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Albedo

Albedo is a term more often associated with global warming than anything else, but it has a direct impact on solar irradiance and how solar energy is captured. The term itself is simply a reflect coefficient or the reflecting power of a surface. The scale ranges from 0 to 1 where 1 is complete reflection and 0 is complete absorbance. By way of example, fresh snow has an albedo of up to 0.9. That is to say, it reflects 90% of the sunlight that strikes it and only absorbs 10%. That is why snow is so slow to melt, even in bright sunlight. Asphalt, on the other hand, has an albedo of 0.04. That means it reflects only 4% of the sunlight that hits it and absorbs 96%, which is why it gets so hot on sunny days.

Insolation and Albedo

Albedo can directly affect insolation at the surface of the Earth. The reason for this is that the atmosphere is not 100% transparent. The various particles within the atmosphere have various reflective properties. As it turns out, human activity can change the reflective properties of the atmosphere by adding (most often) or removing various substances. In a practical sense, albedo can affect the efficiency of solar capture in various locations.

Examples of human actions that affect overall albedo of an area include contrails from jets and aerosols from industrial and other applications. Contrails are nothing more than clouds that are created from jet engines. Clouds have albedo values that can range from 0 to 0.8. In general, clouds tend to reflect more sunlight than they absorb, acting to decrease the overall insolation to the ground below them. In areas where contrails are abundant due to heavy airline traffic, the temperatures can be as much as 10 degrees Celsius colder; an indication of the decreased amount of sunlight reaching the surface.

Solar Design and Albedo

The use of albedo, both natural and artificial, is an important part of design regarding solar energy capture. Photovoltaic cells that reflect 90% of the sunlight that strikes them will be useless compared to cells that absorb 90% of sunlight.

Albedo is very important in both passive and active solar applications. In passive applications, the maximization of absorbance is not always desired. For instance, in hot summer months, reflecting sunlight is more desirable. In these cases, light colors are desirable, such as backing shades and blinds with light colors.

In active solar applications, the need to maximize exposure to sunlight does not necessarily mean that low albedo is the key. In the case of photovoltaic cells, it is certainly true that reflecting light is undesirable. However, in the case of parabolic reflectors, the light needs to be reflected and concentrated by a high albedo substance and then absorbed at a point by a low albedo substance. It’s all about absorption and reflection.

The last point to make about solar design and albedo is that cloud cover and industrial pollution can greatly diminish the effectiveness of a collection system. Whether active or passive, if the choice is available, then locating a home even a few miles away from a particular location may have tremendous impact on the efficiency of a solar system.
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