This work presents a model to optimize a parallel plate heat exchanger (ITC2), which corresponds to the evaporator of a secondary thermal circuit, through the NSGA-II approach. The results found that, when applying the methodology proposed for the design of this evaporator by means of multi-objective optimization and the selection of the best configuration of the five possible solutions through the TOPSIS method, point D was the best solution according to the established criteria. It was possible to minimize the number of entropy generation (ESN = 0.058) and the acquisition cost of the equipment (USD 10,385.55), with an inclination angle (20.44), plate height (2070.32 mm), plate width (205.16 mm), plate length (800.49 mm) and heat transfer area of 22.04 m2. This ensures that the heat transferred from the thermal oil (Therminol 75) to the toluene is 693.87 kW. The motor efficiency of the ORC cycle is 41.6%, and the pressure drop is 980.32 mbar, which is within the limits of the admissible engine backpressure range. this proposed methodology can be applied to the thermodynamic and economic optimization of plate heat exchangers in any type of heat recovery system with indirect evaporation of fluid organic matter. This methodology is always more relevant for cases where there are limitations on the heat source. This is the case for industrial engines with medium and high exhaust gas temperatures, and is applicable in cases where the ORC technology has not been widely applied commercially.
Abstract: A multiobjective optimization of an organic Rankine cycle (ORC) evaporator, operating with toluene as the working fluid, is presented in this paper for waste heat recovery (WHR) from the exhaust gases of a 2 MW Jenbacher JMS 612 GS-N.L. gas internal combustion engine. Indirect evaporation between the exhaust gas and the organic fluid in the parallel plate heat exchanger (ITC2) implied irreversible heat transfer and high investment costs, which were considered as objective functions to be minimized. Energy and exergy balances were applied to the system components, in addition to the phenomenological equations in the ITC2, to calculate global energy indicators, such as the thermal efficiency of the configuration, the heat recovery effciency, the overall energy conversion effciency, the absolute increase of engine thermal efficiency, and the reduction of the break-specific fuel consumption of the system, of the system integrated with the gas engine. The results allowed calculation of the plate spacing, plate height, plate width, and chevron angle that minimized the investment cost and entropy generation of the equipment, reaching 22.04 m2 in the heat transfer area, 693.87 kW in the energy transfer by heat recovery from the exhaust gas, and 41.6% in the overall thermal ecfficiency of the ORC as a bottoming cycle for the engine. This type of result contributes to the inclusion of this technology in the industrial sector as a consequence of the improvement in thermal efficiency and economic viability.
The full version of the investigational product is available below: entropy-21-00655 (1). The original article is published in the magazine energies Vol. 12 No.8 (2019): entropy.