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.