Our fluidized bed reactor process

Although photovoltaic solar energy is widely available and environmentally friendly, it is still more expensive to produce than many competing sources.  One reason for this is the large amount of energy needed to make silicon that is pure enough for solar products. 

Six years ago, REC decided to take on this challenge.  Since then, we have developed our unique Fluidized Bed Reactor (FBR) silicon refining process, a next-generation method of refining silicon. With the FBR process, we can produce solar-grade silicon at a lower cost, while using 80-90 percent less energy that the traditional Siemens method of silicon refining.  We expect our first FBR-based silicon refinery to begin full commercial production by Q3 2009.
 

Reducing energy consumption

There are many existing methods of refining silicon, all of which use significant amounts of energy. This is a challenge, given the rise in both global environmental concerns and energy consumption.  The Siemens process, which is used by most of the industry including REC, can basically be described as superheating silicon gas and seed rods inside a giant refrigerator. In this method, a gasified silicon compound, being either silane gas (SiH4) or thriclorosilane (SiHCl3) is released into a superheated chamber that contains seed rods of silicon. The heated gas deposits pure silicon on these heated rods, and at the same time the chamber walls need to be chilled in order to avoid silicon deposition on them. Obviously, this process consumes large amounts of energy per unit of silicon produced.

With the Fluidized Bed Reactor process, REC has taken a major step forward in silicon purification, using less energy.  Instead of using seed rods, FBR uses seed granules of purified silicon.  The seed granules are fed into a chamber that has heated silane gas entering from below and exiting above. The flow of gas “fluidizes” the silicon granules, causing them to flow like a liquid, as the silane gas breaks down and deposits silicon layers on them. The granules grow larger and heavier and exit when they are sufficiently large. As they do so, new seed granules and gas are introduced into the chamber and the process continues.

FBR is more efficient for several reasons. First, it does not waste energy by placing heated gas and silicon in contact with cold surfaces. Secondly it produces more silicon per cubic meter of reactor space, because the silicon crystals have a larger total surface area than the rods used in the Siemens process. And finally, it is done a continuous process rather than in batches, so there is less wasted downtime or setup effort required.

Preparing for FBR production

REC is now in final stages of implementing the FBR technology. We successfully operated a test facility for four years and have now completed construction of a new, full-scale FBR-based silicon refinery at our Moses Lake plant in Washington, USA. The new FBR plant is expected to begin production by Q3 2009.