Evaluation of a hybrid system for a nearly zero energy greenhouse

Greenhouses are widely used in the World, especially in the Mediterranean climate, to provide suitable environment in cultivation of different agricultural crops. Significant amount of energy is necessary to produce, process and distribute these crops. Various systems, including steam or hot water radiation system and hot air heater system, are being used in greenhouse heating. A ground source heat pump system, generally seen as a favorable option since it can provide both heating and cooling energy, is considered for a greenhouse in this study. The aim of this study is to evaluate a renewable energy option for the required total energy need of a greenhouse. Grid connected solar photovoltaic panels are selected to assist a ground source heat pump, and generate sufficient electrical energy for lighting. In this way, a nearly zero energy greenhouse concept is foreseen for three different agricultural products. Monthly and annual heating, cooling and lighting energy load of the greenhouse for these agricultural products were computed. The monthly average electricity generation of 66 photovoltaic panels, which cover 50% of the southern face part of the asymmetric roof, was calculated. Annual photovoltaic electricity generation was found as 21510.4 kWh. It was observed that photovoltaic electricity generation can meet 33.2-67.2% of greenhouse demand in summer operation months. Nevertheless, the coverage ratio, calculated by dividing the photovoltaic panels electricity generation to the electricity demand of the greenhouse (heating, cooling and lighting) for each crop, were very high in winter operation months. Yearly coverage ratio values were 95.7% for tomato, 86.8% for cucumber and 104.5% for lettuce. These high coverage ratio values justify the nearly zero energy concept for the considered greenhouse. Economic and environmental evaluation of the considered system were also accomplished. A simple payback time of the crop cultivations was computed between 7.0 and 7.4 years. The energy payback time of the system was found to be 4.9 years and the greenhouse gas payback time value of 5.7 years and 2.6 years were calculated, based on natural gas and coal based electricity generation, respectively.