| Questions and answers on photovoltaics |
We would like to answer the most frequently-asked questions on photovoltaics here. Clicking on the relevant question will take you directly to the answer. (All information is provided to the best of our knowledge and conscience, subject to errors.)What is photovoltaics and how do photovoltaic systems work?Photovoltaics is the conversion of the sun's energy into electrical energy or electricity. Photovoltaics is now used to generate electricity all over the world. The Greek word "photos" means light. 'Volta' was the surname of Alessandro Volta (1745 - 1827) who invented the battery and is one of the founders of the modern era of electricity. The unit of electrical tension, the 'volt', is named after him.Photovoltaic systems have large modules, which combine a number of individual solar cells. The solar cells (normally 36 units each) are connected in series within the modules. A solar cell provides 0.5 volts and a current of 3 to 7 amps, depending on its size. 36 cells in series produce 18 volts. The main element of a solar cell is silicon, extracted from melted sand. Silicon is literally as plentiful "as sand on the beach". However, the extraction process is very complicated. As soon as a solar cell is exposed to sunlight, a direct current is produced, which is then fed through a cable to the inverter, converted into alternating current and passed into the electricity network through a meter. This is an ongoing process, i.e. there is an uninterrupted flow of electrons for as long as the cell is exposed to light. Solar cells do not lose mass or get used up. How efficient is it?Efficiency is the relationship between the energy generated by a solar module and the radiated energy from the sun. In optimum conditions, the sun radiates about 1000 watts of radiated energy per square metro of module surface. However, the module only converts around 120 watts of this into electrical energy. This corresponds to an efficiency of 12 percent. What happens to the rest? Over half of the light that shines onto the surface is reflected. The conductor paths also have intrinsic resistance, plus there is loss of heat etc. This means that the efficiency of individual cells is greater than that of the module as a whole. The cells are spaced out within a solar module. Depending on the cell type and design of the modules, there can also be different levels of efficiencybetween different modules. For the user, only the module efficiency is relevant. What is the current level of technology?Currently, there are mono-crystalline, poly-crystalline and thin-layer modules available.In the crystalline modules, technical developments are already highly refined and only minor improvements can be made to these modules. The efficiency of solar cells is around 15 percent and any slight increase would be extremely complex. This would cause the already considerable costs to increase even further. Output from mono- and poly-crystalline modules is now the same. Mono-crystalline cells are drawn into a cell structure, poly-crystalline modules are cast in a block. In poly-crystalline modules, the cell walls hamper the flow of electrons, but this is no longer a major factor thanks to special techniques that are now available (diffusion with hydrogen atoms). Thin layer modules should soon become the most cost-effective option. In this module, the silicon is just sprayed onto a carrier material. The benefits of this are clear – automatic production involves a printing process, which is less time-consuming and extremely affordable. Current thin-layer modules have an efficiency of around eight percent – but this does not reduce yield, as they are much cheaper to produce. It is also possible that we will see technological developments for thin-layer modules. Where is development work leading?The trend is currently towards producing poly-crystalline modules in high unit numbers. It is likely that Japanese manufacturers will dominate the market with mass-produced goods. But the big step towards 'cheap solar modules' is still a long way off. Production of the necessary solar cells still makes up a massive 70 percent of the overall cost of a photovoltaic system.Why photovoltaics?Photovoltaics is a major global technology for producing electricity in the 21st century. However, there is still room for improvement.The melting process for extracting silicon is very complicated. Expensive silicon from microchip production is currently still used. Numerous module factories are currently being built, but the solar silicon required is still produced using complicated processes in too small quantities. Major investment would be necessary, but all manufacturers are nervous about this because of the uncertainty surrounding long-term political market backing. Making photovoltaics systems suitably ready for the market is a task whose completion will make a significant and sustainable contribution to preservation of the environment around the world. The market is crying out for decentralized energy units (especially in emerging countries). The technology itself is now refined. More than half of the world's population currently lives without any electricity. Photovoltaics is the perfect solution to provide fair, environmentally friendly and peaceful energy. Even in this country, a 2000-watt photovoltaic unit could meet half the energy requirements of a four-person household. How long does a photovoltaic system last?The life expectancy of a photovoltaic system is over 30 years.The solar cells themselves can go on working forever but the solar modules can get old and leaky. The cells are sealed in artificial resin between layers of safety glass and completely resistant film. The manufacturers guarantee that the solar modules will continue to produce around 80 percent of their rated output as per the data sheet after 25 years. We are often asked, "should I wait a while?" The answer is simple – "no, waiting is not worth it." Prices will not drop significantly in the short term. However, the long-term subsidies are still high but are being cut every year. So you will be paying the same amount for a photovoltaic system in the future. Is my roof suitable?Roofs need to face south-east to south-west and have a pitch of between 10 and 60 degrees. The roof must not be in the shade. You should check whether shadows are cast on the roof in spring or autumn. However, in winter, shadows are not relevant as only a fraction of the annual output is produced during the cold season. Throughout the rest of the year, there should be now shadows over the intended site. For a flat roof, shadows are also a consideration. In this case, the modules are made from individual aluminum sheet consoles and placed on the flat roof. They are then aligned towards the south with a 15-30 degree incline. On a façade (vertical mounting), the output is around 25 percent lower.Will it make my roof leak?No! The pan tiles on a pan tiled roof are not damaged. The roof hooks are screwed into the rafters and protrude beyond the pan tiles. The unit can therefore be removed and your roof is the same as before. With a flat roof, the unit is placed on the roof on a rubber mat so there is no contact. The units are then secured against windage with ballast (concrete blocks of between 50 and 100 kg).Can I use the photovoltaic system instead of roof tiles?In principle, yes, but the work involved and the costs are currently more than the saving. Incorporating the system into the roof is complex. Transitions between the modules and tiles need to be permanent. The modules are sealed from above using aluminum strips which form a rim of dirt. Silicon solar modules currently have a temperature-dependent voltage response. The voltage drops as the temperature increases since the electrons can migrate as a result of the heat energy and are lost in terms of the solar cell output. So the modules need to be back-ventilated, as their efficiency decreases with increasing temperature.How does an inverter work?Inverters automatically convert the direct current into alternating current and always feed the maximum output into the network. The maximum performance point (MPP) is also calculated automatically. The sinus wave (~) = alternating current (230 volt) of our electricity supply is measured by the inverter several thousand times a second and the alternating current generated is fed into the power network accordingly.Inverters are maintenance-free and work on a transistor basis (IGBT). Their life expectancy is comparable to that of other electronic devices. SMA inverters do not need to be maintained or inspected. The guarantee period is five years. After the end of the guarantee period, faulty devices can be replaced for a small repair fee. How big should a photovoltaic system be?A photovoltaic system must display at least 1000 watts of installed module performance in order to receive subsidies. There are no upper limits. We advise a minimum size of around 2000 watts. The price to benefit ratio pays off from around this level. An optimum 1000-watt photovoltaic unit generates between 800 and 1000 kWh per year. A four-person household consumes an average of 4000 kWh per year.A 5000-watt photovoltaic unit would generate enough to cover the annual consumption of a four-person household. The photovoltaic system must be permanently connected to the mains. (no isolated or switchable systems) Can I install a photovoltaic system myself? Installing a photovoltaic system is relatively simple. However, working on the roof is a major obstacle as it requires particular skill and dexterity. The frame and modules are fitted over two days. The electrical connections have socket connectors and are very simple and safe. All that remains to do is screw the inverter to the wall and the system is ready. An electrician can connect it to the mains. The energy provider will install a feed-in meter. It is also worth checking with your electrician whether he is happy to fit the whole system. We can provide all the know-how (and he gets a discount). What is the annual profit?As of 01/10/2010, systems of up to 30 kW are paid 33.03 cents per kWh fed into the system for 20 years.end faq |
