# Hydraulic energy recovery for high-pressure, low flow (non-steady state) conditions

What would be the best way to implement hydraulic energy recovery while de-pressurizing a pressurized volume of silicone oil initially at 1500 bar. Can a piston pump be concievably repurposed as a hydraulic motor/turbine? From what I understand, power recovery turbines are generally designed for low pressure high flow applications, and here we have the opposite. We also don't have a steady state flow, but instead just occasional "bursts" of flow as the vessel is depressurized and oil comes out.

• There isn't any energy to recover. The oil is nearly incompressible. The tank may shrink and cause some fluid to flow (hopefully not at 1500bar), but the energy is really trivial. Jan 23, 2021 at 19:17

My perspective -

1. Phil's point should be considered first. The amount of energy available to recover is probably small relative to the cost of energy recovery equipment. However there are special cases with compressible vessels where energy storage may be significant. I'll answer the question anyway assuming you did all the homework on energy storage.
2. Recover energy @ 1500 bar. There are only a handful of manufactures for rotating hydraulic equipment anywhere near that pressure. Here is one I can recommend from having installed their equipment before: https://www.dynexhydraulics.com/product/pf4200-series-pumps/ The 4200 series lists a maximum pressure of 1380 bar. However Dynex often designs for custom applications with extreme pressures or temperatures (down to -50C). Also the 4200 series is listed as a pump but you can ask them about converting the unit for motoring operation.
3. Recover energy @ lower pressure. You could use a simple piston / cylinder to transform pressures. Take a cylinder with 3:1 area ratio. Input flow of silicone @ 1500 bar to the end with smaller surface area. The output flow will be @ 500 bar. Then you have many more options for hydraulic motors to recover energy. You would need a small secondary system to reset piston / cylinder position after each cycle.
4. One of the comments suggested expanding your 1500 bar fluid into a tank containing inert gas. This is certainly possible with significant energy dissipation as the comment noted. If you wanted to go this direction, here is one option for equipment - https://accumulators.com/piston-accumulators/available-models/#1496162639880-4e25dcdc-014b. Again these are rated up to 1380 bar but they should be able to get comfortably above that range with custom accumulators.
5. Side note - most hydraulic motor types are not worth checking for your application. Common varieties based on vane / gear / geroller / lobe / etc are rarely usable above 300 bar. Piston-type motors are the only option worth spending time on.

This looks like an application for a hydraulic motor, these are typically camring, external lobe or internal lobe/geroller. Since (some) hydraulic machines work in short increments too, this should be doable.

You should hunt around for one the works with your silicon oil.

You should try to even out the flow as much as possible, since this will allow you to go for a smaller motor, smaller generator etc. Find out the longest time you have for unloading your system, and size the equipment for that.

Also i really doubt the 1500 bar. Are you sure you don't mean 1500 psi?

• Thank you for your helpful suggestion! Indeed 1500 bar is not a typo. We use a stepper motor to drive a high pressure pump in a system that's been adapted from common rail fuel injection used in diesel engines (where pressures can be as high as 2000 bar). Are there hydraulic motors that are available in the market that can work at such high pressures? Unfortunately, the use case is such that the flow cannot really be evened out. Dec 18, 2020 at 20:55
• I don't think so but I don't know, you'd have to talk to suppliers. if the volume of oil is always the same, you could expand it into a tank containing an inert gas, compress the gas to a few undred bar so the pressure becomes managable and then use the hydraulic motor. This would loose some energy of course.
– mart
Dec 21, 2020 at 7:13