Performance simulation of solar-boosted ocean thermal energy conversion plant
Introduction
Ocean thermal energy conversion (OTEC) is a power generation method wherein the heat energy associated with the temperature difference between the warm surface water and cold deep water of the ocean is converted into electricity [1], [2], [3], [4]. Considerable research effort has been directed to the development of OTEC. Uehara [5], [6], [7], [8] conducted numerous theoretical and experimental studies on the major components of an OTEC plant. The results of these studies revealed that ammonia is one of the suitable working fluids for a closed-Rankine-cycle OTEC plant. However, due to a small temperature difference (approximately 15–25 K) between the surface water and deep water of the ocean, the Rankine-cycle efficiency is limited to be only 3–5%. This results in a high cost of the electricity generated by an OTEC plant. In order to improve the cycle efficiency, other thermodynamic cycles such as Kalina cycle and Uehara cycle that use an ammonia–water mixture as the working fluid have been developed and reported to have better thermal efficiency than the Rankine cycle at the same temperature difference. However, it is evident that increasing the temperature difference between the hot and cold heat sources is the most effective solution to improve the thermal efficiency of a thermodynamic power generation cycle. Saitoh and Yamada [9] have described a conceptual design of the multiple Rankine-cycle system using both solar-thermal energy and ocean thermal energy in order to improve the cycle efficiency. This concept is quite reasonable because good seasonal solar radiation would be expected at many OTEC candidate sites. Further, in order to reduce material cost and attain low electricity cost, Straatman and Van Sark [10] have reported the conceptual description of a unique OTEC system combined with an offshore solar pond called the OTEC-OSP hybrid system. We consider that the combination of OTEC and typical low-cost solar collectors could be another possible way to improve the cycle efficiency and to attain low-electricity cost.
In this study, we describe a first-order simulation model of the OTEC system that utilizes not only ocean thermal energy but also solar-thermal energy; the latter is used as a secondary heat source. A solar collector used in a residential application is simply installed to the conventional OTEC component. This power generation system is termed as SOTEC (Solar-boosted Ocean Thermal Energy Conversion). The performance simulation of a 100-kWe SOTEC plant with three typical low-cost solar-thermal collectors, which increase the turbine inlet temperature of the working fluid, is carried out under the actual weather and sea-water conditions at Kumejima Island in the southern part of Japan. The simulation results of the SOTEC plant are discussed and compared with that of the conventional OTEC plant.
Section snippets
Simulation model of SOTEC plant
Fig. 1, Fig. 2 show schematics of the conventional closed-Rankine-cycle OTEC operation and the proposed SOTEC operation, respectively; these figures show the general arrangement of the heat exchangers, pumps, piping, turbine generator, and solar collector. In SOTEC, we present two probable ways to install solar collector into the cycle, as shown in Fig. 2(a) and (b). In Fig. 2(a), the warm sea water is pumped from the ocean surface and is heated by a solar collector; then, the working fluid is
Results and discussion
First, the conventional 100-kWe OTEC operation was simulated and the heat transfer areas of the evaporator and condenser were determined in advance, the SOTEC daytime operation was then simulated by optimizing the flow rates of warm and cold sea water in order to avoid the required heat transfer capacities of the evaporator and condenser from exceeding the determined values for the OTEC operation: AE = 514 m2 and AC = 478 m2. In SOTEC, the increase in the turbine inlet temperature, i.e. the increase
Conclusion
A solar-boosted ocean thermal energy conversion (SOTEC) system was proposed and first-order performance simulation was carried out. The results reveal that the installation of a solar collector enhances the thermal efficiency of an OTEC plant, particularly in daytime operation. Net thermal efficiency of SOTEC operation with 20-K solar boost is 2.7 times higher than that of OTEC operation under the daytime conditions at Kumejima Island. This results in approximately 1.5-times higher annual net
Acknowledgments
This study was conducted under the Cooperative Research Program of the Institute of Ocean energy, Saga University (Research No.05001A). N. Yamada thanks Dr. Graham L. Morrison, Emeritus Professor of University of New South Wales for his valuable assistance for the TRNSYS simulation and Dr. Takeo S. Saitoh, Emeritus Professor of Tohoku University, for his valuable support.
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