Concentrating Solar Power


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 Gemasolar CSP plant in Spain
The Gemasolar CSP plant in Spain is using molten salt to collect solar energy concentrated by an array of mirrors. This molten salt acts as a thermal battery enabling the generation of electricity even when the sun is not shining.

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

video iconEarth: 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)

Click here for a video transcript of "Lightbulbs in the Desert".

Narrator: Planet Earth is awash in renewable energy. The oceans store heat and offer wave and tidal power. Plants harvest sunlight and store its energy. The Sun warms the atmosphere and sets air in motion, and we're getting better at tapping wind power. But the biggest and most promising energy source is the nearby star that lights our days and warms our world. Sunlight reaching the Earth's surface offers about 120,000 terawatts. If the Sun's energy were spread around the world, it would average around 240 watts per square meter. Richard Alley brings that huge number down to earth.

Alley: If I walk out into this little patch of this great desert, and I hold out my arms about like this-- And then another of me does the same thing-- And each of me is holding two 60 watt incandescent light bulbs, or 10 compact fluorescents, that's 240 watts per square meter that I'm marking out here. That's a lot of energy. And averaged across the globe, day and night, summer and winter, that's how much sunlight is available to power the planet. Let's see what it takes to turn that vast potential into energy we can use. It doesn't take a genius to know that a mirror reflects the Sun, but it does take an inventor and engineer to make the next step. Use the mirror to focus the Sun's rays on a tank filled with liquid to make steam, to drive a turbine, to make electricity, and you have concentrated solar power. That's not a new idea, but one that a little-known American inventor, Frank Shuman, pursued around 1910.

Narrator: In his Philadelphia workshop, Shuman invented safety glass for skylights and automobiles. He also came up with designs that could concentrate sunlight on metal tubes, heat liquid, and drive a steam turbine. But in Pennsylvania, back then, it was all about coal. Shuman had difficulty finding local backers. So in 1912, he set off for Egypt. His prototype solar farm used parabolic troughs to concentrate sunlight and boil water. The steam ran a 75 horsepower engine that pumped water from the Nile to irrigate cotton fields. The idea was right, but ahead of its time. Hobbled by both a lack of government support and adequate private capital, the experiment ended with the outbreak of World War One. These parabolic troughs look very similar to Shumans' designs, though they didn't come online until a century later.

This is Solnova 3, at one of the world's first commercial solar power plants. Just as in Shuman's experimental station, the troughs concentrate solar radiation on a pipe that contains a heat-bearing fluid. When completed there'll be three almost identical plants, each with an output of 50 megawatts, large enough to support about 26,000 households. While the Sun powers the Solucar platform, it was the Spanish government that helped develop solar power. The central government set a specific target of 500 megawatts of concentrated solar power and committed to price supports for 25 years. That, in turn, unleashed inventors and industry to prototype plants like this one. The technology works, though changing government policies and the budget crisis have impacted the industry. But, Abengoa, the company building Solucar, is a part of a consortium planning the world's largest solar power project. Formed by a group of European and North African companies and the DESERTEC Foundation, this consortium has energy ambitions that are revolutionary for both Europe and the Middle East.

Unlike some of its neighbors, Morocco has little oil or other fossil fuels. But it does have sun, sand, and empty spaces. The Moroccan government has encouraged the use of distributed solar power by small businesses and individuals. Already, out on the edge of the Sahara, you can see photovoltaic panels on top of tents. But the Desertec vision goes beyond this by including concentrated solar power plants, photovoltaic installations, and wind turbines, linked with low-loss, high-efficiency transmission cables back to Europe. The Desertec project estimates that solar power from the Sahara could provide more than 80% of North Africa's needs, and 15% of Europe's electricity, by 2050. In a single generation, Morocco's young and growing population could go from energy poverty to energy independence. The energy created by this proven technology could generate both electricity and income for some of the world's poorest nations. And updated versions of Shuman's century-old designs and a smart grid could go a very long way toward meeting our species' need for energy. Collecting just 10% of the Sun's energy from a 600-mile-square of low-latitude desert would supply roughly twice today's human consumption of energy.

Source: Earth the Operators' Manual

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.