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Reflectance & Solar Power

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Solar radiation through a glazing material is either reflected, transmitted or absorbedA usual safety concern of neighbors/cohabitants near the planned PV system installation is the glare and reflectance from PV systems. The article demonstrates that the solar glass has less glare and reflectance than other common residential and commercial glasses thus, making them safe for surrounding environments.

The underlying concept of efficient solar power is to absorb maximum sunlight while reflecting as little light as possible.  Solar panels use “high-transmission, low-iron” glass which absorbs more light, producing smaller amounts of glare and reflectance than normal glass. In addition, PV solar-glass is often stippled and has a light-trapping, very photon-absorbent solar cell attached to its’ back side contributing additional factors which result in even less light energy being reflected.

Reflection, Refraction and Angles-of-incidence

Reflection Characteristic exampleWhen a beam of light falls on a piece of glass, some of the light is reflected from the glass surface, some of the light passes through the glass (transmitted) and some (very little) is absorbed by the glass. The measure of the proportion of light reflected from the surface is called reflectance (reflection) and the measure of the proportion transmitted is the transmittance. This is where the term high light transmission glass comes from because the glass is formulated to allow more light to pass through its surface than would pass through a standard glass surface.

The reflection/refraction behavior of a medium is directly related to its index of refraction. The lower the index of refraction for a medium, the less light it reflects because the medium is allowing more of the incident beam to pass directly through (in PV systems, directly through the glass to the solar cells).

For instance, the study of the reflected energy percentages of sunlight, off of some common residential and commercial surfaces shows that ‘steel’ reflects more energy than ‘snow’ while ‘snow’ reflects more energy than ‘standard glass’, etc. The reflected energy percentage of Solar Glass is far below that of standard glass and more on the level of smooth water.

Stippled Glass” and “Light Trapping”

Light Trapping. More light energy is absorbed by the cell with each ensuing reflection of the initial light beam In addition to the superior refractive/reflective properties of solar glass versus standard glass, many PV suppliers use stippled solar glass for their panels. Stippled glass is also used with high powered telescopes and with powerful beacons and flashlights. The basic concept behind stippling is for the surfaces of the glass to be “textured” with small types of indentations. As a result, stippling allows more light energy to be channeled/ transmitted through the glass while diffusing (weakening) the reflected light energy.

“Light trapping” is also used by more high-quality PV suppliers. It is the practice of using additional techniques like mirrors and natural surface textures to “trap” light within the layers of the solar cell, allowing even less light to escape by reflection.

This is the reason why a reflection off a high-quality solar panel will look hazy and less-defined than the same reflection from standard glass. The stippled and lighttrapping PV glass and cell texture transmit a larger percentage of light to the solar cell while breaking-up the intensity of the reflected energy.

Courtesy:
Sunpower Corporation
USA

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