Plastic collector for 25 euros hardly lowers the cost of solar thermal systems

Plastic collector on a extrusion basis (1), extruded part of an absorber for flexible collector widths (2), end cap for plastic collectors (3). (Photo: Franhofer ISE)
Plastic collector on a extrusion basis (1), extruded part of an absorber for flexible collector widths (2), end cap for plastic collectors (3). (Photo: Franhofer ISE)

As part of the ExKoll project, scientists at the German Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) have calculated that collectors made of extruded polypropylene (PP) can be manufactured for 25 €. An 80 x 160 cm collector with a back panel made of several double-walled plastic sheets served as a model.

The collector is designed for use in pressureless drain-back systems without glycol. The material can withstand temperatures of approximately 100 °C. To avoid overheating, the cavities in the back wall are adapted to the local climate and allow heat loss in controlled 'doses'.

Yield is lower than a conventional solar collector

The specific yield is approximately 20% lower compared to a conventional flat-plate collector. A similar collector with an additional absorber made of polyphenylene sulphide (PPS) that can withstand up to 250 °C would cost about € 45 to manufacture. The collector design used in ExKoll is a synthesis of the results of various research projects and was not the actual goal of the project.

However, even though the solar collector might seem to be a price cutter at first glance, the effect on the price of the entire system is not that significant. Based on the typical cost structure of flat plate collectors, scientists projected that the PP collector could be sold on the market for less than a hundred euros per m2, which seems reasonable. At a market price of 170 € / m2, the PPS collector would probably be too expensive for a plastic collector.

Savings melt away due to cost of sales and installation

According to a survey in Europe, flat plate collectors cost around 230 € / m2. When the low efficiency is factored in and the almost identical costs for installation and distribution of the system are added, the savings for a thermosiphon system, such as the ones commonly used in southern Europe, shrinks to between 8 and 16%.

The ExKoll project, which was completed in December 2014, was the German contribution to the IEA SHC Task 39 'Polymeric Materials for Solar Thermal Applications', which had already ended in October last year. In order to pursue the issue of cost reduction further, two follow-up projects in the IEA SHC programme are planned for the summer to address the reduction of system costs.

Patent on plastic collector

As part of the SCOOP project, the Fraunhofer ISE has also developed a thermosiphon system with an integrated plastic tank. Together with the Norwegian company Avanta, which already has plastic collectors for roof integration in its product range, Fraunhofer ISE has applied for a patent on a plastic collector with double-walled sheets.

In Austria, the company Sunlumo previously announced the 'one-world collector'. Using this collector, a pumped solar system with 4 to 6 m2 collector area could be made available for less than 1000 euros before VAT in Central Europe. Sunlumo intends to grant licenses for manufacturing, but so far has not announced any partners.

Eva Augsten