'Energy' is the ability to do work. Energy is a property of all things which can be turned into heat, and is measured in heat units (British thermal units (BTUs), calories, or joules).

'Emergy' is the availability of energy (exergy) of one kind that is used up in transformations directly and indirectly to make a product or service. The unit of emergy is the 'emjoule' (see below), a unit referring to the available energy of one kind consumed in transformations. For example, sunlight, fuel, electricity, and human service can be put on a common basis by expressing them all in the emjoules of solar energy that is required to produce each. In this case, the value is a unit of 'solar emergy' expressed in 'solar emjoules' (abbreviated sej). Although other units have been used, such as coal emjoules or electrical emjoules, in most cases all emergy data are given in solar emjoules.

'Emjoule' is the unit of measure of emergy; the term is short for emergy joule. An emjoule is an expression of the units of energy previously used to generate a product; for instance, the solar emergy of wood is expressed as joules of solar energy that were required to produce the wood. Solar emjoules is abbreviated as 'sej'.

'Empower' is a flow of emergy (ie emergy per time). Emergy flows are usually expressed in units of solar empower (solar emjoules per time).

'Unit emergy values' (UEVs) are based on the emergy required to produce something. UEVs are calculated by dividing the sum of all emergy required by the units of product output. There are two types of UEVs appropriate for this chapter as follows:

• 'Transformity' is defined as the emergy per unit of available energy (exergy). For example, if 4000 sej is required to generate 1J of wood, then the solar trans-formity of that wood is 4000 solar emjoules per joule (abbreviated sejJ-1). Solar energy is the largest but

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