UC Berkeley

Advanced Multiple Effect Distillation Processes forNuclear Desalination

Per F. Peterson

Nuclear Engineering Department, University of California, Berkeley

American Nuclear Society Winter MeetingAlbuquerque, NM

November 7th, 2006

Haihua Zhao

Idaho National Laboratory

UC Berkeley

Overview

Introduction to desalination and MultipleEffect Distillation (MED)

The Advanced MED system for coupling toclosed gas Brayton cycles

Economic analysis Conclusions

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Where is desalination needed?

UNEP Water Availability Projection for 2025

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Major desalination technologies

There are four different types of desalination plants in existence: multi-stage flash (MSF), multi-effect distillation (MED), multi-effect vapor compression (MEV), and reverse osmosis (RO).

Currently, a very large desalination plant is 240,000 m3/day (thecapacity of Taweelah A1); to compare, in 2002 waterconsumption by the City of San Diego (population 1,256,000) was800,000 m3/day.

The waste heat from a nuclear power station the same size as thenearby 2,329 MW(e) San Onofre Nuclear Generating Station,could potentially provide a quarter of San Diegos current watersupply if it used a closed Brayton cycle for power conversionwith GOR = 2.5.

2.50.5 4800MWt( ) 3600sec/hr( ) 24hr/day( )

2.4MJ/kg( ) 1000kg/m3( )= 220,000m3/day

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Schematic of a conventional MED system, using steamas a heat source, with four effects stages

Steam(~105C)

CondensedFreshwater

Brine

Boiler

Effect 3 Effect 4Effect 2

Condensate(~105C)

Effect 1 Seawater(~15C)

Seawater

Heat RejectionCondenser

Some basic concepts for MED:Number of effectsGOR: gain output ratioTop brine temperature: LT-MED (90C)

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Combining MED with closed gas cycles - AMED

Seawater(~15C)

Seawater

Intermediatecoolingloop return(~25C)

Intermediatecoolingloop supply(~70C)

CondensedFreshwater

Brine

Boiler

Effect 2 Effect 3 Effect 4Effect 1Heat RejectionCondenser

Closed gas Brayton cycles reject heat across a range oftemperatures Can use this energy without affecting power cycle efficiency

Two desalination cases considered LT-MED system with TBT about 70C HT-MED system with TBT about 120C (small efficiency penalty)

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Combining MED with closed gas cycles - AMED

To achieve the same GOR AMED requires roughly twice as manyeffects as MED. As shown in the following figure, for the same GORand Top Brine Temperature (TBT) the larger number of effectsresults in smaller temperature differences across the heatexchangers, and as a consequence the AMED system will alsorequire approximately 80% more heat exchanger surface area (ameasure of capital cost) to provide the same GOR.

Tin = Tc1

Ts1

Tseawater

Ts2

Ts3Tout = Tc6

Tc4

Tc3

Tc2

Position in heat exchangers

AMED (n=4, GOR=~2.0) Tin = Tc1

Ts1

Tseawater

Ts2

Position in heat exchangers

Conventional MED (n=2, GOR=~2.0)

IntermediateCoolant

Brine

Steam

Brine

Tc5

Seawater

CondensateTs4

Seawater

Condensate

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MED using plate-type heat exchangers

Alfa-Laval MED desalination unit using platetype heat exchangers

Alfa-Laval titanium stampedplates for MED desalination

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AMED using plate-type heat exchangers

Seawater(~15C)

Seawater

Intermediatecoolingloop return(~25C)

Intermediatecoolingloop supply(~70C) Condensed

Freshwater

Brine

Effect 1 Effect 2 Effect 3 Condenser/Cooler

Vacuum

Schematic diagram of flow inside a plate-type AMED system

Compact, high surface density, reduced vacuumvessel size; therefore, potential lower cost

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AMED using plate-type heat exchangers

Schematic gasket configurations for AMED with plate-type heat exchangers

Coolant Evaporation Condensation

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Economic analysis of coupling closed gas cycles withAMED (Multiple reheat AHTR case)

The cost of water (COW) from a MED desalinationplant: Varies significantly with design, size, location, brine water type

and other factors.

For a modern large MED desalination plant, $0.70 per m3water production can be achieved.

The COW includes water plant installation, thermal energycost, capital cost, maintenance cost, electricity cost, and others.

For a MED plant, the electricity cost in COW is only 1% andcan be ignored.

For a tower type MED, the water plant cost is about 37% andthe thermal energy cost is 27% .

For an AMED plant coupling with a closed gas cycle, thethermal energy cost can be zero if the system is optimized.

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Economic Analysis

As a first-order approximation,the MED water plant capitalcost can be assumedproportional to the total heattransfer area.

A function relating relativespecific heat transfer area withthe number of effects and TBTcan be obtained throughmultivariate regression [1].

With the increase of thenumber of effects, more specificheat transfer area is needed forone effect.

With higher TBT, less specificheat transfer area is needed forone effect.

1) Narmine H. Aly, Adel K. El-Fiqi, Thermal Performance of Seawater Desalination Systems, Desalination 158 (2003) 127-142.

Relative specific heat transfer area pereffect as the function of number of effectsand TBT (142C for HT-MED and 86C forLT-MED).

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Net revenues per day for a combined 1200 MWe power and AMEDdesalination plant for a water price of $0.29/m3

LT-MED: 6% net revenue increase with 95,000 m3/day waterHT-MED: 4% net revenue increase with 140,000 m3/day water

Low water price, $0.29 per cubic meter

3.0E+05

3.1E+05

3.2E+05

3.3E+05

3.4E+05

3.5E+05

3.6E+05

3.7E+05

3.8E+05

3.9E+05

4.0E+05

2 3 4 5 6 7 8 9 10 11 12

Number of effects

Net

reven

ues p

er

day, U

S$

HT-MED

LT-MED

Electricity only

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Net revenues per day for a combined 1200 MWe power and AMEDdesalination plant for a water price of $0.5/m3

LT-MED:13% net revenue increases with 110,000 m3/day waterHT-MED:14% net revenue increases with 150,000 m3/day water

Middle water price, $0.5 per cubic meter

3.0E+05

3.1E+05

3.2E+05

3.3E+05

3.4E+05

3.5E+05

3.6E+05

3.7E+05

3.8E+05

3.9E+05

4.0E+05

2 3 4 5 6 7 8 9 10 11 12

Number of effects

Net

reven

ues p

er

day, U

S$ HT-MED

LT-MED

Electricity only

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Net revenues per day for a combined 1200 MWe power and AMEDdesalination plant for a water price of $0.7/m3

LT-MED:21% net revenue increases with 120,000 m3/day waterHT-MED:24% net revenue increases with 170,000 m3/day water

High water price, $0.7 per cubic meter

3.0E+05

3.1E+05

3.2E+05

3.3E+05

3.4E+05

3.5E+05

3.6E+05

3.7E+05

3.8E+05

3.9E+05

4.0E+05

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Number of effects

Net

reven

ues p

er

day, U

S$ HT-MED

LT-MED

Electricity only

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Conclusions

By using an advanced multi-effect distillation (AMED) system,the waste heat from closed gas Brayton cycles could be fullyutilized to desalinate brackish water and seawater withoutaffecting the power cycle thermal efficiency.

For higher water prices, the net revenues from a combinedelectricity and LT-AMED plant, could be as much as 20%greater than the production of electricity alone, without affectingthe electricity efficiency.

Even at relatively low water prices, where the optimal GOR isrelatively small, with an AMED system HTR power stationswould still generate large quantities of desalinated water (90,000m3/day for a 1200 MW(t) station).

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Back-up

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Closed gas Brayton cycles

Closed gas Brayton cycles have an advantage over steam Rankine power cycles becauseclosed Brayton cycles reject heat at substantially greater average temperature.

In contrast to a conventional steam system, the cooling water from a closed Braytoncycle intermediate loop delivers heat across a range of temperatures.

For a turbine inlet temperature of 900C, the net thermal efficiency is 54% for theconfiguration with one compression and intercooling for each reheat and expansionstage. The helium outlet and inlet temperatures in the coolers are 35C/142C.

With two stages of compression and intercooling for each reheat and expansion stage,the net thermal efficiency is 56%. The intercooler helium inlet temperature for this caseis 86C.

G

C

MPG

LP

T T

C

GHP

T

C

R

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If we assume $0.70/m3 COW for a regular MED plant with 14effects and 105C TBT, the specific water cost except for thermalenergy for an AMED system can be estimated by the followingequation:

Economic Analysis

( ) ( )

+

+

++=KK

TBTnp

nrcrrcTBTnc e

eiwieweo

10515.273,8.15.0

)114(

)1(1,

2

2

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The GOR for a modern large MED plant is directly related to the numberof effects and weakly related with TBT. For conventional MED the GOR isa function of the number of effects. The water production rate can then becalculated as:

Economic Analysis

( )( )

31000

15.0,

m

kgh

QnnQ

fg

gor

gorw

=

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Economic Analysis

The net daily desalination revenues from combined AMED andelectricity production can be calculated from the followingequation:

Assuming that the electricity price is $0.04 per kWh and thegeneration cost is $0.03 per kWh, the net electricity earnings perday from a large MCGC power cycle can be calculated by:

The total earning per day for a combined power and MEDdesalination plant then is

( ) ( ) ( )( )TBTncvdaynQvTBTnE eowgorwwew ,,,,, =

( ) ( )03.004.0

=hrkW

dayQEe

( ) ( ) ( )ewewwe

EvTBTnEvTBTnE += ,,,,,,