Solar House - Solar Power Off-the-Grid Prefab Home

Winner of Solar Decathlon 2007

Project participants
Innovative materials manufacturers

Design: Students of the TU Darmstadt, Department of Design and Energy Efficient Construction, Prof. Manfred Hegger (Project Management)
Project: Solar House
Location: El-Lissitzky-Straße 3, 64287 Darmstadt, Hesse
Year: 2007
Client: TU Darmstadt (+ operator + user)
Gross floor area: 72 m²
Heated net floor area: 50 m²
Gross volume: 182 m³
Heating requirement (EnEV): 12,00 kWh / m²a
Usable area (according to EnEV): 58 m²
A / V ratio: 1.15 m-1
Architecture and engineering systems: thermal insulation, façade systems, glazing + windows, daylight planning, ventilation + heat recovery, active cooling, regenerative + passive cooling,
thermally activated component systems, heat pump, heat / cold storage, control engineering, plant management, building automation, solar thermal energy, photovoltaics, biomass utilization, building materials ecology

The solar power off-the-grid prefab home "Solar House" designed by students of the TU Darmstadt has won the international competition "Solar Decathlon 2007" for the most attractive and energy-efficient solar house. The energy self-sufficient prefab home was built on the campus of the TU Darmstadt and transported to the USA after completion.

The house is a wooden lightweight construction with low heat storage mass compared to massive new buildings. In order to combine maximum living comfort with the lowest energy consumption, a compact and highly insulated building envelope was chosen. During the hot summer days, the shading of the windows help to avoid overheating the interiors. The shading elements consist of the southern roof overhang and the lamellar shell of oak, which opens and closes when needed.

Plasterboard with integrated phase change material (PCM) compensate for heat peaks. They store the heat energy generated during the day and release it at night. Per volume unit, the material can store six times as much heat as concrete and thus acts as a latent heat storage. Once the wax contained in the PCM has completely melted, no further heat can be absorbed. Cooling can be achieved by cross-ventilation between the north and south sides during the night or by passive cooling system of the PV modules on the roof. For this purpose, a built-in double soil water reservoir is used, which is connected to a heat exchanger. On the opposite side of the heat exchanger is the cooling circuit, which consists of capillary tube mats in the ceiling. These absorb the heat of the phase change material and transport it into the water tank. This allows the PCM to actively cool constantly.

Heating / energy plan

The basic concept of the building technology is based on a passive low-tech system, which is supported by high-tech. All technical building components, pipes and connections are combined in the core zone. Only when the passive systems (insulation, solar gains, night cooling) are no longer sufficient to meet the required living comfort, they are supplemented by the active energy systems.

In winter, the sun heats. Due to the large glazing in the south, a large part of the required heating energy enters the building. The residual heat demand is provided by the controlled ventilation system, which also ensures the necessary hygienic air exchange. A compact unit combines ventilation, cooling, heating, water heating and storage in a 60 x 60 x 230 cm housing. A cross-countercurrent heat exchanger uses the heat of the exhaust air in the heating season, the cool outdoor air
preheat. If this is not sufficient or if there is a higher heat requirement, a reversible air/air or air/water heat pump is connected. This takes 75% of the heat from the exhaust air (exhaust air after the cross countercurrent exchanger) and requires 25% electricity to produce the desired room heat. As the heat pump is a series of two condensers, hot water is automatically prepared when the air is heated.

If the shading elements are insufficient in the summer as a passive cooling measure, the compact device can also cool. For this, the functions of the evaporator and the condenser in the heat pump cycle are reversed. The supply air thus passes the evaporator and is cooled. The waste heat is used for heating the 180 liter hot water tank.

The electricity for the heat pump is supplied by the photovoltaic system on the roof and on the facade. Two solar collectors in the middle of the flat roof also supply the drinking water storage tank.

Phase Change Material

Phase change materials, also called PCMs, are latent heat storage devices that can store a high proportion of heating and cooling energy over a long period of time and release it without loss. Their function is based on the exploitation of reversible thermodynamic state changes of a storage medium, such as in the phase transition from solid to liquid (solidification / melting).

Upon melting, the storage medium - usually salts, e.g. Glauber's salt or sodium acetate and organic compounds, e.g. paraffins or fatty acids - a lot of heat energy. After reaching the phase transition temperature (paraffins, for example, have their phase change at room temperature) takes place so no increase in temperature until the material is completely melted. Only when solidifying the storage medium gives exactly the amount of heat absorbed. Since no significant increase in temperature occurs for a long time despite heat supply, the heat stored during the phase transition is called "hidden" or "latent" heat. Compared with the heat storage capacity of building materials such as gypsum, wood, cement or stones - ranging from 0.8 to 1.5 kJ / kg in a 1°C interval - PCM can store a multiple of heat during melting.

Because phase change materials liquefy when used, they are typically microencapsulated. With a diameter of about 10 microns (ie 10-thousandths of a millimeter) they can be added as aggregate various building materials (such as interior plasters, fillers, gypsum plasterboard, aerated concrete blocks). In plastic or glass panes, for example, they are integrated in the form of encapsulated salt crystals. PCMs are also used in component temperature control, where they can be used to cut off temperature peaks without costly air conditioning. The examples given are for passive temperature stabilization. The PCM stores excess thermal energy during the day and discharges it at night.

In separate heat and cold storage systems, on the other hand, PCMs require active components such as fans and pumps, as well as control. However, this variant has the advantage that the stored heat or cold can be selectively retrieved when needed. As an example, actively flowed surface cooling systems in combination with PCM building materials called, so cooling ceilings in which capillary tube mats are integrated into a PCM plaster layer and are traversed with water.

"SmartBoard" building board with PCM heat storage - BASF

PCM can also be integrated into bricks - Celbloc Plus from H + H, Wittenborn

Building board with integrated heat storage function based on PCM materials, which can be thermally activated with the heat transfer medium water - Bine information service

Plastering of a thermally activated cooling ceiling, also visible in the blue capillary tube system - Fraunhofer ISE

Cross ventilation

Principle of ventilation of an apartment with a ventilation system is, outside air is supplied to the living rooms (living room, bedroom and children's room) and the exhaust air is removed from kitchen, bathroom and guest toilet. The corridor usually serves as overflow zone. Cross-ventilation results in effective use of the fresh air.

In the sense of DIN EN 12792: 2003 also: free ventilation, as a result of the differential pressure, which arises due to wind pressure on the external building surfaces and in which thermal buoyancy in the building is of lesser importance.

Water heating

The term hot water preparation (also dhw heating) refers to the heating of drinking water in a water heater. These are differentiated according to their design and type of heating. Examples include: instantaneous water heaters, adjacent hot water storage tanks, indirectly heated storage water heaters, solar combi storage tanks or hot water stratified storage tanks.


Air extracted from a building at the location before entering a ventilation unit. The exhaust air usually has the temperature of the room air of the exhaust air rooms. It can be energetically used in a heat recovery system.

In the sense of DIN EN 12792: 2003 generally defined as: air leaving the treated room (marked in yellow in plans).

Heating season

The heating period is understood to be the time of heating a building during which the mean outside air temperatures are less than the heating limit temperature. The heating season depends on meteorological parameters and on the thermal properties of the building.

Exhaust air

Exhaust air is the air that leaves the building, at the point behind the outlet of a ventilation unit.

The exhaust air is warmer than the ambient air even with a ventilation unit with heat recovery. The residual heat contained in it can still be meaningfully used in an exhaust air heat pump.

Heat pump

Heat generator, the addition of energy (usually electricity or heat at high temperature) can also absorb environmental energy at low temperature and makes both energies usable for heating purposes. To recover the energy from the environment, the soil or the outside air is often used as the heat source. For a high number of years of operation of the heat pump, a small temperature swing (temperature difference between the temperature of the environmental energy and that of the heating circuit) is decisive.

Supply air

The supply air is the air that is supplied to a building, at the point behind the exit from a ventilation unit. The supply air is often heated by heat recovery.

For the purposes of DIN EN 12792, the supply air is defined as: air flow entering a room or air entering the room from the system after treatment.

Supply air is indicated in plans with a color that corresponds to the number of thermodynamic treatments. Number of thermodynamic treatments / color:

- none / green
- 1 / green
- 2 or 3 / blue
- 4 / purple


Component of a heat pump in which a liquid (refrigerant) is evaporated by supplying heat. The evaporation usually takes place at temperatures around zero degrees Celsius, whereby the heat of the environment can be withdrawn.


The environmentally friendly production of electrical energy by solar energy is called photovoltaic (PV). Solar cells convert incidental sunlight directly into electricity. Solar modules consist of interconnected solar cells, which are framed in a frame and provided with a glass cover.

The solar power generated is fed via an inverter into the public grid of the electricity supplier. The Renewable Energy Sources Act (EEG) guarantees the operator of a photovoltaic system the acceptance and payment of the electricity by the locally responsible energy supply company.

Description from Solar Decathlon

1st prize of the international competition "Solar Decathlon" for the most attractive and energy-efficient solar house went in 2007 to Germany. The competition demands completely energy self-sufficient buildings and also limit the floor area of ​​the house on 75 m². The prototype is peppered with new technologies and concepts: vacuum insulation (VIP), thermoactive component systems with phase change materials (PCM), solar power generation (PV) integrated into the façade and the roof, and much more ensure energy autonomy. The team of the TU Darmstadt around the architect Prof. Manfred Hegger sat down in Washington D.C. against 19 top universities from the USA, Canada and Spain.

Building concept

The design is based on three basic ideas. On the one hand, this is the principle of stratification: the layout in different zones divided, which lay like onion skins around an inner core. The differently tempered layers allow a differentiated presentation of the floor plan depending on the season (summer and winter house). Secondly, the platform concept for furniture and technology comes into its own, similar to what the car manufacturer has done: a double bottom takes over building technical components that can be inserted in the plug-in process in the overall system. A building services room is no longer necessary. The double bottom also serves as storage space for the furniture, which is crucial for that. Thirdly, it should create a quiet space, in its actual, pure form can be perceived. The use of space remains flexible and sustainable in that sense.

Space concept

The constructed area was limited by the competition rules to about 75 m². In the competition contribution of the TU Darmstadt divides a core with all supplying and sanitary uses 59 m² large heated interior into one living and working area in the west and dining and sleeping area in the east. In the south, dining and living areas spatially connected by the flexible kitchen. The core of the building includes the vertical technical installations, the bathroom and the kitchen, as well as further storage space and a cloakroom. In the contracted state, the bath is sufficient for quick body care, the kitchen can only be used as a single kitchen. Are walls or worktop extended, results each a generous space for the appropriate use. The previously internal bathroom becomes a daylight bath, and the kitchen to the room for cooking together.


The façade is simple and at the same time adaptable. With the slats, which have a strongly horizontally embossed structure and determine all sides of the house, a living and at the same time homogenous external appearance shall arise. To South, this lamellar level is detached from the facade and defines a protected, but unheated gap as transition to the outside world.


In order to meet the transport requirements, the solar power off-the-grid prefab home is constructed in a modern timber construction. The modular design is a combination of timber frame construction with sandwich and box elements. The load case transport was taken up by steel components and considered from the outset and aesthetically integrated.

Use of solid wood as a visible construction in conjunction with modern architectural aesthetics determine performance and degree of innovation of wood-based materials. The material-appropriate and innovative use of renewable or locally produced building materials is also in the foreground in interior design. Aesthetics, comfort and simplicity of use are criteria both for the architecture-integrated, as well as for the selection of freestanding furniture.


The building was designed and built by a student team. Together with the manufacturer of the window facades with quadruple glazing and the facade parts filled with vacuum insulation in the north as well as for passive house-compatible sliding windows in the south façade were designed and assembled. A new solution is the component integration of photovoltaic into the surrounding wooden lamella layer of the solar power off-the-grid prefab home. Students of the TU Darmstadt in cooperation with the Technical University of Munich and a medium-sized facade construction company were involved.

Plus energy solar power prefab home

Solar power

Three different types of photovoltaic are used: monocrystalline modules with high efficiency and 8.4 kW power are mounted on the opaque flat roof. Above the porch are also monocrystalline, perforated solar panels. Cells embedded in a special glass provided with an innovative antireflective coating. The point-mounted modules thus simultaneously provide weather protection, sun protection and create an exciting light and shadow play. The wooden lamella façade, the defining element of the prefab home, which both shaded and offers privacy and burglar protection, is equipped with amorphous silicon modules with about 2 kW power. The slats are automatically adjusted to the sun or changed in the north and south by pressing a button. The louver doors in the north and south, however, each can be moved by hand, opened or closed. The solar electricity stored in batteries. An isolated inverter ensures intelligent control, when power is consumed directly or initially stored in the battery or taken from there. After the final installation in Germany, the solar power is fed into the grid.

The photovoltaic elements and the solar thermal collectors were considered in favor of the aesthetics in the flat roof to integrate. With the horizontal arrangement, only about 10% of the annual income has to be waived. During the competition in October 2007 there was enough sun power for off-the-grid prefab home functioning. In addition, the vertical facades in the east, south and west are covered with photovoltaic.

Solar heat

Two flat-plate collectors are fitted over the core area between the PV modules and the roof. They and an innovative compact ventilation unit generate hot water. The compact ventilation unit can be used on the smallest room, with a reversible heat pump in combination with heat recovery, in one device for ventilation, heating, cooling and hot water.



The "Light" is a separate discipline in the Solar Decathlon competition. It is both about light and energetic factor as well as visual comfort and aesthetics. As part of the competition, therefore, both continuous measurements of the illuminance at different points in the building were made, as well as the daylight and artificial lighting situation subjectively assessed by a jury.

Assessment criteria for the jury are: correspondence of the lighting concept with different lighting situations (light to see, look at), architectural integration, color rendering, innovation, design of the luminaires, energy consumption and flexibility, distinction between indoor and outdoor space, use of a light control system, lighting mood, artificial light. In addition, the design of the lighting system directly affects the discipline "energy balance", since the house had to be completely lit in the evening hours.

In solar power off-the-grid prefab home, all luminaires are integrated into the architecture - except the desk lamp. Shelves made from Plexiglas become luminous objects by combining them with light-emitting diodes (LEDs). When choosing the bulbs, aesthetics, adequacy and energy consumption were considered. Result is a combination of LED, Halogen and compact fluorescent lamps. A BUS system enables the programming of different light scenarios and the intelligent control of daylight and artificial light. The transparent Nordfasse makes it possible to comply with the prescribed minimum illuminance at the workplace: 50ftc - 538 lx must be reached there on four days between 9 and 17 o'clock - while the south facade, depending on the incidence of the sun, partially or completely shaded. Also evaluated was the lighting concept in the outdoor area: Here were LED installed as orientation lights and behind polycarbonate panels, and thus lend the surrounding deck a floating character. The solar power off-the-grid prefab home itself shines through the fine slits in the lamellar facade from the inside out and thus becomes a luminous object.

Thermal protection and temperature control

First of all, traditional methods of temperature control are used: use of solar and internal gains in winter, summer heat insulation through shading, cross-ventilation, thermal storage masses and a dense, very well-insulated shell. The largely transparent north façade enables optimal illumination with daylight.

In addition, high-tech materials are used, such as vacuum insulation panels in ceilings and walls and
thermal storage mass in the form of PCM (Phase Changing Materials) in walls and ceilings, to avoid the temperature peaks in the interior.

Simulation and modeling

The design and interaction of the systems was calculated using various static and dynamic models. In the Dechatlon competition, these simulations were also crucial to the strategy. Energy should be used optimally in each case in order to comply with as many competition criteria as possible.

Optimization measures and possibilities

The energy concept is based on the optimized interaction of passive measures and energy-efficient, active systems. With the monitoring, the systems should be optimized in continuous operation with regard to energy consumption and comfort. In addition, a calibration of the calculations that took place beforehand can also take place.

Another point of optimization will be the lamella facade. Here it is necessary to check whether the energy gained justifies the associated effort by the tracking.

Construction costs and economy

The investments amounted to approximately 550,000 euros. The costs were financed by industrial sponsoring, by donations, as well as through research funding. The comparatively high costs of 10,000 € / m² heated area are due to various factors: the prototype development, an extremely small area, the use of novel systems and materials, the complex technology for building control and monitoring, the competition-based grid-connected solar system and last but not least the competitive quality equipment.




Project participants

The development and construction of the solar power off-the-grid prefab home was done in collaboration and with the help of generous direct and indirect sponsorship. From the very beginning, the approach of integrative planning and the harmonization of the requirements of architecture, competition and technology was in the foreground in order to create a viable Energy Plus residential building concept.

TU Darmstadt

The Technische Universität Darmstadt, commonly referred to as TU Darmstadt, is a research university in the city of Darmstadt, Germany. It was founded in 1877 and received the right to award doctorates in 1899. In 1882, it was the first university in the world to set up a chair in electrical engineering.

AddressKarolinenpl. 5, 64289 Darmstadt, Germany

About Solar Dechatlon Team Germany

AddressEl-Lissitzky-street 3, 64287 Darmstadt, Germany

Designing and Energy Efficient Building

Prof. Manfred Hegger
Isabell Schä

Monitoring partners: soap architecture GbR sustainability office for architectural projects


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Solar Decathlon Europe

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SDE advocates are working together to propel the mandate on sustainability.
There is an ambitious group of people with the same goals and vested interest
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Foundation, located in Rotterdam, Netherlands.

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Solar House - Solar Power Off-the-Grid Prefab Home