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Optimizing air-conditioning for greenhouse horticulture

The drive to lower greenhouse energy consumption is spawning new techniques and applications. For instance, air-conditioning systems commonly used in buildings are also finding their way into greenhouses, where they control temperature, humidity and air circulation. However, the systems have to be adapted to the totally different climate conditions. TNO is investigating how to evaluate the systems effectively.

Greenhouse growers are turning increasingly to air-conditioning systems or units, which use the mechanical mixing ventilation principle. The air drawn in from outside by a fan eventually enters the greenhouse. First, the inlet air is heated to greenhouse temperature to prevent any internal temperature differences, and only then distributed evenly around the greenhouse through hoses. The systems also control air humidity, which affects how well plants absorb nutrients, for instance. And if humidity is too high, plants are unable to transpire properly, and there is a greater risk of disease. The traditional way for greenhouse growers to lower humidity is to open vents at the top of the greenhouse. With an air-conditioning system the vents need opening less often and less wide, since the outside air drawn in by the fan happens to be dryer than the air inside the greenhouse. An additional advantage is that less CO2 is lost, which means greater production with less energy consumption. The flow of incoming air also keeps the air moving around the plants, which affects the temperature distribution and the water vapour discharge.

More effective use

The optimum control of these greenhouse climate factors with air-conditioning systems calls for further development. TNO is now performing a study on site at Themato, a tomato grower with a closed greenhouse system, on the climate effects of the Climate Optimizer system from the manufacturer and project partner Priva. This study is intended to lead to more effective use of the system in practice. The research data will also be used for developing a model for evaluating studies of other systems, with a view to making reliable comparisons. The model is expected to be available in 2010.

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