Entropy generation minimization of one and two-stage tube in tube ammonia evaporators cooling high pressure gaseous hydrogen for vehicle refuelling

摘要

Gaseous hydrogen as an automotive fuel is reaching the point of commercial introduction.To facilitate this coming fleet of hydrogen vehicles,refuelling stations must be implemented.To ensure a costumer acceptable refuelling duration without overheating the vehicle's hydrogen tank,hydrogen must be supplied at a temperature of-40XC.This paper presents a study on the design of coaxial tube in tube ammonia evaporators for three different concepts of hydrogen cooling,one one-stage and two two-stage.A multi objective optimization has been imposed to minimize the entropy generation rate and evaporator size.For the two-stage concepts,the optimal intermediate temperatures have been found by minimizing the thermally driven entropy generation rate.A zero-dimensional numerical heat transfer model of the tube in tube evaporator is developed in Engineer Equation Solver using heat transfer and pressure drop correlations from the open literature.With this heat transfer model the optimal choice of tube sizes and circuit number is found for all three concepts.The results showed that cooling with a two-stage evaporator after the pressure reduction of hydrogen yields the lowest entropy generation rate,49％lower than the highest,which was encountered with an one-stage evaporator after the pressure reduction.This entropy generation reduction requires an increase in evaporator size of 59％compared to the one-stage cooling.Two-stage cooling with the high-stage before the pressure reduction and the low-stage after,resulted in a 42％reduction of entropy generation but equally 59％of evaporator size increase.