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How much sunlight do we get on earth?

”The sun keeps shining. Only, we earthlings turn our faces from it”, said a wise man once. Solar energy is there for everyone to use but some of us do not care to benefit from it.

From an average distance of about 93 million miles at a temperature of 5800K (nearly 5500°C) the sun is showering the earth’s atmosphere continuously with approximately 1367 W/m2 on an average day.

The atmosphere cuts off some of the energy and at the earth’s surface the power density is about 1050 w/m2. This means a panel the area of 1m2 will receive a solar power of about 1kW/m2 provided it is facing the sun directly.

How much of it the panel and the balance of system convert to useful power? That depends on the system efficiency, particularly, the panel conversion efficiency.

These figures are average values over the year. Actually, they vary. One of the reasons is ellipticity of our planet's orbit around the sun. The irradiance increases by about 3% in February, and reduces by about 3% in July each year due to change in distance from the sun.

World Insolation GlobalYearly sum of global irradiance (source: GreenRhinoEnergy)

 

Apart from this annual variation, there may be changes due to sun spot activity which happens every eleven and a half years approximately. Minor variations over much longer cycles e.g., 88 years, 208 years, and 1000 years have also been detected.

The panel must be at right angles to the rays for receiving maximum energy. Any misalignment will reduce the intercepted power. This misalignment comes on a daily basis due to rotation of the earth, and an annual basis due to the inclination of the polar axis with the plane of the orbit.

Ideally, a two axis solar tracker should be used. However, it is not cost-effective, nor very aesthetic in most situations, and fixed installations are preferred with panel slopes adjusted to achieve best results around the year. In some cases it may be possible to arrange for adjusting the panel slope once or twice in a year. Luckily, a misalignment of up to 15° will cause a loss of less than 10 percent.

The azimuth orientation is normally kept south (in the northern hemisphere) but it may be useful to think about offsetting it by a few degrees depending on your daily load variation, the local demand curve and rate variations. Generally, usage will be more in the afternoon and evening than in the morning.

Value it or not, the sun will shine till Doomsday.

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