An Exergy Footprint Metric Normalized to US Exergy
Consumption per Capita
Reggie J. Caudill, Sun Olapiriyakul, and Brian Seale

1. Introduction
2. Background
3. Approach
4. United States Exergy Analysis
5. Individual Exergy Footprint
6. Discussion
7. References

I. INTRODUCTION
The primary objective of this research is to combine the basic concepts and structure of carbon footprints with exergy analysis to create a sustainability metric called the exergy footprint. The goal is to establish a streamlined exergy calculator that is simple to use for individuals and businesses and allows direct comparison of consumption patterns with national and international standards or baseline values. Essentially, this approach focuses on simplicity of use and ease of understanding as opposed to extending the accuracy or broadening the scope of exergy analysis.With a global concern for climate change and carbon dioxide emissions, carbon footprints have moved beyond the domain of environmental science and lifecycle assessment into the realm of public awareness and personal decision making. The methodology underlying carbon footprints has several
intriguing characteristics: First, the technique is quantitative and has wide application to individuals, businesses,
communities, products and processes. Second, various websites have implemented carbon calculators with extensive databases which help to minimize user input and make the calculations available to anyone, anywhere. And, thirdly, the results of a carbon footprint can be easily normalized using both global and national-level per capita emission values, so that results can be benchmarked, areas for improvement identified and trends monitored.

Although the carbon footprint can be very useful, the limited scope restricts its ability to serve as a broad environmental metric. The carbon footprint calculates greenhouse gas emissions associated with the consumption of energy carriers: electricity, natural gas, petroleum and renewable sources [1]. However, other environmental impacts and resource concerns are equally important and should be considered. The exergy footprint presented here offers the potential to overcome these limitations and more broadly address environmental performance, resource consumption and other issues of sustainability.

The exergy footprint is structured similar to the carbon footprint but includes all resource consumption categories: materials, water, energy, food and, with the additional research currently underway, human and monetary capital. Consequently, the exergy footprint provides a comprehensive, sustainability view of the impact of a process or activity. By using exergy to quantify each of the resource consumptions and flows, the need to define, normalize and weight various impact categories is avoided. The rationale for using exergy as the common measure for the sustainability footprint is as follows:

• Exergy is based on sound scientific and engineering principles—the first and second laws of thermodynamics; consequently, the theoretical basis is well understood and universally accepted.
• All systems—physical, biological and ecological—are governed by the same fundamental principles; consequently, a single, unified theory underlies the analytical framework.
• Exergy provides a common set of units to evaluate material flows, energy flows, process behavior and human body interaction with the surrounding environment.
• Exergy consumption has been widely accepted as an environmental metric relating increased entropy to environmental chaos.
• Exergy analysis has been used extensively to identify potential efficiency improvements in complex processes. Often times, energy analysis is misleading and fails to identify the inefficient process operation. Consequently, the carbon footprint method may also lead analysts to misjudge areas for improvement or contribute to other misleading results.

II. BACKGROUND
Exergy analysis is well developed and has been widely used in recent years to assess process efficiency and evaluate environmental impacts. Exergy analysis is more informative and meaningful than energy analysis alone due to its ability to measure losses associated with thermodynamic irreversibility of a process [2-6]. According to these studies, process performance characteristics, such as exergy loss (consumption) and exergetic efficiency, have been used as indicators for environmental impacts [7-8]. Exergy is related to the entropy, a measure of irreversibility in a system, and quantifies the maximum possible work that can be done by a system as it is brought from its initial state into equilibrium with a reference state [2]. The following expression for exergy is used in this study as it focuses on the chemical exergy associated with material resources and energy carriers [9]:

The exergy footprint metric proposed here uses nationallevel exergy consumption on a per capita basis as the normalization factor. The normalization factor essentially indicates whether a specific consumption quantity is large or small by comparing to a national baseline value. For this study the target country is the United States and the normalization factor is the annual per capita exergy consumption based on data from 2008. National consumption patterns for several countries have been reported based primarily on the work by Reistad [10] and Wall [11]. These methods are also followed here to allow for international comparison and consistency. Reistad's study on US available energy consumption considered exergy consumption from energy carriers only. This study divided the consumption into three sectors based on the economic end-use sectors of Industry, Transportation, and Residential/Commercial.

Another approach takes into account all flows of materials and energy. In Wall's analysis of exergy in Sweden flows outside of energy use were considered [3]. Wall Accounted for wood products used in construction and paper products. Agriculture was taken into account, as well as Minerals and Ore. Energy carriers not directly used for energy were also included.

Based on the work of Reistad and Wall, Ertesvag [12] analyzed the exergy consumption of the Norwegian society, considering material and energy flows much like Wall. Ertesvag then compared exergy consumption across many societies [13]. The Reistad approach has been applied to Finland [14], Canada [15], Brazil [16], and Turkey [17]. Similarly, Ertesvag and Mielnik performed an exergy analysis of the Norwegian society [12]; Wall studied exergy conversion in the Japanese society [18]; and, Chen and Chen analyzed the Chinese society [19]. These studies extended Reistad’s work to include exergy of materials as well as just energy carriers. Chen and Chen applied many of the same principles used by Wall and Reistad but extended the analysis to include fluxes of labor services and monetary capital based upon the previous work of Sciubba [19]. In addition, Ukidwe and Bakshi examined resource intensities of chemical sectors in the US using input-output models [4]. The work of Bakshi and his team at Ohio State University has led to the development of a software tool called EcoLCA which accounts for exergy flows across the entire US economy using a 500x500 economic input-output sector model. This work also incorporates labor and monetary capital.

III. APPROACH
As discussed in the previous section, calculating exergy consumption is well established and widely explored with numerous applications both inside and outside the environmental research community. The objective of the work presented here is to formulate a metric that is similar to the activity-based nature of carbon footprints but is expanded to include the impacts of material resources and water consumption, in addition to energy use. The following sections describe the data and assumptions for calculating exergy consumption for energy carriers, transportation, water, food, materials and paper.

Fossil Fuel and Renewable Energy. The exergy associated with energy carriers is taken to be the product of the gross heating value of the carrier and the quality factor. The higher heating value, referred to as the gross heating value, is used by the Energy Information Administration (EIA), as a representation of the heat content of combustible materials. The net heating value, or lower heating value is used for the representation or exergy contained in fuel wood and fossil fuels according to Wall's approach [14]. These heating values represent the amount of heat released during the process of combustion. The two differ in the accounting of the latent heat of water vaporization that is released during combustion. The higher heating value includes this latent heat
while the lower heating value does not. These two values can be considered to be approximately equal when the moisture content is low. The heating value represents the energy content of the material and using the quality factor will give the total exergy of the given material. The exergy consumption for each energy carrier is given in Table 1, in terms of GJ per capita. The 2008 Annual Energy Review [21] from the EIA of US DOE is the reference for energy consumption by carrier and sector. The quality factors used in Table 1are taken from various sources as indicated.

Exergy associated with energy use in the Transportation sector is shown in Table 2. Of course, the primary energy carrier is petroleum based; however, biomass is growing in importance.

Fresh and Saline Water. A recent report from the USGS estimates that the total water usage for the US is approximately 410 billion gallons per day [22]. Of this amount, withdrawals from freshwater sources account for 349 billion gallons per day, or 85% of the total, while saline water represents the remainder. The report indicates that most saline-water withdrawals were seawater and brackish coastal water used to cool thermoelectric power plants. Since the exergy reference state for water is assumed to be seawater, the chemical exergy for saline is defined to be zero. The chemical exergy for freshwater is given in Szargut [2] as 0.77 kJ/mol or 50 MJ/m3. Consequently, the annual US consumption of freshwater is approximately 127,385,000 m3 which corresponds to a total exergy amount of 24,100 PJ or 79.3 GJ per capita

Food. The usefulness for the food supply, to a society, is in the nutritional content of the food or its ability to supply energy to people. This is represented in the caloric content of the food. The caloric content is the potential for the food to provide energy for people to live and do work. Therefore, caloric content will be considered as the exergy content of the food. The caloric data for the US is obtained through the United States Department of Agriculture's National Nutritional Database [23].

Food is divided into eight primary categories as follows: Red Meat and Poultry; Fish and Shell Fish; Dairy Products and Eggs; Fruits and Nuts; Vegetables; Flour and Cereal Products; Fats and Oils; and Beverages and Sweeteners. The US food consumption pattern and associated caloric data are given in Table 3.

Construction Materials, Metals and Plastic. In accordance with Walls approach, exergy content of construction material and metals will be considered as the chemical exergy of the material. The chemical exergy of the material is multiplied by the amount of the material that is consumed. With the exception of plastics, the annual consumption of materials shown in Table 4 is taken from the 2008 USGS report [24], which tracks the annual US material flows and consumption patterns. Consumption data on plastics is taken from the American Plastics Council of the American Chemical Society [25].

The chemical exergy values for the broad categories of construction materials and metals are approximated based on weight-factions of composition materials and corresponding individual element chemical exergy values, as given in equations 2 and 3, above.

Paper and Wood. Wood is divided into three categories of use: wood used for fuel, construction, and paper. These three use categories of wood correspond to different exergy contents. Wood used for fuel will be assumed to have an exergy content of 10.44 GJ/m3 [11]. This wood will be assumed to have 20% moisture content and a density of 750 kg/m3. The exergy content of paper will be assumed to have the value of 17 GJ/ton. Construction wood will be assumed to have exergy content of 8 GJ/m3 with a density of 450 kg/m3. The exergy values for construction wood and paper were used in many previous exergy studies, including Ertesvag's study on Norway [12].

IV. UNITED STATES EXERGY ANALYSIS
For the study of exergy in the US the apparent consumption of the different materials as described above will be considered. The consumption of various exergy carriers is based on the Annual Energy Review 2008 [21] and the quality factors shown in Tables 1 and 2. Table 1 shows the exergy based on the electric power and other energy sources consumption rates. This table does not include the exergy associated with transportation fuels, which are included in Table 2. Table 1 is separated into Electric Power Generation and Other Energy Sources, including fossil fuels and  renewable energy. Total exergy consumption in 2008, in these two sectors, is 341 GJ/capita. There is reasonable agreement  with this value compared to Reistad's 1970 value of 321 GJ/capita [10]. Tables 3and 4 show the exergy consumption as related to food and materials, respectively.

The total US exergy consumption per capita for the year 2008 can now be determined as the sum of each exergy sector: Energy Carriers (239.3 GJ/capita), Transportation (102.1 GJ/capita), Water (79.3 GJ/capita), Food (7.1 GJ/capita), and Materials (20.7 GJ/capita). The result is approximately 135,400 PJ or 448 GJ/capita. As indicated in the individual calculations, exergy consumption is dominated by energy carriers; however, the exergy of water and materials are significant contributors and should be carefully considered in any environmental study.

As indicated earlier, similar societal exergy studies have been performed for other countries with the following results: Norway: 278 GJ/capita [12]; Sweden: 310 GJ/capita [11]; Italy: 140 GJ/capita [28]; Japan: 150 GJ/capita [18]; China: 100 GJ/capita [19]; UK: 275 GJ/capita [29]; and, Saudi Arabia: 400 GJ/capita [30]. This information is presented in Figure 1 overlaid with national average values obtained from the carbonfootprint.com website. A couple of interesting observations can be made from these trends: (1) US exergy consumption on a per capita basis is the highest of the exergy values shown, which is consistent with the similar observation of the carbon footprint result. (2) While the Norwegian carbon footprint is similar in magniture to the average American, the exergy value in Norway is much less than in the US. (3) Sweden’s exergy consumption is similar to Norway’s but the Swedes carbon footprint is significantly lower. This is due to the fact that Norway has substantial petroleum refinery operations and exports almost all of their petroleum products; consequently, significant CO2 emissions are added to the carbon footprint while only minimal resource consumption is included in the exergy analysis.

V. INDIVIDUAL EXERGY FOOTPRINT
The structure of exergy footprint, as depicted in Figure 2, is similar to carbon footprints but expanded to include all resources. From data presented in the previous section and other sources referenced, the basic formulation of the exergy footprint metric is expressed in terms of the following intensity factors and activity drivers:

Housing (Input: Number of occupants and spending patterns)
Electricity: 11.36 MJ/kWh
Natural Gas: 110 MJ/Therm
Heating Oil: 156 MJ/gal
Propane: 102 MJ/gal
Water: 0.19 MJ/gal
Construction: 10.8 MJ / sq. ft. (energy + materials1)
1 Calculated from standard single-family house specification

Transportation  (Input: Auto characteristics & travel patterns)
Automobile: 139 MJ / gal
Bus Transit: 4.83 MJ/ passesger-mile
Rail Transit: 2.73 MJ/ passesger-mile
Air Travel: 3.53 MJ / passesger-mile
Auto Production: 130 GJ/veh (energy + materials2)
Note: reference [31] for basic mode efficiencies
2 Calculated from standard vehicle material list

Agriculture (Input: Household spending patterns [32])
Food Content: 5.82 GJ /capita & adjusted by spending
Production: 31% of freshwater used for irrigation

Goods and Services (Input: Household spending patterns)
Purchases: 3.9 MJ/ dollar spent 3

3 Note: Monetary exergy based on total exergy from Indistrial and Commercial sectors and 2008 US GDP
This model was implemented as an Excel spreadsheet and used to calculate the exergy footprint of a typical university professor.  The annual exergy consumption for the individual is 421 GJ as compared with the US per capita average of 448 GJ, calculated in the previous section.   Also, the exergy footprint reveals the allocation of exergy consumption by activity to be 33% for housing, 35% for transportation, 11% for food, and 21% for goods and services. An additional comparison was made with the Cal-Berkeley CoolClimate carbon footprint calculator [33]. Overall, the individual carbon footprint was 60 mtCO2e or 88% of the US average which correlates fairly well with the exergy footprint result shown here…94% of the national average.

VI.  DISCUSSION
A  streamlined  exergy  analysis is  combined  with  the structure and concept of carbon footprints. The result is an interesting metric that considers resource consumption more broadly than traditional carbon footprint. And, by normalization to national-level exergy consumption,  individuals and businesses can better understand how to improve their operations and life style. Currently, the technique is being expanded to include human capital and monetary exergy so that a broader perspective on sustainability can be reflected. In addition, our website www.exergyfootprint.com is being developed so that others can use this metric to compare behavior, benchmark performance, identify actions for improvement, and reduce resource consumption.

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