Academia.eduAcademia.edu

Figure 16 – uploaded by Dahiru Lawal

See full PDFdownloadDownload figure
Fig. 14. Second law efficiency of all three systems as a function of mass ratio.

Figure 14 Second law efficiency of all three systems as a function of mass ratio.

Related Figures (22)

irreversibility analysis has also been applied by Mistry et al. [13] to characterize HDH desalination cycles and identified ways to improve the HDH cycles and its components. Al-Sulaiman et al. [14] performed exergy analysis on a proposed hybrid TVC-HDH-RO (humidification dehumidification system coupled with reverse osmosis operated by thermal vapor compressor) desalination system and introduced new performance parameter called the total true specific exergy lost. The new parameter was found to be useful in assessing the exergetic per- formance of the considered system. Ashrafizhadeh and Amidpour [15] performed an exergy analysis on a HDH system and developed equa- tions for exergy analysis of the system. The model developments in- volved a sink and source model, as well as basic relations characterizing the system. Besides exergy analysis, another useful way of analyzing desalina- tion systems is through exergo-economic analysis, which combines exergy analysis with cost by calculating the exergetic as well as the monetary cost of each stream in the system. Jamil et al. [16] presented an exergo-economic analysis of HDH desalination system of the fol- lowing configurations; a basic open-water open-air (OWOA) and a modified closed-water open-air (CWOA) configuration. The analysis was extended by coupling an HDH unit with reverse osmosis (RO) system having different retrofit options. Their results show that the modified cycle has the higher exergetic efficiency and lower product cost ($2.82/m?) when compared to the basic cycle ($6.56/m°). A low product cost of $0.13/m* was achieved from the integrated HDH-RO
irreversibility analysis has also been applied by Mistry et al. [13] to characterize HDH desalination cycles and identified ways to improve the HDH cycles and its components. Al-Sulaiman et al. [14] performed exergy analysis on a proposed hybrid TVC-HDH-RO (humidification dehumidification system coupled with reverse osmosis operated by thermal vapor compressor) desalination system and introduced new performance parameter called the total true specific exergy lost. The new parameter was found to be useful in assessing the exergetic per- formance of the considered system. Ashrafizhadeh and Amidpour [15] performed an exergy analysis on a HDH system and developed equa- tions for exergy analysis of the system. The model developments in- volved a sink and source model, as well as basic relations characterizing the system. Besides exergy analysis, another useful way of analyzing desalina- tion systems is through exergo-economic analysis, which combines exergy analysis with cost by calculating the exergetic as well as the monetary cost of each stream in the system. Jamil et al. [16] presented an exergo-economic analysis of HDH desalination system of the fol- lowing configurations; a basic open-water open-air (OWOA) and a modified closed-water open-air (CWOA) configuration. The analysis was extended by coupling an HDH unit with reverse osmosis (RO) system having different retrofit options. Their results show that the modified cycle has the higher exergetic efficiency and lower product cost ($2.82/m?) when compared to the basic cycle ($6.56/m°). A low product cost of $0.13/m* was achieved from the integrated HDH-RO
To simplify the analysis, the following assumptions are made in the current analysis [10]: Heat losses to the surroundings are neglected. The system runs in a steady-state condition. The kinetic and potential energy terms are neglected in the energy balance. The condensing (high) pressure is 1378kPa (200 psi), while the evaporating (low) Fig. 1. Schematic diagram of an integrated HP-HDH-WH plant.
To simplify the analysis, the following assumptions are made in the current analysis [10]: Heat losses to the surroundings are neglected. The system runs in a steady-state condition. The kinetic and potential energy terms are neglected in the energy balance. The condensing (high) pressure is 1378kPa (200 psi), while the evaporating (low) Fig. 1. Schematic diagram of an integrated HP-HDH-WH plant.
Fig. 2. Schematic diagram of an integrated HP-HDH-AH unit.
Fig. 2. Schematic diagram of an integrated HP-HDH-AH unit.
The mass and energy balance for the main components of the pro- posed HDH desalination systems are as follows [10]: The Energy balance for the evaporator (pre-cooler): Fig. 3. Schematic diagram of a conventional E-HDH-WH system.
The mass and energy balance for the main components of the pro- posed HDH desalination systems are as follows [10]: The Energy balance for the evaporator (pre-cooler): Fig. 3. Schematic diagram of a conventional E-HDH-WH system.
Fig. 4. Effect of dehumidifier effectiveness on Gain Output Ratio and Specific Energy Consumption Fig. 4 indicates that both HP-HDH-AH and HP-HDH-WH units por- trayed superior performance in terms of GOR and SEC as compared to the E-HDH-WH system, with HP-HDH-AH attaining the highest GOR and lowest SEC. This is because a lower amount of energy is required to heat the humidified air as compared to heating water in the case of the water-heated cycle. Furthermore, the improvement in performance of HP-HDH-AH and HP-HDH-WH systems over the E-HDH-WH cycle is because of better condensation of water vapor in the dehumidifier, leading to higher productivity, and consequently higher GOR and lower SEC. Increasing the effectiveness of the dehumidifier leads to a better system performance especially the HP-HDH-AH. This is primarily due to better vapor condensation process in the dehumidifier. The annual capital cost (Z): This cost can be obtained by For the purpose of comparison, we also present cost analysis de- scribed in [33]. This approach is simple and treats the whole plant as a single unit instead of components-wise. The production cost depends on the processing capacity, site characteristics, and design features. The product cost is divided into the direct/indirect cost and annual oper- ating cost. In E-E-M, product cost is obtained by dividing the total cost (comprising the cost of equipment, the labor cost, operation and maintenance costs) by the plant capacity.
Fig. 4. Effect of dehumidifier effectiveness on Gain Output Ratio and Specific Energy Consumption Fig. 4 indicates that both HP-HDH-AH and HP-HDH-WH units por- trayed superior performance in terms of GOR and SEC as compared to the E-HDH-WH system, with HP-HDH-AH attaining the highest GOR and lowest SEC. This is because a lower amount of energy is required to heat the humidified air as compared to heating water in the case of the water-heated cycle. Furthermore, the improvement in performance of HP-HDH-AH and HP-HDH-WH systems over the E-HDH-WH cycle is because of better condensation of water vapor in the dehumidifier, leading to higher productivity, and consequently higher GOR and lower SEC. Increasing the effectiveness of the dehumidifier leads to a better system performance especially the HP-HDH-AH. This is primarily due to better vapor condensation process in the dehumidifier. The annual capital cost (Z): This cost can be obtained by For the purpose of comparison, we also present cost analysis de- scribed in [33]. This approach is simple and treats the whole plant as a single unit instead of components-wise. The production cost depends on the processing capacity, site characteristics, and design features. The product cost is divided into the direct/indirect cost and annual oper- ating cost. In E-E-M, product cost is obtained by dividing the total cost (comprising the cost of equipment, the labor cost, operation and maintenance costs) by the plant capacity.
Fig. 5. Exergy destroyed in the main components of the systems. As presented in Figs. 5 and 6(c), the order of magnitude of entropy generation within the humidifier, dehumidifier and the heater is iden- tical to those obtained in the previous study [13]. It is worth noting that exergy destroyed within the compressor can be greater than any other component if we change the operating conditions or consider lower values of compressor isentropic efficiency, as presented in Fig. 7(a and b) for the case of HP-HDH-WH and HP-HDH-AH, respectively. In fact, entropy generated in the compressor can be > 130% higher than that generated in the evaporator, if we consider a compressor isentropic efficiency of 50%, dehumidifier effectiveness of 95% and humidifier effectiveness of 90%. Such case of high exergy destruction in the compressor can be attributed to the reciprocating compressor being used in the system. The losses are huge due to friction between the cylinder and piston rings, and losses due to wire drawing effect during suction and delivery of the refrigerant, which changes entropy of the system and increases the irreversibility of the system. Although, the lubricant is used to minimize these losses, still frictional losses are more prominent in reciprocating compressors. It should be noted that exergy destruction in the dehumidifier can be higher or lower than that in humidifier depending on the mass flow rate ratio. For both HP-HDH water heated cycle and HP-HDH air heated system, the exergy destruction associated with the humidifier tends to approach zero as MR increases. This shows that the limiting component depends on the mass flow rate ratio. Hence, the effectiveness of the humidifier cannot be further increased, therefore, the designer should
Fig. 5. Exergy destroyed in the main components of the systems. As presented in Figs. 5 and 6(c), the order of magnitude of entropy generation within the humidifier, dehumidifier and the heater is iden- tical to those obtained in the previous study [13]. It is worth noting that exergy destroyed within the compressor can be greater than any other component if we change the operating conditions or consider lower values of compressor isentropic efficiency, as presented in Fig. 7(a and b) for the case of HP-HDH-WH and HP-HDH-AH, respectively. In fact, entropy generated in the compressor can be > 130% higher than that generated in the evaporator, if we consider a compressor isentropic efficiency of 50%, dehumidifier effectiveness of 95% and humidifier effectiveness of 90%. Such case of high exergy destruction in the compressor can be attributed to the reciprocating compressor being used in the system. The losses are huge due to friction between the cylinder and piston rings, and losses due to wire drawing effect during suction and delivery of the refrigerant, which changes entropy of the system and increases the irreversibility of the system. Although, the lubricant is used to minimize these losses, still frictional losses are more prominent in reciprocating compressors. It should be noted that exergy destruction in the dehumidifier can be higher or lower than that in humidifier depending on the mass flow rate ratio. For both HP-HDH water heated cycle and HP-HDH air heated system, the exergy destruction associated with the humidifier tends to approach zero as MR increases. This shows that the limiting component depends on the mass flow rate ratio. Hence, the effectiveness of the humidifier cannot be further increased, therefore, the designer should
Fig. 6. Exergy destruction by percentage in the components of (a) HP-HDH water heated, (b) HP-HDH air heated, and (c) E-HDH water heated.
Fig. 6. Exergy destruction by percentage in the components of (a) HP-HDH water heated, (b) HP-HDH air heated, and (c) E-HDH water heated.
Fig. 7. Effect of compressor isentropic efficiency on the exergy destroyed within the major components. The variation in feed water temperature with the degree of irre- versibility in the humidifier, dehumidifier, expansion valve, compressor, condenser evaporator, and the heater are demonstrated in Fig. 10. It can be noticed that the irreversibility within the humidifier, dehumidifier, evaporator, and condenser is very sensitive to variation in feed water temperature, with the highest exergy destruction recorded in the evaporator for HP-HDH water-heated and HP-HDH air-heated cycles, due to a large temperature difference, which causes greater losses during the heat transfer process. Therefore, the temperature difference should be kept as small as practical e.g. in evaporators and condensers to minimize entropy generation in those components. Ex- ergy lost in both the humidifier and dehumidifier are observed to be more sensitive to the change in feed water temperature for the case of HP-HDH air-heated cycle and E-HDH water-heated cycle. In all the
Fig. 7. Effect of compressor isentropic efficiency on the exergy destroyed within the major components. The variation in feed water temperature with the degree of irre- versibility in the humidifier, dehumidifier, expansion valve, compressor, condenser evaporator, and the heater are demonstrated in Fig. 10. It can be noticed that the irreversibility within the humidifier, dehumidifier, evaporator, and condenser is very sensitive to variation in feed water temperature, with the highest exergy destruction recorded in the evaporator for HP-HDH water-heated and HP-HDH air-heated cycles, due to a large temperature difference, which causes greater losses during the heat transfer process. Therefore, the temperature difference should be kept as small as practical e.g. in evaporators and condensers to minimize entropy generation in those components. Ex- ergy lost in both the humidifier and dehumidifier are observed to be more sensitive to the change in feed water temperature for the case of HP-HDH air-heated cycle and E-HDH water-heated cycle. In all the
Fig. 8. Effect of dehumidifier effectiveness on the exergy destroyed within the major components.
Fig. 8. Effect of dehumidifier effectiveness on the exergy destroyed within the major components.
Fig. 9. Influence of mass ratio on exergy destroyed within the major components.
Fig. 9. Influence of mass ratio on exergy destroyed within the major components.
Fig. 11. Second Law efficiency of all three systems. Fig. 10. Effect of feed water temperature on exergy destroyed within the major components.
Fig. 11. Second Law efficiency of all three systems. Fig. 10. Effect of feed water temperature on exergy destroyed within the major components.
Fig. 12. Second law efficiency of all three systems as a function of feed tem perature.
Fig. 12. Second law efficiency of all three systems as a function of feed tem perature.
Results of some exergetic performance parameters for the HP/E-HDH systems. Table 2
Results of some exergetic performance parameters for the HP/E-HDH systems. Table 2
The rate of investment cost for the main components of HDH-HP/E plants.
The rate of investment cost for the main components of HDH-HP/E plants.
The monetary costs of each stream from the HP-HDH and E-HDH plants.
The monetary costs of each stream from the HP-HDH and E-HDH plants.
Fig. 13. Second law efficiency of all three systems as a function of dehumidifier effectiveness.
Fig. 13. Second law efficiency of all three systems as a function of dehumidifier effectiveness.
Fig. 15. Cost of freshwater of all three systems.
Fig. 15. Cost of freshwater of all three systems.
Fig. 16. Effect of unit electricity cost on desalted water cost for all the system: investigated.
Fig. 16. Effect of unit electricity cost on desalted water cost for all the system: investigated.
Fig. 17. Influence of plant life expectancy on freshwater cost for all the systems investigated.
Fig. 17. Influence of plant life expectancy on freshwater cost for all the systems investigated.
Related topics:
Wast heat recoverySolar Desalination of WaterSeawater DesalinationHumidification and dehumidification
580 California St., Suite 400
San Francisco, CA, 94104
© 2025 Academia. All rights reserved