Study On The Combined Power And Refrigeration Cycle Based On The Ocean Thermal Energy Conversion

With the problem of the relatively low energy utilization efficiency and environmental pollution,exploitation of the ocean energy has been a major topic in our country,and Ocean Thermal Energy Conversion(OTEC)plays a pivotal role in the utilization of ocean energy.However,thermal efficiency of the OTEC is very low and there are still many technical problems,such as the small temperature difference between the warm sea water and cold sea water,low thermal efficiency,the low heat transfer coefficient in the heat exchangers,a large consumption of the sea water pump,the difficulty in the arrangement of the pipe in the system and so on.This paper mainly focuses on the first three problems to provide theoretical support for practical application.Considering the small temperature difference between the warm sea water and cold sea water,this paper uses solar energy as assisted energy to improve the temperature of the warm sea water.Talking about the low thermal efficiency of the system,this paper proposes the combined power and refrigeration cycle to provide both power and cooling capacity for residents.In this combined cycle,the refrigeration cycle works on the exhaust heat extracted from the turbine in the Kalina cycle.Ammonia-water and isobutane are adopted as the working fluid of the Kalina cycle and the refrigeration cycle respectively.The primary energy ratio of the combined cycle has been almost doubled compared with that of the Kalina cycle.Besides,the exergy efficiency and the primary energy saving ratio in the proposed cycle reach as high as 41.88% and 8.3% respectively.It is also found that when the inlet pressure of the turbine exceeds 2.9MPa and the outlet pressure of the turbine keeps between 1.3 and 1.6MPa in the numerical range,the proposed cycle shows better performance than the stand-alone systems in view of the energy saving.Furthermore,parametric analysis demonstrates that the basic concentration of ammonia-water,the generation pressure,the inlet and outlet pressure of the turbine,the inlet temperature of the turbine,ejector entrainment ratio and the outlet temperature of the condenser II have significant impacts on the coefficient of performance,primary energy ratio,primary energy saving ratio,the output of the turbine,refrigeration output as well as exergy efficiency.As for the low heat transfer coefficient in the heat exchangers,this paper combines the grooved channel which is usually used as the fundamental part of the heat exchangers and the pulsatile flow together to improve the heat transfer rate of the heat transfer plates.The present paper investigates the flow behavior and heat transfer characteristics in the grooved channel for pulsatile flow with a reverse flow through experimental and numerical approaches under different Strouhal numbers(from 0 to 0.125)and different oscillatory fractions(from 0.6 to 1.4).The pulsatile flow patterns are visualized using the aluminum dust method,and the numerical model is validated by the experimental results.It is seen that the flow is less stable in the reverse acceleration phase.At the same time,streamlines,temperature and vorticity in the upper and lower grooves are asymmetrical.Besides,this instability leads to a striking mix between the groove and the main flow,and then contributes to the enhancement of heat transfer.In addition,the onset of the unstable state gradually delays and the duration of the unstable state shortens with the increase of Strouhal number during the numerical range when the net Reynolds number is 375 and the oscillatory fraction is 1.4.Also,it demonstrates that the time-averaged Nusselt number increases with oscillatory fraction.It is further revealed that the maximum heat transfer enhancement factor is 2.74 when the oscillatory fraction is 1.4.These all suggest that the imposed reverse flow is capable of improving the heat transfer.