Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
As the climate becomes hotter and wetter, loss of vegetable and fruit crops is expected to increase due to heat stress and damage from heavy rains.
Several projects across the country are researching the synergistic benefits of co-locating photovoltaic arrays on vegetable and fruit farms. Potential benefits to the crops will derive from lower plant temperatures, reduced sunburn and improved fruit set. Additional benefits are expected to be reduced evapotranspiration, increased soil water use efficiency and reduced damage from heavy rain or hail.
As the climate is expected to become wetter with more extreme rainfall events, losses of vegetable and fruits to excess rain will become more of a concern. Passive rain shelters using plastic coverings have been employed throughout the world to protect sensitive fruits and vegetables such as strawberries and tomatoes from excess rainfall. Research at the USDA Beltsville has shown that there is improved productivity and quality of repeat blooming strawberries under low tunnels serving as rain shelters. Rain shelters are also used for tomatoes which are susceptible to bruising and cracking. They also reduce foliar wetting and rain splash and therefore can reduce fungal and bacterial diseases if left on for the whole growing cycle. Solar panels could be used to provide this rain sheltering effect.
Artificial shading is a known strategy to mitigate heat stress. Commonly, shade cloth or netting is used to provide 20-30% shade levels during periods when the plant is most sensitive to heat (such as tomato fruit development). Research at the Universities of Maryland, Georgia and Kansas has shown that passive shading can improve marketability and quality of tomatoes, peppers and lettuce, respectively.
Research at UD with 20-40% shading of tomato, pepper and strawberries for summer production showed mixed effects, benefiting in some years but not in others depending on sunlight and temperature . In 2019, which was sunnier and hotter than 2018, the shade treatments produced significantly more marketable peppers than the unshaded plots, e.g. yield of marketable first harvest for 30% shading was 18 times higher than unshaded. Lettuce trials conducted at UD over similar range of shade found shading reduced soil temperatures by 3 °C and reduced bitterness.
Only recently (in the last 3 years) have studies incorporated actual solar arrays to evaluate their shading. Oregon State University applied fixed tilt solar arrays over grazing land and found a substantial increase in moisture retention but concluded the economics of solar production as active shading devices should be studied as well. University of Massachusetts just initiated a study of solar module shading and found while vegetable production was lower and solar output was lower, the combined payback was greater than either alone. Arizona researchers found that some pepper and tomato varieties had 2-3 times higher yield under solar modules while other varieties had same yield but used half as much water. Agrivoltaics research is now underway at more than a dozen universities.
Shading is commonly used in nursery production of high value ornamental shade plant in areas such as Florida and Southern California. However, use of artificial shading in vegetable and fruit production has been limited in the temperate areas of the US due to the expense of shade house structures and limited profit potential. But the need for this is changing due to hotter and wetter weather.
Climate change has the potential to impact fruit and vegetable production as temperatures increase. Many vegetable crops have suffered losses due to the heat in recent years because photosynthesis rapidly decreases above 94°F, so high temperatures will limit yields in many vegetables and fruits. In flowering and fruiting crops, high night temperatures will affect pollen production, often reducing viable pollen numbers resulting in reduced set and lower yields. Sunburn of fruits increases on hot sunny days, especially where leaves wilt resulting in reduced fruit cover. Shading from solar panels could alleviate these problems.
Providing adequate moisture through irrigation is critical in high heat periods. However, irrigation cannot completely compensate for extreme heat. High soil temperatures can damage surface roots, limiting water and nutrient uptake. This is particularly an issue in crops grown on black plastic mulch, a common cultural practice. Solar shading can reduce the local temperature under the modules by blocking the direct sunlight on the hottest days of the year, mitigating many of these harmful effects
High value crops could be grown in the partial shade of solar panels or in areas between solar panels while simultaneously generating significant income from sales of clean electricity. If successful, this could also boost yield and quality of specific vegetable and fruit crops on farms.
Issues with current solar fields and agrivoltaics include:
- Panels are low to the ground making them hard to work under. Panels will need to be higher for agrivoltaics to work for under panel production.
- Fixed solar arrays cut light significantly and will limit crops that can be grown under them. Panels will have to have gaps to allow enough light.
- Tracking solar arrays are always shaded limiting the used under the panels. Counter tracking could be used to solve this problem (track opposite to let sun in the morning and afternoon).
- Growing in lanes between panels is possible but makes movement challenging. Equipment could be modified for between panel production mechanization.
Between fixed panel production.
Growing under solar panels with gaps.
Growing under and in-between tracking solar panels.
The University of Delaware has received funding to create agrivoltaic user-facilities at UD, in Newark and in Georgetown. We will study the benefits of co-locating uniquely designed sun-tracking PV arrays with crop production. The test crops will be high-value vegetables and fruits impacted by adverse climate change related weather (e.g., tomatoes, peppers, lettuce, and strawberries). Demonstrating mutual benefits could justify the use agricultural land for PV installations. This equipment allows separate control of module orientation and position on the tracker to optimize electrical and vegetable/fruit production. Another innovation is control of the solar panel orientation to serve as a shelter to keep damaging rain from crops.
System to be constructed at 2 University of Delaware research farms. Diagrams courtesy of SolAgra. This system allows for below panel production with common farm equipment.