Improved Method of Desalination of Seawater with Electric Power Generation using Solar Energy

Mukund.N final year- department of electrical and electronics engineering, St.Joseph’s college of engineering,

Chennai-119, Tamilnadu, INDIA. itsmukundhere@yahoo.com

Muthu Raman.V final year- department of electrical and electronics engineering, St.Joseph’s college of engineering,

Chennai-119, Tamilnadu, INDIA. muthusa186@gmail.com

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.

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.

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 are-

1. 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.

Fig 1: Block diagram of PPW and EPG

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2011 2nd International Conference on Environmental Science and Development IPCBEE vol.4 (2011) © (2011) IACSIT Press, Singapore

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:- Solar energy + Seawater 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.

Fig 3: Block Diagram of the Solar Heating System

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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. 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. 2.3. Electrodialysis Process: The Electrodialyser is shown in fig 4.

Fig 4: Electrodialyser

Electrodialysis is defined as the transport of ions, from

one solution to another solution through the ion exchange

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, ion-selective 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, 2e- + 2 H2O H2 + 2OH-

At the Anode,

H2O → 2 H+ + ½ O2 (g) + 2e-

This can also be written as:- Condensed Liquid Pure Water +

Brine

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. 2.3.1. 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

Compressed air is sent into the concentration compartment in order to prevent membrane fouling and reduction of concentration polarization. Multi-stage

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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.

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. POWER CALCULATIONS 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

Equipments Specifications Power Consumption

(KWh)Pump-initial

intake 1HP 1.4

PTSC 3 ED 5.3[6]

Pump-Turbine 1HP 2 Others 3

TOTAL 14.7

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

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.

Electrical power generated = 2.943*5 = 14.715 KW

IV. MERITS 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 re-circulating 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.

V. 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.

REFERENCES [1] The chemical composition of seawater homepage [Online].

Available: http://www.seafriends.org.nz/oceano/seawater.htm [2] 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.

[3] 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.

[4] Electrodialysis Membrane Design Calculator homepage. [Online].Available: http://www.ajdesigner.com/phpelectrodialysis /electrodialysis_equation_current.php

[5] Indian institute of science-hydel power-CEDT homepage. [Online] Available: http://www.classle.net/sites/default/files/25006 /student_slides05.pdf

[6] Seawater Desalination homepage.[Online] Available: http://www.pccell.de/appli/desalen.htm

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