The purpose of this study was to determine and compare the performances of conventional and heat pump systems for energy and water conservation in open and confined greenhouse systems. Conventional gas furnaces and evaporative cooling, respectively, provided heating and cooling in the conventional system. In the heat pump systems, gas-fired heat pump units provided both heating and cooling. The greenhouse with heat pump units also had an option to be operated as a completely confined system, using one of the heat pump units as a dehumidifier. Average energy consumption in winter was 56.9, 23.5, and 11.3 MJ/day.m² in the conventional, open, and closed loop (confined) heat pump systems, respectively. In spring, the same greenhouse systems averaged 20.6, 8.8, and 5.3 MJ/day.m², respectively. In summer, energy consumption was considerably less than those predicted for winter and spring. The major portion of the total energy consumption was for heating. Daily transpiration rates averaged for winter, spring, and summer were 2.09, 2.02, and 1.02 kg H2O/day.m² in the conventional, open, and closed loop heat pump systems, respectively. In the closed loop system, almost all the transpired water was recovered on the cooling coils, making the overall water consumption in this system essentially zero. This study concluded that greenhouse operation with the heat pump system has a great potential for enormous energy, water, and CO2 savings, as well as cooling and dehumidification. A confined greenhouse with a heat pump system would make it easier to control the humidity and keep high CO2 levels, while reducing the risk of insects and diseases. A combination of open and closed loop systems would be the most energy conserving system, operating the system as a closed loop system in winter, spring and fall, and as an open system in summer.


Bioresource and Agricultural Engineering | Horticulture

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