According to SolarPower Europe, 49.5% of the world’s solar PV capacity additions in 2023 were on rooftops, rather than in utility-scale solar farms. As well as policy incentives for installing rooftop solar, the rise has been attributed to the increased energy prices of the last two years and a growing desire for energy independence amongst consumers.
Simultaneously, over half of the world’s population now lives in cities. The World Bank says that by 2050 – when all of the requisite renewable energy for net-zero emissions will ostensibly be deployed and solar capacity will be comfortably in the Terawatts – that number will be more like 70%.
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With a morphing population distribution, challenges morph too, regarding infrastructure and job provision and how the sprawl of urbanised land and security affects both energy and food. One potential solution could be the nascent but emerging field of urban agrivoltaics.
Jennifer Bousellot, assistant professor of urban horticulture at the University of Colorado, told PV Tech Premium that: “I see rooftop agrivoltaics as providing two of the renewable resources we’re going to need, and possibly even three into the future. Producing food and power in the same place and, based on our data, even saving water.”
By ‘rooftop agrivoltaics’, Bousellot means solar panels deployed on city roofs in conjunction with plant growth or rooftop farming.
The PV industry is already beginning to reap the benefits of ‘traditional’ agrivoltaics, where agricultural production and solar PV are combined on the same land and can offer increased yields, panel efficiencies and security for both farmers and project developers.
Ground-mounted agrivoltaics with crop production or grazing animals are more expensive than traditional PV systems in many cases – notably in the increased amount of steel required for elevating panels above animals, crops or farm equipment – and there is often a need for a cost-benefit analysis and a pricing structure that incorporates the agricultural production, power production and the sustainability of both in tandem.
Bousellot points out that the same applies for rooftop agrivoltaics: “The biggest cost is getting the infrastructure for the solar up on the rooftop, and once it’s installed its lifetime is very long. So I suspect that, if we can overcome that cost, the rest can follow.”
That cost will fall, not least because of the rising tide of rooftop solar adopters and, with them, rooftop solar suppliers. Nonetheless there is still a premium for rooftop solar compared with larger installations, partly because panel efficiency and quality is a greater priority.
Green roofs also carry a premium, but bring benefits too. “One of our biggest problems in this area [Colorado] is urban heat island effect, so we’re trying to reduce our urban temperature,” Bousellot says. “One of the best ways is green roofs.” Plants transpire moisture and oxygen and absorb heat, lowering temperatures around them. This cooling effect also happens to increase solar module efficiency.
Green roofs with solar panels also use less water than alternative projects – urban agrivoltaics needs irrigation in some climates, and Bousellot found that the water required is much less than straight ground-level farming:
“We have about 15 centimetres of irrigation water over the entire growing season [on the Denver test site], compared with two and a half centimetres every week on turf grass.” Furthermore, the rate of water use under solar panels is about a third of the use in full sun.
She continued: “There was a Canadian study that ran all of the benefits of green roofs, and found an average, with multiple factors, return on investment of about seven years. So, to me, I see that if we can line up the resources for the solar array install the rest comes quickly.”
How might rooftop agrivoltaics gain traction?
Speaking to PV Tech Premium about community solar recently, Jason Spreyer, executive vice president, business development at US solar company Summit Ridge Energy suggested that rooftop agrivoltaics are becoming more enticing.
“A lot of [US] states are exploring different avenues such as pollinator-friendly environments and agrivoltaics … and some of those options allow for you to have a priority position, potentially, in capacity awards,” says Spreyer. “We’re pushing for some of these avenues to be considered on rooftops.
“You can be creative on rooftops,” Spreyer adds, “and rooftops is a key part of the penetration of the community solar market.”
“I suspect it’s going to be partnerships surrounding botanic gardens, city municipalities, food pantries, those kind of things that would help us set up the systems,” Bousellot says, when asked about scaling up rooftop agrivoltaics.
“It’s the organisation more than anything that needs work: I could see, say, one of the big ones like bp who has an entire solar division taking on [urban agrivoltaics] and partnering with a food non-profit; that would be an incredible poster child for rooftop agrivoltaics and helping secure food, water and energy in urban spaces.
“It’s going to be grassroots until you have somebody as big and serious as that that can take it on.”
Bousellot also gives the example of Gotham Greens, a specialist rooftop greenhouse and urban agriculture company that grows crops in cities in Canada and the US. The company has managed to scale their operation and now has a supply deal in place with Whole Foods, the grocery chain.
“They’ve scaled it so they can operate and make a profit in most locations. The scale can go from that wide, as big as it gets on a commercial scale, all the way down to the food pantry level.” The principle, then, could be applied to incorporating solar PV into urban farming as well.
Food and energy
US think tank the Center for Strategic and International Studies published a piece in 2020 examining the way that food insecurity can affect urban poor populations in rapidly urbanising areas of India. Notably, the lack of access to independently grown food, and a reliance on cash economy, exposes poor urban populations to food – and particularly nutrition – shortcomings.
Gabrielle Hayes, an analyst for sustainable solar development at US solar developer BlueWave, points out that a co-operative relationship between solar companies and other land custodians can open new avenues for solar developments and also contribute to food security in local and urban communities. The company has developed an agrivoltaics plant in an urban cooperative farm, to demonstrate the effectiveness of this idea.
“It was a piece of land that was cooperatively stewarded by a group of immigrant farmers [and] first generation Americans,” says Hayes. “That would have been the first time BlueWave has developed an agrivoltaic array in such a complex agricultural setting, where it’s not just one tenant farmer, or one landowner, but a group of people who are growing different foods for their communities.”
“The land tenure they had was a little tenuous, and so the process of building the array would have resulted in a 30-year lease and would maintain the ability of these people to farm this land with security.”
In microcosm, this demonstrates the potential of solar companies to find diverse and innovative ways to deploy their products, whilst also answering the needs of an increasingly urban population in energy and food security.
The principle crosses over with community solar projects, which Wood Mackenzie said will add at least 6GW of capacity in the next four years, while the Coalition for Community Solar Access has given a target of 30GW of installed US community solar capacity by 2030.
On an individual basis these types of agrivoltaic projects are small potatoes for a developer, but their cumulative effect is already growing significantly. In an agrivoltaics plant two of the central concerns for our transitioning society converge – food and energy – on a valuable shared resource: land.
“I think agrivoltaics is a solution for farmland preservation”, Hayes says. She refers to the global trend of farmland consolidation, the term for the increasing share of agricultural land that is being bought and owned by large, single operators rather than medium- or small-sized farms. And whilst the amount of land that solar PV will need has been overstated and then corrected before, it is still something to be considered.
Spreyer makes a similar argument for preservation regarding rooftop agrivoltaics: “You’re not using farmland, so if you ultimately can put a project on a rooftop and add agrivoltaics or pollinator-friendly environments as part of these rooftop projects, you’re bettering the environment overall because you’re not using farmland and you’re adding the pollinator-friendly [or agrivoltaic] aspect.”
SolarPower Europe’s report above, which found that 49% of solar PV additions were on rooftops in 2022, follows a number of reports that consumers from Australia to Germany and the US are looking to shore themselves up against energy price turbulence and increase their personal energy security. Jennifer Bousellot says the same is happening with food.
Community agriculture and community solar
Community-supported agriculture operates on a similar principle as community solar, providing food where the latter provides energy to local communities without direct access to personal resources. “I suspect that would be the most resilient model for using rooftop agrivoltaics,” Hayes says.
It might not really be possible to produce a lot of food in cities like New York, where she lives, according to Hayes: “Urban farming; I’m a huge advocate for it and it has a lot of benefits like climate benefits, urban heat island and biodiversity benefits and a lot of community benefits, but in a city as dense as New York City, the impact that urban farms can have on food security is honestly quite limited.”
“So I think that speaks to how important it is to maintain farmland.” Agrivoltaics is a way to do that and to diversify the utility of land to allow a new industry to maintain an old one.
Hayes says of BlueWave’s community agrivoltaics projects: “With agrivoltaics for cities, you’re making sure there’s a food supply that is regional to a city. [BlueWave is] creating solar arrays on farmland but maintaining the agricultural viability of that land.”
As city populations grow and their footprint spreads, alongside the rollout of solar PV as a keystone of the renewable energy transition, there is a clear opportunity for urban agrivoltaics. Small-scale solar projects built on rooftops, in urban farms or community projects near cities offers new deployment options for solar PV, and plays a broader role in combating climate change.