Effects of the photovoltaic roofs on the greenhouse microclimate

The installation of photovoltaic (PV) arrays on the greenhouse roof allows the farms to increase their competitiveness, by producing income from both crops and renewable electricity generation. This led to the spread of PV greenhouses in Southern Europe, often aiming at maximise the income deriving from energy production. In this study we investigated the solar radiation and temperature inside an east-west oriented greenhouse with 50% PV coverage, located in Sardinia, Italy (39°19’59”N, 8°59’19”E). The south-oriented roof was completely covered with multi- crystalline silicon PV panels, amounting to 68 kWp rated power. A high-light demanding crop (cherry tomato, Solanum lycopersicon L. ‘Shiren’) was chosen for comparing the environmental data with the achieved yield. The PV array decreased the yearly sunlight availability inside the greenhouse by 64%, compared to the situation without PV panels, while the temperature was averagely 2.8°C higher than outside. The temperature remained uniform over the greenhouse area, while the solar radiation was distributed following a north-south gradient, characterised by higher values on the sidewalls, and decreasing towards the centre of the span. The solar radiation under the conventional plastic roof was 305% higher than under the PV roof, causing a high variability of total production between the plant rows, which ranged from 1.9 kg m-2 in some rows under the PV cover, where plants showed a negative photosynthetic rate (up to -3.72 mmol CO2 m-2 s-1), to 5.1 kg m-2. The results suggested new design criteria for PV greenhouses, concerning the decrease of the PV array coverage and different installation patterns of the PV panels on the roof. Furthermore, the crop management in terms of irrigation should be adjusted for every plant row, according to the observed yield variability and the actual incident solar radiation. These measures can contribute to increase the agronomic sustainability of PV greenhouses.