Waste heat - AbsoluteAstronomy.com

Waste heat sometimes called Secondary heat or Low-grade heat refers to heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

produced by machine

Machine

A machine manages power to accomplish a task, examples include, a mechanical system, a computing system, an electronic system, and a molecular machine. In common usage, the meaning is that of a device having parts that perform or assist in performing any type of work...

s, electrical equipment and industrial processes for which no useful application is found. Energy is often produced by a heat engine

Heat engine

In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

, running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to the second law of thermodynamics

Second law of thermodynamics

The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...

, waste heat is regarded as a waste by-product

By-product

A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced.A by-product can be useful and marketable or it can be considered waste....

of this process. When produced by human

Human

Humans are the only living species in the Homo genus...

s, or by human activities, it is a component of anthropogenic heat, which additionally includes unintentional heat leakage, such as from space heating

Space heating

A space heater is a self-contained device for heating an enclosed area. Space heating is generally employed to warm a small space, and is usually held in contrast with central heating, which warms many connected spaces at once...

. Waste heat is thought by some to contribute to the urban heat island

Urban heat island

An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...

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.

Conversion of energy

Machines converting energy

Energy conversion

Transforming energy is when the energy changes into another form.In physics, the term energy describes the capacity to produce changes within a system, without regard to limitations in transformation imposed by entropy...

contained in fuels to mechanical work

Mechanical work

In physics, work is a scalar quantity that can be described as the product of a force times the distance through which it acts, and it is called the work of the force. Only the component of a force in the direction of the movement of its point of application does work...

or electric energy 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

HVAC

HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

, mechanical energy

Mechanical energy

In physics, mechanical energy is the sum of potential energy and kinetic energy present in the components of a mechanical system. It is the energy associated with the motion and position of an object. The law of conservation of energy states that in an isolated system that is only subject to...

and electric power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

. Often, these additional forms of energy are produced by a heat engine

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

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 and vehicle engines. The largest single sources are power stations and industrial plants such as oil refineries and steelmaking

Steelmaking

Steelmaking is the second step in producing steel from iron ore. In this stage, impurities such as sulfur, phosphorus, and excess carbon are removed from the raw iron, and alloying elements such as manganese, nickel, chromium and vanadium are added to produce the exact steel required.-Older...

plants.

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%.
The images show cooling tower

Cooling tower

Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...

s 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 or glass making are major sources of waste heat.

Electronics

Although small in terms of power, the disposal of waste heat from microchips

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

and other electronic components, represents a significant engineering challenge. This necessitates the use of fans, heatsinks, etc. to dispose of the heat.

Biological

Animals, including humans, create heat as a result of metabolism

Metabolism

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

. In warm conditions, this heat exceeds a level required for homeostasis

Homeostasis

Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

in warm-blooded

Warm-blooded

The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...

animals, and is disposed of by various thermoregulation

Thermoregulation

Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

methods such as sweating

Sweating

Perspiration is the production of a fluid consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals...

and panting. Fiala et al. modelled human thermoregulation.

Disposal

Low temperature heat contains very little capacity to do work (Exergy

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. When the surroundings are the reservoir, exergy is the potential of a system to cause a change as it achieves equilibrium with its...

), 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

Sea

A sea generally refers to a large body of salt water, but the term is used in other contexts as well. Most commonly, it means a large expanse of saline water connected with an ocean, and is commonly used as a synonym for ocean...

, lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

or river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

. If sufficient cooling water is not available, the plant has to be equipped with a cooling tower

Cooling tower

to reject the waste heat into the atmosphere.
In some cases it is possible to use waste heat, for instance in heating homes by cogeneration

Cogeneration

Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....

. However, by slowing the release of the waste heat, these systems always entail a reduction of efficiency for the primary user of the heat energy.

Cogeneration and Trigeneration

Waste of the by-product heat is reduced if a cogeneration

Cogeneration

Cogeneration is the use of a heat engine or a power station to simultaneously generate both electricity and useful heat....

system is used, also known as a Combined Heat and Power

Combined Heat and Power

Combined Heat and Power may refer to:* Cogeneration* Combined Heat and Power Solar...

(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

Swimming pool

A swimming pool, swimming bath, wading pool, or simply a pool, is a container filled with water intended for swimming or water-based recreation. There are many standard sizes; the largest is the Olympic-size swimming pool...

heating, paper mills. In some cases cooling can also be produced by the use of Absorption refrigerator

Absorption refrigerator

An absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling system...

s for example, in this case it's called Trigeneration

Trigeneration

Trigeneration, also called CCHP , refers to the simultaneous generation of electricity, useful heating and useful cooling from the same original heat source such as fuel or solar energy...

or CCHP (combined cooling, heat and power).

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

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

before heating in homes or power plants.

Electrification of waste heat

There are many different approaches to transfer thermal energy to electricity, and the technologies to do so have existed for several decades. The organic Rankine cycle

Organic Rankine Cycle

The Organic Rankine cycle is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as...

, offered by companies such as Ormat, is a very known approach. The ORC is an electricity generation process where an organic substance is used as working medium instead of water. The benefit is that this process can utilize 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.
Another established approach is by using a thermoelectric, such as those offered by Alphabet Energy, where a change in temperature across a semiconductor material creates a voltage through a phenomenon known as the Seebeck effect.

Greenhouses

Waste heat (along with carbon dioxide from combustion) can be used to provide heat for greenhouses particularly in colder climates.

Anthropogenic heat

Anthropogenic heat is heat generated by humans and human activity. The American Meteorological Society

American Meteorological Society

The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, the American Meteorological Society has a membership...

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."

Estimates of anthropogenic heat generation can be made by totaling all the energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

used for heating and cooling, running appliances, transportation, and industrial processes, plus that directly emitted by human metabolism

Metabolism

Environmental impact

Anthropogenic heat is a small influence on rural temperatures, and becomes more significant in dense urban

Urban area

An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.Urban areas are created and further...

areas. It is one contributor to urban heat island

Urban heat island

s. Other human-caused effects (such as changes to albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

, 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

Global warming

Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

than are greenhouse gas

Greenhouse gas

A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

es. 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

Energy flux

Energy flux is the rate of transfer of energy through a surface. The quantity is defined in two different ways, depending on the context:# Rate of energy transfer per unit area...

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.

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.