Tag Archive for: Solar greenhouse

When it comes to conversations surrounding energy and water use in the modern world, the agricultural industry’s consumption of both is often at the forefront. As the world’s population continues to grow, humanity is tasked with the challenge of finding ways to meet both food and energy demands across the globe. “I really believe that greenhouse growing is the epitome of sustainable agriculture,” says Soliculture cofounder and CEO Dr. Glenn Alers. Whether it is the ability to greatly increase crop yields when compared to traditional open field growing, or the potential for increased water-use efficiency in combination with hydroponics, greenhouses could play a key role in addressing these concerns. Solar greenhouses also could also play a role in mitigating future energy crises. 

Soliculture began in 2012 as a startup in the Physics Department at the University of California Santa Cruz. Dr. Alers and his cofounder were conducting research on luminescent solar concentrator panels when he realized the technology’s agricultural potential. Luminescent solar panels utilize a luminescent dye that selectively absorbs a portion of the solar spectrum and readmits light at a different wavelength. The dye used in Soliculture panels absorbs the green portion of sunlight with low photosynthesis efficiency and converts it to red light with much higher photosynthesis efficiency. The panels enhance the light quality inside a greenhouse by optimizing the light spectrum for improved plant growth. Moreover, the panels contain bifacial cells that collect the light reflected from the crops planted below them. The red luminescent dye also enhances the power output of embedded cells by 15 to 32%, compared to a conventional panel. 

Installed Soliculture luminescent solar panels.

In 2019, Soliculture began a research project on Whiskey Hill Farms in Watsonville, California, aimed at developing these solar panels for use on hybrid high-tunnel greenhouses. As an active organic farm already growing produce in both field and greenhouse settings, Whiskey Hill Farms served as an ideal host for the project. The Soliculture research greenhouse was constructed from the ground up with help from a local high tunnel installer, measuring 120’ long and 25’ wide upon completion. Additional bracing was added to the roof structure, forming a “queen style” truss to support the weight of the panels. One half of the high tunnel was covered by a semi-clear plastic film that served as the control for the upcoming crop growth study, and the other was covered by Soliculture solar panels. These panels were specially designed for high tunnel greenhouses and had a cell coverage of 42%.  

Interior of research greenhouse with panels installed.

The project hit a temporary snag when waterproofing the panel racking system proved to be more of a challenge than expected. At first, horizontal mounting bars were attached to the tubing of the greenhouse’s roof frame and foam weather stripping was installed between the panels to create a watertight seal. Water was still able to leak through at the corners and where the mounting bolts connected the panels to the roof. Knowing the potential for these leaks to cause erosion and negatively impact crop growth, the Soliculture team returned to their laboratory and created a modified racking system specifically for high tunnel application. This new system used mounting brackets that attached to the bottom of the frame and utilized a rubber “T” gasket inserted between the panels to create a seal. Finally, the plastic film portion of the roof was attached to the panels using an aluminum channel screwed into the panel frame and “wiggle wire” to hold the plastic film in position. With a waterproof roof in place, the crop trail was ready to commence.  

The following crops were selected for planting following the completion of the high tunnel in mid-November: strawberries, red romaine lettuce, red butter head lettuce, cilantro, mustard greens, and turmeric. To ensure the trial’s results would translate to commercial production, the research team used common commercial growing methods throughout the duration of the trial. These methods included drip irrigation with untreated well water, sand filtration, and liquid organic fertigation. By the end of the trial, the majority of the crops grown under Soliculture panels matured close to two weeks ahead of those grown under the clear film portion of the high tunnel. The fresh weight for the under-panel crops was superior as well, with red butter head lettuce seeing the greatest benefit at 145% higher weight. Mustard greens weighed in at 95% higher, cilantro at 35%, romaine lettuce at 32%, and turmeric at 25%. The strawberry fruit showed no statistically significant difference in fresh weight, but the single 5’ by 120’ planted bed yielded more than 350 pounds of fruit by the end of July.  

Crops grown under Soliculture panels.
Crops grown under plastic film.

On top of the very successful crop trial, the power generated by the greenhouse panels was used by Whiskey Hill Farms to power their day-to-day operations. A total of 58 Soliculture panels provided the farm with a 6kW system, which was connected to an inverter. The AC power was then fed back into the farm’s power system, a testament to how greenhouse solar can benefit the farm beyond improving plant growth.  

The field of agrivoltaics is constantly evolving, with numerous researchers and farmers searching for the ideal nexus between the agricultural industry and energy production. Soliculture’s contributions to agrivoltaics is important for farmers who have reservations about growing food underneath and around solar panels. “We haven’t seen any negative effect on plant growth,” Dr. Alers says, referring to the Whiskey Hill Farms project and several other successful Soliculture installations across the United States and Canada. Greenhouse production has always had the potential to help alleviate the water crisis and increase the amount of food grown per acre, but Soliculture’s technology is giving it a bright future in energy production, as well. 

All photos courtesy of Soliculture 

Wavelength-Selective Photovoltaic Systems (WSPVs) combine luminescent solar cell technology with conventional silicon-based PV, thereby increasing efficiency and lowering the cost of electricity generation. WSPVs absorb some of the blue and green wavelengths of the solar spectrum but transmit the remaining wavelengths that can be utilized by photosynthesis for plants growing below. WSPVs are ideal for integrating electricity generation with glasshouse production, but it is not clear how they may affect plant development and physiological processes. The effects of tomato photosynthesis under WSPVs showed a small decrease in water use, whereas there were minimal effects on the number and fresh weight of fruit for a number of commercial species. Although more research is required on the impacts of WSPVs, they are a promising technology for greater integration of distributed electricity generation with food production operations, for reducing water loss in crops grown in controlled environments, as building-integrated solar facilities, or as alternatives to high-impact PV for energy generation over agricultural or natural ecosystems.

The use of renewable energy in modern greenhouse management is important to achieve efficient and sustainable food supplies for a world with increasing population. This study assessed the performance of a blind-type shading regulator that can automatically rotate semi-transparent photovoltaic (PV) blades installed on the greenhouse roof in response to sunlight variation. The PV blind oriented parallel to the roof partially blocked intense sunlight penetration into the greenhouse, but it transmitted sunlight during cloudy time by turning the blind bearing to be perpendicular to the roof. A stable irradiation environment is therefore producible in the greenhouse under variable sky conditions. Annual operations demonstrated that the blinds’ own generated electrical energy can sustain PV blind operation and produce surplus electrical energy. The PV blind electricity generation and sunlight availability for crops below the PV blind roof were calculated based on a mathematical model developed using theoretical sunlight parameters and the experimentally obtained PV blind system parameters. Assuming cloudless skies and threshold irradiance for blind rotation set at 500 W m−2, 13.0 and 12.3kWh m−2 yr−1 surplus electrical energy can be generated, respectively, by north–south and east–west oriented model greenhouses. Cloudy skies reduce surplus electrical energy production by 50%, but PV blinds can supply greenhouse electrical energy demands partially or completely, depending on the degree of greenhouse electrification. Below the PV blinds, 8–10 MJ m−2 day−1 of insolation is expected to irradiate crops under actual sky conditions. This insolation is sufficient to cultivate major horticultural crops. Regulating the threshold irradiance level for PV blind turning can control the sunlight apportionment ratio for cultivation and electricity generation, thereby enabling sustainable energy–food dual production in a greenhouse.

This report reveals that increasing the sustainability of food production will require development of new mixed-use technologies. Also discussed is that novel electricity-generating windows (Wavelength-Selective Photovoltaic Systems, WSPVs) are suitable for use in greenhouses for growing plants. Results show minimal lasting effects of growth under WSPVs on plant physiology and development, thus WSPVs represent a new wedge for decarbonizing the food system.

This AgriSolar Best Practices Guide is intended to assist farmers, PV developers, regulators, and other stakeholders in developing high quality Agrisolar projects. The guide provides Best Practices for Agri-PV systems, PV on agricultural buildings, and open-field applications. Also included in this guide are discussions of trends and innovations in the AgriSolar community. This guide defines the key actions required of all parties involved in project development to maximize the sustainability of Agrisolar projects, from an agronomical, ecological, and financial perspective.

This study assessed the climate conditions inside a greenhouse in which 50% of the roof area was replaced with photovoltaic (PV) modules, describing the solar radiation distribution and the variability of temperature and humidity. The distribution of the solar radiation observed in this study is useful for choosing the most suitable crops and for designing PV greenhouses with the attitude for both energy and crop production. The study also includes suggestions for a better agronomic sustainability of agrivoltaic systems.

The long-term analysis in this study demonstrated a good capability of the numerical model to predict the shading effect inside a photovoltaic greenhouse combining the daily calculated exposed percentage with measurements of solar radiation. Photon flux daily values inside a PV greenhouse were calculated and measured from April 18th to June 8th in 2014. Commercial software was used to calculate the exposed percentage values for the greenhouse being studied. This study shows that modern software can be utilized in optimizing PV greenhouse operations.

Overall, this study demonstrated that the use of semi-transparent OPVs as a seasonal shade element for greenhouse production in a high-light region is feasible. However, a higher transmission of PAR and greater OPV device efficiency and durability could make OPV shades more economically viable, providing a desirable solution for co-located greenhouse crop production and renewable energy generation in hot and high-light intensity regions.

This study investigated the feasibility of a greenhouse roof with an integrated semi- transparent PV-blind system to provide moderate shading conditions to greenhouse crops along with simultaneous electrical energy generation. The results in this study can be used to optimize variations of agrivoltaic operations and their development in the future.

This study shows that adding semitransparent organic solar cells (ST-OSCs) to a greenhouse structure enables simultaneous plant cultivation and electricity generation, thereby reducing the greenhouse energy demand. To lower the energy footprint of greenhouses, there has been growing interest in integrating solar cells onto the greenhouse structure. In this approach, a portion of light is captured by the solar cells to generate power, while the remaining light transmits into the greenhouse for crop production. The results of this study can benefit development of agrivoltaic operations by maximizing the amount of sunlight reaching plants grown in greenhouses.