Evaluating Energy Savings Strategies Using Heat Pumps and Energy Storage for Greenhouses

As energy costs are increasing, many greenhouse operators are re-evaluating energy consumption and savings strategies. In most cases, updating older heating systems to more efficient units in addition to the use of double layer glazing, insulation materials, and energy curtains significantly reduces fuel consumption. Insulating greenhouses must not conflict with the need for high light transmission through the structure. Since solar radiation loads often significantly exceed the instantaneous heat requirement of a greenhouse, many ideas have been proposed to capture this excess heat and store it for later greenhouse heating. Heat pumps are promising for use in an integrated cooling and heating system. In the study described in this paper, a simple spreadsheet approach was used to evaluate the performance of a system utilizing a heat pump and water storage. The evaluation bases its calculations on historic hourly weather data to determine hourly cooling and heating rates and storage status. The calculations allow for evaluations of the appropriate size of the heat pump, storage device, and heat exchangers. The calculations are used to investigate storage capacities that are sized for one to a few days harvest of surplus heat from the greenhouse for a range of percentages of peak cooling requirement. The model includes the option of utilizing a geothermal source for the heat pump to charge the storage during periods when greenhouse cooling is not required. The first study presented examines the impact of increasing thermal storage capacity on heat utilization from a generic co-generation system. The second considers a specific, natural gas fired, fuel cell system for various sizes of greenhouse at two different locations and includes the utilization of CO2 from the reformer section. The heat pump study looks at the relationships between capacities of the heat pump and storage for two different locations. Provided hourly weather data are available for other sites, the spreadsheet approach can be used for other locations across the world.