Process-based humidity control regime for greenhouse crops

Modern greenhouses in The Netherlands are designed for efficient use of energy. Climate control traditionally aims at optimal crop performance. However, energy saving is a major issue for the development of new temperature regimes. Temperature integration (TI) results in fluctuating and often high relative humidity (RH) levels in modern, highly insulated greenhouses. At high temperature, water vapour pressure deficit (VPD) is usually high and RH consequently low and vice versa. Relatively lows fixed set points (80-85% RH) for air humidity as is common practice, may strongly influence the efficiency of TI, because heating and/or ventilation actions are required to control humidity rather than temperature. This requires much energy. Fluctuating RH may affect crop performance in several ways. Too low VPD may reduce growth due to low transpiration and associated physiological disorders. Water vapour pressure above the dew point leads to condensation on the relative cooler plant tissue and this may give rise to diseases. High VPD, on the other hand, may induce high stomatal resistance and plant water stress (PWS). The aim of the present research was the design of a process-based humidity control concept for a reference cut chrysanthemum crop cultivated with TI. RH control set points were generated as function of underlying processes. Greenhouse performance with this humidity regime and different temperature regimes were simulated with respect to greenhouse climate, energy consumption and photosynthesis. Compared with a fixed 80% RH set point, annual energy consumption of a year-round cut chrysanthemum cultivation could be reduced by 18% for TI with +/-2 degreesC temperature bandwidth as well as for regular temperature control. For separate 12 week cultivations with planting date 1 March, energy saving could increase up to,27 or 23% for TI and regular temperature, control, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.