Corrosion-free solar collectors for thermally driven seawater desalination

Seawater desalination can be carried out by a variety of techniques, some of which are thermally driven at operating temperatures in the range of 70 to 120degreesC. At this temperature level, these systems can be supplied with thermal energy gained from solar flat-plate collectors. In order to increase the efficiency of such systems and simultaneously reduce costs, a heat exchanger between the collector loop and the desalination unit can be omitted if the seawater flows directly through the collector absorbers. Since hot seawater is very aggressive, this configuration requires corrosion-free absorbers. We developed collectors which fulfil this requirement, They were installed in a pilot plant in Pozo Izquierdo, Gran Canaria, which produces about 600 1 of drinking water per day. The main task was to find materials which are resistant to seawater and can withstand stagnation temperatures up to 200degreesC, out of which appropriate absorber constructions had to be developed. Moreover, it had to be ensured that it was possible to apply a selective coating on the absorber. A further limitation was given by the aim for the resulting water costs. After investigating different materials and collector designs, we developed a prototype consisting of an array of selectively coated glass tubes mounted in a conventional flat-plate collector housing. The efficiency was increased by adding a specially shaped reflector. The design was developed by carrying out various optical and thermal simulations, and indoor as well as outdoor tests. We obtained appropriate collectors which fulfil the technical and economical requirements. Finally, first results of the in-situ measurements in Gran Canaria will be presented. (C) 2002 Elsevier Science Ltd. All rights reserved.