Air conditioning units use electricity which ends up as heat

Waste heat refers to heat produced by machines, electrical equipment and industrial processes for which no useful application is found, and is regarded as a waste by-product. When produced by humans, or by human activities, it is a component of anthropogenic heat, which additionally includes unintentional heat leakage, such as from space heating. Waste heat is thought by some to contribute to the urban heat island effect. The biggest point sources of waste heat originate from machines such as electrical generators or industrial processes, such as steel or glass production. The burning of transport fuels is a major contribution to waste heat.

[edit] Conversion of energy

Machines converting energy contained in fuels to mechanical work or electric energy produce heat as a by-product

[edit] Sources

The largest proportions of total waste heat are from power stations and vehicle engines.^{[citation needed]} The largest single sources are power stations and industrial plants such as oil refineries and steelmaking plants.^{[citation needed]}

[edit] Power generation

The electrical efficiency of thermal power plants is defined as the ratio between the input and output energy. It is typically only 30%.^{[citation needed]} The images show cooling towers which allow power stations to maintain the low side of the temperature difference essential for conversion of heat differences to other forms of energy. Discarded or "Waste" heat that is lost to the environment may instead be used to advantage.

Coal-fired power station that transform chemical energy into 36%-48% electricity and remaining 52%-64% to waste heat

[edit] Industrial processes

Industrial processes, such as oil refining steelmaking or glassmaking are major sources of waste heat.

[edit] Electronics

Although small in terms of power, the disposal of waste heat from microchips and other electronic components, represents a significant engineering challenge. This necessitates the use of fans, heatsinks, etc. to dispose of the heat.

[edit] Biological

Animals, including humans, create heat as a result of metabolism. In warm conditions, this heat exceeds a level required for homeostasis in warm-blooded animals, and is disposed of by various thermoregulation methods such as sweating and panting. Fiala et al. modelled human thermoregulation.^[1]

Cooling towers evaporating water at Ratcliffe-on-Soar Power Station, United Kingdom.

[edit] Disposal

It is often difficult to find useful applications for large quantities of low temperature heat energy, so the heat is qualified as waste heat and rejected to the environment. Economically most convenient is the rejection of such heat to water from a sea, lake or river. If sufficient cooling water is not available, the plant has to be equipped with a cooling tower to reject the waste heat into the atmosphere.

[edit] Uses

[edit] Cogeneration

Waste of the by-product heat is reduced if a cogeneration system is used, also known as a Combined Heat and Power (CHP) system. Limitations to the use of by-product heat arise primarily from the engineering cost/efficiency challenges in effectively utilizing small temperature differences to generate other forms of energy. Applications utilizing waste heat include swimming pool heating, paper mills and cold chain logistics (by the use of Absorption refrigerators).

[edit] Electrification of waste heat

There are many different approaches to transfer thermal energy to electricity, these approches are mostly still in development. The organic Rankine cycle is a very known approach, it is an electricity generation process where an organic substance is used as working medium instead of water. The benefit is that this process can utilise lower temperatures for the production of electricity than the regular water steam cycle. By help of ORC-modules it is possible to turn this previously wasted energy economically into electricity.^[2]. Another approach is by using thermogenerator, the delta change in temperature causes a small electric current to flow between two plates by phenomenon such as the Seebeck effect. Another way for the electrification of heat is the Thermoacoustic hot air engine. ^[3]

[edit] Anthropogenic heat

Anthropogenic heat is heat generated by humans and human activity. The American Meteorological Society defines it as "Heat released to the atmosphere as a result of human activities, often involving combustion of fuels. Sources include industrial plants, space heating and cooling, human metabolism, and vehicle exhausts. In cities this source typically contributes 15–50 W m⁻² to the local heat balance, and several hundred W m⁻² in the center of large cities in cold climates and industrial areas."^[4]

Estimates of anthropogenic heat generation can be made by totaling all the energy used for heating and cooling, running appliances, transportation, and industrial processes, plus that directly emitted by human metabolism.

[edit] Environmental impact

Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in dense urban areas.^[5] It is one contributor to urban heat islands. Other human-caused effects (such as changes to albedo, or loss of evaporative cooling) that might contribute to urban heat islands are not considered to be anthropogenic heat by this definition.

Anthropogenic heat is a much smaller contributor to global warming than are greenhouse gases. In 2005, although anthropogenic waste heat flux was significantly high in certain urban areas (and can be high regionally. For example, waste heat flux was +0.39 and +0.68 W/m² for the continental United States and western Europe, respectively) globally it accounted for only 1% of the energy flux created by anthropogenic greenhouse gases. Global forcing from waste heat was 0.028 W/m² in 2005. This statistic is predicted to rise as urban areas become more widespread.^[6]

Although waste heat has been shown to have influence on regional climates,^[7] climate forcing from waste heat is not normally calculated in state-of-the-art global climate simulations.^[6]

[edit] See also

• Waste heat recovery unit
• Heat recovery steam generator
• District heating
• Relative cost of electricity generated by different sources
• Pinch analysis

[edit] References