Here's how solar power plants make energy from sunlight

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 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 consist 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 panel type is ideal for applications as flexible films over existing surfaces or for integration within building materials like roofing tiles.

These solar power panels generate electricity directly fed into the national grid or stored in batteries. mPower plants using these types of panels tend to have the following basic components:

• Solar panels convert sunlight into useful electricity. They tend to generate DC 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 more than 500 kW, it usually employs 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 must then 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 needing 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 convert photons into electricity transmitted somewhere else.

What is a solar thermal power plant?

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: 

• Linear
• 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 facilities have a large 'field' of parallel rows of solar collectors. They tend to consist of three discrete types of systems:

1. Parabolic trough systems are very effective

Parabolic troughs use parabola-shaped reflectors that are able to focus between 30 and 100 times normal sunlight levels onto the collector. The method is used to heat a fluid, which is then collected at a central location to generate high-pressure, superheated steam.

These systems tilt to track the sun throughout the day. The longest-operating solar thermal plant in the world, the Solar Energy Generating Sytems (SEGS) in the Mojave Desert, California, is one of these power plants. The first plant, SEGS 1, was built in 1984. 

The last plant built, SEGS IX, with an electricity generation capacity of 92 megawatts (MW), began operation in 1990. Today there are currently nine operating SEGS plants on the site with a combined capacity of around 354 MW net (394 MW gross) installed capacity - this makes it one of the largest solar energy thermal electric power projects in the world. These kinds of solar thermal power plants 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 fluid continuously flowing 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. Linear concentrating systems are simple, yet effective

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.

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 the 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 systems to focus sunlight approximately 30 times the normal intensity.

3. Solar dishes and engines are increasingly popular

Solar dishes also use mirrors to focus the sun's energy onto a collector. These tend to consist of dishes like oversized satellite dishes that are clad in a mosaic of small mirrors that focus energy onto a receiver at the focal point.

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 then transfers the heat generated to an engine generator. The most common type of heat engine used in dish/engine systems is a type of Stirling engine. Heated fluid from the dish receiver moves 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 dish's focal point. A solar dish's concentration ratio is much higher than linear concentrating systems, with a working fluid temperature above 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 using 429 Stirling engine solar dishes. 

4. Solar power towers can generate a lot of power

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 can concentrate sunlight as much as 1,500 times what 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 square km and 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. In the U.S., three solar power tower plants have been constructed. These are the 392 MW Ivanpah Solar Power Facility in Ivanpah Dry Lake, California, the 110 MW Crescent Dunes Solar Energy Project in Nevada (which is not currently in operation), and the 5 MW Sierra Sun Tower in the Mojave Desert, California (which has been shut down). 

The concentrated solar energy is used to heat the air in the tower 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 being researched and tested using nitrate salts because of their higher heat transfer and storage properties than water and air. 

The thermal energy-storage capability allows the system to produce electricity during cloudy weather or at night. These solar power plants are ideally suited for operations in adverse weather conditions. They're used in the Mojave Desert in California and have withstood hailstorms and sandstorms. However, two plants built so far are too expensive to run. 

5. Solar ponds are an interesting type of solar power plant

Solar pond power plants use a pool of salt water to collect and store solar thermal energy. It uses a technique called salinity-gradient technology.

This technique creates a thermal trap within the pond where the energy generated can be used directly or stored for later use. This kind of power plant was used 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; 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 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 electricity generation through a turbine or as a source of thermal energy.

And that is all for now, folks. 

As you can see, solar power is not just about PV panels. There are, in fact, various ways the sun's energy can be harnessed and used for our benefit.