Energy plays an important part in social and economic develop. As the oil price sharply increases, it stimulates the step of researching the renewable energy field. The advantages of using renewable energy reduce the dependence on fossil fuel resources and carbon emission to the atmosphere. The total incoming solar radiation is about 3.8 million EJ per year and it can satisfy the entire energy demand if utilise correctly . The use of solar energy is usually identified in two contexts: passive solar energy systems, which use natural processed without mechanical equipment and no additional energy and active solar energy systems, which utilise solar energy with the assistance of mechanical equipment . Solar thermal collectors, photovoltaics, and photovoltaic-thermal systems are the active solar energy system. This part will first introduce the relative knowledge of the sun and three technologies will describe at the following.
It is necessary to acknowledge the information of the properties of sunlight before to optimize the performance of photovoltaic and other systems which convert sunlight into electricity or heat energy. In the following part, it will introduce the Physical source of sunlight, the solar constant, solar intensity at the earth’s surface, direct and diffuse radiation.
Physical source of sunlight
The sun is essentially a sphere of gas heated by a nuclear fusion reaction at its centre. Hot body emit electromagnetic radiation with a…