Waste heat

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

Low temperature heat contains very little capacity to do work (Exergy), 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 can be equipped with a cooling tower or air cooler to reject the waste heat into the atmosphere. In some cases it is possible to use waste heat, for instance in district heating systems. ==Uses==

Conversion to electricity There are many different approaches to transfer thermal energy to electricity, and the technologies to do so have existed for several decades. 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. Cogeneration and trigeneration 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 exploiting small temperature differences to generate other forms of energy. Applications utilizing waste heat include swimming pool heating and paper mills. In some cases, cooling can also be produced by the use of absorption refrigerators for example, in this case it is called trigeneration or CCHP (combined cooling, heat and power). District heating Waste heat can be used in district heating. Depending on the temperature of the waste heat and the district heating system, a heat pump must be used to reach sufficient temperatures. These are an easy and cheap way to use waste heat in cold district heating systems, as these are operated at ambient temperatures and therefore even low-grade waste heat can be used without needing a heat pump at the producer side.{{citation|author=Simone Buffa |display-authors=etal |periodical=Renewable and Sustainable Energy Reviews|title=5th generation district heating and cooling systems: A review of existing cases in Europe|volume=104|pages=504–522|date=2019|doi=10.1016/j.rser.2018.12.059 Pre-heating Waste heat can be forced to heat incoming fluids and objects before being highly heated. For instance, outgoing water can give its waste heat to incoming water in a heat exchanger before heating in homes or power plants. ==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/m2 to the local heat balance, and several hundred W/m2 in the center of large cities in cold climates and industrial areas." In 2020, the overall anthropogenic annual energy release was 168,000 terawatt-hours; given the 5.1×10 m surface area of Earth, this amounts to a global average anthropogenic heat release rate of 0.04 W/m. Environmental impact Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in dense urban areas. 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 greenhouse gases 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. that have been actualized recently 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. ==See also==