Energy House Where Solar Collectors Are Plants


by the CADDET Danish National Team


The Danish Folkecentre for Renewable Energy in North Jutland has recently concluded a project integrating well-known technologies of different kinds. The project is innovative in its specific design features and combination of well-known components and materials. The dwelling house project was originally planned in 1990 and details of the construction were defined during the following years. In early 1994 the house was completed, and a testing and measurement programme was carried out until November 1995. A full report is available in English.


The aims of the project were:

  • to demonstrate a building as a heat production source with surplus energy;
  • to achieve energy savings in comparison to traditional greenhouse design;
  • to achieve cost savings on dwelling house prices due to more efficient climate shielding.


The building is a 400 m2 low-energy house with a 200 m2 living and working area and 200 m2 for recreational use and grocery production (a greenhouse). A 240 m2 insulated glass facade, made of 2 layers of tempered 5 mm glass, faces south. This facade has mobile insulation provided by 50 m3 of polystyrene beads between the layers.

The walls of the house are 125 mm leca concrete with 200 mm insulation. The floor separation is 240 mm concrete with 75 mm insulation. Below the roof on the north side there is 200 mm insulation. Plant lighting in the greenhouse part of the house is supplied by 14 24 400 W sodium lamps.


When the sun shines through the south-facing glass facade, the evaporation of the plants increases. The total plant leaf area is the "solar collector" of the project.

Plastic pipes filled with cold water are mounted under the ceiling. Vapour from the plants condenses on the pipes and gives off evaporation heat to the water inside. The condensation falls into a gutter leading back to the plants. Water in the pipes is circulated to a heat pump, which sends the energy either into the heating system of the house or for "sale" in the Folkecentre's district heating system.

The glass roof is made of two layers of tempered glass and the space can be partly or completely filled up with polystyrene beads. By the use of this mobile insulation it is possible to change the house from a greenhouse (day) to a well-insulated low-energy house (night/winter) as shown in the diagram.

The Heat Pump

Electrical power

5 kW

Cold effect

10 kW

Heat effect

15 kW

Cold Storage

20 m3

Heat Storage

20 m3


600 m
50 mm
PVC pipe

Radiators (surface)

50 m3

Note: Humidity is regulated by aheat pump condensing the water-laden air to produce 50oC hot water



The total cost of the project was DKK 3,755,000 (where DKK is the Danish krone). Planning, drawings and tendering cost DKK 963,000, construction and implementation DKK 1,711,000, testing and measurements DKK 1,070,000 and reporting DKK 38,000.

Houses similar to this demonstration project can be built for less than DKK 6,000 per m2 which is considered to be in the lower-end of prices for new-built Danish houses. The production of energy in the form of heat for hot water and space heating gives a simple payback time of 3.7 years. The calculated unit cost of energy produced is DKK 0.37/kWh and this gives a difference between the normal energy price and the cost of energy produced of DKK 0.27/kWh.


The project has demonstrated that it is possible to combine a greenhouse with a traditional dwelling. Furthermore, it opens new perspectives to the integration of greenhouse facilities into a district heating system. The design is also very interesting for greenhouse facilities in colder climates.

The project has, in addition, demonstrated new possibilities for the integration of plants as solar collectors in greenhouses and winter gardens. This opens possibilities for business premises, swimming baths and other public buildings

For more information contact the Danish National Team in Tølløse.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.