Solar panels on your rooftop can slash an electricity bill, but also sell energy to the power companies.

Solar Panel

roof solar panelsThe power of individual solar cells is small up to 1-2 Wp, and as such cannot be in a wider use. Therefore, PV cells mechanically and electrically are connected into larger units called solar module or solar panel (set of one or more electrically connected modules). In order to obtain the higher power, the modules are connected by the same principle in the so-called photovoltaic panels, and then to an array of solar panels , which power can be several tens of MW. Thus, it is a field of interconnected solar modules of 30-36 serial connected solar cells. Each cell in the solar radiation produces a voltage of about 0.5V, so the total voltage of the module 15-18V.

If two 40Wp photovoltaic panels, each one provides a nominal voltage of 16V and a nominal current of 2.5A, are connected in parallel (Fig. 1a), it will be given the voltage of 16V and current of 5A on the circuit terminals, and if we tie them in series (Figure 1b) the terminal voltage will be 32V and current 2.5A. In both cases, it is the same power 80W. So the parallel panel bonding increases the power of the whole system, the serial binding makes the higher output voltage. Each solar panel has a specified value (50Wp, 70Wp, 80Wp, read Watt-peak), which means how much power might give to the loads at nominal radiation from 1000W/m2. The total amount of the output voltage of the system is proportional to the number of series-connected panels, while the output current is proportional to the number of parallel connected PV panels.

parallel and serial connection of solar panels

Figure 1. a) Parralel and Serial connection b) of solar cells

Solar panels use solar energy to generate electricity through the photovoltaic effect. The majority of modules is crystalline, polycrystalline silicon cells or thin-film cells based on cadmium telluride, amorphous silicon, or organic material. The conducting wires that take the current off the panels may contain silver, copper or other conductive transition metals.The solar cells must be connected electrically to one another, and to the rest of the solar power system. Also they must be protected from mechanical damage and moisture. Most solar panels are rigid, but the thin-film cells are semi-flexible. And, like mentioned above, electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired current capability.

american solar panels

Figure 2. American solar panels

Separate Schottky diodes are needed to avoid reverse currents, in case of partial or total shading, and at night. Reverse currents waste electrical power and can also lead to overheating of shaded solar panels. Solar cells become less efficient at higher temperatures and installers try to provide good ventilation behind them. Some PV modules include concentrators in which light is focused by lenses or mirrors onto an array of smaller cells. This increase the efficiency factor.

Depending on construction and basic material, photovoltaic panels can produce electricity from a wide range of wavelength, but usually cannot cover the entire solar range, specifically ultraviolet and infrared. Hence much of the incident solar energy is wasted by solar panels.Therefore, another design concept of pn junctions is to split the light into different wavelength ranges, different pn junction for each range. This has been projected to be capable of raising efficiency by 30-50% (depands on nuber of pn junctions).

NASA Satelite Solar PanelsFigure 3. NASA Satellite Solar Panels

Currently the best achieved solar panel efficiency is around 43% in labs. The energy density of a PV moduls is the efficiency described in terms of peak power output per unit of surface area, commonly expressed in units of Watts per square meter (W/m2).

Solar panel performance is generally rated under standard test conditions (STC): irradiance of 1,000 W/m², solar spectrum of AM1.5G and its temperature at 25°C. Electrical characteristics include nominal power (PMAX, measured in W), open circuit voltage (VOC, measured in V), short circuit current (ISC, measured in A), maximum or nominal power voltage (VMPP), maximum or nominal power current (IMPP), peak power, kWp, and efficiency η (%).

solar power carFigure 4. Solar Power Car

Nominal voltage refers to the PMAX. The actual voltage output of the PV module changes as lighting, temperature and load conditions change, so there is never one specific voltage at which it operates. Nominal voltage allows users, at a glance, to make sure the solar panel is compatible with a electrical system. Open circuit voltage, VOC is the maximum voltage that the panel can produce when not connected to an electrical circuit – with no current to it. The peak power rating, kWp, is the maximum output according under standard test conditions and that is not the maximum possible output power .

Solar panels must withstand heat, cold, rain and hail for many years: crystalline, both mono and poly silicon module manufacturers offer a warranty that guarantees electrical production for 10 years at 90% of rated power output and 25 years at 80%.

Leipzig Germany solar array

Figure 5. Leipzig Germany solar array

Average pricing information divides in three pricing categories: small quantities in the kilowatt range, mid-range up to 1 MWp, and large quantity with access to the lowest prices. Over the long term there is clearly a systematic reduction in the price of solar panels and desired ROI. For example in 1998 it was estimated that the quantity cost per watt was about $4.50, which was 33 times lower than the cost in 1970 of $150, currently is about 0.87 – 1.5$/Wp. The price of solar panels are increasingly reduced, and as example, the current medium price panel is about 1.5 $/Wp, which means that for the nominal power of 100Wp panel, should be payed around $ 150, and prediction is that the crystalline silicon modules in 2012 will be at the average price of $1.17 per watt.

See also: Off-grid PV system, Grid-tied PV system, Solar Calculator.

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