Thermally Boosted Concept for Improved Energy Storage Capacity of a Hydro-Pneumatic Accumulator

Abstract

Hybrid powertrain concepts provide a critical path for improving the fuel efficiency of vehicles. The energy storage capacity plays a critical role in maximizing the hybrid powertrain improvements. This is especially true for hydraulic hybrid vehicles which use a hydro-pneumatic accumulator as the energy storage device. Accumulators have great power density, but low energy density. This dissertation first presents a fundamental thermodynamic analysis that assesses potential benefits and theoretical limitations of accumulators used as energy storage devices. This analysis suggests that isobaric accumulator operation yields the best possible energy storage capacity. The accumulator should be heated and cooled during the expansion and compression stages, respectively, to achieve isobaric operation. This idealized method of heating and cooling yields significant energy storage improvements, but requires further analysis to determine how this can be accomplished in a practical device. A thermally boosted accumulator concept is introduced as a feasible means of improving energy storage by using excess heat from the powertrain system to act as a high temperature heat source. A customized one-dimensional computational fluid dynamics model is developed to investigate the effect of low speed flows on heat transfer between the heat source/sink and gas. Advanced thermodynamic models are used to predict the state of the real gas under high pressure conditions. In addition, a sophisticated metal foam heat exchanger model is incorporated to capture the overall convective heat transfer characteristic. The analysis of the thermally boosted accumulator indicates that isobaric operation can only be maintained for a brief period of time, after which the process transitions into the isentropic conditions in the constant volume chamber due to poor heat distribution. However, the transition into the isentropic conditions in the constant volume chamber can be avoided with the use of a metal foam connected to the metal shell of the accumulator to increase the localized heat capacity of the system. Ultimately the thermally boosted accumulator simulation indicates that more than 60% improvement can be achieved during high power demands, and that its energy storage capacity and efficiency are decoupled from the power demands of the system.