The largest pit thermal storage of its kind is about to be put into service

On Wednesday this week, the ribbon was cut, and Denmark’s largest weekly pit thermal storage is now officially put into use. The pit thermal storage is located in Høje-Taastrup and contains 70,000 cubic meters of water and has an energy content of 3.300 MWh. It is 180 meters long, 14.5 meters deep and between 52 and 72 meters wide. A large hole in the ground in Høje-Taastrup that will help make better use of district heating in the capital region.

Pit thermal storage is not a new idea, but according to Brian Vad Mathiesen, professor of energy planning at Aalborg University, it is still a very good idea.

“One of the most important Smart Energy System components is being built here,” Brian Vad Mathiesen recently wrote about the project on LinkedIn, after VEKS presented a time-lapse video of the construction.

The thermal storage makes it possible for VEKS to store excess heat through the transmission network when the electricity price is low, while Høje-Taastrup Fjernvarme can extract the heat and supply its district heating customers when the electricity prices are high. 

Weekly thermal storage with brand new materials

The pit thermal storage is quite simply a hole in the ground, covered with a plastic liner and topped with an insulating lid, but this simple model also hides new developments. 

The project in Høje-Taastrup has thus received a grant of DKK 10 million from the EUDP to use newly developed materials, which they have done in the form of a brand new kind of a plastic liner and lid.

“We have decided to go with a new type of plastic liner from an Austrian innovation project, because it looks as if it can last 35 years at 95 degrees, whereas the traditional plastic liners run the risk of being degraded within 15–20 years,” says Thomas Hartmann, energy planner at VEKS.

The liner is a new type of polypropylene liner, and in addition, it has a leakage control system to detect any leaks.

The new lid type was developed by Aalborg CSP and is designed as a diffusion-open construction, where any vapours can be discharged without damaging the insulating structure. With the new materials, the heat loss from the storage is expected to be between 8 and 9 percent.

Replacement on a weekly basis

The water exchange in the storage will take place on a weekly basis, whereas other heat storages in Denmark are seasonal. Conventional thermal storages are filled with hot water in the summer, typically from solar cells, and emptied for use in the cold months.

The daily and weekly distribution of heat is calculated by advanced systems from the company Varmelast, which, among other things, take into account weather forecasts, fuel prices, technical prerequisites, and restrictions on the network.

“Their optimization systems then make a calculation every day of how much value there will be in having some heat in stock in a week’s time, and if there is great value in having heat in stock in a week’s time, then it starts being stored. It is necessary to look a week ahead, because it takes four days to fill up and almost four days to empty,” Thomas Hartmann says.

Based on system calculations and over 100 simulations, VEKS and Høje-Taastrup Fjernvarme have found that the facility will add an annual value of DKK 6–7 million to the district heating system in the capital region. According to the calculations, there is much higher value in emptying and filling the storage on a weekly basis compared to seasonal storages that can be found around Denmark in smaller distribution networks.

More thermal storage in the future

Thomas Hartmann says that the future will only bring more value to pit thermal storages like this one, because more heat pumps will be added when some of the large cogeneration plants are to be decommissioned. With an increasing number of heat pumps powered by electricity, it therefore becomes really important that the large electric heat pumps do not run when the electricity price is high.

“All our calculations say that the value of thermal storage is going to increase over the next 10–15 years,” Thomas Hartmann says. 

Thomas Hartmann would like to see more large pit thermal storages, also some that are much larger than this one, because he believes that it is clearly the cheapest way to store energy. 

He points out, however, that very large thermal storages can present a challenge.

“These pit thermal storages take up a lot of space, so it comes down to finding a plot of land near the district heating grids which is large enough and also available for sale. High land prices can ruin the business case,” Thomas Hartmann says.

VEKS is apparently good at spotting suitable locations, because there are already plans for an even larger pit thermal storage facility in Roskilde. According to the first analyses, the thermal storage in Roskilde will be 200,000 cubic meters in size if it is approved.