100% renewable district heating

Example of SUNSTORE4

The integrated new energy system consists of the following elements:

  • 15,000 m2 solar system;
  • 4 MW wood chips boiler, including thermal oil boiler for ORC;
  • 75,000 m3 pit heat storage;
  • 1.5 MW (thermal) CO2-driven heat pump;
  • 750 kWel ORC;
  • Estimated yearly heat production: 32,000 MWh.

SUNSTORE4 scheme-english

Main principles of the proposed innovative system:

In the summer period the solar system loads the storage and provides Marstal with district heating. From end of September the stored heat has to be supplied with heat from the wood chip boiler or the heat pump. The boiler runs with about 4 MW heat output and the integrated ORC will produce 750 kW electricity. The heat pump runs, when the electricity prices are low, to cool the storage and heat up the return temperature to 75 °C.

In the winter period the back up boilers will have to supply a few hours and/or the heat pump has to run also when the electricity price is higher.

In February the solar plant will begin to heat up the storage again. The wood chip boiler will run approximately full time until April.


Depending on what is added to the core system beside solar, a limited use of biomass and electricity is needed. Marstal Fjernvarme is situated on the Ærø island in Northern Europe, where the biomass resource is already utilised. A limited production of energy crops (willow) is possible and will be utilised in the wood chip boiler. For production of 13,900 MWh wood chips/year about 280 hectares are needed. Thus the total wood chip consumption is produced on the island.

Energy production data

The production system has been implemented in 2011-12 and the following has been achieved for the single elements:

  • 10% better efficiency of solar collectors (15,000 m2)
  • 11.7% lower price for the solar collectors
  • COP 3.1 from heat pump with CO2 as refrigerant delivering 78 °C cooling capacity 0.962 MW
  • Price for heat storage of 39 Euros/m3 water equivalent (including in-and outlet, transmission pipes, pumps, geo-technical support etc). 28 Euros/m3 for the storage alone (75,000 m3)
  • A wood chip boiler with total efficiency of 109% (4 MW total)
  • An ORC with efficiency of 19,5% (0.75 MW)
Solar collectors

13,400 MWh

Heat pump

1,300 MWh

Wood chip boiler

19,500 MWh

Existing oil boiler

1,000 MWh

Heat loss, storage

-3,200 MWh

Electricity production

3,250 MWh

Calculated production of heat and electricity