The water-food-energy nexus on urban rooftops
Nurturing the intersection of multiple adaptation strategies
As California gets blasted with successive storms - closing out 2022, and ringing in 2023, it feels apropos to think about how to manage all that water and build more resilient strategies looking ahead. Headlines across the state warn of flooding, downed trees, and compromised infrastructure. When phrases like ‘atmospheric rivers’ and ‘polar vortexs’ get bandied about with such regularity, leading to devastating global climate events, it feels like an avoidable case of deja vu.
In cities, much of the water damage stems from impervious surfaces that intensify runoff, lack of green space to absorb excess rainfall, and climate change itself, where previously parched soil simply can’t absorb moisture at such high rates. Instead, heavy rainfall overwhelms storm drains, causes sewer overflow where combined systems are present, and leeches harmful bacteria and runoff into our waterways.
In our dense urban centers, where space is at a premium, adaptation techniques, particularly around stormwater management, are being rerouted to rooftops. With all of that unused space, green roofs can add a powerful stormwater detention and retention mechanism,
helping to keep 65-80% of water from storm cycles from ever reaching road-level, overwhelming storm drains, and causing widespread flooding.
Green roofs, on their own, provide substantial climate mitigation and adaptation options, but what if we double down? Like other strategies in both design and engineering thinking, combining two seemingly unconnected elements can make the whole much stronger. In this newsletter, we’re looking at the combination of two vastly different climate technologies to wield surprising and unexpected results.
Green roof vs. solar: why can’t we be friends?
When considering a climate mitigation strategy for our rooftops, it can feel like we have to choose one option, at the expense of another. How do we know which is the better approach? Each has its own associated costs and tradeoffs, but how do you compare the outcomes?
Green roofs have seen steady adoption as more and more people become aware of how many mitigation and resiliency boxes they tick. They are true workhorses: reducing heat island effects, capturing stormwater to prevent flooding, increasing biodiversity, and reducing GHG emissions.
On top of that, they offer respite for tenants escaping the typical hardscapes of city surfaces. Nature exposure effects are hard to quantify, but no less impactful to health outcomes.
But green roofs still have an identity problem. They’re seen as one-off, unique projects with high price tags. Solar, meanwhile, has benefitted from skyrocketing demand, increased incentives, commoditized products, and broad financing mechanisms. This has driven costs down to approachable levels for many property owners.
The drive to electrify everything in the built environment has also aided solar’s rise in popularity. This is justifiable and important work, but it often renders the rooftop inaccessible to tenants. With changing tenant demands around nature based solutions (NBS), many property owners only see tradeoffs when confronted with an either/or decision, and most often the price tag drives decisions.
What if we don’t have to choose?
A step back
For years solar and green roofs have been pitted against/alongside one another, either by accident or default, as once unused rooftop surfaces gained recognition as a vital climate adaptation opportunity. We previously neglected to address the front line in climate adaptation on our roofs because they had long been viewed as nothing more than a parking lot for the pigeon hoards, HVAC mechanicals, and telecommunications equipment - out of sight, out of mind.
Policy and incentives abound for choosing one or the other climate strategy, but the vast majority of property owners’ decisions lean toward solar.
For one, the solar market in the US, however flawed and inaccessible it appears - due to murky and drifting incentives from both state and federal sources and utility companies who appear determined to orchestrate its implosion - has developed into a much more mature market. The debt vehicles are now well understood and exercised through a variety of options - purchase, loan, lease, power purchase agreements (PPAs). Friction points have been, and continue to be reduced, despite the aforementioned hurdles.
And really, who doesn’t want to generate abundant clean energy from a natural resource and wean themselves off of a bloated, environmentally damaging, and inefficient grid with skyrocketing costs?
ROIs are easy to calculate based on projected kWh and many can benefit from a sound business decision that very rightly affects the environment. But a rooftop solar project alone, effectively removes tenant access and along with it, the tangible/intangible health and biodiversity benefits that a green roof can provide.
Of course, different strokes for different folks - some property owners will prefer the tried and true solar model of building it and leaving it, while they reap the passive energy rewards. But many more want to add even more to their resiliency footprint. They want to see clear connections between their decarbonization efforts and the health outcomes of their communities.
Greening rooftops comes with triple bottom line benefits; including, additional ESG reporting and disclosures, water, air, and energy upgrades, and community food security segments.
But let’s take it a step further. What if you don’t have to choose one resiliency project over the other? What if we pair solar with rooftop farming? I don’t mean divvying up the rooftop into separate sections, I mean one above/below the other.
Here comes the sun
The nascent field of rooftop Agrivoltaics explores the coexistence and co-benefits of photovoltaics (PV) and agriculture on urban buildings. In other parts of the world the combination of a green roof and PV is referred to as BioSolar roofs. These systems focus more specifically on solving biodiversity challenges, while Agrivoltaics’ focus is on the water-food-energy nexus.
To date, most Agrivoltaics research has focused on the dual sectors coming together in rural agriculture fields, with startling results. But in the last five to ten years, there has been movement coalescing around the potential for urban rooftops where maximizing space and efficiency is paramount. What if we could derive all the energy we needed to cover each building’s operational energy usage requirements and feed entire communities without fighting over roof access?
The co-benefits of solar AND crops are varied but compelling:
Plants aid in cooling the PV panels and boosting efficiency through evapotranspiration - ET. For the PV panels, the temperature effect drives performance. Not only is temperature important in predicting power output, but the material can have varied levels of efficiency based on the temperature. Research has pointed to a nearly 10% increase in production of energy when paired with green roof/crops.
PV panels provide shade, water capture, and a wind block for plants growing beneath them. Early studies have shown a 25% increase in plant growth, when located below translucent PV panels and, in some cases, an increase in their leaf size as they spread out to capture more light. The shade helps in soil moisture retention, keeping the plants hydrated during excessive heat days.
More solar energy and bigger crops. Win, win.
Grow baby, grow
To understand how these co-benefits work from a scientific perspective, we need to look at the light saturation point for plants. Plants have an upper limit on how much sunlight they can use. Beyond this point, the extra sunlight doesn’t increase photosynthesis, meaning, it doesn’t help the plants grow.
What it does do is increase the water demand. For growing in water challenged regions, Agrivoltaics can lead to more sustainable resource management and diversify crop selection, while boosting energy efficiency.
Having a PV panel above crops means filtered sunlight actually helps them to grow larger with less water demand, as the shading effects keep the soil below cool and moist for longer durations. That equates to reduced irrigation demand and better resource utilization.
Of course, these conditions will be better suited for certain crops (shade and fruit bearing), but ongoing global studies are challenging what’s possible.
For rooftop applications there are additional considerations when evaluating viability, particularly identifying the infrastructure capable of supporting these systems. PV + soil = substantial wet loads (weight bearing capacity), which makes it applicable for roofs with the right load capacity (125+ lbs/sf).
As well, wind uplift is a serious challenge on taller buildings, as the logarithmic velocity profile of wind over height can lead to damaging gusts. In response, many Biosolar roof systems are designed to use soil as ballast without penetrating the roof membranes. These systems help to prevent uplift and avoid potential failure points for water penetration.
For those buildings with viable rooftops, capable of carrying an Agrivoltaics or Biosolar system, the impacts can be profound. These developments personify the food-energy-water nexus, turning urban rooftops into climate adaptation machines and surfacing significant hidden ROI.
Coming to a city rooftop near you!
As we look to reduce our urban carbon footprints, Agrivoltaics offers a remarkable opportunity to capture the best of two technologies. Hybrid models are gaining traction, as systems thinking illuminates opportunities to amplify previously siloed strategies. But the comingling of inputs and outputs doesn’t stop at solar and green roofs.
Other rooftop crop research has hinted at using captured CO2 from exhaust vents as fertilizer inputs for leafy greens (higher temps also likely contribute to bigger growth). Combining strategies can not only amplify outputs, but also move the needle closer to operating our buildings as circular systems. Free energy is captured to power operational needs, and waste in the form of CO2 is used to augment crop growth.
Much more research is needed to fine tune these strategies, but what is abundantly clear is the staggering diversity of returns green roofs and solar panels can offer in the fight to decarbonize our buildings. Both technologies are available today and can be installed over the course of days, not months, providing immediate returns and impact.
Combined with both strategic financing models and supportive policy mandates, these systems can be implemented quickly and cost effectively across multiple communities to ensure equitable deployment and benefits. Let’s get to work!
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