In the journey towards establishing a fossil-free energy system, maintaining an equilibrium between energy consumption and production is pivotal. This balance needs to be economically viable, especially regarding the inclusion of a high share of fluctuating renewable energy sources.

Taking a significant step towards harnessing the potential of waste heat, Copenhagen has become the home of a collaborative initiative aimed at creating an innovative heat storage facility The Copenhagen district heating system consists of four Combined Heat and Power (CHP) plants, as well as three waste incineration plants. These CHP plants generate both heat and electricity simultaneously from a single source of fuel, and they capture and utilise the waste heat, which is otherwise lost in conventional power generation.

This system is, however, inflexible, as the CHP plants produce electricity during periods of high wind and low electricity prices and can be forced to produce heat in periods with high electricity prices.

In an effort to add flexibility to the district heating system, Høje Taastrup District Heating and Vestegnens Kraftvarmeselskab I/S, VEKS, collaboratively opted to build a heat storage facility. This joint venture manifested itself in a subterranean cavity lined with a plastic membrane and a sealed lid on top. While Denmark already features several pit thermal energy storages (PTES), these predominantly serve as seasonal storage solutions linked with solar.

By effectively storing heat during periods of low electricity production costs and distributing it during times of high electricity cost, the facility extends electricity production incomes and contributes to the Copenhagen metropolitan area’s green energy transition through better integration in the electricity system. However, this mechanism is not designed to supply heat back to the transmission grid.

Image by: Loannis Sifnaios

The storage facility accommodates 70,000 m2 of water able to withstand a temperature of up to 90°C, which is new and requires a newly developed PP membrane to seal the storage facility and a change in the management of the Copenhagen district heating system. Additionally, it contains a charging and discharging capability of 30 MW and can store 3,300 MWh, contributing to a retained estimated annual value of approximately EUR 1,1m (DKK 8m) to the district heating systems across the entire Copenhagen metropolitan area from more flexible operation of CHP plants, replacement of natural gas used as peak load and more efficient utilisation of heat from  waste incineration during summer. Currently, the site has been in operation since the beginning of 2023.