Effects of Solar Energy Intermittency on a PV/Thermal Powered Membrane

Distillation System

Amanda J Hughes, Tapas K. Mallick and Tadhg S O’Donovan

4th ICAER 10-12th December 2013 IIT Bomday, Mumbai, India

- Provide a reliable and competitive means of water desalination that can be implemented in developing countries.

-  Development of a membrane distillation system with concentrating photovoltaic/thermal energy source for seawater desalination .  

Motivations and Aims

0.9 billion people without clean drinking water

1.5 billion people without access to electricity

0.8 billion without water and electricity

“Water, water, everywhere, Not any drop to drink.” –S. T. Coleridge

Solar based solutions are particularly suitable for desalination purposes, given the availability of such source in most of the water stressed areas

(i.e. the more the water scarcity, the more the solar radiation)

Reverse Osmosis Electro Dialysis

MED MVC Solar stills MSF

Desalination Processes

Membrane Thermal

Membrane distillation

Membrane Distillation

 What it is?  Thermally driven membrane

separation process

 Hydrophobic membranes allow pure water vapor to pass through, whilst salt is retained

Membrane Distillation Advantages

  100% theoretical salt retention   Low operating temperature, 30-800c, when compared with

conventional distillation   Reduced vapour space compared to conventional thermal

processes, thus reduced plant volume.   Low operating pressure when compared with conventional

pressure driven membrane processes   The membranes used in MD are tested against fouling and

scaling.   Chemical feed water pre-treatment is not necessary.

  System efficiency and high product water quality are almost independent from the salinity of the feed water.

Membrane Distillation

• Problems

• Membrane wetting • High thermal energy consumption • Low permeate flux capacity when compared with established membrane technologies. (MD capacity 1-10 m3/d compared with 100-1000 m3 for Wind-RO)

This is still an emerging technology….

Developing a membrane module

Condensing plate

Membrane

Spacers

Cold Channel

Hot Channel

Distillate stream

Stainless steel end plate

Stainless steel end plate

Developing a membrane module

SEM Image of membrane

Energy system

Parabolic dish with a PV/thermal receiver   Aperture area of 1m2 and a receiver area

0.002m2, giving a concentration ratio approximately 500×.   The photovoltaic cells have an efficiency of

36%   2 axis tracking required

Evacuated tube collectors • The evacuated tube collectors have an area of 5.7m2 and an efficiency of 65%

PV/T receiver – Temperature effect

  PV absorbs a section of the solar spectrum, the rest is converted to heat

  PV cells have a nominal operating temperature, usually around 250C

  Any increase in temperature above this cause a decrease in efficiency of the PV.

  0.4% per degree for silicon

Heat collected from the PV cells will provide the driving force for membrane distillation

System overview

How does the membrane cope with a fluctuating power supply?

•  Solar intensity is

known to fluctuate

throughout the day,

subsequently so will

the inlet seawater

temperature of the MD

unit.

•  Transient operation

will affect the quantity

and quality of the

distillate produced.

Temperatures from energy system

Output temperatures from the energy system, calculated via a mathematical model

Geometry & Boundary Conditions of 3D Densely Packed Receiver

No Region Boundary condition

1 On top of cells Inflow heat flux as found from numerical model

2 Ambient Ambient temperature of 20-45oC

3 Cell’s surface Surface to ambient radiation and natural convection

4 Sides of cell Heat is conducted through the layers

5 Heat Sink Surface to ambient radiation and convection

2

1

4

3

5

6

2PV receiver components 1: Frame 2: Cover glass 3: Al2O3 ceramic 4: Solar cells 5: Copper plate 6: Aluminium heat sink

5

1

4

3

The distillate flow rate showed a time delayed response to the solar fluctuations, due to the thermal mass of the MD unit

Results from the distillate yield

The distillate quality varied, but always remained well within the guidelines set by the World Health Organisation.

The result show that there is nothing to rule out transient operation of the MD module

Conductivity of the distillate yield

Discussion & Conclusions

  A Membrane distillation module was developed at Heriot Watt

  The system was tested under fluctuating operating conditions, as would be the case were it powered by a solar energy system.

  The quantity of drinking water produced was greatly reduced when the power supply reduced, as is to be expected. A delay was seen in this effect due to thermal mass of the system.

  The quality of the drinking water produced, its conductivity, remained within the safe drinking guidelines set by World Health Organisation, even during fluctuating operation… something not seen in a renewable energy powered membrane system to date!

Thank you for your attention !

Any Questions?