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This paper describes the basic operation of a new type of closed greenhouse for solar thermal energy storage, water recycling, water desalination and advanced horticultural use. A system of constant air humidification enables the transfer of large amounts of energy via latent heat from the greenhouse to a thermal storage. To accomplish this, only little air velocity is required and can be provided by a buoyancy driven air circulation system. It is realised by a combination of a cooling tower and a secondary heat collector. In this heat collector the air is humidified to the maximum level before the process of heat exchange. INTRODUCTION Greenhouse production is a highly profitable technology for food production in Central and Southern Europe, but is facing growing challenges caused by increasing energy prices (for heating, transport) and limited water resources (for irrigation). Desalination is a technology used to supply future growing demands for water, but common technologies have a large demand for primary energy. A widely discussed technological solution for these challenges is the idea of heat and water recovery from greenhouse exhaust air, or from the inside of closed greenhouses, aided by heat exchangers and heat accumulation systems (Warshall, 1996). The major challenges of such endeavours can be described as follows: Usually, a large amount of electric energy is needed for ventilation to reach a sufficient transport of hot air to a heat exchanger. The primary energy needed for this transport is close to the primary energy of the heat that can be captured. Because of this, such a system is only marginally effective. (Mackroth, 1982) Air has little heat capacity. Therefore, any transfer of heat from air to water or to any other storage media is little effective. Greenhouse temperatures are limited due to crop tolerances, a situation that leads to very low supply temperatures and therefore low efficiencies of heat exchangers and thermal storage systems. Placement of heat exchangers in the roof zone, representing the hottest area of a greenhouse, originates unwanted shading of the plants. Additionally, the heat transfer between the rising hot air and the falling cooled air from the roof zone results in insufficient cooling in the lower plant zone. NEW GREENHOUSE ELEMENTS The Watergy system (Buchholz, 2000) contains two innovative elements: a greenhouse cooling tower and a secondary collector element. These elements aim at working against the challenges mentioned above: The cooling tower is used to overcome the need for electric fans by creating free, buoyancy driven ventilation. It consists of an air duct that drives heated air from the greenhouse to the top of the tower and an internal cooling duct with a large air-to-water heat exchanger made of plastic, which is connected to a heat accumulator. The air passes, driven by the buoyancy force, from the greenhouse into the chimney. After being cooled Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 510 by the heat exchanger, the air falls back ground level. Vapour from the greenhouse condenses, releasing thermal energy and producing distilled water. The resulting cool dry air can circulate within the system. The cycling process can be used for greenhouse cooling, air dehumidification and energy transport to the heat exchanger. Using such a cooling duct, the produced cool air can be directed to the lower volume of the vegetation area without being mixed with the hotter air, which rises inside the greenhouse. The constant process of cooling and condensation along the whole duct also constitutes an effective means of dehumidification of the greenhouse air. A secondary collector aims at further heating and humidification of the greenhouse air while on its way to the cooling tower, resulting in a higher efficiency of the heat transfer and –storage processes. Higher temperatures result in higher heat exchanger supply temperatures and in a higher loading capacity of the storage system. A higher absolute humidity means a larger amount of thermal energy available, while it is transported with the same amount of air movement. In other words, less total air flow is needed for the transfer of solar energy from the secondary collector to the heat exchanger. Additionally, the higher humidity also results in a larger amount of energy that is released directly at the heat exchanger surface during the condensation process without thermal barriers of laminar air zones at the surface, and thus resulting in a higher total transfer rate of the heat exchanger. Since the secondary collector is separated from the vegetation area, salty water can be used to feed the evaporation process of the secondary collector for desalination purposes. Through working with large amplitudes of salinity between the storage loading and de-loading phase, also endothermic and exothermic effects within the brine can be used as a part of the energy storing process. PLACEMENT OF THE SECONDARY COLLECTOR The placement of the secondary collector is dependent on the amount of irradiation. In areas with high irradiation, the collector can be placed above the vegetation zone and can be operated like a flexible shading system: Components of the greenhouse roof like a plastic layer and a water film can already filter a relevant part of the incoming radiation. If needed, additional shading can be provided by conventional shade curtains underneath the plastic film, or by mixing shading additives into the water film that can be added/removed mechanically in a controlled way. Both methods aim at using the solar radiation to heat up the water film for a consistent proportion of air heating and air humidification. The secondary collector is limited to the basal surface and to a certain capacity, because of its position above the crop area and the need for as much radiation as possible for plant growth. On the other hand, the advantage of an installation above the plants is the possibility of having large, continuous greenhouse surfaces. For areas with limited irradiation conditions (e.g., Central Europe), light is most of the time already a limiting factor for plant growth. Therefore, the secondary collector surface has to be placed outside the greenhouse. The resulting flexibility in the size of this collector surface allows for it to be sized in relation to the crop production area and to the desired crop production temperatures. Since this area without plants is required in such a configuration, it is useful to think about an additional use of such a surface. A logical application can be integrating building facades and –roofs in combination with greenhouses within an urban context. In the Watergy project, both versions will be tested within two different prototypes. The first one is a single greenhouse for which the main focus is thermal control and water production. The thermal energy, that is stored during the day will be utilised for greenhouse heating at night during wintertime and for further water evaporation/condensation at night during summer, in particular evaporation of salty water for desalination purposes on the heat exchanger and condensation on the greenhouse cover. By unloading the storage at night, coolant for the next day is made available in both cases. (Saitoh, 2000)