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The Watergy project is funded by the European Community's Vth Framework in its Energy, Environment and Sustainable Development programme. It consists in the development of a humid air solar collector system that follows the principle of a closed two phase thermosyphon. A combination of evaporation and condensation allows to use solar thermal energy in a much more efficient way. The main advantage is not only the reduction of costs in space cooling and heating, but the possibility of water purification, as the system can be fed with low quality water to obtain distilled water. The decentralization of heat and water supply opens the possibility of residential areas where greenhouses fed with grey water could be used to produce distilled water as well as heat and fruits.

The project contemplates the development of two prototypes: one application for arid climates in Southern Europe with an emphasis on water production in the context of greenhouse horticulture, and another for temperate Central European climate focused on heat and water production for sustainable architecture.

The first prototype is a closed greenhouse for solar thermal energy capture, water recycling, water desalination and advanced horticultural use. It is already constructed in Estacion Experimental de Cajamar in Almeria (Spain), and functioning since the fall of 2004. The system allows to control the climate inside the closed greenhouse as well as closing the water cycle with the recovery of all the evapotranspiration from the plants. This opens a very interesting possibility for sustainable management of water in intensive horticulture, as the greenhouse irrigated with grey water becomes a means of producing not only of fruits but also clear water. Alternatively, if grey water is left out of the system, the greenhouse can reduce greatly its water consumption with the reuse of the recovered distilled water.

The second prototype is under construction in Berlin (Germany), and it is a building with an autonomous supply of heat and also of clear water. In this case, the closed greenhouse is connected to the building and purifies its residual grey water. Beside its main function as solar collector and water distiller, the greenhouse provides fruits and can be fed with residual air from the building. The more efficient collection of solar thermal energy in the system and its seasonal storage allow for a passive climatization of the building. In the context of sustainable architecture, the Watergy system means that this concept of zero energy is complemented with that of water autarchy.