Researchers from Soochow University have developed a prototype solar panel that can harvest energy from rain. Could this be a new direction for solar energy or is it merely a neat science project?

For the past two decades, green energy sources have been heavily developed and invested in. It was not long ago that there where many contenders in the energy market including solar, wind, geothermal, hydroelectric, and even fusion. It seems, however, that fusion is always 20 years away, and that wind, geothermal, and hydroelectric systems each have their own set of hurdles or limitations that make them challenging to implement.

Solar, for its part, has largely proven to be a reliable electrical source for many places around the world, but even solar has drawbacks. Firstly, solar energy is dependent on sunlight, so it can only generate electricity during the day (excepting bright moonlit nights where some solar panels can produce as much as 0.02% of their capacity). Secondly, solar energy output also drops during overcast and rainy days which makes solar energy most efficient in hot, dry places where there is little rain or clouds. Since a good portion of the northern hemisphere has a wet overcast climate, solar panels in places such as the UK and US face some serious energy penalties.

If only engineers and scientists could find a way to harness more energy during rain…

Harnessing the Energy of Rain with Nanogenerators

Scientists from Soochow University in Taiwan have created a solar panel that combines triboelectric nanogenerators with the top layer of the solar cell. A “triboelectric nanogenerator” is a nanodevice that converts mechanical energy into electrical energy—but, unlike piezoelectric devices, triboelectric nanogenerators take advantage of dissimilar materials producing a static charge when rubbed together.

Older designs of solar panels with integrated nanogenerators had the nanogenerator layer on top of the panel but this resulted in a lack of transparency and therefore a reduction in solar energy efficiency. The Soochow team instead created a nanogenerator layer that doubles as the top layer for the solar panel. The resulting panel can generate electricity when it is sunny and when raindrops hit the solar panel.

Solar panels are one of the most popular renewable energy sources. Image courtesy of Mike Buckawicki

But Soochow University is not the only entity attempting to create an all-weather solar panel; a second team from Yunnan Normal University and Ocean University of China have created a panel based on graphene. When raindrops hit the aqueous graphene, it causes the raindrop to disassociate into positive (salts, etc.) and negative ions (free electrons in the graphene). These ions, along with the graphene layer, interact to create a capacitive effect which can store electrical energy in the form of a potential difference. But this design is still in the early days and has yet to advance beyond “proof of concept” before it can be considered as a viable source of electricity.

How Much Energy Can You Get from Rain? A Thought Exercise

So, Soochow University has created a panel that can generate electricity from rain—but how much? Interestingly, the team announced a solar panel efficiency of 13%, which falls within the standard 10-15% efficiencies of commercial panels. This means that the nanogenerators are clearly not impeding typical solar energy harvesting, but how much energy is actively generated from the rain? Currently, there is no answer but we can determine what energies we can expect and if they are viable at all with a little bit of mathematics and a good amount of speculation.

To calculate how much energy we could get from raindrops in the form of mechanical energy, let's try to calculate the kinetic energy of each raindrop and the average number of raindrops falling per square meter per unit time. I'll use the UK—my home and a famously rainy area—as an example area.

Raindrops are rather small—this 2004 paper from the American Meteorological Society says that raindrops do not typically exceed an average diameter of 2.5mm. Since raindrops are almost perfectly spherical, we can calculate the volume and therefore the mass of the raindrop. A diameter of 2.5 mm gives us a volume of 8.18mm3, which contains 0.00818ml of water and thus the mass of said raindrop will be approximately 0.00818 grams or 8.18e-6 kg. The terminal velocity of a typical raindrop is 10 meters per second and therefore the energy that a falling raindrop has is approximately 0.000409J or 4.09e-4 J.

Each raindrop contains some amount of mechanical energy.

For the UK the average rainfall is 885mm per year and, since this measurement is irrespective of area, then we can make a broad calculation of how much rain makes up a square meter of rainfall! The total volume of a 1 x 1 meter area whose annual rainfall is 885mm is 0.885m3 and therefore the number of raindrops in this area is approximately equal to (using the raindrop volume of 8.18mm3) 105,000,000 and therefore the combined kinetic energy is 42,945J. Considering that there are 133 rainy days in the UK then the total energy rate output can be calculated. There are 11491200 seconds in 133 days and, knowing the time and total energy, the average energy output from rain's kinetic energy is 0.0037W / m2.

This energy reading is almost insignificant, which may suggest that the raindrop interaction with the material produces more energy. However, energy is always conserved and rubbing two materials together to create a static charge cannot be greater than the work done needed to overcome the friction force between the materials. The mathematics here rely on assumptions and approximations—but, for rain harvesting to be economical, the energy figure above would need to be several orders of magnitude greater. That may be a tall order. Rain would probably produce more energy if it was collected into a water storage unit and then allowed to empty and pass a turbine in a similar fashion to hydroelectricity.

Again, this is a thought experiment and any actual calculations of these panels' energy harvesting capabilities will need to come from the researchers, themselves. If any stray meteorologists have input on my maths here, please do let me know in the comments below.

So where does this leave “all-weather” solar energy?

In order to see how these new panels could be used to help tap more energy from the environment, we need to see data on the nanogenerators and how much electricity they generate. There are several techniques through which this nanogenerator technique could contribute to the renewable energy industry, possibly by improving the efficiency of solar panels or by being incorporated into energy harvesting facilities in other ways.

The concept of rain-energy harvesting has sparked imaginations and innovations for years, oftentimes using piezoelectric polymers to capture the mechanical energy of raindrops. This technique has been demonstrated by researchers at CEA/Leti-Minatec in France and even an enterprising 14-year-old in 2014 for a Google Science Fair project.

This most recent research is rather unique in that it represents a will to combine solar with rain-generated energy while utilizing triboelectric nanogenerators. We're not exactly likely to see nanogenerators incorporated into solar panels on every roof any time soon—but this work can provide important context for future innovations.