Apart from harnessing solar thermal energy to directly (through heated greenhouse air) or indirectly (through heated water) meet the greenhouse heating demands, energy from the incident sunlight can also be converted to electricity through the photovoltaic (PV) effect. Despite the abundant amounts of solar energy reaching the earth, on the local scale, it is intermittently available over the day and around the year - this means that the electricity generated during periods of high availability would need to be stored using battery systems to supply it during times of deficient availability.
Moreover, a huge challenge is the conversion efficiency from solar energy to electrical energy of the various available PV modules - for commercially available crystalline-silicon PV modules this is in the range of 15 - 25%. PV modules based on other materials such as perovskites and organic materials are still under research and development.
The overall photovoltaic electricity production depends on the efficiency of the PV modules used, geographical location (determining solar radiation incidence), time of the year, orientation of the PV array, shading, temperature, and maintenance (i.e. removing dust/dirt).
The performance of solar panels, such as their reported power rating (in watts) is calculated under Standard Test Conditions (STC) - at a cell temperature of 25°C and an irradiance level of 1000 W/m2. In real-world use, cell temperature is affected by the air temperature, wind speed, and solar radiation on the cell, and is generally more than 25°C. During sunny weather, the internal cell temperature can typically go 20-30°C higher than the ambient air temperature, reducing the power output by ~8-15%, based on the type of solar cell. This loss is quantified by the ‘temperature coefficient', which gives the percentage change in power generation for every degree of temperature increase/decrease from 25°C, measured in %/°C. In the best-performing hetero-junction PV modules, this goes as low as -0.25% /°C, while typical values range from -0.29 and -0.5 %/°C.
For application to greenhouse horticulture, an important consideration is that PV modules and crops, both compete for the same resource i.e. solar radiation. It is therefore not recommended to mount PV panels on the greenhouse roof.
Recently, there is significant research dedicated to semi-transparent PV panels that allow partial transmission of wavelengths considered important for photosynthesis, while using the rest for electricity generation. However, such panels still decrease the overall PAR transmission, compromising with crop production. PV panels can be mounted on administrative buildings, offices, or parking spaces associated with greenhouses.
Contribution to energy balance and resource use of greenhouses:
Electricity generated from PV modules can be used to supply power to climate-conditioning equipment such as heat pumps, forced ventilation, mechanical dehumidification, as well as for artificial illumination, which contribute to the internal climate of the greenhouse and its energy and resource use.
However, with limited efficiencies of PV modules and with constraints on the area available for mounting the PV panels, the total electrical demand of the greenhouse is difficult to be met completely by PV modules alone, and other sources of electrical power are required.
Possible steps towards sustainable, energy-efficient greenhouses
The development of more efficient PV modules would enable much better utilization of the freely available solar energy to power the greenhouse climate-management in a sustainable and energy-efficient way.
If electrical power is generated in excess of the electrical demand of the greenhouse, this can be stored in batteries to be used during times of lower PV power output due to reduced availability of solar energy (e.g. at night-time and in winter months).