Concentrated solar power (CSP) is an approach to generating electricity in which mirrors are used to reflect, concentrate and focus sunlight onto a specific point. The concentrated light is converted into into heat, which in turn is used to create steam. The steam is then used to drive a turbine that generates electrical power.
CSP technology is used to store solar power so it can be used on cloudy days and in the hours after sunset or before sunrise. As of this writing, CSP is still an emerging approach to generating electricity and is most commonly used in utility-scale projects.
Concentrated solar power and the ability of CSP technology to store thermal energy has made the renewable energy option especially popular in the United States Sun Belt region. The Sun Belt region covers states in the southern and southwestern parts of the country, stretching from California to Florida; this includes: Georgia, South Carolina, Alabama, Mississippi, Louisiana, New Mexico, Arizona, Nevada and Texas.
The United States has been reliably operating CSP plants for over 15 years. For example, the Crescent Dunes power plant in the Mojave Desert -- which spans across southeast California and southern Nevada -- was developed with U.S. Department of Energy (DoE) funding in the 1990s. The power plant uses molten salt as their heat transfer fluid and storage medium, thereby benefiting from enhanced efficiency and cost-effectiveness.
Key requirements for concentrated solar power
Several factors are essential to the successful implementation of a concentrated solar power system. The key requirements include:
- Proper financing - CSP technologies and systems are expensive. Receiving adequate project financing can be difficult, but necessary.
- Available water sources - CSP systems need access to a water source for cooling and to wash the collection and mirror surfaces. CSP plants have the ability to employ wet, dry or hybrid cooling techniques in order to optimize efficiency in the generation of electricity and conservation of water.
- Locations with high solar radiation - Optimal concentration of the sun's energy requires that the light be intense and not too spread out. The sunlight's focus can be measured by the direct normal intensity (DNI) of the sun's energy. The sun's DNI is stronger in the Sun Belt region than any other area of the U.S., thus making the production potential with CSP stronger here than anywhere else in the country.
- Adjacent plots of land with minimal cloud coverage - While the exact amount of land required by a CSP plant will vary based on the technologies used, CSP plants typically require five to ten acres of land per megawatt (MW) of capacity.
- Accessible and proximate transmission access - The land used for CSP must be suitable for power generation as well as provide access to an increasingly stressed and antiquated transmission grid. High-voltage transmission lines are also a key requirement for the utility-scale projects to transfer electricity from the CSP system to the end user.
Types of concentrated solar power systems
The four main types of CSP systems are:
- Parabolic dish systems
- Parabolic trough systems
- Compact linear Fresnel reflector systems
- Power tower systems
Parabolic dish systems employ u-shaped -- or parabolic -- mirrored dishes that are about ten times the size of a residential satellite dish to focus sunlight onto the receiver which is mounted at the focal point of the dish. This receiver is integrated into a high efficiency external combustion engine that utilizes tubes holding hydrogen or helium gas that run along the outside of the engine and open into the four piston cylinders.
As the concentrated sunlight is collected by the receiver, it heats the gas in the tubes to extreme temperatures, causing the hot gas to expand within the cylinders. This expanding gas drives the pistons which turn a crankshaft which, in turn, powers the generator that produces electricity.
In a parabolic dish system, the receiver, engine and generator form a single, combined assembly that is placed at the focal point of the mirrored dish. In order to capture the peak amount of solar energy, the dish system follows the sun's path across the sky, similar to a tracking array.
Parabolic trough systems incorporate u-shaped mirrors that have oil-filled pipes running along the focal point. The mirrors are pointed towards the sun and concentrate the sunlight onto the pipes in order to heat the oil inside; temperatures can reach as high as 750°F. The heated oil is then used to boil water and produce the steam that is used to drive turbines and generators.
Compact linear Fresnel reflector systems use long parallel rows of flat mirrors that cost less than the curved mirrors in the parabolic trough system. The flat mirrors in the compact linear Fresnel system concentrate the solar energy onto raised receivers that incorporate a collection of tubes filled with flowing water. The focused sunlight is used to boil the water, thus producing steam that can be used to generate power.
Power tower systems, sometimes called central receivers, use numerous large, flat mirrors to track the sun and focus the solar energy onto a receiver. In this system, the receiver sits at the top of a tall tower and collects sunlight that is used to heat fluids, such as molten salt; temperatures can reach up to 1,050°F. The heated fluid can either be used immediately to make steam and generate electricity, or it can be stored and saved for use at a later point.
Benefits of concentrated solar power
The reduction of carbon emissions is one major benefit presented by all solar power systems. Some other benefits include:
- Energy security - Renewable energy is domestic, whereas oil and gas sources are concentrated in specific areas. A diverse energy supply throughout a nation, provided by domestic energy sources, strengthens energy security and contributes to a sustainable, long-lasting energy strategy that can protect the power supply from detrimental market fluctuations and vulnerabilities. Decreasing dependence on imported sources and replacing foreign energy with reliable, clean domestic electricity can provide local economic opportunities while increasing energy security.
- Longstanding reliability - On average, solar power plants are built for up to 25 to 30 years of operation. The operator of the plant should understand that the technology and equipment will need to be maintained, refurbished and replaced over time. It is expected that each upgrade is more efficient with a lower cost. Therefore, longstanding reliability is established as the renewable energy source produces electricity for a long time while continuously improving efficiency.
- Increasingly competitive prices - While the cost of gas, fossil fuels and other energy alternatives continues to fluctuate across regions, renewable energy has seen a continuous decrease in price. The most significant drop has been in the solar industry, with prices dropping around 80% over the last seven years. Therefore, solar power presents a cheaper, more cost effective energy source.
History of concentrated solar power
According to the ancient Greek myth, the concept of concentrated solar power was first discovered by Archimedes in 214 - 212 BC. He developed a defense tactic where soldiers used bronze shields to concentrate the sunlight onto the invading Roman ships, causing the vessels to catch on fire. However, the first documented use of CSP was in 1866 when August Mouchout used a parabolic trough system to heat water and produce steam in order to drive the first solar steam engine.
In 1912, Frank Schuman, an inventor from Philadelphia, Pennsylvania, established a parabolic trough system in a small farming community in Meadi, Egypt. The troughs were used to generate steam which, in turn, was used to drive large water pumps, delivering 6,000 gallons of water per minute to vast areas of desert.
In 1968, the first operational CSP plant was built in Sant'Ilario, Italy by Professor Giovani Francia. The plant utilized a power tower system surrounded by a field of other solar energy collectors.
In 1982, a collection of solar industry organizations and the U.S. DOE began operating Solar One. Solar One was a 10 MW demonstration project of the power tower system that established the system's feasible use and operation.
In 1986, the world's largest solar power facility was commissioned in Kramer Junction, California. The system used rows of mirrors to focus the solar energy onto a system of pipes and heat the transfer fluid inside. The steam produced powered a turbine that produced electricity.
Between 1996 and 1999, the U.S. DOE and a collection of solar industry organizations worked on Solar Two -- a project that was supposed to be an improvement of the Solar One power tower project.
Future of concentrated solar power and space-based solar power
Advancements and inventions continue to be made which improve the use and accessibility of concentrated solar power. Improvements are continuously developed and applied to CSP systems, allowing them to operate at increased efficiency and power a variety of utility-scale projects.
Since the mid-20th century, research has been conducted on the use of space-based solar power (SBSP). SBSP is the concept that solar energy can be captured in outer space and then transferred to Earth or other planets to use as electricity. SBSP has the ability to solve the world's greenhouse effect with minimal impact on the environment.
One way to construct a SBSP system is by using concentrated solar power. These SBSP systems employ the technique of focusing solar energy using mirrors which heat up a liquid and drive a turbine that creates electricity.
One obstacle that is preventing the widespread experimentation and deployment of space-based solar power systems is the steep cost of acquiring and sending all the necessary materials. SBSP has the ability to provide constant, clean, reliable power at cheaper costs, but it is going to take many years of building, testing, investment and successful deployments before the system can even begin to make up its initial costs.
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