Innovation award for solar process steam

13.05.2015
The collector field on the roof of the University of Karlsruhe produces solar process steam for heating and cooling purposes. (Photo: University of Karlsruhe – Technology and Economics)
The collector field on the roof of the University of Karlsruhe produces solar process steam for heating and cooling purposes. (Photo: University of Karlsruhe – Technology and Economics)

Process heat for industry is predicted to be a very important application for solar thermal in the future. The Ritter Group has now been awarded the Otti Innovation Prize for developing an important building block in this area.

Up till now, solar thermal process heat has only played a minor role in central Europe. One reason for this is that industry often uses steam as a heat carrier, and solar thermal energy cannot provide it, at least not in northern and central Europe. The amount of solar radiation in the region is usually insufficient to make concentrating collectors a viable solution.

Process steam without heat exchangers

The German company Ritter XL Solar has now been awarded the Otti Innovation Prize for solar thermal energy for a research project that uses CPC vacuum tube collectors in Germany to create process steam to drive a steam jet ejector chiller

The system was installed at the end of 2013 and consists of 80 vacuum tube collectors with an area of 400 m². The collectors are equipped with plasma technology, which is basically an anti-reflective coating on the vacuum tubes. This makes the collectors approximately 20% more efficient than the best models that Ritter had in its range up till now. In addition, CPC mirrors ensure that even diffuse light is concentrated on the tubes, for example when the sky is cloudy.

Water as a heat transfer medium

The collectors use pure water as the heat medium, which is turned into steam over a large part of the collector field. The steam, which is still wet up to this point, becomes superheated in the last part of the collector field. In its present configuration, the collector field can deliver superheated steam between 100 and 140 °C at a pressure of 3 to 4 bar. Because of the high temperatures in the system, the insulation had to be improved to reduce energy losses.

The distance between the collector array and the chiller is also relatively long, at least at the test facility in Kassel. At a more favourable location, the source of heat and the consumer could be placed closer together, which would increase the yield even further.

High annual yield thanks to intelligent control technology

Despite the high temperatures and the long distance that the steam has to travel, an annual collector yield of approximately 460 kWh / m² is achievable. Generally speaking, it would also be viable to use significantly higher temperatures. The stagnation temperature of the collector is 360 °C. The control technology, which Ritter developed itself, automatically increases or decreases the flow rate to maintain stable temperatures, even when there is less sunlight. An overall daily utilisation rate of approximately 40% can be achieved, according to Ritter.

The heat that is produced is stored in a latent heat storage system based on polyethylene that was developed at the University of Karlsruhe. The polyethylene melts at temperatures between 125 °C and 135 °C, enabling it to store heat. The cooling water is also stored using latent heat. A paraffin-water medium is used for this purpose. It provides a higher storage capacity than conventional water storage systems can achieve.

A wide range of applications for process heat from solar thermal

The storage systems are part of the research project. At the moment, the most economical solution would be to feed the solar steam directly into an existing steam circuit. It also has a positive effect on the economic efficiency of the system if there are both cooling and heating requirements, as is the case in Karlsruhe. This eliminates the need to get rid of excess heat in the summer, and the heat can be used for space heating when the air conditioner is inactive in the winter.

Jan Gesthuizen

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