Joachim Koschikowski, Matthias Rommel, Marcel Wieghaus

Fraunhofer-Institut fur Solare Energiesysteme ISE
Heidenhofstr.2, D-79110 Freiburg, Germany
Tel +49-761-4588-5294
Fax +49-761-4588-9000
email ioako@jse. fhg. de

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. Each day about 25 Mio. m3 of the world water demand is produced in desalination plants. These “water factories” are in the capacity range up to 230.000 m3/d and can provide big cities with drinkable 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. The use of renewable energy sources as wind or solar radiation can compensate this lacks.

The absence of drinkable water in arid and semi-arid regions often corresponds with a high solar insolation, this speaks for the use of solar energy as the driving force for water treatment systems. These systems must be adapted to the special conditions required by the alternating operation conditions caused by solar energy powering and to low water demands, challenging ambient conditions and the lacks of well trained technicians for setup and maintenance. So the main focus of the development work is on the construction of robust systems which operate maintenance free in a stand-alone mode. The systems have to be modular in order to resize them to a wide range of user profiles and they must be able to withstand different raw water compositions without chemical pre-treatment in order to develop standardised stand-alone systems for all current types of sea and brackish water. Mainly two different options are given for using solar energy as the driving force for desalination: PV (photo voltaic) coupled RO (revere osmosis) systems and solar thermally driven distillation systems. Also PV and thermally driven VC -(vapour compression) systems are possible.

This paper reports on an ongoing development of solar thermally driven stand­alone operating desalination systems for a capacity ranges of 0.1 to 20 m3 per day. To achieve these aims separated desalination units based on the membrane distillation technique (MD) with internal or external heat recovery function are coupled with high effective solar thermal collectors. The implemented heat source for very small capacities is a corrosion free, sea water resistant thermal collector developed by Fraunhofer ISE in 1999. For larger systems a design is used which is based on a separated collector loop coupled to the brine loop by a seawater resistant heat exchanger. In that case standard flat-plate collectors or vacuum tube collectors are used.

Two options for small-scale stand alone operating systems

For seawater desalination two principal different techniques are existing, thermal driven systems and pressure driven systems. Also for small-scale stand alone operating systems this two options are given.

The most common type of thermally driven stand-alone operating desalination Figure 1: Example for a simple solar still. system is the solar still type (figure 1). It

consists of a basin with a dark bottom in

order to absorb solar radiation. The top is covered by a solar radiation transparent plate made from glass or polymer. The saltwater is evaporated by the heat transformed from the solar insulation. The vapour condenses on the cold surface of the top plate and the formed distillate is collected in a trough. This construction is quite simple and many different designs which are all based on the same principle exist. Due to the fact that its thermal efficiency is very low, the specific collector area per cubic meter desalted water is very high. The experience with simple solar stills were negative, especially with respect to the low system efficiency (V. Janisch, 1995). In advanced solar thermally driven desalination systems the desalination unit must be separated from the solar collector in order to achieve high efficiencies on the heat generator site and to integrate a heat recovery into the desalination unit.

An other option for decentralised, stand alone operating desalination is the use of reverse osmosis (RO) modules and photo voltaic (PV) coupled high pressure pumps.

Osmotic pressure results from a concentration gradient between two salt solutions. If there are two solutions of different concentration separated by a permeable membrane with a diameter of pores smaller than salt ions, then the liquid from the lower concentration site permeates through the membrane to the higher concentration site until the hydrostatic pressure of the water column (see figure 2) is equal to the osmotic pressure. The osmotic pressure mainly depends on the height of the concentration gradient.

To produce water with a low salt concentration from a higher concentrated solution, an external pressure on the concentrate site is necessary which is higher than the osmotic pressure. This principle is called reverse osmosis (RO). For technical applications RO is used to produce fresh water from sea or brackish water. For sea water desalination pressure pumps are operated between 60 and 80 bar. Today circa 42% of the world wide installed plant capacity for sea and brackish water desalination is based on RO.

While these grid coupled RO systems are very well developed, it is known that difficulties exist to operate small scale stand — alone systems which are supplied by PV or wind energy. The comparison between solar thermally driven evaporation systems and PV driven RO systems with respect to the long-term system efficiency, reliability and appropriateness can not finally be assessed.

Membrane distillation (MD)

Membrane distillation is another technique which is operated with thermal energy but also uses a membrane for the separation of pure water from the concentrated solution. Apart from some experimental systems the MD-technique is not used for desalination up to now, but with regard to the implementation in solar driven stand alone desalination systems it holds important advantages. The most important advantages are:

• The operating temperature of the MD-process is in the range of 60 to 80 °C. This is a temperature level at which thermal solar collectors show a good performance.

• The membranes used in MD are proved against fouling and scaling.

• Chemical feed water pre-treatment is not necessary.

• Intermittent operation of the module is possible without heat storage.

• The system efficiency and the high product water quality is almost independent from the salinity of the feed water.

1992, Findley 1967, Schofield 1987) is

Contrary to membranes for RO, which have a pore diameter in the range of 0,1 to

3,5 nm, membranes for membrane distillation have a pore diameter of about 0,2 pm. The separation effect of these membranes is based on the fact that the polymer material it is made from, is hydrophobic. This means that up to a certain limiting pressure liquid water can not enter the pores. Molecular water in the form of steam can pass the membrane. In figure3 the principle functioning of membrane distillation is discribed.

On the one side of the membrane there is salt water, for example at a temperature of 80°C. If there is a lower temperature at the other side of the membrane, for example by cooling the condenser foil to 75°C, then there exists a water vapour partial pressure difference across the membrane. This is the driving force that makes the water passes through the membrane. The water vapour condenses on the low temperature side and distillate is formed.

For the design of a solar powered desalination system the question of energy efficiency is very important since the investment costs mainly depend on the area of solar collectors to be installed. Also the power consumption of the auxiliary equipment (for example the pump) which will be supplied by PV has an important influence on the total system costs. Therefore, the system design has to be focused on a very good heat recovery function to minimise the need of thermal energy. Heat recovery can be carried out by an external heat exchanger or by an internal heat recovery function were the feed water is directly used as coolant for the condenser channel.

Heat-

exchanger1

The principle of the internal set-up of the MD-module with internal heat recovery function is shown in figure 4. All together, there are three different channels: the condenser channel, the evaporator channel, and the distillate channel. The condenser and the distillate channel are separated by a impermeable condenser foil, while the evaporator and the distillate channel are separated by a hydrophobic, steam permeable membrane. The hot water (e. g. 80°C inlet temperature) is directed along this membrane, passing the evaporator channel from its inlet to its outlet while cooling down (e. g. 30°C evaporator outlet temperature). The feed water (e. g. 25°C inlet temperature) passes the condenser channel in counter flow from its inlet to its outlet while warming up (e. g. 75°C outlet temperature). The partial pressure difference caused by the temperature difference on both sides of the membrane is the driving force for the steam passing the membrane. The heat of evaporation is transferred to the feed water by condensation along the condenser foil. Thus the heat of evaporation is (partly) recovered for the process. Because the energy for evaporation is removed from the brine, the brine temperature decreases. The liquid distillate is gained from the distillate outlet on a temperature level between feed in — and brine — outlet. The heat input which is necessary for the required temperature gradient between the two channels (e. g. 5°C) is introduced into the system between the condenser outlet and the evaporator inlet. Thus the thermal energy consumption of the system is given by the volume flow rate and the temperature lift of the feed water between these two points. The heat recovery function has an important influence on the energy consumption of the system. In thermal desalination processes the "Gained Output Ratio” (GOR) is an important parameter for the evaluation and assessment of the heat recovery function. The GOR can be calculated as the quotient of the latent heat needed for evaporation of the produced water and the energy input supplied to the system from out site. A possible design for a MD module is a spiral-wound module construction. A sketch of the channel assembly is shown in the cross section in figure 5. The picture on the right hand shows a test module in the performance test facility.

The technical specifications of the MD module are:

• hydrophobic PTFE membrane, mean pore size 0.2 pm

• height 650 mm

• diameter 300 mm

• membrane area 7 m2

• feed temperature at evaporator inlet 60-85 °C

• specific thermal energy consumption 100-150 kWh/m3distinate (GOR about 4 to 6)

• distillate output 20-30 l/h

• all parts are made of polymer materials (PP, PTFE, synthetic resin)