Performance of ocean hydrate-based engine for ocean thermal energy conversion system

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.

The performance of a theoretical thermodynamic cycle using clathrate hydrate as the working fluid (later we call it the hydrate cycle) is analyzed with emphasis on the application to “Ocean Thermal Energy Conversion (OTEC)” after organizing the processes in this cycle thermodynamically. OTEC is an electric power generation system utilizing a temperature difference of about 15 K to 25 K between the top and the bottom layers in the ocean. The organic Rankine cycle with fluorocarbons, ammonia or propane is considered to be one of the most effective methods for OTEC. There is, however, a growing tendency to avoid fluorocarbons in industries since they are powerful greenhouse gases. Ammonia is also improper because of not only its corrosion to metals but being a source of acid rain. This research proposes that the hydrate cycle may be an alternative to the above cycles for OTEC. In this cycle hydrate is formed at low temperature and dissociated at high temperature. Energy is generated through the alternate repetition of hydrate formation and dissociation. Various guest substances, neither corrosive nor influential in the environment like noble gases can form hydrates. When the hydrate cycle is operated with the high and low reservoirs at 280 K and 295 K, assuming the use in a temperate climate, the thermal efficiency of the hydrate cycle is 2.15 % for Kr hydrate and 2.58 % for Xe hydrate, which are comparable to that of the organic Rankine cycle: 2.24 % for CH2F2, 3.31 % for C2H3F3, 3.34 % for C3H8. These results indicate the prospects of the hydrate cycle for OTEC as more environment-friendly than the organic Rankine cycle.