Green Roofs Make Solar Panels Work Better

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TLDR: a photovoltaic stormwater tool with cooling biodiversity that pays for itself

No technology comes without side effects. This is also the case with photovoltaics systems which, in many studies, have been shown to increase urban heat island effects. However, combining photovoltaics with green roofs can alleviate this issue as the green roof evapotranspiration can cool the panels and the surrounding area. Add green roof stormwater detention, and you also have a clear ROI.

The importance of mandate wording

In many places, mandates have been put in place for either solar panels or green roofs on new commercial buildings, e.g., NYC or, more recently, France. This is something to celebrate, but we need to be aware of uncareful mandate wording that could pitch one green industry against another. In NYC, the mandate inadvertently pitched photovoltaics companies against green roofs for shares of the market, which likely was not the intended effect of the mandate.

Situations like this one can be highly damaging, as we, in this case, need a collaborative effort and not a competitive one. Especially as a combination of these two technologies might be an essential ingredient for sustainable urban areas.

Photovoltaics – sorely needed for energy production

Europe is currently in a massive energy crisis. Furthermore, the warming climate demands fossil-free green energy such as wind power or photovoltaics. The transition to green energy is critical. Nonetheless, we need to consider the exacerbation of urban heat island effects through photovoltaic panels before we cover the roofs of entire metropolitan areas in PV, especially since there are green nature-based solutions that can alleviate this issue.

Solar panels and the urban heat island effect?

A heat island effect describes the localized and dangerous heating up of urbanized areas as buildings, roads, and other infrastructures absorb and re-emit heat more efficiently than vegetated areas.

Photovoltaic panels absorb around 90% of the sun’s energy, but they are only approximately 20% efficient. This can result in significant heat loss and heating up of the environment. Furthermore, you end up with two hot surfaces: first the PV panel itself and then the roof underneath.

Learn more about urban heat islands in this article: How Green Roofs Mitigate Urban Heat Island Effects

A review of 116 papers on the topic showed that photovoltaic panels heat cities during the day. Still, it should be noted that they can often cool the surrounding area during nighttime. The energy balance of these systems is complex, which is why this review was sorely needed.
The relative effect of the PV panels on the local heat flux depends on many factors. One of them is what type of roof you put them on.

If you put a solar panel on a black roof, you are not having a substantial adverse effect on the thermal environment. But we must ask ourselves if this is the comparison we want to make. A black roof should be a thing of the past due to the poor thermal performance concerning the urban heat island effect. Hence, I personally do not find it appropriate to use a black roof as a point of reference.

On the contrary, if you add photovoltaics panels to a white roof, you will most definitely disturb the thermal performance of that roof. A white roof, or a cool roof, is usually used to alleviate urban heat islands. It generally works well when the roof is new, but dirt can, over time, reduce the cooling efficiency, essentially turning it into a semi-black roof.

To further complicate matters, PV panel efficiency is typically tested at 25°C. Due to the roof conditions, this temperature is often exceeded. Already at 60°C, a 10-15% efficiency drop has been reported.

Nevertheless, many of these issues can be solved using green infrastructure in combination with photovoltaic panels.

How green roofs can support photovoltaics

Green roofs are roofs that are covered with vegetation. Vegetation is highly effective in cooling due to the evapotranspiration process. We all know the difference between the burning sand in the sun and the cool grass next to it.
Hence, the green roof vegetation can cool photovoltaics panels and reduce the loss of efficiency. Vegetated roofs are well known for significantly reducing heat island effects, and plants have been used for the cooling of buildings since the beginning of civilization. This is no new technology.

Nonetheless, there are some things to consider. A green roof only cools as long as there is access to water. This means that it is critical that you design the roof to be optimized for your climate. You can do this using our evapotranspiration/retention modeler (free of use). Also, you should consider emergency drip irrigation lines, preferably connected to harvested rainwater to cover extreme drought phases.

Let us know if your city is not yet included in the modeler.

It is also vital that you select an appropriate plant palette. Sedum plants are hardy and drought tolerant and are generally good in combination with PV panels as they rarely grow high enough to disturb the panels. Sedum plants also do well in the shade below the panels, and data show that they transpire sufficiently for a significant cooling effect.

A green roof can also act as ballast for the photovoltaic system removing the need for anchorage and penetration of the waterproofing membrane. This leads to a reduced leakage risk. A green roof also extends the lifespan of the waterproofing membrane significantly and protects it from foot traffic, hail damage, and other mechanical damage. Contrary to what some people may think - a green roof can protect you from leaks compared with a bare roof or a PV roof.

What about the cost?

Yes, adding a green roof will add cost, and though there is an efficiency increase in the PV panels, it will take time before you see a return on investment. However, this is not all. You will also save on building energy consumption due to the cooling effect of the green roof. Also, due to the roof’s retention capacity, your sewage fees will be reduced.

Nonetheless, a clear ROI can be added to this equation if you live in a densely populated urban area: green roof detention. This is a particular class of green roofs that can be described as stormwater tools that double as green roofs. Using detention-type green roofs, you can reduce or remove at-grade stormwater solutions, which clears up floor space that can be used for income-generating activities.

You end up with a photovoltaic stormwater tool with cooling biodiversity that pays for itself.

If you want to learn more about green roof detention and the economic possibilities, take a look at this article:
Economic Case Study: Green Roof ROI