The solar energy industry is dominated by solar PV, with solar farms, rooftop solar, and residential installations at an all-time high. But photovoltaics aren’t the only form of solar energy. Concentrated solar power has traditionally stood far behind PV in terms of implementation, being relatively unrecognized in the larger realm of renewable energy. But as technological innovation makes concentrated solar more viable, CSP plants can become a competitor to solar PV, and even fossil fuels, over the next few decades.
Concentrated solar power, also known as CSP, is the conversion of sunlight to thermal energy. CSP plants utilize a field of solar collectors to concentrate a large area of sunlight onto a small focal point. At the focal point is a receiver filled with a liquid medium that is heated by the sunlight. This superheated medium is used to transfer heat that can be used for several purposes, from generating electricity to driving machinery.
Concentrated Solar: How It Works
Whereas photovoltaics rely on a chemical reaction to convert sunlight into electricity, concentrated solar harnesses the raw thermal energy from the sun. They do this by using a series of concave or parabolic mirrors to focus sunlight onto a small area. This magnifies the sunlight and creates an area of extremely high temperatures, which is used to heat a liquid medium. The heat from the superheated liquid medium is used to create steam, which drives a turbine and produces electricity. Since CSP uses thermal energy, it has more in common with fossil fuel plants than solar PV farms.
Concentrated solar plants rely on liquids that are good at storing heat, like oil or molten salt. The liquid is stored in a central receiver placed at the focal point of the solar collectors, absorbing the concentrated sunlight. The immense amount of energy focused on a small area raises the temperature of the liquid to between 700 and 1,000 degrees Fahrenheit.
The superheated liquid is then pumped through a heat exchanger, where the thermal energy is transferred to a steam engine. The liquid medium heats up the water in the engine, creating pressurized steam that drives a turbine and generates electricity. The steam is then cooled and condensed back into water, and the liquid medium is pumped back towards the receiver at the focal point. CSP plants can store thermal energy in the liquid medium, allowing them to continue generating electricity even after the sun sets.
Concentrated solar power is a relatively simple concept, and so it’s been used by numerous civilizations throughout history. As early as the 7th century BC, humans were using magnifying glasses to start fires. The Romans and Greeks used mirrors to light torches, and the Greeks even had a “death ray,” which consisted of a parabolic mirror that used concentrated sunlight to set fire to enemy ships.
In 1767, Swiss scientist Horace de Saussure invented the first known solar cooker. It was made of a series of glass boxes that fit inside one another. It concentrated the sun’s heat into the innermost boxes, which would get hot enough to boil water and cook food. His solar cooker was used by Sir John Herschel during his South African expedition.
French physicist Augustin Mouchot invented the first solar-powered steam engine in the 1860s. It consisted of a dish-shaped solar mirror that concentrated sunlight on a glass tube filled with water. The water was brought to a boil, and the resulting steam drove the engine. Mouchot received financial support from the French government, who saw his invention as a viable form of energy. But a decrease in the price of coal made his invention less economically appealing, and so the government cut his funding.
Over the next few decades, scientists like Frank Schuman conducted research into using concentrated solar for irrigation, locomotion, and refrigeration. In 1913, Schuman designed and installed a 41 kW parabolic solar plant in Egypt to pump irrigation water. His invention was highly regarded by scientists and inventors of the day. Unfortunately, interest in concentrated solar power was hampered by two world wars and a decrease in the price of fossil fuels.
In the 1960s and 70s, interest in concentrated solar once again peaked as oil prices increased and environmental concerns came into the public discourse. The first electricity-generating CSP plant was built in Italy in 1968. Designed by solar pioneer Giovanni Francia, the solar plant used a central solar receiver in a field of solar collectors. It had a capacity of 1 MW.
The 1980s and 90s saw several CSP plants erected in California. Solar One was built in 1981 and had a capacity of 10 MW. It was upgraded to Solar Two in 1995, incorporated a new design using molten salt as the liquid medium for heat transfer. SEGS was commissioned in 1984. With a rated capacity of 354 MW, SEGS was the largest CSP plant in the world until 2014.
There was a lull in new CSP plants until the mid-2000s, when several solar plants were built in sunny regions across the globe. Spain took the lead, building over 40 parabolic trough systems between 2009 and 2013. CSP plants also went up in Australia, the Middle East, and the Southwestern US.
Types of Concentrators
There are several different ways to concentrate the sun’s energy, though all methods rely on focusing sunlight onto a single point to heat some kind of fluid. Whereas water was once the fluid medium of choice, molten salts are usually employed as they can operate at higher temperatures, though direct steam systems still do exist. The main differences between CSP systems lie within the shape and configuration of the solar collectors and receivers.
Parabolic trough systems use long rows of curved parabolic mirrors to focus sunlight on a thin receiver tube at the mirror’s focal point, which hangs over the center of the mirror. This tube is filled with a superheated liquid medium, which can reach temperatures as high as 750 degrees Fahrenheit. The liquid runs through a heat exchanger, which heats water and creates pressurized steam. Parabolic CSP plants consist of hundreds of parallel rows of solar collectors. They’re oriented north to south so they can follow the sun across the sky. Some parabolic trough systems incorporated a greenhouse-like design, where the curved mirror becomes an enclosed tube with the receiver in the center. Parabolic troughs are the dominant form of CSP around the world, though solar power towers have recently become the preferred system.
Solar Power Towers
Solar power towers use a central receiver system, where hundreds to thousands of solar collectors are aimed at a single central tower. The focal point is set high at the top of the tower, and the liquid medium drives a steam turbine inside the tower. The mirrors in the plant are flat and track the sun as it moves through the sky. Because they only have one receiver, they can reach higher temperatures than parabolic troughs, making them more efficient. The liquid medium in a solar tower can reach temperatures over 1,000 degrees Fahrenheit. Solar power towers are taking over parabolic trough systems because of their higher efficiency and the ability to more easily integrate energy storage.
Fresnel reflectors use a series of long, narrow mirrors to focus sunlight on a central receiver. They’re similar in design to parabolic troughs, but they use flat mirrors, which are significantly cheaper. All the mirrors share a central receiving tube which runs the length of the mirrors at their focal point. Fresnel reflectors are cheaper and have a simpler design when compared to parabolic troughs.
Solar Dish Stirling Engine
Solar dish collectors use a single large parabolic dish to concentrate the sunlight onto the focal point. The receiver at the focal point is usually attached to a Stirling engine, which uses the superheated liquid to create mechanical energy. The dish-engine system is notable for its high efficiency compared to other CSP designs.
Concentrated solar power is incredibly useful for a wide range of applications. The sun’s thermal energy can be used as is for heating, or it can be used to generate electricity or drive mechanical engines. In some sense, CSP can be more practical than solar PV.
Most CSP plants are used to generate electricity. They concentrate sunlight to create heat. This heat is then converted into mechanical energy in a turbine, which is then converted into electrical energy. The Ouarzazate Power Station in Morocco is the world’s largest CSP plant, with a capacity of 510 MW. It combines a 150 MW solar power tower with 360 MW of parabolic troughs. There are also plans to install a 72 MW PV plant as well.
Concentrated solar power can be used as thermal energy to drive the machinery for heavy industrial processes. Solar thermal energy has been used in mineral processing, chemical production, food processing, and desalination. Solar thermal enhanced oil recovery is a technique used in oilfields with unusually heavy petroleum. Steam generated from concentrated solar is pumped into the ground to make the oil less viscous and easier to pump. This method has made normally unprofitable oil wells more economical for extraction.
Low-temperature solar thermal can also be used as passive heating in residential and commercial applications. Solar collectors can be used to heat pools, homes, and buildings. They’re often integrated with sustainable building designs.
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Concentrated Solar vs Photovoltaics
CSP is often compared to PV solar panels. They both use the sun, which makes it an easy comparison, but they work on very different principles. Concentrated solar turns sunlight into thermal energy to generate electricity using a steam-powered turbine. PV panels use sunlight to stimulate a chemical reaction within a semiconductor material that produces electricity. In this respect, CSP has more in common with other forms of thermal energy, like fossil fuels.
Concentrated solar has a few advantages over PV. Whereas both solar panels and CSP are limited by the availability of sunlight, concentrated solar relies on heat, which can be stored. Stored thermal energy can be used to produce electricity for several hours after sunset or during cloudy days. While PV arrays can store energy in batteries, the capability to produce excess energy during the day is based on the size of the system and the amount of energy being used.
But there is a reason PV is way ahead of CSP in terms of utility-scale use. Concentrated solar is expensive, with the cheapest CSP plants still about 5 times more costly than PV plants. Whereas a solar farm can cost as little as 1.3 cents per kWh, a solar power tower can cost a minimum of 7 cents per kWh. Concentrated solar is becoming cheaper as technology advances, and experts expect CSP to be competitive with PV in a few decades.
Concentrated solar also has a larger environmental footprint compared to PV. Like other types of thermal energy, CSP is a large consumer of water. Large volumes of water are needed to cool down the internal machinery and wash off the solar collectors. Many CSP plants are in dry regions where water is scarce, which adds to the overall environmental cost. Many solar power towers also burn fossil fuels at the beginning of the day to get the liquid medium up to the operating temperature. But solar PV has an environmental impact as well. Some types of solar panels require toxic chemicals to manufacture, and old solar panels can become hazardous waste if not disposed of properly.
Concentrated solar power is an increasingly attractive option to generate clean energy. It’s a more direct form of solar energy, one that harnesses the heat energy of the sun to drive a steam turbine. Even though the use of solar thermal energy goes back centuries, solar farms that use PV panels are still cheaper to install and operate. Yet, solar power towers are being built across the globe, with Spain and the US taking the lead. Even with the benefits of thermal energy storage, CSP still lags behind solar PV in cost, and the comparatively large environmental footprint is another big hindrance to further development.
Frequently Asked Questions
Concentrated solar power, also known as CSP, is the the capturing of sun’s thermal energy to generate electricity. CSP uses reflective solar collectors combined with receivers filled with a liquid medium. The receiver is placed at the focal point of the solar collectors, and the medium is heated to extremely high temperatures. This heat is then used to drive a steam turbine.
There are four types of concentrated solar plants; parabolic troughs, solar power towers, Fresnel reflectors, and solar dish engines. Parabolic troughs and solar power towers are the most common types.
CSP plants benefit from increased energy storage capacity when compared to PV, as the thermal energy can be stored for several hours to continue generating power after sunset. But solar PV is still cheaper and less environmentally impactful than CSP.
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