to methods for sustainability assessment of technological systems has been drawn up. All assessment methods cover the operational phase of installations, equipment and infrastructure including the amounts of inputs and outputs. Not all methods take into account the phases of construction and decommissioning of the installations, equipment and infrastructure and the following components of sustainability: the depletion and/or scarcity of the inputs, the distinction between renewable and non-renewable inputs, the disposal and/or abatement of emissions and waste flows, land use, exergy losses, economic aspects and social aspects. In addition, methods for the calculation of sustainability indicators should be objective and sufficient data should be available to calculate these indicators. The sustainability assessment methods found in the literature appear to be incomplete with respect to the list of requirements. The environmental life cycle assessment methods are not fully objective because they make use of weighting factors and because no consensus exists about all models used for quantifying environmental impact. The economic methods do not include all indirect costs and their indicators change over time because of market developments. The social methods suffer from the limited availability and qualitative or semi-quantitative nature of many data. The exergy analysis methods found in literature do not consider all components of sustainability and/or make use of indicators, equations and weighting factors that are not commonly accepted. It was therefore decided to develop a new exergy analysis method on the basis of fundamental scientific equations. The newly developed exergy analysis method has been named the Total Cumulative Exergy Loss (TCExL) method and takes into account as many of the designated components of sustainability as possible. The TCExL is the summation of the exergy loss caused within the technological system including its supply chains, the exergy loss caused by abatement of the resulting emissions and the exergy loss related to the land occupied by the technological system including its supply chains. The latter is relevant because land use prevents capturing new exergy from sunlight by the ecosystem. Components of the list of requirements that can only indirectly be considered when calculating the exergy loss caused by a technological system are the depletion and scarcity of resources as well as the economic and social aspects of sustainability. The TCExL method is an improvement compared to existing exergy analysis methods in the sense that it is solely based on the calculation of exergy losses and that it takes into account all exergy losses caused by a technological system during its life cycle. However, until now the abatement exergy loss of only a few emissions is included because of the lack of data regarding other emissions. The value of exergy analysis in sustainability assessment of technological systems has been investigated by conducting two case studies that comprise several power generation systems and subsequently comparing the results of the assessment methods with and without exergy of the systems of each case study. Power generation was chosen as the subject of the case studies because of the major role of electricity in our society. The choice of the systems of the case studies is not meant to indicate that these systems are preferable and/or desirable compared to other central or decentral power generation systems, nor that it is not important to look at the transport, distribution, use and/or storage of electricity. The first case study consists of the following systems for coal-fired power generation in combination with LNG evaporation: a power plant of which the waste heat is used for LNG evaporation, an oxyfuel power plant that is combined with air separation and LNG evaporation, and a stand-alone power plant plus the combination of LNG evaporation with an Organic Rankine Cycle (ORC). The other case study concerns power generation from fossil and renewable sources and compares the co-firing of coal and wood pellets with a wind farm and with power generation from the combustion of bioethanol that originates from the fermentation of verge grass. The method applied for determining the environmental sustainability is the ISO-certified environmental Life Cycle Assessment (LCA) method with ReCiPe endpoint indicators as the result. The present worth ratio (PWR) has been calculated to determine the economic sustainability. A newly developed method based on man-hours and the Inequality-adjusted Human Development Index (IHDI) reported by the UNDP has been used to assess the social sustainability, because a standard method for social LCA is still under development and because it would be too time-consuming and costly to gather site-specific social data. From the case studies, it is concluded that the sustainability of our society can be improved by applying exergy analysis in the assessment of technological systems, but that in the case of comparing technological systems with different inputs, a technological system that is preferred from an exergetic point of view is not always preferred from the economic and social points of view. If according to the results of the TCExL method a system is preferred that has a lower economic sustainability, it must be realised that economic indicators do not include all indirect costs and change over time. In the case of comparing technological systems with different inputs or with inputs from different locations, the calculation of a social sustainability indicator like the IHDI_overall indicator introduced in this research can have an added value compared to calculating only the TCExL. From a sustainability point of view, it is important to use exergy wisely. The higher the amount of exergy that is available on earth, the better people will be able to meet their needs. Therefore, the TCExL can be used as a fundamental indicator in the operationalization of the definition of sustainable development by the Brundtland commission. It is also concluded that exergy analysis leads to more fundamental insights into which process or part of a system has the largest potential for improvement than the standard sustainability assessment methods. It is recommended that exergy losses be taken into account when striving for a more sustainable society and that the TCExL method be used in decisions between technological systems. Furthermore, it is recommended that a working group be set up to investigate the possibilities for increasing the use of exergy analysis and that the TCExL method be implemented in software tools.