Concentrated Solar Power
If you have ever left a cold drink out in the sun during the summertime (or if you have children, if you have ever left water in the kiddie pool out in the sun for a long time), you would notice that the formerly cold water gets warm – maybe even hot. If it happens to be summertime where you are living right now, try it! Whether you realize it or not, this little science experiment is the basis for a second way of harnessing the sun’s energy to produce electricity, called “concentrated solar power” or CSP. (This technology is also sometimes called “solar thermal.”)
The following video explains how CSP works. The basic idea is that a collection of mirrors reflects the sun’s light (and heat) onto a large vessel of water or some other fluid in a metal container. With enough mirrors reflecting all of that sunlight, the fluid in the metal container will get hot enough to turn water into steam. The steam is then used to power a turbine just like in almost any other power plant technology
Earth: The Operators' Manual
To get started, please watch the video below. This particular video will discuss the history of the idea of concentrated solar power.
Video: Lightbulbs in the Desert (Powering the Planet) (5:55)
Recently, more advanced CSP systems have begun to replace the water or synthetic oil with molten salt as the fluid that is heated molten salt can remain as a liquid from 290 to 550°C. Once it is heated in the tower at the center of the array of mirrors, the hot liquid salt is stored in a highly insulated tank and when there is a demand for electricity, it is sent to a heat exchanger where it turns water into steam, driving the turbine to generate electricity. When the molten salt passes through the heat exchanger, it gives up heat, so it cools off. It is then recirculated to the tower at the center of the mirrors, where the concentrated sunlight heats it back up. These systems have enough liquid salt so that it can act as a thermal battery, storing the solar energy for more than a week before it cools off to the point where it cannot make steam. These kinds of power plants are expensive at the moment, but the technology is still quite new and so we expect prices to drop quickly, as they have for other renewable energy technologies. In fact, a CSP system in Spain using molten salt is now capable of producing energy on demand, 24 hrs a day rather than being limited to times of peak sunlight. The ability to schedule power production versus having to take the electricity when it comes is of great value to the folks that operate electricity systems. Nevertheless, there are still a few obstacles for CSP:
- CSP is difficult to make work on a small scale. A lot of land, usually in sunny deserts, is typically needed. So CSP does not scale up and down to large and small installations like Solar PV can.
- CSP is currently quite expensive — roughly twice as much per unit of energy as Solar PV. However, this is a very new technology and prices are expected to go down in the future.