Modelling solar energy storage systems for greenhouses

Abstract This paper presents the development and validation of a computer model used to describe a greenhouse with a thermally coupled energy store. The physical system modelled consisted of a 6·7 × 12·2 m greenhouse with a 3·0 × 10·0 × 1·8 m rockbed attached via insulated ducts. Provisions were made to circulate air from the greenhouse through the rockbed and back whenever heating or cooling were required. When the capacity of the bed was reached, heating and cooling were accomplished in the normal manner. The computer model was based upon two existing models, one for the greenhouse and one for the rockbed. Modifications were made to each to increase their efficiency and to account for the unique aspects of this system. Validation was accomplished for three separate days comparing observed data with predicted results. Fossil fuel consumption was predicted to within 8·3% and solar energy storage and recovery to within 10·6% and 9·6% respectively. Mean differences between predicted and observed inside temperature, inside relative humidity, and rockbed temperature were 0·4°C, 4·0%, and 0·4°C respectively. A sensitivity analysis was conducted by varying the rate of moisture evaporation within the rockbed, the rate of air infiltration within the greenhouse, the outside convection coefficients of the greenhouse cover, and the rate of moisture input to the system from the plants (stomatal diffusion resistance). The results showed that only greenhouse relative humidity was greatly affected by the evaporation rate within the rockbed, but that relative humidity and rockbed temperature were very sensitive to infiltration rates and outside convection coefficients. Rockbed temperature, and therefore total energy stored, was only slightly sensitive to the rate of moisture input from the plants; however, fossil fuel energy consumed was significantly affected. Inside temperature was relatively insensitive to all of the factors varied.