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Solar power is becoming increasingly popular as a clean and renewable source of power generation all over the world. The cost of production of commercial solar panels has dropped by more than half since 2012. In fact, the International Energy Agency forecasts that renewable energy sources will contribute no less than 40% of all generated power in the world, by 2040.
In such a scenario, solar panels are a preferred choice among many customers. Solar panels have been undergoing advances in development over the decades, but so far a major drawback has been that they are not functional under rainy conditions. This limitation has now been overcome, admittedly to a minute extent, by a proof-of-concept experiment by researchers in China’s Soochow University.
The heart of the new technology used in hybrid-type solar panel is a triboelectric nanogenerator (TENG), and this allows power to be generated by the cell not only when sunlight falls on it but also rain.
What is a TENG?
A nanogenerator is a machine which is capable of converting the mechanical energy of a moving particle into electricity, and the TENG performs this function for raindrops by using the friction of the sliding raindrop to knock off some electrons and produce usable electrical charge. TENGs are already in use to generate power from car tyres rolling over a road surface, from the friction of clothing materials against each other, and other forms of friction. However, compared to these applications, it is quite difficult to develop a TENG that would work for raindrops without increasing the size or complexity of the solar panel beyond a limit. These panels are designed to fit upon rooftops, in most cases, and they could not cross a size limit.
Structure of a Hybrid Solar Panel
The Polymer Solar Panel
The basic structure of the newest hybrid solar panel is one which is designed to combine both a heterojunction silicon solar cell and a TENG device composed of paired polymer layers positioned above a photovoltaic cell or conventional solar panel. These are grooved in a manner identical to that of a standard DVD, using a DVD imprint, to enhance the efficiency with which it can gather frictional energy from the falling raindrops as well as helping the solar cell to capture more sunlight. This is necessary because on a rainy day the sunlight has only one-tenth of the intensity it has on a normal day.
The upper imprinted polydimethylsiloxane (PDMS) is the actual TENG (operating with a single electrode as mentioned above). The grooved polymer layer below it is the mutual electrode that connects both the solar panel and the TENG. It is made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film. The imprinted PEDOT:PSS film decreases the level of reflection of light causing increased short-circuit density of current. Once raindrops hit the topmost layer, it loses an electron and becomes positive, while the surface of the PDMS film becomes negative. This leads to the buildup of differential charge between the negative surface and the positive raindrop, which can be trapped and conducted to the lower layer by the nanogenerator. As more of the imprinted PDMS comes into contact with the rain drops, the TENG output goes up, peaking at about 33 nA short-circuit current, and at 2.14 V at peak open-circuit voltage. This provides both the high level of current that is generated by a solar cell with the high voltage produced by a TENG, which optimizes energy gathering efficiency under a range of weather conditions.
At the same time, the transparency of the polymer layers meant that sunlight could pass through them to create energy transfer in the conventional manner of a solar cell. However, the efficiency of energy capture is reduced to about 14% by the presence of the additional layers, compared to the 20% that is more characteristic of a good-quality solar cell on the market today. Even this model is an advance over the earlier sandwich-type build which had a TENG layer above, an insulator in between and the solar panel below. This led to the loss of too much solar energy for the photovoltaic unit to function well. This has now been replaced by the single electrode, and the imprinting with DVD grooves pushes up the efficiency of power conversion to almost 14% in sunlight, and about 2 volts in simulated rain. This could be magnified hundred-fold by increasing the surface area of the cell.
The Graphene Solar Panel
The first step in this direction was taken in 2016, by researchers at the Ocean University of China. They used a layer of graphene, a marvelous 2D material which is making waves in many applications, along with an ordinary solar cell, to generate electricity from falling rain. In this case the energy was not frictional but due to the ability of graphene (which has a sea of delocalized electrons) to act as a pseudocapacitor. That is, it acted by binding to positive ions in salty rain, which created two layers that had varying levels of energy relative to each other, and thus generated an electric current because of the separation between the two layers. Natural rainwater is not neutral but contains ammonium, calcium and sodium cations.
The amount of electric current generated in this manner is measured in microvolts, which is far from being anywhere close to that created even by an ordinary AA battery cell. Obviously, this experiment is only an early step and much more work remains to be done to bring hybrid solar cells to the brink of commercial viability.
Another limitation was the use of seawater instead of true rain, because the former is much more salty and contains far more ions. The cost of production of these models is also prohibitive at present but could come down with the use of large-scale manufacture of the materials and perhaps the use of newer materials. For instance, fern-like graphene sheets might increase the efficiency of the panel.
Despite the many difficulties, these primitive projects show great promise because if they ever develop to a marketable level, we could watch solar panels generating power both during the day, from solar energy/rainfall, and even at night if it happened to rain. The renewable and clean energy source represented by solar cells would indeed take a huge leap forward if the research in this field is successful.