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Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility (or in thermodynamics lexicon a lower exergy or higher entropy) than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation.
Instead of being "wasted" by release into the ambient environment, sometimes waste heat (or cold) can be used by another process (such as using hot engine coolant to heat a vehicle), or a portion of heat that would otherwise be wasted can be reused in the same process if make-up heat is added to the system (as with heat recovery ventilation in a building).
Thermal energy storage, which includes technologies both for short- and long-term retention of heat or cold, can create or improve the utility of waste heat (or cold). One example is waste heat from air conditioning machinery stored in a buffer tank to aid in night time heating. Another is seasonal thermal energy storage (STES) at a foundry in Sweden. The heat is stored in the bedrock surrounding a cluster of heat exchanger equipped boreholes, and is used for space heating in an adjacent factory as needed, even months later.Andersson, O.; Hägg, M. (2008), "Deliverable 10 - Sweden - Preliminary design of a seasonal heat storage for IGEIA – Integration of geothermal energy into industrial applications , pp. 38–56 and 72–76, retrieved 21 April 2013 An example of using STES to use natural waste heat is the Drake Landing Solar Community in Alberta, Canada, which, by using a cluster of boreholes in bedrock for interseasonal heat storage, obtains 97 percent of its year-round heat from solar thermal collectors on the garage roofs.Wong, Bill (June 28, 2011), "Drake Landing Solar Community" , IDEA/CDEA District Energy/CHP 2011 Conference, Toronto, pp. 1–30, retrieved 21 April 2013Wong B., Thornton J. (2013). Integrating Solar & Heat Pumps. Renewable Heat Workshop. Another STES application is storing winter cold underground, for summer air conditioning.Paksoy, H.; Stiles, L. (2009), "Aquifer Thermal Energy Cold Storage System at Richard Stockton College" , Effstock 2009 (11th International) - Thermal Energy Storage for Efficiency and Sustainability, Stockholm.
On a biological scale, all organisms reject waste heat as part of their Metabolism, and will die if the ambient temperature is too high to allow this.
Anthropogenic waste heat can 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) and heat loss through building envelopes. The burning of transport fuels is a major contribution to waste heat.
For example, data centers use electronic components that consume electricity for computing, storage and networking. The French CNRS explains a data center is like a resistor and most of the energy it consumes is transformed into heat and requires cooling systems.
An established approach is by using a thermoelectric device, where a change in temperature across a semiconductor material creates a voltage through a phenomenon known as the Seebeck effect.
A related approach is the use of thermogalvanic cells, where a temperature difference gives rise to an electric current in an electrochemical cell.
The organic Rankine cycle, offered by companies such as Ormat, is a very known approach, whereby an organic substance is used as working fluid instead of water. The benefit is that this process can reject heat at lower temperatures for the production of electricity than the regular water steam cycle. An example of use of the steam Rankine cycle is the Cyclone Waste Heat Engine.
Anthropogenic heat is a much smaller contributor to global warming than are. In 2005, anthropogenic waste heat flux globally accounted for only 1% of the energy flux created by anthropogenic greenhouse gases. The heat flux is not evenly distributed, with some regions higher than others, and significantly higher in certain urban areas. For example, global forcing from waste heat in 2005 was 0.028 W/m2, but was +0.39 and +0.68 W/m2 for the continental United States and western Europe, respectively.
Although waste heat has been shown to have influence on regional climates, climate forcing from waste heat is not normally calculated in state-of-the-art global climate simulations. Equilibrium climate experiments show statistically significant continental-scale surface warming (0.4–0.9 °C) produced by one 2100 AHF scenario, but not by current or 2040 estimates. Simple global-scale estimates with different growth rates of anthropogenic heatR. Döpel, "Über die geophysikalische Schranke der industriellen Energieerzeugung." Wissenschaftl. Zeitschrift der Technischen Hochschule Ilmenau, , Bd. 19 (1973, H.2), 37-52. ( online). that have been actualized recentlyH. Arnold, "Robert Döpel and his Model of Global Warming. An Early Warning – and its Update." (2013) online. 1st ed.: "Robert Döpel und sein Modell der globalen Erwärmung. Eine frühe Warnung - und die Aktualisierung." Universitätsverlag Ilmenau 2009, . show noticeable contributions to global warming, in the following centuries. For example, a 2% p.a. growth rate of waste heat resulted in a 3 degree increase as a lower limit for the year 2300. Meanwhile, this has been confirmed by more refined model calculations.
A 2008 scientific paper showed that if anthropogenic heat emissions continue to rise at the current rate, they will become a source of warming as strong as GHG emissions in the 21st century.
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