Family-owned Watlow was founded in 1922 and holds more than 50 patents, employing 2,000 employees, working in 11 manufacturing facilities in the US, Mexico, Europe and Asia. The company has sales offices in 14 countries around the world and a global distributor network. Jake Lindley is a senior engineer at the company, where he specialises in the design of complete thermal systems including heaters, sensors and controls. Here he tells PES about the company’s latest innovations for the industry …
As photovoltaic cell manufacturing methods continue to evolve, achieving thermal uniformity across a target (typically consisting of silicon wafers, large float glass plates or a metallic web) persists as a common theme in the application of process heat. Excessive variation in temperature can lead to a myriad of problems, including warped or broken substrates, inconsistent layer thickness during deposition, poor penetration during diffusion or incomplete chemical reactions. All of these issues can lead to increased production times, lower yields and increased cost per module. Here we explore some of the more common issues in thermal system design that can lead to poor thermal uniformity as well as offering some basic guidelines for the design of thermal systems in order to avoid common pitfalls.
Typical photovoltaic substrate heating systems often utilise large radiant or conductive heater panels to impart energy on a target. Intuition may lead one to the conclusion that a uniform power density on the heating system will result in a uniform temperature distribution on the substrate, be it continuous (metallic or polymer web) or discrete (silicon wafer or glass plate). In fact, due to heat losses at the edges, the resulting temperature profile would be highest in the centre and would decrease with proximity to the edge. These edge effects would be most severe at the corners of discrete targets.
To create a uniform temperature on the substrate, the heater should be designed to prevent or offset heat loss at the edges. A variety of techniques can be used independently or in combination for greatest effect. One way is to increase the size of the heat source with respect to the target thus allowing for some edge margin. The necessary size of this margin is directly dependent on the thermal uniformity desired by the manufacturer. Alternatively a picture frame approach to power distribution may also be employed to offset the edge losses. This power distribution can be produced by creating two discrete zones of control or by changing the power distribution of the heater element itself. A third method of dealing with this phenomenon is the use of insulation or heat shields at appropriate locations around the source to minimise edges losses.
