A conventional accumulator stores energy for hydraulic systems by compressing an enclosed mass of air, but this air takes up too much volume at low pressure to be practical in applications such as a hydraulic hybrid passenger vehicle. An open accumulator compresses air from the atmosphere to store energy, eliminating the need to store low-pressure air but creating large temperature swings if the heat transfer during compression and expansion is poor. This thesis investigates thermodynamic and heat transfer aspects of an open accumulator to assist in its design.
A thermodynamic model was created to determine the efficiency and required heat transfer for open accumulator designs with a volume 1/5th that of a comparable conventional, or “closed,” accumulator. A heat transfer parameter, Z = hA/V, describes how easy it would be to implement the required heat transfer, with low required values of Z being desirable. A design with only one stage of compression and high wall temperature had a lower required value for Z than the high pressure stages in multi-stage designs. For an open accumulator that provides 20 kW of power in expansion and 840 kJ of energy storage at a pressure of 350 times atmospheric conditions, the volume target was 15.7 ℓ and the required Z values for compression and expansion were approximately 6.2×104 W/m3K.
A computational fluid dynamics model using the program FLUENT was created to investigate whether the required Z could be achieved in a more practical, three-stage open accumulator design. The expansion case of the lowest-pressure stage was simulated, with a required Z value from the thermodynamic model of 3.83×104 W/m3K. The
computational domain was a symmetrical, 3-D, diaphragm-bounded chamber of approximately 0.5 ℓ displaced volume, and a realizable k-ε model was used to model the effects of turbulence. The flow pattern generated during the air intake period dominated the flow during expansion, and peak local heat fluxes occurred where the intake flow patterns drew cold fluid next to the walls. The peak heat transfer for the simulation was 386 W. The mean Z value calculated was 9.79×103 W/m3K, around 1/4th of the required value.