Let's Clean the Air! The Greening of Solar Panel Production
By Carolyn Byszewski, Group Manager, The Clean Air Group

Solar energy is one of the "green" sources that have a chance to become mainstream within the next decade. Keeping it green from start to finish as film production grows will require increasingly innovative solutions to handle air quality issues in panel manufacturing as well as meeting other environmental concerns.

For years, supporters of alternative energy sources, such as solar power, have made the price of a barrel of crude oil the tipping point to make solar and other forms of renewable energy more attractive. The oil market's volatility and anticipated government support in the United States for developing renewable energy sources may have tipped the balance even more. Efforts to manufacture thin-film solar panels to replace carbon-based fuels for providing electricity, heat and hot water continue to ramp up worldwide, especially in the United States, China, India, Germany and Spain, where there are strong investors, government subsidies or both.

After all, the reasoning goes, isn't it "cleaner" to harness the power of the sun? The answer is: Yes, but…

The manufacturing process of the high-grade silicon for solar panels involves highly toxic materials. The raw materials and thin-film manufacturing processes generate a number of effluent gases that contain contaminants such as silane, trichlorosilane, dichlorosilane and hydrochloric acid (HCl), all of which are hazardous to the environment and people who may be exposed to them. The industry has made vast strides in improving the silicon purification process, but meeting air quality needs will be an on-going challenge for those who advocate for the use of solar energy collection technologies.

The Clean Air Group of Croll Reynolds knows the problem well. As an engineering firm specializing in the research, design, and manufacture of scrubbers and packed towers for air pollution control systems, we are frequently asked to scrub - or clean - contaminants from the effluent of solar-grade silicon processing.

The stakes are high as the solar energy industry scales up to replace part of the market now dominated by carbon-based fuels. Thin-film technologies are expected to grow at an annual rate of 45 percent over the next five years to account for 19 percent of the photovoltaic market. Thin films are less expensive than the crystalline films found on most solar collectors today, and they are expected to represent a $32 billion market by 2012. This is according to research from BCC Research, which charts technology markets.

But in this growth stage, the solar energy industry is facing a moment of green truth.

As more companies begin to develop efficient solar energy collection technologies and higher purity manufacturing systems, the price of solar energy will become more competitive in the growing marketplace. However, solar energy's economic and environmental benefits could be wiped out if thin-film manufacturers don't design, install and maintain pollution control systems to reduce harmful effluents from their gases. We'll only be trading one set of problems for another and won't make the progress we should in improving and sustaining air quality in the US and around the world.

Process and Gases
New solar energy panel plants in the US fall under the regulatory scrutiny of Tier 3 of the Clean Air Act of 1990, which tightens air quality regulations considerably. Compliance essentially requires manufacturers to control a growing list of harmful contaminants while threshold limits are reduced to ever smaller parts-per-billion levels. New technologies and manufacturing methods, such as the thin films for solar energy collection, present newer, more difficult pollution control challenges.

Although solar energy collector panel manufacturers keep their production processes proprietary for the most part, there are commonalities. The panels are essentially semiconductors that convey electrons from one place to another. Thin-film solar panels use less than 1 percent of the raw material of wafer-based solar cells and are made through an automated manufacturing process, which helps significantly lower the price per kilowatt hour. Indeed, thinner, more flexible solar panels provide more architectural and installation options for all types of buildings.

Even though the most critical step, depositing silicon onto steel sheets, takes place inside a vacuum chamber, the manufacturing process generates dangerous vent gases that include silanes, which are pyroforic, and chlorosilanes, which release HCl, harmful to humans if inhaled or contacted. They are very acidic and can damage skin and lung tissue.

Silane, which has a repulsive smell, acts as superconductor under extremely high pressures. Chlorosilanes are a class of liquids that range in color from clear colorless to light yellow colored. Slightly denser than water, they may have a sharp odor. Contact with water releases HCL gas, which may severely irritate skin, eyes, and mucous membranes, and they may be toxic by ingestion, inhalation or skin absorption.

Controlling the Gas Stream
Controlling the gas stream calls for integrated technologies and configurations that meet the specific needs of the manufacturer. The solution depends on the requirements a plant must meet, its manufacturing process and any physical limitations associated with its site.

PACKED TOWER SCRUBBER Utilizing the most current packing designs to provide contact between gas and liquid streams, the Packed Tower Scrubber achieves the extremely low toxic gas discharge limits required to meet emission standards.

Some processes generate a liquid acid stream that can be treated in a scrubber and in downstream neutralization tanks. We know how to safely contain and neutralize both continuous waste gas effluent from the process containing chlorosilanes and HCl as well as intermittent releases from periodic process shut downs or even massive emergency releases with the properly designed spray, venturi or packed tower systems. Our packed tower scrubbers handle the HCL and the Si O2 solids that are formed by the production process. Because these soluble solids are abrasive and can cause plugging, our custom-designed systems use specially designed spray towers with high-flow, low-plugging nozzles for high efficiency.

The key is having the design engineers get the most accurate data possible to specify the best type of packing, scrubbing liquid and tower size. Keeping designs open to minimize plugging is key. Our proprietary jet venturi designs are perfectly suited to this application. The packing type is the most critical of these specifications. It should be very open in design to minimize pressure drop while maintaining a high surface where the gas absorption will take place. The choice of a specific packing depends on the gas stream's temperature, pressure and concentration and the project's efficiency requirements.

Construction materials can be FRP (fiberglass-reinforced plastic), stainless steel, Alloy 20 or any other suitable, specified material, and internal components can be made from a variety of plastics. Some materials for internal components will have lower initial costs, and budgets as well as process needs will guide material selection.

Counter-flow design towers offer the highest possible performance, but space limitations or design limits may require the use of a cross flow or co-current flow design.

Controlling and collecting silanes and chlorosilanes that form HCl as well as Si O2 must work with particulate solids that can be very small in size - in the micron and sub micron ranges. We provide multi stage wet scrubbers with finishing towers equipped with high efficiency mist eliminators to address these particulates and to minimize particulate carryover between systems. Our systems also add wash sprays and ME sprays to minimize maintenance and plugging

Multi-stage venturi scrubbers typically treat the dust generated by the thin-film manufacturing process. Because these scrubbers need to remove wet dusts, nozzle designs must minimize plugging, and construction materials must be able to withstand corrosion to maximize the system's service life.

These systems can exceed 99 percent efficiency by paying careful attention to pH control and reagent addition and by having an excellent liquid distribution system. Theoretically, the system can get HCl down to 1 part per million (ppm).

Solar panel plants with compliance needs or requirements for production expansion have options for adding to their existing systems if the real estate is there. We can retrofit updated technology to an existing system if the space is available and help plants meet higher production volumes by running parallel units, enhancing mist eliminators or using higher efficiency packing.

The Solution for a Growing Industry
The Clean Air Group can help smaller installations scale up into the large mega plants required to supply current and future demands. In addition to space-availability, other factors to consider are tightening environmental emissions regulations, good neighbor programs for industry in residential neighborhoods, "responsible care" programs and whether existing air pollution permits limit production.

Advances in air pollution control for the growing number of solar panel manufacturing will require coordination of current solutions and new process configurations for higher removal efficiencies and improved maintenance.

We are well along this path. We are developing alliances with preferred equipment vendors with high quality standards and large scale experience. We are forming teams with our suppliers to select the best products and methods of control for challenging new applications. Our team approach requires all parties buy in to the success of each new application.

Our proven systems will remain as a basis for air pollution control for solar panel manufacturing, but more advanced computer solutions will provide process control logics for tighter pH controls and less reagent waste and less hazardous liquid purge effluents. We have developed feed-forward control loops on pH to adjust reagent additions based on actual consumption and effluent gas concentrations. This will minimize the waste of valuable reagents and minimize wastewater treatment loads.

The green revolution is here. The Clean Air Group is dedicated to working with our customers and partners to make it sustainable.