Integrating photovoltaics into greenhouses is not simply a matter of determining how much electricity a solar roof can generate. It also requires assessing how much radiation reaches the plants, under what microclimatic conditions, and how this affects their water requirements. A project developed by the University of Jaén (UJA) in Spain has investigated this interaction using two commercially available semi-transparent photovoltaic module technologies.
The research, funded by the Regional Ministry of Universities, Industry, Energy and Innovation of the Government of Andalusia, compared cadmium telluride (CdTe) and amorphous silicon (a-Si) modules installed as roofs on small experimental greenhouses. The results indicate that photovoltaic generation can significantly alter the indoor microclimate, with temperature reductions of up to 5 C and lower water losses through evaporation.
The study, published in AgriEngineering under the title “Spectral Selectivity and Microclimatic Buffering of Semi-Transparent Photovoltaics in Greenhouses: A Comparative Analysis of CdTe and a-Si Technologies for Agrivoltaic Applications,” builds on the development of agrivoltaics (APV) by applying the concept to protected agriculture, where radiation management is particularly important.
To investigate the trade-off between electricity generation and the amount of radiation available to crops, the researchers built two greenhouses fitted with semi-transparent modules and a third with a conventional transparent roof that served as a reference.
The first technology analyzed was CdTe, which transmitted approximately 50% of incident solar radiation, while the a-Si modules used in the experiment transmitted around 20%.
Five successive tomato and lettuce growing cycles were conducted over approximately one year. The primary objective was to characterize how the different roofs altered the growing environment, rather than to rank the technologies according to crop productivity.
The prototypes incorporated a monitoring system that recorded data at five-minute intervals. The researchers measured variables including solar radiation and its spectral distribution, air and soil temperature and humidity, CO2 concentration, and electricity generation.
This temporal resolution enabled the researchers to study how the greenhouses responded to changes in radiation and temperature and determine how the photovoltaic roofs altered environmental conditions compared with the reference greenhouse.
The reduction in radiation caused by the modules had a direct impact on the greenhouses’ thermal balance. Both photovoltaic systems produced cooler and more humid indoor environments, reducing evaporative demand from the plants and soil.
According to the project results, water losses fell by approximately 31% under the CdTe modules and by more than 60% under the amorphous silicon modules.
The effect is particularly relevant in Mediterranean regions, where high summer temperatures and limited water availability are among the main constraints affecting protected agriculture.
The findings also point to a second potential source of economic value for greenhouse operators. Electricity generated by the roof could reduce energy costs or provide additional revenue, while changes to the microclimate could help reduce irrigation requirements.
In addition to quantifying the total amount of radiation passing through the modules, the researchers analyzed its spectral composition.
The CdTe technology displayed a potentially valuable agronomic characteristic. Although it reduced the total amount of radiation available to the plants, it allowed a relatively higher proportion of certain wavelengths to pass through, including blue and red regions of the spectrum that are relevant to photosynthesis.
This suggests that assessments of photovoltaic greenhouse roofs should not be based solely on their transparency levels. The spectral quality of transmitted radiation may be equally important when evaluating the physiological response of crops.
Subsequent trials conducted by the research team also point to plant adaptation mechanisms under lower-radiation conditions. Crops can modify their architecture through greater leaf expansion and stem elongation to improve their ability to capture the available light.
The next stage of the research will involve further testing in Murcia, southeastern Spain, using additional photovoltaic technologies and environmental conditions representative of the region’s intensive agricultural sector.
The researchers aim to determine the extent to which photovoltaic generation, spectral transmission, temperature, humidity and water consumption can be optimized simultaneously without compromising crop yields.
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