03 September 2014 | Posted by Zachary
Following up on my article about different types of solar PV, here’s one on different types of concentrating solar thermal power, aka concentrating solar power or concentrated solar power (CSP). CSP is commonly confused with CPV, but they are actually very different. Whereas CPV concentrates sunlight onto solar PV cells in order to make them generate more electricity, CSP concentrates sunlight in order to concentrate heat and eventually produce electricity (as is done in coal and nuclear power plants).
While CSP and the most cost-competitive forms of solar PV were comparable in cost a handful of years ago, the cost of solar PV has fallen so fast that CSP is rarely competitive today. However, CSP has some clear benefits that make it a better choice in some markets.
As with solar PV, there are several different types of CSP technology. However, as with solar PV, one type dominates the current market.
Parabolic Trough: There are slight variations of parabolic trough CSP systems, but the basic system is the same. Here’s the description from Wikipedia: “A parabolic trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The receiver is a tube positioned directly above the middle of the parabolic mirror and filled with a working fluid. The reflector follows the sun during the daylight hours by tracking along a single axis. A working fluid (e.g. molten salt) is heated to 150–350 °C (423–623 K (302–662 °F)) as it flows through the receiver and is then used as a heat source for a power generation system.”
Here are some pics from Masdar’s Shams 1 CSP plant:
(Note that the parabolic troughs were not in operation in the pictures above. If they were oriented toward the sun and in operation, no one could be in front of them like that or they’d get fried.)
The heat from the CSP system is generally used to heat water and produce steam that then turns a turbine that is connected to an electrical power generator. This latter process is the same process used in coal, nuclear, and natural gas power plants. Some CSP plants are actually partially natural gas power plants (hybrid plants) as a result.
Enclosed trough: “Enclosed trough systems are used to produce process heat. The design encapsulates the solar thermal system within a greenhouse-like glasshouse. The glasshouse creates a protected environment to withstand the elements that can negatively impact reliability and efficiency of the solar thermal system,” Wikipedia writes. “Lightweight curved solar-reflecting mirrors are suspended from the ceiling of the glasshouse by wires. A single-axis tracking system positions the mirrors to retrieve the optimal amount of sunlight. The mirrors concentrate the sunlight and focus it on a network of stationary steel pipes, also suspended from the glasshouse structure. Water is carried throughout the length of the pipe, which is boiled to generate steam when intense sun radiation is applied. Sheltering the mirrors from the wind allows them to achieve higher temperature rates and prevents dust from building up on the mirrors.”
Solar power tower (aka heliostat power plants): Instead of concentrating sunlight (and thus heat) on pipes, solar power tower CSP systems have mirrors surrounding a tower and directing light onto it. The mirrors (aka heliostats) again heat a liquid inside the tower. Originally, that liquid was just water, but companies have more recently used sodium and even more recently molten salts (40% potassium nitrate, 60% sodium nitrate). These liquids can store the heat for some time and then that heat can later be used to heat water and produce steam. As above, the steam produced from these systems is used to turn a turbine that is connected to an electrical power generator.
The big benefit with the molten salt systems is its ability to store heat, since these solar power plants can produce electricity once the sun goes down. With peak electricity demand often being in the early evening, this can be quite useful and valuable.
Dish sterling: Dish sterling CSP plants might be the most “science fiction” looking type of CSP plants. As you can see in the pics below, dish sterling CSP plants have large, reflective, parabolic dishes that focus the sunlight/heat on a central point not far from the parabolic dish. “Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used,” Wikipedia currently writes.
This parabolic sterling dish design reportedly has the highest heat-electricity conversion efficiency amongst all CSP plants developed so far. That can rise up to about 30%.
Yiting Wang writes: “One dish costs around $250,000 averagely, depending on the capacity of it. Once production rates rise, they could cost less than $150,000. Southern California Edison Electric Company cannot give away the actual price per kWh, but they say it is well below the 11.33 cents seen currently.” Well below 11.33 cents per kWh is not bad for a nascent technology that hasn’t yet benefited from economies of scale. Even if it’s at 10 cents per kWh currently, the cost reduction noted above from higher production rates could bring that down to 6 cents per kWh, which is competitive with other electricity sources in many markets.
These CSP options are quite fascinating, in my opinion, and generate a lot of enthusiasm from some people. They generally aren’t competitive with solar PV, but with growth, incremental scientific or engineering improvements, and economies of scale, some of them may become cost competitive in the coming decades and see a bright future.
Image Credits: Zachary Shahan / CleanTechnica x 2 (CC BY-SA 4.0); Masdar; Glasspoint x 4; afloresm (CC BY 2.0); afloresm (CC BY 2.0); GoShow (CC BY-SA 3.0); BrightSource Energy x 3; Sandia National Laboratories; International Rivers; Infinia