Implications of PV Manufacturing and Production
A researcher checks a sample on an x-ray diffraction (XRD) system. XRD is used to measure structural characteristics of cadmium telluride and other films.
Because manufacturers use a wide variety of processes to make PV cells, a wide range of chemicals—some of them toxic or hazardous—are employed in PV cell production. In terms of worker safety and health, simple protective and administrative measures can be used effectively to protect those who produce PV systems. In terms of the environment, the PV production process produces small amounts of waste materials, but this is minimal relative to the emissions from conventional energy sources.
Most of today's PV cells consist of crystalline or multicrystalline silicon. Silica particles can be released in the mining and refining stage, but these present a hazard only to workers—one that can easily be avoided. Silicon PV module production can include fluorine, chlorine, nitrates, isopropanol, sulfur dioxide, carbon dioxide, silica particles, and solvents. According to a report from Utrecht University, "Estimated air emission is maximally 0.16 [kilograms of fluorine] and 430 [kilograms of chlorine] per [1000 megawatt-hours] of electricity supplied by PV modules, which is orders of magnitude smaller than the corresponding emissions of a coal plant."
According to the same report, neutralizing etching and texturing solutions and flue gases can yield water-borne fluorides and chlorides. But these are still three to five times smaller per unit of electricity than those produced by a coal-fired plant. Alternative etching, texturing, and purification methods can reduce those emissions. Using sulfur-containing carbon sources in the reduction of silica also produces negligible amounts of sulfur dioxide and carbon dioxide.
Solvents and alcohols used in the process could contribute to ozone and smog if they are emitted into the atmosphere. But EPA regulations limit the amount of such chemicals that can be released into the air, and simple emission controls can filter out those pollutants in a plant's exhaust system.
The SPI-Assembler 5000 and the SPI-Buffer 350 are automated equipment used in manufacturing PV modules. The Assembler solders solar cells together with interconnecting ribbon to form cell strings. These strings are later laminated, usually to a glass sheet that acts as the front surface of the module. After lamination, the Buffer provides automated module storage between process steps such as edge trimming, framing, junction box attachment, and testing. The Buffer can also feed modules, glass, or other sheet materials into a process or accumulate them after a process.
Although crystalline silicon is the primary material used today to produce PV cells, a growing number of PV products are being produced from other materials. The primary concerns with each of these materials are as follows:
-
Amorphous silicon — Silane, an explosive gas, is used in making amorphous silicon. Toxic gases such as phosphine and diborane are used to electronically "dope" the material.
-
Copper indium diselenide — Toxic hydrogen selenide is sometimes used to make copper indium diselenide, a thin-film PV material.
- Cadmium telluride — Cadmium and its compounds, which are used in making cadmium telluride PV cells, can be toxic at high levels of lung exposure. More information about cadmium can be found at the National Renewable Energy Laboratory's Web site.
Besides the semiconductor material, PV modules also use palladium and silver or nickel or nickel chromium (and possibly tin) for metallic contacts and usually a tin/lead solder for electrical connections. Of these materials, the one of greatest concern is lead, a toxic chemical that can affect mental development in children.
Although it is a problem common to all electronic devices, the topic of lead-free solder was tackled in a recent PV workshop. Participants discussed an alternative solder made primarily of tin with a small amount of silver. ASE Americas is using this solder, and Steel Heddle, a solder manufacturer, is making an alternative solder ribbon available to its customers at the same price as the standard tin/lead solder ribbons. See the workshop summary.