Denmark tests storage technique using basalt granules

In many respects, Denmark is a trailblazer of the energy transition, including the use of thermal energy storage systems. Pit stores, for one, are already fairly widespread in the Scandinavian country. During the summer, water heats up in them so it can be used in the winter, for instance to heat buildings. Now, Denmark-based Andel and Stiesdal aim to connect a thermal storage system to the grid in a project named GridScale.

Old idea, new technology

The concept of storing electricity as thermal energy in order to use it to produce power on demand isn’t novel as such. In Germany, various projects are underway, involving heating volcanic rock using excess electricity generated from renewables. Recovery is performed with thermal energy used to evaporate water, which generates electricity via a conventional steam turbine-and-generator combination.

Conversely, GridScale splits thermal energy into heat and cold, each of which is stored in a tank full of basalt granules. To charge the system, the electricity powers a cycle in which a heat pump heats air, which heats up a heat tank. On the flip side, a chilling unit fills the second basalt tank with cold air.

To generate electricity, the cycle is reversed and produces electricity via the electric motor, which acts as a generator. This brings the temperatures in the two tanks back closer to each other.

“Storing heat and cold increases round trip efficiency from about 40 percent to some 60 percent,” inventor and company founder Henrik Stiesdal tells en:former. This means that up to 60 percent of the energy used as electricity to charge the system can be fed back into the grid as electricity.

Uncomplicated and safe

GridScale is one of the projects in which engineers have come up with an intelligent combination of tried-and-tested technologies in order to create something new. An electric motor works together with a heat pump and a compressor sandwiching two rock granule containers.

Such a system is capable of feeding two megawatts into the grid. Each pair of tanks stores the energy needed to supply this power for five hours. This means that four tanks operating at capacity can cover the night shift of a solar farm.

The developers claim that basalt maintains the temperature long enough to bridge several days of lulls during which wind turbines stand still. To increase storage capacity, a GridScale pumped storage power plant can simply be fitted with multiple pairs of tanks. “The modular design contributes to the cost reduction we can achieve through mass production,” explains Stiesdal.

Profitable and flexible

Unlike refrigerators and air conditioning systems, GridScale uses air instead of refrigerants that are harmful to the environment. Its engineers add that no rare natural resources are used and that there is no danger of explosion.

These factors will enable the technology to be operated at low cost. Its efficiency of 55 to 60 percent is far below that of redox flow storage (75 to 80 percent) and lithium-ion batteries (over 95 percent).

But manufacturers are convinced that this product will be profitable because they believe that the low construction, material and maintenance costs, the modular design and the stable storage capacity over several years will provide for an extremely favourable cost profile.

Based on company estimates, serial production would put the cost of one kilowatt hour of recovered electricity at approximately ten euros. According to GridScale, the costs of large battery storage systems are 20 times higher.

Danish government on board

The innovative project is scheduled for an initial three years and has a budget of 35 million Danish crowns, equivalent to approximately 4.7 million euros. Just over half of it is a subsidy from the Danish government.

Stiesdal claims that the planned demonstration plant will have an electric generation capacity of two megawatts (MW) and an output of 20 megawatt hours. It is slated to be connected to the grid next year and go into normal operation. The waste heat it produces will be fed into the local district heating network.