In summer, greenhouses have to deal with an excess of solar energy which is mostly discharged by ventilation. In moderate climates, on a yearly base this discharge of energy is comparable to the energy demand for heating. Thus, in times of growing awareness of the scarcity of fossil fuels, harvesting and storing of summertime heat excesses for application in winter seems to be a promising technique. Preferably the harvesting units are integrated in the greenhouse design because this enables the shared use of space and supporting constructions and the extraction of excess heat can improve the inside climate conditions, especially when one is trying to increase the inside CO2-concentration to above outside levels. However, although the concept sounds easy, in practice a lot of difficulties have to be overcome since there are strong limits to the affordable expenses, giving the value of the energy harvested. Moreover, the harvesting of summertime excesses and the application of the (low thermal) heat results in an important electricity demand for driving ventilators and a heat pump. This means that the ratio between heat and electricity demand shifts to the latter, which is unfavourable because of the much higher value of electricity compared to heat. Nevertheless, with a carefully designed energy harvesting greenhouse, promising opportunities appear to be achievable, especially when smart choices are made around the greenhouse air temperatures and humidity control. This paper presents the reasoning of such a design called the Sunergy Greenhouse. The proposed design was built as a 550 m2 demonstration object and has been in operation since June 2008. In this paper a number of results are presented and commented. Moreover, based on the observations, a simulation model was developed. With this model, amongst lots of other things, the impact of the prices of gas and electricity on the affordable costs of energy harvesting can be studied. The results, presented in this paper, help to understand business economical considerations.
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