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What is photovoltaics? » How photovoltaics works » The photovoltaic effect » How does a solar cell work? » What is a solar cell comprised of? » From the cell to the module » Which parts make up a photovoltaic system? » What does Kilowatt Peak (kWp) mean? » Are there different degrees of efficiency? » What happens when weather conditions are cloudy? » Modules - the cooler the better! This is how photovoltaics works With the photovoltaic effect, photovoltaic systems use the most environmentally-friendly forms of generating electrical energy. "Photovoltaics" is the direct conversion of sunlight into electrical energy. The technology behind it is actually quite simple: Electricity from sunlight can be generated by solar cells whose primary component is a semiconductor, generally silicon. A semiconductor is a material that can not be categorised as either insulator or conductor and whose electrical properties can be influenced greatly by adding impurities (contamination). A solar cell comprises of two neighbouring semiconductor layers with separate metal contacts. The semiconductors are contaminated to create an "n" layer (n = negative) with an excess of electrons and a "p" layer (p = positive) underneath with an electron shortage. Because of the concentration difference, electrons flow from the n-field to the p-field such that an electrical field, known as the "space charge zone", forms inside this semiconductor structure. The photovoltaic effect On a solar cell, the upper n-layer is so thin that photons of the incident sunlight can penetrate it, not releasing its energy to an electron until in the space charge zone. The excited electron is free to move, follows the inner electrical field and exits the space charge zone onto the metal contacts of the n-layer. The circuit is closed on connection of a consumer: Electrons flow via the consumer to the rear-side contact of the solar cell and then back to the space charge zone. This effect is called "photovoltaic" (from the Greek word for light, "phos", and the name of the physicist Alessandro Volta). The DC current produced by the solar cells is converted into AC current by an inverter, the "heart" of the system. How does a solar cell work? Electricity from sunlight can be generated by solar cells whose primary component is a semiconductor, generally silicon. A semiconductor is a material that can not be categorised as either an insulator or a conductor and whose electrical properties can be influenced greatly by adding impurities (contamination). A solar cell comprises of two neighbouring semiconductor layers with separate metal contacts. The semiconductors are contaminated to create an "n" layer (n = negative) with an excess of electrons and a "p" layer (p = positive) underneath with an electron shortage. Because of the concentration difference, electrons flow from the n-field to the p-field such that an electrical field, known as the "space charge zone", forms inside this semiconductor structure. The photovoltaic effect On a solar cell, the upper n-layer is so thin that photons of the incident sunlight can penetrate it, not releasing its energy to an electron until in the space charge zone. The excited electron is free to move, follows the inner electrical field and exits the space charge zone onto the metal contacts of the n-layer. The circuit is closed on connection of a consumer: Electrons flow via the consumer to the rear-side contact of the solar cell and then back to the space charge zone. What is a solar cell comprised of? Over 95% of all solar cells manufacturer across the world comprise of the semiconductor material silicon (Si). As the second most common element in the earth's crust, silicon has the benefit of being available in sufficient quantities and the processing of the crystal is eco-friendly. A distinction is made between three cell types, depending on type of crystal: monocrystalline, polycrystalline and amorphous. The different cell types can again be differentiated based on manufacturing cost and degrees of efficiency. The degrees of efficiency of amorphous cells (so-called "thin-layer cells") lie below those of the other two cell types, but they are cheaper because the manufacturing process is not as complicated. From the cell to the module At maximum intensity of the solar irradiation (approx. 1000 Watt per m2), a radiated power of about 10 Watts falls onto a solar cell measuring 10x10cm. Depending on the quality, one such cell can release electrical power of between 1 and 1.5 Watt. To increase power, several cells are combined to make up a solar module. A solar generator is the term used for the connection of several modules. The menu bar in the top right provides more information on "photovoltaics". Which parts make up a photovoltaic system? A photovoltaic system generally comprises of solar cells (combined in solar modules), the inverter (converts DC to AC current) and the supply meter. In addition, systems are often configured with system monitoring and data visualisation functions. What does Kilowatt Peak (kWp) mean? Kilowatt Peak stands for peak performance. This figure specifies the power a solar module can reach at maximum insolation (under defined standard test conditions). Optimal solar irradiation of 1000 Watt per m2 is used as a standard condition - a figure attained in the mid-day heat on a summer's day in Germany. Most manufacturers also call peak performance "nominal rating" or "nominal power". Because they are based on measurements taken under ideal conditions, the peak power does not equate to the power under real irradiation conditions (in practise, these are approx. 15-20% lower because of the severe warming-up of solar cells). Are there different degrees of efficiency? The degree of efficiency essentially specifies the ratio of usable to used energy. The higher the degree of efficiency, the higher the ability to convert light rays into electricity. A differentiation is made between the degree of efficiency of cells, modules and systems. In commercial mass production, a cell efficiency of up to 18.3% is currently being attained (this is dependent on technology deployed). Module efficiency relates to the entire module surface and is therefore always slightly lower than cell efficiency. One of the reasons for this is the presence of non-usable gaps between neighbouring solar cells in the module. System efficiency relates to the solar power system as a whole. A further reduction in module efficiency is attributable to transmission losses, e.g. through cables. What happens when weather conditions are cloudy? PV modules do not only use direct sunlight under cloudless conditions, they also use diffuse lighting in cloudy conditions. The brighter it is outside, the higher the performance of the modules - irrespective of whether the sun is directly visibly or not. In Central Europe, diffuse lighting makes up for a good 50% of the insolation. Modules - the cooler the better! It is often an overlooked fact that PV systems usually work at their best at 25°C. The percentage of direct sun rays at, say, the equator is high compared to our latitudes, but the high outside temperatures considerable reduce the yield of the system as a result of the modules heating up. The drop in performance for prevalent silicon cells is around 0.4% per °C. So the cooler temperatures in Germany compensate noticeably for the weaker sun rays. Source: Solarpraxis AG |
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