Photovoltaic System Technology

Transcription

1 Photovoltaic System Technology Photovoltaic Cells What Does Photovoltaic Mean? Solar electricity is created using photovoltaic cells (or PV cells). The word photovoltaic is made up of two words: photo and voltaic. Photo comes from the Greek word, phos, which means light. The term voltaic comes from Alessandro Volta ( ), an Italian scientist, who invented the first chemical battery. The volt is the unit we use to measure the strength of electrical energy, so photovoltaic could be read as meaning light electricity. What Are Photovoltaic Cells? Photovoltaic cells can turn solar energy from sunlight directly into electricity. This electricity can be used immediately or stored in a battery for later use. To create more power, PV cells can be linked together. Photovoltaic panels are usually made of many PV cells linked up. In the past, PV cells have been especially useful for creating electricity in remote areas where it is too costly to link up power cables with the main power supply. A single photovoltaic cell. A typical PV cell is about 5cm in diameter and 1mm thick. Inside the Cells Silicon Photovoltaic cells are often made of two layers of silicon. The silicon used has a crystal-like structure. The most common PV cells use a different kind of silicon for each layer. The top layer of silicon has a metal contact grid on top of it like a wire netting. This grid links up with an outside circuit that returns to a metal sheet sitting below the bottom layer of silicon. 1

2 Outside circuit going from the metal contact grid to the metal contact plate metal contact grid top layer of silicon bottom layer of silicon metal contact plate A cross section of a silicon PV cell. On Top of the Cells A glass cover sits on top of the PV cells to protect them from the weather. Because silicon is a very shiny material, it would normally reflect some of the sun s light. Reflected light can t be used by the PV cells to create electricity. This is because the sun s reflected light would not be absorbed by the silicon. To solve this problem, the PV cells are covered with a layer of anti-reflective material that absorbs light. When Sunlight Hits Silicon Silicon does not usually conduct electricity very well, but this changes when silicon comes into contact with sunlight. Energy from the sun s rays knock free some of the silicon s electrons. This means they move around the silicon more easily, which creates an electric charge. Because the two silicon layers are slightly different, the electric charge moves from the bottom layer of silicon to the top layer. This is a bit like the electricity which zaps you if you have been rubbing wool on plastic. The spark or zap is the electricity jumping the field between the plastic and your body. We can talk about the strength of this field using the word voltage. Some of the free electrons also cross onto the metallic contact grid. This is because the electrons crossing from bottom to top create an electron traffic jam between the two layers. Like cars taking the off-ramp to avoid a traffic jam, some of the electrons flow on to the metal contact grid and along the circuit back to the metal sheet at the bottom of the PV cell. This flow is the electric current that is used to power lights, calculators and other electrical gadgets. When the electrons arrive back at the bottom layer of silicon, they join up with the atoms there once more, before crossing to the top layer of silicon again. 2

3 This diagram shows how a PV cell works. Free electrons flow to the metal contact grid and along the circuit. Schoolgen: Renewable Energy Energy that Escapes Some of the sun s energy will pass through the silicon without knocking electrons free. The maximum amount of energy a PV cell absorbs from the sun s light is about 25%, but usually PV cells absorb about 15% of the sunlight s energy. This is because not all of the sunlight has the right amount of energy to affect the electrons in the silicon. Most of the sunlight that strikes a PV cell has either too much or too little energy to knock the electrons free. Other energy is lost while the electrons move across the silicon or because the light hits the contact grid. Much of this lost energy becomes heat. Different Types of Silicon Photovoltaic cells can be made of a number of different kinds of silicon. Some PV cells are very expensive to make and highly efficient. Others are cheaper to manufacture but generate less electricity. Single Crystal (monocrystalline) Silicon Some PV cells are made by slicing silicon from a single large crystal that has been specially grown to make the PV cells. These crystals are pure and almost flawless. The crystals are expensive to grow and a lot of silicon is wasted in the saw dust. This means that these kinds of single crystal PV cells cost more to buy. Polycrystalline Silicon Polycrystalline PV cells are much cheaper to make. Melted silicon is poured into moulds, which sets. It is then sliced into fine wafers. These crystals have more flaws inside them so the PV cells are not as efficient at generating electricity as single crystal cells. 3

4 Amorphous Silicon Another kind of PV cell is made from amorphous silicon. This kind of PV cell is made by coating a backing such as glass with a thin layer (or film) of silicon. Rather than having a crystal structure, this kind of silicon has a make-up similar to glass. PV Systems A PV system can simply be a panel of PV cells linked to an appliance such as a light or fan. But most PV systems need to store energy or convert energy into a more suitable electric current. The electric current produced from a PV cell is direct current (dc). The direct current can be used to power special dc appliances. However, most appliances use alternating current (ac). An inverter changes the dc current into ac current. A battery can be used to store extra electricity that is not used immediately. This stored electricity can then supply energy when there is not enough sunlight or at night time. A control box is used to make sure the electricity from the PV panel goes into the battery with the correct voltage or current to charge the battery. Panel of PV cells Control box Power inverter Battery A simple stand-alone PV system that uses a power inverter to power an appliance, and a battery to store extra electricity. Stand-Alone PV Systems A PV system can be set up to run without any other power supply. This can be done by using a battery, which stores any extra electricity generated during the day. This electricity can be used later when there is not enough electricity being produced by the solar panels. 4

5 Australian Business Council for Sustainable Energy, Electricity From The Sun Solar PV Systems Explained From%20The%20Sun.pdf Research Institute for Sustainable Energy, CASE STUDY SYSTEM 3: Small Solar Home System or Caravan System A power generator (running on fuel, such as diesel) can also be used to back up the solar electricity supply. Research Institute for Sustainable Energy, CASE STUDY SYSTEM 1: Small Stand-Alone Power System Australian Business Council for Sustainable Energy, Electricity From The Sun Solar PV Systems Explained, page 17 the%20sun.pdf PV Systems Connected to Outside Power Lines A PV system can be connected to the main power grid. Any extra electricity generated by the PV system and not used can be fed into the electricity grid and sold to the power company. If there is not enough sunlight for the PV system to supply the energy needed, then electricity can by provided by the power company. Research Institute for Sustainable Energy, CASE STUDY SYSTEM 4: Grid-connected System Australian Business Council for Sustainable Energy, Electricity From The Sun Solar PV Systems Explained, page 7 the%20sun.pdf 5

6 Stationary and Tracking PV Panels To gain maximum light, the most effective PV panels track the sun as it moves across the sky. Compared to a stationary panel, a tracking device can increase the amount of electricity produced by as much as 40% in summer. Ideally, the tracking device would have two axes. One axis would follow the passage of the sun cross the sky and the other axis would adjust to the angle (or height) of the sun above the horizon. Some PV panels use mirrors or lenses to increase the intensity of the sunlight on the PV cells. This is to increase the amount of electricity generated. Factors that Limit PV Cell Effectiveness There are many factors that can limit the amount of electricity produced by a PV system. The amount of sunlight and the intensity of the light falling on the panel are two of these factors. A PV panel facing in the wrong direction, with shadows falling across it, or not kept clean will not produce as much electricity. When a school applies to have Schoolgen PV panels installed on its building s roofs, Genesis Energy checks to see if the school has roofs that are suitable. Shadows Usually, the amount of electricity generated by a PV panel is in proportion to the intensity of the sunlight that falls on it. But this is only if the entire PV panel is exposed to the sun. If even a small part of a PV panel is dirty or in shade, this will significantly reduce the amount of electricity produced by the whole panel. This is because a panel s PV cells are all linked in a series the electric current travels through each PV cell in a line. The weakest link in the chain will limit the amount of electrical current produced by the whole panel. Even if only one tenth of a PV panel is in shadow, the electrical current produced can be reduced by as much as one half. The amount of power lost will also depend on the size and darkness of the shadow. All schools that install Schoolgen PV systems set up a maintenance plan for the PV system as part of their agreement with Genesis Energy. The plan sets out regular times for someone to check the panels to make sure they are kept clean and in good repair. You may like to ask if you can see the maintenance plan. 6

7 Snow and Ice Ice or snow can affect the amount of electricity generated by PV cells. If there is a layer of snow on a PV panel, the sunlight will often travel through the snow. This will make the panel heat up enough to melt the bottom layers of snow. Once this happens, the snow will usually slide off the panel. For this reason, there must be no ledges or gaps across PV panels that are used in regions where it snows. Design problems like these can trap ice or snow and stop it from sliding off the panels. High Temperatures If a silicon PV panel gets very hot, the amount of voltage it creates will be reduced. Photovoltaic panels can also be damaged by very hot temperatures, so the back of the panels need ventilation to remove heat. (Silicon PV cells need to be kept below 70 degrees Celsius to avoid heat damage.) Panels that use mirrors or lenses, or that track the sun, can reach very high temperatures. These panels usually need their temperatures checked and often have to be cooled down. Over a long period, high temperatures mixed with wet weather can cause rust in some parts of a PV system. For this reason PV systems need to be designed to keep out the damp. High Winds It is unusual for PV panels to be blown off roofs, but it is important to be sure that the support structures for the panels are strong enough to withstand high winds. To keep a panel secure, its position on a roof is also important. For example, if the panels are installed near the edges rather than in the centre of roofs, the power of the wind pushing against them can increase by a factor of three. Damage Because it is important that the PV systems are left uncovered, it also makes them easy targets for vandals. PV systems are often used in remote areas and this means that the panels are sometimes easy targets for thieves too. Panels can also be damaged by lightening strikes. To protect them, most PV systems are linked to the lightning strike protection already in a building s electrical designs. 7

8 Purchasing PV Systems Factors for Consideration For building owners interested in installing PV panels, there are a number of factors that will influence their decision. It is likely they will compare the advantages and disadvantages, and the costs and savings of PV systems with other sources of electricity. Although this is not an exhaustive list, potential buyers may consider some of the following factors. Cons Pros Installing a PV system Cost of buying a PV system Hassle of installing a PV system Cost of building permit Possible power shortages Income from surplus energy can be sold to a power company Reduced power expenses Low regular maintenance costs Greater awareness of power use through metre reading Relying on power company grid system for electricity Cost of setting up connection to a power company s grid Regular line service fees Regular meter reading fees Vulnerability to increases in electricity costs Ready and continuous supply of power Customer service and support Easy to set up The high initial costs of PV systems may mean that those interested in setting up a PV system may want to weigh these up against long-term costs of relying on traditional energy sources. Energy Efficiency and Conservation Authority, Photovoltaic Fact Sheet photovoltaics-fact-sheet-04.pdf Cost Variations The cost of purchasing and installing PV systems can differ from country to country. The cost of purchasing electricity from power companies is more expensive in some countries than others. 8

9 Government Help In many countries, local and central governments have set up programs and incentives to encourage their citizens to invest in solar power and PV systems. One way governments try to encourage people to invest in renewable energies is by regulating the price for renewable electricity. This makes sure that the producer makes a good profit when they sell electricity that comes from a renewable energy source. In some countries, governments have set up Green Markets, which give renewable energy like electricity from PV systems added value when it is sold to power companies. Some governments and organisations offer consumers soft loans (low interest or no interest loans) to help people pay for buying and installing PV systems. Other governments, such as the Australian Government, provide rebates (refunds) to those who have bought and installed PV systems. Australian Government, Department of the Environment, Water, Heritage, and the Arts, Photovoltaic Rebate Program The New Zealand Government provides financial support to help people on lower incomes or landlords with tenants on lower incomes insulate their homes. It has also set up programmes to help businesses cut back on energy use. Businesses that use a lot of energy can receive money from the New Zealand Government to help pay for new energy saving technologies, but this does not include photovoltaic technology. Energy Efficiency and Conservation Corporation eeca.govt.nz The New Zealand Government also provides money to help New Zealanders pay for solar water heating systems and contributes towards the cost of energy efficient light bulbs. Some local governments in New Zealand are also trying to encourage people to install solar water heating systems by no longer charging for building consent fees. Solar Water Heating solar.energywise.govt.nz 9

10 Location PV Cell Efficiency and Light Intensity PV system efficiency in generating electricity is largely the same in all countries and in all temperatures. But the intensity of the sun s light can be very different depending on where you are in the world. The light s intensity directly affects the amount of electricity generated by a PV system. This may influence a person s decision about whether or not to purchase a PV system. Other Factors Environmental Concerns For some people the decision to purchase PV solar systems may be influenced by their desire to invest in an environmentally friendly way of generating electricity rather than relying on non-renewable sources of power. The fact that PV systems generate on-site electricity from the sun without causing environmental harm is a major motivating factor for many consumers. Other factors consumers may value include: Photovoltaic systems create electricity silently without major waste from by-products or great loss of natural or mineral resources. The generation of electricity through PV technology does not contribute to green house gases. (Over 30 years, a 2kW PV system on a house will mean about 40 tonnes of CO 2 will not be released into the atmosphere.) Energy from the sun will continue to be available for millions of years to come. Increased Independence Installing a PV system provides the benefit of giving users increased independence from utility company power supplies. It can also ensure electricity availability in areas where the power supply is unreliable. 10