Shared and district heat reuse: Science and thermodynamics

Turning 25-40 C waste heat into network-ready warmth

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

Shared heat schemes collect low-grade heat from one site (a data centre, treated sewage, a metro tunnel, a factory or a supermarket) and deliver it to buildings through a district heating network, usually after a large heat pump has raised its temperature.

Grade of energy

Mixed

Almost all of these sources are lukewarm. Air-cooled data centres typically reject heat at about 25 to 40 C, and treated wastewater, river water and metro ventilation air are similar or cooler, while district networks need hotter water, so a heat pump is nearly always needed. Liquid-cooled servers run warmer and need less temperature lift. The resource is large: German data centres alone turn more than 13 TWh of electricity a year into heat, most of it wasted. Industrial symbiosis, as at Kalundborg in Denmark, can also trade higher-grade steam; its partners say local industry makes enough surplus heat to potentially warm more than 40,000 households.¹¹¹

Typical data centre waste heat 25-40 C¹ German data centre heat more than 13 TWh/yr¹ Kalundborg surplus heat potential enough for more than 40,000 households (partnership claim)¹¹

Exergy versus energy efficiency

Favourable

In energy terms these schemes are attractive: a heat pump delivers several units of heat for each unit of electricity because most of the heat comes from the source. In exergy terms the source is poor, and electricity supplies the quality. Helen's Katri Vala plant in Helsinki shows the dual value: its heat pumps produce both district heat (160 to 165 MW) and district cooling (100 MW) from treated wastewater, so hot and cold outputs are both sold. Helen's Eiranranta plant, opened in August 2026, then takes the same wastewater after Katri Vala and still extracts heat from water as cold as about 5 C. Public COP figures for these plants were not found.⁸⁹¹²

Katri Vala heat output 160-165 MW⁸⁹ Katri Vala cooling output 100 MW⁹ Eiranranta (opened 2026) about 90 MW heat, 60 MW cooling, about 300 GWh/yr¹²

Media compatibility and corrosiveness

Mixed

Data centre heat comes as clean air or water, so fouling is minor. Sewage and river water carry solids and biofilm, which is why plants such as Katri Vala use treated rather than raw wastewater; even so, exchangers need regular cleaning. Industrial symbiosis trades steam, cooling water and by-products whose quality must be fixed in contracts.⁸

Variants compared⁵⁶⁹¹⁰¹¹¹³¹⁴¹⁵²¹

Variant	Named example	Key figure	Note
Data centre heat reuse	Meta, Odense (Fjernvarme Fyn)	Up to 100,000 MWh/yr, about 6,900 homes (2020 goal)	EU duty above 1 MW unless not feasible
Open district heating from many data centres	Stockholm Exergi and Stockholm Data Parks	Over 100 GWh/yr from 20 suppliers (2022)	Paid about EUR 190,000 per MW per year
Hyperscale data centre region	Microsoft and Fortum, Espoo and Kirkkonummi	Heat pumps up to 180 MW, started May 2026; about 40% of a 2 TWh/yr network	Fortum investment about EUR 225 million
Treated sewage heat pumps	Helen Katri Vala, Helsinki	160 MW heat, over 1,000 GWh/yr	Also gives 100 MW cooling
Metro tunnel heat	Bunhill 2, Islington, London	1 MW heat pump; network serves more than 1,350 homes (2020)	Council tenants get 10% off heating charges
Industrial symbiosis	Kalundborg, Denmark	17 partners; about 586,000 t CO2/yr saved (partnership claim)	Started 1972
Supermarket refrigeration heat	No sourced example	Not sourced	Small; suits local networks

Sources

Waste heat recovery from data centres (Innovation landscape for smart electrification) IRENA, 2023
Odense Data Center 2 Meta (Facebook Engineering), 2020
Stockholm, Sweden: heat recovery from data centres (Covenant of Mayors case study) EU Covenant of Mayors, 2023
Katri Vala heating and cooling plant Helen, 2025
Waste heat plays a significant role in a sustainable energy system Helen, 2023
Bunhill Heat and Power: capturing waste heat from the London Underground Euroheat & Power, 2017
Numbers are worth a thousand words (Kalundborg Symbiosis key figures) Kalundborg Symbiosis, 2026
New-technology heat pump plant starts at Eiranranta, recovering heat from water as cold as 5 C Helen, 2026
Bunhill Heat and Power Network brochure Islington Council, 2021
Fortum has started heat production at two large data centre sites in Finland Fortum, 2026
Kalundborg Symbiosis: six decades of a circular approach to production European Circular Economy Stakeholder Platform, 2024
LSBU academics help develop a world-first scheme using waste heat from the Tube to warm homes in Islington London South Bank University, 2020