Waste heat isheat that is produced by amachine, or other process that usesenergy, as a byproduct of doingwork. All such processes give off some waste heat as a fundamental result of thelaws of thermodynamics. Waste heat has lower utility (or in thermodynamics lexicon a lowerexergy or higherentropy) 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, aninternal 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 withheat 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 isseasonal 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.[1] An example of using STES to use natural waste heat is theDrake Landing Solar Community inAlberta, Canada, which, by using a cluster of boreholes in bedrock for interseasonal heat storage, obtains 97 percent of its year-round heat fromsolar thermal collectors on the garage roofs.[2][3] Another STES application is storing winter cold underground, for summer air conditioning.[4]
On a biological scale, all organisms reject waste heat as part of theirmetabolic processes, and will die if the ambient temperature is too high to allow this.
Anthropogenic waste heat can contribute to theurban heat island effect.[5] 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 also a significant contribution to waste heat.
Machinesconverting energy contained infuels tomechanical work orelectric energy produce heat as a by-product.
In the majority of applications, energy is required in multiple forms. These energy forms typically include some combination ofheating, ventilation, and air conditioning,mechanical energy andelectric power. Often, these additional forms of energy are produced by aheat engine running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to thesecond law of thermodynamics, therefore a heat engine will always produce a surplus of low-temperature heat. This is commonly referred to as waste heat or "secondary heat", or "low-grade heat". This heat is useful for the majority of heating applications, however, it is sometimes not practical to transport heat energy over long distances, unlike electricity or fuel energy. The largest proportions of total waste heat are frompower stations and vehicle engines.[citation needed] The largest single sources are power stations and industrial plants such asoil refineries andsteelmaking plants.[citation needed]
Conventionalair conditioning systems are a source of waste heat by releasing waste heat into the outdoor ambient air whilst cooling indoor spaces. This expelling of waste heat from air conditioning can worsen theurban heat island effect.[5] Waste heat from air conditioning can be reduced through the use ofpassive cooling building design and zero-energy methods likeevaporative cooling andpassive daytime radiative cooling, the latter of which sends waste heat directly to outer space through theinfrared window.[6][7]
Theelectrical efficiency ofthermal power plants is defined as the ratio between the input and output energy. It is typically only 33% when disregarding usefulness of the heat output for building heat.[8] The images showcooling 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.
Industrial processes, such asoil refining,steel making orglass making are major sources of waste heat.[9]
Although small in terms of power, the disposal of waste heat frommicrochips and other electronic components, represents a significant engineering challenge. This necessitates the use of fans,heatsinks, etc. to dispose of the heat.
For example, data centers use electronic components that consume electricity for computing, storage and networking. The FrenchCNRS explains a data center is like a resistor and most of the energy it consumes is transformed into heat and requires cooling systems.[10]
Humans, like all animals, produce heat as a result ofmetabolism. In warm conditions, this heat exceeds a level required forhomeostasis inwarm-blooded animals, and is disposed of by variousthermoregulation methods such assweating andpanting.[11]
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 asea,lake orriver. If sufficient cooling water is not available, the plant can be equipped with acooling tower or air cooler to reject the waste heat into the atmosphere. In some cases it is possible to use waste heat, for instance indistrict heating systems.
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 athermoelectric device,[12] where a change in temperature across a semiconductor material creates a voltage through a phenomenon known as theSeebeck effect.
A related approach is the use ofthermogalvanic cells, where a temperature difference gives rise to an electric current in an electrochemical cell.[13]
Theorganic Rankine cycle, offered by companies such asOrmat, is a very known approach, whereby an organic substance is used asworking 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.[14] An example of use of the steamRankine cycle is theCyclone Waste Heat Engine.
Waste of the by-product heat is reduced if acogeneration 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 includeswimming pool heating andpaper mills. In some cases, cooling can also be produced by the use ofabsorption refrigerators for example, in this case it is calledtrigeneration or CCHP (combined cooling, heat and power).
Waste heat can be used indistrict heating. Depending on the temperature of the waste heat and the district heating system, aheat pump must be used to reach sufficient temperatures. These are an easy and cheap way to use waste heat incold 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.[15]
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 aheat exchanger before heating in homes orpower plants.
Anthropogenic heat is heat generated by humans and human activity. TheAmerican 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."[16] In 2020, the overall anthropogenic annual energy release was 168,000 terawatt-hours; given the 5.1×1014 m2 surface area of Earth, this amounts to a global average anthropogenic heat release rate of 0.04 W/m2.[17][18]
Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in denseurban areas.[19] It is one contributor tourban heat islands. Other human-caused effects (such as changes toalbedo, or loss of evaporative cooling) that might contribute to urban heat islands are not considered to beanthropogenic heat by this definition.
Anthropogenic heat is a much smaller contributor toglobal warming thangreenhouse gases are.[20] In 2005, anthropogenic waste heat flux globally accounted for only 1% of theenergy 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.[21]
Although waste heat has been shown to have influence on regional climates,[22]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.[21] Simple global-scale estimates with different growth rates of anthropogenic heat[23] that have been actualized recently[24] 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.[25]
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 asGHG emissions in the 21st century.[26]
Although uptake may increase autonomously in the future, relying on air conditioning to deal with the risk is a potentially maladaptive solution, and it expels waste heat into the environment - thereby enhancing the urban heat island effect.
One such promising alternative is radiative cooling, which is a ubiquitous process of losing surface heat through thermal radiation. Instead of releasing waste heat into ambient air as conventional cooling systems, radiative cooling passively discharges it into outer space.
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