Energy, exergy analysis and working fluid selection of a Rankine cycle for subsea power system

In this paper, a subcritical/transcritical Rankine cycle for subsea power system is investigated. By balancing the pressure between the inner and outer side of the rotary seal, the dynamic sealing problem for the subsea power system is minimized. This Rankine cycle is electrical heated and utilizes the surrounding deep seawater as the cooling source. The mathematical model was established based on the first and second law of thermodynamics and the simulation validation was made. Water-steam, CO2, C9H20, C10H22 and C12H26 are selected as the working fluid and the comparison was made. The results show that the subsea Rankine cycle can obtain a higher thermal efficiency as the working depth increases between 1000 and 5000 m. The thermal efficiencies are obtained as: 19.4-26.7% for water-steam cycle, 9.6-31.2% for CO2 cycle, 9.1-14.7% for C9H20 cycle, 8.0-13.5% for C10H22 cycle and 6.3-11.8% for C12H26 cycle, depending on the working depth. With the same working depth, the water-steam cycle has a higher thermal efficiency, exergy efficiency and turbine power output but requires higher total energy input; while the CO2 cycle can obtain a high thermal efficiency, exergy efficiency with lower energy consumption but produces less turbine power output. In addition, it is found exergy destruction mainly occur in the boiler and condenser, accounting for more than 90% in total.