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Waste heat is
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
that is produced by a machine, or other process that uses energy, as a byproduct of doing
work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal t ...
. All such processes give off some waste heat as a fundamental result of the
laws of thermodynamics The laws of thermodynamics are a set of scientific laws which define a group of physical quantities, such as temperature, energy, and entropy, that characterize thermodynamic systems in thermodynamic equilibrium. The laws also use various paramet ...
. Waste heat has lower utility (or in thermodynamics lexicon a lower
exergy In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir, reaching maximum entropy. When the surroundings are the reservoir, exergy is the pot ...
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 An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal c ...
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 Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...
, 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 Seasonal thermal energy storage (STES), also known as inter-seasonal thermal energy storage, is the storage of heat or cold for periods of up to several months. The thermal energy can be collected whenever it is available and be used whenever nee ...
(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. An example of using STES to use natural waste heat is the Drake Landing Solar Community in
Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...
, 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. Another STES application is storing winter cold underground, for summer air conditioning. On a biological scale, all organisms reject waste heat as part of their metabolic processes, and will die if the ambient temperature is too high to allow this. Anthropogenic waste heat can contribute to the
urban heat island An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
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.


Conversion of energy

Machines converting energy contained in
fuels A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
to
mechanical work In physics, work is the energy transferred to or from an object via the application of force along a displacement. In its simplest form, for a constant force aligned with the direction of motion, the work equals the product of the force stre ...
or
electric energy Electrical energy is energy related to forces on electrically charged particles and the movement of electrically charged particles (often electrons in wires, but not always). This energy is supplied by the combination of electric current and electr ...
produce heat as a by-product.


Sources

In the majority of energy applications, energy is required in multiple forms. These energy forms typically include some combination of: heating, ventilation, and air conditioning,
mechanical energy In physical sciences, mechanical energy is the sum of potential energy and kinetic energy. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical energy is ...
and electric power. Often, these additional forms of energy are produced by a heat engine, running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to the second 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 from
power stations A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many po ...
and vehicle engines. The largest single sources are power stations and industrial plants such as oil refineries and
steelmaking Steelmaking is the process of producing steel from iron ore and carbon/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon (the most important impurity) are removed from the sourced iron, and alloy ...
plants.


Air conditioning

Conventional
air conditioning Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
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 the
urban heat island An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
effect. Waste heat from air conditioning can be reduced through the use of passive cooling building design and zero-energy methods like evaporative cooling and
passive daytime radiative cooling Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zer ...
, the latter of which sends waste heat directly to outer space through the infrared window.


Power generation

The
electrical efficiency The efficiency of a system in electronics and electrical engineering is defined as useful power output divided by the total electrical power consumed (a vulgar fraction, fractional Expression (mathematics), expression), typically denoted by the Gr ...
of thermal 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. 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.


Industrial processes

Industrial processes, such as oil refining,
steel making Steelmaking is the process of producing steel from iron ore and carbon/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon (the most important impurity) are removed from the sourced iron, and alloy ...
or
glass making Glass production involves two main methods – the float glass process that produces sheet glass, and glassblowing that produces bottles and other containers. It has been done in a variety of ways during the history of glass. Glass container ...
are major sources of waste heat.


Electronics

Although small in terms of power, the disposal of waste heat from
microchips An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny M ...
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 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.


Biological

Animals, including humans, create heat as a result of
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
. In warm conditions, this heat exceeds a level required for
homeostasis In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
in
warm-blooded Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The onl ...
animals, and is disposed of by various thermoregulation methods such as sweating and panting. Fiala ''et al.'' modelled human thermoregulation.


Disposal

Low temperature heat contains very little capacity to do work (
Exergy In thermodynamics, the exergy of a system is the maximum useful work possible during a process that brings the system into equilibrium with a heat reservoir, reaching maximum entropy. When the surroundings are the reservoir, exergy is the pot ...
), 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 A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
or
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...
. 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 District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heating a ...
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 The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
device, where a change in temperature across a semiconductor material creates a voltage through a phenomenon known as the
Seebeck effect The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when ...
. 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 Ormat Technologies, Inc. is an international company based in Reno, Nevada, United States. Ormat supplies alternative and renewable geothermal energy technology. The company has built over 190 power plants and installed over 3,200 MW. As of J ...
, is a very known approach, whereby an organic substance is used as working medium 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 The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sourc ...
is the
Cyclone Waste Heat Engine The Cyclone Waste Heat Engine (WHE) is a small steam engine developed to produce power from steam created from waste heat. It is an offshoot of the development of the Cyclone Mark V Engine by the company Cyclone Power Technologies of Pompano Be ...
.


Cogeneration and trigeneration

Waste of the by-product heat is reduced if a
cogeneration Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time. Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elec ...
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 A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags, and other ingredients. Prior to the invention and adoption of the Fourdrinier machine and other types of paper machine that use an endless belt ...
. In some cases, cooling can also be produced by the use of
absorption refrigerator An absorption refrigerator is a refrigerator that uses a heat source (e.g., solar energy, a fossil-fueled flame, waste heat from factories, or district heating systems) to provide the energy needed to drive the cooling process. The system uses tw ...
s for example, in this case it's called trigeneration or CCHP (combined cooling, heat and power).


District heating

Waste heat can be used in
district heating District heating (also known as heat networks or teleheating) is a system for distributing heat generated in a centralized location through a system of insulated pipes for residential and commercial heating requirements such as space heating a ...
. Depending on the temperature of the waste heat and the district heating system, a
heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...
must be used, to reach sufficient temperatures. 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.


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 A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...
before heating in homes or
power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...
s.


Anthropogenic heat

Anthropogenic heat is heat generated by humans and human activity. The
American Meteorological Society The American Meteorological Society (AMS) is the premier scientific and professional organization in the United States promoting and disseminating information about the atmospheric, oceanic, and hydrologic sciences. Its mission is to advance th ...
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."


Environmental impact

Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in dense
urban Urban means "related to a city". In that sense, the term may refer to: * Urban area, geographical area distinct from rural areas * Urban culture, the culture of towns and cities Urban may also refer to: General * Urban (name), a list of people ...
areas. It is one contributor to
urban heat island An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
s. Other human-caused effects (such as changes to
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...
, or loss of evaporative cooling) that might contribute to urban heat islands are not considered to be
anthropogenic heat 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 ( ...
by this definition. Anthropogenic heat is a much smaller contributor to
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
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 Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things). ''C ...
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 heat 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. One research 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 Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and larg ...
in the 21st century.


See also

* Waste heat recovery unit *
Heat recovery steam generator A heat recovery steam generator (''HRSG'') is an energy recovery heat exchanger that recovers heat from a hot gas stream, such as a combustion turbine or other waste gas stream. It produces steam that can be used in a process (cogeneration) or us ...
* Pinch analysis *
Relative cost of electricity generated by different sources Different methods of electricity generation can incur a variety of different costs, which can be divided into three general categories: 1) wholesale costs, or all costs paid by utilities associated with acquiring and distributing electricity to ...
*
Urban metabolism Urban metabolism is a model to facilitate the description and analysis of the flows of the materials and energy within cities, such as undertaken in a material flow analysis of a city. It provides researchers with a metaphorical framework to study ...


References

{{DEFAULTSORT:Waste Heat Heat transfer Thermodynamics Energy conversion Climate forcing Atmospheric radiation Waste