Photovoltaic panels generate electricity when sun shines on them. However, the energy crisis to operate under partial or shaded sun conditions is met through the design of Maximum Power Point Tracker (MPPT) system. MPPT systems generate electricity from solar panels irrespective of the alterations in environmental conditions. The extensive applications surveyed for PV panels using MPPT explores that solar panels have now been triggered to produce electricity even under rain. The incredible material ‘Graphene’ is analyzed in this context with its properties and promising use in generating electricity through solar panels. There are a number of applications available in different fields based on Graphene technology. In this paper, the mechanism of Graphene based application systems using rain drop utilization technology is analyzed.

Graphene Properties and Uses

Graphene is a two-dimensional form of carbon in which the atoms are bonded into a honeycomb arrangement. Graphene is just one atom thick, and can both simplify and revolutionize tissue technology to drug delivery applications. When combined with salt water, it has unexpected potential to generate electric potential. The unique properties of graphene that makes it appropriate in terms of strength, electricity and heat conduction are:

1. The crystalline structure of graphene comprises carbon to carbon bonds. The chemical bonding, of each graphene atom is such that it is connected to 3 other carbon atoms on the two dimensional plane, leaving 1 electron freely available in the third dimension for electronic conduction. These highly-mobile electrons are called pi (π) electrons located above and below the graphene sheet. These pi orbital’s overlap and help to enhance the carbon to carbon bonds preventing thermal fluctuations from destabilizing it.
2. It possesses very thin atomic thickness of about 0.345Nm. As a result, it acts very much like photons in their mobility due to lack of mass. This enables charge carriers to travel sub-micrometer distances without scattering; known as ballistic transport.
3. Another useful semiconducting property of graphene is its zero-overlap Energy band gap. As such, both holes and electrons available as charge carriers with very high electrical conductivity.
4. The Dirac point in graphene, electrons and holes has zero effective mass. This occurs because the spectrum for excitations or energy movement is linear for low energies near the 6 individual corners of the Brillouin zone. These electrons and holes are known as Dirac fermions, or Graphinos, and the 6 corners of the Brillouin zone are known as the Dirac points. Due to the zero density of states at the Dirac points, electronic conductivity is quite low whose Fermi level can be changed by doping (with electrons or holes) to create potentially better material.
5. Another of graphene’s stand-out properties is its inherent strength. Due to the strength of its 0.142 Nm-long carbon bonds, graphene is the strongest material ever discovered, with an ultimate tensile strength of 130,000,000,000 Pascals (or 130 gigapascals), compared to 400,000,000 for A36 structural steel, or 375,700,000 for Aramid (Kevlar).
6. Graphene is also very light at 0.77milligrams per square metre (for comparison purposes, 1 square metre of paper is roughly 1000 times heavier). It is often said that a single sheet of graphene (being only 1 atom thick), sufficient in size enough to cover a whole football field, would weigh under 1 single gram.
7. Another notable characteristic feature of graphene is that it also contains elastic and absorbing property. It retains its initial size after strain. It can absorb 2.3% of white light due to its electrons that act like a mass less charge carriers with very high mobility. Once the optical intensity reaches a certain threshold (known as the saturation fluence) saturable absorption takes place (very high intensity light causes a reduction in absorption). This is an important characteristic with regards to the mode-locking of fibre lasers.

Earlier uses of graphene were limited to boost solar energy technologies through creation of a effective absorbing material for ambient heat and light. Nowadays, it is being looked upon to eventually to lead to applications using solar panels that can work with the diffuse sunlight and finds its way indoors. 

Working Operation

In order to allow rain to produce electricity under rain as well, solar cells have been dye-sensitized i.e. they have coated solar cell with a whisper-thin film of Graphene. It can readily be prepared by the oxidation, exfoliation, and subsequent reduction of graphite. Graphene is characterized by its unusual electronic properties. Moreover, it conducts electricity and is rich in electrons that can move freely across the entire layer (delocalized). It also possess features like 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 . 

There are number of materials used in chemical composition of Graphene. Chemically derived Graphene oxide (GO) is an atomically thin sheet of graphite that has traditionally served as a precursor for Graphene. It is covalently decorated with oxygen-containing functional groups either on the basal plane or at the edges so that it contains a mixture of sp² and sp³ hybridized carbon atoms. In particular, manipulation of the size, shape and relative fraction of the sp² hybridized domains of GO by reduction chemistry provides opportunities for tailoring its optoelectronic properties. Furthermore, in contrast to pure Graphene, GO is fluorescent over a broad range of wavelengths, owing to its heterogeneous electronic structure.

       The electrical conductivity of reduced graphemes oxide (rGO) obtained from graphene oxide (GO) using sodium borohydride (NaBH₄) as a reducing agent has been investigated as a function of time (2 min to 24 h) and temperature (20 °C to 80 °C) by researchers. The conductivity increases with increase in time but saturates during the residual hours of reaction. The reduction rate decreases significantly due to the elimination of oxygen. The steep increase in conductivity is recorded during the first 8-12 min due to the reduction of C-O (e.g., hydroxyl and epoxy) groups. The reduction of epoxy or hydroxyl groups had a greater impact on the restoration of the conductive nature of the graphite structure in rGO.

Polyethylene terephthalate (ethylene terephthalate)  PET, PETE, or PET-P, is the most common thermoplastic polymer resin of the polyester family and is used in fibers for clothing, containers for liquids and foods, thermoforming for manufacturing, and in combination with glass fiber for engineering resins.

The scientists have added Graphene to a dye-sensitized solar cell, a kind of inexpensive thin-film solar cell, then put them on a flexible, transparent backing of indium tin oxide and plastic. The resulting flexible solar cell demonstrated a solar-to-electric conversion efficiency of up to 6.53 percent, and generated hundreds of microvolt from slightly salty water that was used to simulate rainwater. The same technology can be used by future solar cells to produce electricity in all weather. 

The current is generation from rain impinged on surface of solar panel. The raindrops are not pure water but contain salt that splits up into positive and negative ions. The grapheme electrons can attract the positively charged ions, such as sodium, calcium and ammonium towards itself. The result is that layers of positive and negative ions are separated and act much like a capacitor to store energy. 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 result in a double-layer made of electrons and positively charged ions, a feature known as a pseudo-capacitor.

The difference in potential associated with this phenomenon produces a voltage and a current. In general, raindrops would form a layer of positive ions, which are found in the salt of rainwater. Meanwhile, Graphene is rich with delocalized electrons that are free to move around. This forms a double layer where positive and negative charges are separated thereby creating a voltage, just like in a battery. The voltage output generated as such can be used for various electricity generated applications. 

Application Systems using Graphene

 The commercially available Graphene MPPT systems are in China, Britain and Mexico. These are stimulated by incident light on sunny days and raindrops on cloudy days. Amongst few systems, SMILE (Solar Mobile Independent Low cost Energy System) is developed by using transparent Graphene based solar panels to be embedded on walls of buildings to develop BMS (Building Management System). The panels on building would be generating electricity even under rainy weather. The SMILE technology is a first attempt to bind Graphene that binds electrons of solar panel with positively charged ions present in rain water. The salt of rain separates this bonding and as such natural water gets combined with Graphene. This results in dual layer between Graphene and natural water whose energy difference generates electricity