Projekt - Sunstore 3

Ivan  Katic

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Projekt - Sunstore 3

Sunstore 3

Project start 2008. Ended 2014.


The objective of the SUNSTORE 3 project has been to optimize, design and implement a full scale demonstration plant with 35,000 m2 solar thermal collectors, 60,000 m3 pit heat stor-age and a heat pump that utilizes the storage as heat source. The demonstration plant opened in May 2014. The cost is a little lower than budgeted and the production from the plant seems to be as expected.


The SUNSTORE 3 project started in 2008 where EUDP supported a design project for a large pit heat storage with solar collectors and heat pump in Dronninglund. This part is reported in “Slutrapport, Fase 1” [Ref. 1]. The present report covers Phase 2, implementation of the project, supported in 2009 and in 2010 from EUDP.

The design project ended up with a production plant of 35,000 m2 solar collectors, 60,000 m3 pit heat storage and an electrical driven heat pump with heat capacity of 3 MW. But also with a problem with bad economy for the electrical driven heat pump due to the Danish electricity taxes.

This problem was not solved before the implementation of the plant. The electrical driven heat pump therefore had to be replaced by a heat driven absorption heat pump. TRNSYS calculations showed an optimum of 2.1 MW cooling capacity for the absorption heat pump.

Also the design of the storage lid was changed. The main reason was that Marstal District Heating in 2012 developed and implemented a new type of lid, where the cost was in same level as for the type designed for Dronninglund in Phase 1 of the SUNSTORE 3 project. The design in Dronninglund was improved according to ventilation and anchoring of the insulation material. GSE, supplier of the HDPE-liner for the storage, offered a new type of liner, with a durability guarantee of 20 years if temperatures in the storage don’t exceed 90 oC.

In the solar collector field a new type of Logstor pre-fabricated pipes for solar fields gave room for extra solar collectors. The collector area thus ended at 37,573 m2 instead of 35,000 m2 because calculations showed that it would be feasible to extend the solar collector area if the existing natural gas fired CHP plant should join the open electricity market and therefore have only a few running hours in the summer period.

After these changes the budget for the project could be fixed and implementation started in March 2013. During the implementation period there was only miner unexpected problems except for corrosion protection in the storage. Marstal District Heating found corrosion in the first year of operation and therefore a corrosion expert was consulted and it was desided a.o. to treat the water with reverse osmosis to remove all salts and to raise pH to 9.6-9.8 in the storage water.


The technical objectives in the project were:

  • To demonstrate a full scale pit heat storage in combination with solar thermal and a heat pump. The plant is implemented.
  • To cover 50 % of the district heat consumption in Dronninglund with solar thermal. Calculations show a solar fraction of 41 %. The reason why the 50 % is not reached is that the electrical driven heat pump was part of the 50 % in the original design concept.
  • To demonstrate a new cover solution where disadvantages from the 10,000 m3 pit heat storage in Marstal are solved (air pockets under the cover, water pits near the edge, moisture in insulation and demolition of convection stop. The new cover solution seems to have solved these problems.
  • Implement a storage that also can be utilized to store excess heat from industries, incineration plants, power plants and biogas plants. Since the HDPE-Liner used in Dronninglund has a durability of min. 20 years for temperatures up to 90 oC the whole year round, t




Danish Technological Institute

Marstal Fjernvarme

Dronninglund Fjernvarme

SOLITES (subcontractor to PlanEnergi)


The project is supported by EUDP.

More information

Final report

Link toproject on Energiforskning