IMPROVED CONTROL STRATEGIES FOR GREENHOUSE THERMAL SCREENS
A comparison was made of the simulated performance of 3 control strategies with normal temperature regimes which could be used to operate greenhouse thermal screens. The energy required to produce an early tomato crop in the UK was calculated using a model of the heat loss from a greenhouse and hourly meteorological data. Reductions in yield, caused by reductions in light reaching the crop, were quantified using data from recent horticultural experiments. The most effective method of controlling both transparent and high performance aluminised thermal screens was to close the screen whenever the value of the energy saved exceeded the value of the crop lost. Almost as good was operating the screen at the most appropriate fixed value of solar irradiance; delaying the opening and advancing the closing of the screen by a fixed time interval was least satisfactory. An alternative heating strategy involved reducing the day heating setpoint temperature and increasing the night temperature, when the thermal screen is used, to achieve the same 24 hour average temperature as occured in the unscreened greenhouse. This gave a greater energy saving with no loss in yield provided the plants respond to the average temperature. The best control strategy was a combination of optimised control with reduced day and increased night temperatures. This was the most profitable giving net reductions in heating costs of 26–41% depending on the screen and the reduction in day temperature compared to 21–33% when the screens were used from sunset to sunrise.
Bailey, B.J. (1988). IMPROVED CONTROL STRATEGIES FOR GREENHOUSE THERMAL SCREENS. Acta Hortic. 230, 485-492