Performance Analysis of OTEC Plants With Multilevel Organic Rankine Cycle and Solar Hybridization

Among the renewable energy sources, ocean energy is encountering an increasing interest. Several technologies can be applied in order to convert the ocean energy into electric power: among these, ocean thermal energy conversion (OTEC) is an interesting technology in the equatorial and tropical belt, where the temperature difference between surface warm water and deep cold water allows one to implement a power cycle. Although the idea is very old (it was first proposed in the late nineteenth century), no commercial plant has ever been built. Nevertheless, a large number of studies are being conducted at the present time, and several prototypes are under construction. A few studies concern hybrid solar-ocean energy plants: in this case, the ocean thermal gradient, which is usually comprised in the range 20-25 degrees C in the favorable belt, can be increased during daytime, thanks to the solar contribution. This paper addresses topics that are crucial in order to make OTEC viable, and some technical solutions are suggested and evaluated. The closed cycle option is selected and implemented by means of an organic Rankine cycle (ORC) power plant, featuring multiple ORC modules in series on the warm water flow; with a three-level cycle, the performance is approximately 30% better if compared to the single-level cycle. In addition, the hybrid solar-OTEC plant is considered in order to investigate the obtainable performance during both day and night operation; this option could provide efficiency benefit, allowing one to almost triplicate the energy produced during daytime for the same prescribed water flow.