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THE STUDY ON COMBINED DESALINATION

AND POWER GENERATION SYSTEM USING

SOLAR POND

Xiaolan Ge, Rengshao Liu2, Zhanfang Jiao, Yuchun ZhaoThe School of Mechanical Engineering , Jiangsu University

Zhenjiang, [email protected], [email protected]

Abtr- new combined desalination and power generation

system using solar pond was presented in this paper and the

principle of the new system was introduced. A new discconvergent-divergent nozzle was desgined and used in the simple

reaction turbine. According to theoretical analysis of the

thermodynamics process, the power production and fresh water

rate were obtained at tapical parameters. To prove the feasibility

of the principle a small experimental rig was set. Aer simple

experiment it is found that the actual fresh water rate is a little

higher than the theoretical value, but the actual power is too

much lower, which means that the hot brine couldn't expanded

insuciently in the nozzle and the ow passage of nozzle needs to

be improved farther to increase the power production.Also the

other inuencing factors should be considered such as the

condenser efciency, drag losses,chamber pressure and etc.

Kywrdr pd; twp w; d vrtdvrt zz; dt; isentropic expansion

I. NTRODUCTON

It is known that the world is now facing three serious

problems: fresh water shortage, energy crisis and degradationof soil. Although the people has invented the desalinationtechnology to solve the water crisis, the traditional methodsconsume huge of electricity energy produced by fossil elhf,lg w hg um b b hful h l hg whh h d h vl

S, nding solutions which are sustainable andenvironment kind is the main task for us at present.

A w lh d l gy, h l gy

lg h h The paper will present a novel system powered entirely by solar energy to produce both fresh waterand electrical power from saline groundwater or seawater. It iscalled the CDP system which stands for Combined

Desalination and Power Generation. Historically, most previous work on desalination powered by sustainable energyhas focussed on solar thermal collectors (parabolic solarcollector) coupled to multistage ash or multiple effectevaporation systems with temperature higher than 300°C toobtain high eciency in power generation [1]. A number of workers in Israel have investigated combined desalination and

power production using solar thermal collectors to producewater vapour from hot brine and then passing the vapourthrough an expander to produce power and in tu fresh wateras the vapour is cooled and condensed, but as for the lowtemperature resource through non-parabolic solar collector,

78--6284-752-8//$26.00 ©20 IEEE

119

there is still lack of useful information, especially combinedwith reaction turbine [2].

The other problems in combined desalination and powergeneration concept to be highlighted are the applicability of the Hero Turbine for energy conversion from low quality, two phase inlet uids. Although the Hero' turbine was investigatedextensively during 1973 to 1980 aer the Middle East oil

crisis, especially by Laurence Livermore Laboratory and JetPropulsion Laboratory in USA, most research work wasfocused on geothermal application because there is enoughgeothermal resource in American and relatively it is highquality energy due to higher temperature and pressurecomparing to energy from the solar pond and solar collector[3-4]. It isn't so easy to nd the same system using lowtemperature and pressure solar energy at present.

In this paper, the idea of combined desalination and powergeneration was conformed by use of low temperature heatsource coming from soalr pond in this system. The systemneedn't to produce high temperatre and pressure vapor to rnthe reaction turbine. So it decreases the running cost of thewhole device. Furthermore low temperature industrial waste

water can be employed as the work uid in the system.f thistechnique can be applied in practice,it is a good choice tosolve the energy crisis and water shortage simultaneously.

II. THE PRINCIPLE OF COMBINED DESALINATIONAND POWER GENERATION SYSTEM

In the CDP system, the tasks of fresh water production and power generation are performed using solar energy with zerogreenhouse gas emissions. The principle of the CDP system isrelatively simple and the principle schematic of system isshown in Fig..

Before rnning the system, the air of vacuum chamber is

drew off by the vacuum pump or the ejector and the chamberis maintained to be a constant low pressure state. The lowtemperature brine is heated to be a scope of 50�80 througha heat exchanger in solar pond. Then the hot brine is suckedinto a disc covergen-divergent nozzle through a hollow shain the reaction turbine. As a result of the difference betweenatmospheric pressure outside and that in of the vacuumchamber, the hot brine in the nozzle will expand and vaporizeinto a mixture of water droplets and vapor. The vapor rises inthe chamber and is condensed into fresh water by thecondenser at the top through which a stream of cooling watercontinually ows. At the same time, the mixture exiting thenozzle at high velocity will exert a couple of reaction forces

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on the nozzle. The reaction forces can create a torque thatmakes the hollow sha tu and then the reaction turbine will

drive the generator rotating at high velocity and the nctionof power generation is realized. At last, the collected freshwater by container and the residual concentrated brine arepumped out of the chamber respectively. The simple reactionturbine and the disc nozzle are shown in Fig.2 and Fig.3resectively.

oowr ie t �

� wr outlet t <:-

Vau cher t l t Va

coru- 6 So Pn

l=6_1� -�IBre Frewr

Figure 1. Schematic showing the principle of the CDP system

igu 2 Th l ction tubin

igu 3 Th disc convgnt-divgnt

Spp y f J Uy (N9JDG69)

0

III. THE HEAT SUPPLY SYSTEM

The focus of design for the CDP system is that the systemis entirely based on solar thermal energy, with particularemphasis on low temperature heat resources. Solar pond is asalty water pond with concentration gradient. Its simplestructure, easy maintenance and low cost are very suitable forlarge-scale using [5]. For the common desalination and

distillation device, the work uid temperature is about 40-50°C. The multiple-effect evaporation system can also work well

in the temperature scope of 70-80 °Cand the solar pond can

absolutely satisfy the operating requirment. Therefore, choosing solar pond as the heat supply source is a goodconsideration for the CDP system. For no-convection type of solar pond, the up convection zoon is ll of clean water andthe temperature of water is approximate to ambienttemperature. So the clean water can be the work uid in thecondenser, which is a important reason to choose solar pond.

IV. THE SIMPLE REACTION TURBINE ANDCONVERGENT- DIVERGENT NOZZLE

The reaction trbine is the main part in the system and itcan make the heat from the uid transform into themechanical energy. In fact, the turbine in the CDP system canbe called a reaction two-phase turbine and realizes thenction of synchronal desalination and power generation bymeans of the expansion and evaporation of hot brine in theconvergent-divergent nozzle.

The most important part in reaction trbine is theconvergent-divergent nozzle. It not only affects the recoveryrate of esh water, but also inuences the power generation.hough the research on the internal ow characteristics innozzle for the single-phase gas has been relative mature, theproperties of the two-phase uid through the nozzle have notbeen studied thoroughly. In the last 50 years, there have beennumerous models proposed for calculation of the critical owrate of a two-phase one-component ow [6]. Some modelshave no theoretical support and are based solely on semiempirical formulae [7]. The diculty in theoretical treatmentsis that the ashing process is very complicated. The traditionalmass, momentum and energy conservation equations are morecomplex, since they must include terms relating to theinteraction between the two phases, uid and gas properties, and the friction of the wall of the nozzle [8].

Therefore, the design for two-phase nozzle has to be basedon the present two-phase theory and repeated experiments. Toreduce the energy loss coming from the shock wave when thehot brine expanding in the divergent part of the nozzle and

improve the power production, a new disc convergentdivergent nozzle is used in the experiment and its dimensionof divergent part is much longer common Laval nozzle, whichmay make the hot brine expansion more sucient.

V. THE THEORETICAL ANALYSIS OF THECOMBINED DESALINATION AND POWER SYSTEM

A The possibly suitable to-phase fow model in the system

Choosing the suitable two-phase model is vital beforeanalyzing the theoretical process for theCDP system. The

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model choosed must be convenient to simplify the wholeanalysis process and the caculation of typical parameters forthe critical ow in the throat of nozzle. Basically, thetheoretical modeling of two-phase ow can be divided intotwo main categories: Theories that assume thermodynamicequilibrium throughout the expansions, which can be furthersubdivided into homogeneous theories in which the vapor isassumed to be mixed with liquid homogeneously and there is

enough time for the two phase transfer in mass, momentumand energy; and non-homogeneous theories in which the two phases exist separately and non-equilibrium theories [9-10].

The other categor comprises non-equilibrium theories.

In the present work homogeneous theories based onthermodynamic equilibrium are adopted because this model isrelatively simple while it can still provide an accurate andsatisfactory predictions in the application of simple Hero'sturbine with steam pressure of 242.3 kPa, for the pressurelower than 101.3 kPa, Henr claimed that it should beapplicable but no test was reported. Many practicalapplications have used it as the basic model, for example, incalculations relating to the water cooling systems of nuclear power plants[-13].

The Isentropic-Homogenous Expansion(IHE) Model is based on the following assumptions:

1 The velocities of the two phase are equal.2 Thermal equilibrium exists.3) The expansion in the nozzle is isentropic.4 Data on the thermodynamic properties correspond to

those of a static , equilibrium, two-phase system with planeinterface[14].

B The theoretical analysis based on the THE model for anisenttropic process in the reaction turbine.

Theoretical analysis of the CDP system will stem from

the assumption of isentropic expansion of the nozzle in thissection. The performance of the whole system consideringmechanical eciency, generator eciency and drag forceacting on the surface of nozzle will be investigated in the test.

Setting the typical parameters:

The brine mass rate ms =O.kg/s.

The cool brine temperature t =25°.

The hot brine temperature aer being heated tI 75°.

The vacuum chamber pressure Pc =3 .17kPa.

The vacuum chamber temperature (maintained by the

condenser and is equal to the cooling temperature in

theory) tc =25°.

The ambient temperature is 25°.

The physical quantities in different position points can beseen in Table I. The thermodynamic cycle can be analyzed asfollows referring to the Fig.and the munoy hof hmodynmi y in CD ymhon in FigA .

2

TABLE 1. THE PHYSICAL QUANTITIES IN DIFFERENT POSITIONPOITS

i essue

P kI 0

-----

5

4 75 7

0

i Qiieperaure ropy

kJ/kg·K))

75 055

75 055

75 055

25 05525 074

25 075

L e L< Pbe

alpyh kJ/kg9

9

9

95049

0495

7YC lOl.3a

7YC 38Ise n

4 Ta

E [( K)]

ryex

0

0

0

00790

0

Figure 4 The Temperatre-Entropy graph of thermodynamic cycle

c -: The cold brine is heated by the solar pondfrom 25°t75° and this process can be treated as constant pressure heating. The state of hot brine is in unsaturation. Theheat needed is the difference of the enthalpy between points 6

and 1.

Q=�S(hl -h6)0.1 X (313.93 104.89)20.904kW

c -: The hot brine is drawn into the hollow shain the turbine for the pressure difference and ows to the inletof the disc nozzle (point 2). The calculated formula of pressurein point 2 for rotating homogeneous uid is introduced here:

1=+r

w

(2)2

Here, p- the density of hot brine (kg/m ), r - the distance

between the rotor centre and inlet of nozzle (m), O - theangular velocity of the disc nozzle(ra/s).

ccording to Formula (2), it is seen that the hot brine pressure of nozzle inlet will rise as a result of the highrotational speed, but the entropy of hot brine keeps constant.

c-: The sub-cooled brine goes through theconvergent part of the nozzles, and accelerates, decreasing in pressure to that corresponding to the local saturatedtemperature of the input solution (assumed to be 75°) . Thehot brine at throat of nozzle (point 3) turns into saturated statehere. ctually, the local saturation temperature in the vacuumchamber is a little higher than the theoretical assumption (25

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°C), which is relation to the cooling ability of the condenser.

In the test, this temperature was 2-4 °Cabove 25°C.

3-: Saturated brine begins to vaporize after thethroat and continues to vaporize and to be accelerated as ittravels through the divergent part of the nozzle. At last themixture of brine and vapor ejecting at high speed from theoutlet of nozzle (point 4). The process can be treated as anisentropic process if the energy losses due to the ineciency

of the nozzles and bearings, and the reaction of the mixture of vapor and droplets with the surrounding enironment in thechamb�r are neglected. The process3-4 is the actalexpann.

From TableI, it is seen that the enthalpy of brine hasdecreased in the process from point 1 to point 4. If the hot

brine keeps isentropic expansion in the nozzle, the reducedenthalpy should transform into the mechanical energy of mixture we need. If all the mechanical energy can drive thegenerator to produce power, the maximal power generation intheory can be obtained by the following formula:

p = �S(hl - h4) (3)

=0.1 313.93298.15 =1.578kW

Therefore the gross eciency of power generation is got :

rp P /Q X 100%

=1.57820.904 =7.55%

(4)

According to Table I, it also can be seen that the dryness inoutlet of nozzle is 0.0791, which means that the mass

percentage of vapor is 7.91% in mixture. So the maximal freshwater production is obtained:

m=ms,x4

(5)=0.1O.0791=0.00791kg/s

-5: The vapor is condensed by the condenser, and

then collected in the tray and stored in the fresh water tank.The remaining droplets of liquid ow into the brine tank. Theheat taken out by the condenser is the difference in theenthalpy between points 4 and 5.

Qc(6)

=0.1 298.15104.89 = 19.326kW

5-6: The fresh water collected and remaining brineare pumped out of the tanks respectively. The mechanicalenergy required for this pumping is the difference of enthalpy between points 5 and 6.

M=qm(h6 -hJ(7)

O. (10.91510.89)=0.0025 kW

At the same hot brine mass rate(O.kg/s), the fresh water production and power generation in theory can be obtained under the condition of different cooling temperature and hot brine temperature, as is shown in Table II. It is found that thefresh water production and power generation increase with thedecrease of cooling temperature and increase of hot brinetemperature. Therefore, the condenser is ver important forthe combined system to keep contant low temperature state.

122

ALE II. E RES AER RODUION AND OERRODUION IN EORY

T H B T e C Cl 75 85

T Freh water Power Freh water C production production production

(kg/h) (kW) (kg/h)25 28.476 1.578 33.69627.5 27.252 1.401 32.544

30 25.992 1.270 31.23032.5 24.696 1.131 30.09635 23.4 1.001 28.836VI THE EXPERIMENT AND ANALYSIS

Powerproduction

(kW)2.2332.046

1.8651.6961.536

In order to prove the feasibility of the new principle forDP system, a small simple rig was set and is shown in Fig 5.In the test, the solar pond was replaced by the electricity toheat the cold brine. The condenser was composed of 26m longand diameter of 16 copper tube. The rotor of reactionturbine was connected to a 90W D generator for power producing. The chamber pressure was kept at 4.25kPacorresponding to the temperature of 30°C by running coolingwater owing in condenser. The inner diameter of hollowsha in trbine was 14mm. The mass rate of brine was kept atO.kg/s.

5.

Through repeated testes covering a temperature range of 40 to 80°C. The feasibility of the DP concept was conrmedsuccessfully and the useful results on the production of fresh

water and power generation above the tests was shown in Fig6 and Fig 7.

It is can be seen from the above two gures, the percentage of fresh water in the tests varies linearly with theincreasing temperatre of hot brine as expected, and in someinstances higher than that predicted in theory. While the power produced in tests is much less than that of theoretical prediction, though the actual power generation increases withthe increasing of hot brine temperature as predicated.

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9r-:===========-----1. The eorecfreshw ernte I /8

;. The�clfresh w�ter r e /�'y�

0//� l

SC 6 1 8

Ho be tm pene C)

Figure 6. The recovery rates for fresh water compared with the theoretical prediction

D

1C,: 1.s"u;o 1.b

ID

80

.1 00J

Q2

OD

2,*:;.-_/ 1

=- 5 6

Hot me tepe e C)

3

8

6

'

8

6

U

8

28

Figure 7. The actual power generation compared with the theoretical prediction

�9.�

��

�

��a

Many reasons result to the low power generation, but themain reason lies in that the insucient expansion of hot brinein the nozzle casues the shock wave and then leads to lowefciency of the nozzle. In other words it is reasoned that mostof the phase change between vapor and liquid and the resultingexpansion takes place outside of the disc nozzle, so the desiredmomentm isn't transferred to the rotor of trbine and causeslots of mechanical energy losses in the process -4

In fact the successful design of nozzle should make sure

that the nozzle always work at critical mass ow at throat andthere is no any shock wave in or outside the nozzle ,that meansthe pressure at exit plane at the end of the nozzle should beequal to the chamber pressure exactly,so there is no any overexpansion or under expansion happening. But there is noreliable method to design the two phase nozzle. So in order to utmostly increase the power generation, the dimensions of nozzle need to be improved rther. Of course ,the otherinuencing factors should be considered such as the condensereciency, drag losses,chamber pressure and will not beintroduced detailedly as a result of the pages limit.

23

VII. CONCLUSIONS AND FURTURE WOKS

In the paper, the new concept of combined desalinationand power generation has been proved to be feasible, and that itmay help solve the challenges of water shortage and salinationcurrently encountered in many arid areas. According to anumber of tests, it is found that the fresh water production isapproximate to that of theoretical prediction, but the powergeneration is much lower compared with predicated value.

Therefore further work in future that needs to beconducted includes:

1 The ow passage of nozzle needed to be improvedfarther to increase the power production.

2) The new condenser need be designed to improve thecooling eciency.

3 The actual solar pond should be constrcted as the heatsupply resource.

4) The economical analysis about the system should bedone compared with other combined systems.

CNOWLEDGMENT

The authors would like to take this opportunity to thank

the nancial support by Mechanical Engineering School of Jiangsu University. Also, we would like to give our thanks toMechatronics plant in Jiangsu University who helped us toconstrct the small experimental rig.

EFERENCES

[] Kalogirou S.A. Seawater desalination using renewable energy sources.Progress in energy and combustion science 2005; 31:242-281.

[2] Sagie .D, Mandelberg, E., commercial scale solar-powered desalination,http://www.rotemi.co.il/solar powered desalination/ Viewed on 3 July2005.

[3] Comfort I, W.J., Modeling the Performance of a Two-Phase TurbineUsing Numerical Methods and the Results of Nozzle, Static Cascade, And Wind age Experiment, ASME, and June 21, 1978.

[4] Elliott, D. G, March 1982, Theory and Tests of Two-Phase Turbine, JPL81-105, Califoia Institute of Technology, Pasadena, Califoia, USA.

[5] Zangrando, F., 1986, Hydrodynamics and Thermodynamics of SolarPonds, SERI/TP-252-3088, and UC Category: 62, DE97001124.

[6] Dauria, F. and Vigni, P., Two-phase critical ow models, CSNI Report No.49 (1980), Roma.

[7] Xu Jijun, Boiling heat transfer and gas-liquids two-phase ow (Beijing,China: Nulear Energy Publishing Press, 1996).

[8] Xu, L.J.,1997, Critical ow in convergent-divergent nozzles with cavitynucleation model, Experimental Thermal and Fluid Science, 14:166-173.

[9] Starkman, E.S., Expansion of a Very Low Quality Two-Phase Fluidthrough a Convergent-Divergent Nozzle, Joual of Basic Engineering,June, 1964.

[10] Schrock, V.E., Starkman, E.S. and Brown, R.A.,Flashing Flow of

Initially Sub-Cooled Water in Convergent-Divergent Nozzles, Joual of Heat Transfer, May 1977, Vo.99 page 263-268.

[] Len, Taylor, Preliminary Design and Calulation on the Mill Rotor,Unpublished Investigation, 2005.

[12] Henry, R. E., 1968, A Stdy of One- and Two-Component, Two-PhaseCritical ows at Low Qualities, ANL-7430

[13] Henry, R.E., the Two-Phase Critical Discharge of Initially Satrated orSub-Cooled Liquid, Nuclear Science and Engineering, Vol. 41, Page336-342, 1970.

[14] Starkman, E.S., Expansion of a Very Low Quality Two-Phase Fluidthrough a Convergent-Divergent Nozzle, Joual of Basic Engineering,June, 1964.

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