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

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Can renewable energy power the world?

2.2 Solar radiation

The Sun is an enormous nuclear fusion reactor, in which at very high temperature hydrogen atoms fuse to form helium at the rate of 4 million tonnes per second. This fusion reaction causes the Sun to radiate energy, due to the resulting high surface temperature (around 6000 °C).

A very small proportion of the Sun’s radiated energy reaches the Earth, 150 million kilometres away, and if you look at Figure 1 you will see that approximately one-third of it is simply reflected back to space. You will also see that the rest is absorbed by the planet and eventually re-transmitted to space as long-wave infrared radiation.

On average, the Earth re-radiates as much energy as it receives and sits in a stable energy balance at a temperature suitable for life.

Figure 1 Radiation of energy to and from the Earth

As Figure 1 shows, solar radiation spreads over a wide spectrum of wavelengths - from ultraviolet light (shorter wavelength than visible violet light) to ‘short-wave’ infrared light (longer wavelength than visible red light). This wavelength distribution is determined by the surface temperature of the Sun.

Earth temperature and solar collection

The Earth has an average atmospheric temperature of –20 °C and a surface temperature of 15 °C. It re-radiates energy as long-wave infrared to deep space, the temperature of which is only a few degrees above absolute zero, –273 °C.

Activity 1

How is solar energy collected differently at low or high temperatures?


Low-temperature solar energy collection mainly depends on using glass and other surfaces with selective properties that allow solar radiation to pass through but block the re-radiation of long-wave infrared. Glass is transparent to visible light and short-wave infrared radiation, but is opaque to long-wave infrared re-radiated from a solar collector or building behind it.

Over recent decades enormous effort has been put into improving the performance of glazing, both to increase its transparency to visible radiation, and to prevent heat escaping through it.

Gathering solar energy for high-temperature applications, such as driving steam engines to power electricity generators, mainly involves concentrating the Sun’s energy using complex mirrors, as you will see in Section 2.9

Yiu will now go on to discuss direct and diffuse solar radiation.


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 over 40 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, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 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 prospectus