LNG as cold heat source in OTEC systems

In Ocean Thermal Energy Conversion systems (OTEC), the production of electric energy is achieved in tropical areas exploiting the warmer surface seawater as hot source for the vaporization of a specific working fluid, and the coldest water pumped from the depths for its condensation. These systems are very expensive due to both the required large infrastructures and the limited values of thermal efficiency. To reduce the payback period of investment costs, greater values of thermal efficiency are recommended and they could be obtained by an augment of the temperature difference between the two heat sources. In this paper the possibility of using Liquid Natural Gas (LNG) stored at atmospheric pressure and temperature of 113 K in regasification terminals is investigated. With reference to data concerning an LNG regasification plant designed for Gioia Tauro (Southern Italy), the energy performances of a novel OTEC system which operates through two thermodynamic cycles in cascade, using pure ammonia and LNG as working fluids, were evaluated. The proposed plant allows an increase of the electric power recovering the LNG exergy during the gasification process and, at the same time, guarantees the complete gasification of LNG for the immission in the gas pipeline network.