In this form of transfer, liquid or gas, such as air is heated, then moves away from the source of heat, being replaced by cooler material which repeats the process of carrying away heat. In natural convection, the heated fluid becomes lighter due to expansion and rises away from the source, being replaced by cooler, heavier fluid. In forced convection, the fluid is driven by some outside force, such as a fan, pump, or wind. It is heated by contact with the source of heat and carries the heat away as it is moved by the outside force. The third method of heat transfer or loss.
Radiation occurs by transfer of energy through empty space.
The amount of heat transferred by radiation is proportional to the difference between the fourth powers of the absolute temperatures of the radiating surface and the radiation receiver surface. When solar collectors are operated at higher temperatures, the radiation losses increase very rapidly with temperature and are the largest losses responsible for loss of efficiency. At lower temperatures, radiation losses are small and conduction and convection losses are the main sources of loss. Both convection and conduction losses are proportional to temperature differences. A therm is equal to 100,000 BTU, or 29.3 KWH Temperature vs. Energy Knowledge of temperature is necessary to knowledge of thermal energy, but is not enough.