An investigation of heat recovery of submarine diesel engines for combined cooling, heating and power systems

Energy Conversion and Management 108 (2016) 50–59

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

An investigation of heat recovery of submarine diesel engines for combined cooling, heating and power systems Roonak Daghigh ⇑, Abdellah Shafieian Department of Mechanical Engineering, University of Kurdistan, Kurdistan, Iran

a r t i c l e

i n f o

Article history: Received 11 June 2015 Accepted 2 November 2015

Keywords: Submarine Diesel engines Heat recovery CCHP Absorption chiller

a b s t r a c t High temperature and mass flow rate of the exhaust gases of submarine diesel engines provide an appropriate potential for their thermal recovery. The current study introduces a combined cooling, heating and power system for thermal recovery of submarine diesel engines. The cooling system is composed of a mixed effect absorption chiller with two high and low pressure generators. The exhaust of the diesel engine is used in the high pressure generator, and the low pressure generator was divided into two parts. The required heat for the first and second compartments is supplied by the cooling water of the engine and condensation of the vapor generated in the high pressure generator, respectively. The power generation system is a Rankine cycle with an organic working fluid, which is considered a normal thermal system to supply hot water. The whole system is encoded based on mass stability, condensation and energy equations. The obtained findings showed that the maximum heat recovery for the power cycle occurs in exhaust gas mass ratio of 0.23–0.29 and working fluid mass flow rate of 0.45–0.57 kg/s. Further, for each specific mass ratio of exhaust gas, only a certain range of working fluid mass flow rate is used. In the refrigerant mass flow rate of 0.6 kg/s and exhaust gas mass ratio of 0.27, the power output of the cycle is 53 kW, which can also be achieved by simultaneous increase of refrigerant mass flow rate and exhaust gas mass ratio in a certain range of higher powers. In the next section, the overall distribution diagram of output water temperature of the thermal system is obtained according to the exhaust gas mass ratio in various mass flow rates, which can increase the potential of designing and controlling the thermal system. The effect of parameters of this system on output water temperature was also analyzed. Finally, the performance of mixed effect absorption chiller was examined and the findings indicated that in the exhaust gas mass ratio of 0.375, the system has a coefficient of performance equal to 0.94. The output water temperature of evaporator in this state is 3.64 °C and the cooling system power is 176.87 kW. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction All submarines are equipped with diesel engines, either as a main source of power or for emergency use. They are also possessed of air induction and diesel exhaust systems. Air induction systems take in outside air for combustion within the diesel engines, while exhaust systems discharge the combustion by products overboard. The engines are being cooled by water injection before discharge [1]. Based on the discharge features, exhaust flow rate and cooling water, the injection rates vary for each class of vessel, from about 6116 m3/h to 14,611 m3/h and from 0.44 kg/s to 0.95 kg/s, respectively. The exhaust flow temperature can also escalate to 650 °C [2]. Based on the abovementioned figures, merely a small portion of combustion energy turns into output ⇑ Corresponding author. Tel.: +98 8716664600 8. E-mail addresses: [email protected], [email protected] (R. Daghigh). http://dx.doi.org/10.1016/j.enconman.2015.11.004 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

from the crankshaft; most of the energy is, however, wasted in the exhaust and coolant systems. Therefore, there is a tremendous potential for energy recovery from submarine engines. There is constant production of heat by submarines, which is released into the air by hot engines, storage batteries, galley stove, electric lights, electric heaters, other devices and human occupants. Moreover, there exists continuous production of humidity, discharged into the air by evaporation from four main sources: storage batteries, cooking, human occupants, and the bilges. In addition to the problem of air quality, if the temperature of air reaches the dew point, the interior surfaces start to perspire; i.e. there is always a potential danger from short circuits or grounds in electric systems. Thus, the cooling and air-conditioning systems play a pivotal role in submarines [3]. Hamilton Standard Division conducted a study to determine the type or types of airconditioning systems, perfectly suited to large submarines. The performance of the systems was examined based on quantitative

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and qualitative evaluation of the systems according to five criteria: noise creation, maintenance-free life, volume, refrigerant type and power. The findings of overall rating indicated that the centrally located absorption-cycle, air-conditioning system was the optimum cooling system for a submarine. Absorption-cycle airconditioning system increases the effectiveness of submarine, decreases it’s size and noise. This system requires minimum number of the crews and provides the most comfortable atmosphere for them. Absorption system shows the quietest submerged patrol among all compared systems. Finally it shows the best thermodynamic performance [4]. Haywood et al. investigated the possibility of harvesting data center waste heat to drive an absorption chiller [5]. The initial heat source was waste heat that was produced by CPUs on each server blade. The main challenge of the system was to cool the data center simultaneously and produce sufficient exergy to drive the cooling process, regardless of the thermal output of the data center equipment. Ochoa et al. performed an energetic and exergetic study of an absorption chiller integrated into a microgeneration system [6]. They introduced a thermodynamic cogeneration model that coupled single-effect LiBr/H2O absorption chiller to a microturbine and analyzed energetic and exergetic behavior of the system. A computational algorithm was developed on the EES-32 platform to assess the impact of the main operating parameters of the cogeneration system. The COP values were reported to be between 0.24 and 0.74. The overall energy and exergy efficiencies of cogeneration were around 50% and 26%, respectively. The concept of automobile waste heat-driven adsorption cooling seems to be quite appealing, on which several studies have already been carried. Zhu et al. used the exhaust gas from a diesel engine of a fishing boat, to drive a zeolite–water adsorption refrigeration system, for preserving aquatic products [7]. Suzuki stated the combination of adsorption cooling system with exhaust heat from engines as a solution to reduce the environmental problems related to current automobiles [8]. Zhang and Wang studied the waste heat-driven adsorption cooling system for automobiles. The effects of some parameters on the performance of the system was also studied [9]. Zhang designed and examined an experimental intermittent adsorption cooling system driven by the exhaust gas of a diesel engine [10]. The experimental findings indicated that the prototype could be successfully utilized for waste heat driven air conditioning. The COP of the system was reported to be 0.38. Kristiansen and Nielsen studied the potential of applying thermoelectric generators on ships. They intended to develop a thermoelectric generator powered by waste heat recovery from ships. In the study, the potential of system in a bulk carrier was evaluated. It was shown that the exhausts exits the engine and the sludge oil incinerator are the most favorable heat recovery sources [11]. Chen et al. computationally studied the application of thermoelectric generators in marine power plants. Computational Fluid Dynamics modeling is used to evaluate the feasibility of applying thermoelectric generators in the boiler section of marine plants. Finally, the results showed that by supplying 300 kW of the waste heat produced by boiler, thermoelectric generators can produce more than 600 W power [12]. Zhao and Tan reviewed materials, modeling and applications of thermoelectric cooling systems. First, ancient improvements of these systems have been briefly introduced. After that, historical accomplishments in the field of thermoelectric materials have been stated. Finally, different modeling techniques have been summarized in detail [13]. Thermo-electric cooling systems are compact and light in weight. They have no moving parts and are powered by direct current. In spite of mentioned advantages over conventional cooling devices, they have considerable shortcomings for submarines, which omits this system from consideration in studies. Application of large electric generators necessitates using large amounts of highly filtered,

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high-voltage, d-c power. Also, because of having low efficiency, thermoelectric devices require more sea-water flow in comparison with other systems. This results in considerable noise increase inside submarine. Finally, thermoelectric materials can’t afford the required current, used in large-capacity cooling units. Wang et al. reviewed the studies carried out on thermal exhaust heat recovery with Rankine cycle [14]. Rankine cycle (RC) was viewed the most favorite basic working cycle for thermodynamic exhaust heat recovery systems. It was also reported that designing a systemic structure and selecting both the expander and the working fluid (medium) are critical in order to achieve the highest possible efficiency of the system. Further, Larjola showed that up to 50% of exhaust gas exergy can be recovered by employing a Rankine cycle exhaust heat recovery system [15]. Hung et al. compared different cryogens, that are used as working fluids in Organic Rankine Cycle exhaust heat recovery systems [16]. An organic Rankine cycle (ORC) can be used to recover the waste heat, thereby improving the thermal efficiency of an internal combustion engine. A number of studies have recently been conducted to analyze the ORC performance. Kang studied characteristic performance of Organic Rankine Cycles experimentally [17]. He et al. investigated the optimal evaporation temperature in an organic Rankine cycle [18]. Sun and Li investigated the organic rankine cycle as heat recovery power plant [19]. Wang et al. compared organic Rankine cycle different working fluids for engine waste heat recovery [20]. Generally, the qualities of dry and isentropic fluids do away with the concerns of damage generated by liquid droplets on the turbine blades as a result of wet steam and application of superheated apparatus [21]. Zhang et al. suggested this as one of the major reasons for organic working fluid being adopted as the working fluid of RC [22]. Teng et al. conducted a study and demonstrated that a carefully selected organic fluid can reduce the temperature difference between the waste heat and working fluid. They concluded for diesel engines from which a moderate level of waste heat temperature is generated, the most efficient and highest power output is usually achieved by utilizing an appropriate organic fluid instead of water as the working fluid of RC [23]. As pointed out previously, there is a great potential for energy recovery from submarine engines. So, in this work, a new combined cooling, heating and power system for energy recovery from submarine engines was introduced. A mixed effect absorption chiller was applied to recover the jacket water and exhaust gas waste heat of engines. An organic Rankine cycle (R245fa was selected as the working fluid) and an ordinary heating system were also considered to supply power and hot water demand. The whole system was mathematically modeled and real discharge characteristics of submarine engines were used to investigate the performance of the system.

2. System description The simple schematic of the proposed system used for heat recovery of a submarine diesel engine is shown in Fig. 1. The system consists of three main sections. First, cooling section is a mixed effect absorption chiller, which is designed to recover the waste heat of both exhaust gas and cooling jacket water. Second, power section is an organic Rankine cycle that utilizes R245fa as working fluid. Finally, a common heating system is considered to provide the hot water requirement of submarine. Based on the demand, the exhaust will be applied to the cooling, power and heating sections in proper ratios. As stated before, the cooling jacket water is only applied to power the low pressure generator of mixed effect absorption chiller. The current and future submarine designs are not limited to weight, but are critically restricted to equipment

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Fig. 1. The simple schematic of the proposed system.

space. However, space is not as crucial in the aft engine room or machinery room as it is in other compartments [4]. Cooling and heating units already exist inside submarines. So, combining heat recovery system with these units does not require a considerable extra space. The power section is placed in the aft engine room, where the space limitation is not very critical. Therefore, on the basis of space criteria, the whole system is justified. 2.1. Mixed absorption chiller The schematic of mixed effect absorption chiller is illustrated in Fig. 2. The mixed effect absorption chiller is different from the dual system in that the low pressure generator is divided into two sections. The required heat for the first section is supplied by the engine cooling water and the required heat for the second section is provided by the vapor condensed in the high pressure generator. It has been proven that water–lithium bromide solution has the optimum performance in absorption refrigeration systems [24]. Thus, water–lithium bromide solution was used as the working fluid of the absorption chiller. As illustrated in the figure, the diluted outlet solution enters the heat converter 1 from the absorber, where through heat recovery it takes in some heat from the concentrate outlet of the lower part of low pressure generator. This diluted solution is then divided into two parts. The first part enters the upper section of high pressure

generator. As explained above, the heat produced from the engine cooling water evaporates some of the refrigerant existing in this solution. This solution then enters the second section of low pressure generator. The second part of dilute solution enters high pressure generator after heat recovery in converter 2, where it loses some of its refrigerant as vapor by taking in the heat of engine exhaust gas, and is converted into a semi-dilute solution. The refrigerant vapor enters the lower part of low pressure generator. The semi-dilute outlet solution enters the heat converter number 1 from the high pressure generator and enters the lower part of low pressure generator following temperature reduction, where it is mixed with the solution discharged from the upper part of high pressure generator. Here, the semi-dilute solution loses some of its refrigerant during the vapor condensation of refrigerant in high pressure generator and is converted into concentrate. Following heat recovery in converter 1, the concentrate enters the absorber. The refrigerant vapor generated in both sections of low pressure generator enters the condenser while combining. In condenser, the refrigerant vapor is condensed while giving its extra heat to the cooling water, and is combined with the condensed refrigerant coming from the lower part of the low pressure generator. Following the pressure-breaking process, this refrigerant enters the evaporator and by absorbing the cooling water heat reduces the temperature of cooling water and simultaneously evaporates.

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Fig. 2. Mixed effect absorption chiller.

Fig. 3. Organic Rankine cycle.

2.2. Organic Rankine cycle Owing to high safety and appropriate functional and environmental properties, R245fa is used as the working fluid of the power recovery cycle [25]. As shown in Fig. 3, the energy of engine exhaust gas is transferred to working fluid through evaporator. The heat transferred to working fluid changes the organic fluid phase in high pressure. Then, the organic fluid with high enthalpy drives the turbine and generates power. Based on the aforementioned discussion, it is necessary to point out that evaporator is the most important part of recovery cycle. The efficiency of thermal converter affects the performance of the whole cycle. In common systems, two types of thermal converters are used, the plate type and the shell-and-tube type. However, in thermal recovery

systems, finned-tube converter is recommended to be used due to high pressure and temperature [26]. The characteristics of finned-tube thermal converter are presented in Table 1. The working fluid from the turbine is condensed in condenser and returns to thermal converter. 2.3. Water heating Before entering the cooling system, the engine exhaust gases enter the thermal converter of the cooling system. As stated above, due to high pressure and temperature, a finned-tube thermal converter was used. The heat of exhaust gases in thermal converter is transferred to the cold water coming from the tank, causing it to be heated. The heated water is returned to the hot water tank.

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Table 1 Geometric dimensions of the finned-tube evaporator.

0

hðT; xÞ ¼ ð1  xÞh ðTÞ þ hc

Item

Parameter

Unit

Total number of tubes Tube outside diameter Tube inside diameter Row pitch Tube pitch Fin density Fin height Fin thickness Fin material Tube material Tube length Tube row alignment

50 27 20 46 54 50 27 4 Stainless steel Stainless steel 40 Staggered

– mm mm mm mm m1 mm mm

cm

PðT; xÞ ¼ Pr ðhÞ h¼T

xM LiBr xM LiBr þ ð1  xÞM w


All processes are in a stable state.
The refrigerant vapor discharged from high pressure generator turns into a saturated liquid after the transfer of heat to solution in low pressure generator.
The refrigerant discharged from condenser and evaporator is in saturated.
The pressure drop in the pipes is ignored.
The pressure drop between low pressure generator and condenser is disregarded.
In spite of being a non-equilibrium process, by considering an adiabatic process, the enthalpy at the inlet and at the outlet of a valve is assumed to be constant.

X X

mi ¼

X

mi xi ¼ mi hi 

ð1Þ

mo

X

X

ð2Þ

mo xo

mo ho ¼ Q

ð3Þ

To calculate the properties of water–lithium bromide solution, the thermodynamic formula presented by Klomfar Pa´tek was used [30]. The formulation was obtained by using experimental data and presented in terms of two variables of temperature and molar ratio, as presented below:

C p ðT; xÞ ¼ ð1 

xÞC 0p ðTÞ

 ti 8 X Tc þ C p;t ai xmi ð0:4  xÞni T  T0 i¼1

ð4Þ

ð8Þ

ð9Þ

As previously mentioned, the required heat for the second part of low pressure generator is supplied by the vapor condensation of the refrigerant generated by high pressure generator. So, the following relation is used to calculate the heat:

Q ¼ mðho  hi Þ

ð10Þ

Temperature difference in each part is calculated by the logarithmic mean temperature difference presented below:

DT LMTD

    T i;1  T o;2  T o;1  T i;2      ¼ Ln T i;1  T o;2 = T o;1  T i;2

Q ¼ UADT LMTD

ð11Þ ð12Þ

The overall performance of mixed effect absorption chiller is analyzed by the following relation:

COP ¼ There are inflow and outflow in every section of absorption chiller. Based on the hypothesis of presence of a stable state, the equations for mass stability, concentration and energy for each part of absorption chiller are as follows:

ð7Þ

The data related to water in saturated and condensed liquid states were obtained by REFPROP software and the properties of water in superheated state were acquired using the equations presented by Lachkov et al. The presented equations have been formulated to calculate the enthalpy, viscosity and density of superheated vapor under different performance conditions and in real piping [31]. The transferred heat in all components in relation to external fluid is obtained via following relation:

Q ¼ mC p ðT o  T i Þ

The following hypotheses were formulated for simulation of mixed effect absorption chiller [28,29].

X

 ti 8 X T ai xmi ð0:4  xÞni Tc i¼1

ð6Þ

where the pressure of solution is equal to the pressure of saturated liquid in hypothetical temperature h. The fixed coefficients of ai, mi, ni and ti can be found in the tables of 30. The relationship between mass ratio and molar ratio of the solution is presented in the following equation:

3. Mathematical model

3.1. Mixed effect absorption chiller

ð5Þ

where h0 (T) and C 0p are properties of water in saturated liquid state.

x¼ Calculation of the characteristics of exhaust gas such as viscosity and thermal capacity is necessary for thermal analysis. These characteristics are used for computation of Reynolds and Nusselt numbers. The main components of diesel engine exhaust gases are CO2, H2O, N2 and O2. The properties of each component were obtained by Refprop software [27]. Then, by taking into account the mass ratio of each component in combustion process, the properties of the exhaust gas were computed.

 ti 30 X Tc ai xmi ð0:4  xÞni T  T0 i¼1

Qe Q ex þ Q jw þ W p

ð13Þ

3.2. Organic Rankine cycle As explained before, a finned-tube converter is used as Rankine cycle evaporator. The real state of converter is cross flow arrangement, but the converter can be analyzed as counter flow due to adequate number of rows [32]. Considering the engine exhaust gas in the external part of converter, the heat transfer coefficient of evaporator gas is obtained through the following relation [33]:

hex ¼ 0:1378

 0:718  s 0:296 kex do mex Pr 1=3 ex d do lex A

ð14Þ

The organic fluid in the evaporator section undergoes singlephase and dual-phase states. The following relation is used to calculate heat transfer coefficient in the single-phase state [34]:

hf ¼

f ðRe 8

 1000ÞPrf qffiffi 1 þ 12:7 8f Pr 2=3 1 f

ð15Þ

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55

Fig. 4. The overall process of simulation.

It has experimentally been proven that in the dual-phase state, the following relation provides a good approximation from the heat transfer coefficient of R245fa organic fluid [35],

h

2

hpc ¼ ðEhf0 Þ þ ðShnb Þ

2

i0:5

ð16Þ

where E and S are modified heat transfer coefficients for the film and nucleate boiling, respectively [36].

"

ql E ¼ 1 þ wPrl 1 qg

!#0:35 ð17Þ

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Fig. 5. Exhaust gas temperature in superheated area based on gas mass ratio in different mass flow rates of working fluid. Fig. 7. Exhaust gas temperature in preheated region based on the gas mass ratio in different mass flow rates of working fluid.

Fig. 8. Power output of Rankine cycle based on the working fluid mass flow rate in different exhaust gas mass ratios.

Fig. 6. Exhaust gas temperature in dual-phase region based on the gas mass ratio in different mass flow rates of working fluid.

S¼

1 1 þ 0:055E0:1 Re0:16 f0

ð18Þ

The overall heat transfer coefficient is calculated through the following relation according to the outer surface of converter.

U¼

Ao hi Ai

1 þ kt AAmo þ h1o

ð19Þ

3.3. Water heating A finned-tube converter is used for heat recovery of exhaust gases. The exhaust gas exists in the outer part of the converter and the water flows inside the tubes. The same as previous section, the gas-side heat transfer coefficient is calculated using Eq. (14) and tube-side heat transfer coefficient is obtained

Fig. 9. Outlet water temperature changes of the heating system according to exhaust gas mass ratio in different mass flow rates.

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Fig. 10. Exhaust gas temperature of thermal converter of heating system according to exhaust gas mass ratio.

Fig. 11. Coefficient of performance of mixed absorption chiller according to heat transfer coefficient.

by applying Eq. (15). The thermodynamic properties of gas and water are calculated by the methods presented in previous sections. The overall process of simulation is illustrated in Fig. 4.

4. Results and discussion Since the mass flow rate of engine exhaust gas depends on the functional requirements of engine; no constant value can be considered for it. Therefore, the gas mass ratio (X) is defined as inlet mass flow rate of the system divided by maximum mass flow rate of engine exhaust gas. As explained before, the Rankine cycle evaporator has three sections, preheated, two-phase and superheated. The exhaust gas temperatures in superheated, two-phase and preheated areas according to gas mass ratio in different mass flow rates of the working fluid are shown in Figs. 5–7. To recover the maximum heat of the engine exhaust gas, the thermal converter should be designed such that it has minimum level of outlet temperature. However, it should be noted that this temperature must not be so low to reach acidic condensation temperature. Hence, the

57

exhaust gas temperature of converter must not be lower than 100 °C. Analysis of the changing trend of exhaust gas temperature in the three regions shows that the thermal gradient is increased from superheated to preheated region. This trend indicates that heat transfer coefficient is also increased during movement from superheated to preheated area. According to Fig. 7, it can be concluded that the minimum outlet temperature occurs in exhaust gas mass ratio of 0.23–0.29 and working fluid mass flow rate of 0.45–0.57 kg/s. So, based on what was discussed, this region has the maximum level of heat recovery. The findings show that a certain limit of working fluid mass flow rate is used for each specific mass ratio of exhaust gas. For example, for 0.25 gas mass ratio, the working fluid mass flow rate is between 0.51 and 0.63 kg/s. Also, in the area related to a certain mass ratio of exhaust gas, the outlet temperature is declined by a reduction in working fluid mass flow rate. Thus, reduction of working fluid mass flow rate in a specific range of exhaust gas mass ratio causes higher level of heat recovery and vice versa. In the fixed mass flow rate of working fluid, a certain range of exhaust gas mass ratio can be used. For instance, for working fluid mass flow rate of 0.45 kg/s, the exhaust gas mass ratio is between 0.16 and 0.22. Moreover, contrary to the previous condition, the outlet temperature is reduced by an increase in the mass ratio of exhaust gas in a specific range of working fluid mass flow rate. Therefore, the increase of exhaust gas mass ratio in a specific range of working fluid mass flow rate causes higher level of heat recovery. The power output of Rankine cycle according to mass flow rate of working fluid in various mass ratios of exhaust gas is illustrated in Fig. 8. As observed, in the refrigerant mass flow rate of 0.6 kg/s and exhaust gas mass ratio of 0.27, the power output of the cycle is about 53 kW. The changing trend of power is ascending with simultaneous increase of refrigerant mass flow rate and exhaust gas mass ratio. On the other hand, the powers higher than 53 kW can be achieved by simultaneous increase of refrigerant mass flow rate and exhaust gas mass ratio. The mentioned power has been obtained in a rather low mass ratio; therefore, increasing this ratio can have a great impact on increasing the generated power of cycle. However, it is necessary to point out that increasing the above parameters causes an increase in the dimensions of system components, and given the spatial limitations inside submarine, the system must be justifiable in terms of dimensions. Another point to highlight is that the refrigerant mass flow rate and exhaust gas mass ratio are subordinate to each other, and increase of each parameter must be performed within a certain range of another parameter. For example, if the exhaust gas mass ratio is increased from 0.21 to 0.24, the working fluid mass flow rate should be between 0.6 and 0.48 kg/s. The analysis of Figs. 5–7 shows this is completely compatible with the results obtained from the analysis of evaporator. The outlet water temperature changes of the heating system according to exhaust gas mass ratio in various mass flow rates are presented in Fig. 9. This figure enhances the potential of designing and controlling the heating system. For example, the water mass flow rate of 0.2 kg/s and exhaust gas mass ratio of 0.23 can be used to obtain the outlet water temperature of 60 °C. This temperature can also be obtained by water mass flow rate of 0.3 kg/s and exhaust gas mass ratio of 0.41 or water mass flow rate of 0.26 kg/s and exhaust gas mass ratio of 0.32. As illustrated in Fig. 9, the increase of exhaust gas mass ratio has caused an increase in outlet water temperature of thermal converter in different mass flow rates. In a fixed exhaust gas mass ratio, reduction of water mass flow rate causes an increase in outlet water temperature of thermal converter. This increase is especially evident in high mass ratios of exhaust gas. For instance, reducing the inlet water mass flow rate of thermal converter by half in the mass ratio

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Table 2 Performance of mixed absorption chiller. X

xa,i

xa,o

mev,v (kg/s)

Tex,o

Trefw,o

Qcooling (kW)

COP

0.0625 0.25 0.375

0.556 0.578 0.589

0.51 0.573 0.56

0.02 0.072 0.11

176.5 173.5 170

7.8 4.97 3.64

57 147.6 176.87

0.74 0.85 1.02

of 0.15 causes 15 °C increase in the water temperature. However, this increase is about 30 °C in the mass ratio of 0.5. Fig. 10 shows the changes of exhaust gas temperature of thermal converter according to exhaust gas mass ratio in different inlet water mass flow rates. Since the gas entering the thermal converter of heating system enters the absorption cooling system after heating recovery, it is necessary to analyze the temperature of exhaust gas from this converter. As indicated, the changes of inlet water mass flow rate have little impact on exhaust gas temperature in low exhaust gas mass ratios. The recovered heat has also the same trend of changes. The recovered heat level in all mass ratios is approximately the same in low exhaust gas mass ratios. The amount of this heat becomes dependent on the inlet water mass flow rate with an increase in exhaust gas mass ratio. Fig. 11 illustrates the effect of exhaust gas mass ratio on the performance of mixed effect absorption chiller according to heat transfer coefficient. The heat transfer coefficient is defined as:

HR ¼

UAhg UAeV










ð20Þ

In all heat transfer ratios, increase of exhaust gas mass ratio causes an increase in the temperature entering the system, thereby enhancing the performance of absorption chiller. The performance coefficient of the system in the exhaust gas mass ratio of 0.3 and heat transfer coefficient of 4.8 is equal to 0.94. Moreover, the performance and several parameters of the system such as cooling power, exhaust gas temperature, outlet cooling water temperature, mass ratio of solution in input and output absorber and mass flow rate of output refrigerant in high pressure generator based on exhaust gas mass ratio are presented in Table 2. As it can be observed, increasing the exhaust gas mass ratio causes a reduction in outlet cooling water temperature of the system evaporator. Since there is a direct relationship between cooling water temperature and cooling power of system, the less is outlet cooling water temperature, the higher is the cooling power of system. In exhaust gas mass ratio of 0.375, the cooling water temperature and cooling power of system are 3.64 °C and 176.87 kW, respectively. 5. Conclusions The current study introduced a combined cooling, heating and power system for heat recovery of submarine diesel engines. The cooling system is composed of a mixed absorption chiller with two high pressure and low pressure generators, the power generation system is a Rankine cycle with an organic working fluid and the heating system is a normal system to supply hot water. The whole system is coded according to mass stability, concentration and energy equations with the following results:
Heat transfer coefficient increases while moving from superheated to preheated area in evaporator of Rankine cycle. Also, the minimum output temperature occurs in exhaust gas mass ratio of 0.23–0.29 and working fluid mass flow rate of 0.45– 0.57 kg/s. Thus, this area has maximum heat recovery.
In the power cycle, a specific range of working fluid mass flow rate can be used for each specified exhaust gas mass ratio. For instance, for mass ratio of 0.25, the working fluid mass flow rate ranges from 0.51 to 0.63 kg/s. Further, the outlet temperature is




reduced in the area related to a specific exhaust gas mass ratio with a reduction in working fluid mass flow rate. Therefore, reduction of working fluid mass flow rate in a specific range of exhaust gas mass ratio causes higher level of heat recovery; the converse is also true. In the fixed mass flow rate of working fluid, a specific range of exhaust gas mass ratio can be used. Also, the outlet temperature increases by increasing the exhaust gas mass ratio within the area of working fluid mass flow rate. Therefore, increasing the exhaust gas mass ratio within a specific working fluid mass flow rate causes higher heat recovery. The power output of the cycle is about 53 kW in the refrigerant mass flow rate of 0.6 kg/s and exhaust gas mass ratio of 0.27. The changing trend of power is ascending by simultaneous increase of working fluid mass flow rate and exhaust gas mass ratio. On the other hand, the powers higher than 53 kW can be achieved by simultaneous increase of working fluid mass flow rate and exhaust gas mass ratio. Increase of exhaust gas mass ratio causes an increase in output water temperature from thermal converter of heating system in different mass flow rates. In a fixed exhaust gas mass ratio, reduction of water mass flow rate increases output water temperature of thermal converter. This increase is especially evident in higher exhaust gas mass ratios. In low exhaust gas mass ratios, the changes of input water mass flow rate in the thermal converter of heating system have little impact on output gas temperature. There were more changes in output gas temperature with an increase in gas mass ratio. The trend of changes for the recovered heat is the same. In low exhaust gas mass ratios, the amount of recovered heat in all mass ratios is almost the same. By an increase in exhaust gas mass ratio, however, the level of this temperature is dependent on input water mass flow rate. The absorption chiller has a coefficient of performance equal to 0.94 in exhaust gas mass ratio of 0.375, where the output cooling water temperature of evaporator is 3.64 °C and the power of cooling system is 176.87 kW.

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