Controlling evapotranspiration, ground reflectivity and module height can enhance agrivoltaics performance – study

Hot and arid climates benefit most from the combination of agriculture and PV farms. Image: Iberdrola.

Combining solar modules and crops in an area is not a new idea in the PV industry since it could be a solution to enhance crop yields and produce energy in a large area.

However, the benefits of combining both could be more, as crops on solar farms can help increase solar module performance.

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Recently, a group of scholars at Cornell University studied the microclimate of solar farms, quantifying the cooling effects of evapotranspiration, module height, and reflectivity of the ground. 

In general, solar panels mounted over vegetation demonstrate surface temperature drops compared to arrays mounted over bare ground due to evapotranspiration from vegetation. From an agricultural perspective, using vegetative cover to increase surface reflectivity can reduce the amount of solar energy absorbed by the ground. 

The cooling effect of reflectivity

The study showed that reflectivity could affect ambient temperature, as a 10% increase in reflectivity could result in a temperature drop of 0.09°C.

Also, lower surface reflectivity leads to higher module surface temperature because the ground absorbs more radiation. Therefore, radiation is emitted as sensible heat to solar panels. However, when it comes to the correlation between reflectivity and evapotranspiration, increased reflectivity could generate a greater cooling effect without evapotranspiration. This is likely a result of water particles playing a role in temperature reductions by absorbing reflected radiation from the ground.

When considering reflectivity and panel height, there is a consistent negative correlation. This is because when module height increases, more radiation will be reflected into the sky compared to when the panels are placed close to the ground, where they would be incident with more reflected radiation.

Reflectivity also offers an extra benefit for bifacial solar systems: Higher reflectivity increases shortwave radiation reflected off the ground can improve power production. 

The cooling effect of evapotranspiration

The research team also examined the relationship between evapotranspiration and reflectivity, unveiling that there is also a negative correlation between both factors. This can be attributed to water particles rising from the ground, absorbing radiation reflected and emitted off the ground, and cooling solar arrays above. The research highlighted this as a clear benefit for power production when solar arrays are mounted over vegetation.

The cooling effect of solar module height 

Speaking of the cooling effect of solar module height, there is, once again, a negative correlation between reflectivity and module height. This results from the radiative heat exchange between solar modules and the ground. When ground reflectivity is low, a large amount of radiation is emitted from the ground as heat. Therefore, increasing module height has a larger cooling effect compared to a scenario where less heat is emitted from the ground.

However, the cooling effect of ground clearance is slightly more prominent combined with evapotranspiration, as water particles absorb more radiation emitted from or reflected off the ground.

In a more prominent example, an agrivoltaic system mounted at four metres above soybeans sees up to 10°C reductions in module surface temperature compared to a traditional solar power plant. 

“We’re showing dual benefits. On the one hand, you have food production for farmers. On the other hand, we’ve shown improved conversion efficiency for solar developers,” said Henry Williams, lead author of the study. 

However, the study could not cover every aspect when it comes to combining crops and solar modules. The research team said more studies about crop models and thermal-fluid simulations in the context of co-locating agriculture are needed in the future. 

“Research studies so far have shown that regions in hot and arid climates benefit most from the combination of agriculture and PV farms. We expect more and more extreme heatwaves in many regions of the world. In terms of PV panel conversion efficiency, many regions across the globe can benefit from the combination of agriculture and PV modules,” said Max Zhang, professor of the Sibley School of Mechanical and Aerospace Engineering at Cornell University. 

The research also said social factors that could heavily influence the practicality of solar developments should be studied in the future. 

Previously, PV Tech reported that the UK’s then-prime minister Liz Truss once said that “fields should be full of our fantastic produce (and) it shouldn’t be full of solar panels”, while current prime minister Rishi Sunak stated that under his leadership Westminster would understand the needs of rural communities, “making sure our fields are used for food production and not solar panels”.

Their comments caused significant concern in the UK solar sector, which has been growing at pace thanks to the low cost of the technology and the need to transition to low-carbon energy sources to tackle the climate crisis.

In response to the comments, the Association for Renewable Energy and Clean Technology said the solar industry aimed to work in conjunction with, not against, agricultural use of land, commonly by either building on marginal land or ensuring multi-land use applications. 

“We need to take a holistic view of the energy system and the biophysical environments to design future solar farms, which I believe that it will be very different ten years from now,” added Zhang.

Agrivoltaics are in focus in the most recent edition of our quarterly journal PV Tech Power (vol.34), along with a broad range of other industry topics, including the potential for offshore floating solar and the evolution of business models for energy storage in Europe. You can download the journal as part of your PV Tech Premium subscription.

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