Rooftop agrivoltaics: Is it here yet?
There’s nothing really new concerning the individual components of rooftop agrivoltaics (RAV). The first solar electric panel was placed on a New York City rooftop in 1884 by Charles Fritts. Fritz was the American inventor who created the world’s first practical solar cell the previous year. He did so by coating selenium wafers with a thin layer of gold. Efficiencies were just 1 to 2%, but, hey, you have to start somewhere. The first rooftop commercial solar water heaters appeared in places like California and Florida during the last decade of the 19th century. Active roof-mounted solar space heating equipment arrived in the 1930s and 40s.
The use of rooftops for growing food and ornamental plantings goes much further back in history, to the period between 4000 and 3000 BC. The very earliest recorded rooftop gardens are found in ancient Mesopotamia, followed by Egypt, around 2500 BC, then during the Roman era, then gardens created in Mexico by the Aztecs, and so on.
So why has it taken so long for mankind to begin looking at the possibilities of using roof space to produce both food and electricity? We already have some early work underway in what is called Agrivoltaics, which combines the growing of food and the generation of electricity on the same piece of land. Is there anyone out there trying to develop such a hybrid, not on land but on a roof? The answer is yes, but just barely.
Let’s give it a name – Rooftop Agrivoltaics – RAV for short. The world leader, and possibly one of its only participants, is Colorado State University (CSU) in the U.S. CSU is already an established leader, along with the Amherst campus of the University of Massachusetts, in standard agrivoltaics (APV) at its Foothills campus, and undertook its first, limited pursuit of RAV there.
More recently, it began researching RAV at its Spur campus in Denver in 2022. To date. CSU/Spur has been growing tomatoes, peppers, zucchini, pollinator plants, and such herbs as cilantro, saffron, lemongrass, mint and basil. The CSU lead researcher in this work is Horticulture Assistant Professor Jennifer Bousellot. The CSU team cites the benefits of RAV as including lowered water usage, reduced energy costs, increased local food security, the production of clean energy, and the creation of climate-resilient green spaces for public usage.
CSU/Spur has between 900 and 1000 square feet of silicon solar panels on the roofs of two buildings. Some of their panels are single-sided; some are bi-facial. The panels are installed above an 18-inch deep green roof system from American Hydrotech (www.hydrotechusa.com) SemperGreen sedum mats (www.sempergreen.com) surround the crop research plots, providing separation between different food growing plots.
Other RAV findings to date include:
- Substrate moisture is higher under solar panels than in full sun conditions.
- Immediate area temperatures are also lower
- The combination of greater humidity and lower air temperatures is advantageous to the growing of many edible food plants.
- Water usage for plant irrigation is reduced.
- Plants grown under the partial shading of solar panels evapotranspire moisture, helping cool the panels and resulting in higher operating efficiencies, enabling 5 to 8% more kilowatt-hours of electricity to be generated.
- Lightweight cadmium telluride solar panels seem to work best for RAV. These semi-transparent panels can protect the plants under them from harsh environmental conditions like inclement weather, intense solar radiation, and strong winds.
- Leafy greens have shown that they produce higher yields under 40% semi-transparent solar panels compared to receiving full unshaded sunlight.
- Culinary saffron, an exceptionally high-value crop, with RAV, continues to grow weeks, even months longer than when grown in the field.
Research found online for RAV projects, other than the work going on at CSU, is scanty. But progress is being made, here and there, around the world:
- France: In France, some progress has been made by that country’s National Research Institute for Agriculture, Food and Environment (INRAE). The Institute has pioneered in RAV research with pilot projects which integrate semi-transparent solar PV panels with rooftop farming. Their studies demonstrate an increase of up to 15% in lettuce yields under controlled shading while achieving a bonus-level solar energy output of 100 kilowatt hours per square meter per year.
- Japan: In Japan’s Nagano prefecture, rooftop agrivoltaic systems have been installed to grow shade-tolerant crops such as herbs and leafy greens. These systems utilize bi-facial PV panels to optimize energy output and to provide diffused light for agriculture. In addition to electricity production, the Nagano model has increased agricultural productivity by 10% compared to rooftop farming without solar panels.
- India: In Pune, India, a pilot RAV system on the roof of a commercial building demonstrated its feasibility in tropical climates. In addition to its production of electricity, it supported the cultivation of spinach and coriander. Reduced water consumption and higher crop yields were attributed to microclimate changes beneath the panels.
- USA: In New York City, the Brooklyn Grange has integrated rooftop solar into some of its urban agriculture.
Rooftop agrivoltaics: What’s next?
Once we’ve succeeded in getting a respectable share of our grown produce off the ground and up on the roof, and successfully integrated with solar panels, what’s the next logical step? Why, it’s Hydroponic RAV (HRAV?), of course. And if the amount of online info on RAV itself is scanty, all the more so when the rooftop growing medium is not soil but a liquid nutrient solution. I’ve found one example of HRAV online. A startup company in Israel called Bing Klima. Bink Klima’s stated intent is to combine Green Roof, Blue Roof, and Solar Roof features together on a single roof.
The Green Roof is partially or fully covered with vegetation and a growing medium of some sort. The Blue Roof is designed to temporarily store rainwater and to then release it, or to use it in a controlled and perhaps useful and productive way. And the Solar Roof, of course, enables the generation of electricity from sunlight. The result is an entire green roofing system, within a single patented module, that contains a number of solar panels and a hydroponic growing system, which includes a nutrient solution tank. The tank serves two purposes: it irrigates the hydroponically grown plants and anchors the entire module to the roof.
A possible variation of HRAV could be to dispense with the pumps, filters, and even with the electric power required to operate standard hydroponic growing systems. This is already being practiced on the ground through the use of what is called the Kratky hydroponic system. The method is named for Bernard Kratky, a researcher at the University of Hawaii, who first proposed his unique technique in 2009 in an article in the journal Atca Horticulturae. The Kratky method has since been found capable by its many practitioners of producing all kinds of food plants grown in anything from one-liter jars up to 5, 10, 30-gallon and larger containers.
The level of the nutrient solution in these containers is initially set just high enough to make an inch or so of contact with the roots of the tiny plants or seedlings. Over time, as the water-based nutrient solution gradually evaporates, the plant’s root system continues to grow deeper into the container, so the plant can continue to benefit from the nutrients. The oxygen required for the plant to grow to maturity is not provided by a pump, but solely by the increasingly large airspace between the the surface of the diminishing volume of solution and the top of the container.
When employing the Kratky methods out of doors, as in a rooftop situation, steps need to be taken to prevent rainwater from leaking into the container, which would dilute the strength of the nutrient solution and perhaps also eliminate the required air pocket. HRAV’s utilization of solar panels above the Kratky growing containers might solve – wholly or partially – this problem.
Beyond the technical aspects of producing electricity while growing food on a rooftop, RAV and its variant, HRAV, raise a number of unique and practical questions. These would include, but are not limited to:
- Can the roof of the existing building handle the increased weight of the solar and the growing systems, and of the regular foot traffic of those individuals operating the equipment and devices?
- If RAV or HRAV is being viewed as a desirable feature for a yet-to-be-built building, what will be the cost premium that will enable this facility to be constructed?
- How and by whom will both the initial RAV feature be designed and built, and how and by whom will it be managed? Are these people already in the building – eg., a university building, or some sort of science-type building – or will it need to be operated by others who will need to come and go regularly, as in a multi-family apartment building or a standard office or industrial building? Note: the Kratky approach is close to a “set it and forget it” type of growing method, thereby minimizing the frequent coming and going of agricultural personnel.
- Will there be other additional costs with a roof that hosts RAV activities? Such as higher insurance costs, or for unforseen maintenance & repair issues. Will the rooftop activity center result possibly in higher local property taxes than the building’s owners might otherwise need to pay?
- Answers to questions like these will only be forthcoming with the gradual spread of RAV over more and more rooftops.
Follow the science
The online journal: Living Architecture Monitor: A Green Roofs for Healthy Cities Publication
Also, keep an eye on Colorado State University’s Source website.
There is a YouTube video as well.
