An alternative regenerative braking system is being developed by the Ford Motor Company and the Eaton Corporation. It's called Hydraulic Power Assist or HPA. With HPA, when the driver steps on the brake, the vehicle's kinetic energy is used to power a reversible pump, which sends hydraulic fluid from a low pressure accumulator (a kind of storage tank) inside the vehicle into a high pressure accumulator. The pressure is created by nitrogen gas in the accumulator, which is compressed as the fluid is pumped into the space the gas formerly occupied. This slows the vehicle and helps bring it to a stop. The fluid remains under pressure in the accumulator until the driver pushes the accelerator again, at which point the pump is reversed and the pressurized fluid is used to accelerate the vehicle, effectively translating the kinetic energy that the car had before braking into the mechanical energy that helps get the vehicle back up to speed. It's predicted that a system like this could store 80 percent of the momentum lost by a vehicle during deceleration and use it to get the vehicle moving again [source: HybridCars.com]. This percentage represents an even more impressive gain than what is produced by current regenerative braking systems. Like electronic regenerative braking, these kinds of brakes -- HPA systems -- are best used for city driving, where stop-and-go traffic is common.
So far, HPA systems have been used primarily as proofs of concept and in demonstration projects only. They aren't quite ready for production models just yet. At present, these hydraulic brakes are noisy and prone to leaks; however, once all of the details are ironed out, such systems will probably be most useful in large trucks weighing 10,000 pounds (4,536 kilograms) or more, where these types of brakes may prove to be a more optimal system than electronically controlled regenerative brakes.
Eventually, this technology may trickle down to smaller vehicles. One company, Hybrid-Drive Systems, LLC, of Michigan, has retrofitted a 1968 Volkswagen Beetle with a hydraulic regenerative braking system. However, the accumulators take up a considerable amount of space, and future production plans are focused more on using the technology in larger vehicles, like vans. Meanwhile, the U.S. Environmental Protection Agency (EPA) has partnered with Eaton Corporation to install hydraulic regenerative braking systems on UPS delivery trucks.