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Plus - Energy House in Denmark
By Lars Yde, Folkecenter for Renewable Energy, Denmark
The Plus-Energy House Concept
Low-Energy Houses are increasingly common nowadays. Zero-Energy
Houses are still in the experimental stage. The idea of a
Plus-Energy House was originally introduced by the Danish
engineer Jean Fischer, and later, the Folkecenter for Renewable
Energy took it up from a different angle. We not only wished to
build a house that produced more energy than it consumed, we also
wanted to create a quite new concept of a house. It would be a
house in which you, so to speak, take the garden, or rather the
greenhouse, indoors during the winter time, and thereby create
subtropical surroundings in the dwelling house. A winter garden.
The dream of all Scandinavians, but, with current techniques, a
nightmare with regard to energy consumption.
Since 1985, the Folkecenter for Renewable Energy has worked on
a greenhouse concept based on mobile insulation (bead wall layer)
with control of air humidity and regeneration of solar heat by
means of a heat pump. This work has formed the basis for
development of the Plus-Energy House concept and of the project
to build a Plus-Energy House.
Around the beginning of this year, the building of the
Plus-Energy House was finished at the Folkecenter for Renewable
Energy in Denmark.
The house was built from conventional building materials. It is
to function as a combined greenhouse and office building. A
240-m^2 glass roof facing south allows sunbeams to get in and
heat the concrete floors, walls, and ceiling.
After only few hours of sun, the house is heated up. With the
mobile insulation (polystyrene "flamingo" beads) in the glass
roof, it can manage without solar energy for about two days.
The glass roof consists of two layers of tempered glass. It is
possible to fill the 20-cm space between the glass plates fully
or partly with polystyrene beads. This gives the glass roof an
insulation value comparable to that of an outer wall of a
low-energy house. The flamingo beads provide the mobile
insulation. They are moved with the use of air blowers to
provide appropriate insulation, day or night. During daylight
hours, they can shade the house; at night, they trap the heat,
keeping the house warm.
Energy Process
Half of the house is used for growing greenhouse plants. The
plants, e.g., fruits, vegetables, and flowers, evaporate water.
The energy in the water vapour (evaporation heat) is an important
part of the heating system.
When the sun shines on the plants, evaporation will increase.
Just below the ceiling, plastic "condenser" pipes have been
mounted and filled with cold water. Water vapor from the plants
condenses onto the pipes and transfers heat energy to the water
within the pipes. The condensation drips off the pipes into a
gutter that leads the water back to the plants.
The water in the pipes is circulated to the cold side of a heat
pump. From the hot side of the heat pump, heated water is sent
out into the heating system of the house and out of the house for
sale in the form of district heating. The mass of plants, or
rather, the total leaf area, serves as the solar collector in the
system. From here, the evaporation occurs, and it is here that
the solar heat is converted to evaporation heat. The heat is
released later, when the water vapor is recaptured by the
condenser pipes. Therefore, the heat-producing capacity of the
heat pump is determined by the leaf area of the plants and their
ability to evaporate water.
Energy - Economics
On the basis of the now-complete data acquisition, it can be
concluded that one can build a house that consumes the same
quantity of energy as other houses of the same size, and that at
the same time, produces energy at the rate of 300 kWh/m^2/year.
This corresponds to what a solar collector produces per m^2 under
Scandinavian conditions.
The project has also proven that a Plus-Energy House can be built
for a price per square meter that does not exceed 6,000 DKK, the
normal price for Danish houses. The direct pay-back time of the
energy plant (heat pump system, silos, pipes, and blowers for the
mobile insulation) is calculated to be 3.7 years. Meeting Danish
standards for fire prevention increases break-even time to 5.8
years. The house, which has 400 m2 floor space, produces 75,000
kWh of heat per year, which would cause an emission of 20,000 kg
of CO2 if produced from fossil fuels. The energy consumption,
which is covered by a 22-kW windturbine, amounts to 50,000 kWh
per year.
Gardeners' Support
The Plus-Energy House project originally started as a greenhouse
project, but due to lack of interest in the concept from the side
of the gardeners, the emphasis of the project was shifted towards
the use of this technology for, e.g., houses, business premises,
swimming baths, and winter gardens. In the decade since,
gardeners have shown increasing interest in environmentally
benign growing methods. These gardeners differ from traditional
greenhouse gardeners in their view of energy consumption. Now,
it looks as if the project is returning to its starting point.
The Board of Organic Agriculture under the Ministry of
Agriculture and Fisheries, the Danish Directorate for Development
in Agricultural and Fisheries, has granted 686,000 DKK for
further development of the concept to a low-energy greenhouse for
growing of organically grown cucumbers and tomatoes.

- Technical Data of The Plus Energy House
- Ground floor: 10x20 m = 200 m^2
- Floor area: 400 m^2
- Window + door area: 22 m^2
- Walls: 125 mm leca concrete 1,500 kg/m^3
- Floor separation: 240 mm concrete 1,500 kg/m^3
- Insulation, roof facing North: 200 mm, lambda = 0.039
- Insulation, walls: 200 mm, lambda = 0.039
- Insulation, floor: 75 mm, lambda = 0.035
- South facade, area: 20x12 m = 240 m^2
- South facade, rafter: laminated wood 200x60 mm
- South facade, glass: tempered glass, 2 layers of 5 mm
- Mobile insulation: polystyrene, › 3-5 mm, 50 m^3
- The heat pump
- Electrical power: 5 kW
- Cold effect: 10 kW
- Heat effect: 15 kW
- Cold storage: 20 m^3
- Heat storage: 20 m^3
- Radiators (surface): 50 m^2
- Condenser: 600 m, › 50 mm PVC

- Plant light: 14-24 sodium lamps, 400 W each.
- Annual import of electricity: approx. 50,000 kWh.
- Annual export of heat: approx. 75,000 kWh.

More Info: Lars Yde, Folkecenter for Renewable Energy, PO Box
208, 7760, Hurup Thy, Denmark.
Ph/fax: +45-97956600/-97956565.

Pictures above and down: The mobile insulation consists of
polystyrene, 3-5 mm 'flamingo beads' between the 2 layers of the
glass roof. The insulation is moved with air blowers to provide
appropriate insulation day and night.
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