Energy 36 (2011) 1442e1459 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy An exergy-based framework for evaluating environmental impact Adam P. Simpson*, Chris F. Edwards Stanford University, Department of Mechanical Engineering, Stanford, CA 94305, USA article info abstract Article history: The analysis framework introduced in this paper utilizes the recognition that exergy is a form of envi- Received 25 July 2010 ronmental free energy to provide a fundamental basis for valuing environmental interactions inde- Received in revised form pendent from their secondary impacts (e.g., global warming, photochemical smog). In order to extend 7 December 2010 exergy to analyze environmental interactions, modifications are required to the traditional representa- Accepted 17 January 2011 tion of the environment and definition of the dead state used in technical exergy analysis. These are Available online 27 January 2011 accomplished through a combination of logical extensions and use of non-equilibrium thermodynamic principles. The framework is comprised of two separate components: (1) environmental exergy analysis Keywords: Exergy and (2) anthropocentric sensitivity analysis. Environmental exergy analysis is based on fundamental Exergy analysis thermodynamic principles and analysis techniques. It extends the principles of technical exergy analysis Reference state to the environment in order to quantify the locations, magnitudes, and types of environmental Dead state free energy impactdstate change, alteration of natural transfers, and destruction change. Anthropocentric sensitivity Environmental change analysis is based on the concepts of anthropocentric value and anthropocentric sensitivity. It enables the Environmental impact results of environmental exergy analysis to be interpreted for decision making, but at the expense of introducing some subjectivity into the framework. A key attribute of the framework is its ability to evaluate and compare the environmental performance of energy systems on a level playing field, regardless of the specifics of the systemsdresources, by-products, sizes, time scales. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction sulfur dioxide, carbon dioxide, and chlorofluorocarbons (CFCs). The emission of these particular species are monitored because of With the supply of conventional energy resources being identified adverse environmental consequencesdphotochemical depleted at their fastest rates, and the environmental impacts of smog, acid rain, global warming, and ozone depletion. Species- their use better understood, there is an increasing desire to develop based techniques typically communicate an energy system’s envi- and employ alternative energy technologies that lessen depen- ronmental performance by the mass of a species emitted per unit of dence on these resources and reduce environmental impact. This desired output (e.g., kg-CO2/MWhelec). leads to the question: Which alternative energy technologies Extensions have been made to species-based techniques for should we pursue? In order to answer this question, the potential comparing a range of species on the basis of their potential to cause technologies need to be compared on a level playing field based on a common environmental consequence. The most prominent of their environmental performance. these extensions is the use of Global Warming Potentials (GWPs) by There currently exist several analysis techniques for evaluating the government agencies such as the U.S. Environmental Protection environmental performance of energy systems. These can be cate- Agency (EPA) and the Intergovernmental Panel on Climate Change gorized as being based on species, economics, or thermodynamics. (IPCC). GWPs provide a measure of a species’ potential to cause Species-based techniques measure the environmental perfor- global warming relative to that of carbon dioxide. The details of and mance of energy systems based on the amount of a particular standard methods used to measure GWPs are defined by the IPCC species emitted by a system. The species of interest are typically [1]. When GWPs are employed to measure an energy system’s those that have been identified as causing adverse environmental environmental performance, the results are communicated consequences. Example species include oxides of nitrogen (NOx), through a mass of carbon-dioxide-equivalent emission per unit of desired output (e.g., kg-CO2-eq./MWhelec). Economics-based techniques measure environmental perfor- mance by assigning a monetary value to the undesirable outputs * Corresponding author. Tel.: þ1 650 725 2012; fax: þ1 650 723 1748. E-mail addresses: [email protected], [email protected] from energy systems that are known to cause adverse environ- (A.P. Simpson). mental consequences. In economics, the adverse environmental 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.01.025 A.P. Simpson, C.F. Edwards / Energy 36 (2011) 1442e1459 1443 consequences are referred to as environmental damages, and the Cumulative Exergy Extraction from the Natural Environment cost assigned to these damages are referred to as an environmental extends the embodied exergy concept used in CEC to include the external cost. There are several methods employed to define what exergy of renewable resource inputs, such as solar radiation, wind, is considered environmental damage and to prescribe an environ- and geothermal exergy. CEENE measures the environmental mental external cost. A detailed review of these methods is performance of energy systems through the total amount of provided by Riva and Trebeschi [2]. Economists and policy makers embodied exergy (measured through the CEC approach) and typically use the environmental external cost of an energy system renewable exergy required to produce a desired output (i.e., as a measure of its environmental performance. The environmental through the total amount of exergy removed from the natural external cost of an energy system can be combined with its internal environment). Dewulf et al. [7] describe the details and applica- cost (i.e., internalized) to provide a measure of the system’s real tions of CEENE. cost. It is often argued that the real cost of an energy system Extended Exergy Accounting tracks the extended exergy provides a more appropriate measure of economic performance content of all inputs and outputs of an energy system over its entire than internal cost when comparing energy system options [2,3]. life, including capital and labor. The extended exergy content of A significant shortcoming of species- and economic-based non-capital and non-labor inputs is calculated through the analysis techniques is that the environmental transfers of concern embodied exergy methods used in CEC. The extended exergy must be identified as being undesirable a priori of the analysis. content of capital is calculated by assigning an exergy value to Environmental transfers from an energy system are typically only a unit of a country’s currency based on the country’s gross domestic identified as being undesirable after they have caused some type of product (GDP) and the total amount of natural resource exergy adverse consequence. Neither technique is capable of evaluating consumed by the country in a given year (e.g., MJ/$GDP). The the environmental performance of an energy system without prior extended exergy content of labor is calculated by assigning an knowledge of the system’s environmental transfers causing exergy value to a hour of work (a “man-hour”) based on the total adverse consequences. Furthermore, both techniques only provide amount of natural resource exergy consumed by a person to proxy measures of environmental performance. They do not support 1 h of work (e.g., MJ/man-hour). EEA assigns an extended provide the ability to evaluate the environmental change driven by exergy content to an energy system’s undesired outputs (e.g., an energy system’s environmental interactions (upstream and emissions, waste) based on the extended exergy content of all downstream environmental transfers). inputs that would be required to reduce their exergy (thermo- Thermodynamics-based techniques employ thermodynamic mechanical and chemical) to zero. EEA measures the environ- concepts to evaluate and measure the environmental performance mental performance of energy systems through the total amount of of energy systems. The noteworthy thermodynamics-based anal- extended exergy input to a system in order to produce a desired ysis techniques are: Life Cycle Assessment (LCA), Cumulative output and reduce the exergy of undesired outputs to zero. The Exergy Consumption (CEC), Cumulative Exergy Extraction from the details and applications of EEA are described by [8e14]. Natural Environment (CEENE), Extended Exergy Accounting (EEA), Emergy Analysis tracks the solar emergy of all inputs to an energy and Emergy Analysis. All of these analysis techniques extend the system over its entire life, including capital and labor. Solar emergy is control boundary of an energy system and the time scale of the defined as the total amount of solar exergy directly or indirectly analysis to include all components and processes required to required to produce a given flow, and is measured in solar equivalent support the system over its entire life (often referred to as “cradle- joules (seJ). The solar emergy of capital is calculated by assigning to-grave” analysis). The key differences between the five