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District energy

District heating


World largest solar heating plant in Silkeborg

12. October 2016
world's largest solar heating plant

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

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The large-scale solar heating is booming these years. The world record for plants is doubled every year: Marstal 20.000 m2 in 2013, Chile 39.000 m2 in 2014, Vojens 70.000 m2 in 2015 and now Silkeborg 156.000 m2 in 2016. Who is next ?

Silkeborg municipal-owned district heating company in Denmark, supplies most of the city Silkeborg with heat. The 400 GWh annual heat production has for 20 years been provided with a gas fuelled CC plant and gas boilers. Due to lower electricity prices, this production has been less cost effective recent years.

Therefore the company and the city looked for better solutions. Various more green solutions, including combining the existing system with solar heat and biomass. A result of detailed feasibility studies was to use the symbiosis of combining large-scale solar water heating and a more efficient gas CC plant by sharing a new absorption heat pump and larger heat storages.

The solar heating plant, delivered by Arcon-Sunmark will be completed end of 2016.

The transformation has in brief been the following


  • Gas fuelled CC plant                       108 MW elec./85 MW heat
  • Total efficiency of the plant          87%
  • Heat storage tank                           2*16.000 m3 = 32.000 m3
  • Heat production from CHP          80 GWh
  • Heat production boilers               320 GWh


  • Gas fuelled CC plant                      106 MW elec./120 MW heat
  • Total efficiency of the plant          102%
  • Heat storage tanks                         4 x 16.000 m3 = 64.000 m3
  • Large-scale solar heating              156.000 m2
  • Absorption heat pumps                25 MW cooling capacity
  • Heat production solar                   70 GWh
  • Heat pump from solar                  10 GWh
  • Heat production CHP                   210 GWh
  • Heat pump condensation           30 GWh
  • Heat production boilers              80 GWh

The symbiosis between large scale solar and the CHP plant is the following:

  • The storage is mainly used for solar in three summer months and for CHP optimization the rest of the year
  • The heat pump increases the production of solar heat autumn and spring and increases the efficiency of the CHP plant with flue gas condensation the rest of the year, thereby improving the competitiveness of the plant in the electricity market and its operation hours

The total investment has been around 230 mill DKK for the solar heating plant and 450 mill DKK for heat pump, the flue gas condensation and the tanks.

Our analysis shows that the heat production cost of the district heating will be reduced by around 100 DKK/MWh or around 20%.