Category Solar Electric Power Generation – Photovoltaic Energy Systems

Solar Facades

While solar facades are less favorable from the technical point of view (e. g., 30% less irradiance in Germany), they are very attractive in terms of visual appearance. Companies and institutions which want to express their environmental competence and conscience, (e. g., energy suppliers, construction industry, banks, insurances) often apply PV facades to call public attention and recognition. Beside the losses by lower irradiance, higher reflection losses occur also. In locations close to the equator, where the sun’s elevation is high, reflection losses may amount 42% of the incoming irradiance (see Krauter 1994a)...

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Solar Roof Tiles

Figure 10.9 shows a roof covered by solar roof tiles made out of colored acrylic glass. In each of the tiles 24 single-crystalline silicon solar cells are integrated, the wiring among the solar roof tiles is carried out by a plug system. A similar system, but consisting of amorphous silicon solar cells, is manufactured by the Swiss company Atlantis. The costs are around 4.30 €/m2 and thus a new roof a system of such type is considerably less expensive than a conventional roof with an additional mounted PV generator.

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Fig. 10.9. A roof covered by solar roof tiles manufactured by the Swiss company Newtec (Wildnau).

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Fig. 10.10. Single solar roof tile from Newtec, Switzerland.

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Substitution of Building Components

Substituting a conventional component of a building -, e. g., roof tile, window or facade – with an adequate solar energy component, results in significant material, energetic and financial cost savings, while the structural components, such as framing and glass sheets, do not have to be considered as part of the solar generator in the balance.

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Reduction of Expenses for Mounting

A new mounting system presented below (Fig. 10.7) allows to install PV modules without tools just using simple stainless steel cramps (see Fig. 10.8). The bending stress of the support structure fixes the components together and secures a tight mount of the solar module.

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Fig. 10.7. Reduction of mounting cost by the Solbac®. support structure.

The support structure is made of Eternit® (see Fig. 10.7) and is very suitable for flat roofs, while the fixation to the roof could be achieved just by a pebble stone filling without the necessity of roof perforation. Substitution of the conventional metal support structure by Eternit allows to reduce primary energy consumption in the vicinity of 90%...

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Additional Anti-Reflective Coating

To achieve an optimal matching of the module surface with the air, an additional anti-reflective coating (ARC) should have a refractive index of n =1.3. Unfortunately, no solid material has got such properties. Nevertheless, experiments using a liquid film as an AR-layer have been carried out: Using water (n =1.33) as an AR coating resulted in a 2% increase of the short-circuit current (Krauter 2004).

However, optically thin layers (e. g., A/4), that allow a reduction of reflection losses even for relatively refractive indices, have been successfully applied at solar thermal collectors and are commonly used at high-quality optical equipment (camera lenses, binoculars, etc.). First application tests for PV modules (incl. evaluation of durability and cost-benefit ratio) are on the way.

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Improved Matching of the Refractive Indices of the Module Encapsulation Layers

Using the optical model presented above as a simulation for the optical system consisting of front glass, EVA, anti-reflective coating, and silicon solar cell, a parameter variation leads to the following results: A better optical matching of the two upper layers (glass and EVA) allows an increase optical transmittance (and consequently electrical yield) by 3.2% for materials with ideal properties, and 1.9% for real materials (see Krauter 1993c). Figure 10.6 shows the transmittance for perpendicular, unpolarized irradiance for a variation of the parameters (refractive index of front layer) and n2 (refractive index of second layer). Also the transmittance of a real PV module (PQ 40/50) is shown.

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Fig. 10.6...

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Selective Structuring

A significant advantage obtained by selective, partial structuring of the cover is its deflection capability. For example, for a 1.5 mm wide front contact (“bus bar”), the irradiance onto a plane surface over it is useless, but the V- structure can direct it (completely within an incidence angle area of ± 5° to the normal of the plane) onto active areas. As structuring of the front layer is only done partially, the value for transmittance of the whole area is formed by the weighted average of structured and unstructured areas. The gain to be expected is about 60% of the front contact and busbar area (depending on the profile and reflectivity of the contacts) and 80% to 95% of the space area between the cells.

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Fig. 10.3. Left: Conventional, plane module surface...

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Structuring of the PV Module Surface

A suitable structure for an optical surface, which prevents irradiance reflected from a structure flank of the surface to be lost, allows it to re-enter (in major parts) the PV-module when hitting the neighboring affronted flank of the structure.

In order to obtain indications about the entire module performance a three – layer flat-plate simulation program was modified to also consider V-structured surfaces. The results for three different grooving angles are presented in Figure 10.2 that shows the relative irradiance on the cell as a function of the incidence angle for different grooving angles at 90 °; 120° and 150°. Over a relatively large range of angles of incidence an increased level of irradiance on the solar cell (compared to the planar surface) can be noted.

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Fig. 10.2...

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