Emergy is the amount of energy consumed in direct and indirect transformations to make a product or service. [1] Emergy is a measure of quality differences between different forms of energy. Emergy is an expression of all the energy used in the work processes that generate a product or service in units of one type of energy. Emergy is measured in units of emjoules, a unit referring to the available energy consumed in transformations. Emergy accounts for different forms of energy and resources (e.g. sunlight, water, fossil fuels, minerals, etc.) Each form is generated by transformation processes in nature and each has a different ability to support work in natural and in human systems. The recognition of these quality differences is a key concept.
The theoretical and conceptual basis for the emergy methodology is grounded in thermodynamics[ citation needed], general system theory [2] and systems ecology. [3] Evolution of the theory by Howard T. Odum over the first thirty years is reviewed in Environmental Accounting [1] and in the volume edited by C. A. S. Hall titled Maximum Power. [4]
Beginning in the 1950s, Odum analyzed energy flow in ecosystems (e.g. Silver Springs, Florida; [5] Enewetak atoll in the south Pacific; [6] Galveston Bay, Texas [7] and Puerto Rican rainforests, [8] amongst others) where energies in various forms at various scales were observed. His analysis of energy flow in ecosystems, and the differences in the potential energy of sunlight, fresh water currents, wind and ocean currents led him to make the suggestion that when two or more different energy sources drive a system, they cannot be added without first converting them to a common measure that accounts for their differences in energy quality. This led him to introduce the concept of "energy of one kind" as a common denominator with the name "energy cost". [9] He then expanded the analysis to model food production in the 1960s, [9] and in the 1970s to fossil fuels. [10] [11]
Odum's first formal statement of what would later be termed emergy was in 1973:
Energy is measured by calories, btu's, kilowatthours, and other intraconvertable units, but energy has a scale of quality which is not indicated by these measures. The ability to do work for man depends on the energy quality and quantity and this is measurable by the amount of energy of a lower quality grade required to develop the higher grade. The scale of energy goes from dilute sunlight up to plant matter, to coal, from coal to oil, to electricity and up to the high quality efforts of computer and human information processing. [12]
In 1975, he introduced a table of "Energy Quality Factors", kilocalories of sunlight energy required to make a kilocalorie of a higher quality energy, [13] the first mention of the energy hierarchy principle which states that "energy quality is measured by the energy used in the transformations" from one type of energy to the next.
These energy quality factors, were placed on a fossil-fuel basis and called "Fossil Fuel Work Equivalents" (FFWE), and the quality of energies were measured based on a fossil fuel standard with rough equivalents of 1 kilocalorie of fossil fuel equal to 2000 kilocalories of sunlight. "Energy quality ratios" were computed by evaluating the quantity of energy in a transformation process to make a new form and were then used to convert different forms of energy to a common form, in this case fossil fuel equivalents. FFWE's were replaced with coal equivalents (CE) and by 1977, the system of evaluating quality was placed on a solar basis and termed solar equivalents (SE). [14]
The term " embodied energy" was used for a time in the early 1980s to refer to energy quality differences in terms of their costs of generation, and a ratio called a "quality factor" for the calories (or joules) of one kind of energy required to make those of another. [15] However, since the term embodied energy was used by other groups who were evaluating the fossil fuel energy required to generate products and were not including all energies or using the concept to imply quality, embodied energy was dropped in favor of "embodied solar calories", and the quality factors became known as "transformation ratios".
Use of the term "embodied energy" for this concept was modified in 1986 when David Scienceman, a visiting scholar at the University of Florida from Australia, suggested the term "emergy" and "emjoule" or "emcalorie" as the unit of measure to distinguish emergy units from units of available energy. [16] The term transformation ratio was shortened to transformity in about the same time. It is important to note that throughout these twenty years, the baseline or the basis for evaluating forms of energy and resources shifted from organic matter to fossil fuels and finally to solar energy.
After 1986, the emergy methodology continued to develop as the community of scientists expanded and as new applied research into combined systems of humans and nature presented new conceptual and theoretical questions. The maturing of the emergy methodology resulted in more rigorous definitions of terms and nomenclature and refinement of the methods of calculating transformities. The International Society for the Advancement of Emergy Research Archived 2016-05-13 at the Wayback Machine and a biennial International Conference at the University of Florida support this research.
Years | Baseline | Unit Emergy Values | Units | Reference |
---|---|---|---|---|
1967–1971 | Organic matter the baseline. All energies of higher quality (wood, peat, coal, oil, living biomass, etc.) expressed in units of organic matter. | Sunlight equivalent to organic matter = 1000 solar kilocalories per kilocalorie of organic matter. | g dry wt O.M.; kcal, conversion from OM to kcal = 5kcal/g dry wt. | [9] [17] |
1973–1980 | Fossil fuels and then coal the baseline. Energy of lower quality (sunlight, plants, wood, etc.) were expressed in units of fossil fuels and later in units of coal equivalents. | Direct sunlight equivalents of fossil fuels = 2000 solar kilocalories per fossil fuel kilocalorie | Fossil fuel work equivalents (FFWE) and later, coal equivalents (CE) | [10] [11] |
1980–1982 | Global solar energy the baseline. All energies of higher quality (wind, rain, wave, organic matter, wood, fossil fuels, etc.) expressed in units of solar energy | 6800 global solar Calories per Calorie of available energy in coal | Global solar calories (GSE). | [3] [18] |
1983–1986 | Recognized that solar energy, deep heat, and tidal momentum were basis for global processes. Total annual global sources equal to the sum of these (9.44 E24 solar joules/yr) | Embodied solar joules per joule of fossil fuels = 40,000 seJ/J | Embodied solar equivalents (SEJ) and later called "emergy" with nomenclature (seJ) | [19] |
1987–2000 | Further refinements of total energy driving global processes, Embodied solar energy renamed to EMERGY | Solar Emergy per Joule of coal energy ~ 40,000 solar emjoules/ Joule (seJ/J) named Transformity | seJ/J = Transformity; seJ/g = Specific emergy | [1] |
2000–present | Emergy driving the biosphere reevaluated as 15.83 E24 seJ/yr raising all previously calculated transformities by the ratio of 15.83/9.44 = 1.68 | Solar emergy per Joule of coal energy ~ 6.7 E 4 seJ/J | seJ/J = Transformity; seJ/g = Specific emergy | [20] |
Emergy— amount of energy of one form that is used in transformations directly and indirectly to make a product or service. The unit of emergy is the emjoule or emergy joule. Using emergy, sunlight, fuel, electricity, and human service can be put on a common basis by expressing each of them in the emjoules of solar energy that is required to produce them. If solar emergy is the baseline, then the results are solar emjoules (abbreviated seJ). Although other baselines have been used, such as coal emjoules or electrical emjoules, in most cases emergy data are given in solar emjoules.
Unit Emergy Values (UEVs) — the emergy required to generate one unit of output. Types of UEVs:
Term | Definition | Abbreviation | Units |
---|---|---|---|
Extensive Properties | |||
Emergy | The amount of available energy of one type (usually solar) that is directly or indirectly required to generate a given output flow or storage of energy or matter. | Em | seJ (solar equivalent Joules) |
Emergy Flow | Any flow of emergy associated with inflowing energy or materials to a system/process. | R=renewable flows; N= nonrenewable flows; F= imported flows; S= services |
seJ*time−1 |
Gross Emergy Product | Total emergy annually used to drive a national or regional economy | GEP | seJ*yr−1 |
Product-related Intensive Properties | |||
Transformity | Emergy investment per unit process output of available energy | Τr | seJ*J−1 |
Specific Emergy | Emergy investment per unit process output of dry mass | SpEm | seJ*g−1 |
Emergy Intensity of currency | Emergy investment per unit of GDP generated in a country, region or process | EIC | seJ*curency−1 |
Space-related Intensive Properties | |||
Emergy Density | Emergy stored in a volume unit of a given material | EmD | seJ*volume−3 |
Time-related Intensive Properties | |||
Empower | Emergy flow (released, used) per unit time | EmP | seJ*time−1 |
Empower Intensity | Areal Empower (emergy released per unit time and area) | EmPI | seJ*time−1*area−1 |
Empower Density | Emergy released per unit time by a unit volume (e.g. a power plant or engine) | EmPd | seJ*time−1*volume−3 |
Selected Performance Indicators | |||
Emergy released (used) | Total emergy investment in a process (measure of a process footprint) | U= N+R+F+S (see Fig.1) |
seJ |
Emergy Yield Ratio | Total emergy released (used up) per unit of emergy invested | EYR= U/(F+S) (see Fig.1) |
— |
Environmental Loading Ratio | Total nonrenewable and imported emergy released per unit of local renewable resource | ELR= (N+F+S)/R (see Fig.1) |
— |
Emergy Sustainability Index | Emergy yield per unit of environmental loading | ESI= EYR/ELR (see Fig.1) |
— |
Renewability | Percentage of total emergy released (used) that is renewable. | %REN= R/U (see Fig.1) |
— |
Emergy Investment Ratio | Emergy investment needed to exploit one unit of local (renewable and nonrenewable) resource. | EIR= (F+S)/(R+N) (see Fig.1) |
— |
Emergy accounting converts the thermodynamic basis of all forms of energy, resources and human services into equivalents of a single form of energy, usually solar. To evaluate a system, a system diagram organizes the evaluation and account for energy inputs and outflows. A table of the flows of resources, labor and energy is constructed from the diagram and all flows are evaluated. The final step involves interpreting the results. [1]
In some cases, an evaluation is done to determine the fit of a development proposal within its environment. It also allows comparison of alternatives. Another purpose is to seek the best use of resources to maximize economic vitality.
System diagrams show the inputs that are evaluated and summed to obtain the emergy of a flow. A diagram of a city and its regional support area is shown in Figure 1. [21]
A table (see example below) of resource flows, labor and energy is constructed from the diagram. Raw data on inflows that cross the boundary are converted into emergy units, and then summed to obtain total emergy supporting the system. Energy flows per unit time (usually per year) are presented in the table as separate line items.
Note | Item(name) | Data(flow/time) | Units | UEV (seJ/unit) | Solar Emergy (seJ/time) |
---|---|---|---|---|---|
1. | First item | xxx.x | J/yr | xxx.x | Em1 |
2. | Second item | xxx.x | g/yr | xxx.x | Em2 |
-- | |||||
n. | nth item | xxx.x | J/yr | xxx.x | Emn |
O. | Output | xxx.x | J/yr or g/yr | xxx.x |
All tables are followed by footnotes that show citations for data and calculations.
The table allows a unit emergy value to be calculated. The final, output row (row “O” in the example table above) is evaluated first in units of energy or mass. Then the input emergy is summed and the unit emergy value is calculated by dividing the emergy by the units of the output.
Figure 2 shows non-renewable environmental contributions (N) as an emergy storage of materials, renewable environmental inputs (R), and inputs from the economy as purchased (F) goods and services. Purchased inputs are needed for the process to take place and include human service and purchased non-renewable energy and material brought in from elsewhere (fuels, minerals, electricity, machinery, fertilizer, etc.). Several ratios, or indices are given in Figure 2 that assess the global performance of a process.
Other ratios are useful depending on the type and scale of the system under evaluation.
The recognition of the relevance of energy to the growth and dynamics of complex systems has resulted in increased emphasis on environmental evaluation methods that can account for and interpret the effects of matter and energy flows at all scales in systems of humanity and nature. The following table lists some general areas in which the emergy methodology has been employed.
Emergy and ecosystems
|
Emergy and Information
|
Emergy and Agriculture
|
Emergy and energy sources and carriers
|
Emergy and the Economy
|
Emergy and cities
|
Emergy and landscapes
|
Emergy and ecological engineering
|
Emergy, material flows and recycling
|
Emergy and thermodynamics
|
Emergy and systems modeling
|
Emergy and policy
|
|
The concept of emergy has been controversial within academe including ecology, thermodynamics and economy. [23] [24] [25] [26] [27] [28] Emergy theory has been criticized for allegedly offering an energy theory of value to replace other theories of value.[ citation needed] The stated goal of emergy evaluations is to provide an "ecocentric" valuation of systems, processes. Thus it does not purport to replace economic values but to provide additional information, from a different point of view.[ citation needed]
The idea that a calorie of sunlight is not equivalent to a calorie of fossil fuel or electricity strikes many as absurd, based on the 1st Law definition of energy units as measures of heat (i.e. Joule's mechanical equivalent of heat). [29] Others have rejected the concept as impractical since from their perspective it is impossible to objectively quantify the amount of sunlight that is required to produce a quantity of oil. In combining systems of humanity and nature and evaluating environmental input to economies, mainstream economists criticize the emergy methodology for disregarding market values.[ citation needed]
{{
cite web}}
: CS1 maint: archived copy as title (
link).
Emergy is the amount of energy consumed in direct and indirect transformations to make a product or service. [1] Emergy is a measure of quality differences between different forms of energy. Emergy is an expression of all the energy used in the work processes that generate a product or service in units of one type of energy. Emergy is measured in units of emjoules, a unit referring to the available energy consumed in transformations. Emergy accounts for different forms of energy and resources (e.g. sunlight, water, fossil fuels, minerals, etc.) Each form is generated by transformation processes in nature and each has a different ability to support work in natural and in human systems. The recognition of these quality differences is a key concept.
The theoretical and conceptual basis for the emergy methodology is grounded in thermodynamics[ citation needed], general system theory [2] and systems ecology. [3] Evolution of the theory by Howard T. Odum over the first thirty years is reviewed in Environmental Accounting [1] and in the volume edited by C. A. S. Hall titled Maximum Power. [4]
Beginning in the 1950s, Odum analyzed energy flow in ecosystems (e.g. Silver Springs, Florida; [5] Enewetak atoll in the south Pacific; [6] Galveston Bay, Texas [7] and Puerto Rican rainforests, [8] amongst others) where energies in various forms at various scales were observed. His analysis of energy flow in ecosystems, and the differences in the potential energy of sunlight, fresh water currents, wind and ocean currents led him to make the suggestion that when two or more different energy sources drive a system, they cannot be added without first converting them to a common measure that accounts for their differences in energy quality. This led him to introduce the concept of "energy of one kind" as a common denominator with the name "energy cost". [9] He then expanded the analysis to model food production in the 1960s, [9] and in the 1970s to fossil fuels. [10] [11]
Odum's first formal statement of what would later be termed emergy was in 1973:
Energy is measured by calories, btu's, kilowatthours, and other intraconvertable units, but energy has a scale of quality which is not indicated by these measures. The ability to do work for man depends on the energy quality and quantity and this is measurable by the amount of energy of a lower quality grade required to develop the higher grade. The scale of energy goes from dilute sunlight up to plant matter, to coal, from coal to oil, to electricity and up to the high quality efforts of computer and human information processing. [12]
In 1975, he introduced a table of "Energy Quality Factors", kilocalories of sunlight energy required to make a kilocalorie of a higher quality energy, [13] the first mention of the energy hierarchy principle which states that "energy quality is measured by the energy used in the transformations" from one type of energy to the next.
These energy quality factors, were placed on a fossil-fuel basis and called "Fossil Fuel Work Equivalents" (FFWE), and the quality of energies were measured based on a fossil fuel standard with rough equivalents of 1 kilocalorie of fossil fuel equal to 2000 kilocalories of sunlight. "Energy quality ratios" were computed by evaluating the quantity of energy in a transformation process to make a new form and were then used to convert different forms of energy to a common form, in this case fossil fuel equivalents. FFWE's were replaced with coal equivalents (CE) and by 1977, the system of evaluating quality was placed on a solar basis and termed solar equivalents (SE). [14]
The term " embodied energy" was used for a time in the early 1980s to refer to energy quality differences in terms of their costs of generation, and a ratio called a "quality factor" for the calories (or joules) of one kind of energy required to make those of another. [15] However, since the term embodied energy was used by other groups who were evaluating the fossil fuel energy required to generate products and were not including all energies or using the concept to imply quality, embodied energy was dropped in favor of "embodied solar calories", and the quality factors became known as "transformation ratios".
Use of the term "embodied energy" for this concept was modified in 1986 when David Scienceman, a visiting scholar at the University of Florida from Australia, suggested the term "emergy" and "emjoule" or "emcalorie" as the unit of measure to distinguish emergy units from units of available energy. [16] The term transformation ratio was shortened to transformity in about the same time. It is important to note that throughout these twenty years, the baseline or the basis for evaluating forms of energy and resources shifted from organic matter to fossil fuels and finally to solar energy.
After 1986, the emergy methodology continued to develop as the community of scientists expanded and as new applied research into combined systems of humans and nature presented new conceptual and theoretical questions. The maturing of the emergy methodology resulted in more rigorous definitions of terms and nomenclature and refinement of the methods of calculating transformities. The International Society for the Advancement of Emergy Research Archived 2016-05-13 at the Wayback Machine and a biennial International Conference at the University of Florida support this research.
Years | Baseline | Unit Emergy Values | Units | Reference |
---|---|---|---|---|
1967–1971 | Organic matter the baseline. All energies of higher quality (wood, peat, coal, oil, living biomass, etc.) expressed in units of organic matter. | Sunlight equivalent to organic matter = 1000 solar kilocalories per kilocalorie of organic matter. | g dry wt O.M.; kcal, conversion from OM to kcal = 5kcal/g dry wt. | [9] [17] |
1973–1980 | Fossil fuels and then coal the baseline. Energy of lower quality (sunlight, plants, wood, etc.) were expressed in units of fossil fuels and later in units of coal equivalents. | Direct sunlight equivalents of fossil fuels = 2000 solar kilocalories per fossil fuel kilocalorie | Fossil fuel work equivalents (FFWE) and later, coal equivalents (CE) | [10] [11] |
1980–1982 | Global solar energy the baseline. All energies of higher quality (wind, rain, wave, organic matter, wood, fossil fuels, etc.) expressed in units of solar energy | 6800 global solar Calories per Calorie of available energy in coal | Global solar calories (GSE). | [3] [18] |
1983–1986 | Recognized that solar energy, deep heat, and tidal momentum were basis for global processes. Total annual global sources equal to the sum of these (9.44 E24 solar joules/yr) | Embodied solar joules per joule of fossil fuels = 40,000 seJ/J | Embodied solar equivalents (SEJ) and later called "emergy" with nomenclature (seJ) | [19] |
1987–2000 | Further refinements of total energy driving global processes, Embodied solar energy renamed to EMERGY | Solar Emergy per Joule of coal energy ~ 40,000 solar emjoules/ Joule (seJ/J) named Transformity | seJ/J = Transformity; seJ/g = Specific emergy | [1] |
2000–present | Emergy driving the biosphere reevaluated as 15.83 E24 seJ/yr raising all previously calculated transformities by the ratio of 15.83/9.44 = 1.68 | Solar emergy per Joule of coal energy ~ 6.7 E 4 seJ/J | seJ/J = Transformity; seJ/g = Specific emergy | [20] |
Emergy— amount of energy of one form that is used in transformations directly and indirectly to make a product or service. The unit of emergy is the emjoule or emergy joule. Using emergy, sunlight, fuel, electricity, and human service can be put on a common basis by expressing each of them in the emjoules of solar energy that is required to produce them. If solar emergy is the baseline, then the results are solar emjoules (abbreviated seJ). Although other baselines have been used, such as coal emjoules or electrical emjoules, in most cases emergy data are given in solar emjoules.
Unit Emergy Values (UEVs) — the emergy required to generate one unit of output. Types of UEVs:
Term | Definition | Abbreviation | Units |
---|---|---|---|
Extensive Properties | |||
Emergy | The amount of available energy of one type (usually solar) that is directly or indirectly required to generate a given output flow or storage of energy or matter. | Em | seJ (solar equivalent Joules) |
Emergy Flow | Any flow of emergy associated with inflowing energy or materials to a system/process. | R=renewable flows; N= nonrenewable flows; F= imported flows; S= services |
seJ*time−1 |
Gross Emergy Product | Total emergy annually used to drive a national or regional economy | GEP | seJ*yr−1 |
Product-related Intensive Properties | |||
Transformity | Emergy investment per unit process output of available energy | Τr | seJ*J−1 |
Specific Emergy | Emergy investment per unit process output of dry mass | SpEm | seJ*g−1 |
Emergy Intensity of currency | Emergy investment per unit of GDP generated in a country, region or process | EIC | seJ*curency−1 |
Space-related Intensive Properties | |||
Emergy Density | Emergy stored in a volume unit of a given material | EmD | seJ*volume−3 |
Time-related Intensive Properties | |||
Empower | Emergy flow (released, used) per unit time | EmP | seJ*time−1 |
Empower Intensity | Areal Empower (emergy released per unit time and area) | EmPI | seJ*time−1*area−1 |
Empower Density | Emergy released per unit time by a unit volume (e.g. a power plant or engine) | EmPd | seJ*time−1*volume−3 |
Selected Performance Indicators | |||
Emergy released (used) | Total emergy investment in a process (measure of a process footprint) | U= N+R+F+S (see Fig.1) |
seJ |
Emergy Yield Ratio | Total emergy released (used up) per unit of emergy invested | EYR= U/(F+S) (see Fig.1) |
— |
Environmental Loading Ratio | Total nonrenewable and imported emergy released per unit of local renewable resource | ELR= (N+F+S)/R (see Fig.1) |
— |
Emergy Sustainability Index | Emergy yield per unit of environmental loading | ESI= EYR/ELR (see Fig.1) |
— |
Renewability | Percentage of total emergy released (used) that is renewable. | %REN= R/U (see Fig.1) |
— |
Emergy Investment Ratio | Emergy investment needed to exploit one unit of local (renewable and nonrenewable) resource. | EIR= (F+S)/(R+N) (see Fig.1) |
— |
Emergy accounting converts the thermodynamic basis of all forms of energy, resources and human services into equivalents of a single form of energy, usually solar. To evaluate a system, a system diagram organizes the evaluation and account for energy inputs and outflows. A table of the flows of resources, labor and energy is constructed from the diagram and all flows are evaluated. The final step involves interpreting the results. [1]
In some cases, an evaluation is done to determine the fit of a development proposal within its environment. It also allows comparison of alternatives. Another purpose is to seek the best use of resources to maximize economic vitality.
System diagrams show the inputs that are evaluated and summed to obtain the emergy of a flow. A diagram of a city and its regional support area is shown in Figure 1. [21]
A table (see example below) of resource flows, labor and energy is constructed from the diagram. Raw data on inflows that cross the boundary are converted into emergy units, and then summed to obtain total emergy supporting the system. Energy flows per unit time (usually per year) are presented in the table as separate line items.
Note | Item(name) | Data(flow/time) | Units | UEV (seJ/unit) | Solar Emergy (seJ/time) |
---|---|---|---|---|---|
1. | First item | xxx.x | J/yr | xxx.x | Em1 |
2. | Second item | xxx.x | g/yr | xxx.x | Em2 |
-- | |||||
n. | nth item | xxx.x | J/yr | xxx.x | Emn |
O. | Output | xxx.x | J/yr or g/yr | xxx.x |
All tables are followed by footnotes that show citations for data and calculations.
The table allows a unit emergy value to be calculated. The final, output row (row “O” in the example table above) is evaluated first in units of energy or mass. Then the input emergy is summed and the unit emergy value is calculated by dividing the emergy by the units of the output.
Figure 2 shows non-renewable environmental contributions (N) as an emergy storage of materials, renewable environmental inputs (R), and inputs from the economy as purchased (F) goods and services. Purchased inputs are needed for the process to take place and include human service and purchased non-renewable energy and material brought in from elsewhere (fuels, minerals, electricity, machinery, fertilizer, etc.). Several ratios, or indices are given in Figure 2 that assess the global performance of a process.
Other ratios are useful depending on the type and scale of the system under evaluation.
The recognition of the relevance of energy to the growth and dynamics of complex systems has resulted in increased emphasis on environmental evaluation methods that can account for and interpret the effects of matter and energy flows at all scales in systems of humanity and nature. The following table lists some general areas in which the emergy methodology has been employed.
Emergy and ecosystems
|
Emergy and Information
|
Emergy and Agriculture
|
Emergy and energy sources and carriers
|
Emergy and the Economy
|
Emergy and cities
|
Emergy and landscapes
|
Emergy and ecological engineering
|
Emergy, material flows and recycling
|
Emergy and thermodynamics
|
Emergy and systems modeling
|
Emergy and policy
|
|
The concept of emergy has been controversial within academe including ecology, thermodynamics and economy. [23] [24] [25] [26] [27] [28] Emergy theory has been criticized for allegedly offering an energy theory of value to replace other theories of value.[ citation needed] The stated goal of emergy evaluations is to provide an "ecocentric" valuation of systems, processes. Thus it does not purport to replace economic values but to provide additional information, from a different point of view.[ citation needed]
The idea that a calorie of sunlight is not equivalent to a calorie of fossil fuel or electricity strikes many as absurd, based on the 1st Law definition of energy units as measures of heat (i.e. Joule's mechanical equivalent of heat). [29] Others have rejected the concept as impractical since from their perspective it is impossible to objectively quantify the amount of sunlight that is required to produce a quantity of oil. In combining systems of humanity and nature and evaluating environmental input to economies, mainstream economists criticize the emergy methodology for disregarding market values.[ citation needed]
{{
cite web}}
: CS1 maint: archived copy as title (
link).