Page 45 - Alternative Energy Systems in Building Design

22 ENERGY SYSTEMS

Since electromagnetic (EM) radiation can be conceptualized as a stream of photons,
radiant energy can be seen as the energy carried by these photons. EM radiation also
can be seen as an electromagnetic wave that carries energy in its oscillating electric
and magnetic fields. Quantum field theory reconciles these two views.
EM radiation can have a range of frequencies. From the viewpoint of photons, the
energy carried by each photon is proportional to its frequency. From the viewpoint of
waves, the energy of a monochromatic wave is proportional to its intensity. Thus it can
be implied that if two EM waves have the same intensity but different frequencies, the
wave with the higher frequency contains fewer photons.
When EM waves are absorbed by an object, their energy typically is converted to
heat. This is an everyday phenomenon, seen, for example, when sunlight warms the
surfaces it irradiates. This is often associated with infrared radiation, but any kind of
EM radiation will warm an object that absorbs it. EM waves also can be reflected or
scattered, causing their energy to be redirected or redistributed.
Energy can enter or leave an open system in the form of radiant energy. Such a
system can be human-made, as with a solar energy collector, or natural, as with the
earth’s atmosphere. Greenhouse gases trap the sun’s radiant energy at certain wave-
lengths, allowing it to penetrate deep into the atmosphere or all the way to the earth’s
surface, where it is reemitted as longer wavelengths. Radiant energy is produced in the
sun owing to the phenomenon of nuclear fusion.