LafueSol – Laser Welding of Glass Tubes for Solar Thermal Collectors

Project duration:

1 June 2009 - 31 May 2012

Sponsored by:

German Federal Ministry of Education and Research (BMBF)

The goal of the joint project is to provide process and system technology for the energy-efficient production of solar thermal collectors with glass tubes by using lasers. With the help of new technologies, the process restrictions of the gas burner technology used today will be overcome. The goals of the project are:

Energy-efficient welding of glass tubes
Development of an online analysis of the the residual stresses generated in glass
Development of a unit to record temperature fields using an IR camera for process control
Reduced cycle time
Construction of a prototype to demonstrate the complet concept/process

In particular, the project goals can be achieved by constructing a complete system existing of a glass turning system, a CO₂ laser, a glass-specific infrared camera and a system for online analysis of the residual stresses in glass. The welding process developed should be integrated in a prototype, together with the peripheral technologies, so that at the end of the project a fully functional unit to demonstrate the process will be completed. The complete concept should be based on energy efficient production, making it possible to save energy and at the same time to increase production capacity of solar glass tube collectors. An important step in achieving energy efficiency is to become independent of gas burner technologies, concerning not only production-technical restrictions in glass tube processing, but also the necessary measures for cooling the work areas. A comprehensive view of the welding process, including upstream and downstream production steps, is necessary.

For example, it is possible to transpose 80% of CO₂ laser radiation into heat, whereas only 10% of the energy in the flame of gas burners can be used to heat glass.

Figure 1: Joining glass tubes with CO2 laser radiation

Since the laser can easily be controlled, the glass tubes can be heated according to their material characteristics. Investigations have shown that by heating the tubes rapidly, up to 50% of the energy necessary for heating can be saved, as the time-dependent thermal losses can be minimized. Typical thermal heating rates for glass tubes are currently at 15.000 K/min, which means that the processing temperatures are reached within a few seconds. For a better understanding, the heating process of glass tubes using the CO₂ laser was simulated, in order to characterize, for example, the width of the heat-affected area in comparison to the thermal loss flow. Furthermore, precise statements concerning the formation of the temperature pattern in the axial direction as well as statements concerning the heating of the glass thickness. Additionally, simulation can be used to make a prediction about the laser output needed for welding.

Simulation der Einflüsse der Laserstrahlbreite
Figure 2: Simulation of the temperature pattern and the heat flux density

Last not least, the temperature control can be used in the joining process to achieve high reproducility. This results in uniform tension levels in the joined glass tubes, so that the final and the most energy consuming tempering process can be adjusted appropriately.

Results and User Potential

The prototype to be developed should allow the end-user to directly integrate the new technology into their production line. The goal is to achieve the highly automated production of glass tube solar collectors with low energy costs. The peripheral technologies (residual stress analysis, infrared camera) will also be available to the glass branch outside of this specific application. The aim is to analyse the connected problem areas concerning energy use for other typical thermal glass processing applications, and to improve energy efficiency.

Apart from use in processing solar collectors, the machine technology developed here should also be able to be used for other applications. For example, by adapting the process, based on the absorption of the CO₂ laser radiation on the glass surface, it can also be used for quartz glass. Currently, quartz glass processing calls for special burner gasses in order to reach the high processing temperatures typical for this kind of glass.