Interactions of several energy flows

Miscellaneous symbol for subsystem

Money transaction

Production subsystem

Money transaction

Figure 1 Energy system symbols.

Three other energy symbols are used: interactions indicating interaction of two or more different types of energy flows; the miscellaneous symbol (a rectangle) that represents complex systems, money transaction indicating flows of money to pay for energy, materials, and services. Money flows in opposite direction to energy, materials, and services. Using these symbols, it is possible to model natural and human ecosystems and understand their characteristics. An energy value can be assigned to each pathway.

c HI


Figure 2 Hierarchical webs and energy flows.

position in the chain represents different capacities for control and quality. For example, directly and indirectly, it takes about 1000J of sunlight to make a joule of spatially dispersed organic matter, about 40 000 to make a joule of coal, and usually even more to make a joule of electrical energy. A joule of sunlight, a joule of coal, and a joule of human work have different qualities and different energy convergences to make them. According to this concept, Odum sustained that a more meaningful way to express quality is not to consider the energy content of a system but the energy embodied (emergy) in it, that is, how much energy was used to make or sustain the system starting from the lowest level of the web. For this reason, emergy is sometimes referred to as 'energy memory'.


From its definition emergy is an extensive quantity; its unit is the solar emjoule [(sej)] and its physical dimensions are those of energy ([ML2T_2] of solar type, where M = mass, L = length, T = time). It is not a state function, since it depends on the pathway that the process follows. In fact, the emergy of a product is related to the way it is produced. A joule of electricity generated from oil has a different emergy to one generated by wind power.

Emergy is a donor-referenced concept rather than a receiver-referenced one. It measures the source energies converging at system boundaries into processes or products. The total energy flowing through a system per unit time is its empower, with units sej/[time] and physical dimensions [ML2T_3] of solar type.

The basis of emergy evaluation is the conversion of all process inputs, including energy of different types and energy inherent in materials and services, into emergy by means of a conversion factor called transformity.


Transformity is defined as the energy of one type directly and indirectly required to generate 1J of another. Solar transformity is currently used in emergy evaluation. Unlike emergy, transformity is an intensive quantity, representing the inverse of classical energy efficiency and is measured in sej J_1. It is dimensionless and system specific. To evaluate increased organization of concentrated matter, a mass-specific emergy (sej g_1) is sometimes used instead of transformity. The emergy of a certain type of material is obtained by multiplying its mass with the emergy-to-mass ratio. As with energy-based transformities, matter evaluations are also system and process specific.

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