P2: Collecting energy from the Sun

Advantages of photocells:

• robust
• do not need much maintenance
• no fuel needed
• do not need long power cables
• cause no pollution
• do not contribute to global warming
• use a renewable energy resource

Disadvantages of photocells:

• they do not produce electricity when it is dark or too cloudy

How photocells work:

• a photocell contains two pieces of silicon joined together to make a p-n junction
• n-type has an impurity added to it to produce an excess of free electrons
• p-type has a different impurity added to produce an absence of free electrons
• photons (energy packets in sunlight) cause free electrons to move producing an electric current

The output from a photocell depends on:

• light intensity
• surface area exposed
• distance from light source

Passive solar heating uses glass to help keep buildings warm - it re-radiates the very short wavelength from the Sun to make it last longer.

Solar reflectors are moved by computer so that they are always facing the Sun.

Advantages of wind farms:

• do not contribute to global warming
• do not pollute the atmosphere

Disadvantages of wind farms:

• can be noisy
• take up lots of space
• people complain that they spoil the view