Technology: Waste Heat

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Waste Heat for District Heating

Waste heat from industries have a long history in district heating supply. In recent years, the introduction of heat pumps and cheaper power have enlarged the potential for waste heat in district heating with the many heat streams that are too cold to feed directly into district heating. In addition, careful analysis of smaller sources are leading to an increased use of waste heat in district heating. In certain cases, new technologies have increased the potential. For instance, new cooling systems in supermarkets with CO2 as refrigerant have enabled the use of waste heat from refrigerators and freezers to be fed directly into local district heating networks. In other cases, new, more efficient technologies in industries have reduced the waste heat potential for district heating.

Among the sources for waste heat for use in district heating are:

  • Iron smelters.
  • Smelters for other metals.
  • Foundries.
  • Cement factories.
  • Glass factories.
  • Brick and tile factories.
  • Refineries.
  • Food industries with boiling as part of the process. Sometimes heat pumps are required.
  • Cold stores and freezer storages with CO2 as refrigerant in trans-critical process.
  • Supermarkets, where cooling circuit is with CO2 as refrigerant in trans-critical process. In the country of Denmark, more than 3000 supermarket can potentially deliver waste heat for local district heating.
  • Production of compressed and liquified gases.
  • Data centres, heat pumps are required.
  • Waste water above 20’C, heat pumps are required.
  • Future technologies as hydrogen production, bio-refineries.

In 2021, waste heated provided 6% of the district heating supply in Denmark.

Technologies for waste heat recovery varies with industry and processes. In some cases, the waste heat is in the form of hot water that can be sent to district heating via a heat exchanger or via a heat pump. In other cases, the waste heat is in the form of warm air or flue gas. Then the usage requires installation of air-water heat exchangers.

Examples

Skjern Paper Mill

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This paper producer is now providing waste heat to cover a large part of the heat for the town of Skjern with 7800 inhabitants and 3200 consumers (families, businesses etc) for district heating. The paper mill has waste heat in the form of hot air, 50-58’C. The hot air is first used to heat the return water from the district heating and is then used for to heat the cold side of three heat pumps with a combined capacity of 4 MW. The district heating water is heated from 37’C to 70’C. Because of the high temperature on the cold side, the COP is 6,9 (annual average) compared to COP=3 for heat pumps using ambient air. A heat storage in the form of a hot water tank is used to match heat demand of heat users with heat supply from the factory. Investments were 3,7 mill. € of which 0,7 mill. € was for pipes between the factory and the district heating main lines in Skjern.

Supermarket Waste Heat in Skjern

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The supermarket SuperBrugsen (member of the cooperative chain Coop Denmark) in the Danish town of Skjern has renovated the cooling system for refrigerators and freezers in the shop and has included waste heat usage. This heat is covering both the heating needs in the shop for room heating and hot water and is also providing heat to Skjern District Heating company to heat at least 20 single family houses. The investments were paid back in less than 3 years in saved electricity (the new system is more efficient) and in saved and sold heat.

Sources for information

Calculation of energy and economy
Cases
Technology suppliers and installers

The technology used for waste heat use is standard technology for industry and district heating, not from specialized suppliers

There are several suppliers of refrigeration for supermarkets with use of waste heat. One of them is advansor.com

Technical advice
Other sources

Sources of waste heat (branches)