The town Vojens is known all over the world for Vojens Speedway Center, starting with Ole Olsen, world champion several times.
Today Vojens is also 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, 70,000 m2, solar heating plant and the world largest, 200,000 m3, underground thermal seasonal heat storage in an old sand pit.
Both records is a factor two larger than the next largest. However, the record will not last long, as several similar projects combining solar and seasonal storage are in the pipe line. Soon the this storage technology may also store surplus useful heat from industries and waste-to-energy plants, which else would be wasted.
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 remain 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 the 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 interseasonal 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 solar plant is almost completed, and the pit almost filled with treated water. We hope that the storage pit will be ready when the sun starts to shine and warm up the panels this summer.
For more information, contact Flemming Ulbjerg firstname.lastname@example.org