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Abstract: Hydraulic accumulators are used in a variety of applications to minimize the pressure variation in hydraulic circuits and to store energy. Conventional hydraulic accumulators suffer from two major limitations, the hydraulic system pressure varies with the quantity of energy stored and the energy density is significantly lower than other energy domains. In this paper, a novel hydraulic accumulator is presented that uses a piston with an area that varies with stroke to maintain a constant hydraulic system pressure while the gas pressure changes. The variable area piston is sealed with a fabric reinforced rolling diaphragm. In this work, the piston radius profile is developed as a function of the piston displacement and then transformed into a function of the axial contact location between the piston and the diaphragm. The piston profile was solved numerically for a variety of conditions using both transformation methods to illustrate the geometric design trade-offs. Using a variable area gas piston with a fixed cylinder area, the maximum gas volume ratio was 1.8:1. An analysis of the energy density revealed that the constant pressure accumulator provides a 16% improvement in energy density over a conventional accumulator at a volume ratio of 2.71:1 and also exceeds the maximum energy density of a conventional accumulator at the lower volume ratio of 1.8:1. This new promising technology maintains a constant hydraulic system pressure independent of the quantity of energy stored, easing system control and allowing other circuit components to be downsized to meet the same power requirements, while also increases the energy storage density. [Copyright &y& Elsevier]

Abstract: A liquid piston concept is proposed to improve the efficiency of gas compression and expansion. Because a liquid can conform to an irregular chamber volume, the surface area to volume ratio in the gas chamber can be maximized using a liquid piston. This creates near-isothermal operation, which minimizes energy lost to heat generation. A liquid piston eliminates gas leakage and replaces sliding seal friction with viscous friction. The liquid can also be used as a medium to carry heat into and out of the compression chamber. A simulation is presented of the heat transfer and frictional forces for a reciprocating piston and a liquid piston. In the application of an air compressor, with a pressure ratio of 9.5:1 and a cycle frequency of 20Hz, the liquid piston decreased the energy consumption by 19% over the reciprocating piston. The liquid piston and the reciprocating piston exhibited a total efficiency of 83% and 70% respectively. The liquid piston demonstrated significant improvements in the total compression efficiency in comparison to a conventional reciprocating piston. This gain in efficiency was accomplished through increasing the heat transfer during the gas compression by increasing the surface area to volume ratio in the compression chamber. [Copyright &y& Elsevier]

A high pressure, efficient and power dense air compressor/expander is a critical element in an isothermal compressed air energy storage (ICAES) system. Heat transfer is often the limiting factor in realizing efficient and power dense compression and expansion processes. Liquid piston compressor/expanders with porous media inserts have been proposed, in which the porous media serves as heat exchangers. While this concept has been studied through modeling and simulation, it has only been validated experimentally at low pressures (10 bar). This paper studies experimentally the effect of porous media in a high pressure (7–210 bar) liquid piston air compressor/expander, which is the proposed pressure for the ICAES system. Cases with the porous media uniformly distributed and non-uniformly distributed in the chamber are studied with various compression and expansion rates. Results show that at 93% efficiency, the uniformly distributed 2.5 mm interrupted plate porous medium increases power density by 10 times in compression and 20 times in expansion; or at the same power, efficiency is increased by 13% in compression and 23% in expansion. Moreover, the porous medium, if deployed at the top of the chamber, is shown to be more effective than if deployed at the bottom. The results indicate that the added surface area provides the dominant benefits but the porous media also increase the heat transfer coefficient at the same efficiency regime. These results are consistent with and extend the findings from previous low pressure experiments. [ABSTRACT FROM AUTHOR]

The efficiency and power density of gas compression and expansion are strongly dependent on heat transfer during the process. Since porous media inserts can significantly increase heat transfer surface area, their addition to a liquid piston compressor/expander has been hypothesized to reduce the time to complete the compression or expansion process and hence the power density for a given thermodynamic efficiency; or to increase the thermodynamic efficiency at a fixed power density. This paper presents an experimental investigation on heat transfer with porous inserts during compression for a pressure ratio of 10 and during expansion for a pressure ratio of 6. A baseline case without inserts and five cases with different porous inserts are tested in a compression experiment: 3 interrupted ABS inserts with plate spacing of 2.5, 5, and 10 mm and 2 aluminum foam inserts sized with 10 and 40 pores per inch. The 2.5 mm and 5 mm interrupted plate inserts were also tested in expansion experiments. Porous inserts are found, in compression, to increase power-density by 39-fold at 95% efficiency and to increase efficiency by 18% at 100 kW / m 3 power density; in expansion, power density is increased three fold at 89% efficiency, and efficiency is increased by 7% at 150 kW / m 3 . Surface area increase is found to be the predominant cause in the improvement in performance. Thus, a liquid piston compressor/expander together with a porous medium may be used in applications requiring high compression ratios, high efficiencies, and high power density such as in an open-accumulator compressed air energy storage (CAES) system or a compressor for compressed natural gas (CNG). [ABSTRACT FROM AUTHOR]