A solar power plant is any type of facility that converts sunlight either directly, like Photovoltaics, or indirectly, like Solar Thermal plants, into electricity.
They come in a variety of 'flavors' with each using discretely different techniques to harness the power of the sun.
In the following article, we'll take a quick look at the different types of solar power plants that harness the sun's life-giving sunlight to produce electricity.
Photovoltaic power plants use large areas of photovoltaic cells, known as PV or solar cells, to directly convert sunlight into usable electricity. These cells are usually made from silicon alloys and are the technology most people have become familiar with - chances are you may have one on your roof.
The panels themselves come in various forms:
- Crystalline solar panels - As the name suggests these types of panels are made from crystalline silicon. They can be either monocrystalline or poly- or multi-crystalline. As a rule of thumb monocrystalline versions are more efficient (about 15-20%) but more expensive than their alternatives (tend to be 13-16% efficient) but advancements are closing the gap between them over time.
- Thin-film solar panels - These types of panels consist of a series of films that absorb light in different parts of the EM spectrum. They tend to be made from amorphous silicon (aSi), cadmium telluride (CdTe), cadmium sulfide (CdS), and copper indium (gallium) diselenide. This type of panel is ideal for applications as flexible films over existing surfaces or for integration within building materials like roofing tiles.
These types of plant generate electricity that is then, usually, directly fed into the national grid.
These kinds of power plant tend to have the following basic components:-
- Solar panels that convert sunlight into useful electricity. They tend to generate DC current with voltages up to 1500 v;
- These plants need investors to transform the DC into AC
- They usually have some form of a monitoring system to control and manage the plant and;
- They are directly connected to an external power grid of some kind.
- If the plant generates in excess of 500 kW they will usually also employ step-up transformers.
1.1 How does a solar PV power plant work?
Solar PV power plants work in the same manner as small domestic-scale PV panels or the tiny one on your calculator but on steroids.
Most solar PV panels are made from semiconductor materials, usually some form of silicon. When photons from sunlight hit the semiconductor material free electrons are generated which can then flow through the material to produce a direct electrical current.
This is known as the photo-effect in physics. The DC current then needs to be converted to alternating current (AC) using an inverter before it can be directly used or fed into the electrical grid.
PV panels are distinct from other solar power plants as they use the photo-effect directly without the need for other processes or devices. For example, no liquid heat-carrying agent, like water, is needed as in solar thermal plants.
PV panels do not concentrate energy they simply convert photons into electricity that is then transmitted somewhere else.
2. Solar Thermal Power Plants
Solar Thermal power plants, on the other hand, focus or collect sunlight in such a manner as to generate steam to feed a turbine and generate electricity. Solar thermal power plants can also be subdivided into a further three distinct types: -
2.1 Linear, Parabolic Trough Solar Thermal and Solar Dish Power plants
This is the most common form of a solar power plant that is characterized by their use of fields of either linear U-Shaped, parabolic trough collectors pr solar dishes. These types of facility tend to consist of a large 'field' of parallel rows of solar collectors.
They tend to consist of three discrete types of system:
2.1.1. Parabolic trough systems
Parabolic troughs use parabola-shaped reflectors that are able to focus between 30 and 100 times normal sunlight levels on to the collector. The method is used to heat a special type of fluid, which is then collected at a central location to generate high-pressure, superheated steam.
These systems tilt to keep track of the sun throughout the day. Because of their parabolic shape, these kinds of reflectors are able to focus between 30 and 100 times the normal sunlight intensity on the collector.
The longest operating solar thermal plant in the world, the Solar Energy Generating Sytems (SEGS) in the Mojave Desert, California, is one of these types of power plants. The first plant, SEGS 1, was built in 1984 and operated to 2015 with the second, SEG 2, operating between 1984 and 2015.
The last plant built, SEGS IX, with an electricity generation capacity of 92 megawatts (MW), began operation in 1990. Today there are currently seven operating SEGS plants with a combined capacity of 357 MW - this makes it one of the largest solar energy thermal electric power plants in the world.
2.1.2. How does it work?
These kinds of solar thermal power plant work by focussing sunlight from long parabolic mirrors onto receiver tubes that run the length of the mirror at their focal point. This concentrated solar energy heats up a fluid that continuously flows through the tubes.
This heated fluid is then sent to a heat exchanger to boil water in a conventional steam-turbine generator to generate electricity.
2.2. Linear concentrating systems
Linear concentrating systems, sometimes called Fresnel reflectors, also consist of large 'fields' of sun-tracking mirrors that tend to be aligned in a north-south orientation to maximize sunlight capture. This setup allows the banks of mirrors to track the sun from east to west throughout the day.
2.2.1. How does it work?
Much like their parabolic mirror cousins, linear concentrating systems collect solar energy using long, rectangular, U-shaped mirrors. Unlike parabolic systems, however, linear Fresnel reflector systems, place the receiver tube above several mirrors to allow the mirrors greater mobility in tracking the sun.
These types of systems use the Fresnel lens effect that allows for the use of a large concentrating mirror with a large aperture and short focal length. This setup allows these kinds of systems to focus sunlight approximately 30 times the normal intensity.
2.3. Solar Dishes and engines
Solar dishes also use mirrors to focus the suns energy onto a collector. These tend to consist of oversized satellite dishes that are clad in a mosaic of small mirrors that focus energy onto a receiver at the focal point.
2.3.1. How does it work?
Like the parabolic and linear systems, the dish-shaped, mirror clad, surface directs and concentrates sunlight onto a thermal receiver at the dish's focal point. This receiver transfers the heat generated to an engine generator.
The most common type of heat engine used in dish/engine systems is the Stirling engine. Heated fluid from the dishes receiver is used to move pistons in the engine to create mechanical power.
This mechanical power then runs to a generator or alternator to generate electricity.
Solar dish/engine systems always point straight at the sun and concentrate the solar energy at the focal point of the dish. A solar dish's concentration ratio is much higher than linear concentrating systems, and it has a working fluid temperature higher than 749 degrees Celsius.
Power generating equipment can either be directly mounted at the dish's focal point (great for remote locations) or collected from an array of dishes and electrical generation occurring at a central point.
The U.S. Army is developing a 1.5 MW system at the Tooele Army Depot in Utah with 429 Stirling engine solar dishes.
3. Solar Power Towers
Solar power towers are an interesting method in which hundreds to thousands of flat sun-tracking mirrors (heliostats) reflect and concentrate solar energy onto a central tower. This method is able to concentrate sunlight as much as 1,500 times than would normally be possible from direct sunlight alone.
One interesting example of this kind of power plant can be found in Juelich, North-Rhine Westphalia, Germany. The facility is spread over an area of 18,000 km squared that houses more than 2,000 heliostats that focus sunlight onto a central 60-meter high tower.
The U.S. Department of Energy and other electric utility companies built and operated the first demonstration solar power tower near Barstow, California, during the 1980s and 1990s.
Some are currently in development in Chile too.
Today, in the U.S., there are three solar power tower plants in operation. These are the 392 MW Ivanpah Solar Power Facility in Ivanpah Dry Lake California, the 110 MW Crescent Dunes Solar Energy Project in Nevada and the 5 MW Sierra Sun Tower in the Mojave Desert, California.
3.1. How does it work?
The concentrated solar energy is used to heat the air in the tower to up to 700 degrees Celsius. The heat is captured in a boiler and is used to produce electricity with the help of a steam turbine.
Some towers also make use of water as the heat-transfer fluid. More advanced systems are currently being researched and tested that will use nitrate salts because of their higher heat transfer and storage properties compared to water and air.
The thermal energy-storage capability allows the system to produce electricity during cloudy weather or at night.
These kinds of solar power plants are ideally suited for operations in areas with adverse weather conditions. They're used in the Mojave Desert in California and have withstood hailstorms and sandstorms.
4. Solar Pond
Solar pond solar power plants make use of a pool of saltwater that collects and stores solar thermal energy. It uses a technique called salinity-gradient technology.
This technique acts as a thermal trap within the pond that can either be used directly or stored for later use. This kind of power plant has been in use in Israel at the Beit HaArava Power Plant since 1984.
There are also other examples at the Bhuj in India and was completed in 1993.
4.1. How does it work?
Solar Ponds use a large body of saltwater to collect and store solar thermal energy. Saltwater naturally forms a vertical salinity gradient, known as a halocline, with low-salinity water on the top and high-salinity water at the bottom.
Salt concentrations levels increase with depth and, therefore, density also increases from the surface to bottom of the lake until the solution becomes uniform at a given depth.
The principle is fairly simple. Solar rays penetrate the pond and eventually reach the bottom of the pool.
In a normal pond or body of water, water at the bottom of the pond is heated, becomes less dense and rises setting up a convection current. Solar ponds are designed to impede this process by adding salt to the water until the lower levels become completely saturated.
As the high-salinity water doesn't mix easily with low-salinity water above it, convection currents are contained within each discrete layer and minimal mixing between them occurs.
This process concentrates thermal energy and reduces heat loss from the body of water. On average the high-salinity water can reach 90 degrees Celsius with low-salinity layers maintaining around 30 degrees Celsius.
This hot, salty water can then be pumped away for use in electricity generation, through a turbine or as a source of thermal energy.