The Sahara Desert, and the Sahel region, could be set to take advantage of large-scale wind and solar power projects covering huge surfaces, as these would increase coverage by vegetation, which would itself determine increased surface friction and reduced albedo. Both these two factors, in turn, would be responsible for an increase in rainfall, which in the region is strictly dependent by vegetation.
A new study, published in the journal Science, shows that solar power plants and wind farms covering large surfaces may be able to contribute to increased rainfall in the Sahara Desert, and particularly in the Sahel region, an ecoclimatic and biogeographic zone of transition between the Sahara to the north and the Sudanian Savanna to the south.
The study, based on a climate model with dynamic vegetation, shows that the construction of large-scale wind and solar facilities occupying wide surfaces may have as an initial consequence a local temperature increase, which would result in increased surface friction and reduced albedo. These two factors, in turn, could increase coverage by vegetation, creating a positive feedback loop that would increase rainfall in the region, the authors of the study claim.
According to them, in fact, installed solar plants and wind turbines, if large enough, have the power of modifying land surface properties and, in particular, surface roughness and albedo, respectively.
The researchers specified that the wind and solar facilities simulated in their study would generate approximately 3 and 79 TW of electrical power, respectively, averaged over a typical year.
“Compared with the control experiment, a 50% increase in precipitation (+0.13 mm/day) is observed in solar farm locations in the Sahara, and an increase of +0.57 mm/day is recorded in the Sahel,” the scientists found.
The report also notes that changes in climate are especially enhanced when wind and solar plants are deployed together in the Sahara. “The precipitation in the Sahara increases from 0.24 mm/day in the control run to 0.59 mm/day in the case of combined wind and solar farms, a ~150% increase, whereas the temperature increase (+2.65 K) is only slightly larger compared with that for the solar farm alone,” they further explained.
If wind farms contribute to more rainfall, due to the fact that higher surface roughness strengthens low-level convergence, solar plants cause more precipitation thanks to the decreased albedo associated with panels, which results in more absorption of solar radiation and, hence, surface warming, which leads to low pressure at the surface, as well as convergence, rising motion.
The authors of the report, however, warn that the increase in rainfall linked to solar plants depends on the low conversion efficiency (around 15%) of the solar panels used in the experiment and the background environment albedo. “If solar panel efficiency and the associated effective albedo are high enough to lead to an albedo increase relative to the background environment (as, for example, a 45% efficiency would), the climate impact would be surface cooling with precipitation suppression, similar to the impact of overgrazing in the desert,” they stated.
The researchers concluded, however, that installations of wind and solar farms with current conversion efficiency, if carefully planned, may trigger more precipitation in the Sahara Desert, while also enabling these two energy sources to have minimal competition for land surface area against natural and other human land uses, such as agriculture
Source PV Magazine