Concentrating solar collectors use reflective surfaces to concentrate sunlight on a small area, where it gets absorbed and converted into heat.
In the case of solar photovoltaic (PV) devices, the sunlight is converted into electricity.
Concentrators are capable of increasing the radiant power of sunlight a few hundred times. This type of solar collector is generally used for high-temperature applications, including steam production for generating electricity and thermal detoxification.
Concentrating collectors are ideal for climates with primarily clear sky days.
Concentrating solar collectors in Concentrated Solar Power (CSP) systems concentrate sunlight on a receiver where it heats a heat transfer fluid. Subsequently, it exchanges the absorbed heat to water to produce steam for powering a steam turbine-generator (STG) to generate electricity.
Concentrating collectors function based on direct sunlight, and they work the best in clear, dry climates. Most concentrating collectors are mounted on free-standing structures, which allow them to track.
There are, however, some collectors that resemble flat plate collectors and can be mounted directly onto building surfaces.
In this post, we will mainly talk about different types of concentrating solar collectors and their overall advantages and disadvantages. Here we go.
Types of Concentrating Solar Collectors
Primarily there are four types of concentrating solar collectors, which are:
- Parabolic trough collector
- Power tower receiver
- Parabolic dish collector
- Fresnel lens collector.
Parabolic Trough Collector
A parabolic trough comprises a linear parabolic reflector that concentrates sunlight on a receiver that is positioned along the focal line of the reflector. The receiver is a tube placed directly over the middle of the parabolic mirror and filled with a working fluid.
The heat absorbed by the working fluid transfers to water for producing steam. The focus of solar radiation changes with the change in the Sun’s elevation.
The reflector keeps following the sun during the day by tracking along a single axis. A working fluid (e.g., molten salt is heated between 150 and 350 °C (302–662 °F) as it flows through the receiver and is then used as a source of heat for generating electricity.
Among all the concentrated solar collectors, trough systems are the most developed technology.
Acciona’s Nevada Solar One near Boulder City, Nevada, and Andasol were first commercial parabolic trough plants of Europe.
Also, the Solar Energy Generating Systems (SEGS) plants in California and Plataforma Solar de Almería’s SSPS-DCS test facilities in Spain are other examples of such plants.
Solar Power Tower
A solar power tower comprises an array of dual-axis tracking reflectors or heliostats that concentrate sunlight on a central receiver placed at the top of the tower. The receiver contains a heat-transfer fluid, which contains water-steam or molten salt.
The heliostats are installed around the central tower. Each heliostat rotates into two directions to track the sun. The solar radiation that reflects from heliostats is absorbed by the receiver mounted on a tower of about 500 m height.
Optically a solar power tower is similar to a circular Fresnel reflector. The working fluid in the receiver is heated to 500–1000°C (932–1,832°F or 773-1,273 K). Subsequently, it is used as a heat source for generating electricity or storing energy.
An advantage of the solar tower is that the reflectors can be adjusted instead of the whole tower. The technology of power tower is less advanced than trough systems. However, they provide higher efficiency and better capability of storing energy.
The Ashalim Power Station, Israel, once completed, will be the tallest solar tower in the world. The construction of the tower began in 2014. The tower will have concentrated light from more than 50,000 heliostats.
The Planta Solar 10 (PS10) in Sanlúcar la Mayor, Spain, is the first commercial utility-grade solar power tower in the world.
The Ivanpah Solar Power Facility, located in the Mojave Desert (377 MW capacity), is the largest CSP facility in the world and has three power towers.
Fresnel reflectors contain many thin, flat mirror strips to concentrate sunlight on tubes through which working fluid is pumped.
Flat mirrors accommodate more reflective surfaces in the same amount of space than a parabolic reflector. They capture more available sunlight and also much cheaper than parabolic reflectors. Fresnel reflectors can be used in various sizes of CSPs.
Fresnel reflectors are often said to be a technology with the lowest output than other methods.
Some new models of Fresnel reflectors with ray tracing capacity have recently been tested and initially proved to provide higher output than the standard version.
Parabolic Dish Collector
A parabolic dish collector or dish stirling has a stand-alone parabolic reflector that concentrates light on a receiver placed at the focal point of the reflector.
The reflector tracks the Sun along dual axes. The working fluid in the receiver is heated up in the temperature between 250°C and 700°C (482–1, 292 °F) and then used in a stirling engine.
Parabolic-dish systems provide a high level of solar-to-electric efficiency (between 31% and 32%), and their modular nature provides scalability.
SES dishes at the National Solar Thermal Test Facility (NSTTF) in New Mexico set a world record for solar-to-electric efficiency at 31.25% on January 31, 2008.
In 2015, Ripasso Energy, a Swedish firm, the developer of parabolic dish collector, tested the system in the Kalahari Desert in South Africa and recorded 34% efficiency.
Some of the examples of this technology are Stirling Energy Systems (SES), Science Applications International Corporation (SAIC) dishes at UNLV, and United Sun Systems (USS) and Australian National University’s Big Dish in Canberra, Australia.
Because of the limitations of size and the small quantity of fluid, parabolic dish collectors are suitable for small-scale power generation (up to a few kW).
Advantages of Concentrated Solar Collectors
- Zero Fuel Cost: Concentrated solar collectors do not need any fuel like most other renewable energy sources. This is undoubtedly a significant advantage over fossil fuels, the cost of which is going up rapidly every year. Due to this reason, electricity prices are also increasing fast in more parts of the world faster than general inflation.
- Can Produce Both Electricity and Heat: Concentrating solar collectors deliver heat at a much higher temperature. Due to higher temperatures, it is possible for the power generation equipment to generate both electricity and heat.
- Round-the-Clock Availability of Electricity: Concentrated solar collectors make it possible to produce electricity 24-hours a day by storing the energy. Other forms of Renewable energy, like wind energy, are intermittent.
- No Carbon Emission: Concentrated solar collectors do not cause any carbon emission, which is a great advantage.
- Job Creation: Concentrated solar power production can create more permanent jobs and boost the economy as compared to other types of renewable energy resources.
- Economy of Scale: The effects of a significant economy of scale can be observed when shifting to large concentrating systems, which makes the technology cost-effective.
Disadvantages of Concentrated Solar Collectors
- High Costs: The average production cost of concentrated solar thermal energy is much higher than other renewable resources. Though during the past few years, the average cost has dropped to $0.20/kWh. Still, the costs are high as in comparison, the average production cost of solar PV is in the range of $0.05 to $0.10/kWh. The average cost of solar PV production will drop even further in the future.
- Future Technologies can potentially make CSP obsolete: Solar energy is witnessing new innovations quite frequently. Companies all over the world are experimenting to find more efficient and cost-effective ways to produce solar energy by making technological breakthroughs. Especially, Chinese solar companies are dominating the solar market by providing low-cost energy using advanced methods. These new innovations may potentially make concentrated solar technology outdated.
- May Aggravate Water Shortage: CSP plants use significant amounts of water, and that may pose a major problem in dry regions. Using non-water cooling increases the cost of CSP projects quite a bit. Though using seawater has been suggested; still, its feasibility remains to be seen.
- Environmental Issue: Using massive arrays of mirrors may negatively impact the wildlife in the dry regions, especially endangering the rare species. For example, there have already been conflicts in California on this issue with project Developers reducing the size of their plants and bearing the additional cost to move wildlife.
- As concentrated solar collectors can focus only on direct solar radiation, their performance is poor during cloudy days.
- The cost of building and maintaining concentrated solar collectors is high.
- Concentrated solar collectors are practical for implementation only in areas with high direct insolation, such as arid and desert regions.
The Way Forward
Concentrating solar collectors use mirrored surfaces to concentrate the sunlight on an absorber called a receiver. The solar collectors can achieve high temperatures, but they can do so only when direct sunlight is available.
China has currently installed 114 million square meters of rooftop solar collectors for heating water. Their target is to increase the installation to 300 million by 2020.
European countries, on the other hand, have set a target to install 500 million square meters of solar collectors by 2020, and for the time period, the target of the US is to install 300 million square meters.
All these global targets installing rooftop solar collectors, the total installation my exceed 1.5 billion square meters by 2020. This would increase the global solar thermal capacity to 1,100 thermal gigawatts, which is the equivalent of 690 coal-fired power plants.
Besides the utility-scale CSP production, it is important that residential consumers all over the world become more aware of this energy. It is because concentrated solar power is capable of providing significant amounts of clean, renewable energy for sustainable use.
Since the early days, Sumit has been deeply concerning for the climate crisis and always felt hurt seeing how the human intervention is disrupting the ecological balance. He 100% believes that solar energy is the missing puzzle to our energy transition, and we have to go all out to implement this energy solution all over the world.