District heating companies should consider incorporating storage as a strategic tool that interacts with heat production and the electricity market, says Chief Consultant Jens Chr. Nielsen from the Danish District Heating Association.
The green transition is primarily based on renewable energy from the sun and wind. This means that energy production and electricity prices fluctuate – and that combination gives storage a role in district heating companies.
That is the conclusion of Jens Chr. Nielsen, Chief Consultant at Danish District Heating.
– Storage should be seen in conjunction with other energy production. In itself, storage provides no energy, but it adds value, as companies can store heat for hours with high energy prices or better utilize the district heating company’s own production facilities, he says, adding that storage is particularly interesting for heating companies that use or sell electricity at variable prices.
The fluctuating electricity market
In the 2000s, when Combined Heat & Power was on the rise, the plants operated when electricity prices were high, and then the heat was stored in large accumulation tanks. Depending on the season and heat demand, energy could be stored for anything from a few hours to several days. The largest storage facilities can store energy from summer to winter.
As more heating companies invest in electric boilers and heat pumps, heat storage is once again a central tool to ensure low prices and balanced production. Heat pumps and electric boilers are activated when electricity prices are low. Again, the heat can be stored for later.
Interaction with other heat production
In addition to considering the opportunities of the electricity market, the storage must be seen in conjunction with the production facility.
– In winter, for example, the storage can be used strategically to reduce morning load because they can step in instead of peak load systems with expensive, and sometimes fossil, fuels, explains Jens Chr. Nielsen.
Often, the smallest production facilities of heating plants will be larger than the heat demand in the summer. In some cases, it may make sense to operate the boiler with optimal efficiency, fill the energy into storage, and then let the boiler stand by for a period until there is space in the storage again.
In other cases, pushing a heat pump a little extra may make sense when the electricity price is low, even if it may mean lower efficiency. With a low electricity price, it can still be a good deal – because when the heat is delivered at a higher temperature, a larger amount of heat can be stored. This results in a lower overall heat price despite the efficiency not being optimal.
When it comes to choosing technology, according to Jens Chr. Nielsen, district heating companies should, for example, consider the temperature level. Storage with temperatures below 100 °C is often sufficient for district heating, while storage with higher temperatures can be used for steam production and, thus, for example, for electricity production via a turbine.
– If the heat is stored at a low temperature, it typically requires more space than stored at a higher temperature. This should be a central consideration in using excess heat and heat pumps, which often operate best at lower temperatures, says Jens Chr. Nielsen.
Several thermal storage systems based on different technologies, such as salt, stone, and water storage, are on the market and under development. Some technologies operate with heat transmission, while others are based on phase shifts in, for example, salt or oil.
The technologies have different levels of maturity and various advantages and disadvantages. Storage capacity, charging and discharging efficiency, physical size, space, and price are crucial parameters for the choice of technology.
– Thermal storage is relatively inexpensive per stored energy unit compared to electrical batteries. And they often make sense regarding the power they are charged and discharged with. However, competition from electrical batteries will be interesting to follow in the coming time. As long as the heat demand and energy prices fluctuate, there will be good sense in storing heat and incorporating it into the district heating company’s setup, says Jens Chr. Nielsen.
This article was published as part of a theme on storage in the magazine “Fjernvarme” in December 2023, by the Danish District Heating Association