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Photovoltaic power plant
Electric current from photovoltaic panels is direct, so it has to be converted for the electric grid’s need for alternated current, which Inverters do. The efficiency of AC/DC conversion in inverters highly exceeds 90%. Inverters also ensure stable AC frequency and minimize voltage fluctuations. Thanks to additional circuitry, they can also provide data about actual energy production and a panel’s status. There can be one inverter for the whole plant, or they can be installed on each panel (microinverters), or the output of one string of panels can be conducted into a combiner box and then into a so-called string inverter.
Each photovoltaic panel is placed in a stainless steel frame which orientates it optimally towards the sun (south in the northern hemisphere). The support structure is usually fixed, but movable panels also exist which allow the following of the sun throughout the day. This can raise efficiency but is more complicated and expensive.
underground power lines
AC current is flows from inverters via underground cables to the substation.
There is grass naturally growing between panels (if the plant is not situated in the desert). It must be kept so that it does not shade the panels or impede the maintenance of panels. The grass is mowed, or sheep are allowed to graze.
Individual solar cells are coupled in series into larger complete solar panels. The solar panels are usually designed to deliver energy of a certain voltage, for example, 12 or 24 Volt. In this way, the produced electric current is directly proportional to the amount of light captured by the panel. Solar panels are usually wired together in series to form strings. Because they are often exposed to harsh weather all year round, they have to endure a lot in terms of mechanical and thermal stress. High temperature decreases the panels' efficiency.
The photovoltaic cell is a core part of the photovoltaic power plant. It converts solar energy into useful electricity through the photovoltaic effect process. The PV cell is composed of semiconductor material; the 'semi' means that it can conduct electricity better than an insulator but not as well as a good conductor like metal. When the semiconductor is exposed to light, it absorbs the light’s energy which is transferred as electrons which flow through the material as an electrical current. The most used semiconductor material in PV cells is silicon (monocrystalline or polycrystalline), but also thin-film, perovskite, organic, quantum dots, or other types of PV cells are made.
Spaces between the rows are set up to prevent the modules from shading one another and to allow vehicles to drive in between the rows, e.g. when cleaning the panels.
To connect a photovoltaic power plant to the grid, it is necessary to increase the voltage level from a medium to a high voltage. The purpose of a substation is to convert low voltages from electricity generated to high voltages using power transformers. It also consists of the power plant’s main switch, current, and voltage measuring array, switches for the station service transformer, and a disconnecting switch, which allows the shutting off of current under a full load.
Power output from the photovoltaic power plant is usually 22kV high-voltage AC.
The fence prevents the entry of unwanted visitors, either humans or animals. Cameras usually monitor the area.
Photovoltaic power plant
Photovoltaic solar power plants can directly generate electric energy from incoming solar radiation utilizing solar panels. Panels generating direct current are situated in vast fields and occupy remarkably large pieces of land. The direct current of the output of such power plants is transformed using inverters to alternate the current, and the voltage is converted to the distribution grid level. The average density of electricity generation reaches only roughly 140 W/m². A photovoltaic power plant of 1000 MW would need approximately 50 km² of the used field. A conventional coal-fired power plant of the same capacity would do with an area of 2.5 km².
Parabolic trough collectors fall into the category of so-called concentrators. They consist of parabolic mirrors, which concentrate sunlight from a large area into a small focus plane through which the absorber tube runs. Collectors are in parallel rows typically aligned in a north-south orientation to maximize solar energy collection. This way, they reach temperatures of around 400 °C and are suitable for generating electricity in large, farm-type plants. An alternative use may be to supply heat to industrial-scale technologic processes.
A heat absorber (also called a receiver) is a long tube running through the focal plane of the parabolic mirror. The tube is often designed as a Dewar tube (vacuum insulated) to minimize heat loss. Inside the tube, the heat transfer medium (mostly oil) flows, heated to 400°C by concentrated sunbeams.
Thermal storage tanks
The thermal energy of the heat transfer fluid could immediately be turned into electricity or stored for later use in thermal storage tanks. The solar power plant uses molten salts, where the heat is transferred and stored. The heat from the storage system is used to produce electricity in the evening or during cloudy weather.
Cold salt tank
A tank with cold molten salts with a temperature of about 200°C. Salts from this tank flow to a heat exchanger where they are warmed up to approx. 400°C via heat transfer fluid (oil) and then flow to a hot salt tank where they are stored.
The power plant uses concentrated solar heat to produce energy. It consists of long parabolic mirrors which follow the position of the Sun and concentrate and focus the solar radiation which runs through the absorber tube. The tube is filled with a heat transport medium heated to temperatures in the range of thousands of degrees Celsius. The heat is used for the generation of electric energy in solar power park stations.
Steam coming out from a turbine needs to be cooled and condensed. This is done by cooling towers where fans or cooling water is used. Cooling by water is more efficient than by air, but this water needs to be delivered to the power plant site which could impact plants located in the desert.
Parabolic mirrors have engines that gradually tilt them to follow the sun to get maximum solar energy throughout the whole day. This is true for mirrors aligned in a north-south orientation. There could also be lines of mirrors aligned in an east-west direction. They get enough solar irradiation without movement during the day, and they need only slight adjustment through the year according to the sun's relative position in the sky. The thermal efficiency of parabolic trough collectors can be as high as 90% if the mirror surface is good. Still, in the desert, the mirror's surface may be scratched by sand which could decrease their reflectivity and thus energy conversion efficiency.
Hot salt tank
A tank with hot molten salts where the heat is stored for later use. Heat is then passed through the heat exchanger to water, creating steam and propelling the turbine.
The power output from a parabolic trough collector power plant is usually 22kV high-voltage AC.
The heat generated by parabolic trough collectors needs to be turned into electricity. The heat transfer fluid passes its thermal energy to the water inside the steam generator, creating steam. Steam propels a turbogenerator producing an electric current. Heat exchangers, turbines, and generators are located inside the powerhouse, with all necessary pipes, valves, pumps, and other devices.
Central tower solar power plant
Heat transporting medium
The modern designs of solar-tower power plants use heat transporting medium molten salts composed of 40 % potassium nitrate and 60 % sodium nitrate. Older, less efficient methods used water. Some are experimenting with sand-like particles to maximize the power cycle temperature. The heat transporting medium is either superheated directly in a receiver to drive a turbine with an electric generator, or a part of the solar energy is deposited in heat accumulators containing molten salts.
Thermal storage tank
The heat produced by the receiver could be stored in storage tanks filled with molten salts and used later for electricity generation so that the plant could produce electricity on cloudy days or evenings. A solar-tower power plant without any accumulation reservoir can continue producing approximately 25 percent of a year's operation time. Still, with a reservoir, it may be possible to reach a rate of up to 65 percent.
On the top of the tower is a receiver - a cavity filled with a wall of tubes through which a heat transporting medium flows which heats up to temperatures above 500 °C. In this way, the effectiveness of heat utilization in conversion to electric energy is upgraded.
Central tower solar power plant
Solar-tower power plants concentrate solar beams reflected from a large surface onto a receiver placed on the top of a tower. Onto the small space of the receiver, the solar energy is concentrated using plane mirrors – heliostats placed around a central tower. Every heliostat has a computer-controlled two-axis turning system. Heat is then used to generate electricity or for industrial purposes.
Steam coming out from the turbine needs to be cooled and condensed. This is done by cooling towers where fans or cooling water is used. Cooling by water is more efficient than by air, but this water needs to be delivered to the power plant site, which could impact plants located in the desert.
The power output from a solar-tower power plant is usually 22kV high-voltage AC.
The heat generated by concentrated solar beams needs to be turned into electricity. The heat transporting medium passes its thermal energy to the water inside the steam generator, creating steam. Steam propels a turbogenerator producing electric current. Heat exchangers, turbines, and generators are located inside the powerhouse with pipes, valves, pumps, and other devices.
Solar energy is concentrated onto the small absorber area (receiver) by plane mirrors – heliostats, placed around a central tower. Every heliostat has its two-axis system enabling it to follow the Sun’s position. A computer continually tracks the position of the Sun and every few seconds assures a correction of the heliostat’s position in such a way that the solar beams are reflected onto one point on the top of the tower.
The tower holding the receiver on its top is made mostly from concrete. Its height reaches about 100 - 200 meters.