Methods of generating electricity from the sun
- Solar field (discerning system)
- Solar tower (unified system)
- Solar pool
- Solar chimney
The thermal energy of the fluid heated by solar energy is converted into mechanical energy and ultimately to electrical energy in the generator. It is the most widely used fluid water. With the help of the collector, solar energy concentrated in a steam generator vaporized water. It runs a steam turbine generator group on steam.
In solar tower technology, rays from the sun focus on the top of the solar tower with plane mirrors (heliostats). Condensed solar energy is stored as heat energy and electrical energy is produced by being cycled.
The system is an expensive system because heliostats need to position according to the angle of arrival of the sun and perfectly focus the sun's rays on the focus of the tower.
This solar tracking system of heliostats is either supplied from a main control center with automation systems, and the software required for this is expensive. It is also used in sensors that detect the angle of arrival of the sun's rays and is an expensive and difficult technology to implement. Heliostats act on two-axis with the help of motors placed under them.
The layout of heliostats around the tower is important. It should be in the direction that ensures optimum efficiency of the plant. In fact, when more than necessary heat energy is obtained, electricity can be produced even during hours when this energy is stored and the sun cannot be utilly utiled. Thus, the continuity of production is better ensured.
In the solar power tower system, liquid salt melt at 290°C is pumped from the cold storage tank to the receiver. Here, the temperature is lowered to 565°C and sent to the hot storage tank.
When power is drawn from the plant, hot salt is pumped into a steam generation system that produces over-fried steam for a classic Rankine cycle turbine-(generator) system. The salt in the steam generator is stored by returning to the cold tank and eventually re-ingested in the receiver.
Molten salt solution storage
In some collectors, solar energy is operated by the Stirling engine, inginging mechanical energy. The Stirling engine runs the generator, inginging electrical energy. The efficiency of the collector system can be around 76%. The salt melt used in the system can be sodium nitrate, potassium nitrate.
• The fact that these energy carrier liquids used are not flammable or poisonous provides an advantage for the safety of the system.
• If it is seen that salt is spilled by accident, the salt will freeze before making significant effects on the soil.
• Salt is removed with the help of a shovel and, if necessary, joins the cycle for re-use.
• Dangerous gases or liquid emissions/radiations are not released during the operation of solar power tower plants.
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- Flat mirrors (heliostat) are used.
- Heliostat follows the sun in two-axis motion.
- Temperatures of 565°C can be reached.
- If oil is used, steam at a maximum temperature of 400°C
- Each heliostat moves separately.
- The need for land preparation is low.
- 100 MW required an area of 2.5 –3 km².
- Investment costs > $5/w
Various examples from around the world;
- Ps 10 Spain 11 mw
- – 75000 m² area, 624 heliostats
- – 24.3 GWh annual production
- – One-hour steam backup
- – 27% of the time
- – Total yield 17%
- – Temperature 250ºC
- – Mirrors Abengoa Solar,
- – Tower ALTAC
- – Partnership of Inabensa, Fichtner, Ciemat, DLR
Parabolic cylindrical collectors
Parabolic dish collectors
Power plants with parabolic cylindrical collectors
It consists of power plants with parabolic groove collectors, solar field, steam and electricity generation systems. For process temperature in these plants, linear intensification is achieved from solar energy over 300 øC and thermal oil resistant to high temperatures is used as heat transfer fluid.
It's a solar field; it is an area consisting of parabolic groove collector groups connected parallel to each other in the form of independent units. These units reflect incoming solar energy on the receiving pipe in focus, through mirrors that are 4 mm thick and have a high reflecting rate (94%). Groups of parabolic groove collectors are supported by metal structures that do not prevent them from turning along the horizontal axis. The system has a sensor that allows mirrors to monitor the sun.
Heat Collection Element
Heat collection element; The glass tube consists of steel receiver pipes and glass-metal defragmenters with an absorption of approximately 97% of the surface. To reduce heat losses caused by the high temperature occurring on the receiving pipe, the air between the glass tube and the receiving pipe is vacuumed. This clearance pressure is about 0.1 atm.
The heat-resistant glass tube has a high permeability and an antireflective structure to minimize radiation losses. Bellows glass-metal defragmenters are used to eliminate the effects of temperature-intensifications.
Solar field control system; general control system and local control units in each collector's group. The general control system monitors sunbathing status and accordingly turns the system on or off completely or partially. This is done in communication with local control units. Local control units control each group of collectors individually, so that the sun is monitored.
Steam production system; preheating, steam production and super heating sections. The steam, which is passed through these sections and raised to 371 o C and 100 bar pressure, is sent to the turbine for electricity generation. Steam, which does not cool enough after production, is heated back to the same temperature and sent back to the turbine before being sent to a new cycle. After this second cycle, the steam, which is now cooling, is compressed and liquidized and then sent to a new cycle.
In solar power plants, in cases where solar power is insufficient, additional heaters are used to ensure uninterrupted electricity production. Additional heaters powered by oil or natural gas produce steam at the same temperature and pressure.
Parabolic groove power plant diagram
When solar energy is insufficient, a natural gas heater system is used to ensure uninterrupted energy production. Depending on whether the solar energy is sufficient, insufficient or not at all, the system works in three different ways.
In cases where solar energy is sufficient, the heat transfer fluid passes directly through the solar field. In case of insufficient or none, they are supported by natural gas heaters or fully activated. When both energy sources are used, the by-pass valve is left open to take advantage of both solar and natural gas.
In this case, the liquid heated in the solar field is heated until it reaches working temperature with the help of support heaters.
Electricity generation with parabolic dish collectors
Parabolic dish collectors point-intensifies the solar radiation coming to their surfaces in their focus. The surfaces of these collectors are covered with reflective mirrors, such as the surfaces of parabolic groove collectors. The incoming solar energy is concentrated on the Stirling engine in focus through these mirrors. The Stirling engine converts heat energy into the mechanical energy required for the electric generator.
Electricity obtained by parabolic dish collectors is used in other ways to support power plants and to meet the electricity needs of mines, radar stations or remote villages. Because these plants are made up of small modules, they can be installed near places where energy is needed and at the capacity needed.
Stirling engine operating principle;
Pistons connected to the same shaft with 2 separate bolts work antagonistly with each other.
On the one hand, the piston, which is widened by hot gases, has a piston on the bottom side that is trapped by the cooling gases, resulting in a rotation. By moving the shaft to a generator, electrical energy is produced.
Luz Solar Electric Generating Systems parabolic groove solar plant at 354 MW-California
Methods of Generating Electricity from the Sun – SOLAR CHIMNEYS
The rise of the warm air and the effect of the venturi form the basic operating principle. Under the collector, the warm air moves towards the chimney, where mechanical energy is obtained by rotating the wind turbine. This mechanical energy is converted into electrical energy with the help of generators.
The solar chimneys proposed by Prof. Schlaich, who served at the University of Stuttgart in 1978, are made up of a greenhouse collector's section (collector) that collects solar energy and transmits it into the circulating air, and a long chimney section with a wind turbine power generation system. The collector part is usually covered with thousands of square meters of glass at an altitude of 5-6 meters.
Greenhouses can be made in the area of 75% below the solar collector roof area. The larger the collector area, the more energy is absorbed. To minimize air friction, the collector ceiling rises towards the chimney entrance, reaching a speed of 15m/s when it reaches the turbine.
Collector coatings may be different. The most efficient is the glazed one. It can turn 70% of the annual solar glow into heat. In other coatings, the average rate is 50%. They have a 60-year operating life with good care. In addition, channels with liquid (usually water) in the floor of the collectors during the day store solar energy and the air under the collector is heated by the heat of the fluid there when there is no solar energy at night and production continues 24 hours a day.
The most important part of the plant is the chimney, resembling a giant stove pipe. Pressure losses should be designed to be minimum. The efficiency is directly proportional to the height of the chimney and the air temperature of the environment. There are 2 types of chimneys for the plant, one is a free-standing chimney and the other is a Guyed type chimney. There are different types of turbines placed in the chimney. These are one large turbine, six small turbines, a large number of turbines. The efficiency of a chimney 1000 meters high is approximately 3%, which is a very low value.
Methods of Generating Electricity from the Sun – SOLAR POOLS
The black floor of the pool covered with water at a depth of approximately 5-6 meters is used to capture the sun's radiation and obtain hot water at a temperature of 90°C. The distribution of heat in the pool is regulated by the concentration of salt added to the water, the concentration of salt increases from the top to the bottom. Thus, even if there is a cold water surface at the top, the temperature is high in the area with saturated salt concentration at the bottom of the pool.
This hot water can be pumped into a heater and used as heat or used in electricity production through rankin cycle. Most studies and applications have been carried out in Israel on solar pools. In addition to the two 150 kW-power 5 MW-powered system, there are solar pools in Australia at 15 kW and 400 kW in the US.
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