Solar panels can heat the local urban environment, systematic review reveals
Why do solar panels have this heat effect on the urban environment?
It’s important to put all of my discussion on this topic in the context that it depends on what you’re comparing a particular application to. So, the example would be if you put photovoltaics on an existing black roof, you’re not having as much of an adverse effect on the urban thermal environment as if you put those photovoltaics on a white roof, because a white roof would normally be a relatively cool surface.
When you put photovoltaics on that white roof, the PV panels typically absorb in the order of 90% of the energy of the sun. And the photovoltaic panels then do convert some of that energy to electricity, but typical panels today are only maybe 16–20% efficient. These panels are absorbing a tremendous amount of energy from the sun, converting some of it into electricity, but then warming up because they’re not able to use all of the energy.
So, these photovoltaic panels tend to be rather hot surfaces in the environment. They’re almost always installed in an elevated format – above a roof surface or above ground level in a field. And as a result, you end up having two hot surfaces, the top surface of the panels and the underside surface of the panels. And so, as air flows over these panels, it readily picks up that heat essentially twice as effectively as it would if you had the same temperature on a building surface or a ground surface.
What effect does this heating have on the local urban environment?
There are several studies out there that have looked at panels from a modelling perspective and others have looked at observational data.
I’ve been involved in one project where we went out into the field and did measurements in and around a photovoltaic array in the desert, and then in an area not too far away that was a similar desert environment [the reference site]. What we found in that observational study was that the average air temperature at 1.5 m in the PV array site was about 1.3 °C warmer than the reference site, which is the non-PV site. At night we found almost no effect. And so, our observational studies lead us to conclude that PVs do, in fact, have this warming effect during the day, whereas at night the effect can either be very small or negligible and difficult to measure.
Other studies, particularly modelling studies, had previously suggested a daytime cooling effect of photovoltaics. But those had a flawed representation of the PV panels, where they ignored the fact that photovoltaics are able to convect heat from both the top and bottom surfaces.
Why do solar panels have this heat effect on the urban environment?
It’s important to put all of my discussion on this topic in the context that it depends on what you’re comparing a particular application to. So, the example would be if you put photovoltaics on an existing black roof, you’re not having as much of an adverse effect on the urban thermal environment as if you put those photovoltaics on a white roof, because a white roof would normally be a relatively cool surface.
When you put photovoltaics on that white roof, the PV panels typically absorb in the order of 90% of the energy of the sun. And the photovoltaic panels then do convert some of that energy to electricity, but typical panels today are only maybe 16–20% efficient. These panels are absorbing a tremendous amount of energy from the sun, converting some of it into electricity, but then warming up because they’re not able to use all of the energy.
So, these photovoltaic panels tend to be rather hot surfaces in the environment. They’re almost always installed in an elevated format – above a roof surface or above ground level in a field. And as a result, you end up having two hot surfaces, the top surface of the panels and the underside surface of the panels. And so, as air flows over these panels, it readily picks up that heat essentially twice as effectively as it would if you had the same temperature on a building surface or a ground surface.
What effect does this heating have on the local urban environment?
There are several studies out there that have looked at panels from a modelling perspective and others have looked at observational data.
I’ve been involved in one project where we went out into the field and did measurements in and around a photovoltaic array in the desert, and then in an area not too far away that was a similar desert environment [the reference site]. What we found in that observational study was that the average air temperature at 1.5 m in the PV array site was about 1.3 °C warmer than the reference site, which is the non-PV site. At night we found almost no effect. And so, our observational studies lead us to conclude that PVs do, in fact, have this warming effect during the day, whereas at night the effect can either be very small or negligible and difficult to measure.
Other studies, particularly modelling studies, had previously suggested a daytime cooling effect of photovoltaics. But those had a flawed representation of the PV panels, where they ignored the fact that photovoltaics are able to convect heat from both the top and bottom surfaces.
