Thermal Efficiency Enhancement of Ocean Thermal Energy Conversion (OTEC) Using Solar Thermal Energy

Compared to the restriction of intermittency of solar power generation, ocean thermal energy conversion (OTEC) is not only 24/7 base-load, but also comprehensive utilization of fresh water production, air-conditioning, mariculture etc. However, limited temperature difference between warm surface seawater and the cold deep seawater is a crucial factor that restricts the thermal efficiency of OTEC. But today, with the appliance of solar collector in OTEC net power output and the net thermal efficiency have been significantly improved. In this study theoretical analysis and performance simulation of 1MW solar-ocean thermal energy conversion (SOTEC) in South China Sea area is conducted. Net power output and net thermal efficiency of SOTEC with solar-boosted temperature of 20K and OTEC under the condition of weather conditions in South China Sea are compared and analyzed. The results show that the net power output and net thermal efficiency of SOTEC have been significantly improved by combining the solar collector. This study is practical for autonomous supply of islands and coastal areas, and instructive for the comprehensive utilization of renewable energy.

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Finite-Time Thermodynamic Model for Evaluating Heat Engines in Ocean Thermal Energy Conversion

Entropy ◽

10.3390/e22020211 ◽

2020 ◽

Vol 22 (2) ◽

pp. 211 ◽

Cited By ~ 11

Author(s):

Takeshi Yasunaga ◽

Yasuyuki Ikegami

Keyword(s):

Energy Conversion ◽

Thermal Energy ◽

Temperature Difference ◽

Thermal Efficiency ◽

Atmospheric Condition ◽

Ocean Thermal Energy Conversion ◽

Dead State ◽

Heat Engines ◽

Thermal Energy Conversion ◽

Ocean Thermal Energy

Ocean thermal energy conversion (OTEC) converts the thermal energy stored in the ocean temperature difference between warm surface seawater and cold deep seawater into electricity. The necessary temperature difference to drive OTEC heat engines is only 15–25 K, which will theoretically be of low thermal efficiency. Research has been conducted to propose unique systems that can increase the thermal efficiency. This thermal efficiency is generally applied for the system performance metric, and researchers have focused on using the higher available temperature difference of heat engines to improve this efficiency without considering the finite flow rate and sensible heat of seawater. In this study, our model shows a new concept of thermodynamics for OTEC. The first step is to define the transferable thermal energy in the OTEC as the equilibrium state and the dead state instead of the atmospheric condition. Second, the model shows the available maximum work, the new concept of exergy, by minimizing the entropy generation while considering external heat loss. The maximum thermal energy and exergy allow the normalization of the first and second laws of thermal efficiencies. These evaluation methods can be applied to optimized OTEC systems and their effectiveness is confirmed.

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Thermodynamic Analysis of Ocean Thermal Energy Conversion System With Different Working Fluids

Volume 6: Energy ◽

10.1115/imece2017-71764 ◽

2017 ◽

Author(s):

Dashu Li ◽

Li Zhang ◽

Xili Duan ◽

Xiaosuai Tian

Keyword(s):

Energy Conversion ◽

Thermal Energy ◽

Thermal Efficiency ◽

Sea Water ◽

Working Fluid ◽

Ocean Temperature ◽

Water Pump ◽

Ocean Thermal Energy Conversion ◽

Thermal Energy Conversion ◽

Ocean Thermal Energy

A thermodynamic model is developed for ocean thermal energy conversion (OTEC) systems. Considering the narrow temperature range in the evaporator, different refrigerants including R717, R134a and R600 were analyzed and compared under sub-critical state with practical ocean thermal conditions. The results show that larger ocean temperature differences will lead to higher evaporation pressures, and less pumping power requirements for all pumps, i.e., warm sea water pump, cold sea water pump and pumps for the working fluid. The thermal efficiency of different systems and the net power output were found to be closely related to ocean temperature difference, with a positive linear relationship. It was also found that R717 provides the highest thermal efficiency with the least pump power requirement. This working fluid could potentially be used for OTEC system development. This study provides useful insights to the design and equipment selection of OTEC systems.

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Performance Evaluation Concept for Ocean Thermal Energy Conversion Toward Standardization and Intelligent Design

Energies ◽

10.3390/en14082336 ◽

2021 ◽

Vol 14 (8) ◽

pp. 2336

Author(s):

Takeshi Yasunaga ◽

Kevin Fontaine ◽

Yasuyuki Ikegami

Keyword(s):

Performance Evaluation ◽

Energy Conversion ◽

Thermal Energy ◽

Thermal Efficiency ◽

Equilibrium Condition ◽

Evaluation Methods ◽

Exergy Efficiency ◽

Ocean Thermal Energy Conversion ◽

Thermal Energy Conversion ◽

Ocean Thermal Energy

Ocean thermal energy conversion (OTEC) uses a very simple process to convert the thermal energy stored mainly in tropical oceans into electricity. In designs, operations, and evaluations, we need to consider the unique characteristics of OTEC to achieve the best performance or lower the electricity cost of projects. The concept and design constraints of OTEC power generation differ from those of conventional thermal power plants due to the utilization of a low temperature difference. This research theoretically recognizes the unique characteristics of the energy conversion system and summarizes the appropriate performance evaluation methods for OTEC based on finite-time thermodynamics and the equilibrium condition of the heat source. In addition, it presents the concept of normalization of thermal efficiency for OTEC and exergy efficiency based on the available thermal energy in the ocean defined as the transferable thermal energy from the ocean and the equilibrium condition as the dead state for exergy. The differences between conventional thermal efficiency and the effectiveness of the evaluation methods are visualized using the various reference design data, and it is ascertained that there is no clear relation between the conventional thermal efficiency and exergy efficiency, whereas the normalized thermal efficiency is definitely proportional to the exergy efficiency. Moreover, the exergy efficiency shows the effectiveness of the staging Rankine, Kalina, and Uehara cycles. Therefore, the normalized thermal efficiency and the exergy efficiency are important to analyze the heat and mass balance as well as improvement of the system.

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