The challenge of the energy systems in a low carbon economy is to use fluctuating and low quality energy sources, such as wind energy, solar thermal and surplus heat from the thermal power production. District heating and heat storage tanks have the ability to utilize and store these energy sources. Compared with individual electric heat pumps, we find that the district heating system including large heat pumps and large thermal heat storages has the same impact on the power sysem as a battery or a pump storage.

The more we want to use these resources, the larger storages we need and the cheaper storage we have, the more resources we can get.

The large-scale thermal storage pit is a new opportunity, which is under development these years.

Marstal District heating in Denmark started to establish a 15,000 m3 test plant and has now a second 75,000 m3 test plant in operation. The largest commercial storage, so far, is a 200,000 m3 pit storages which is being established by Vojens District heating in Denmark in 2014. The storage will allow the company to supply more than 50% of the annual heat production demand from a 70,000 m2 solar plant in a cost effective way.

The technology is developed on the basis of experience from heat storage tanks and landfills plus a number of other practical arrangements. The storage is formed as a pit with an insulated cover on top. The water in the storage is circulating in a closed circuit separated from the district heating circuit by a heat exchanger and protected from soil and oxygen by a water-proof plastic liner on the soil and under the isolated cover. Inside the storage is established the same kind of diffusers on top and bottom, which we have in the steel tanks.

Some of the many challenges we face and find various solutions for are to drain the horizontal surface, to prevent corrosion and to provide sufficient water at low cost.

So far we have only designed pit storages to store solar heating from summer to winter replacing expensive gas boilers, but we have feasibility studies showing that it in the future could also be an opportunity in the larger district heating systems in which there could be several load cycles during a year. The most valuable could be to avoid expensive peak boilers on the coldest days.

In energy systems in which the electricity price fluctuates, like in Denmark it is of interest that the storage has available capacity from september to may which at no more cost can store heat from electric boilers, heat pumps and gas CHP, which are optimized with respect to the electricity price. Moreover it can be cost effective to increase the storage capacity in order to supplement the solar heat with other cheap heat sources from surplus heat and surplus electricity all year. The marginal cost of extending the storage is roughly only 30% of the average cost of the pit.

The only disadvantage of this kind of storage is that all buildings shall accept preferably not more than 80 dgr. C from the district heating on the coldest day. Although the storage could contain water up to 95 dgr.C, it is better to keep the temperature lower in order to prolong the life time of the liner. At a maximal temperature of 80 dgr.C the life time of the liner should be not less than 20 years.

On the picture we se the 200,000 m3 storage in Vojens under construction, autumn 2014 - ready to be filled-up with water in spring 2015.

The 70.000 m2 solar heating plant is almost completed and will start loading the storage early summer 2015.

The first two commercial storages in Vojens and Gram (established without subsidy) has now been in operation in two summers according to the plan. The surrounding dry soil has now been heated up and takes now part in the insulation to bring down the heat losses to an level.

More storages are in the pipeline, e.g. in another danish town Toftlund.

Many delegations from all over the world have visited the storage pits in the small danish towns and we wonder when we will see a new world record.