Yasunaga Takeshi, Ikegami Yasuyuki
Institute of Ocean Energy, Saga University, 1 Honjo-Machi, Saga 840-8502, Japan.
Entropy (Basel). 2020 Feb 13;22(2):211. doi: 10.3390/e22020211.
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.
海洋热能转换（OTEC）将存储在温暖表层海水和寒冷深层海水之间的海洋温差中的热能转化为电能。驱动OTEC热机所需的温差仅为15 - 25K，理论上其热效率较低。人们已经开展研究以提出能够提高热效率的独特系统。这种热效率通常用于系统性能指标，并且研究人员一直专注于利用热机更高的可用温差来提高该效率，而没有考虑海水的有限流量和显热。在本研究中，我们的模型展示了一种用于OTEC的新热力学概念。第一步是将OTEC中可转移的热能定义为平衡态和死态，而非大气条件。其次，该模型通过在考虑外部热损失的同时最小化熵产生，展示了可用最大功，即火用的新概念。最大热能和火用使得热效率的第一和第二定律能够归一化。这些评估方法可应用于优化的OTEC系统，并且其有效性得到了证实。
