Photovoltaic blinds to improve greenhouse energy performance

Scientists from Iran’s Shiraz University and Mälardalen University, in Sweden, have analyzed how a PV blind system can reduce the gas and electric demand of a rose greenhouse while driving down CO₂ emissions.

The researchers considered 14 configurations of PV blind in checkerboard arrays 1m above a greenhouse roof modeled with EnergyPlus building energy simulation software, in Shiraz, Iran. “Utilizing PV panels in a checkerboard array provides intermittent shadows on plants but has no effect on the plant growth and using PV trackers above the greenhouse generates more electricity,” the group stated adding, the panels also reduce greenhouse temperature.

The greenhouse modeled included heating from three natural gas, direct-combustion boilers and two pumps. The panels used in the model were 60-cell devices measuring 1.64×0.99×0.04m and offering power output of 260 W at 15.9% efficiency. The devices would rotate on a longitudinal axis and be able to shift into shade mode, if illumination on the plant canopy exceeded a maximum value, or into unshaded operation if illumination was below a minimum value.

The researchers analyzed the modeled building’s indoor temperature, relative humidity, infiltration ratio – which measures the movement of air into and out of greenhouses – maximum illumination of plant canopy, electricity and natural gas demand and PV generation.

The results indicated a PV system with 22.4% roof coverage would be able to generate 47.1 kWh/m² annually – 1.29 times more than the modeled greenhouse’s power demand. Grid electricity would still be needed, though, as storage was not considered in the modeling, due to costs. The researchers said grid power costs could be offset by the sale of surplus energy into the power network.

The best greenhouse configuration modeled saw the PV system meet 46% of power demand. The weakest configuration offered 31.9%.

“Results indicate that covering 19.2% of the roof, with no significant change in the illumination level on the plant canopy, will annually reduce natural gas consumption, electricity demand and CO₂ emission by 3.57%, 45.5% and 30.56kg/m², respectively,” the group stated.

The findings were presented in the paper Thermo-environomic assessment of an integrated greenhouse with an adjustable solar photovoltaic blind system, published in Renewable Energy and on the ScienceDirect website.