Solar modules encapsulation

Finally, considering that, in general, ZnO is known to be particularly sensitive to a humid environment (see paragraph 9.4.1.2), this feature should also be checked, in more detail, for the case of ZnO films deposited by CVD. In fact, Sang et al. [84] reported that in a humid environment, ZnO B films deposited by LP-CVD showed higher degradation than sputtered ZnO Ga films. On the other hand, Oerlikon Solar (formerly Unaxis Solar) [79] has proven that thin him silicon solar modules using LP-CVD ZnO as TCO layers can successfully pass the standard damp-heat test, provided they are encapsulated in an appropriate manner. 

Reliable stability data of the p-i-n solar cell itself are not easily obtained, especially for non-encapsulated cells or modules. One of these tests e.g. for EN/IEC 61646 certification of modules is the so-called damp-heat test (85°C, 85% humidity, up to lOOOh). Recent studies were performed by Stiebig et al. [50, 51] exposing different types of cells to harsh conditions. One of the most important results was the excellent stability of silicon thin film solar cells. Remarkably, this is also valid for small area modules even without encapsulation [52]. This is of high interest because costs and efforts for module encapsulation strongly depend on the inherent stability of the solar cells. As a more detailed treatment of this subject is beyond the scope of this chapter, the reader is referred to the original papers [50,51]. 

Photovoltaic (Solar) Module or Panel - A solar photovoltaic product that generally consists of groups of PV cells electrically connected together to produce a specified power output under standard test conditions, mounted on a substrate, sealed with an encapsulant, and covered with a protective glazing. Maybe further mounted on an aluminum frame. A junction box, on the back or underside of the module is used to allow for connecting the module circuit conductors to external conductors. 

Although photovoltaic conversion is nonpolluting, environmental, health, and safety aspects must be considered, especially with regard to harmful emission and waste products resulting from the production of the solar cell modules. It has been shown that, with proper encapsulation and a proactive recycling program, it should be possible to minimize environmental concerns. 

Most of the chapters in the third section are concerned with photovoltaic (PV) applications (conversion of light into electrical energy). Because of the diffuse nature of solar energy, the photovoltaic collection devices must be very large or else the light that strikes them must be concentrated. The first chapter in this section gives an overview of luminescent solar concentrators that can be used with the PV collectors. Most PV collectors or modules are multilayered systems containing a photovoltaic cell element. The next four chapters consider the use of various plastics as encapsulant or pottant materials in the PV modules. 

Solar cell modules must undergo substantial reductions in cost in order to become economically attractive as practical devices for the terrestrial production of electricity. Part of the cost reductions must be realized by the encapsulation materials which are used to package, protect, and support the solar cells, electrical interconnects, and other ancillary components. As many of the encapsulation materials are polymeric, cost reductions necessitate the use of low-cost polymers. This article describes the current status of low-cost polymers being developed or Identified for encapsulation application, requirements for polymeric encapsulation materials, and evolving theories and test results of antlsolllng technology. 

The analyses for structural adequacy Identified that the thermal expansion or wind deflection of photovoltaic modules can result In the development of mechanical stresses In the encapsulated solar cells sufficient to cause cell breakage. The thermal stresses are developed from differences In the thermal expansion properties of the load carrying panel, and the solar cells. However, the analysis Interestingly Identlfed that the solar cell stresses from either thermal expansion differences or wind deflection can be reduced by Increasing the thickness t of the pottant, or by using pottants with lower Young s Modulus E. 

Cuddihy, E.F., Baum, B., and Willis, P., "Low-Cost Encapsulation Materials for Terrestrial Solar Cell Modules", Solar Energy, Vol. 22, p. 389 (1979). 

Encapsulants are necessary for electrical Isolation of the photovoltaic circuit. They also provide mechanical protection for the solar cell wafers and corrosion protection for the metal contacts and circuit interconnect system over the 20-year design life of a photovoltaic array. The required components Include the solar cell circuit, the rigid or structural member, the pottant, and the outer cover/insulator. Surface modifications may be needed to develop strong, stable bonds at the Interfaces in the composite. If the module is to be framed, edge sealants may also be required. The functions of the Individual components and the performance requirements as they are now known are described. Costs are ccmipared where possible and candidate materials identified. 

A few years ago, most terrestrial photovoltaic (PV) modules were assembled by casting the cells in a transparent silicone substance, using a metal substrate for support. When this approach was reviewed by the Flat-Plate Solar Array Project (FSA), development was begun on new materials that would reduce the cost and quantity of material required for encapsulation. 

Laboratories, Low-Cost Encapsulation Materials for Terrestrial Solar Cell Modules, JPL Internal Document No. 5101-78, Pasadena, California, September 1978. 

Such a panel of solar cells is encapsulated by pol5rmeric encap-sulants to form a solar cell assembly. This assembly may be further sandwiched between two protective outer layers to form a weather resistant module. The protective outer layers are formed from sufficiently transparent material to allow photons to reach the solar cells. In modules, where PVB is used as the encapsulant material, it has been foimd that the PVB tends to discolor over time, when in contact with an oxidizable metal component. 

Dhete, N.G., Gadre, K.S. Comparison of mechanical properties of EVA encapsulant in new and field-deployed PV modules. In Proceedings of the 2nd Worid Photovoltaic Solar Energy Conference and Exhibition, Vietma, Austria, 6-10 July 1998... 

One important application of EVA (Ethylene Vinyl Acetate) material is the encapsulation of solar cells in photovoltaic (PV) modules, where the material has to fulfill several basic functions. Unfortunately, the lEC standard does not provide any information on the changes in the internal stmcture of the EVA encapsulant material as a consequence of encapsulation process or environmental exposure. 

Another desirable optical characteristic for the encapsulant would be rejection of infrared (IR) sunlight of wavelengths longer than usable by the solar cells. The rejection of the infrared sunlight would raise module performance and increase module lifetime by reducing operating temperatures. However, a cost-effective method for rejecting the infrared heat has not been developed. 

Cells are normally grouped into "modules," which are encapsulated with various materials to protect the cells and the electrical connectors from the environment. The manufacturers supply PV cells in modules, consisting of Nj parallel branches, each with solar cells in series, as shown in Figure 12.9. 

A new silicone encapsulant for the solar industry is available that can enable flexible thin-film modules to be laminated cost effectively using roll-to-roll processing. Marketed under the trade name ELASTOSIL Solar 2200, this new product is transparent, pourable and non-corrosive. It vulcanizes rapidly at elevated temperature and reportedly features outstanding adhesive properties. In its cured state, it can provide all types of thin-film solar cells with effective long-term protection from chemical and mechanical stresses. Web www.wacker.com... 

---