Ongoing developments in greenhouse climate control

Passive (unheated) greenhouses are typical of mild winter climate areas. Passive greenhouses seek environmental sustainability by reducing inputs of energy and materials. Besides, they have to be economically viable. This paper reviews recent studies on passive techniques and their effect on the night time greenhouse climate: The effect of covering materials properties on temperature and humidity, humidity issues in semi-closed greenhouses and the role of thermal inertia are examined. Research studies show that most passive techniques give moderate temperature rise (in the range of 1 to 4°C). Even though such effect may seem meager, relevant benefits are derived by extending the growing period, increasing yield and ensuring frost protection. In active high technology greenhouses of cold areas, one of the main focuses is energy saving, and for that purpose, a new generation of semi-closed greenhouses is under development. Main efforts for energy saving are the reduction of heat losses by making greenhouses tighter (with multiple covers), intensive use of screens to minimize radiative losses at the expense of maintaining higher ambient humidity values. Canopy condensation is prevented by means of different dehumidification systems, such as fans that drive cold/dry air from above the screens to the canopy area or systems based on the use of heat exchanges to drive external preheated dehumidified air to the canopy area with the help of perforated sleeves, among other systems which are preferred over rising the heating set point.