Study of available thermal energy on OTEC

Abstract

Thermal efficiency is commonly applied in the power plants for the purpose of evaluating heat engines. However, the maximization of the thermal efficiency does not lead the maximization of the available power output from ocean thermal energy conversion (OTEC), which converts the finite thermal energy stored between different temperature seawaters into work. The maximum work efficiency based on the theoretical maximum work from the ideal heat engine has been identified for evaluation; however, the implication in the available thermal energy has not yet been substantially identified. This paper theoretically reveals the available thermal energy from the ocean thermal gradient and the effective energy, which is known as the exergy, using the reference of the equilibrium temperature as the dead state. By employing the finite-time thermodynamics, the available energy and the exergy calculation methods for OTEC are obtained. For the evaluation of the systems, the normalized thermal efficiency is proposed instead of the conventional thermal efficiency and the exergy efficiency for OTEC is proposed as well. The results indicated that increases in the normalized thermal efficiency increase the power output from the OTEC system and the maximum normalized thermal efficiency conditions correspond to the maximum power output heat balance. They also showed the effectiveness of the proposed exergy for OTEC derived by entropy generation minimization.