SIMULATED PERFORMANCES OF A HEAT PUMP SYSTEM FOR ENERGY AND WATER CONSERVATION IN OPEN AND CONFINED GREENHOUSE SYSTEMS

Abstract

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 2 consumption in winter was 56.9, 23.5, and 11.3 MJ/day.m in the conventional, open, and closed loop (confined) heat pump systems, respectively. In spring, the same green­ 2 house systems averaged 20.6, 8.8, and 5.3 MJ/day.m , 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 2 H2O/day.m in the conventional, open, and closed loop heat pump systems, respec­ tively. 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. INTRODUCTION To provide economically optimal micro-environments for plant growth, producers can use or control the number of glazing layers, insulation curtains or screens to reduce long-wave radiation losses at night, reduced ventilation rates, evaporative coolers, and shading devices to control incoming solar radiation. In addition, a Rankine cycle heat pump that was developed by Yildiz (1993), and Yildiz et al. (1993) holds promise for reducing winter heating requirements and warm weather cooling loads. These systems involve complex tradeoffs between initial and operating costs for cooling and heating, plant responses to various environmental factors and the strategies used to regulate temperature, humidity and CO2 levels in the crop canopy. Special attention must also be given to the operational strategies associated with the use of heat pumps, especially in maintaining acceptable relative humidity levels within greenhouses. A dynamic simulation model was developed and validated to provide an accurate prediction of greenhouse energy and moisture exchanges as a function of dynamic environmental factors (Yildiz and Stombaugh, 2006). This model was used to predict heating and cooling loads, water use, and to evaluate the operational strategies associated with heating and cooling using the proposed heat pump and a conventional system. The heat pump was evaluated for both open and closed loop systems, and these were compared to a conventionally ventilated and heated greenhouse. The overall objective of this study was to compare conventional, open loop and closed loop heat pump greenhouse systems in terms of energy and water conservation.