The electrons at the surface of a substance vibrating with thermal energies emit electromagnetic waves.The higher the temperature, the shorter the wavelengths emitted.
Darker surfaces are better at radiating and absorbing heat than bright or shiny ones.
Solar radiation Heat from the Sun is radiated heat. It travels to Earth (as well as in all other directions) by means of electromagnetic radiation.2
SOLAR IRRADIANCE
Solar Irradiance is the average amount of the Sun’s energy falling per second per- pendicularly on 1 metre squared of the Earth’s atmosphere.
The value of solar irradiance is 1.35 kW m2. SCIENCE,TECHNOLOGY & SOCIETY
Solar heating Many of the solar heating systems that are used in Ireland comprise two main parts: a solar panel (consisting of a series of black metal pipes) and a water tank. The water in the pipes is heated by sunlight, this rises to the top of the water tank by convection and is replaced by cooler water from the bottom of the tank as illustrated.
Another type of ‘solar panel’ that can be used to heat a building efficiently is one made of photovoltaic cells.3 Photovoltaic cells, of the type also used by solar-powered calculators, convert the Sun’s energy into electrical energy. This electrical energy can be used for heating or other purposes.
Water tank Hot water
Cold water Sunlight
Black metal pipes
Circulating fluid (convection currents)
A solar heating system Fig 9.12: Solar heating system
2Electromagnetic radiation will be dealt with in more detail in Chapter 14. 3Photovoltaic cells are often referred to simply as photocells. However, we must take care to avoid confusion with photo- emissive cells which may also be referred to as photocells.The distinction between the two will be dealt with in Chapter 21.
