EME 807
Technologies for Sustainability Systems

3.2 Examples of environmental metrics

Environmental metrics are designed to assess the environmental impact of technology or activity. The impact is primarily related to using natural resources (renewable or non-renewable) and generating waste. The ultimate sustainability goal is to minimize the environmental impact via using less non-renewable resources and generating less waste and pollution. Since the complete elimination of these factors is hardly possible, it is also important to evaluate the rate at which environment can absorb the impacts and become remediated.

Embodied energy / Emergy / Transformity

The concepts of embodied energy, emergy, and transformity are introduced as universal measures in the environmental accounting theory. The basic approach in that theory is to use energy units for assessing inputs and outputs of various natural and industrial systems. All types of energy and real wealth products are related to the primordial source - solar energy - through transformity. It is stated that going through multiple transformations via both technological and natural converters, available energy acquires new quality, while the load on the environment due to those transformations increases.

For example, on a hot day, we use an air conditioner. The energy to power the air conditioner comes from the electricity grid. Prior to that, the electric energy is converted by an electromechanical generator at a power plant from the kinetic or thermal energy of steam, which is in turn produced by combustion of fossil fuels. Energy stored in the fossil fuels (which were originally biomass) was the solar energy transformed by plants to organic matter via photosynthesis. So, solar energy is the original component of the final energy used by the air conditioner (Figure 3.1).

Image is adequately described in the text above
Figure 3.1. Energy transformations resulting in a new quality of usable energy.
Credit: Mark Fedkin

Transformations of the primary types of energy through an array of energy conversion technologies described in the example above demonstrate the idea of transformity applied to a particular type of usable energy, such as electricity. In other words, transformity indicates how many transformations are necessary to obtain a particular sort of energy in a usable form and also show how “costly” those transformations are for the environment. Quantification of the energy inputs and outputs in terms of primary solar energy may be a tricky task; however, a number of studies provided such data and enabled energy flow analysis for environmental systems.

Check Your Understanding

Probing Question: Which of the following types of resources, in your opinion, has the lowest transformity based on the concept outlined above:

(A) Hydrogen
(B) Natural gas
(C) Wastewater
(D) Gasoline
(E) Ammonia fertilizer

Click for answer.

ANSWER: (B) From this list, natural gas requires fewer transformations than others to become a usable resource, especially when directly piped from a formation. Hydrogen is produced from natural gas, and gasoline is produced from crude oil, so they should have a higher transformity. Fertilizer and wastewater are products of longer chains of industrial processes and therefore should have the highest transformity.

Emergy is defined as the available solar energy used up directly or indirectly to make a service, product, fuel, or another form of usable resource. This term essentially means the solar energy equivalent. Transformity, in this case, is the equivalence factor:

Emergy [seJ] = Energy stored or available [J] x Transformity [seJ/J]

Emergy is usually measured in solar energy joules [seJ], and transformity is therefore expressed as a ratio of solar energy Joules to regular Joules.


Here we use tree logs as an example for expressing transformity. The key energy transformation involved in the production of tree logs is photosynthesis, the natural processes that convert CO2 gas and solar radiation into biomass.

This calculation is done for 1 Ga of forest. Here, the Solar emergy flow essentially indicates how much solar energy is supplied by the sun onto that 1 Ga area. This would depend on solar irradiance, or insolation, which in turn would depend on the geographic location of the forest, local weather profile, and other factors. The Energy flow, in this case, is the energy content of the wood produced by the 1 Ga forest per year. The result can be read as: 3846 joules of solar energy is used per each joule of energy stored in the logs. We understand that in order to be accurate, transformity has to be evaluated taking into account the larger surroundings of the system and specific conditions. We also see from this example that only a part of the available solar energy is captured and converted to the usable stored energy (logs), while the rest of it is dissipated or redirected in this system.

Several quantitative environmental metrics have been defined based on the emergy theory (see system diagram in Figure 3.2).

Emergy-based indices described in text below
Figure 3.2. Emergy-based indices, accounting for local renewable emergy inputs (R), local nonrenewable inputs (N), and purchased inputs from outside the system (F)
Credit: Brown, M.T. and Ulgiati, S., Ecological Engineering 9 (1997) 51–69

Figure 3.2 is a system diagram showing the energy flows and transformations within a generic locality (surrounded by the system boundary). The Economic Use box can be seen as a "transformer" of the available energy and resources into some Yield (Y), i.e., some product needed to sustain this system. The inputs to the system are classified as renewable resources, non-renewable resources, local resources, and non-local (purchased) resources. It is presumed that system sustainability is favored by using renewable energy resources and local energy resources. The sum of both of these categories is denoted by R on this diagram. On the contrary, non-renewable (N) and non-local (purchased, F) resources work against sustainable development. These presumptions set the background for devising certain sustainability metrics in this study.

One of such metrics, which characterizes the environmental impact of a transformational process, is Environmental Loading Ratio (ELR):

ELR = (F + N) / R

From this relationship, we can see that the more non-renewable and outside resources are involved in the process, the higher the ERL index. An increase in renewable energy use translates into a lower ELR value. As you can guess, lower ELR is benefitial for the environment.

Another useful index introduced here is Energy Yield Ratio (EYR):

EYR = Y / F

This metric characterizes system's capability to exploit local resources (renewable or not). The more the system depends on imported resources or services (increasing F), the lower EYR, and the higher system vulnerability.

Finally, the Sustainability Index (SI) combines both ELR and EYR as follows:


Obviously, for higher sustainability “score”, we are interested in having the highest EYR versus the lowest ELR. Within this approach, SI can be used as an aggregate measure to characterize sustainability function of a given process, technology, or economy.

Please see further explanation of this method and example calculations of metrics in the reading material referenced below.

Reading Assignment:

Journal article: Brown, M.T., and Ulgiati, S., Ecological Engineering 9 (1997) 51-69.

This paper explains the calculation of environmental and sustainability indices based on the available energy flows. It illustrates the process of devising sustainability metrics and applying them to a number of technologies and products. Please study this article. In this lesson activity, you will be asked to perform a simple calculation of the environmental metrics based on the approach described herein.

The article is available as PDF file in the Lesson 3 Module on Canvas or can be accessed through the databases of the PSU Library system.

Supplemental Reading:

Book: Odum, H.T., Environmental Accounting, John Wiley & Sons 1996. pp.1-15.

This book introduces the emergy theory and method for evaluation of environmental and economic use. Chapters 1 and 2, especially, would help you understand the basics of this approach. This is an optional reading, and the book is not provided in the electronic format. However, if you are interested in this topic and would like to use this approach in your own assessments, it is a proper resource to explore.

Note that the above-described approach to assessing a system sustainability is just a single illustration of how sustainability metrics can be devised. The parameters chosen by the authors were specific to their objectives. Calculations they provide answer some of the questions, but may not answer other questions that a stakeholder may have. In that respect, setting the objectives for your assessment and stating clear definitions and assumptions is a very important step in any assessment study in order to make the results meaningful.

Kaya Equation

Kaya Equation (introduced by the economist Yochi Kaya) is another example of environmental metric, which helps to recognize different contributions to the total CO2 emissions of a country. It incorporates population, level of economic activity, level of energy consumption, and carbon intensity:

Where P = population, GDP = gross domestic product, (GDP/P) = GDP per capita, (E/GDP) = energy intensity per unit of GDP, and (CO2/E) = carbon emissions per unit energy consumed. GDP is the market value of all officially recognized final goods and services produced within a country in a given period of time. GDP per capita is often considered an indicator of a country's standard of living.

The last two terms on the right side of the equation are metrics that need to be lowered in order to benefit the environment.

The term (CO2/E) is technology dependent. The more fossil fuel burning is involved in the production of consumable energy (energy conversion), the higher the “carbon cost” of that energy. This carbon cost is indicated by this factor. Cleaner technologies are characterized by lower (CO2/E), or even zero in an ideal case. From the systems perspective, though, there may be no zero emission technologies, since manufacturing, maintenance, and support system operation of such energy conversion systems may still require a certain amount of energy from fossil fuels.

For example, a “green bus” uses a hydrogen fuel cell stack as an engine and emits only water from H2 + 0.5O2(air) = H2O reaction. However, manufacturing of such a bus requires equipment operated from the grid, which distributes electricity from a fossil fuel power plant. Furthermore, maintenance of this bus over its lifetime may require other non-renewable resources. Therefore, its carbon “footprint” may be quite low, but still non-zero (unless we decide on different system boundaries).

The Kaya model allows estimating how changing technological solutions for energy conversion can help the economy in terms of emission reduction. Determination of the CO2/E factor provides a quantitative scale for measuring environmental impact in terms of “carbon cost”. The CO2/E metric is common in many assessment studies discussing alternative energy sources. We need to keep in mind reported values usually reflect the lifecycle, "from cradle to grave" emissions, i.e., those related to technology manufacturing, operation, and decommissioning altogether (not only operational emissions).

Reading Assignment:

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Take a look at this example of a National Renewable Energy Laboratory (NREL) study that had a goal to compare the lifecycle greenhouse gas emissions of various energy technologies. The study took into account the total estimated emissions from more than 2100 LCA publications and related those to the total amount of energy generated by those systems during their lifetime operation. The results are summarized on the bar graph (p.2 of the fact sheet). Interpreting the graph, answer the following questions for yourself:

  • What units are used to express the CO2/E metric?
  • Energy from which technology (out of those studied) has the highest "carbon cost"? Which one has the lowest "carbon cost"?
  • Which stage of the technology lifecycle does result in the most CO2 emissions: in case of renewable energy systems and in case of fossil fuel energy systems?

Ecological footprint

Various Internet sites use combinations of environmental metrics to calculate the so-called ecological footprint. This is an illustration how environmental metrics can be used to compare human lifestyles, which essentially comes down to the comparison of technologies people use. These calculators are far from being specific and use generalized information on environmental impacts. Here are a couple of calculators you can check just for fun: