I’ve got an idea. Tell me what you think. You have an application where you need to store 20 liters of hydraulic fluid to be used at 120 bar. You know that using a conventional accumulator will do it but it is not the most efficient way. The minimum pressure in the accumulator will have to be higher than the working pressure to get any flow and the pressure will increase as the accumulator fills. That means that extra hp will be needed to push the fluid into the accumulator and the extra power will be lost as heat when the fluid is used. You know that a weighted accumulator would be more efficient because all the fluid could be stored at the working pressure but the application would be awkward. The facility uses compressed air at 6.6 bar.
You can take a regular hydraulic cylinder with an 80 mm bore and a 4000 mm stroke and use it to hold the 20 liters of fluid. Then connect the rod of an air cylinder with a 350 mm bore and a 4000 mm stroke to the rod of the hydraulic cylinder. The rod end of both cylinders will be vented to atmosphere. The blind end of the air cylinder will be connected to the unregulated air supply while the blind end of the hydraulic cylinder will be connected to the high pressure fluid source. Because of the relative areas of the cylinder pistons, the hydraulic fluid will have to be at about 130 bar to move the air cylinder. However, as hydraulic fluid enters the blind end of the storage cylinder, the exhausting air from the pneumatic cylinder is pushed back into the air supply and remains at 6.6 bar. This means that all the fluid can be stored at 130 bar. No compressed air will be consumed and the energy will be stored as though you were using a weighted accumulator.
Practical? You decide.
Energy efficient? Absolutely!