Estimation and analysis of exergy loss and performance evaluation of marine freshwater generating sy

Abstract

Keywords: Evaporator; Exergy Efficiency; Exergy Loss; Plate Type Heat Exchanger; Waste Heat

Introduction

Freshwater is required for domestic and technical purposes in marine vessels. The entirety of the generated heat

energy serves as propulsion shaft power, with 5.2% of it dissipated through the jacket cooling water of the main engine. This heat energy from the jacket water is employed to produce freshwater through a low-pressure shell and tube-type

*Corresponding author. *E-mail address: jitendrasinghpal2011@gmail.com This paper was recommended for publication in revised form by Regional Editor Hafiz Muhammad Ali Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

freshwater generator [1]. This low-pressure evaporation of seawater equation of state at one atmosphere was presented by Millero and Poisson [2], shown in Figure 1. Kahraman et al. [3, 4], applied a cerci model to examine multi-stage flash desalination, achieving a second law efficiency of only 4.2%. They highlighted that the exergy associated with salinity turns negative when the stream salinity exceeds the incoming seawater salinity. Nafey et al. [5] utilized a sophisticated visual design and simulation package to assess the exergy loss of 4.04 mW in the Eoun Mousa MSF plant. Additionally, they employed a mathematical model to calculate the cost balance equation. Sharqawyet al. [6, 7, 8], calculated seawater exergy of reverse osmosis desalination plant by using MATLAB and thermodynamic properties of seawater given in IAPWS [9]. The two approaches are the first one by Cerci, the ideal mixture model, and the second approach by Mostafa seawater in which the chemical exergy is taken into consideration for the calculation of exergy of the desalination plant. The calculation of seawater enthalpy differs significantly between the models for seawater and aqueous sodium chloride solutions. The Mostafa model is used in this study to measure stream exergy at different plant locations, as shown in Figure 1 and Table 1. The derived equations are used to determine the hybrid components’ irreversibility (exergy destruction). In the study conducted by Hosseiniet al. [10], the steam generated and fuel were described and expressed completely in the exergy analysis of the hybrid system. This description will explain how exergy efficiency is truly realized.

Gude [11] conducted an exergy analysis of desalination plants, identifying condensers as the primary source of the highest losses. Choudhari and Sapali [12] investigated the exergy aspects of a water-cooled refrigerator, concluding that the water-cooling system results in lower energy consumption and discharge gas temperatures for the compressor. Shikalgar and Sapali [13] presented experimental results comparing a water-cooled condenser unit with a hotwall air-cooled condenser unit to identify exergy losses in refrigeration. The findings indicated higher exergy losses in the water-cooled condenser unit, and the analysis involved both energy and exergy methods. Koroglu and Sogut [14] examine a marine power plant utilizing an organic Rankine cycle energy and exergy assessments are performed to discover components and parameters that affect the system’s efficiency. Rawabawale and Sapali [15] noted that the analytical exergy loss is less than the experimental exergy loss. They also found that the exergy loss fluctuates based on the duration and height of the cross-current cooling tower. According to Yuksel et al. [16] parametric ‘s analysis, seawater temperature of 31.88 0C and a jacket water mass flow rate of 72,000 kg/h are necessary to achieve the maximal freshwater generation capacity. According to Solanki and Pal [17], numerical execution uses pinch methods to fuse the solar energy system to generate thermal power in the dairy industry. The aim of this study on freshwater generators for marine vessels are shell and tube type and plate type is to find the most efficient use of waste heat on board as well as for water generation. To locate the components that are more exergy deficient, a method is developed to quantify

Figure 1. Balancing mass and energy in a shell and tube-type freshwater generator.

the thermal characteristics and specific exergy of a fluid stream. Exergy analysis is used to propose a freshwater generator using a plate-type heat exchanger, compared to the existing shell and tube-type freshwater generator [18]. Theory The low-pressure steam distillation system was installed on several commercial and navy ships between 1940. Auxiliary exhaust steam was delivered at 5 kg/cm2 and 160°C to these units. During that time, three fundamental types of low-pressure distillation units are available: shell and tube type heat exchangers, multi flash-type units, and vertical shell units. The most popular, the submerged tube unit, shell-tube type freshwater generator, was employed in this research. Submerged tube distillation machines with capacities ranging from 4,000 to 50,000 litres per day were designed and manufactured. The effects could be single, double, or triple. A stage of the distillation process is called as an effect. A single effect evaporator evaporates seawater using the low-pressure steam fed in the submerged tube bundle. In a double-effect unit, the steam produced in the first effect is used to generate lower pressure steam from seawater in the second effect. The steam from the second effect is utilized to evaporate seawater in the third effect if a third effect is added. The vacuum maintained in the evaporator shell by the water ejector pump, the water ejector for air brine, etc., makes up the freshwater generator, which is represented by the evaporator shell and condenser in Figure 1. The distillate water is pumped out by the pump and then goes via the solenoid pump, electrical salinity indicator, and water flow meter, as seen in Figure 2.

The research begins with examining the freshwater generator, auxiliary boiler, and composite boiler in the PrabhuVidya ship-in-campus. After the investigation is completed, a steady-state model was created in Microsoft Excel. After the steady-state model in Microsoft Excel performs as predicted, a steady-state model in “C” is built. Each software’s plant model produces a set of data that were validated with data received from intern cadet’s ships data. The exergy of freshwater at a specific point in the system is expressed as system-specific exergy and represented by Eq. (1) [19]. a. Methodology b. Material balances and energy balances c. Boiling point elevation and thermodynamic losses d. Exergy calculation of evaporator & other components of the freshwater generator e. Boiler saturation temperature and pressure drop calculation of marine boiler f. Exergy parameter calculation of freshwater generator and marine boiler. As per the assumptions that freshwater generator process is steady-state: Ψ (Specific Exergy) can be represented in a mathematical model as given by Sharqawy et al. [8]:

(1) g. In the RUN mode, the screen asks for the data in the same order given in the flow chart. Once the data are fed in, the computer goes on to perform the various checks and iterations and calculate the specific exergy. h. Print the values of chemical, physical, and total exergy of the steam. The values are shown in Figure 2 and Table 1.

Figure 2. Flowchart to calculate the properties of the stream of freshwater generator.

If no beneficial work is accomplished during the process, this alteration represents a depletion in available work as indicated by Eq. (2). (2) Mustafa and AI Ghamdi [20] introduced the Performance Ratio (τratio). (3) Li [21] presents the second law efficiency (ϑex), defined as the ratio of exergy output to exergy input. (4) When evaluating different processes, the Improvement Potential proves valuable. The most significant enhancement in exergy efficiency for a process or system occurs when the rate of exergy loss or irreversibility is minimized. Hammond [22] introduces this improvement potential on a rate basis through Equation (5). (5)

Experimentation The shell-and-tube freshwater generator achieved a steady-state condition within one hour of operation, with the fluid flowing steadily through a controlled volume in this processWater from the jacket enters the evaporator at approximately 85°C and exits at 71.1°C, facilitating freshwater production under vacuum conditions maintained by the ejector. Experiments were conducted to study parameters such as pressure, jacket water temperature, and the motive fluid (seawater) for the ejector in a parametric investigation. The readings from the experiments were taken at the freshwater generator’s maximum capacity of 20 tons per day. Table 2 and Figures 2 and 3 demonstrate the various parameters of the experimental study. The condensate produced is if less than 10 PPM total dissolved solids then it will be delivered to the freshwater tanks.

Results And Discussion

The integration of exergy analysis into the freshwater generation system entails incorporating exergy-block calculations subsequent to conventional energy-balance calculations. A ‘C++’ program is employed to determine the exergy characteristics of a shell-and-tube freshwater generator. Exergy input and losses for shell-and-tube-type

freshwater generators are illustrated in Tables 1 to 2 and Figure 4. Additionally, Figure 5, along with Tables 3 and 4, presents a specific flow exergy diagram for a plate-type freshwater generator, depicting its exergy input and losses. Figures 4 and 5 highlight the origin of errors associated with considering processes solely from an energy balance perspective. By utilizing waste heat from the propulsion engine’s fuel energy, the energy derived from the propulsion engine jacket cooling water is utilized to convert seawater into freshwater. Cross-hatched triangles represent the exergy streams of major components of the freshwater generator, illustrating the distribution of exergy losses. This depiction visually represents the available work as it moves through various system components. The different widths of the flow work lines serve as a visual indicator of the extent of streamflow, encompassing both useful work and exergy losses within these components. Figures 4 and 5 vividly and precisely depict the size and locations of exergy losses in crucial components such as the evaporator, condenser, and ejector. The flow diagram for the freshwater production system depicted in Figure 1 is derived from

the first-law thermal balance. A comparison of these two flowcharts underscores the substantial impact of the exergy approach in precisely identifying the location and extent of inefficiencies within the energy system. Figures 4 and 5 accurately depict the actual distribution of losses, avoiding inaccuracies inherent in the energy-balance approach. Quantitative values for individual losses contributing to the overall component loss are outlined in Tables 2 and 4. These visuals will play a crucial role in discussions on energy-saving strategies utilizing exergy analysis, emphasizing that shell-and-tube-type freshwater generators exhibit a higher improvement potential of 83.28 kW, whereas platetype freshwater generators require only 27.07 kW. The flow exergy diagram highlights a significant exergy loss of 90.63 kW in the ejector section of the shell-and-tube-type freshwater generator. It should be noted that the divergence is acknowledged when transitioning from the ideal reversible cycle to the actual irreversible cycle. The analysis of this freshwater generation system elucidates the exergy losses inherent in a particular plant configuration. Figure 6 presents a comparison of the second-law efficiency between plate-type and shell-type freshwater generator systems. It

Table 1. Thermal characteristics of streams 1 to 11 at different points within the shell-and-tube freshwater generator Stream

Table 2. Exergy characteristics rate calculations at various components of the shell and tube type freshwater generator Component

Figure 4. Exergy characteristics flow diagram of shell and tube type freshwater generator system.

Table 3. Thermal characteristics properties of streams 1 through 11 at various locations of the plate type freshwater generator Stream

Table 4. Exergy characteristics rate calculations at various components of the plate type freshwater generator Component

Figure 5. Exergy flow diagram of plate type freshwater generator system.

Figure 6. Comparison of II law efficiency of shell and tube and plate type freshwater generator system. is evident that the plate-type FWG system exhibits higher efficiency than the shell-type FWG system.

Conclusion

The energetic and exegetic experimental performance analysis of a freshwater generator with shell-tube type and plate-fin type heat exchanger is studied to check the possibility of energy conservation and for the improvement of exergy efficiency.: -

1. The plate-type freshwater generator has an exergy

input of 313.86 kW and a total exergy loss of 92.08 kW, whereas the shell and the tube-type freshwater generator has an exergy input of 413.3 kW and a total exergy loss of 185.5 kW.

2. It reveals that freshwater generator plate type heat

exchangers have 15.32% less exergy destruction than shell and tube type freshwater generators. The plate type freshwater generator has an improvement potential of 27.07 kW with a second law efficiency of 70.66

%, whereas the shell and tube type freshwater generator have an improvement potential of 83.28 kW with a second law efficiency of 55.11 %. Which make freshwater generator plate type heat exchanger is more suitable for marine vessels. 3. The fraction of exergy loss in the pump-motor of seawater is 3.1% and the distillate pump is 7.7%. Using a variable frequency drive pump in place of existing pumps is recommended. This exergy analysis of freshwater generation systems is to minimize the irreversible losses through an exergy analysis is a prime area for effective improvement of the efficient use of energy.

Acknowledgement

The authors thank the management of Tolani Maritime Institute for providing the facility of Ship-in-campus freshwater generator for this study purpose.

Data Availability Statement

This study makes use of data from a freshwater generator. All graphics collected or generated throughout the study are included in the published publication.

Conflict Of Interest

The authors declared no potential conflicts of interest concerning this article’s research, authorship, and/or publication.

Greek symbols Ψ Exergy flow: kJ/kg Second law efficiency Chemical exergy of water in seawater: J/kg Chemical exergy of water in sea water: J/kg Performance ratio Improvement potential: kW Subscripts m Mass of stream s Salt w Water sw Seawater in Inlet stream out Outlet stream Superscripts * Restricted dead state o Global dead state

References

1. Ibrahim S, Al-Mutaz I, Wazeer I. Development of ETHICS a steady-state mathematical model for MEE-TVC There are no ethical issues with the publication of this desalination plant. Desalination 2014;351:9-18. manuscript. [CrossRef]
2. Sharqawy MH, Zubair SM, Lienhard V JH. Second [15] Rawabawale N, Sapali SN. Exergy analysis of the law analysis of reverse osmosis desalination plants: cross current cooling tower. J Therm Engineer An alternative design using pressure retarded osmo- 2020;6:499-510. [CrossRef] sis. Energy 2011;36:6617-6626. [CrossRef] [16] Yuksel O, Gulmez Y, Konur O, Aykut S, Erdogan KA,
3. Sharqawy MH, Lienhard VJH, Zubair SM. On Colpan CO. Performance assessment of a marine exergy calculations of seawater with applications in freshwater generator through exergetic optimiza- desalination systems. Int J Therm Sci 2011;50:187- tion. J Clean Prod 2019;219:326-335. [CrossRef]
4. International Association for the Properties of Water and implement solar energy in dairy industries. J and Steam. Release on the IAPWS Formulation 2008 Therm Engineer 2021;7:1216-1238. [CrossRef] for the Thermodynamic Properties of Seawater. [18] Ahern JE. The exergy method of energy systems Available at: https://www.teos-10.org/pubs/IAPWS- analysis. New Jersey, US: John Wiley & Sons; 1980.
5. pdf. Accessed August 2, 2024. pp. 102-106.
6. Kotas TJ. The Exergy Method of Thermal Plant Thermoeconomic analysis with reliability consid- Analysis. London, UK: Exergon Publishing eration of a combined power and multi-stage flash Company; 2012. pp. 33-44. desalination plant. Desalination 2011;278:424-433.
7. Gude VG. Exergy evaluation of desalination pro- mal seawater desalination - A case study. Renewable cesses. Chem Engineer 2018;2:28. [CrossRef] Energy Powered Desalination Handbook. Oxford,
8. Choudhari CS, Sapali SN. Testing of environ- Butterworth-Heinemann; 2018. pp. 491-515. ment-friendly refrigerant R290 for water cooler [CrossRef] application. Int J Engineer 2018;31:157-163. [CrossRef] [21] Li KW. Applied Thermodynamics: Availability
9. Shikalgar ND, Sapali SN. Energy and exergy analysis Method and Energy Conversion. New York: Taylor of a domestic refrigerator: Approaching a sustainable & Francis; 1989. refrigerator. J Therm Engineer 2019;5:469-481. [CrossRef] [22] Hammond GP. Engineering sustainability:
10. Koroglu T, Sogut OS. Advanced exergy analysis of Thermodynamics, energy systems and the environ- an organic Rankine cycle waste heat recovery sys- ment. In: Winnett A, ed. Towards an Environment tem of a marine power plant. J Therm Engineer Research Agenda. London: Palgrave Macmillan; 2016;3:1136-1148. [CrossRef] 2004. [CrossRef]

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