While the amount of process heat produced from solar sources is still low, the potential is high. Heating chemical baths, generating solar cooling and using hot air in a drying oven: these are classic examples of process applications. Now a metal finishing plant near Wuppertal has achieved first time worldwide by connecting its own steam network to parabolic trough collectors in which steam is generated directly in the absorber pipe. This simple technology eliminates the need for thermal oil as a heat transfer medium and saves on fossil generated energy.

German industry accounts for more than 40% of the country’s energy requirements, and a quarter of this energy is used for process heat. Fossil fuels are used in most cases, with additional flat plate or evacuated tube collectors in some plants which generate heat for industrial processes or for heating industrial premises. Saturated steam is often used as a transport medium to carry the heat from a central generator to various points of use in the factory. The solar thermal collectors which are available on the market are mostly suitable for heating water in a temperature range below 100 °C. In most industrial process heat applications they are connected directly to selected processes which require heat at a corresponding low temperature level. Concentrating collectors, such as parabolic troughs, are used for higher temperatures.

At Alanod Aluminium-Veredelung GmbH & Co. KG in Ennepetal, Germany, a “pilot system for process heat generation using parabolic trough collectors” (P3) was tested directly in operation. Alanod’s products include aluminium strips with selective absorption layers, which the solar industry uses for various collector types. As part of the P3 project, a solar steam generator was integrated into this production facility. Scientists from the German Aerospace Centre (DLR), Solar- Institut Jülich (SIJ) at the Aachen University of Applied Sciences, and the Institute for Thermodynamics and Thermal Engineering (ITW) at the University of Stuttgart designed the experimental system together with industry partner Alanod and collector manufacturer Solitem.

For the first time, process steam is generated directly in the absorbers of the parabolic trough collectors. As saturated steam at up to 200 °C is often required for industrial production processes, the researchers also chose steam as the heat transfer medium for the pilot system so that it could be utilised directly. Saturated steam is steam at saturation temperature. Using thermal oil as the transport medium results in higher heat losses than with steam, as it needs to be sent through a heat exchanger and higher temperatures occur in the collector array. Saturated steam can be piped directly to the process, saving fossil energy in the central steam generator.

To make the raw aluminium source material more resilient, it is provided with an oxide coating (anodisation) in the finishing process resulting in a porous oxide layer. In a sealing bath at 95 °C, the pores on the surface of the aluminium oxide layer are sealed. This process requires saturated steam at around 140 °C and 4 bar. Saturated steam is necessary in order to heat the baths rapidly to the required temperature. The 9-bar steam line that exists in the production facility, which is used to operate the strip dryer, is not suitable for the solar generated steam because the solar radiation at Ennepetal is not intense enough. It would take a long time to reach the required pressure and temperature, which would mean higher system costs and greater losses. The 4-bar steam line is suitable, however. Seven baths are connected to it in parallel, and the continuous strip of aluminium runs through them. The aluminium goes through the following process steps: degreasing, etching, polishing bath, stripping bath (intermediate cleaning), anodising bath (oxide layer), dye bath and sealing bath. Each bath has a volume of between 10 and 20 m³ and requires temperatures between 60 and 110 °C. Pipes transfer the heat into heat exchangers and pipe coils, which then heat the baths.