A New Wrinkle in Hybrids Does Away With Batteries

For instance, a United Parcel Service delivery truck the government rolled out in Washington last week was equipped with a prototype hybrid system using hydraulic fluid and a high-pressure pump instead of electrical current and a generator. In this design, energy is stored in a series of pressurized tanks, rather than in nickel-metal hydride or lithium-ion batteries; the energy moves not as high voltage current in copper wires but as hydraulic fluid pressurized to thousands of pounds per square inch.

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