Cogeneration approach for near shore internal combustion power plants applied to seawater...

Cogeneration approach for near shore internalcombustion power plants applied to seawater desalination

T.C. Hung a,*, M.S. Shai a, B.S. Pei b

a Department of Mechanical Engineering, I-Shou University, 1, Section 1, Hsueh-Cheng Road, Ta-Hsu Hsiang,

Kaohsiung County 840, Taiwan, ROCb Department of Engineering and System Sciences, Tsing-Hua University, Taiwan, ROC

Received 8 February 2002; accepted 18 May 2002

Abstract

The present study utilizes the waste heat streams, jacket water and exhaust gas from a Diesel engine as

the heat source for desalination of seawater. The seawater is preheated to a saturated state, and then,

throttling and heat exchange processes are alternately employed for generation of fresh water. The exit

brine is eventually crystallized to salt via the wind. In the evaluation, the temperature differences among the

stages of the evaporator significantly influence the generation rate of fresh water. Accompanying the use ofplastic heat exchangers, the brine related dirt problem could be avoided. The appropriate arrangement of

the waste heat utilization could not only omit installation of the warm water discharge system but also

prevent damage to the underwater ecology. The study successfully shows the feasibility of application of

waste heat from combustion engines in the desalination of seawater.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Waste heat stream; Desalination; Brine; Combustion engines

1. Introduction

Since irreversibility in energy conversion is unavoidable by the second law of thermodynamics,a great fraction of energy must be exhausted via the form of waste heat. Usually, it is about 40%conversion efficiency from fuel to work for a Diesel engine. The fraction of heat exhaustedthrough the flue gas and jacket water could be as high as 55–60%. In general, the internalcombustion Diesel engine has a medium temperature of the exhaust gas stream with temperaturearound 400 �C. Though this temperature is not good enough to be directly employed as the heat

Energy Conversion and Management 44 (2003) 1259–1273www.elsevier.com/locate/enconman

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E-mail address: [email protected] (T.C. Hung).

0196-8904/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

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source for a bottoming steam cycle, its quality in temperature is relatively good for use as the heatsource for desalination.Bourouni et al. [1] theoretically and experimentally studied the heat transfer of air–water–vapor

mixtures in a desalination plant using the aero-evapo-condensation process. This process, con-sisting of a polypropylene falling film evaporator and condenser, showed promise for cooling anddesalting geothermal water in areas where geothermal waters are available.A modular type simulation software, EVAPOLUND, was discussed and simulated by Jernqvist

et al. [2]. This program is a flexible tool for the design and evaluation of most types of ther-mal desalination processes. An abundant database for the physical properties of seawater andcorrelations of the heat transfer coefficient for different surfaces and flow regimes is included.Based on a well developed set of materials and energy balance equations, as well as correlationsfor evaporation in physical properties, heat transfer coefficients, and thermodynamic theory.Ettouney and Eldessouky [3] have developed a computer package, which includes models forsystems of single effect evaporation, multi-stage flash (MSF) evaporation and multi-effect evap-oration.

Nomenclature

Cp specific heat (kJ/kgK)_EEcon water heat released from condensate water (kW)_EEjacket heat released from jacket water (kW)_EET;gas total heat released from flue gas (kW)_EEH;gas recoverable heat from high temperature flue gas (kW)_EEL;gas recoverable heat from low temperature flue gas (kW)hf;Tmax

specific enthalpy of saturated liquid at Tmax (kJ/kg)hf;Tmax�nDT specific enthalpy of saturated liquid at Tmax � nDT (kJ/kg)hfg;Tmax�nDT latent heat at Tmax � nDT (kJ/kg)_MM incoming flow rate of seawater (kg/s)_mm total production rate of fresh water (kg/s)_mmn steam generated in nth stage of evaporator (kg/s)Si, So inlet and exit salinity of seawaterTatm atmospheric temperature (�C)Tdp dew point temperature (�C)Tgas exh exhaust gas temperature of engine (�C)Tgas;min lowest temperature of flue gas at exit of low temperature waste heat boiler (�C)Tjacket;in temperature of jacket water at inlet of preheater (�C)TSW;env environmental seawater temperature (�C)TSW;max highest seawater temperature in system (�C)TSW;pre seawater temperature at exit of preheater (�C)DT fluid temperature difference between two stages (�C)xn dryness at nth stage of evaporatorgpreheat utilization fraction of heat of jacket water

1260 T.C. Hung et al. / Energy Conversion and Management 44 (2003) 1259–1273

Cogeneration applied to thermal seawater desalination plants should be very efficient and re-duce the cost of water to a level more competitive than the reverse osmosis technique. Degunz-bourg and Larger [4] presented an original concept of cogeneration, relating a gas turbine to amulti-effect desalination unit and an absorption heat pump. They concluded that the estimatedcost of distilled water at the outlet of the plant is very attractive. Safi and Korchani [5] investi-gated the efficiency of two desalination low temperature technologies: MSF and multi-effectdistillation (MED) in dual purpose power/desalination plants based on gas turbines. A MEDdesalination plant is used to recover part of the heat from the turbine exhaust gases and permitsreaching attractive costs. Other applications of waste heat from cogeneration also reflect lowerproduction costs and better overall economics than other seawater desalination processes [6].In the study of Milow and Zarza [7], a 14 effect MED solar energy driven desalination system

hooked to a field of solar parabolic trough collectors was evaluated and showed a high reliability.The technical and economic feasibility of absorption heat pumps for seawater desalination pro-cesses was proven. Since a large quantity of heat is lost in the condensers of steam power plants,Slesarenko [8] arranged an absorption heat pump to couple with a low temperature vacuumdesalting unit and has shown an interesting design of desalination system at power stations.Dvornikov [9] also implemented the feasibility study for a combined power and MED desali-nation plant.Being a sample case herein, Peng-Hu County of Taiwan is an island with population of about

70 thousand. It is not economically feasible to establish a steam cycle for power demand on theisland. Based on the conditions of energy resources and fuel-to-power price structure in Taiwan,an internal combustion Diesel engine could be the best choice. The plant is composed of 12 setsof 10 MWe Diesel engines. The shortage of fresh water is always a headache for this area. Thepresent study plans to utilize the waste heat streams from the Diesel engines, jacket water andexhaust gas as the heat source in seawater desalination. The jacket water has the lower temper-ature and is used to preheat. The exhaust gas with higher temperature is then introduced to heatthis preheated seawater to a saturated state. Thereafter, throttling and heat exchange processesare alternately employed for generation of fresh water. In the present study, the brine is notexhausted to the off shore ocean but is further dried to be the by-product salt or considered to beemployed for the CO2 reduction purpose. The exit concentrated brine from the last stage of theevaporator is then crystallized to be salt via the sea wind or the residual warm gas.

2. Mathematical model

The H2O–NaCl system is the most studied of all water salt systems, owing to its importance notonly in geologic studies, but also in many other industrial and engineering applications. For H2O–NaCl solutions having salinities from 0–40 wt.% NaCl, salt is basically well dissolved in the water.In the present study, the influence on the thermo-physical properties by the salt concentration hasbeen neglected.A mathematical model for utilization of waste heat in desalination has been developed based on

the internal combustion Diesel engine plant. Fig. 1 illustrates the flow processes and the energytransfer mechanisms schematically. First of all, the inlet seawater should be pressurized to thedesired level in order that its pressure and the associated saturation temperature are satisfactory

T.C. Hung et al. / Energy Conversion and Management 44 (2003) 1259–1273 1261

to drop along the heat exchange stages. In order to prevent the occurrence of corrosion, the fluegas does not directly heat the evaporators. The waste heat from the exhaust flue gas is sequentiallyemployed to heat the water in the high temperature and low temperature waste heat boilers insubsystems A and B shown in Fig. 1. Part of the water in these waste heat boilers is then evap-orated to vapor, which is used as the heat source to the first stage evaporator. The saturated liquidwater in the waste heat boilers is guided to the seawater preheater, which is subsystem C shown inFig. 1. Each stage of the evaporator takes the steam from the last stage. This steam is thencondensed to hot water, which is still hot, and is then employed as the heat source to the preheaterof subsystem C. The circulated jacket water is primarily used as the energy source to heat theseawater in the preheater, and the evaporators operate under low pressure.The dew point temperature Tdp is selected to distinguish between the high and low temperature

waste heat boilers. In order to prevent condensation of NOx and SOx, which would cause thecorrosion to the metal material, the low temperature waste heat boiler is suggested to be made ofplastic. A plastic evaporator is also employed when the stream temperature from a prior stageof the seawater evaporator is less than about 70 �C, at which temperature fouling will happenin ordinary pipes. The amount of waste heat from the high temperature exhaust gas can beexpressed as

Fig. 1. The schematic diagram of the desalination system.


_EEH;gas ¼ _EET;gas Tgas exh

�� Tdp

�= Tgas exh

�� Tatm

�When the flue gas temperature is lower than Tdp, the recoverable heat is

_EEL;gas ¼ _EET;gas Tdp�

� Tgas;min

�= Tgas exh

�� Tatm

�The lowest allowable flue gas temperature through the low temperature boiler is generally set atTgas;min ¼ 60 �C. The temperature of the seawater will be raised to TSW;max after being heated in thepreheater and the waste heat boiler.

TSW;max ¼ _EEH;gas=ð _MMCpÞ þ TSW;pre

Alternating heat transport between the steam and seawater in the evaporator is the primaryprinciple employed in the present study of seawater desalination. After being heated to TSW;max, aconstant enthalpy process will be employed to release the pressure of the seawater, and its tem-perature will reduce by a value of DT in the mean time. As shown in the T–h diagram at Fig. 2, thenew state will located at the two phase region with dryness x1:

x1 ¼ hf;Tmaxð � hf;Tmax�DT Þ=hfg;Tmax�DT

where the seawater is controlled at the saturated liquid state. Therefore, an amount of steam, _mm1,is produced:

_mm1 ¼ _MMx1

This steam is guided to the next stage, where a throttling process is imposed on the seawater sothat its temperature and pressure were decreased. The new state at stages nP 2 will yield a drynessas

xn ¼hf ;Tmax�ðn�1ÞDT � hf;Tmax�nDT

hfg;Tmax�nDT

Fig. 2. The T–h diagram of the desalination principle along the saturated liquid water curve.


The steam from the upstream stage will release latent heat to heat the seawater in the current stagein order to obtain steam. Therefore, after and including the second stage, i.e. nP 2, the pro-duction rate of steam in the nth stage of the evaporator, _mmn, can be expressed as

_mmn ¼ _MM

�Xn�1

j¼1

_mmj

!xn þ _mmn�1

hfg;Tmax�ðn�1ÞDT

hfg;Tmax�nDT

� �

The state will still be located at the saturated liquid state after condensation. The temperature isstill high in the upstream stages, and therefore, these stages of condensate can be employed as theheat source to heat the seawater to TSW;pre. It has been assumed that the seawater absorbs heatfrom the jacket water as its temperature is lower than Tjacket;in. The condensate is only employed toheat the seawater that has a temperature greater than Tjacket;in by the quantity of heat:

_EEcon water ¼Xn

_mmnCp½ðTSW;max � nDT Þ � Tjacket;in�

If the heat of the jacket water has not been fully used in the preheater, the surplus energy canstill be utilized in the downstream stages with lower operation pressure, which could be lower than1 atm. Therefore, the utilization fraction of the heat of the jacket water is

gpreheat ¼ _MMCpðTjacket;in � TSW;envÞh i.

_EEjacket

The excess steam, _mmjacket, produced at the stage with temperature Tjacket;in (i.e. n ¼ jacket) by thesurplus energy of the jacket water will be

_mmjacket ¼ ½ð1� gpreheatÞ _EEjacket�=hfg;Tjacket

Fig. 3. Excel calculation model.


The total amount of fresh water produced is, hence, the summation over all the stages:

_mm ¼Xn

_mmn

The exit salinity, So, must be controlled by the condition of crystallization. At this moment, theflow rate of the brine is ðSi=SoÞ _MM , Therefore, the fresh water generation could be expected to beless than ð1� Si=SoÞ _MM .Microsoft Excel has been employed as the software and Visual Basic was used as the Macro for

programming the mathematical model. After the required parameters and conditions of theprogram are given, this program will link the Marcos to perform the logic judgment and thencomplete the computation. The calculated results of a sample case are presented in Fig. 3. Detailinformation, such as the temperature, flow rate, pressure, salinity, enthalpy, amount of steamgenerated and so on, are described at various locations.

3. Results and discussion

In the present study, a 10 MWe Diesel engine is selected. Its thermal energy to power conversionefficiency is assumed to be gth ¼ 44%. The energy fractions of the exhaust gas, jacket water andradiation heat loss are assumed to be 29%, 26% and 1%, respectively. The lowest temperature andthe highest exit salinity at the last stage of the steam generator are set at 40 �C and 0.4, respec-tively. Possible temperature changes between stages are selected between DT ¼ 1–20 �C. Alfa-Laval in Sweden, a plate heat exchanger maker, has announced that DT ¼ 1 �C is achievable. Thetemperature of the exhaust gas at the exit of the stack is set no less than the dew point,Tdp ¼ 150 �C so that the byproducts of NOx and SOx will not be condensed. In the followinganalyses, the pressure of the high temperature waste heat boiler is set at 8 kg/cm2, providing asaturated temperature of about 170 �C. The inlet temperature of seawater is set no higher than 160 �C.

3.1. Case A: jacket water as the only heat source

If the energy of the jacket water can be completely used, not only can the installation cost of thedischarge system of the power plant be saved, but also the impact on the near shore ecology canbe prevented. Therefore, the jacket water is considered as the only heat source for desalination, inwhich the waste heat boilers, i.e. subsystems A and B in Fig. 1, are not used. The highest tem-perature of the seawater occurs at the exit of the preheater. The pressure of this stream is droppedvia a throttling process and then guided into the first stage of the steam generator. The tem-perature difference between stages is DT . By repeatedly dropping the pressure and guiding thestream into the next stages, the latent heat will be released and used as the heat source toevaporate the brine. Steam will condense to hot water after the release of its latent heat. Thisstream of hot water, i.e. the desired product, fresh water, can also be employed in preheating theseawater. The brine will be collected when the salinity reaches the set point.The relation between DT and the maximum generation rate of fresh water with exit salinity as a

parameter is shown in Fig. 4. Basically, the trends are similar among the various exit salinitiesof the brine with So ¼ 10%, 20%, 30% and 40%. For small DT �s, the change of the maximum


generation rate of fresh water is more significant, i.e. greater o _mm=oðDT Þ, however, o _mm=oS is notquite significant. Because of the limit of the lowest temperature, the allowable amount of a stage isinversely decreased with respect to the increase of DT . Therefore, the total produced amount offresh water is reduced. The generation rate of fresh water is much less affected by DT when DT isgreater than 10 �C.The fresh water production rate as a function of DT and the required stages of the evaporator

have been plotted in Fig. 5 when So ¼ 40%. Figs. 4 and 5 have obviously shown that the value ofDT is the primary factor in the production of fresh water. For instance, the situation of DT ¼ 1 �Cprovides the greatest range of drinking water production rate, covering 0–25.9 kg/s, but, it is 0–12.03 kg/s for DT ¼ 5 �C. The stage amount is not strictly proportional to the generation amountof fresh water. There exists the phenomenon that the generation rate is increased but the amountof a stage remains unchanged.

Fig. 4. Maximum fresh water production versus DT for various So (Case A).

Fig. 5. Stages of evaporator versus fresh water production for various DT when So ¼ 40% (Case A).


When the temperature difference between stages is 10 �C, Fig. 6 shows the generation of freshwater under various inlet flow rates of seawater. In the first stage, it is seen that the amount offresh water increases and then decreases when the incoming seawater flow rate is greater than 3kg/s. The reason is that less seawater inflow does not need a large amount of heat. Therefore, theenergy of the jacket water could efficiently evaporate the water in the first stage. Greater seawaterflow needs a greater amount of heat to preheat from the subcooled to the saturated state, so thegeneration of fresh water in the first stage is less. However, the overall amount of the process isincreased due to the contribution in the downstream stages.

3.2. Case B: jacket water and high temperature exhaust gas as the only heat source

Here, the high temperature exhaust gas and the jacket water are used as heat sources for de-salination, in which the low temperature waste heat boiler, i.e. subsystem B in Fig. 1, is not in-cluded. The jacket water has the lower temperature, so it is used in preheating. The energy fromthe exhaust gas of the engine with higher temperature is employed to heat the first stage of thesteam generator after the transformation in the high temperature waste heat boiler. The tem-perature here has the highest value in the system. Once the energy from the jacket water is morethan needed for preheating, the extra amount is then used as the source in the lower temperatureevaporator. By doing so, the amount of stages could be reduced, and the system can still efficientlyoperate. The function of the other portion of the system is similar to that described in case A.

Fig. 6. The relation diagram amongst fresh water production, inlet seawater flow rate and stages of evaporator when

DT ¼ 10 �C and So ¼ 40% (Case A).


The relation between DT and maximum generation rate of fresh water with exit salinity as theparameter is presented in Fig. 7. Basically, the trend of the curves is similar to that of case A. Theincrease of production is obviously attributed to the heat of the flue gas. The fresh water pro-duction rate as a function of DT and the required stages of the evaporator have been shown inFig. 8 where So is 40%. It is interesting to see that in some curves of DT and in some ranges offresh water production, the needed stages of the evaporator is decreasing, but the production rateof fresh water is still increasing. This results from the jacket water being used as the heat sourcefor preheating with the extra heat being employed as the heat source in the low temperatureevaporator. Therefore, the generation of fresh water at this stage of the evaporator will increasesignificantly. The next right stage will receive the energy from this stage to further enhance theproduction of fresh water. The needed stages, will, hence, be decreased.The generation of fresh water under various inlet flow rates of seawater associated with the

stage numbers of the evaporator for DT ¼ 2 and 5 �C are shown in Fig. 9a and b, respectively.The trend of the production of fresh water is similar for both DT �s. In the case of the unrealistic

Fig. 7. Maximum fresh water production versus DT for various So (Case B).

Fig. 8. Stages of evaporator versus fresh water production for various DT when So ¼ 40% (Case B).


situation of DT ¼ 2 �C, it was assumed that the evaporators are nearly perfect in heat transfer.It is interesting to see that the production rate rapidly increases after several stages. When the


So ¼ 40% and DT ¼ 2 �C (a) and 5 �C (b) (Case B).


incoming flow rate of seawater is less than 30 kg/s and greater than 21 kg/s, the production rate offresh water is basically increasing in the downstream stages of the evaporator. When the incomingflow rate of seawater is less than 21 kg/s, i.e. with lower flow rates of seawater, the workingtemperatures in all stages are higher than 80 �C. Therefore, the energy carried by the jacket watercould not be used. When the incoming flow rate of seawater is greater than 30 kg/s, i.e. with

Fig. 10. Maximum fresh water production versus DT for various So (Case C).

Fig. 11. Stages of evaporator versus fresh water production for various DT when So ¼ 40% (Case C).


greater flow rates of seawater, the energy of the jacket water is primarily utilized in preheating.Therefore, more fresh water has been generated in the upstream stages.


So ¼ 40% and DT ¼ 2 �C (a) and 5 �C (b) (Case C).


3.3. Case C: jacket water, high and low temperature flue gas as the heat source

The system has been schematically shown in Fig. 1. The case here includes low temperature heatrecovery, which was not used in case B. By doing so, this portion of heat can be applied forpreheating, and the heat from the jacket can be efficiently used in the low temperature evapo-rators. This approach can reduce the stages of the evaporators and increase the production offresh water as well. The application of plastic evaporators could prevent the pipe corrosion due tothe form of the compounds from water and the NOx molecules. Therefore, a wider temperaturerange of heat can be employed for the purpose of desalination.The relation between DT and the maximum generation rate of fresh water with exit salinity as

a parameter is shown in Fig. 10. The fresh water production rate as a function of DT andthe required number of stages of the evaporator have been plotted in Fig. 11 when So is 40%.The trend between the number of evaporator stages and the production rate of fresh water issimilar to that of the previous cases. The results indicate that there is about 20% more pro-duction than that of case B. It proves the economic profit from the contribution of the lowtemperature flue gas. Fig. 12 shows the generation of fresh water at various inlet flow rates ofseawater for DT ¼ 2 and 5 �C. Like that shown in Fig. 9 for case B, the optimal amount ofevaporator stages can be obtained according to the peaks of the curves. Of course, the totalproduction rate of the present case is more than that of case B because of the use of the lowtemperature flue gas.Some reports suggested using the ocean as the final deposit for CO2 [10–12]. In order to

lengthen efficiently the residency time of CO2 in the depths of the ocean, it is urgent to find afeasible technique to fix it firmly but not just resolve it. Since the concentrated brine supplies largeamounts of Ca2þ and Mg2þ for CO2 absorption and fixation in the form of carbonates, the presentstudy suggests that the brine could be drawn into the depths of the sea to dissolve the CO2

dramatically. By introducing the flue gas into the deep ocean, the seawater can remarkably dis-solve CO2 by its own pressure. This idea would prevent the discharge of the brine to pollute thesea and may contribute to CO2 reduction too.

4. Conclusion and suggestion

A mathematical model for the purpose of waste heat recovery from an internal combus-tion power plant applied to seawater desalination has been established and analyzed via Mi-crosoft Excel and Visual Basic. The results tell us that an appropriate arrangement of parameters,such as DT , stages of evaporator, inclusion of waste heat subsystem, incoming seawater flow rateand so on, could obtain the amount of fresh water for the specific population of near shoreregions. Plastic evaporators can be employed once the temperature is low. The analyzed resultsindicate that fully using the waste heat could eliminate installation of the warm water dischargesystem. The study successfully shows the feasibility of application of waste heat from combustionengines in the desalination of seawater. It is suggested to establish a detailed phase diagrambetween CO2 and various concentration of brine in order to contribute in CO2 reduction. Bydoing so, we can approach the goal of both efficient utilization of energy and protection of en-vironment.


Acknowledgement

Financial support for this research was provided by the National Science Council, Taiwan,ROC under the grants NSC 87-TPC-E-007-006.

References

[1] Bourouni K, Martine R, Tadrist L, Chaibi MT. Appl Energy 1999;64:129.

[2] Jernqvist J, Jernqvist M, Aly G. Desalination 1999;126:147.

[3] Ettouney HM, Eldessouky H. Desalination 1999;125:277.

[4] Degunzbourg J, Larger D. Desalination 1999;125:203.

[5] Safi MJ, Korchani A. Desalination 1999;125:223.

[6] Kronenberg G. Desalination 1997;108:287.

[7] Milow B, Zarza E. Desalination 1997;108:51.

[8] Slesarenko VV. Desalination 1999;126:281.

[9] Dvornikov V. Desalination 2000;127:261.

[10] Cole KH, Stegen GR, Spencer D. Energy Convers Manage 1993;34:991.

[11] Masutani SM, Kinoshita CM, Nihous GC, Ho T, Vega LA. Energy Convers Manage 1993;34:865.

[12] Morishita M, Cole KH, Stegen GR, Shibuya H. Energy Convers Manage 1993;34:841.


Cogeneration approach for near shore internal combustion power plants applied to seawater...

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