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Optimization Model of Water-energy Nexus in the Cooling Towers

Shokri Motlagh, Shamim | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52178 (46)
  4. University: Sharif University of Technology
  5. Department: Energy Engineering
  6. Advisor(s): Avami, Akram
  7. Abstract:
  8. One of the best ways to reduce water consumption is using air cooled heat exchangers, because they work in a completely closed cycle which does not require makeup water. However because of their high surface area and high-technology production method, their production cost is significantly high. Air cooled heat exchangers have relatively high exergy destruction because of low U heat-transfer coefficient. In present research, single-objective and multiple objective optimization problems are developed to minimize total annual cost and exergy destruction. Also the influence of temperature changes on these two objective functions have been evaluated by Monte Carlo simulation. In next step, significance percentage of independent variables is measured by Sobol test. Finally, the influence of each independent variables on objective functions is calculated.Results show that using air cooled heat exchanger instead of wet tower reduce 130 m3 in water consumption daily, but in 5-year period, air cooled heat exchanger imposes 68887 $ more cost than wet tower. The cost of energy consumption of air cooled heat exchanger (76900 $) is more than wet tower (8013 $), which is due to power consumption of cooler fans in exchanger. Therefore, total annual cost of air cooled heat exchanger is more than wet tower. Minimum total cost belongs to 100197 $ and its equal exergy destruction is 359.3 kW. Optimum exergy destruction is 103.8 kW and its equal total cost is 219087 $. Also, Pareto-fronts demonstrate that these two objective functions are in contrast with each other, as increasing one of them leads to reduction of the other one and vice versa. Results of Sobol test show that the diameter, number of fans, number of fins per length, and height of fin are variables which don’t influence on exergy destruction function significantly. Also, the fan diameter doesn’t influence on cost function significantly.Temperature changes of air on the cost objective functions studied by Monte Carlo simulation. The results show that along with temperature improvement, total cost increases. It is due to that by increasing environment temperature, more heat transfer surface area will be required. This problem leads to high cost of exchanger, more pressure loss, and higher operating cost. Results of Monte Carlo simulation for objective function of exergy destruction show that in 120-125 kW, exergy destruction probability is 50 % (the most amount). Probability amount of exergy destruction for the values which are more than 130 kW is relatively low (lower that 15%). Also, results of Monte Carlo simulation for objective function of exergy destruction show that environment temperature improvement leads to reduction of exergy destruction. Because temperature difference of heat-cold fluid will be reduced by increasing environment temperature and heat transfer will be done in lower temperature difference which leads to reduction of exergy destruction. Results of water footprint evaluation for combined cycle technology show that water footprint ratio produced by wet cooler is 88 times more than air cooled heat exchanger.Although using air cooled heat exchanger is more costly than other cooler systems, water consumption of this system is very low, therefore using this system in optimal design conditions in cities like Kerman and Yazd is suggested
  9. Keywords:
  10. Air-Cooled Heat Exchanger ; Optimization ; Monte Carlo Simulation ; Sobol Method ; Exergy Destruction

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