Solar Radiation and its Use in Solar Hot Water Applications

When choosing a solar hot water system, it is important to understand the difference between direct beam radiation and global radiation. This is important because solar concentrators, like the SolarBeam Concentrator rely on direct beam radiation to create solar hot water, whereas flat panels rely on global radiation, which is more diffused energy.

Direct Radiation Versus Global Radiation

Radiation that hits the earths surface directly from the sun, without suffering changes in direction, is known as direct radiation. It is the strongest form of radiation because it is not hindered by cloud cover, high humidity etc. Global radiation on the other hand is the total of direct solar radiation and diffuse sky radiation received by a unit horizontal surface.
So the solar radiation at ground level will consist of a direct component (direct radiation), then comes to us without being deviations, and a diffuse, disseminated throughout the celestial hemisphere (diffused radiation). The sum of these two components in a horizontal plane is called "global radiation"
To see how much direct radiation is found in your area go to this link:

The clouds spread or scatter solar radiation better than air, so on a cloudy day all the radiation we get is diffused radiation. On a clear day direct radiation is several times higher than diffused radiation and so more thermal energy can be produced.
The solar radiation reaching a plane at the earth's surface varies from day to day mainly due to change in cloudiness. To give an indicative figure, on cloudy days when solar radiation received by the surface is mostly diffused, the value of this radiation is usually much smaller in kWh / m², while in a sunny day value radiation is much higher in kWh / m². This will vary depending on time of day, geographic area and season. Solar hot water systems that make use of direct radiation are more effective because they can harvest more energy per hour than systems that rely on global radiation alone. We must also take into account the hourly variation of solar radiation received by a horizontal plane about the Earth's surface is due mainly to the rotation of the Earth. To represent the rotational movement we use the "hour angle." The hour angle measures the distance, expressed in degrees, between the Earth's meridian plane and the Sun.
The thickness of the atmospheric radiation must traverse to reach the surface varies throughout the day in the rotation so that, while early in the morning this layer is thick, at noon-time (solar angle equal 0 °) the thickness is minimal and, therefore, incoming solar radiation is maximum.

We have the example of a technology based company that has designed a solar hot water system called SolarBeam, its a solar concentrating system that directs all direct radiation in a single focal point to concentrate all the energy at that point. This system has dual axis solar tracking, which lets you track the sun throughout the day, being able to harvest more energy.

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