Abstract
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.
Highlights
The Earth absorbs approximately 47% of solar energy as heat
This study proposes an effective performance evaluation method for Ocean thermal energy conversion (OTEC) systems to evaluate the energy conversion performance from heat into work as the normalized thermal efficiency and the effectiveness of the system’s performance, which is defined as the exergy efficiency based on finite-time thermodynamics (FTT) for the productive design of the OTEC system, the practical use of the evaluation method, and to apply the exergy and, calculate the theoretical potential energy of OTEC
The conventional definition of the thermal efficiency certainly recognizes the system’s efficiency; this efficiency only shows the ratio of thermal energy and the work from the heat engine because the maximum work condition and maximum thermal efficiency condition are different
Summary
The Earth absorbs approximately 47% of solar energy as heat. About 71% of the Earth’s surface is covered by ocean. In OTEC, the theoretical thermal efficiency is low due to the small temperature difference in heat sources, which is a maximum of approximately 25–30 K This temperature difference between the two seawaters is the energy required to transfer the heat that drives a heat engine to produce power. Wu first applied FTT to the OTEC system with the assumption of an infinite flow reservoir of seawater [9] Their performance index was the ratio of the work output from the heat engine over the maximum available power as the second law efficiency. To evaluate the system performance of a low-grade thermal energy conversion system (LTEC), Morisaki and Ikegami introduced the maxim power ratio, which is defined as the available power from a heat engine over the maximum power output using a Carnot cycle with an ideal heat exchanger [6]. This study proposes an effective performance evaluation method for OTEC systems to evaluate the energy conversion performance from heat into work as the normalized thermal efficiency (the first law of thermodynamics) and the effectiveness of the system’s performance, which is defined as the exergy efficiency (the second law of thermodynamics) based on FTT for the productive design of the OTEC system, the practical use of the evaluation method, and to apply the exergy and, calculate the theoretical potential energy of OTEC
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