Exergy efficiency is also called second-law efficiency because it computes the efficiency of a process taking the second law of thermodynamics into account. The second law of thermodynamics is an expression of the universal law of increasing entropy. ...

The energy E and exergy B balances of a process are: Exergy is defined differently in different fields of study. ...

and

Efficiency η of the process may be described using the thermodynamic potentials E or B. Efficiency is the fraction of the potential that makes its way into the product. In thermodynamics, four quantities, measured in units of energy, are called thermodynamic potentials: where T = temperature, S = entropy, p = pressure, V = volume Differential definitions The following differential relations hold for the four potentials: If we write the above four equations generally as Then it is seen that yielding expressions for...

and

It should be noted that the energy efficiency quoted by appliance makers is the "first-law" thermodynamic efficiency; it only tells how well the particular appliance converts one form of energy, usually electricity, into another, usually heat. It does not tell how the appliance compares to, say, a Carnot heat engine or any other cyclical process. Thermodynamic efficiency (e) is defined as: where W is the absolute value of the work done in one thermodynamic cycle. ... A Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. ...

Second-law efficiency measures relate existing technological systems to ideal ones by an evaluation of available work rather than energy content (Gilliland 1978, p. 100). It tells us about the quality of energy; it compares the efficiency of a process as compared to a perfect Carnot heat engine operating at the same conditions. For example, instead of using electricity to heat a coil of wire to create heat, one could use the electricity to drive a motor that in turn drives a pump. The pump would move a working fluid in a continuous cycle of compression, heat rejection, expansion, and finally heat absorption. This cycle takes advantage of the very high quality energy more efficiently than a resistive heater. Energy quality the contrast between different forms of energy, the different trophic levels in ecological systems and the propensity of energy to convert from one form to another. ... A Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. ...

Also see exergy. An example of a poor use of the energy availability of a chemical fuel, such as methane gas, is to combust it solely for the purpose of heating water. While this has a high "first-law efficiency", it has a low "second-law efficiency", thus one wastes a tremendous amount of high-temperature, high-quality energy to heat the relatively cold water. A combined heat and power system with inherently higher exergy efficiency, would use the fuel to run a heat engine and use low-temperature waste heat for the water heating requirement. The quantity of energy is the same, but it is of higher quality and higher economic value due to the mechanical output of the engine. Exergy is defined differently in different fields of study. ... Methane is a chemical compound with the molecular formula CH4. ... Cogeneration (also combined heat and power or CHP) is the use of a power station to simultaneously generate both heat and electricity. ... A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. ...

Second law efficiency under maximum power

Neither first or second laws of thermodynamics include a measure of the rate of energy transformation. When a measure of the maximal rate of energy transformation is included in the measure of second law efficiency it is known as second law efficiency under maximum power, and directly related to the maximum power principle (Gilliland 1978, p. 101). This article or section may be confusing for some readers, and should be edited to be clearer. ...

See also

• Energy efficiency
• Maximum power principle

In physics and engineering, including mechanical and electrical engineering, energy efficiency is a dimensionless number, with a value between 0 and 1 or with times 100 given in percent. ... This article or section may be confusing for some readers, and should be edited to be clearer. ...

References

• M.W. Gilliland (1978) Energy Analysis: A New Public Policy Tool, Westview Press.