Investigation of cryogenic energy on condensation and frosting for water generation

Liu, Meixi ORCID: 0000-0003-4979-8925 (2023). Investigation of cryogenic energy on condensation and frosting for water generation. University of Birmingham. Ph.D.

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Abstract

Due to the increasing demand of freshwater in the shipping industry, desalination is seen as an important solution to overcome shortage of freshwater storage on board ships. On LNG powered vessels, the released cooling energy through LNG regasification process is identified as an effective cooling source for various applications, which could maximize utilization of the energy content within the marine fuel, in turn, to further decrease the carbon intensity of LNG powered vessels. One potential usage is freshwater generation assisted with the cryogenic energy recovery from LNG or other emerging liquid marine fuels in a cryogenic state. This thesis focuses on the evaluation of the freshwater generator assisted by cryogenic energy for marine applications. The following objectives are achieved, including obtaining a comprehensive understanding of the effects of the surface treatment and surface temperature on condensation enhancement and anti-frosting phenomena, developing numerical models of the main components of the freshwater generator for system optimisation, and assessing the integration system and its performance on a real-world LNG powered case ship.

To meet these objectives, a test facility to explore the surface characteristics with varying cryogenic temperature is developed. The results and phenomena account for the mechanisms of condensation and frosting, the frost formation and condensation characteristics, and frosting delay with different surface treatment. The hydrophobic coatings have significant effects on the condensation intensification with an enhancement factor of 2.1 times that of untreated. And for the frosting conditions, the tubes with surface treatments help delay droplet freezing for up to 45 seconds compared to plain tubes. A semi-empirical correlation to predict the frost porosity and the frost properties on cold surface is proposed from the experimental data with an error within ±15%.

A verified theoretical model in Siemens Simcenter Amesim software was developed to predict the performance of the evaporator and condenser in the freshwater generator system in terms of the water production and frost characteristics. The impacts of the operation parameters of the evaporator and condenser are evaluated, showing that the heating capacity, pressure, superheat degree, and flow rate have significant effects on the system efficiency. Therefore, the system employs the superheated seawater and operates in the vacuum condition. Then, the parametric analysis was explored on the geometry of the condenser, where the production increases with the contact area and the heat exchanger material. It is seen that the Aluminium alloy 3003-H14 is the best material to use with a greater heat and mass transfer rate.

Finally, regarding the use of the LNG cryogenic energy, the thermodynamics performance of freshwater generator is evaluated systematically in real-world conditions to satisfy the water demand on board. The feasibility of the integrated system of recovering LNG cryogenic energy on the LNG powered case ship with the surface treatment was investigated and optimised by maximize the freshwater production, exergy efficiency and the economic. The comparison between the bare tube, Fluorinated Ethylene Propylene tube, Polytetrafluoroethylene and Fluorine based tube, shows that the Polytetrafluoroethylene is preferred on the heat exchanger surface with a total water production of 14461 kg and water recovery rate of 67.7%. Compared to the published literature, the proposed technology showed a significant increase in the freshwater recovery rate by 3 times.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):