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District energy

District heating

Energy storage


Large-scale solar water heating and seasonal heat storage pit in Gram

22. November 2015

Solution provider


Ramboll is a leading international engineering, design and consultancy company, owned by the Ramboll Foundation.

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The consumer owned district heating company Gram Fjernvarme has in 2015 increased the large scale solar heating plant from 10,000 to 44,000 m2 and established a 120,000 m3 seasonal thermal storage pit. The storage allows the company to increase the share of solar heating from 16% to 61%.

The storage is 15 m deep like the storage in Vojens. But unlike Vojens, which could benefit from an existing sand pit, the storage in Gram had to be established on a flat green field in clayish soil. The soil is balanced by establishing a dam around the pit. Roughly 10 m deep pit and a 5 m high dam provide a 15 m high storage volume. Therefore the unit price is 15% larger per unit than the storage in Vojens. The total investment for the storage is 2.8 million Euro. The total heat production price of the solar heating including the storage covering 65% of the heat production is estimated at 40 Euro/MWh.

The production of all 30 GWh heat will be produced as follows according to the forecast:

  • 44,000 m2 solar panels (61%)
  • A 10 MW electric boiler (15%)
  • A 900 kW heat pump (8%)
  • Industrial surplus heat (8%) and
  • A 5MWe/6 MWth CHP gas engine (8%)

The mix of production will naturally depend on the fluctuating electricity prices. 

Besides designing the solar heating and the storage pits in Vojens and Gram, Ramboll has two more similar storages in the pipe line in the Southern part of Jutland.

Gram District heating has been selected as one of 8 efficient DH&C projects in a report published by EU JRC January 2017, see attached report.