Originally termed embodied energy, the term eMergy can be thought of as Energy Memory, or “the available energy of one kind previously used up directly and indirectly to make a product or service. The energy required for the transformations is no longer in the product or service, but energy carries the memory of the
availability that was used up. Emergy is a new kind of state variable. The units of energy are defined with the prefix em- (e.g., emcalories, emjoules, emBtus)” (Odum, 2007, p. 69).
Flows of energy carry emergy. “Whereas power is useful energy flow per time, empower is the measure of emergy flow over time….The energy that was required [to make the product] is no longer a part of the product, but emergy is a property that represents its history and implies its importance. It remains with the product and the product’s products until the available energy is gone. Emergy disappears when the available energy is used up (degraded) (Odum, 2007, p. 69).
Using emergy, energies of differing qualities such as 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 commonly reported as solar emjoules (abbreviated seJ). The goal of the method is to capture the contributions of nature in economic evaluations.
Energy quality varies, and not all forms of energy have the same ability to do work, depending on the system and the intensity of energy required. Net energy contribution of any process the economy including energy production can be calculated using an Emergy Yield Ratio (EYR) (Odum, 1976, Brown & Ulgiati, 1997). Earlier formulations of net energy such as (Embodied or Net) Energy Analysis or Energy Return on Investment (EROI):
. . . does not include quality correction and other inputs such as labor and environmental contributions. The EYR as its name implies, is the ratio of the yield from a process (in emergy) to the costs (in emergy). . . The EYR is the ratio of the yield (Y) to the costs (F) of getting it. The costs include energy, materials, and human service purchased from the economy, all expressed in emergy. . . Net contributions of energy sources to growth and development is always greatest in early stages of development of systems, and declines as energetic costs of processes and organization (overhead) increase with increasing quantities of structure (Brown, Cohen, & Sweeney, 2009, p. 3426).
The most important difference that results when the two accounting procedures are used is that on pathways from lower order components the embodied energy is about 1.8-times as much as the EMERGY, but on the highest order pathways, the EMERGY is 1.1-2.0-times as great as the embodied energy. The significance is that the differences between lower and higher order pathways are amplified between the two accounting systems. Embodied energy accounting gives more weight (more relative importance) to lower order pathways over higher order ones, while EMERGY accounting gives more relative importance to higher order pathways (Brown & Herendeen, 1996, p. 229 ).
Probably the most significant difference is related to the ‘form’ of EMERGY and embodied energy. EMERGY is defined as the energy of one type (usually solar energy) that is required to produce something. Energies of different types (i.e., solar, tidal, chemical potential energy in rain, fuels, or electricity) are expressed in the equivalent solar energy required to make them. Embodied energy analysis, as practiced, uses strictly the heat energy of fuels and does not include environmental energies. The embodied energy in goods and services, for instance, does not include the environmental support that is derived from solar, geophysical and tidal energies that drive all economies (Brown & Herendeen, 1996, p. 233).
Thus net energy fails to accumulate all of the contributions to complex alternative technologies. It is the idea of hierarchy that is so important. If we do not see the quiet contributions of Nature’s power at the lower ends of the hierarchy, then we devalue the degree of complexity of information and technology at the higher end, thinking that the complexity is a function of man’s technological prowess rather than incredible development of power over time using the powerful multiplier of fossil fuels. Brown and Herendeen clarify:
Fig. 10 [above] illustrates the fundamental reason that recognition of the differences in form energy is necessary. From a thermodynamic perspective the system is correct, having all energy accounted for because the inflows equal the outflows. However, when evaluated in heat energy (as the diagram is) energy flows to the right are so small as to be insignificant when compared with the flows farther to the left. Yet it is apparent from a systems perspective that the processes and flows to the right cannot exist without the inputs, nor since there are feedbacks, can the processes to the left exist without those to the right. In other words since the system is interconnected all components and flows are necessary, yet when evaluated in their heat value, many flows (especially those at the top of energy hierarchies) seem insignificant and of little importance. EMERGY evaluations make the assumption that they are essential for the entire system, their value is the total EMERGY that contributes to them (1996, p. 228).
Net yields from non-renewable energy sources are declining over time, as shown below. It is essential to calculate all costs of energy sources in order to avoid undue optimism regarding potential renewable alternatives. Using EYR to calculate potential alternatives (also shown below) “provides a relative relationship for evaluation of green-house gas emissions. For instance if we assume that ethanol with an EYR of 2 to 1 is used to replace fossil fuels having yields of 8 to 1, the ethanol is actually using energy at four times the rate, and increasing greenhouse gas emissions over the burning of the fossil fuel” (Brown, Cohen, & Sweeney, 2009, p. 3429). Many so-called renewables actually guzzle fossil fuels. The aggregate use of emergy and weighted aggregate EYRs can be calculated. For example, below is the aggregate EYR for the US economy showing a decline of 38% since 1950, from 11 to 1 down to 6.8 to 1.
A bibliography of Emergy Research is available here, with many live PDF links. If you want to know more on the topic of Emergy, the Emergy bible is Environmental Accounting, but it is pricey at $130 US. And these PPTs of Mark’s are really helpful–look at them in order. Once you do that, try tackling Dan Campbell’s short course. Please do not use Wiki as an information source–it appears to have been hijacked.
Denis White at Gresham College on Emergy, November 2012 (28 minute lecture)