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Exergy Efficiency in Industry: Where Do We Stand? Robert U. Ayres,*,† Laura Talens Peiro,† and Gara Villalba Mendez‡ † ‡
INSEAD- Campus Europe, Boulevard de Constance, 77305 Fontainebleau, France Department of Chemical Engineering, Edifici Q, Universitat Autonoma de Barcelona (UAB), ES-08193 Bellaterra, Barcelona, Spain
bS Supporting Information ABSTRACT: Efficiency is a term generally used to determine how well a system performs. However, efficiency can have different meanings and, unaccompanied by a formal definition or taken out of context, can lead to serious misconceptions. In many official publications, efficiency is calculated as the ratio of useful output to energy input. This measure reflects the first law of thermodynamics (conservation of energy) but does not reflect the potential for improvement. A better measure, that also reflects the second law of thermodynamics, is the ratio of the potential useful (exergy) output to the potential useful (exergy) input. We estimate second law efficiencies for the inorganic and organic chemical industries to be 29% and 35% respectively. We also estimate the efficiency of the U.S. industry sector as a whole to be 37.6%, as compared to only 7.7% for the overall U.S. economy. These figures are far lower than the “first law” figures published by the U.S. Department of Energy (80% for industry and 42.5% overall) and they imply a significant potential for improvement.
1. INTRODUCTION The main subject of this paper is “efficiency”, a term often used without a formal definition, as if the term really needed none. Unfortunately, that is not the case. In fact, the term is widely used in three different ways, the more common of which is seriously misleading. For reasons not relevant to this paper, the U.S. Energy Information Agency (USEIA) has published a diagram in its annual energy review since 1950, as shown in Figure 1 and Supporting Information (SI 1). The 2008 version is almost identical to the one for 1970, except that all the flows are proportionally bigger.1 According to this chart, total U.S. energy consumption in 2008 was 99.2 quads (quadrillion BTU) of which 42.15 (42.5%) are classed as “useful”. The remaining 57.5% is classed as “rejected” energy, of which 27.39 quads are from electricity generation and 20.90 quads are from transportation. Remarkably, this chart only shows 4.78 quads of “rejected” energy from industry, as compared to 19.15 quads classed as “useful”; 1.71 quads rejected from commerce as compared to 6.86 “useful” and 2.29 quads rejected from residences (households) as compared to 9.18 quads that were supposed to be “useful”. The fact that the term “efficiency” is not used in the USEIA flowchart does not justify the misleading underestimation of potential for future energy (exergy) savings. The ratio of “rejected energy” to “total energy” is, of course, the loss fraction. Subtracting the loss fraction from unity yields a number that is easily interpreted as a measure of efficiency. We show these efficiency measures by sector for the USEIA energy flowcharts since 1950 in Table 1 below. The definition applied by the USEIA is a ratio between “useful” (in some sense) output and energy input. This is now r 2011 American Chemical Society
called “first law” efficiency because it reflects the fact that massenergy is conserved, so the mass and energy of the inputs and outputs must be the same. However, this definition is misleading because it only distinguishes between “useful” and “rejected” energy but does not tell us how much of the theoretically “useful” output is actually available to do work (is exergy) and how much of that output is unavailable (is anergy) due to entropic irreversibility losses. This point was clarified by a summer study sponsored by the American Physical Society (APS) in 1975.2 “Second-law efficiency”, as defined in the report of the APS summer study is now termed “exergy efficiency” in most textbooks.3 Exergy efficiency is the ratio of potentially useful (exergy) output to potentially useful (exergy) input. The reference to “second law” reflects the fact that losses of potential work result from irreversibilities due to the second law of thermodynamics (e2). Second-law efficiency is usually less than first law efficiency. For instance, a manufacturer may describe a gas-fired hot water boiler as “80% efficient” if for each unit of heat from the flame 0.8 units go into the water and 0.2 units are “rejected” into the exhaust pipe. The ratio of minimum energy needed to raise the temperature of water from 25 to 100 °C, to the heat produced by the flame is 9.92%. Hence, the hot water boiler is, in fact, very inefficient (see SI 2 for examples). The numbers in Table 1 are derived from the Sankey diagrams by USEIA, but most of the “useful” figures in those diagrams Received: June 27, 2011 Accepted: November 1, 2011 Revised: October 31, 2011 Published: November 01, 2011 10634
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Figure 1. U.S. Energy Flow diagram for 2008. Source: ref 4.
Table 1. U.S. Estimated Energy “Efficiencies”. Source: Authors Based on USEIAa sector
1950
1970
1990
2000
2008
electricity generation
25
36
33
31
0.32
residential and commercial
73
75
75
75
0.80
industrial
72
75
75
80
0.80
transport
26
26
25
20
0.24
aggregate
50
50
44
38
0.42
a
Source: LLNL= Lawrence Livermore National Laboratories; DOE = United States Department of Energy.
must have been based on nothing more than guesswork, at best. Only the efficiency of electric power generation (top line) is roughly accurate. None of the other efficiency numbers can be derived from published data. As regards transport, it seems likely that the “useful” numbers are based on the textbook efficiency of internal combustion engines operating in ideal conditions, viz. no part-load penalty, no stop-start penalty, no allowance for power train losses or parasitic loads and no adjustment for the fact that the purpose of transportation is to move people and goods, not vehicles. Calculations made by Dewulf and Van Langenhove of exergy service efficiency for several transport modes, taking all of these limitations into account, yield shockingly different results.5 For trucks, cars, and electric trains, the efficiency of the service is 0.37%, 0.44%, and 1.99% respectively (for a distance greater than 135 km at a speed of 80 km/h). Authors did not do a calculation for aircraft because the transport service provided is different (higher speed). However for obvious reasons, it is unlikely that
aircraft payload efficiency is as high as that for electric trains with a high load-factor. Given the predominance of highway vehicles, the overall service efficiency of the transport sector (excluding the production of fuels) is less than 1%, showing that the potential for improvement is very large. As regards the residential-commercial (buildings) sector, where most energy consumption is for space-heating and lighting, analysis in the APS summer study, indicates that the real (second-law) efficiency of most space-heating systems (by steam or hot water) is around 5% (electric heating is less), whereas water heating and lighting rarely achieve 10%.2 The same efficiency value for lighting and 6% for space heating are given for Norway in 1995.6 A recent paper suggests that the exergy efficiencies of a condensing boiler and a conventional boiler using LNG, and an external airair heat pump are estimated to be between 7% and 9%.7 In Japan, the exergy efficiency in the residential and commercial sectors are 6.33% and 5.74%, respectively.8 For our purposes, it is safe to say that the overall efficiency of the residentialcommercial sector is less than 10%, rather than 80% as implied by Table 1. It is worth noting that Reistad’s efficiency estimates (1975), based on second-law analysis were far more realistic. They were as follows: residentialcommercial (13.7%), industrial (36%) and transportation (20%), for an overall efficiency of 21% for the U.S. economy.9 Reistad’s estimate for transportation was too high, because he considered only the efficiency of the carriers (vehicles), without allowing for the payload. Several authors have undertaken studies comparing the exergy efficiency of different countries, notably Wall and Ertesvag. Wall estimated the exergy efficiency of Sweden (1980) as 22%, Japan (1985) as 19%, and Italy (1990) as 17%.1012 Using Wall’s approach, 10635
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Environmental Science & Technology Ertesvag estimated the efficiencies for Sweden as 22% (1980), Brazil as 23% (1987), Italy as 17% (1990), Turkey as 13% (1995), and Norway as 24% (1995).13 The relatively high efficiencies for Norway and Sweden are partly due to high contribution from hydroelectricity. We focus hereafter on industrial energy (exergy) efficiency, with special emphasis on the chemical sector. The Sections 2 and 3 deal with technicalities and definitions. Section 4 presents numerical results for the production of inorganic and organic chemicals. Section 5 shows exergy efficiency values for other industries and estimates the efficiency of the U.S. economy.
2. EXERGY With the convergence of policies for resource optimization and energy conservation, the idea of analyzing the efficiency of any process or system in terms of inputs and outputs becomes attractive. A brief word about economics: It is traditional to think of physical capital in mass or value terms, but on deeper reflection, capital goods can be divided into two categories: active and passive. Passive capital consists of material goods that provide a service merely by existing in a certain place or situation. The obvious examples of passive capital goods are roads, bridges, tunnels, structures, wires, pipes, and containers. Active capital consists of engines and machines. Active capital requires available energy (exergy) to function. All production (and consumption) in the economic system depend on active capital and available energy. It is important to understand from the outset that not all energy is available. Technically, available energy, now called exergy, is defined as the maximum amount of work that can be done by a subsystem as it approaches thermodynamic equilibrium with its surroundings by a sequence of reversible processes (for example, Szargut et al 198817). Equilibrium is a homogeneous unchanging state in which there are no gradients. This implies uniformity of temperature, pressure, density, chemical composition as well as uniform gravitational and electro-magnetic fields. In practice, the equilibrium end-states for all chemical (and other) processes may be the atmosphere, the oceans or the earth’s crust, depending on the chemical elements involved. Exergy, as potential work, is therefore definable for any subsystem that is not in thermodynamic equilibrium with its surroundings (air, ocean water, and soil). It is also definable as that component of energy potentially capable of doing work, in contrast with energy not capable of doing work (anergy). Fuels can be thought of as exergy carriers (along with process steam, flywheels, storage batteries, and electricity). Energy is conserved in every action or reaction (first law of thermodynamics) whereas exergy is not conserved. Exergy is destroyed when work is done, whereas anergy (like entropy) increases during every process or activity. There are four types of exergy: kinetic, potential (e.g., in the gravitational field of the earth), physical (based on temperature and pressure, with respect to the local environment) and chemical exergy (e.g., the heat output of an exothermic reaction). Electricity is almost pure exergy, as it can be totally converted to high temperature heat in an electric arc furnace or to mechanical work by an electric motor. On the other hand, low temperature heat has very little exergy content, because only a small fraction can be recovered as useful work. From this perspective, all natural and industrial materials can be characterized in terms of exergy “content”. In short all
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materials are exergy carriers, and this fact is particularly relevant to efficiency calculations in industry. Chemically inert materials such as water, carbon dioxide, silicon dioxide, iron oxide (in fact, most metal oxides) have low exergy content. Fuels and biomass, as well as reactive chemicals like industrial acids and caustic soda have high exergy content.
3. EFFICIENCY AND EFFECTIVENESS It is logical, at first glance, to suppose that the efficiency of any activity or process (including chemical or metallurgical processes) can be calculated by comparing the energy content of the useful output with the energy content of the inputs, including materials. This is known as “first law” efficiency, using the terminology of the APS study, because it reflects the fact that the total energy input is equal to the total energy of the output, partly useful and partly “rejected”.2 energy output ð1Þ e1 ¼ energy inputs An obvious extension of this definition is “second-law” efficiency, defined as the ratio of exergy embodied in “useful” products to the total exergy of all inputs. As noted already, the term “second law” reflects the fact that the exergy embodied in most energy flows (whether inputs or outputs) is less than the energy because some of the energy is not “useful”. For example, the exergy of process steam is much less than the energy (heat content or enthalpy) of the steam; because the steam is at a finite temperature (the two measures only coincide exactly at infinite temperature). More generally, second law efficiency reflects the fact that there are losses, due to irreversibilities in every process. e2 ¼
usef ul exergy output total exergy inputs
ð2Þ
As emphasized by the APS group “second law” efficiency is generally a much better measure of potential for improvement than first law efficiency (there is no difference in the case of electric power generation because the output is pure exergy). However, it is easy to demonstrate that “first law” efficiency (as applied to a piece of equipment such as a boiler) can be much greater than “second law” efficiency. The APS study considered a wide range of examples of energy efficiency, including heating systems, refrigeration, heat pumps, engines, generators, and motor vehicles. The study did not consider chemical or metallurgical processes. This paper, by contrast, considers process efficiency, almost exclusively. In this context, it is important to bear in mind that chemical reactions are of two kinds. Endothermic reactions, such as ammonia synthesis, consume (destroy) exergy in a process, so the exergy content of the useful product becomes the numerator of the efficiency fraction. Exothermic reactions, like combustion, generate heat which may be useful in driving a second reaction, or may be wasted. In exothermic reactions there are usually energy-rich byproducts in the output stream. Blast furnace gas from iron smelting is an example. For purposes of calculating efficiency, it depends on whether the energy-rich byproduct (say B.F. gas) is subsequently utilized or not. In addition, there may be unreacted inputs (such as nitrogen from the air) that reappear in the output. In this case, it may be convenient to subtract the exergy content of these unreacted compounds from both the numerator and the denominator, or it may be more practical to count the unreacted inputs as byproducts. 10636
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In exothermic reactions the difference between input exergy and output exergy is divided between byproduct and waste products. But what exactly is a waste? After all, some of it is usually heat, and some heat can be recovered, depending on temperature and equipment. Some of it may be other chemical compounds that have little economic value (chlorination processes are examples). In cases where combustion (or oxidation) is not involved, the definition of a waste is case dependent, and may also be equipment dependent. In some cases, where the inputs can be divided explicitly between “feedstocks” and “utilities”, but where the process is otherwise undefined, one can calculate second law efficiency as the exergy increase in the product (as compared to the exergy of the feedstock, excluding utilities), divided by the exergy of feedstocks consumed to drive the process.14 This applies, for instance, to petroleum refining taken as a whole. e2 ¼
increase exergy between f eedstocks and usef ul products exergy f uel consumed ð3Þ
Many chemical reactions consist of both exothermic and endothermic steps that cannot be physically separated. The exothermic stage produces heat that can drive a later reaction. Examples include ammonia synthesis, methanol synthesis, and carbothermic smelting. For this reason, it is often meaningless to calculate the exergy output of an exothermic reaction in isolation, because the exergy (as heat) may be difficult to recover and utilize, whereas it may be utilized quite efficiently to drive a subsequent endothermic reaction. In fact, this situation is the norm in the chemical industry. In this paper, we apply the second law efficiency calculation to the chemical industry as a whole. As noted above, the second law efficiency is defined as the ratio of the exergy content of the products to the exergy content of the inputs to the process. The exergy inputs are the sum of raw materials or feed-stocks and utilities (mainly electricity) required by the industry. Accounting for all those inputs together helps us to compare processes and product substitutes in terms of resource use, waste and emissions generation.15,16 For calculating second law efficiency, at the product level, it is necessary to have details of the material balances, stream composition and conditions, and flow-sheets of the process under study. The exergy of a product and materials is calculated using the reference state defined by Szargut et al. namely a reference temperature (298 K), pressure (1 atm) and an average conventional composition of the Earth’s litho-, hydro-, and atmosphere.17 The exergy of utilities is based on the amount of electricity, natural gas, diesel, water, and steam supplied to the processes. For electricity, 1 MJ of electrical energy equals 1 MJ of exergy.13 Calculating the efficiency of a process can help quantify how changes in production parameters may change yields and/or reduce emissions and exergy losses.
4. THE EFFICIENCY OF THE U.S. CHEMICAL INDUSTRY Quantitative data about industrial chemicals can be estimated with reasonable accuracy from industry production statistics. In the U.S., production statistics were published annually by the U.S. International Trade Commission (USITC) prior to the mid 1990s. However, in recent years the availability of production data to the public has become progressively worse as the number
of producers of high volume chemicals has declined. Hence, this paper used production statistics from USITC in 19911993 (there are other sources of information such Chemical Engineering News, but unfortunately they do not include statistical production data of hydrocarbons). USITC included production data for virtually all industrial chemicals, including intermediates. While production data for individual chemicals has fluctuated significantly since then, we think that the overall efficiencies have not changed by more than a few percent at most. To compare inputs and outputs for the whole sector, and avoid double counting and ensure the consistency of results, the list can be divided into two groups: (1) basic chemicals, which are made directly from raw materials, and (2) all others, including intermediates. For such classification, some knowledge of the industry is required. For instance, sulfuric acid is mainly made by burning sulfur, but is now also produced as a byproduct of copper smelting. Hydrochloric acid, is no longer made from salt but as byproduct of many downstream chlorination processes and thus is not included as a “basic” inorganic chemical. By combining production and process information of the basic inorganic and organic chemicals, raw material inputs are quantified in mass terms, whence a material balance by elements (C, H, O, N, Cl, S, Na, Ca, etc) can be performed. The difference between mass inputs and useful outputs are wastes and emissions. For the industry as a whole, wastes are characterized by elemental composition but they can be estimated approximately as a mix of compounds (CO2, CO, H2O, NaCl, CaSO4 etc.) based on knowledge of process reactions. For the production of sulfuric acid from sulfur, the inputs to account for are sulfur, oxygen, and hydrogen. For chlorine and sodium hydroxide, the inputs are sodium chloride and water, and for ammonia, the inputs are methane and air. The amount of the input materials is calculated based on their content in the final product. SI 3 shows the elemental mass balance for the production of basic inorganic and organic chemicals in U.S. in 1991. The energy requirement for the production of the inorganic chemicals sector as a whole is estimated based on the standard unit process descriptions for sulfuric acid, ammonia, chlorine and sodium hydroxide, multiplied by the tonnages reported. These calculations have been checked against reported energy consumption by subsectors for sulfuric acid (SIC 2819), ammonia (SIC 2873) and chlorine (SIC 2812) published in the Manufacturing Energy Consumption Survey (MECS) by the USEIA.18 When the material balance is closed and energy inputs are estimated, the exergy of inputs and outputs can be calculated using the procedure explained in Section 3. Once the exergy of inputs and outputs is known, the exergy balance follows directly and the second law efficiency can be calculated. The difference between the exergy of inputs and outputs are wastes and losses from the system. The basic inorganic chemicals made from raw materials are sulfuric acid, ammonia, chlorine and caustic soda. The total production of these four chemicals represents 75% of the total mass of inorganic chemical production.19 Sulfuric acid, the world’s largest-volume industrial chemical, is principally used in the manufacture of phosphoric acid and phosphate fertilizers. Sulfuric acid is also used in making a number of other chemical products, such as hydro-fluoric acid, synthetic detergents, dyes and pigments, drugs, explosives, plasticizers, adhesives, rubbers, edible oils, lubricants and the manufacture of food acids such as citric acid and lactic acid. It is used in petroleum refining (to wash impurities out of refinery products), water treatment, 10637
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Figure 2. Mass (in MMT) and exergy (PJ) balance of the production of inorganic and organic chemicals in U.S. in 1991.
for cleaning iron and steel surfaces before plating, and in mining copper, uranium, and vanadium ores. Ammonia is a starting material used in a wide variety of nitrogen fertilizer materials and industrial products. About 90% of all ammonia production is consumed in fertilizers: urea, ammonium nitrate, ammonium sulfate, and ammonium phosphates.20 The rest is currently used in the manufacture of other inorganic products including nitric acid, nitrates and nitriles, hydrocyanic acid, sodium cyanide, aniline, plastics, fibers (nylon), explosives, hydrazine, amines, amides, and other organic nitrogen compounds that serve as intermediates in dyes and pharmaceuticals manufacturing.21 Most of the world’s ammonia is produced from natural gas by steam reforming, except in China, where ammonia is produced from synthesis gas from coal.22 Chlorine and sodium hydroxide are produced as coproducts by electrolytic decomposition of sodium chloride solutions obtained from brines.19 They are two of the most important inorganic chemical commodities. Most of the chlorine and its organic compounds are converted to other products (as hydrochloric acid) and recycled within industry several times before being embodied in the final product or discarded as waste. Approximately 60% of the value added in the chemical industry involves chlorinated chemicals at some stage, even though chlorine is not contained in the final product. In 1993, 61% was used to make polyvinyl chloride (PVC), 18% went to industrial and commercial solvents (such as dry cleaning fluids), 15% went to other organics, and 6% was used in inorganics such as bleaches.2326 The percentages have not changed very much. Sodium hydroxide was once used mainly in the manufacture of soap (from animal fat), and it is still used in the manufacture of household detergents and cleaners. But caustic soda is now used in many other industrial processes, notably bauxite refining, in the paper pulping process, and in the petroleum and natural gas industry to neutralize acidic contaminants in gas and oil processing. It is also used for pH control, acid neutralization, and off-gas scrubbing. Based on the process descriptions of those basic inorganic chemicals, we assumed that sodium chloride, sulfur, methane, oxygen, nitrogen, and hydrogen are the raw material inputs. Figure 2 shows the material and energy balance for the production of the listed basic inorganic chemicals. The numbers in italics are the mass in million metric tons (MMT) of each input and output to the process. The mass balance shows that about 9%
of the total mass inputs are wasted in compounds of carbon, chlorine, sodium, and sulfur. Exergy inputs and outputs are represented in bold numbers and in PJ. The second law efficiency for the production of inorganic industrial chemicals works out to 29%, corresponding to a loss of more than 1,100 PJ. Based on USTIC statistics for 19911993, the major endproducts from organic chemicals are plastics (as polyethylene, polypropylene, polystyrene and polyvinyl chloride), nylon 6, ethylene glycol (antifreeze), and methyl tert-butyl-ether (a fuel additive that has been largely phased out since 1991). The production of these chemicals, in mass terms, represented about 80% of total U.S. production of organics in 1991.27 From standard unit process descriptions, we have estimated as basic inputs: hydrocarbons, and also inorganic raw materials as chlorine, sulfuric acid, ammonia, and caustic soda. Most organic chemicals are produced from feed-stocks from natural gas or petroleum refineries, with a very small share from coal. There are three categories of feedstock: paraffin, olefins, and cyclic/ aromatics. Paraffins are saturated straight or branched-chain hydrocarbons. Examples include methane, ethane, propane, isobutene, and n-butane. Olefins are unsaturated aliphatic compounds with one or more double bonds. Examples include ethylene, propylene, butylenes, and butadiene. Cyclic aromatics are benzene, toluene, xylene, cyclopentene, cyclohexane, and naphthalene. A detailed breakdown of hydrocarbons included in the assessment is included in the SI 4. Figure 2 illustrates the material and energy balance for the production of the listed basic organic chemicals. Again, mass values represented in italics are in MMT and exergy values in bold numbers are given in PJ. The mass balance shows that about 60% of the total mass inputs are wasted as compounds of carbon, hydrogen, oxygen, nitrogen, chlorine, sodium, and sulfur. The second law efficiency for the production of organic industrial chemicals turns out to be 35%, corresponding to a loss of 2,100 PJ of exergy.
5. THE EFFICIENCY OF U.S. INDUSTRY AND ECONOMY The second law efficiency values obtained for inorganic and organic chemicals together with published second law efficiencies for other industries serve to estimate the overall efficiency of U.S. industry. SI 5 shows “the second law efficiency” for several industrial processes given by different authors. The most significant 10638
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differences are found in exergy efficiency values given for the glass, paper and plastics industries. As regards the glass industry, the difference between the Hall (22.1%) and Ayres (2.9%) efficiency estimates are due to an assumption, in the Hall case, that high temperature waste heat is recovered—to preheat input materials, for instance—whereas in the Ayres calculation no recovery was assumed. For present purposes, we accept the Hall et al. calculation as more realistic. For the paper industry, the exergy efficiency results of Gyftopoulos et al. were calculated based on the exergy change of the feedstock and useful product, independently from the process.14 The efficiency result is misleading for two reasons: the change in the available useful work (exergy) for the sulfate alkaline process is small (only 0.12 GJ/ton) whereas the amount of fuel input is high (41.14 GJ/ton), and the efficiency calculation does not include the fuel or equivalent materials in the form of wood bark and spent pulp liquors generated by the process and used in the boiler. In this paper, we use the exergy efficiency value of 48% published by the U.S. Office of Technology Assessment (USOTA). For plastics, Hall et al. calculate the exergy efficiency based on the production of polymers from monomers whereas Gaines Table 2. Annual Production of Selected End Products (MMT) and Their Exergy Efficiency in 1993 production 1993 (MMT)
exergy efficiency
43.6
23%
ferrous
steel
non-ferrous
aluminum
3.7
24%
copper
1.8
10%
lead
0.3
11%a
zinc
0.2
5%a
industrial chemicals
inorganic
83.9
29%
organic
44.4
35%
other primary
glass
10.6
22%
materials
paper
82.2
48%
cement
67.0
14%b
337.8
30%
totals a
Calculated from Ayres and Ayres 2001.19 b Average exergy efficiency published by U.S. Congress Office of Technology Assessment (USOTA).
and Shen and Dewulf et al. included the production of the monomers.2830 The exergy efficiency of all plastics falls from 87 to 90% to 29 to 54% when including the production of monomers as ethylene, propylene, styrene, and vinyl chloride. In our assessment, we have included plastics as outputs from the organic chemical industry for assessing the U.S. industry efficiency, thus we do not include them as a separate industry. Table 2 shows the second law efficiency values for ferrous, nonferrous, and other primary materials are taken from published sources and based in previous comments. For the nonferrous metals lead and zinc, exergy efficiency values where calculated using data from published exergy flow diagrams.19 The overall industry exergy efficiency (e2) is calculated by adjusting the contribution of each useful product, based on production values, to the production of the whole industry. This methodology gives more importance to useful products generated in larger amounts. The useful products considered for the assessment are ferrous metals, nonferrous metals (aluminum, copper, lead, and zinc), industrial inorganic and organic chemicals, and other primary materials as glass, paper and cement. Table 2 shows the annual production of those products and their second law efficiencies. The annual production data for each industry is directly taken from U.S. annual statistics and technical reports.3136 The second law efficiency of U.S. industry works out to about 30%, an efficiency value well below 7580% estimated by USDOE. The results provides critical information to policy makers as they suggest where there is still a potential for improving present industrial production processes. Further studies about the efficiency of industrial processes are in order to identify potential saving at process level. Potential savings may come from better technologies for heat recovery, and also the recycling of waste. A possible way to increase potential savings at higher level is to promote the exchange of useful flows of materials and energy between industries concentrated within short distances. Potential saving will also depend on the type of industries located nearby, thus data and information about processes is crucial to ensure their success. The results of the exergy efficiency of U.S. industry can also be used to assess the efficiency of the U.S. economy, as a whole. Table 3 includes in the first column the total energy input used by the residential and commercial, industrial and transportation sectors of the U.S. economy in 1991.37 The second and third columns include the efficiency values deduced from USEIA Sankey diagrams (available in SI 4) and the useful energy output for each sector.38 The fourth and fifth columns have the exergy efficiency values for each sector and the useful energy output obtained from them.2,5
Table 3. Efficiency of U.S. Economy in 1991 USEIA
New estimation based on exergy
energy input by sector (quads) efficiency (%) useful energy output (quads) exergy efficiency (%) useful energy output (quads)
a b
residential and commercial
16.20
75
12.20
industrial
16.30
80
13.00
transportation
22.20
25
5.60
7.5a 30 1.0b
1.22 4.89 0.22
useful energy (exergy)
30.80
total energy consumption
82
6.33 82
efficiency
37.6%
7.7%
Average exergy efficiency value based on Carnahan et al. 1975.2 Similar exergy efficiency values are given by Yildiz and G€ung€or 20097 and Kondo 2009.8 Exergy efficiency value given by Dewulf and Van Langenhove 2003.5 10639
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Environmental Science & Technology We note that Reistad estimated that the residential and commercial sector efficiency was 13.7%.9 His estimate was based on two subsidiary estimates with which that we disagree, namely that the direct fuel use (for space and water heating) was 12.5% efficient and that the electrical use was 21.9% efficient, allowing for the efficiency of generation. However, the first figure is too high because the temperature of the water going into the roomheating radiator or the bathtub—as opposed to the water in the tank—is less than 100 °C resulting in a Carnot efficiency of no more than 5%.2 The second figure (electrical use) is also too high because a significant amount of the electrical energy was (and is) used for lighting, which was less than 5% efficient in 1975, and is less than 10% efficient today. For this reason, we think it is more realistic to use exergy efficiency values published by Kondo, which yields an overall efficiency for the sector of less than 10%.8 The efficiency of U.S. economy can be calculated by dividing the useful energy output by the total energy consumption. The efficiency given by USEIA equals 37.6%, compared to 7.7% given using the second law efficiency for the year 1991. The results show that more than 90% of high quality energy extracted from the earth is wasted. The transportation sector is by far the poorer in performance compared to industrial and even commercial and residential. These second law efficiencies indicate where efforts to improve the use of quality energy should be made and show that it is still possible to use resources much more efficiently than they are used, and that there is still a lot of potential for doing so. A recent study estimated that the current efficiency of U.S. economy is about 13% in 2009, but using an estimate of transportation efficiency based on the movement of vehicles, not payload.39,40 USEIA reports suggest that Russia, China, and India remain less energy efficient than the U.S. (at least in the industrial sectors) whereas Japan, the UK, and Austria reach 20% efficiency.41
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional material as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +33 160 724 189; fax: +33 160 745 566; e-mail: robert.
[email protected].
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