Solar Covers 55 % of Heat Demand of 1,500 Households

The new seasonal pit heat storage Sunstore 4 with a volume of 75,000 m3 is at the heart of Marstal’s extended solar district heating network. The town on the Danish island of Ærø receives 55 % of its heat from the 23 MWth solar thermal field, of which 12.6 MWth were installed earlier. The pit heat storage installation, the first of its kind, was a difficult undertaking - the team had to cope with heavy rainfalls in the summer and a thick layer of ice in the winter of 2012, when the underground storage was still not covered. June 2014 was the official end of EU project Sunstore4, which contributed EUR 6.1 million to the extension and the monitoring of Marstal’s district heating plant. The total budget of the Marstal extension project was EUR 15.1 million.
Photo: Sunstore4

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Name of project / installation site

Marstal Fjernvarme / Marstal on the island of Ærø



Start of operation

1996 (former plant) / 2004 / 2012 (new plant)


District heating

Gross collector field size

23 MWth / 33,000 m2

Storage size

75,000 m3

Number of connected households

1,500 households

Predicted specific solar yield per year

406 kWh/m2

Predicted solar share

55 %


Marstal Fjernvarme

Planning / construction

Plan Energi, Denmark / (Various subcontractors)

Collector manufacturer

Sunmark, Denmark


Marstal Fjernvarme (

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