Pneumatic hydraulic energy is the energy stored in the form of pressurized fluid, making it an application of fluid power. Fluid power is the use of pressurized fluids to generate, control, and transfer power. Fluid power can be divided into two parts: hydraulics, which stores energy in the gravitational potential energy of a liquid, typically water, and pneumatics, which stores energy in the compression and pressurization of a gas.
The energy stored in a compressed gas is equivalent to the work done to compress the gas. This work is given by the equation:
•is the volume of the container the fluid is in (cubic meters, m3)
•is the pressure of the atmosphere (pascals, Pa)
•is the natural logarithm
Pneumatic hydraulic systems are widely used in industry. It is an efficient and reliable method of energy storage and easy to transport. Pneumatics also have applications in dentistry, construction, vacuum, and braking systems.
Small-scale energy storage of pneumatic hydraulic power can also be used in small mechanical devices such as the hydraulic regenerative braking system. When the driver steps on the brake, the vehicle's kinetic energy is used to power a reversible pump, which can deliver hydraulic fluid from a low pressure accumulator (a kind of storage reservoir) inside the vehicle into a high pressure accumulator. Conversely, when the vehicle needs to accelerate, the fluid flows from high pressure to low pressure reservoirs and uses the pressure fall to accelerate. The hydraulic regenerative braking system is rarely used - it is still being tested, despite the patent having been filed in 1993.