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Design and Optimization of a Novel Power Cycle and Seawater Freeze Desalination System by Utilizing the Cold Energy of Liquefied Natural Gas

Eghtesad, Amir Saman | 2021

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54209 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Afshin, Hossein; Kazemzadeh Hanani, Siamak
  7. Abstract:
  8. Access to potable water with standard quality is an inevitable component of human’s lives. Seawater desalination is the precedure of removing the impurities and contaminants in water to achieve the suitable quality for human consumption. Freeze desalination, by consuming relatively much lower energy compared to other techniques has shown to be a promising method for desalination. This technique requires the exertion of a cold source to the system which is then accompanied by simultaneous rejection of the impurities from the water. Regarding this, a numerical study on freeze desalination a hollow cylinder is carried out to determine the effects of the design variables such as heat flux, hydraulic diameter, initial salt concentration, and freezing time on the ice mass, ice salinity, ice generation speed, and Nusselt number on a cold surface of the inner tube. Results show that increasing the value of heat flux from -250 W/m^2 to -1000 W/m^2 imposes 2.9 times faster ice generation speed while decreasing the desalination rate by 22%. In opposite, increasing the hydraulic diameter from 2 to 8 cm, reduces the ice generation speed by 41% but improves its quality by 23.5%. Also, ice production and salinity would both increase with the freezing time which imposes a limit for freezing duration. Multi-objective optimization of the design variables with the purpose of obtaining the highest ice mass with the lowest salinity is performed. Finally, multistage freeze desalination is explored to reach WHO standards.Moreover, utilizing the cold energy of liquefied natural gas can reduce the demand of burning fossil fuels for work production and employing refrigeration cycles for seawater desalination. A novel low-temperature cascade power generation cycle combined with a seawater freeze desalination system is proposed to retrieve the cold energy from liquified natural gas. Binary working fluids are utilized to decrease the irreversibilities of the power cycle and enhance the energy recovery efficiency. The effects of the important temperatures of the cycle, pinch temperature of the heat exchangers, number of turbines and their efficiency on the cycle performance are investigated. Optimization of the work production showed that the cycle could achieve 105 kW per unit mass of liquified natural gas while increasing its temperature to -47.48 ℃. An exergoeconomic model is developed to assess the exergy costs of the system operation and minimize the sum of the unit of production cost. A multi-objective optimization is conducted to maximize the work production and minimize the production costs. Results show that the cycle has the best performance while working at its exergoeconomic optimal point. Using the same amount of cold energy that was used for producing a particular work in the literature, the current system can produce 0.537 to 0.441 kilogram of ice along with generating the same work. Using the remaining cold energy of liquified natural gas for work production in another stage in series with the cycle instead of exploiting it in a freeze desalination system would increase the work production between 5.7% to 9% compared to the literature. Although this leads to an addition of 6.7% to the work production of the cycle, the payback period increases by 25%. Hence, from economical point of view, dedicating a specific portion of the cold energy of liquified natural gas for freeze desalination is more effective than using the entire cold energy for work production. Thus, a limit for the maximum temperature of liquified natural gas should be considered at the exit of the cycle for more efficiency
  9. Keywords:
  10. Potable Water ; Exergoeconomics ; Multiobjective Optimization ; Freeze Desalination ; Fresh Water Production ; Economic Analysis ; Water Quality ; Liquified Natural Gas Cold Energy

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