Metallic Films

Parabolic concentrators and reflectors use metallic surfaces to concen­trate light. Parabolic concentrators are composed of two parabolic mirrors, referred to as the first and the secondary. The first mirror reflects light from the Sun to the secondary mirror elevated above the first. The secondary mir­ror focuses light back onto the underlying PV cell. Parabolic concentrators
often exhibit high concentration factors, and they require a dual-axes tracker to follow the course of the Sun and optimize performance. In contrast, reflec­tors have guidance mirrors on each side of the PV cell. Light rays from the Sun hit the mirror and bounce down to the cell. The mirrors are at a fixed position, limiting the concentration ratios to low values of less than 5x Suns when utilizing a single axis of tracking.

Polymers have been proposed as alternative mirror glazing for concen­trators and reflectors since the 1970s; however, they have failed to become widely commercialized due to various technical limitations. The primary focus of the glazings is to act as an environmental barrier to protect the underlying metal, typically silver. In addition, the glazing must be optically clear, impact resistant, weather resistant, and inexpensive to manufacture. Polycarbonate, polyacrylates, and polyesters were surveyed by the industry in the 1970s. Only polyacrylates exhibited no significant degradation after 2 to 5 years of outdoor exposure in Arizona.

Silvered polymer reflectors are a laminate including a heat-sealable polymer film, a tie-layer adhesive, metal foil, and a protective barrier film (Figure 6.6). Their reduced cost, ease of manufacturing, and mechanical flex­ibility make laminates an attractive alternative to silver-plated glass. Their

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FIGURE 6.6

A polymeric, metallic film.

commercialization has been limited by higher than expected production costs, poor optical weathering, and delamination in moisture and thermal cycling.

In the mid-1990s, 3M manufactured ECP-305+, a polyacrylate evaporated with silver, protected with copper, and adhered with a pressure-sensitive adhesive. They also had a separate product line, sold as SS-95, consisting of silver evaporated polyester with a thin protective coating of polyacrylate. The SS-95 film was quickly discontinued due to large optical losses in field tests. Specifically, the National Renewable Energy Laboratory (NREL) con­firmed a 30% reduction in optical properties of SS-95 after less than 5 years of outdoor exposure in Colorado. In contrast, ECP-305+ film demonstrated less than 5% reflection loss after 10 years of outdoor exposure at NREL facil­ities but suffered delamination in the field. The product was discontinued due to high manufacturing costs, insufficient demand, and poor consumer image [20].

NREL has published the most recent weathering data on glazings and over­sees reflector development for the Department of Energy (DOE). Through joint development with commercial vendors, they have been able to extend the life of polymeric mirrors. NREL and ReflecTech® jointly developed an undisclosed formulation that was recently marketed to PV manufacturers. The first attempts significantly weathered under 7 to 8x Suns. The film’s hemispherical reflectance decreased to approximately 70% from 95% in less than two UV equivalent years of simulated weathering. In 2005, after fur­ther material modifications, additional samples were submitted for testing. Currently, the material is rated for 10 years based on Arizona outdoor weath­ering. Despite these improvements, neither this nor any other commercial product has been warranted for the target 25 to 30 years.

Updated: August 22, 2015 — 11:15 pm