Energy conversion efficiency

The input, as well as the useful output may be chemical, electric power, mechanical work, light (radiation), or heat.

Even though the definition includes the notion of usefulness, efficiency is considered a technical or physical term.

It is a ratio of useful heating or cooling provided relative to the work (energy) required.

[4] When talking about the efficiency of heat engines and power stations the convention should be stated, i.e., HHV (a.k.a.

For example, an ideal fuel cell operating at a temperature of 25 °C having gaseous hydrogen and gaseous oxygen as inputs and liquid water as the output could produce a theoretical maximum amount of electrical energy of 237.129 kJ (0.06587 kWh) per gram mol (18.0154 gram) of water produced and would require 48.701 kJ (0.01353 kWh) per gram mol of water produced of heat energy to be removed from the cell to maintain that temperature.

[6] An ideal electrolysis unit operating at a temperature of 25 °C having liquid water as the input and gaseous hydrogen and gaseous oxygen as products would require a theoretical minimum input of electrical energy of 237.129 kJ (0.06587 kWh) per gram mol (18.0154 gram) of water consumed and would require 48.701 kJ (0.01353 kWh) per gram mol of water consumed of heat energy to be added to the unit to maintain that temperature.

[6] It would operate at a cell voltage of 1.24 V. For a water electrolysis unit operating at a constant temperature of 25 °C without the input of any additional heat energy, electrical energy would have to be supplied at a rate equivalent of the enthalpy (heat) of reaction or 285.830 kJ (0.07940 kWh) per gram mol of water consumed.

In Europe the usable energy content of a fuel is typically calculated using the lower heating value (LHV) of that fuel, the definition of which assumes that the water vapor produced during fuel combustion (oxidation) remains gaseous, and is not condensed to liquid water so the latent heat of vaporization of that water is not usable.

Instead of using watts, the power of a light source to produce wavelengths proportional to human perception is measured in lumens.

The effectiveness of a light source to convert electrical energy into wavelengths of visible light, in proportion to the sensitivity of the human eye, is referred to as luminous efficacy, which is measured in units of lumens per watt (lm/w) of electrical input-energy.

[7][8] Calculations for luminous efficiency become more complex for lamps that produce white light or a mixture of spectral lines.

Because the flashtube emits large amounts of infrared and ultraviolet radiation, only a portion of the output energy is used by the eye.

Krypton flashtubes are often chosen for pumping Nd:YAG lasers, even though their wall-plug efficiency is typically only ~ 40%.

Luminaire efficiency refers to the total lumen-output from the fixture per the lamp output.

Similarly, fluorescent lamps also convert the electricity using a ballast (electronic efficiency).

In very similar fashion, lasers also experience many stages of conversion between the wall plug and the output aperture.

Useful output energy is always lower than input energy.
Efficiency of power plants, world total, 2008
The absolute irradiance of four different gases when used in a flashtube. Xenon is by far the most efficient of the gases, although krypton is more effective at a specific wavelength of light.
The sensitivity of the human eye to various wavelengths. Assuming each wavelength equals 1 watt of radiant energy , only the center wavelength is perceived as 683 candelas (1 watt of luminous energy), equaling 683 lumens. The vertical colored-lines represent the 589 (yellow) sodium line, and popular 532 nm (green), 671 nm (red), 473 nm (blue), and 405 nm (violet) laser pointers.
A Sankey diagram showing the multiple stages of energy loss between the wall plug and the light output of a fluorescent lamp. The greatest losses occur due to the Stokes shift.