OPTIMUM CONTROL OF GREENHOUSE HEATING

B.J. Bailey, I. Seginer
A strategy is outlined for changing the greenhouse temperature in a way which reduces energy consumption but maintains the temperature and the temperature integral within prescribed limits relative to the values achieved using conventional heating control based on a fixed temperature. The method is based on the assumption that plants will tolerate limited, short term deviations in temperature and respond to the average value. Optimum control theory was used to define a control parameter representing the marginal cost of obtaining a unit increase in the temperature integral. The optimum value of this parameter was established by simulation using an appropriate thermal model of the greenhouse and actual weather data for the United Kingdom. The control is implemented by calculating the actual marginal cost and comparing it with the optimum value. If the marginal cost exceeds the optimum value the greenhouse temperature is lowered to the limits imposed by the constraints on temperature and temperature integral and vice versa. This control strategy was investigated by simulation for the following greenhouse systems.
  1. Normal greenhouse for which wind speed influences heat loss.
  2. Greenhouse with a thermal screen
  3. Greenhouse with a base load heater using cheap energy and peak demand heater using a more expensive fuel.

Using temperature limits of 15 and 25°C and a maximum deviation of the temperature integral, or temperature exposure, of 50 degree hour gave annual energy savings of 3 and 15% for the first and second greenhouse systems and a 20% reduction in the fuel used by the peak demand heater in the third system. This technique appears to offer useful reductions in energy by shifting some heating to periods when the rate of heat loss from the greenhouse is reduced. As it requires only software, the method should be relatively easy to implement using environmental control computers.

Bailey, B.J. and Seginer, I. (1989). OPTIMUM CONTROL OF GREENHOUSE HEATING. Acta Hortic. 245, 512-518
DOI: 10.17660/ActaHortic.1989.245.69
https://doi.org/10.17660/ActaHortic.1989.245.69

Acta Horticulturae