What is Solar Power Plant?
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 types, 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 energy from the Sun to produce electricity.
What is a PV solar power plant?
Photovoltaic power plants use large areas of photovoltaic cells, known as PV or solar cells, to 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 even have one on your roof.
The panels themselves come in various forms:
1. Crystalline solar panels: As the name suggests these types of panels are made from crystalline silicon. They can be either monocrystalline or polycrystalline (also called multi-crystalline). As a rule of thumb monocrystalline versions are more efficient (about 20% or above) but more expensive than their alternatives (which tend to be 15-17% efficient) but advancements are closing the gap between them over time.
2. 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 (a-Si), 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 solar power panels generate electricity that is then, usually, directly fed into the national grid or stored in batteries.
Power plants using these types of panels tend to have the following basic components:
- The solar panels convert sunlight into useful electricity. They tend to generate DC current with voltages up to 1500V;
- These plants need invertors to transform the DC into AC
- They usually have some form of a monitoring system to control and manage the plant and;
- They are often 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.
How does a solar PV power plant work?
Solar PV power plants work in the same manner as smaller domestic-scale PV panels.
As we have seen, 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 photoelectric effect. 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, they do not use a liquid heat-carrying agent, like water, as in solar thermal plants.
PV panels do not concentrate energy, they simply convert photons into electricity which is then transmitted somewhere else.
Types of Concentrating Solar Thermal Power Plants
Solar thermal power plants, on the other hand, focus on 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:
- Parabolic Trough Solar Thermal
- Solar Dish Power plants
The most common forms of a solar power plant are characterized by their use of fields of either linear collectors, parabolic trough collectors, or solar dishes. These types of facilities tend to consist of a large ‘field’ of parallel rows of solar collectors.
They tend to consist of three discrete types of system:
1. Parabolic trough systems
A parabolic trough collector has a long parabolic-shaped reflector that focuses the sun’s rays on a receiver pipe located at the focus of the parabola. The collector tilts with the sun to keep sunlight focused on the receiver as the sun moves from east to west during the day.
Because of its parabolic shape, a trough can focus the sunlight from 30 times to 100 times its normal intensity (concentration ratio) on the receiver pipe, located along the focal line of the trough, achieving operating temperatures higher than 750°F.
Parabolic trough linear concentrating systems are used in the longest operating solar thermal power facility in the world, the Solar Energy Generating System (SEGS). The facility, with nine separate plants, is located in the Mojave Desert in California.
The first plant in the system, SEGS I, operated from 1984 to 2015, and the second, SEGS II, operated from 1985 to 2015. SEGS III-VII (3–7), each with summer generation capacities of 36 megawatts (MW), came online in 1986, 1987, and 1988. SEGS VIII and IX (8 and 9), each with a net summer electric generation capacity of 88 MW, began operation in 1989 and 1990, respectively.
In combination, the seven currently operating SEGS III–IX plants have a total net summer electric generation capacity of about 356 MW, making them one of the largest solar thermal electric power facilities in the world.
In addition to the SEGS, many other parabolic trough solar power projects operate in the United States and around the world. The other parabolic-trough solar thermal electric projects in the United States and their net summer generation capacity and location are
- Solana Generating Station: a 280 MW, two-plant facility with an energy storage component in Gila Bend, Arizona
- Mojave Solar Project: a 280 MW, two-plant facility in Barstow, California
- Genesis Solar Energy Project: a 250 MW, two-plant facility in Blythe, California
- Nevada Solar One: a 69 MW plant near Boulder City, Nevada
2. Linear concentrating systems
Linear concentrating systems collect the sun’s energy using long, rectangular, curved (U-shaped) mirrors. The mirrors focus sunlight onto receivers (tubes) that run the length of the mirrors. The concentrated sunlight heats a fluid flowing through the tubes.
The fluid is sent to a heat exchanger to boil water in a conventional steam-turbine generator to produce electricity.
There are two major types of linear concentrator systems: parabolic trough systems, where receiver tubes are positioned along the focal line of each parabolic mirror, and linear Fresnel reflector systems, where one receiver tube is positioned above several mirrors to allow the mirrors greater mobility in tracking the sun.
A linear concentrating collector power plant has a large number, or field, of collectors in parallel rows that are typically aligned in a north-south orientation to maximize solar energy collection. This configuration enables the mirrors to track the sun from east to west during the day and concentrate sunlight continuously onto the receiver tubes.
3. Solar Dishes and engines
Solar dish/engine systems use a mirrored dish similar to a very large satellite dish. To reduce costs, the mirrored dish is usually composed of many smaller flat mirrors formed into a dish shape.
The dish-shaped surface directs and concentrates sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to an engine generator. The most common type of heat engine used in dish/engine systems is the Stirling engine.
This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power runs a generator or alternator to produce 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 1,380°F.
The power-generating equipment used with a solar dish can be mounted at the focal point of the dish, making it well suited for remote locations, or the energy may be collected from a number of installations and converted into electricity at a central point.
There are no utility-scale solar dish/engine projects in commercial operation in the United States
4. Solar Power Towers
A solar power tower system uses a large field of flat, sun-tracking mirrors called heliostats to reflect and concentrate sunlight onto a receiver on the top of a tower. Sunlight can be concentrated as much as 1,500 times. Some power towers use water as the heat-transfer fluid.
Advanced designs are experimenting with molten nitrate salt because of its superior heat transfer and energy storage capabilities. The thermal energy storage capability allows the system to produce electricity during cloudy weather or at night.
The U.S. Department of Energy, along with several electric utilities, built and operated the first demonstration solar power tower near Barstow, California, during the 1980s and 1990s. In 2018, there were two solar power tower facilities operating in the United States:
- Ivanpah Solar Power Facility: a facility with three separate collector fields and towers with a combined net summer electric generation capacity of 399 MW in Ivanpah Dry Lake, California
- Crescent Dunes Solar Energy Project: a 110 MW one-tower facility with an energy storage component in Tonapah, Nevada
5. 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 creates a thermal trap within the pond where energy generated can either be used directly or stored for later use. This kind of power plant was in use in Israel at the Beit HaArava Power Plant between 1984 and 1988.
Other solar ponds have been built in Bhuj, India (this is no longer in operation), and El Paso, Texas.
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 concentration levels increase with depth and, therefore, density also increases from the surface to the 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.