It is a challenge to integrate electricity based on renewable energy from wind, solar PV and hydro power. The energy production fluctuates depending on the weather and it has little correlation with the consumption. The hydro power and natural pump storages, which can store the electricity is a limited resource, and we will need more storage capacity in Northern Europe.

But how can we store the electric energy? New pump storages, not to speak about batteries, are expensive.

Fortunately we have an opportunity to store electricity with help from the water-based integrated energy system in the cities. Large hot and cold water storages are much cheaper than batteries and new pump storages, and we have them already available, even in small district heating systems.

The district heating and district cooling grids have with their large thermal storages for hot and cold water in combination with large heat pumps, compressor chillers and electric boilers the same impact on the electricity system as a battery. The system can use a lot of electricity while the prices are low and avoid any electricity consumption for days while the prices are high, whereas the alternative solution, namely small heat pumps and compressor units without storage capacity need electricity every day or even every hour. The water based energy systems and storages are equivalent to a virtual battery compared to the individual solution without storage.

Besides, the district heating can be combined with CHP plants, which generates electricity while prices are high.

The need for storage capacity will in time be reflected in more fluctuating electricity prices, which should encourage investors to invest in smart and “intelligent” energy systems including DH&C, storages and heat pumps instead of “unintelligent” electricity consuming appliances, which can-not respond on prices.

The technology of large hot and cold water storages is fully developed, and the largest hot and cold water tanks have a storage capacity of 75,000 m3.

Almost all DH networks in Denmark are connected to a hot water storage, and the total storage capacity increases dramatically these years, driven by the large-scale solar heating. The largest storage under construction is a 200,000 m3 underground pit storage.