Have you ever flown over a desert in the southwest U.S. and seen a large, other-worldly collection of mirrors with a large tower in the center?
Upon first glance, it’s easy to imagine these are some sort of metallic, alien crop circle or Area 51 experiment. But fret not, for these are actually Concentrated Solar Power (CSP) systems that work by concentrating large amounts of sunlight onto one focal point. It’s similar to using a magnifying glass to start a fire.
Where Photovoltaics (PVs) — the solar panels you see on rooftops — use photon energy (light) from the sun to generate electricity, CSPs use the sun’s thermal energy (heat) to generate electricity.
The CSPs mirrors concentrate heat from the sun onto a focal point, like the tower in our example above, which then powers a steam engine to create electricity.
Voilà! Electricity from the warmth of the sun!
Who Uses CSPs
Unlike the PV systems you see on residential and commercial rooftops, CSPs require large amounts of land and uninterrupted sunlight. Therefore, they are mostly used by utilities and governments with access to large amounts of desert land.
The future is bright, so to speak, for CSP systems in the southwest U.S., northern Africa, the Middle East, China, and India. The current leaders in CSPs by Megawatt (MW) capacity are the U.S. and Spain. Click here to see a list of operational CSPs and CSPs under construction.
Types of CSPs
The most common type of CSP, parabolic troughs use rows of curved mirrors with a focal point on receiver tubes that run the length of the mirrors. The intense sunlight focused from the trough shaped mirrors heats the fluid inside the tubes. The hot fluids then heat water and generate a conventional steam turbine to generate electricity.
Solar Power Tower AKA Heliostat or Central Tower
The solar power tower is the type of CSP exemplified in this article’s introduction. It’s a large collection of mirrors with a fixed, single focal point on a tower above the mirrors. The thermal energy is then used to power a steam turbine to generate electricity.
Linear Fresnel Reflector
Fresnel reflector systems are similar to the parabolic trough systems with long rows of mirrors concentrating solar thermal energy onto a long parallel absorber.
Unlike the curved parabolic troughs that are mostly stationary, fresnel reflectors use flatter mirrors that can rotate its mirrors with the positioning of the sun to maximize thermal energy capture. 
Dish Sterling solar systems
Similar to the power tower but on a smaller scale, dish-sterling systems use a single, dish-shaped mirror on a single focus point.
Integrated Solar Combined Cycle (ISCC)
ISCC is a technology that combines solar thermal energy with traditional fossil fuel powered turbines. Thermal CSP energy is added as auxiliary power to a conventional fossil fuel-powered cycle to produce additional electricity at a relatively low cost. 
The Added Benefit: Energy Storage
A huge benefit of CSPs is the relatively inexpensive way to store thermal energy from the sun. As we’ve discussed before, renewable energy’s big problem is that if you don’t use it, you lose it.
Using either molten salts or heated fluids, CSPs have the ability to store thermal energy from the sun and produce electricity at night or times when solar energy capture is blocked by weather. 
These molten salts are then used to power steam turbines to generate electricity when needed.
Molten salts in particular offer an advantage compared to other energy sources. According to SolarPaces, molten salts can be used efficiently for at least 30 years to generate electricity.
Pros of CSPs:
The fundamental advantage of CSPs is that it’s relatively easy to concentrate sunlight and generate heat. All over the world there are large amounts of desert land ripe for the use of CSPs.
- Mirrors are cheaper than individual PV systems and can possibly last longer
- New solar thermal storage technology is improving the efficiency of CSPs
- Parabolic troughs are passive and don’t need mechanisms to turn them throughout the day, decreasing mechanical costs and failures
Cons of CSPs:
Like all utilities, it comes down to the price. Both a pro and a con for CSPs, the costs of building a new system can vary. The efficiencies of specific systems also affect whether or not CSPs are a viable alternative to PV systems.
Currently, the dropping price of PV systems is beating CSPs. However, CSPs may be a viable option for utility-scale energy production.
- CSPs require large capital investment up front
- CSPs require large amounts of land and uninterrupted sunlight
- CSPs use water to generate steam, clean the mirrors and cooling systems — this poses a problem in deserts where CSPs are viable but have limited access to water
- CSPs use a centralized approach that requires long-distance transmission of electricity
- Weather dependent — if the weather is bad, CSPs do not work well
Bye Bye Birdies?
Early on, bird deaths caused by birds flying through the focal points of thermal heat were an enormous PR nightmare for CSPs.
CSPs create bright light which can attract insects and their predators, most notably birds. If birds fly near the focal point of the CSP they can be killed (burned). There is a large concern for predatory birds like raptors who hunt the smaller birds.
However, new methods have been discovered to drastically reduce and eliminate bird deaths. By not concentrating the light of more than 4 mirrors on any one place at a time during the “standby” period, bird deaths have been reduced to zero. 
PV is currently underpricing CSPs, making it harder to economically justify new, large-scale projects.
However, advancements in CSPs ability to store energy through molten salts and other means may make them more compelling as they can generate more predictable and reliable power at night.
As the tech advances, it’s quite possible that we see new hybrid PV/CSP systems come into use, eliminating some of the CSP’s barriers to entry.
Act on Climate
Need a reminder of why we need to stop climate change? Get outside and enjoy some time in nature. Then read Florence Williams book, The Nature Fix: Why Nature Makes us Happier, Healthier, and More Creative.
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