Solar energy is a term that refers to the direct use of sunlight to produce heat or electric power.  The sun’s energy is plentiful, but it is thinly distributed over a large area. It must be collected and concentrated in order to produce usable power. The amount of solar radiation per unit of flat area is above 1,000 Watts per square meter.

Heat used in a solar thermal system is guided by five basic principles: heat gain; heat transfer; heat storage; heat transport; and heat insulation. Here, heat is the measure of the amount of thermal energy an object contains and is the product of temperature and mass. The higher the temperature of a substance, the more heat it absorbed. Of course, some materials require more heat to become warmer than others — this is caused by differences in so called specific heat of substances.

The angle of the sun depends on the rotation of the earth, and it’s route around sun. Because the earth’s axis of rotation is tilted and not perpendicular to the plane on which the earth travels around sun, sun shines from different angles in winter and summer — an this difference is the main cause for the existence of those seasons.

Heat gain is the heat accumulated from the sun in the system. Solar thermal heat is trapped thanks to the greenhouse effect; the greenhouse effect in this case is the ability of a reflective surface to transmit short wave radiation and reflect long wave radiation. Heat and infrared radiation (IR) are produced when short wave radiation light hits the absorber plate, which is then trapped inside the collector. Fluid, usually water, in the absorber tubes collect the trapped heat and transfer it to a heat storage vault.

Heat is transferred either by conduction or convection. When water is heated, kinetic energy is transferred by conduction to water molecules throughout the medium. These molecules spread their thermal energy by conduction and occupy more space than the cold slow moving molecules above them. The distribution of energy from the rising hot water to the sinking cold water contributes to the convection process.

In the context of solar energy, heat is transferred from the absorber plates of the collector in the fluid by conduction. The collector fluid is circulated through the carrier pies to the heat transfer vault. Inside the vault, heat is transferred throughout the medium through convection. Rate of heat transfer is related to the conductive and convection medium as well as the temperature differences. Bodies with large temperature differences transfer heat faster than bodies with lower temperature differences.

Heat transport refers to the activity in which heat from a solar collector is transported to the heat storage vault. Heat insulation is vital in both heat transport tubing as well as the storage vault. It prevents heat loss, which in turn relates to energy loss, or decrease in the efficiency of the system.