This document describes a solar powered desalination system using membrane distillation. Membrane distillation uses a hydrophobic membrane that allows water vapor to pass through but prevents liquid water from passing. A temperature difference across the membrane causes water to evaporate on the hot side and condense on the cold side, removing salt. The system uses spiral wound modules with an integrated heat recovery system to preheat feed water. It operates at low pressures and temperatures below 80°C, producing pure water for applications like boiler feed water without requiring chemical pretreatment or being susceptible to fouling. The technology has been optimized and is used in countries like Australia, the US and Germany.

2. Introduction:
 Desalination is nothing but removal of salt content
present in the sea water and making palatable.
 But how do we achieve this
 My solution is “ Solar powered Desalination by
Membrane Distillation”.

3.  Why desalination:
 Desalination of sea and brackish water has
become a necessity in many arid and semiarid
regions. Due to the fast growing population and a
correspondingly high water demand in these
regions the few water sources often get
brackish or contaminated.
 The use of fossil energies for desalination leads to an
environmental load.

4. DESCRIPTION OF THE MEMBRANE DISTILLATION (MD)-
PROCESS :
 It is possible to concentrate aqueous solutions of non-
volatile dissolved substances by microporous
membranes impermeable for water but permeable
for water vapour
 Driving force for this "membrane distillation" is a
vapour pressure difference on both sides of the
membrane due to a corresponding temperature
gradient across the membrane.

5. Fig. 1. Principle of membrane distillation

6.  The distillation is performed at ambient pressure
and at a maximum temperature of 80°C (175°"F)
 The system is employing spiral wound
desalination modules. Inside the distillation
modules a thin microporous hydrophobic
PTFE-membrane is used with pore diameters
between 0.051 and 0.2 m.
 This material shows the surprising property of
allowing easy passage of water vapour, but of
completely blocking the flow of liquid water.

7.  In the process one surface (hot side) of the flat sheet
membrane is in contact with the process solution while the
opposite surface (cold side) is in contact with distillate.
 Thus the diffusion gap between evaporating and
condensing surfaces is reduced to the thickness of the
membrane that is only about 30 mm. With an actual
pore fraction of 80% high specific evaporation rates are
possible.
 The recovery of the heat of condensation is done by
utilizing the heat of condensation to preheat the feed
water.

8. DESIGN OF THE DESALINATION
MODULE
 Cold feed water (temperature T l) enters the module
and is progressively heated by the hot condenser
sheet, so that it emerges from flow channel 1
(heat recovery channel) on a significantly higher
temperature level (temperature T2).
 Before the feed water reenters the module into flow
channel 2, the temperature has to be elevated from
T2 to T3 using an external heat source. The
distillation takes place from flow channel 2 across
the membrane into flow channel 3. The feed water is
gradually loosing heat and is getting concentrated.

9. Fig. 2. Principle set-up of the MD-module with an
integrated heat recovery system.

10.  The temperature difference between flow channel 2 and
3 is the driving force for the process and is
maintained along the whole channel length
 The concentrate emerges therefore with a higher
temperature than the incoming feed. The distillate is
collected in flow channel 3 and emerges from the
module almost at ambient temperature (between Tl and
T4).
 The spiral wound design of the module (Fig. 3) allows
high recovery rates of latent heat, eliminates the
need for thermal insulation and mechanical support
and performs as a compact and resistant unit.

11. Fig. 3. spiral-wound membrane distillation
module

12. Optimization of process :
The whole module construction has been optimized
interms of :
 Distillate output
 Pressure losses
 Material stability
 Manufacturing technique

13. APPLICATIONS OF MEMBRANE DISTILLATION :
 Production Of Boiler Feed Water
 Production Of Ultrapure Water For Use In
Medical, Pharmaceutical Or ElectronicIndustry
 Generation Of Pure Water For Rinsing In Surface
Treatment Technology
 Recycling Of Process Solutions By Concentration
 Treatment Of Contaminated Fluids (Poisonous,
Radioactive)
 The industrial applicability for the treatment of
process solutions has been proven in a plant for nickel
electroplating in Dresden/Germany.

14. SYSTEM ADVANTAGES :
 Efficient and compact spiral-wound membrane distillation
modules
 Recovery of the heat of condensation is integrated in the module
design
 Chemical pretreatment of feed water is not required -
 Low system pressure
 Insensitive to dry-running and fouling -
 Neglectible scaling due to process temperatures below 80°C
(176°F).

15. WHICH COUNTRIES
USE THIS METHOD :
 The membrane distillation is
AUSTRALIA being optimized for the
U.S.A production of boiler feed
GERMANY water, the recycling of
electrolytes and for the
production of distilled water
for rinsing.

16. Conclusion :
 Small simple desalination plants operating independent
from the electric grid are either not available or not
economic at all.
 The process of membrane distillation allows the effective
use of low temperature heat sources like solar energy or
waste energy from engines for small to medium scale
desalination.
 In order to achieve an effective membrane distillation
process spiral wound modules have been developed and
optimized during a 6-year R&D program.
 The modules are designed as compact units with
integrated recovery of the heat of condensation,
allowing a highly efficient use of low temperature heat
sources.

17. REFERENCES:
 E. Delyannis. V. Belessiotis Solar desalination, is it
effective? Desalination and WaterReuse Vol. 414
 J. Manwell, J. McGowan, Recent renewable energy
driven desalination system research and development in
North America Desalination, 94 (I 994) p. 229-241
 K. Schneider, T. van Gassel, Membrandestillation Chem.-
lng.-Tech. 66 (1984) Nr. 7, 514-521
 N. Kjellander, Design and fold test of a membrane
distillation system for seawater desalination
Desalination, 61 (1987) p. 237- 243