Experimental Investigations On Solar Driven Desalination Systems Using Membrane Distillation

J. Koschikowski*, M. Wieghaus*, M. Rommel*,

Vicente Subiela Ortin**, Baltasar Penate Suarez**, Juana Rosa Betancort Rodriguez**

* Fraunhofer Institute for Solar Energy Systems ISE
Heidenhofstr.2,79110 Freiburg, Germany
Tel +49-761-4588-5294
Fax +49-761-4588-9000
email ioako@ise. fhg. de

** INSTITUTO TECNOLOGICO DE CANARIAS, S. A.

Playa de Pozo Izquierdo, s/n
35119 — Santa Lucia, Las Palmas
Tel: +34 928 727511 Fax: +34 928 727517
email: baltasarp@itccanarias. org

Abstract

In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future. Missing energy sources and no grid connections complicates the use of standard desalination techniques in these places. Fraunhofer ISE develops solar thermally driven compact desalination systems based on membrane distillation (MD) for capacity range between 100 and 500 l/day and larger systems for the capacity range up to 10m3/day. All systems can be operated energy self sufficient and almost maintenance free. Membrane distillation is a technique which is operated with thermal energy but also uses a membrane for the separation of pure water from the concentrated solution. The physical basics of transport processes in MD are described. Experimental investigations demonstrate that MD keeps important advantages for the operation in solar driven stand alone desalination systems. Altogether eight fully solar driven pilot plants were installed in 5 different countries. Measurements and experimental investigations of these demonstration units are provided.

Keywords: stand alone, desalination, solar thermal, membrane distillation

1. Introduction

In many places world wide drinkable water is already a scarce good and its lack will rise dramatically in the future. Today, sea and brackish water desalination plants are well developed in industrial scales to provide big cities with fresh water. Small villages or settlements in rural remote areas without infrastructure do not profit from these techniques. The technical complexity of the large plants is very high and can not easily be scaled down to very small systems and water demands. Furthermore, the lack of energy sources as well as a missing connection to the grid complicates the use of standard desalination techniques in these places. In arid and semi arid regions the lack of drinkable water often corresponds with a high solar insulation. This speaks for the use of solar energy as the driving force for water treatment systems. Especially in remote rural areas with low infrastructure and no grid connection, stand alone operating systems for the desalination of brackish or sea water are suitable to provide small settlements with clean potable water.

Within the scope of two projects subsidised by the European Union Fraunhofer ISE developed solar driven compact desalination systems for capacity range between 100 and 500 l/day and Two-

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Loop-Systems for capacities up to 10m3/day. All systems are supplied by solar energy only. The energy for the desalination process is provided by solar thermal collectors and the auxiliary equipment as pumps and valves are powered by PV. The main advantage of the compact system is on the very low technical complexity enabling long term maintenance free operation periods. The Two-Loop-System is constructed for low maintenance operation as well, but has a higher technical complexity. A modular design of all systems is important in order to adapt them to a wide range of user profiles.

Membrane distillation is a technology which is operated with thermal energy but also uses a membrane for the separation of pure water from salty water. Apart from some experimental systems the MD-technology is currently not used for desalination, but with respect to the implementation in solar driven stand alone desalination systems it holds important advantages.

Eight fully solar driven pilot plants (2 Two-Loop-Systems and 6 Compact Systems) were installed in 5 different countries. Comprehensive measurements and experimental investigations were carried out on these pilot units for more than 3 years demonstrating on the one hand that membrane distillation is a very suitable technology but discovered on the other hand also significant potentials for improvements.

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