Energy conversion

An important principle of thermodynamics states that no conversion is perfect – there will always be some loss to another form of energy.

The amount of energy available to the animal’s body is much less than the total amount of energy from the sun to make the glucose in the plant. A ball does not bounce back to the same height as before, because some of the energy is ‘lost’ to heat and sound. The total amount of energy is always the same, but not all of the energy can be converted to a single form. The percentage that can be converted is known as efficiency.

η = output/input

This loss of ‘usable’ energy is called degradation of energy.

Efficiency is a technical term for the conversion of one energy from one form to another. Energy is never lost (Second Law of Thermodynamics), so if a fuel is burnt entirely, 100% has been converted to other energy forms. However, much of the new energy may be ‘unusable’, such as sound. If heat is the objective of the conversion, as it is in a boiler and turbine power generator, the efficiency would be a measure of how much electrical power results from the burning of an amount of fuel.

Burning oil in a power plant has the purpose of heating water, which is converted to steam, which drives a turbine, which generates electricity. Burning petrol in a car considers efficiency of conversion to be the kinetic energy of motion, and heat is ‘unusable’, so is lost to the purpose of the conversion.

Efficiencies of energy systems

Energy sourceEnergy typeCurrent rangeTheoretical max.
Fuel cellchemical70-80%85%
Gas turbinechemical30-40%40%
Combined cycle*chemical and thermal40-60%60%
World TotalAll types39% gross33% net


* Two stage production: gas turbine then steam turbine