Thermal performance and economic analysis of saw-tooth and photo-voltaic roof type greenhouse integrated with Earth air heat exchanger

Abstract

In the present study, comparison of single span saw-tooth and PV-roof type greenhouse assisted with Earth air heat exchanger (EAHE) was carried out for tropical climate based on thermal performance, sunlight availability, tomato crop yield and economical aspects. For natural circulation mode in summer, the average temperature at crop height, light illumination level and relative humidity inside sawtooth type greenhouse was 38.5 °C, 10,000 Lux and 30 % respectively. For PV-roof type greenhouse, the average temperature at crop height, light intensity and relative humidity was 40.2 °C, 8000 Lux and 28 % respectively. The sawtooth and PV-roof type greenhouse with EAHE assisted mist fogging was able to maintain inside temperature 10 °C and 12 °C below the ambient conditions respectively. The maximum coefficient of performance (COP) of EAHE was 2.52 in the afternoon hours. The conducive environment with indoor temperature below 35 °C, relative humidity between 60–70 %, sufficient light intensity levels above the light compensation point and duration of exposure of 8–10 h/day led to 32 % higher tomato yield from the saw-tooth and PV-roof type greenhouse compared to the outdoor conditions. The CFD simulations revealed that the presence of recirculation zones within the greenhouse played crucial role for the uniform temperature distribution. In these recirculation zones air followed a circuitous route to the exit and ventilation rate was governed by the air residence time. The reduced incident solar radiation access to the soil for PV roof type greenhouse had created an environment that allowed better exploitation of the earth-to-ground heat exchanger. The discounted payback period for the saw-tooth type and PV roof type greenhouse with EAHE was 5.4 years and 5.2 years respectively. However, for a large-scale PV-roof greenhouse farm integrated with EAHE system, a more lucrative cost benefit analysis and improved payback period can be achieved.