Waste heat

Waste heat is heat that is generated by living beings or technical devices and given off to the environment.

From a thermodynamic point of view, most real processes are irreversible . As a result of the dissipation of energy, heat is inevitably generated during these processes.

biology

Snowdrops penetrate the snow cover using waste heat

In metabolic processes inevitable heat (produced thermogenesis ). Alternating warm organisms release the self-generated heat completely as waste heat to their environment. Because the temperature of such organisms deviates little from their ambient temperature, the low level of waste heat is hardly noticeable. However, the melting of snow in the immediate vicinity of early bloomers can be attributed to the waste heat produced. Snowdrops even actively warm up to above ambient temperature, so that their waste heat melts the snow cover above them and light can penetrate through. Birds and mammals as animals of the same temperature always give off so much energy as waste heat that their body temperature remains almost constant. Your waste heat is noticeable.

In both cases, the organisms have partially developed mechanisms for thermoregulation . If there is a threat of overheating due to strong sunlight or high ambient temperatures, plants increase their heat output by increasing their transpiration and thus dissipating additional heat of evaporation . In higher organisms this function is performed by sweating or panting .

technology

Technical devices and systems cannot be operated without generating waste heat. This usually has to be derived in order to avoid malfunctions due to overheating or to restore the initial state of the working medium in circular processes .

In energy converters , the waste heat always represents a loss. In this case, it is energy that leaves the system and is no longer available for its purpose. The efficiency of the energy conversion can therefore be defined using the waste heat .

Electrical engineering

According to Ohm's law , every conductor through which current flows , i.e. almost every electrical component, generates heat loss. This means that waste heat must be dissipated from every electrical device and every electrical engineering system. In an electric motor, in addition to the mechanical friction of the moving parts, the internal resistance of the current-carrying winding reduces the mechanical power output via the shaft . An energy conversion efficiency can therefore be defined for an electric motor, in which, in addition to the frictional losses , the power loss of the internal resistance of the coils is also negatively included. ${\ displaystyle P _ {\ mathrm {m}}}$${\ displaystyle P _ {\ mu}}$${\ displaystyle R}$${\ displaystyle P_ {R}}$

${\ displaystyle \ eta _ {\ mathrm {m}} = {\ frac {P _ {\ mathrm {m}}} {P _ {\ mathrm {el}}}} = {\ frac {P _ {\ mathrm {el} } -P _ {\ mu} -P_ {R}} {P _ {\ mathrm {el}}}}}$

As a consequence, friction losses are also converted into waste heat ( dissipation ), so that the mechanical power around the entire waste heat power

${\ displaystyle {\ dot {Q}} _ {\ mathrm {ab}} = P _ {\ mu} + P_ {R}}$

is decreased:

${\ displaystyle \ eta _ {\ mathrm {m}} = {\ frac {P _ {\ mathrm {el}} - {\ dot {Q}} _ {\ mathrm {ab}}} {P _ {\ mathrm {el }}}} = 1 - {\ frac {{\ dot {Q}} _ {\ mathrm {ab}}} {P _ {\ mathrm {el}}}}}$

In the same way, losses in other electrotechnical energy converters can be taken into account as waste heat output. Information technology devices (such as CPUs , routers, etc.), on the other hand, are rarely energy converters. Apart from being converted into sound or electromagnetic radiation (light, radio waves ) for signal transmission, they emit all of their electrical power as waste heat. Their efficiency cannot be expressed in terms of efficiency, but rather is determined by the need for electrical energy .

Examples

• In the case of incandescent lamps , the generation of waste heat outweighs the generation of light by far. Their efficiency is therefore a few percent.
• CPUs completely convert the electrical power consumed into waste heat. As this is not sufficiently transported away by free convection , powerful processor coolers are used.

thermodynamics

Heat transfer

In thermal engineering systems, waste heat is often the result of insulation that can only be implemented to a limited extent in real terms, or of losses in the heat transfer that occur in reality . Waste heat here is a loss of heating energy.

Examples

• The heating coil in a kettle not only heats the water that is filled in, but also the material and the surrounding air.
• An internal combustion engine generates waste heat, which is emitted via the cooler in addition to the exhaust system, since combustion heat is also emitted to pistons , valves and cylinder walls, etc.

Circular processes

Heat machines such as thermal power plants , internal combustion engines etc. cannot be operated without releasing waste heat into the environment. According to the second law of thermodynamics , heat machines cannot do any work without creating a temperature difference. Such systems must therefore be able to give off waste heat at a low temperature level in order to function. Since heat transfer occurs spontaneously (i.e. by itself, without any effort), but only in the direction of a temperature gradient and when a power plant is operated, a body of water or the earth's atmosphere (via cooling towers ) can serve as the only natural heat sink with the lowest temperature, waste heat here always falls at temperatures above the ambient temperature.

According to Carnot efficiency , the efficiency of a heat engine increases with decreasing temperature on the cold, the heat-emitting side. The lower the temperature level of this waste heat, the higher the efficiency of the heat engine. With thermal power plants and all variants of using (waste) heat with the help of cycle processes, efforts are therefore made to bring the temperature of the heat output as close as possible to the ambient temperature.

Examples

• An internal combustion engine emits waste heat mainly through the exhaust system, since the combustion heat cannot be fully used during expansion.
• If the exhaust gas heat from the internal combustion engine is used in a block-type thermal power station to heat the water, the waste heat can be reduced and fuel consumption improved. The Carnot efficiency of the overall process (degree of utilization ) increases as the temperature at which the overall system gives off heat to the environment drops.
• In midsummer, the output of thermal power plants, which release their waste heat into rivers or lakes, must be throttled so that the water temperatures do not rise too sharply. Less oxygen is dissolved in warmer water , which would endanger fish and other organisms.
• In winter, the efficiency of most thermal power plants is higher because of the lower ambient temperatures, since waste heat can be given off at lower temperatures than in summer.
• The waste water heat recovery from the sewer system can contribute to a profitable heating operation of a heat pump due to its uniform temperature .

Use of waste heat

In most technical processes, efforts are made to reduce the waste heat output ("waste heat quantity" ) to a technically and economically sensible level through measures such as insulation , heat recovery or the use of suitable materials (e.g. low ohmic resistance). Reducing the waste heat from a system therefore usually means investing more effort in these measures and thus increasing the efficiency of the system.

The most common way of making the heat given off by a technical system usable and thus reducing its heat dissipation to the environment is heat recovery in power plants ( recuperation ), in air conditioning systems, in the blast furnace process ( wind heater ) and many other cycle processes. Here are heat exchangers used that use the heat from hot exhaust gases or hot exhaust air to ambient air or other necessary for the process media, which are present at low temperatures to heat (. Eg fresh air in winter for air conditioning).

Although technical systems emit waste heat with sometimes considerable capacities, for example in the case of cooling towers in power plants, the temperature level of these heat sources is in most cases too low for economic use. A technical possibility of waste heat at a low temperature level, e.g. For example, to convert it into electrical energy, there are circular processes such as the Organic Rankine Cycle , in which hydrocarbons with a particularly low boiling point are used as the working medium, or thermoelectric generators , with which electrical energy can be obtained directly from an existing temperature difference between two bodies.

The temperature level of technical waste heat sources can be used in agriculture, for example, for heating greenhouses , for growing asparagus or for fish farming .

The use of industrial waste heat - both directly and through the heat pump , if the waste heat temperature is not sufficient - is considered an important source of heat for climate-friendly remote - and district heating networks . Heat recovery has great potential for increasing energy efficiency . With the waste heat generated in Europe , the entire needs of the heating sector could be covered. To develop this potential, it would be necessary to expand the district heating supply to transport waste heat to households. In Germany, around 700 to 800 TWh of waste heat is generated every year in industry, of which more than 300 TWh could be used with heat pumps for heating purposes.