Researchers develop all-weather solar cell that generates power from rain as well as from sun
03 April 2016
While many technical advances have made solar cells more efficient and affordable, a disadvantage remains in the fact that solar cells produce no power when it’s raining. Now, however, researchers from the Ocean University of China (Qingdao) and Yunnan Normal University (Kunming, China) have developed an all-weather solar cell that is triggered by both sunlight and raindrops by combining an electron-enriched graphene electrode with a dye-sensitized solar cell.
The new solar cell can be excited by incident light on sunny days and raindrops on rainy days, yielding an optimal solar-to-electric conversion efficiency of 6.53% under AM 1.5 irradiation and current over microamps as well as a voltage of hundreds of microvolts by simulated raindrops. Their work is published as a “Very Important Paper” in the journal Angewandte Chemie.
Graphene is a two-dimensional form of carbon in which the atoms are bonded into a honeycomb arrangement. It can readily be prepared by the oxidation, exfoliation, and subsequent reduction of graphite. Graphene is characterized by its unusual electronic properties: it conducts electricity and is rich in electrons that can move freely across the entire layer (delocalized).
In aqueous solution, graphene can bind positively charged ions with its electrons (Lewis acid-base interaction). This property is used in graphene-based processes to remove lead ions and organic dyes from solutions.
This phenomenon inspired researchers working with Qunwei Tang to use graphene electrodes to obtain power from the impact of raindrops.
Raindrops are not pure water; they contain salts that dissociate into positive and negative ions. The positively charged ions, including sodium, calcium, and ammonium ions, can bind to the graphene surface.
At the point of contact between the raindrop and the graphene, the water becomes enriched in positive ions and the graphene becomes enriched in delocalized electrons. This results in a double-layer made of electrons and positively charged ions, a feature known as a pseudocapacitor. The difference in potential associated with this phenomenon is sufficient to produce a voltage and current.
On rainy days, the new solar cells can be reversed with rGO film upward, creating current and voltage outputs under he persistent dropping of raindrops (NaCl aqueous solution with concentration of 0.6m, 1m, or 2m is used as simulated rain). … Moreover, each droplet can yield voltage and output power of 152.6 mV and 7.17 pW for 9.46 mm, 54.7 mV and 20.7 pW for 7.58 mm, and 25.1 mV and 84.7 pW for 4.52 mm.
… When dropping onto a rGO surface, simulated raindrops will quickly spread to the periphery. In this fashion, the cations are adsorbed at the front end, advancing the pseudocapacitor forward and drawing electrons in the rGO. The migration of drawing electrons creates current. Subsequently, the cations are desorbed at the rear end of the droplets during the shrinking process due to a hydrophobic rGO surface … discharging the pseudocapacitor and releasing the p-electrons to rGO. This discharging process results in a reduced current.—Tang et al.
The research team suggested that the work opens a viable concept of developing all-weather solar cells that can persistently generate electricity.
Tang, Q., Wang, X., Yang, P. and He, B. (2016) “A Solar Cell That Is Triggered by Sun and Rain.” Angew. Chem. Int. Ed. doi: 10.1002/anie.201602114
How does it perform during cloudy days, without sunshine nor rain?
Posted by: HarveyD | 03 April 2016 at 10:51 AM
Considering the 1st law of thermodynamics (conservation of energy) how much "raindrop energy" is lost by the raindrops (What kind of energy is it) and how much of it is converted to electricity?
The amount of energy due to diluted salts in rainwater is next to zero, and it is hardly harvestable (already next to maximal entropy)
This is a nice research work but will always be completely insignificant for energy production. Picowatts indeed.
Posted by: Alain | 03 April 2016 at 01:23 PM
Solar modules are approaching 20% efficiency; this is grad student grant work; want to do something worthwhile? Work on making solar less costly and more efficient when heated in the sun.
Posted by: Lad | 03 April 2016 at 08:53 PM
"Vancouver BC has only 166 days per year with measurable precipitation (rain, snow, hail..) on average, and 289 days of sunshine. [For a total of 455 days - on most rainy days we still get to see the sun.] However, when it is raining it can rain for days. It is not uncommon from November until March to see 20 consecutive days with some amount of rain."
Cloudy days are not that much of a problem for photovoltaics. They don't need direct light to work - they work just fine in ambient light - and will produce significant energy in the fog or on overcast days. In fact, solar panels are actually more efficient at cooler temperatures than hot ones so although they receive less light on cloudy days they use it better.
Having said that: On a cloudy day, typical solar panels can produce 10-25% of their rated capacity. The exact amount will vary depending on the density of the clouds, and may also vary by the type of solar panel; some kinds of panels are better at receiving diffuse light.
Cloudier locations are still a good match for solar.
Germany gets only about as much sunshine as the state of Alaska, but Germans have successfully installed about 25 gigawatts of solar power– half of the entire world’s supply. Portland, Oregon is known for its rainy, dreary winters, but is another good location for solar power: over a full year, despite the winter weather, Portland gets as much sunshine as the average U.S. city. Cities like Portland also have slightly cooler weather than average, which is an advantage for solar panels. Because of the electronics inside, solar panels work best when they aren’t too hot. In a city with extreme summer heat, solar is a little less efficient, which is part of the reason why solar panels in cloudy San Francisco can actually produce more power over a year than the slightly sunnier, hotter city of Sacramento.
A silver lining to that cloud: how the “edge of cloud” effect can produce more solar power than a sunny day.
If you have solar panels and keep a close watch on your power output, you may have noticed a strange phenomenon: on a partly cloudy day, it’s possible to exceed your solar system’s power rating and produce more power that you could on a sunny day. Known as the “edge of cloud” effect, this happens when the sun passes over the outer edge of a cloud, magnifying the sunlight. The intense light causes your solar system to boost power output temporarily, which can help balance out losses from full cloud cover. Solar installers typically select system components that can handle temporary power boosts of this nature (similar effects can occur when sunlight is reflected off snow or water). If you live in a city with frequent partly-cloudy weather, like Seattle, you may choose to install an over-sized solar inverter to take the best advantage of these power boosts.
Another thing to consider - Ultraviolet. UV light also reaches the earth’s surface in abundance during cloudy days (if you’ve ever been at the beach when it’s cloudy and gotten a sunburn, you’ve experienced this firsthand). Some solar cells are in development that can capture UV rays, although these are not out on the market yet.
Posted by: ai_vin | 03 April 2016 at 10:29 PM
April fool, I would think.
Posted by: mahonj | 04 April 2016 at 01:12 AM
Quick & dirty:
".. for a cloudburst it’s 113 (1,220(drops/sq. meter) http://www.kgbanswers.com/how-many-raindrops-fall-in-one-second/4191107
"and 84.7 pW for 4.52 mm.(~biggest drops)
sounds like microwatts
Posted by: kelly | 04 April 2016 at 03:54 AM
Nanowatts actually, about 100/sec. To get to microwatts you'd need for this cloudburst to last a full minute and even then you'd only have about 6 microwatts generated.
Posted by: ai_vin | 04 April 2016 at 10:41 AM
If you want to generate energy from the rainfall that hits your roof. . . try putting a water turbine on your downspout, snark;
At least here; https://www.youtube.com/watch?v=1fJ6E0BEDMU it did something useful.
Posted by: ai_vin | 04 April 2016 at 11:09 AM
ai_vin, it is not a lens effect at the edge of a cloud that causes solar output to spike briefly. Solar panels are more efficient when cooler. So when the sun comes out from behind a cloud, the power spikes until the panel warms back up to equilibrium. This is the same reason that peak sustained output tends to be highest in the cooler windy spring, although total energy is lower due to shorter days.
Posted by: James McLaughlin | 05 April 2016 at 03:23 PM
I'm not talking about the kind of cloud cover that blocks direct light here. (that was earlier) I talking about partly cloudy days. When your panel is in direct light because it's under a part of the sky that is clear it can also receive reflected light from the white clouds in another part of the sky.
Posted by: ai_vin | 05 April 2016 at 08:45 PM