Optimizing the design of heat exchangers to reduce the cost and maximize the production of freshwater using waste heat from a diesel-gen-set

Abstract

• In diesel engines 33% of energy is expelled into the ambient air. • This waste heat can be used to desalinate seawater to produce fresh water. • The cost of freshwater is found to be US$ 0.0107/L or US$ 11/m 3 . • The capital cost is found to be US$ 435(/m 3 /day). • The payback period is found to 9.62 months. The waste heat from diesel engines can be recovered to produce freshwater from sea- or brackish-water. Diesel generators are used in remote locations and in large ships. In this project, a desalination plant run by the exhaust heat from a large generator of capacity 1.1 MW is designed and simulated. The generator used in this study operates 24/7 throughout the year. With the exhaust heat available, the waste heat recovery (WHR) system manages to produce 11,670 kl/year of freshwater. The freshwater is produced from seawater with a salinity of 3.5%. The concentration of salt in the WHR system is kept below 23% which is well below the saturation limit of salt concentration in the water at ambient temperature. During the optimization process, the total weight of the heat exchangers are minimized to reduce the cost of the WHR system keeping the total pressure drops across the superheater, boiler, economizer, and adjoining pipes/fittings at around 200 kPa so that the exhaust gas after passing through these components has enough pressure to be discharged into the atmosphere. Also, the pinch temperatures of hot and cold fluids in the heat exchangers are kept above 20°C for effective heat transfer. All the components needed are considered and added to the cost. The capital cost, cost of fresh water and the payback period of the WHR plant are found to be $435/ m 3 , $11/ m 3 and 9.62 months, respectively.