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Glossary · Energy industry & energy transition

Waste heat

Waste heat heat efficiency conversion losses district heating combined heat and power

Waste heat is heat generated by a technical process without being its intended primary benefit. Examples include industrial furnaces, engines, compressors, data centres, electrolysers and cooling systems. Thermal power plants also release heat that is not converted into electricity. For a practical assessment, the amount of energy alone is not enough: temperature, variation over time, output, location, the heat-carrying medium and the requirements of potential users are also decisive.

In physics, heat refers to the transfer of energy as a result of a temperature difference. A warm cooling water stream contains thermal energy and can transfer some of it to a colder medium. In technical usage, this transferable portion is often referred to as the amount of waste heat. Its size also depends on the temperature to which the stream may be cooled. A figure for the amount can therefore be misleading without this boundary condition.

Energy quantity, output and temperature

Amounts of waste heat are expressed, for example, in kilowatt-hours, megawatt-hours or joules. Thermal output, by contrast, describes how much heat is transferred per unit of time; its units are kilowatts or megawatts. A source with a constant heat output of 10 MW theoretically provides 10,000 MWh, or 10 GWh, over 1,000 operating hours. This is initially the amount at the source considered, not the useful heat delivered to the customer. Losses from heat extraction, transport and, where applicable, storage must still be taken into account.

The temperature level is equally important. Waste heat at 30 °C, 80 °C or several hundred degrees offers different possibilities for use. The temperature must always suit the intended application: a low value may be sufficient for preheating or as a heat source for a heat pump, but not directly for a high-temperature industrial process. Direct transfer through a heat exchanger also requires a temperature difference. A heat source at exactly the desired useful temperature is therefore not necessarily sufficient.

Distinction from useful heat, losses and ambient heat

Heat can be deliberately supplied as useful heat, for example by a boiler or an industrial furnace. Waste heat, by contrast, arises alongside the actual purpose of the process. If it is captured technically and put to beneficial use, it becomes useful heat for another purpose. The term therefore initially describes its origin. In plants expressly designed to generate electricity and heat together, however, heat is an intended co-product. Not all heat extracted from power plants should therefore be described indiscriminately as free waste heat that would exist anyway.

Waste heat and conversion losses overlap. In a thermal power plant, the energy not converted into electricity appears predominantly as heat. If it is discharged unused, it is a loss from the perspective of electricity generation. In combined heat and power, part of the energy input is additionally supplied as usable heat. Efficiency must therefore be stated with a clearly defined system boundary: electrical efficiency and overall efficiency based on electricity and heat actually used are different measures. High overall efficiency does not automatically mean a high electricity yield.

Ambient heat, by contrast, comes from the surroundings, for example from outdoor air, the ground or bodies of water. Low-temperature waste heat can be harnessed using similar technology, but its immediate source is a technical process. A heat pump can raise both types of source to a higher temperature level. The similarity of the technology does not mean that waste heat and ambient heat are classified in the same way legally or in a greenhouse gas assessment.

Role in electricity generation

Thermal power plants derive mechanical and then electrical energy from a temperature difference. In a steam power cycle, heat must be discharged at a lower temperature level after expansion in the turbine. This is not an operating fault but a thermodynamic limit of electricity generation. Waste heat can reach the environment, for example, through cooling water and cooling towers. It does not follow, however, that all the heat discharged can be used for heating without additional measures.

In particular, heat extraction must not be treated as a universally free addition that leaves electricity production unchanged. In an extraction-condensing turbine, steam is extracted for heat supply at a higher pressure and temperature than would be the case with full expansion. This can reduce electrical output. The degree to which electricity and heat are linked depends on the plant technology. Extraction-condensing plants can generate electricity even when there is no demand for their heat; other designs are more closely tied to heat demand.

Heat-led operation can cause a plant to run because of heating demand even though the electricity market signals little demand. Storage and alternative heat generators can loosen this link. At the same time, more wind and solar power changes the mix of sources: these technologies do not require a thermal power plant cycle with comparable central cooling heat streams. They also have losses, but these are generally not available as a similarly easy-to-capture heat supply. Data centres, electrolysers and electric industrial processes, by contrast, can provide other sources of waste heat. This does not automatically replace lost power plant heat in equivalent quantities.

Usability depends on the source and the user

Using waste heat requires investment and effort, even when the heat arises without additional fuel consumption. Equipment required includes, for example, heat exchangers, pipes, pumps, controls, metering and protection against outages. Heat captured in a cooling water circuit is generally easier to harness than heat released diffusely from surfaces. Fouling, corrosion or unacceptable effects on the production process can further limit heat extraction.

At a low source temperature, a heat pump is often also needed. Its electricity consumption forms part of the overall assessment. The heat delivered then consists of waste heat absorbed and drive energy supplied, less any losses within the chosen accounting boundary. It is therefore not entirely recovered waste heat. A greater temperature lift reduces efficiency, all else being equal. Lower network and end-use temperatures can therefore help make a source economically viable to harness.

Location is also decisive. Heat can be transported through pipes, but pipeline construction, pumping energy and heat losses must be appropriate to the amount sold. There is no universally applicable maximum distance. Large, continuous amounts of heat can justify longer pipelines; small or intermittent sources often require nearby users. A theoretical potential in an industrial area is therefore not yet a usable supply for a distant residential neighbourhood.

The pattern over time must also match demand. Residential buildings require substantial space heating in winter and little in summer. Data centres can supply heat relatively steadily, whereas industrial facilities may operate in shifts or shut down for maintenance. Actual load profiles are therefore more important than general assumptions about an industry. The same annual amount can have very different value for supply depending on when and at what output it is available.

Efficiency first, external use afterwards

Before external use is considered, it should be assessed whether the waste heat can be avoided or recovered within the facility. Better process control, insulation and preheating can reduce energy consumption. Use in the original process or in another process at the facility is often more beneficial than supplying third parties. Cascaded use employs heat successively at decreasing temperature levels. A robust estimate of external potential takes foreseeable efficiency measures into account rather than assuming that losses will remain unchanged indefinitely.

Remaining waste heat can be an important source for district heating. It does not, however, automatically replace other generators or reserve capacity. Firm capacity, the required network temperatures and supply during disruptions are decisive. Thermal storage can balance differences in timing between heat generation and demand, but only within its power and capacity limits. A small buffer tank cannot resolve a seasonal surplus.

Waste heat utilisation also relates to flexibility. A heat pump can operate at higher output and charge a storage unit when electricity prices are favourable. When prices are high or the grid is congested, it can reduce its output, provided the heat supply remains secure. This does not automatically make the waste heat source itself controllable: its operation often follows the production or service process. Flexibility then arises primarily from combining the heat pump, storage and other heat generators.

Contracts, economics and climate impact

The heat source and the heat network often belong to different companies. A viable partnership must therefore establish responsibilities and address risks: Who invests in heat extraction? What temperatures and outputs are guaranteed? Who bears the cost of a replacement supply? How is maintenance coordinated, and what happens if the facility closes or relocates? A large annual amount of heat alone is no guarantee of long-term supply.

An economic assessment must consider the entire supply chain. A kilowatt-hour at the point of generation is not equivalent to a kilowatt-hour of usable heat at the customer’s premises. Between the two lie harnessing, temperature lift where required, transport, losses, operation and provision for outages. Large theoretical potentials must therefore be distinguished from quantities that can be technically harnessed, are economically viable and can actually be supplied reliably over the long term.

The climate impact also depends on the comparison system. Waste heat can save fuel and emissions from heat generation that would otherwise be required. It is, however, neither automatically renewable nor automatically emission-free. Factors to consider include additional electricity consumption, possible changes to the source process and the accounting treatment of jointly generated products. For fossil-fuel-based industrial processes, the effect of decarbonisation or closure on future heat supply must also be considered. Eligibility for legal credit does not replace a complete greenhouse gas assessment.

Legal classification in Germany

As at 11 November 2026, the general technical term must be distinguished from unavoidable waste heat under the German Heat Planning Act. Section 3(1), point 13, WPG covers heat produced as a by-product in industry, electricity generation and the tertiary sector that would be discharged unused without a heat network. In addition, it must be unusable in the production process for the stated reasons and impossible to reduce with reasonable effort. Section 3(4) treats certain heat from thermal waste treatment as equivalent to this category. This is a legal classification, not a blanket statement that the heat is emission-free.

Under the version of section 16 of the German Energy Efficiency Act retrieved as at the reference date, companies must avoid waste heat and use remaining waste heat insofar as this is possible and reasonable. Companies with an average annual total final energy consumption of no more than 2.5 GWh over the last three completed calendar years are exempt. For certain installations requiring a permit under pollution control law, more specific requirements take precedence. The provision is therefore not an indiscriminate obligation on every facility to feed heat into a heat network. The Act also contains separate requirements for data centres; their applicability requires a separate assessment.

Waste heat is therefore not a simple reserve that merely needs to be collected. Its value arises from the interaction of temperature, quantity, time profile, proximity, infrastructure and reliable allocation of responsibilities. Precise planning distinguishes between heat that exists, potential that can reasonably be harnessed and a heat supply secured for the long term.

Sources

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Edited by eCMS on 11.11.2026 06:40

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