Concentrated solar power, or CSP, is a technology where mirrors reflect and concentrate the sun's energy onto small areas known as receivers. The receiver then absorbs this heat in different forms, like gases, liquids, or solids.
The heated air is absorbed by electricity generated or stored in an energy storage device, depending on the power source. The heat energy produced by the sun is called solar thermal energy or solar radiation. This type of solar power is different from solar PV, or photovoltaic power generation, which uses sunlight to create electricity through photovoltaic solar panels.
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Difference Between Concentrated Solar and Solar Photovoltaics
The way concentrated solar (CSP) and photovoltaic solar panels (PV) produce energy is what sets them apart.
Photovoltaics utilize electrons to transform solar energy into electricity, while a CSP plant uses solar power to steam turbines.
Produce Electricity by Concentrating Solar Power
The primary element of any concentrated solar power (CSP) system are mirrors. CSP plants use mirrors to bounce and focus sunlight onto one particular area to create heat - and a lot of it! Not only can this heat drive a steam turbine or engine to generate electricity, but it can also store thermal energy to provide power when sunlight is at its weakest.
Not only can geothermal heat be used for power generation, but it also has a range of other applications, including water desalination, enhanced oil recovery, cement production, steel production, and green hydrogen production. CSP, or concentrated solar power, is most commonly used to produce electricity on a large scale. It generally falls into one of four categories.
There are four types of concentrated solar technology.
Parabolic Trough Systems
Parabolic Dish Systems
Power Tower Systems
Linear Fresnel Systems
Parabolic Trough systems
Parabolic trough systems use parabolic reflectors that use the sun's energy to concentrate sunlight onto a central receiver pipe, which is called an absorber tube. The mirrors in a parabolic trough system field are aligned side-by-side in numerous rows that stretch from north to south. This allows the mirrors to follow the sun as it moves across the sky from east to west.
The parabolic, curved shape of the reflector allows it to focus light rays onto a central point, which amplifies its reflection capacity by 30% to 60%. The tube, which is full of man-made oil, gets heated up to around 390°C. This heat can then be used in order to boil water and create steam that will generate electricity.
Of all the Concentrated Solar Power technologies, Solar parabolic trough systems are the most advanced and widely used. The Genesis parabolic trough power plant, located in Riverside County, California, is one of the largest CSP plants in operation within the United States. With an electricity generation capacity of 250MW, it has been running since 2011. The Rankine cycle is a process where fluid flows from a heat source to a cooler space, converting thermal energy into mechanical work.
The CSP uses steam to power a turbine, similar to how most other power plants work. In addition to using renewable energy, the CSP and other parabolic trough power plants can be used in conjunction with thermal-fired power plants that burn coal, natural gas, or biofuels.
Although all CSP technologies boast high efficiency, parabolic trough systems have the lowest at approximately 15%. This is because the fluid temperature doesn't get as hot as with the other methods.
Parabolic Dish Systems
Dish systems are similar in appearance to satellite dishes. The mirrors are parabolic and track the sun to concentrate sunlight onto a power conversion unit that is extended from the center of the disk. The power conversion unit contains thermal receivers, which are a series of tubes that contain a cooling fluid.
This component takes the heat reflected off of the mirror and transfers it to the engine, which then uses that heat to produce energy. Many other concentrated solar technologies use steam turbines, but the receiver in parabolic dishes is instead integrated into a combustion engine. The most common type of combustion engine used is a Stirling engine, which drives an electric generator to produce power.
The engine consists of tubes containing a working fluid, like hydrogen. The temperature inside the tubes can reach as high as 1,200 degrees Fahrenheit. The heat causes the gas to expand and drives pistons that turn a crankshaft, generating electricity in the process.
The average dish system produces 10-25kW per dish and has a conversion efficiency rate of 30%. However, using a reflector in addition to the Stirling engine doesn't make the technology well-suited for thermal storage.
Linear Fresnel Systems
CFNL systems, also known as Compact Linear Fresnel Reflectors, are very similar to parabolic trough systems. Though, they use long rows of flat mirrors that are parallel to each other instead of curved mirrors.
These mirrors reflect sunlight onto a receiver elevated above, where tubes circulate water as a heat transfer fluid.
Concentrated solar energy heats water to produce high-pressure steam, which spins a turbine to generate electricity.
Power Tower systems
Lastly, the most massive CSP systems are the power tower systems. They use large mirrors called heliostats that concentrate and reflect sunlight onto a receiver at the top of a tower. These mirrors on power towers are flat and computer-controlled so that they always follow the sun.
The thermal energy is then used to produce steam by heating a transfer fluid to temperatures above 1,000 degrees Fahrenheit. The steam extracted from boiling water powers a generator turbine to create electricity.
Advanced power tower systems usually use molten salt as the heat-transfer fluid instead of water because it is more efficient.
Molten salt's ability to retain heat means it can be stored and used days later to create energy. Even during dreary weather or after the sun has set, electricity stored in these power towers facilities can still be produced when there is a high demand for it.
The Logistics of CSP technologies
The majority of the world's concentrated solar power plants are located in Spain, making up approximately 42% of all CSP plant projects.
The Ouarzazate concentrating Solar Power Station in Morocco has a capacity of 510 MW, making it the largest CSP plant project globally. With this much power generation, it's able to provide energy to 1.1 million people.
The largest concentrated solar thermal plant in the United States is the Ivanpah Solar Electric Generating System. The 392MW project, located in the Mojave Desert of California, uses 173,500 heliostats and started commercial operations in 2013.
Making CSP technologies more efficient
Yara, a Norwegian company, has developed a new ternary mixture of molten salts that contains calcium, potassium, sodium nitrate, and other natural ingredients. This innovative composition reduces the risk of freezing and solidification while providing superior results.
The temperature of molten salts must stay above 270ºC to remain a liquid - if it gets lower, the salt will freeze and solidify. if it gets too cold, the salt can damage equipment, clog pipes and bring operations to a halt- which creates more risks and maintenance costs.
The new mixture's low melting point (131°C) and wide temperature variation (438ºC ) boost the energy yield/efficiency while simultaneously using less of the molten salt blend. The composition of the salt also reduces corrosion. By adding these features, the plant's lifespan is increased while costs are lowered. This is done due to the lower requirement for materials, potassium calcium nitrate's competitive price tag, and a reduction in corrosion-related maintenance fees. But solar thermal energy storage in a CSP plant isn't restricted to molten salt.
Artificial intelligence to help CSP plants
Heliogen is a California-based company that uses advanced computer vision/AI to precisely align an array of mirrors. By doing this, they are able to store energy in rocks and create a concentrated solar solution. They constantly adjust the position of the mirror edges and angles of reflection 30 times per second.
The average power tower system only produces heat from 400-500ºC, but this machine uses AI and machine learning to produce 1500ºC.
The heat is concentrated on the top of the power tower and moves down an insulated steel tube to a pile of rocks. If it's stored properly, the warmth can last for days or even up to a week. Heliogen's unique technology is not only more efficient but also requires less water than traditional steam turbines.
Moreover, the company's founder, Bill Gross, when discussing costs, stated that its power tower technology aims to provide heat at $0.01/kWh.
New concentrating solar power plant coming to the US
In March 2022, the company signed a project agreement with Woodside Energy to develop a concentrated solar energy project at utility scale on a site in Mojave, California. The planned capacity for the plant is 5 megawatts. After a couple of months, the company announced that they were ready to start testing and implementing the project.
You can find all the current and planned concentrated solar thermal energy plants and thermal storage systems on the national renewable energy laboratory website here.
What is Solar Fluid?
CSP isn't the only technology innovating in storing solar energy as heat. This is entirely different from what CSP offers for large-scale energy production, but it's an indication of where research is going in terms of storing solar energy in more aspects of our lives.
Chalmers University of Technology scientists in Sweden have uncovered a fluid that could harness energy for up to 18 years. This new fluid contains a molecule made up of carbon, hydrogen, and nitrogen.
Electrons are shared among atoms when they form bonds to create molecules. Every type of molecule has a distinctive three-dimensional shape. For instance, methane is shaped like a tetrahedron. When molecules absorb energy, they can change their structure and shape. This enables the atoms to form new bonds that can act as thermal storage for different amounts of energy.
Chalmer University's molecule was specifically designed to be more effective in absorbing various wavelengths of concentrated sunlight. This molecule can utilize energy from ultraviolet and blue-green light waves. When the molecule is hit by sunlight, it changes into a high-energy isomer. An isomer is a molecule made of the same atoms bonded in a new way.
The research team created a catalyst to manage the energy storage and release of energy from the molecule, which puts the molecule back into its original state. When solar fluid changes shape, it increases in temperature by 63ºC. Once it returns to its original state, it is able to capture more solar energy.
Molecular Solar Thermal Energy Storage System (M.O.S.T.)
The system is liquid-based and flows through a panel. It consists of two glass plates with a liquid in the middle. The molecules are exposed to light, which converts them.
The high-energy molecules are stored in a small tank, and later they flow over a catalyst that sits in a device. This device triggers the heat release and sends out the stored energy.
To put it simply, the molecules absorb solar radiation in an energy form and store it as chemical energy. When heat is needed, the molecules release the stored energy in the form of heat. It's like a never-ending cycle that can happen hundreds of thousands of times.
Energy Transportation and MOST
They sent the molecules, which had absorbed energy from Swedish sunlight to China. There, researchers from Shanghai Jiao Tong University released the energy and converted it into electricity using a generator that they developed. This sounds like it could be promising. It's pretty wild that Swedish sunshine was sent to the other side of the world and converted into electricity.
Zhihang Wang, a researcher from the Chalmers University of Technology, explains that: “...The generator is an ultra-thin chip that could be integrated into electronics such as headphones, smartwatches, and telephones. So far, we have only generated small amounts of electricity, but the new results show that the concept really works. It looks very promising...” -Zhihang Wang.
The efficiency of Molecular Solar Thermal energy storage capability
The most optimal systems have an efficiency rate of between 30 to 50% at the wavelength they receive. However, we are able to utilize the full solar spectrum. It consists of numerous wavelengths. Our current best system captures 3% of the incoming solar ray energy from these waves.
According to their theory, the maximum amount of concentrated solar energy that can be converted into another form is between 12 and 16%. It'll not be as much as a photovoltaic, and that is because you have to pay something for the storage of the molecules, and therefore it cannot be as efficient as that.
Optimizing the chemical structure of MOST
The hardest part of the process has been optimizing the molecules. The heat recovery, though, is progressing well. They are successfully extracting the heat they have been storing. In order to minimize losses during energy storage, they want to expand their solar power generation spectrum while also keeping a high inner density.
They are trying to optimize a few parameters at the same time, which can be tough to do when each parameter is pulling in a different direction. The goal is to create systems that work well no matter what aspect you're looking at it from. The research is still in progress, and they are constantly testing out new molecules and making improvements. The next step is to make the system more powerful by increasing it from a few watts to possibly one thousand watts.
Residential Applications of MOST
The molecule may not generate quite as much power as photovoltaics or CSP systems, but it could be added to other things to enhance them.
Now imagine adding a photovoltaic system to your home. Not only would you be turning photons into electrical energy for your home, but you’d also be storing it as heat. This heat can later help with other things like producing hot water in the middle of the night. Not only can this system heat your hot water sans gas or electricity, but it could also be used for a car's cabin. The applications are limitless!
Why aren't there more CSP plants in the US
The last new concentrating solar power project was established in 2016, and only one more is in the works. The reason for this is most likely because CSP only works on a large-scale level, and it's much easier said than done to set one up.
In addition, more utility-scale CSP plants are being abandoned or left to decay because of the colossal scope of the project.
Though it's effortless to get the initial funding required to install them, it's difficult to finance their upkeep. Power generation companies may not view maintaining them as being profitable enough and can end up forgetting about the project altogether.
SEIA Solar Energy Industries Association
For a CSP project to be successful, the SEIA has set out the following requirements.
Convenient Transmission Access
Solar plants need to be situated near a power grid and, for best results, an outdated one that's under current strain.
Siting CSP plants in areas where there is an immediate need for new transmission lines, electrical power providers can earn more money per kilowatt hour.
According to SEIA, The Southwest U.S. is an excellent place to consider under these circumstances for utility-scale application.
Large Parcels of Land
Solar power projects on a utility-scale require significant deeds of land because they are most productive when constructed in bulbous sizes, usually measuring at least 100MW.
The average CSP plant requires 5 to 10 acres of land per MW, as estimated by the SEIA. This is a lot of space for very little output.
Access to Water Resources
CSP systems rely on thermal energy for electricity, which means a lot of water is required to keep the components cool and the steam turbine running. Although other methods are being tested, water cooling is still the best way to go.
The need for water to generate steam and spin the turbines limits where solar concentrator installations can be placed, as these locations are typically areas with little water available.
High Solar Radiation
If a concentrated solar power plant does not receive enough of the sun's energy from direct sunlight, it will be wasteful both financially and in terms of the amount of space it takes up.
CSP experts use sunlight’s direct normal intensity (DNI) to determine an area’s potential for harnessing solar power. And as you would expect, the US southwest is the region with the most promise.
CSP plants are expensive to build, which is why most of them are utility-scale.
One of the reasons for this is that current solar power technologies are considered experimental. This means that there is a high initial investment cost.
The good news is that CSP is reducing in cost, just like solar PV technology.
The cost per kilowatt installed in 2020 was 4,725 U.S. dollars, 50% less expensive than the 2011 10,588 dollar rate.
Viability In CSP technology and its future
The output of CSP systems in terms of energy efficiency lies between 7% and 25%.
Renewable energy sources like hydropower, wind power, and solar PV cells are much more efficient than CSP at 90%, 59%, and 23%, respectively.
However, with more research, we can develop new technologies that make concentrated solar power more efficient.
Concentrated Solar Power Disadvantages
Even though Concentrated Solar Power has many advantages, there have been several disadvantages that have caused some CSP installations to be shut down.
The waste and contamination of the desert environment are a shame, especially because it was all caused by an investment that has now been left to decay.
Disadvantages to having concentrated solar power systems.
CSP is expensive and complicated to set up because it requires molten salt, which is rare and not cheap.
CSP is economically impractical because it requires a large amount of land to function, which precludes establishing these plants in populated areas. Additionally, the land used for CSP could be put to better use for other purposes.
CSP plants rely on freshwater sources for their concentrated solar installations, but these resources are scarce in some of the areas where they choose to build.
Though they don't produce greenhouse gases while relying only on solar power, concentrating solar power plants like Ivanpah burn fossil fuels in the form of natural gas when generating electricity, which releases greenhouse gases into the atmosphere. This is not considered renewable energy.
Although many Concentrated Solar Power (CSP) installations are located in the desert Southwest, people should not assume that the land is useless.
The fragile desert environment is important to the local wildlife and people. These abandoned structures are an ecological disaster.
Concentrated solar power plants attract and kill birds.
Migrating birds die when they fly through concentrating solar power fields along the Pacific Flyway. If these migrating birds don't avoid the solar fields along their path, they will be incinerated during their natural migration.
Every concentrated solar installation causes the yearly death of thousands of birds. At Ivanpah CSP alone, 6,000+ birds die each year from the intense heat in concentrated solar beams. Though they may be seeking food or shelter, the birds that fly into solar fields are putting themselves in grave danger. These plants have caused a death toll among these creatures, and environmental groups are fighting to raise awareness about the issue. The animals aren't safe in their own habitat because of the deadly machines that humans have set up there.
People don't value the land around Ivanpah, but it's directly on the Pacific Flyway - a migration route for many birds, including protected and endangered species. They're not being protected from incineration during their natural migration.
Roadrunners are being killed by coyotes at an unprecedented rate as they get caught outside the fence meant to protect desert tortoises from straying into a solar field.
The Environmental Footprint of Concentrated Solar Power
Furthermore, CSP has negative environmental impacts that cannot be ignored.
For example, power plants need an abundance of water for cooling and generating electricity. However, these power plants are usually built in areas where freshwater is scarce.
This has caused much anger and Skepticism among residents living in places such as the American Southwest, Middle East, and North Africa–where demand for limited water resources is continuing to grow.
CSP plants can have a negative impact on the environment, as they are often set up on lands that are home to endangered species of flora and fauna. These animals and plants are killed or forced to migrate by the sudden change in their natural habitat.
Natural gas is burned in concentrating solar plants.
Ivanpah isn't the only concentrated solar plant that still burns natural gas. In fact, many plants do it in order to get the concentrator hot, especially when there's not enough sun to power the generator.
In Ivanpah's first year, it released 46,000 tons of carbon dioxide gas into the atmosphere. The problem has only gotten worse since then.
The company frequently petitions the California Energy Commission to allow them to use more natural gas. However, this would ultimately lead to an increase in greenhouse emissions, despite the subsidies they receive for being a renewable energy source.
While it is true that the carbon released by even a "dirty" solar plant like Ivanpah pales in comparison to traditional fossil fuel energy plants, we consider it more of a hybrid energy plant than green energy.
Although not all CSP technology is a renewable energy source and might not eliminate environmental issues because some still use fossil fuels, it's still a step in the right direction in terms of greenhouse gases.
The Various Benefits of Using Concentrated Solar Power
Many energy companies have invested in concentrated solar because it usually—but not always—has more advantages or pros than disadvantages.
The primary benefit of CSP is that it's environmentally friendly because it relies on renewable energy. For example, the sun's heat energy is harnessed to generate power, and we'll never use up all the sun's heat.
When relying on solar power alone, concentrated solar power doesn't emit carbon dioxide.
Maintaining CSP plants is considerably less complicated than nuclear or hydrocarbon-based plant counterparts, resulting in lower operating costs for concentrated solar power.
CSP is a more reliable source of energy than solar PV and wind power, especially when molten salt is used as the thermal fluid because it can store energy.
CSP systems can be paired with existing electric power plants that produce steam to run on both fossil fuels and solar. This type of hybrid system saves fossil fuel plants from overconsumption.
It is essential that we find a way to store solar power as we move forward into a more environmentally-friendly future through new solar energy technologies. Though electricity generation from sunlight stored into battery power like solar PV is one method, it isn't the only option.
While there are already some CSP power plants that concentrate sunlight and use molten salts for thermal energy storage, newer methods like storing the sun's energy directly into molecular bonds are still in their infancy. It will likely take some time to figure out how to scale up these newer approaches and find the best applications for them, but the benefits they could offer – like being able to store electrical power for 18 years – are very promising.
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