Download 2011 2nd International Conference on Environmental Science and Development

2011 2nd International Conference on Environmental Science and Development
IPCBEE vol.4 (2011) © (2011) IACSIT Press, Singapore
Improved Method of Desalination of Seawater with Electric Power Generation
using Solar Energy
Mukund.N
Muthu Raman.V
final year- department of electrical and electronics
engineering, St.Joseph’s college of engineering,
Chennai-119, Tamilnadu, INDIA.
[email protected]
final year- department of electrical and electronics
engineering, St.Joseph’s college of engineering,
Chennai-119, Tamilnadu, INDIA.
[email protected]
Abstract—In many places, the potable water is scarce and it is
found that only 0.3% of the total water content present on
earth is alone fit for drinking. To eliminate water crisis, one
must focus on the desalination process. The proposed
technique described below uses solar heating, electrodialysis
process and micro-turbines, to get the outputs – Pure Potable
Water (PPW), Electric power and Brine solution. The Electric
Power Generated (EPG) during this technique is utilized to
carry out the electrodialysis process. Hence, the problems
faced by the existing desalination plants like high power
consumption and feed recovery is eliminated in this proposed
technique.
II.
PRODUCTION OF POTABLE WATER AND ELECTRIC
POWER GENERATION
The different stages in this improved method of
desalination of seawater with electric power generation using
solar energy are1. Process of Evaporation using Solar Energy
2. Condensation
3. Electrodialysis process
4. Electric Power Generation
The complete block diagram and plant layout of this
improved method is shown in fig.1 and fig.2 respectively.
Keywords- Power Shortage,Water Crisis, Brine, EPG, PPW,
Solar energy,Electrodialysis
I.
INTRODUCTION
Desalination method is the best way to meet pure water
demands and electric power requirements in the future as
water and electricity are very much essential for sustenance.
India being a tropical country and a peninsula, has got an
immense potential to use seawater and solar energy as it is
surrounded by oceans on three sides .The input which is used
in this method is Indian seawater whose salinity content
varies from 32 to 36[1] parts per thousand (ppt). According to
World Health Organization (WHO) drinking water standards,
the Total Dissolved Solids (TDS) value for pure water must
be less than 0.5 ppt. So we must reduce the TDS value from
36ppt to 0.5ppt for making it suitable to drink. The presently
existing desalination plants have high power consumption
and for a high TDS value (36 ppt), electrodialysis process is
found to be not suitable. To overcome this problem, the
proposed technique uses both solar heating and
electrodialysis method for converting the seawater into
potable water with electric power generation. In this
technique, the power consumed by the plant for converting
seawater into potable water, is generated by the power
generation unit in the plant itself.
Fig 1: Block diagram of PPW and EPG
158
Fig 2: Complete Plant Layout of Improved Method of Desalination of Seawater with Electric Power Generation using Solar Energy
2.1 Process of Evaporation using Solar Energy:Solar energy is trapped by using parabolic trough solar
collectors (PTSC) in this technique. A pipe is placed along
the focal line of the parabolic trough in such a way that all
the solar rays are focused and concentrated along the focal
line. This pipe is surrounded by glass, forming an outer layer.
The seawater is pumped into the solar heating system by
opening the control valve CV0, closing CV1 and is allowed
to pass through the pipe along the focal line to make contact
with it. By doing this, the sea water gets easily evaporated
into steam after attaining a very high temperature of 220°C at
a pressure of 50 bar. The control valve CV0 and CV1 are
closed now till all the water gets converted to steam. Once all
the seawater gets converted into steam, the control valves
CV1 is opened and CV0 is closed such that the steam is
allowed to enter into a saturated steam chamber.
The equation is given as:Steam
Heat 220°C, 50 bar
2.1.1 Design of Solar Heating System
The design of the solar heating system[2] is represented in
the form of a block diagram in fig 3.
The trough rotates about North-South axis and is driven by a
A.C motor. The tracking system comprises of a variable
speed drive and a Programmable Logic Control (PLC)
which sends commands to the motor. The pump controls the
amount of water to be flown inside the parabolic trough
collectors. The Flow sensor senses the amount of water
present inside the parabolic trough collector and if it
exceeds a permissible limit, it sends signal to the input
operator to control the variable speed drive to stop the
pumping action and as a result the pump stops. This control
signal also controls the action of the control valves CV0 and
CV1.
Solar energy + Seawater
Fig 3: Block Diagram of the Solar Heating System
159
membrane under the applied electric potential difference.
When a salt solution is under the influence of an electric field,
the charge carriers in the solution come into motion. This
means that the negatively charged anions migrate towards
the anode and the positively charged cations towards the
cathode. In order to separate salts from a solution, ionselective membranes, through which only one type of ion can
permeate in an ideal case, are arranged in the solution
perpendicular to the electric field. Thus negatively charged
particles (anions) can pass through an anion exchange
membrane on their way to the anode but are selectively
retained by the upstream cation exchange membrane. This
separation stage results in the concentration of electrolytes in
the concentrate loop which is the Brine Solution and a
depletion of charge carriers in the diluate loop which is pure
Water.
The equations in the Electrodialysis process is given by:At the Cathode,
H2 + 2OH2e- + 2 H2O
The solar radiation absorbed[3] by the receiver is, (on
hourly basis)
Qabs=Aa*ηopt*Idn
Where,
ηopt – optical efficiency,
Idn – direct incident solar radiation.
The useful energy output of a PTSC is,
Qout=Qabs - Qthermalloss
Where,
Qthermalloss – thermal loss value.
PTSC efficiency is given by,
η= (qsolabs-qthermalloss)/Idn*Aa
In the design of PTSC, the above factors are to be
considered to get the desired value. PTSC has thermal loss
due to conduction, convection and radiation. Thermal loss
will occur in the pipe surrounding the glass as it is placed
along the focal line of the parabola. This will exist because
of the temperature difference between the receiver and
surrounding. The glass should be coated with blackened
nickel for intense heating purpose.
At the Anode,
H2O → 2 H+ + ½ O2 (g) + 2eThis can also be written as:Condensed Liquid
Brine
2.2. Condensation:
The condenser is used for condensation to take place and
the steam is converted into liquid. The condenser’s
outermost body and the inner linings are made up of stainless
steel so as to prevent from corrosion and is also made up of
carbon steel to provide the necessary rigidity to the body. By
opening the control valve CV2 and closing CV1, the
condensed liquid is then moved to the electrodialysis
chamber (electrodialyser) for electrodialysis to take place.
Pure Water +
After Electrodialysis, pure water is drawn for
consumption and is given to the consumers by opening the
control valve CV4 and closing the valve CV3. The remaining
portion of pure water is given to the power generation unit
by closing the control valve CV4 and opening CV3.
The Brine which is obtained from the other end of the
electrodialyser can be given to the industries if the TDS
value is high (1.5 to 2 times the initial value). This can be
used as a good pickling agent and a preservative to preserve
vegetables, fish and meat. If the TDS value of the obtained
brine is low, it is re-circulated back to the same system by
opening the control valve CV5.The used water which is
obtained after EPG can be re-used again by sending it back
to the system by opening the control valve CV5.
2.3. Electrodialysis Process:
The Electrodialyser is shown in fig 4.
2.3.1. Design Of Electrodialyser:[4]
The current flow in the electrodialyser can be calculated
by,
I = (FNQE1) / (nE2)
Where,
F - Faraday’s constant= 96487(coulomb/g-equivalent)
N - Solution normality (g-equivalent/liter)
Q - Flow rate (m3/s)
E1- Removal efficiency
n - Number of cells
E2- Current efficiency
Fig 4: Electrodialyser
Compressed air is sent into the concentration
compartment in order to prevent membrane fouling and
reduction of concentration polarization. Multi-stage
Electrodialysis is defined as the transport of ions, from
one solution to another solution through the ion exchange
160
If we increase the height and flow rate for greater power
generation, the initial investment for construction will also be
increased and space factor gets ultimately affected. So, we
have extended the number of turbines to 5 with a minimum
height of 15 m and 40 liters per second.
operation, counter flow mode operation, stack design in the
electrodialyzer can be used to eliminate ohmic losses.
2.4. Electric Power Generation Unit:
The pure water after electrodialysis process is allowed to
enter into the Power Generation Unit by opening the control
valve CV3 and closing the control valve CV4. This water is
then forced to strike the blades of the turbine at a very high
pressure through a nozzle so that the shaft of turbine rotates
along with the shaft of the A.C. generator to produce
electricity. This resembles the operation in a hydel power
plant.
Electrical power generated = 2.943*5
= 14.715 KW
IV.
Though the initial investment (installation of PTSC, ED,
Turbine) done on this technique is high, we are able to get
three useful products – Pure Potable Water, Electric power
and Brine solution all in a single plant simultaneously with
lesser power consumption. We are also able to recover and
re-use the water from the power generation unit by recirculating it back to the same system. By doing this we are
able to get back maximum amount of useful products as
output for the same input without any wastage. So this
technique highlights the importance of recycling the used
water and eliminates the possibility of polluting the aquatic
life.
The hydro power equations[5] can be written as:
Hydro power equations = 9.81*net head*net flow rate
where,
Net head = height of water flow from the ground in meter
Net flow rate = volume of water flowing in cubic
meter/sec
Electrical power= (0.5-0.7%) of hydro power
This is the electrical power generated in the Power
Generation Unit.
III.
V.
The power consumed (for example, by taking 1000liters
of sea water with TDS range of 32 to 36ppt) in this proposed
technique is formulated below:
TABLE 1: AMOUNT OF POWER CONSUMED IN THIS PROCESS
Specifications
Power
Consumption
(KWh)
Pump-initial
intake
PTSC
ED
Pump-Turbine
Others
1HP
1.4
1HP
3
5.3[6]
2
3
14.7
TOTAL
CONCLUSION
A new method has been proposed here for generating
power, pure potable water and brine. The salt which is
deposited in the pipe of PTSC in small amounts can be
removed later. The electric power generated in this proposed
technique can be given for electrodialysis to take place and
hence we are not dependent on any other external supply. In
this method, the TDS value of the input sea water is reduced
from 36ppt to 0.4ppt by which we are able to meet the WHO
drinking water standards.
POWER CALCULATIONS
Equipments
MERITS
REFERENCES
[1]
[2]
[3]
The power generated in the EPG unit is given below:
Net head (h) = 15 m
Flow rate (q) = 0.04 m3/s
Hydro-power= 9.81*h*q
= 9.81*15*0.04
= 5.886 KW/turbine
Electrical power = 0.5*hydro-power
= 0.5*5.886
= 2.943 KW/turbine
[4]
[5]
[6]
161
The chemical composition of seawater homepage [Online].
Available: http://www.seafriends.org.nz/oceano/seawater.htm
MJ Brooks, Mills, T M Harms, Department of Mechanical
Engineering, University of Stellenbosch, “Performance of a parabolic
trough solar collector”, Journal of Energy in Southern Africa ,Volume
17 No 3,August 2006.
Ming Qu, David H .Archer, Sophie V. Masson “A linear parabolic
trough solar collector performance model” in Proc ICEBO2006,
Shenzen, China, Vol.VIII-3-3.
Electrodialysis
Membrane
Design
Calculator
homepage.
[Online].Available:
http://www.ajdesigner.com/phpelectrodialysis
/electrodialysis_equation_current.php
Indian institute of science-hydel power-CEDT homepage. [Online]
Available:
http://www.classle.net/sites/default/files/25006
/student_slides05.pdf
Seawater
Desalination
homepage.[Online]
Available:
http://www.pccell.de/appli/desalen.htm