Photovoltaic Cooling Greenhouse Adapted for Horticulture in the Sahel

Abstract

Photovoltaic greenhouse is a breakthrough technology that creates a synergy between agriculture and energy sectors worldwide. In harsh Sahelian environment, conventional greenhouse generates a microclimate that gets saturated and heats up, becoming unbearable for crops. This work aims to develop an effective and affordable cooling photovoltaic greenhouse for year-round horticulture in the Sahel. The study investigated tomatoes production under the microclimate conditions inside a greenhouse in which 20% of the roof area was replaced by photovoltaic panels and the remaining part covered by a green semi-transparent ligno-cellulosic material in order to reduce the intense local solar irradiance. A cellulosic evaporative cooling pads system powered by poly-crystalline modules of 1.56kWp was used to decrease indoor heat. The greenhouse’s microclimate parameters such as temperature, relative humidity and solar irradiance were recorded with the aid of a weather station, thermo-hygrometer sensors installed inside and outside the greenhouse, and the whole greenhouse system thermodynamic behaviour was simulated through computational fluid dynamic (CFD) software (ANSYS). The carbon dioxide (CO2) contents and grown tomatoes physical parameters inside and outside the greenhouse were also regularly measured. While the outside temperature was around 34.2 – 41.6oC with an average relative humidity of 46.13%, the temperature within the greenhouse was around 29 - 32.8oC with an average relative humidity of 72.24%, causing respectively a temperature drop of 5.2 - 8.8oC for an average relative humidity improvement of more than 26%. The thermal heat distribution and flow patterns showed a well-distributed heat around crop’s coverage area in the greenhouse when analysed under CFD. The CFD analysis allowed to notice that when exposed to the highest solar irradiation event (1355.6 w/m2, 34.3oC and 71% relative humidity), the greenhouse cooling system could create a drop of 4.9 oC and an increase of 15% relative humidity. The lowest temperature attained in the greenhouse was 15.7 oC (at night) whereas the highest temperature (in the afternoon) was 37.8oC against 40.8oC outside. The measured CO2 contents (700 - 920 ppm) and solar irradiance (120 - 220 Wm-2) inside the greenhouse are suitable for plant growth and main horticulture crops production under very sunny Sahelian conditions. A simultaneous tomato crop cultivation (Variety Mongal) inside and outside the greenhouse showed that plants inside the greenhouse could reach 190 cm height with leaves size of Length x Width (LxW) = 10x5cm against 70 cm high and LxW = 6x2.5 cm for outside plants’ physical parameters. This situation resulted from the plant photosynthesis improvement due to sufficient carbon dioxide availability under adequate sun irradiance. Tomato fruits inside greenhouse turned from green to deep red at harvesting stage whereas under ambient conditions tomatoes colours turned green, light yellow then red or light red due to extreme conditions. Energy production for cooling averaged around 4199.016 Wh/day despite the observation of 7620.792 Wh/day of energy loss due to full battery state. Optimization is necessary for cost effectiveness of the greenhouse cooling system.