Figure 4: IV and PV curves of a photovoltaic cell.
The UAVs are equipped with solar cells on top and down of their wings and their energy sources are solar power and rechargeable batteries. Solar cells are so thin and flexible they can easily be adhered directly to a wing or fuselage surface with negligible impact to aerodynamics. It is also possible to integrate the cells directly into carbon fiber or fiberglass resulting in a seamless structure reducing the weight to a minimum while maintaining the right aerodynamics. The solar cells are connected to the system via an electronic switch, so we are able to disconnect the cells when the battery is full or in a failure situation. Solar cells are divided into three main categories called generations. UAV application require:
-Flexible material that can easily be integrated into the wing,
-High efficiency solar due to limited surface area,
-Lightweight material to maintain payload capability
There are many considerations when designing a solar-powered device such as the environment indoor and/or outdoor, maximum power point tracking to maximize power while minimizing weight to optimize power-to-weight efficiency of solar cells in indoor/outdoor and so on. Typical low cost commercial solutions, such as silicon, are high in power, but heavy. The first generation solar cells as monocrystalline, polycrystalline, gain in efficiency but they sacrifice in flexibility.  Important values are reported in table 6.   The second-generation solar cells popularly known as thin-film solar cells (TPSC) or thin-film photovoltaics (TFPV) are about 100 times thinner than materials of first generation. These materials gain in efficiency but sacrifice in efficiency. As far as irradiation is concerned, it must also be possible to provide a high level of direct radiation involving the panels placed on the top of the aircraft and the radiation component reflected by the clouds or the underlying ground (albedo) which instead invests the modules placed below the wings. In all amorphous materials, light should not have a preferential direction to be absorbed: consequently, the yield is not particularly affected even under light conditions (typical of cloudy days) or inclination of panels other than the optimum. Alta Device Company has obtained an advanced Gallium Arsenide (GaAs) lightweight, thin and flexible cell more efficiency that previous GaAs-based technology. Our solar cells have been verified for both single and dual junction cell efficiencies – 28.8% and 30.8% respectively. Another materials for indoor and outdoor conditions are cadmium-telluride (Cd-Te), copper indium gallium diselenide (CIGS), they're extremely thin, light, and flexible, low cost, however both materials are low in power and in efficiency. This means that to get the same amount of energy than GaAs or first generation’s materials you will need to have a larger area on surface of wings. In third generation new materials as perovskite crystals, quantum dot, quantum well, DSSC and feature multiple junctions (made from multiple layers of different semiconducting materials) promise high efficiency solar than to limited surface area cheaper and able to integrate into the wing. The materials operate under real-world operating conditions, generating electricity under a broad range of temperature and cloud-cover conditions and at all times of the day from dawn until dusk.
 
 
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