Soalr energy is the energy produced in the continuous nuclear fusion reaction process inside the sun. Although the energy the sun radiates to the earth aerosphere is only one percent of 2.2 billion of its overall radiating energy(about 3.75*1026W), but it has reached to 173,000TW. That is to say, the energy the sun radiates to the earth per second is equivalent to 5 million tons coal. The wind energy, water energy, ocean temperature difference energy, wave energy, biomass energy and part of tidal power are all rooted in the sun. even if the fosil fuels (such as coal, petrol and gas, etc) in the earth are also the reserved soalr energy since time immemorial at bottom. Therefore, the widesence solar energy includes a very wide range of energy, and the sensu stricto soalr energy limits to the direct transformation of the photothermal, photoelectricity and actinochemistry of the sun radiation energy. The soalr energy is primary energy source as well as a renewable energy. Its source is abundant, it can be used for free, and it needs no transportation as well. It makes no pollution to the environment. 

the speciality of the sun radiation

solar constant

    Day and night are produced by earth rotation, but seasons are produced by the 23 °27 'angle of the earth’s axis of rotation and axis of swivel of the orbit the earth’s revolution surrounding the sun. The earth rotates one circle from west to east by its earth axis through its south pole and north pole every day. Each cycle is one day and night and so the earth rotates 15° per hour. Except for rotation, the earth also orbits one circle surrounding the sun by a elliptic orbit with small eccentricity. The normal between the axis of rotation of the earth and its revolution orbital plane makes a 23.5° forever. When the earth revolves , the direction of its axis of rotation directs to the north pole of the earth all th time. Hence, when the earth is in different position of its orbit, the direction of the from the sun to the earth is different. That makes the changes of four seasons on the earth. The hight of the sun remains the highest every noon of the day. In the low latitudes areas of torrid zone(the area between equator north and south latitudes23 °27 '), there are two times normal incidence of the sun per year. In the more higher latitudes, the sun is always close to equator direction. In the areas of north and south pole(greater than 90°-23 °27 ' in north and south semi - spheres), the time that the sun lower than the apparent horizon is longer in the winter, but in the summerthe time that the sun higher than the apparent horizon is longer.

the sketch map of the earth orbits encircling the sun

    Because the earth orbits encircling the sun by a elliptical orbit, so the distance between the sun and the earth is not a constant and everyday in a year the dstance is different. As we all know, the radiant intensity of one point is inversely proportional to the square of the distance to radiant source. It means that the radiant intensity of the sun above the earth atmosphere will vary as the distance between the sun and the earth changes. However, because the distance between the sun and the earth is too large(the average distance is 150million km), therefore the radiant intensity of the sun outside the aerosphere of the earth is almost a constant. Therefore people adopt the so-called “the sun constant” to discribe the radiant intensity of the sun above the aerosphere of the earth. It means when it is the time of average distance between the sun and the earth, the radiant intensity of the sun received by unit surface area which is perpendicular to the radiation of the sun on the top of the erosphere of the earth. In recent years, the reference value of the sun constant tested by various advanced means is 1353w/m². The variation of the radiant intensity of the sun caused by the changes of the distance between the sun and the earth is not more than 3.4%. 

the radiant intensity of the sun arrives in the earth 

   The radiation of the sun to the horizon or called “insolation” is made up of two parts – nonstop insolation and diffusion insoaltion. When the radiation of sun arrives in the surface of the earth through the aerosphere of the earth, because of the absorb,reflection and dispersion of the radiation of the sun by air molecule, vapor and dust in the atmosphere will not only weaken the radiant intensity but also change the direction of the radiation and the spectral distribution of the radiation. Therefore, the real radiation of the sun that arrives in the surface earth is usually made up of nonstop insolation and diffusion insoaltion. Nonstop insolation means the radiation that directly comes from the sun and the radiation direction never change. Diffusion insoaltion means the radiation that has been reflected and scattered by the atmosphere and the direction has been changed. It is made up of three parts: the diffusion insoaltion around the sun(the sky light around the sun), diffusion of the horizontal circle(the sky light or dull around the sun), and some other sky diffusion radiation. Besides, non-horizontal level also receives the reflected radiation comes from the surface of the ground. The summation of nonstop insolation, diffusion insolation and reflected insoaltion is overall insolation or the whole world insolation. We can focalize nonstop insoaltion by lens or reflectors. If the focusing rate is very high, we can get high-power intensity but waste diffusion insolation. If the focusing rate is low, we can also focaliza part of the diffusion insolation around the sun. The variation range of the diffusion insolation is quite big. When the sky is sunny and without cloud, the diffusion insolation is 10% of overall insolation. But when the sky is full clouds and without sunlight, the overall insolation is equal to diffusion insolation. So the gathering collector gathers less energy than the non-gathering collector. Reflected insolation is generally weak but when the fround is covered by snow and ice, the reflected insolation on vertical plane can be 40% of overall insolation. The radiation of the sun that arrives in the ground mainly affected by thickness of aerosphere. The more thickness of the aerosphere is, the absorb, reflection and diffusion of the radiation of the sun by the aerosphere is more serious, and the radiation of the sun that arrives the ground is less. Furthermore, the condition and the quality of the atmosphere will also affect the radiation of the sun that arrives in the ground. Obviously, the path of the radiation of the sun through the aerosphere is related to the direction of the radiation of the sun. look at t h picture below, A is a point on the sea level on the earth,when the sun is on the vertex position of S, the path of the radiation of the sun drills through the aerosphere to A is OA. When the sun is on the place S, its path that through aerosphere to A is OA. The ratio of O,A and OA is called “the atmosphere quality”. It means the ratio of the path of the sun drills through the atmosphere of the earth and the path when the sun vertically incidents on the vertex position, it is always expressed by m. And enact the standard atmospheric pressure and when the temperature is 0°C the sun incidents vertically on the sea level, the atmosphere quality m=1. from the picture below we can know: 

h is the altitude angle of the sun 

sketch map of the atmosphere quality 

    It is obvious that the sun radiant intensity which arrives in the ground is different in different areas, different seasons, and different weather condition. The form below gives the average sun radiant intensity in torrid zone, temperate zone and cold climate. 

The average sun radiant intensity in different areas

    Generally, we makes the radiant intensity of the sun that arrives in the ground by geography and weather conditionof places into various charts used for projects. They are not only useful for soalr energy ultilization, but also important data for heating and air-conditioning design of the building.