Power plants can generate large amounts of energy in the form of electricity using a variety of resources.
Some require raw materials such as coal, wood, fossil fuel, and metals such as uranium and plutonium. Some depend on the forces of nature such as moving bodies of water and wind.
Among all of the above, sunlight is the most recently exploited source of renewable energy. Solar power plants come in many forms depending on the technology used and the manner in which solar energy is converted.
They can be broadly categorised into three groups: photovoltaic solar power plant, solar thermal energy plant and concentrating solar power plant.
These power plants can operate according to different solar systems. They are off-grid, grid-connected and hybrid solar systems. These three systems will be covered in a separate article.
Photovoltaic Solar Power Plant
Also generally known as solar farms, PV solar power plants utilise great numbers of photovoltaic (PV) arrays to capture solar energy to be converted directly into electricity.
The type of PV cells used for solar farms can vary. Some farms use crystalline solar panels while others may use thin-film solar panels.
The PV material in crystalline solar panels are either monocrystalline, polycrystalline or multi-crystalline. Monocrystalline is known to be more efficient but this material is more expensive than the latter two.
Thin-film solar panels are able to absorb light in different parts of the solar spectrum. Made from a wide range of other materials, thin-film is more flexible and can be flexed into curved structures.
The type of electricity generated is direct current (DC), which can be stored in batteries. However, DC needs to be converted into alternating current (AC) which is the form suitable for use and fed into a power grid.
Because of the need to convert DC to AC, heavy duty inverters are an essential component of PV solar power plants. If a plant generates more than 500kW, it will usually also employ step-up transformers.
This type of power plant usually has some form of monitoring system to control and manage the plant, and the amount of power generated.
More reading on PV solar farms:
- Everything You Need to Know About Starting a Solar Farm
- Solar Panel In China
- Solar Farms In Australia
Solar Thermal Power Plant
This category of solar power plant also collects sunlight but not to convert solar energy into electricity directly. Instead, it uses sunlight to generate heat which is then converted to electricity using different processes.
A miniature scale of solar thermal power plants is the solar-powered water heater which retail consumers are using. These involve the use of PV panels.
An example of a large-scale solar thermal power plant is the solar pond, which also harnesses the power of the sun but by using saline water instead of PV panels.
A solar pond does not use photovoltaic panels to collect solar energy. Instead, it uses salinity-gradient technology.
The technique makes use of a large body of saltwater; say a saltwater pond, to store solar thermal energy. A large body of saltwater naturally has a vertical salinity gradient.
This means the salinity of water differs from the top to the bottom of the pond. The top layer, called halocline, has low salinity.
The salinity progresses to a higher concentration at the bottom. The deeper the saltwater body, the more concentrated the salinity level.
In freshwater, solar rays heat water at the bottom. The warm water becomes less dense and rises to the surface.
But in a saline pond, salt is added until the bottom layer becomes very saturated and dense. This then impedes the movement of heated water to the surface.
Because water of different salinity concentrations doesn’t mix easily, convection currents are contained within each layer of salinity level. This prevents heat loss from the pond.
Highly saline water can reach as high as 90 degrees Celsius while low-salinity layers can maintain around 30 degrees Celsius.
The solar power plant then pumps the hottest layer of saline water through a turbine to generate electricity. Examples of solar ponds can be found in Israel and India.
Concentrating Solar Power Plant
CSP plants do not have photovoltaic arrays either. Instead, they utilise turbines, mirrors, engines and tracking systems.
The principle behind CSP plants is to concentrate solar energy to create heat. The heat generated is then used to drive turbines or engines to produce electricity.
In this sense, this feature also makes them solar thermal power plants. Thermal energy concentrated in a CSP plant can be stored to produce electricity whenever it is needed.
There are a few types of concentrating power plants:
- Solar dish
- Solar tower
- Parabolic trough
- Compact linear Fresnel reflector
Solar Dish Power Plant
Also called dish-engine, this type of CSP technology uses a gigantic parabolic dish lined with mirrors to concentrate sunlight onto a fixed receiver.
The fixed receiver contains a working fluid such as hydrogen. The liquid can be heated to at least 1,200 degrees Fahrenheit or 749 degrees Celsius.
The heated fluid then drives pistons in the engine. The mechanical power from the pistons is channelled to a generator or alternator to produce electricity.
Thus, the name dish-engine. The most common kind of heat engine utilised is the Stirling engine.
A solar dish power plant has at least a few hundred of these dish or engine systems. Each dish in the plant rotates along two axes to track the movement of the sun.
This helps to always face the sun directly and concentrate the solar energy at the focal point of the dish. The concentration ratio of a solar dish is higher than that of linear concentrating systems.
Solar Tower Power Plant
Also called a solar power tower, this type of concentrating solar power plant also uses mirrors, a central receiver system and sun-tracking system like the solar dish.
Instead of parabolic dishes, the mirrors of a solar tower are lined on flat panels which track the sun. These are called heliostats.
The heliostats are controlled by computers which program them to track the sun along two axes so that sunlight is focussed on a receiver at the top of a high tower.
The tower is placed at the center of all the heliostats. It is filled with a medium – either water or air. The heated medium is captured in a boiler which produces electricity with the aid of a steam turbine.
This method of concentrating sunlight can multiply the energy of direct sunlight by as much as 1,500 times. The result is a concentrated heat of up to 700 degrees Celsius or over 1,000 degrees Fahrenheit.
Research is being done on using nitrate salts as the heating medium, which is believed to have higher heat transfer and storage properties than pure water and air.
This energy storage capability will allow the solar farm produce electricity even at night or on cloudy days.
A solar tower facility can spread over an area of 18,000 square kilometres, housing over 2,000 heliostats. The central tower can be as high as 60 meters.
Parabolic Trough Power Plant
The mirrors at a parabolic trough solar power plant are also arranged in long strips but the strips are curved in the center instead, like a trough.
Each trough is usually about 15 to 20 feet tall and measures 300 to 450 feet long.
The curved troughs focus sunlight onto a receiver tube which runs down the center of each trough. The receiver tube contains a high-temperature heat transfer fluid such as synthetic oil.
The fluid absorbs the heat then passes through a heat exchanger where water is heated to produce steam. The steam then powers a conventional steam turbine power system to produce electricity.
The temperature of this fluid can reach at least 750 degrees Fahrenheit or 399 degrees Celsius.
A normal parabolic trough power plant can consist of hundreds of parallel troughs connected in a series of loops. They are arranged on a north-south axis so that they track the sun from east to west.
Compact Linear Fresnel Reflector Power Plant
A compact linear Fresnel reflector (CLFR) is also called ‘linear concentrating system’ in short. The name comes from using the Fresnel lens effect which utilises a large concentrating mirror with a large aperture and short focal length.
The Fresnel lens effect can focus sunlight to about 30 times stronger than the normal intensity.
The CLFR uses the same principles as the parabolic trough system – tracking the sun using U-shaped long rows of mirrors (modular reflectors), and concentrating the sun’s energy on a central receiver.
There are three key differences. One is the use of low-cost mirrors arranged in long parallel rows instead of loops.
The second is that the sun-tracking path is from north to south so as to maximise sunlight capture.
The third difference is that the modular reflectors are elevated. The receiver sits above the modular reflectors which deflect concentrated sunlight onto its surface.
Inside the receiver is a system of tubes filled with flowing water. The heat generated by concentrated sunlight onto the receiver boils the water.
This heated water is then passed through a turbine which generates high-pressure steam for use in power generation and industrial steam applications.
Of all the types of solar power plants, only the photovoltaic power plant makes use of photovoltaic panels to directly convert solar energy into electricity. The other solar power plants are also powered by solar energy. Instead, saline water or mirrors are used to concentrate solar energy to heat a medium. It is the heated medium which is then passed through a turbine or engine to generate electricity.
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