Today Vojens is known to be the solar city number one. The local consumer-owned district  heating company Vojens Fjernvarme is in 2014/2015 in the process of establishing the world largest solar heating plant (70,000 m2) and the world largest underground thermal storage pit (200,000 m3).

The huge storage will be operated as an interseasonal heat storage allowing the solar heating plant to deliver more than 50% of the annual heat production to the network. The rest of the heat will be produced by 3 gas engines, a 10 MW electric boiler, an absorption heat pump, and gas boilers.

The storage is excavated in an old sand pit. The 200,000 m3 water volume will be seperated from the district heating water by a heat exchanger. A huge "plastic bag", formed by a special welded plastic liner, will ensure that the water does not dissapear into the sand and remains clean. The surface of the water will also be covered by the liner and moreover an insulated cover and draining system to remove rain water.

The maximal temperature is in principle 95 dgr. C, but it is planned not to be much higher than 80 dgr. C in order to prolong the life-time of the liner.

On the picture we see that most of the liner is installed and that the pipe construction for warm inlet on top and colder inlet in the bottom is under construction.

Ramboll has in the design introduced several improvements based on experience from the previous smaller pit storages.

Special attention is paid to:

• Protection of steel pipes against corrosion
• Keeping the water clean without organic material
• Damaging the insulated cover
• Installation of the liner
• Production of process water

The cost of heat in winter from the solar heating combined with the interseasonnal heat storage is competitive against the heat from gas boilers, due to economy of scale.

The plant is purely commercial without any subsidies except the indirect subsidy in terms of energy tax on gas.

The plant was put into operation in May 2015. September 2016 the storage has stored heat in two summer seasons according to the plan. In this period the heat balance has been stabilized as it takes time to heat up a large volume of soil around the pit, as the dry soil is part of the insulation and helps to reduce the heat losses to a low acceptable level.

Now it is time to go one step further and establish even larger storages, which can store not only the solar heat, but also heat from the cheapest electricity, which is mainly produced by wind turbines or solar PV.