What are Mechanical Batteries?

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Although every part of a Lightning Hybrids hydraulic hybrid system is essential to the system’s performance, perhaps the signature component is the high pressure accumulator. It’s an energy storage device, and can be thought of as a “mechanical battery”. In this article we’ll take a look at its history, its function and the team who builds it for us.

Lightning Hybrids’ first project was the LH4 sports car, which was designed to deliver fuel efficiency of 100 miles per gallon through the use of a hydraulic hybrid drive. We soon found that even the small hydraulic accumulators that were on the market were too heavy, being made of thick steel. So our team, led by Dan Johnson, designed a lightweight composite accumulator made of a thin steel or aluminum vessel wrapped with carbon fiber composite material. Accumulators have to withstand very high internal pressures, which is where the strength and stiffness of the carbon fiber composite comes in.

In October 2012, Lightning Hybrids sold its accumulator assets, manufacturing equipment and related intellectual property to a CoorsTek group that created our sister company, Steelhead Composites, LLC, located in Golden, CO. This spin-off allowed us to focus on our hydraulic hybrid business, while having a close partner to develop and supply accumulators to meet our increasing demand. Steelhead has grown along with Lightning Hybrids in the past few years, almost mirroring the number of new hires. They have increased their product line to include other products such as fuel storage vessels for vehicles running on compressed natural gas (CNG).

Steelhead Composites’ facility, which consists of two closely located buildings, uses state of the art equipment to manufacture accumulators from aluminum tubing and spun carbon fiber. Sections of aluminum tube are formed into the shape of a vessel, called a liner, in a dedicated machine known affectionately as the “fire-breathing dragon”.

Later in the manufacturing process, the carbon fiber composite overwrap is applied over the liner on a winding machine using a proprietary process.winding

For Lightning’s application, an additional layer of fiber glass composite is applied at the end of the process to provide impact resistance to the accumulators, making them safe to deploy under a truck or bus.

Since the accumulator has to operate reliably at high pressures repeatedly over many thousands of cycles, manufacturing is carefully controlled, with testing and inspection being key parts of the process. The high pressure accumulators are specifically designed such that a failure over repeated cycles results in a leak and not a catastrophic burst. Steelhead’s website lists the tests that are employed to ensure the accumulators aren’t going to fail  in operation, including bonfire testing, hydrostatic proof testing, drop testing, burst testing and gunfire testing.

So how does an accumulator work and why is light weight so important to Lightning Hybrids’ product?

A hydraulic accumulator is a cylindrical vessel for hydraulic fluid. Inside the vessel is a nitrogen-filled rubber bag, called a bladder, which is compressed when hydraulic fluid gets pumped into the accumulator. It takes energy to compress the bladder – just like it takes energy to compress a spring. A fully compressed bladder represents a lot of stored energy which on a hydraulic hybrid system can be released to propel the vehicle – hence the analogy of a “mechanical battery” for the hydraulic accumulator.Accumulator

“Regenerative braking is all about capturing kinetic energy, storing it and releasing it,” says Ian Patterson, our lead hydraulics engineer. “Steelhead’s composite high pressure accumulator is perfect for the job as it gives us the hydro-pneumatic storage we need, in a lightweight, reliable package that scales to even the largest trucks.”

For us, an accumulator needs to provide reliable energy storage with the desirable attributes of low weight and high power density. Power density is a measure of how quickly a storage device can absorb and deliver energy, which is an important parameter for a hybrid system running on a medium or heavy-duty vehicle. (Electric batteries have a relatively low power density, which makes them better suited to light vehicles.) Low weight is important too, since adding weight reduces the payload capacity of the truck or bus (a serious problem with battery-electric hybrids).

Our system is designed so that a “fully charged” accumulator, where the bladder is compressed to its minimum volume, experiences a pressure of 6,000 psi (414 bar). Traditionally, pressures like this would require a steel vessel that is extremely heavy, compromising the benefits of the hybrid system. The design based on composite material from Steelhead delivers an accumulator that weighs about one third to one fourth of its steel equivalent, while safely supporting the maximum operating pressure.

One of the key design features of the Lightning Hybrids hydraulic hybrid system is that it can easily be installed on a wide range of vehicles. To accommodate vehicles of different rated weights, we simply use smaller or larger accumulators, ranging from 10 gallons to 30 gallons (US). (We also add a second pump/motor for larger vehicles, which our modular design easily accommodates.)

As Lightning Hybrids’ customer base expands rapidly, Steelhead will be there for us, with both companies enjoying larger sales volumes and commercial success. “Steelhead’s relationship with Lightning Hybrids is one of the joys of working here,” says Heather Somervill, Steelhead’s sales account manager for Lightning. “Not only are our histories inextricably linked, but we’re still joined at the hip in many ways, commercially, technically and culturally. We’re excited about where Lightning’s headed in the market place, and we’re going to ride this wave together.”