HEAT PUMPS FOR THE HEATING AND NIGHT-COOLING OF GREENHOUSE CROPS: A SIMULATION STUDY
The use of air-to-air heat pumps in greenhouses was investigated using a computer simulation model and weather 
data for Raleigh and Wilmington, North Carolina. A greenhouse model taken from the literature, along with the heat pump subroutines 
developed in this study, were used for the simulations. Heat pumps were investigated for heating only and heating plus 
night cooling, using both standard and time-of-use electrical rates for two North Carolina utilities. Three capacities of heat 
pump were examined: 76.6, 38.3, and 19.2 W m-2 floor area. The results suggest that heat pumps installed at a capacity of 19.2 W 
m-2 may be economically viable for the locations studied, yielding pre-tax returns on investment of 20.4% and 16.3% for 
heating-only in Raleigh and Wilmington, respectively, using LP gas costs of $0.185 per liter ($0.70 per gal). When LP gas costs 
of $0.264 per liter ($1.00 per gal) were used, these values increased to 36.8% and 32.8%, respectively. Using the heat pumps 
for night cooling increased return on investment to as much as 49.7% due to increased yields; however, a market for the increased 
production must exist for this to be realized. Time-of-use rates were an advantage when the heat pumps were used for heating 
only, but when night cooling was employed there was no consistent effect. Heat pumps used for heating only yielded larger 
returns on investment in Raleigh than in Wilmington because the cooler climate in Raleigh allowed for greater heat pump 
utilization. When night cooling was added, the returns were higher in Wilmington than in Raleigh for tomato but slightly lower 
for chrysanthemum. The difference is attributed to the greater heat sensitivity of tomato compared to chrysanthemum.