Can renewable energy sources power the world?
Can renewable energy sources power the world?

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Can renewable energy sources power the world?

9 Solar thermal electricity generation

If the Sun’s rays are concentrated using mirrors, high enough temperatures can be generated to boil water to drive steam engines, which can produce mechanical work for water pumping or, more commonly nowadays, for driving an electric generator.

This solar thermal-electric generation is known as concentrating solar power (CSP). There are trade-offs between the complexity of design of a concentrating system and its concentration ratio. The concentration ratio is the ratio of the power per unit area at the focus to the incoming power per unit area at the aperture. No concentrating collector can deliver in total any more energy than falls on its aperture, but what it does receive is all concentrated into one small area.

How do concentrating collectors work?

One common method of concentrating solar energy is to use a parabolic mirror like the one shown in Figure 21.

Figure 21 Parabolic mirrors for high-temperature applications – principles of focusing

As you can see, rays of light that enter parallel to the axis of a mirror formed in this parabolic shape will be reflected to one point, the focus.

Activity 5 Rays of light and reflection

What happens if the Sun’s rays enter off-axis?


If the rays enter off-axis, they will not pass through the focal point, so it is essential that the mirror is made to track the Sun.

Figure 22 Parabolic mirrors for high-temperature applications – a line focus or ‘trough’ collector

Now look at Figure 22, which shows a line focus or trough collector, mainly used for generating steam for electricity generation. Here the Sun’s rays are focused onto a pipe running down the centre of a trough. The pipe is likely to carry a high temperature heat transfer fluid such as a mineral oil. The trough can be pivoted to track the Sun up and down or east to west.

A line focus collector can be oriented with its axis in either a horizontal or a vertical plane and can produce a temperature of 200–400 °C, usually achieving a 'concentration ratio' of 50, which is adequate for most power plant systems – but the ratio required depends on the desired target temperature.

Figure 23 Parabolic mirrors for high-temperature applications – the point focus or ‘dish’ collector

In the point focus or dish collector shown in Figure 2.24, the Sun’s image is concentrated on a steam boiler in the centre of the mirror. For optimum performance, the axis must be pointed directly at the Sun at all times, so it needs to track the Sun both up and down and east to west. A well-built and well-aimed parabolic dish collector can produce a temperature of over 1500 °C, and achieve a concentration ratio of over 1000.

Another form of concentrating solar electricity generation involves the use of power towers, which we’ll look at next.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371