by Linda Bertelsen
Electric kettle in a green energy system

Once upon a time…

In 1984 the municipalities of Frederiksberg, Gentofte, Gladsaxe, Copenhagen, and Taarnby, in the Greater Copenhagen Area, joined forces to implement and operate a united district heating system: The Metropolitan Copenhagen Heating Transmission Company known as CTR – now aiming to be CO2 neutral in 2025.

The main part of the 18.000 TJ/year of heat from CTR is delivered by Combined Heat and Power plants (CHP), and waste-to-energy plants characterized by low variable cost but relatively large investments.
Today the large CHP plants are converted from coal to biomass (via long-term capacity contracts with Ørsted, and HOFOR Energy Production).

Next step…

The next step is to integrate more green and sustainable energy sources without increasing prices. This is where the electric kettle comes into the picture – or electrically driven boilers as they are called this size.

They are placed in Peak Load Stations (PLS) to smoothen out energy supply and demand peaks. They absorb electrical energy from wind turbines when the wind is blowing, and there is too much electricity in the net, and they supply energy to the DH network when it is cold and the heat demand is too high. This is very important for stabilizing a DH net.

Today, PLS boilers in the CTR system are gas-fired, oil-fired, or electrical. The PLS has low operation hours and fast response time in case of a CHP plant shutdown. Even though they only deliver 5 % of the heat, they contribute significantly to the CO2 emission.

Thus, converting PLS from fossil fuels as gas and oil is important for reducing CO2 emissions and CTR is converting PLS stations to be CO2 neutral in 2025. But the lifecycle of a PLS is 30 -50 years. Many things must be considered!

The future Peak Load Stations

The future PLS must be CO2 neutral, fast response, cheap in investment, and, as the full-load operation hours are low, the variable production cost is of minor interest.

The total heat capacity in CTR is 2100 MJ/S.

In a case of a shutdown or in peak load situations, CTR has 1040 MJ/s of peak load and reserve capacity (ultimo 2020):

  • 450 MJ/s of gas-fired boilers
  • 470 MJ/s of oil-fired boilers
  • 120 MJ/s electrical boilers

Graphical presentations of the technologies are shown in the diagram – they are all possible CO2-neutral solutions. The horizontal axis shows the estimated operations hours on full load, and the vertical axis shows the estimated total expenses pr. installed capacity. As peak load stations in CTR’s system typically operate below 500 hours pr. yearly, electrical-driven boilers and bio-gas boilers will be the cheapest solutions. More energy-efficient heat pumps need more operating hours to be economical.  

Planning the location of the PLS

The shortest distance between the power grid and the district heating system is essential.
This year, two electrical 40 MJ/s boilers will be connected to Energinet’s 132 kV system in Gladsaxe, only competitive as the old PLS was located just beside an Energinet power transformer station.

Transmission and Operation Management

To ensure operability and minimize costs, it is vital to involve a Transmission System Operator1) (TSO) and a Distribution System Operator2) (DSO) in the planning process.

In this case, CTR and the TSO (Energinet) identified and planned the optimal place for the PLS together, considering the shortest possible distance between the power grid and the DH network. The electric boilers/heat pumps are only a small part of the investment. The electrical transmission installations (cable and transformers) and connections to the district heating systems represent significant costs, too.

The lifecycle of a PLS is 30-50 years and CTR are converting stations to be CO2 neutral in 2025.

Since 2018 CTR has a 40 MJ/S electrical boiler in operation in Gentofte.

By the end 2020 another electrical boiler station, containing two 40 MJ/s boilers, will be in operation in Gladsaxe. Investment share of cost in the Gladsaxe PLS:

  • Electrical boilers: 25%
  • Electrical connections: 40%
  • District heating connections: 35%.

CTR’s primary PLS strategy is to deliver CO2-neutral fast operational peak load energy to the DH system. Second, it balances the electrical system using excess electrical energy production and adjusts the frequency. As CTR does not operate in the electrical market, an external partner was necessary in this case.

In Gentofte, Lyngby Kraftvarme (Former Danish Commodities) buys electricity and operates the boiler based on agreed prioritization. The priority is electrical heat available for peak and reserve load in the DH system. The second priority is Lyngby Kraftvarme using the boiler as an ‘energy accumulation tank’ in the reserve-power market.

The success of integrating the DH system and the electrical system depends on the partners’ cooperation and the use of each partner’s strengths and flexibilities.

The DH system can accumulate energy big time. This project is realizing a fine solution for the future excess power in the grid, balancing it out via the DH system – using a very big electrical kettle.

For further information, please contact: Jan Hindsbo, JAH@CT.DK.

Meet the author

Jan Hindsbo
Vice Director, CTR (The Metropolitan Copenhagen Heating Transmission Company)
“Electric kettle in a green energy system?” was published in Hot Cool, edition no. 2/2020. You can download the article here: