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

Abstract

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 °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

Organic Rankine Cycle Power Systems: From the Concept to Current Technology, Applications, and an Outlook to the Future

“…In analogy to steam power plants, multiple pressure-level cycles and reheating of expanded vapor have been considered in order to boost efficiency [10,143]. However, the feasibility of these solutions is challenged by the additional plant complexity they imply.…”

“…The criticality with such systems is related to the fully wet expansion process, still posing technological concerns regarding the expanding device. In case very high electrical efficiency is sought, the cascaded cycle configuration can be attractive and its evaluation has driven some interest, presenting several advantages if compared to a single cycle with large pressure ratio [10,43,136,147].…”

“…One of the main and unique advantages of ORC power systems is that the technology is applicable to virtually any external thermal energy source, 2 with temperature differences between the thermal source and sink ranging from approximately 30 to 500 C [10]. ORC systems are therefore technically suitable for the conversion of renewable or renewable-equivalent energy sources such as • geothermal reservoirs (liquid-dominated or steam-dominated, whereby the steam is too contaminated with acid gases and solid particles to be directly expanded in a turbine) • solar radiation • biomass combustion • industrial waste heat recovery • urban solid waste and landfill gas combustion • thermal energy recovery from other prime movers (reciprocating engines, gas turbines, fuel cells, etc.)…”

Energy and Exergy Analysis of a Cogeneration Cycle, Driven by Ocean Thermal Energy Conversion (OTEC)

“…Their study also pointed out that both the thermal efficiency and the power output of the system increased by increasing the temperature difference between warm and cold seawater. One way of increasing this temperature difference is by increasing the temperature of the heat by using solar heat energy [4,5] and the waste heat from the condenser of existing thermal power plants. Another technique that is effective in improving cycle efficiency involves the integration of other components such as a regenerator to the cycle to maximize the utilization of the available heat resource.…”

“…In the case of OTEC, the upper layer (about 100m deep) of the ocean at a temperature of approximately [25][26][27][28][29][30] o C serves as the heat source (hot reservoir) and cold water drawn from the bottom of the ocean (about 700 -1000 m deep) at a temperature of approximately [5][6][7][8][9][10] o C serves as the heat sink (cold reservoir) [1]. The ability of this technology to achieve power generation with thermal energy from the ocean without the burning of any fuel is what makes it very appealing especially with respect to environmental impact, and the use of a constantly renewable resource.…”