Who would have ever believed that Gilligan’s Island could teach us lessons in engineering? The “professor” on the 1960s sitcom seemed to use coconuts to build everything shy of a boat for his fellow castaways. Now modern engineers are using coconut shells to create ultracapacitors, which are expected to play a key role in the development of energy storage systems for hybrid vehicles.
It is no garden-variety coconut shell, says Brendan Andrews of Maxwell Technologies, which makes energy storage and power delivery systems. Most of those with the required characteristics are harvested in an area close to the equator, generating activated carbon that is eventually rolled and formed into sheets.
“We put it into modules, and these modules have the higher energy density and the higher voltages,” he says, referring to the can-shaped ultracapacitors that are anywhere from two inches in diameter to the size of a Coke can.
While a battery is used to store energy and can offer long-term voltage stability, an ultracapacitor can support short-term bursts of high power that are needed for tasks such as starting a vehicle or operating the tools on a utility truck. Think of the flow of energy like water escaping from a bucket. If you had a traditional battery, the water would escape from a hole as big as a nail. In a system that incorporates an ultracapacitor, the water could escape through a hole as wide as a basketball.
They are components that will be particularly important in the evolution of heavy-duty hybrid vehicles. An electric car may deal with 30 starts and stops per charge, but its heavy-duty counterpart will need to do that 750 times. And while a car may need 30 kW of peak power, a transit bus might need 200kW.
Refuse vehicles present a perfect application for ultracapacitors, says Rob Delcore, ISE Corporation’s director of business development, energy storage systems. The components’ high discharge rate makes it possible to launch away from a curb more quickly, without generating as much heat as a device that offers more resistance. “If your application requires a huge amount of power, ultracapacitors are it,” he says.
“You can almost think of an ultracapacitor as something that can accept a lightning bolt,” Delcore adds, referring to the high charging rate. And while a lead battery can typically offer 3,000 load cycles, an ultracapacitor can be discharged millions of times.
Systems that blend batteries and ultracapacitors will extend the life of the batteries that like to be discharged gradually, and allow for energy storage systems to be downsized. “It’s a really exciting concept,” he says.
These components are hardly the tools of Star Trek. “This is proven technology,” Andrews says. The ultracapacitors are already being used to increase the cranking power of more than 1,000 buses in service in Asia, the US and Europe. He also referred to the prototype of a 16.2-volt truck starter system has been able to start a 12-litre diesel engine six times on a single charge.
If anything, the ultracapacitors will be crucial to increasing the life of lithium ion batteries that the US Secretary of Energy has suggested will be vital to the future of hybrid vehicles.
The use of lithium batteries will be growing substantially with the introduction of vehicles like the Chevy Volt, says Kevin Snow, chief engineer for hydraulic application development at Eaton. But significant improvements in power density are probably at least eight years away.
Unlike lead acid batteries, the resulting hybrid technologies will need to be part of a larger system that is fully integrated into the chassis, Snow adds.
While lithium ion batteries can be safe, their temperatures need to be controlled to protect performance and the life of the battery itself. As energy storage needs increase, ultracapacitor modules might best be ganged together in a junction box that contains everything from relays to fuses and sensors.
Still, batteries are not the only tools being used to store energy in hybrid vehicles. Another form of hybrid technology has come in the form of accumulators that have been incorporated into refuse trucks and delivery vehicles used by UPS and FedEx.
Accumulators come in a variety of forms. Some store the energy from a battery, hydraulic accumulators store the potential energy in a fluid, and hydropneumatic designs store energy in the form of compressed gas within a fluid container such as a piston or bladder.
Pressurized accumulators made of composite materials were first used in NASA’s rocket casings, and moved into commercial applications as an option when making storage tanks for compressed natural gas, says Rafeal Toledo, an applications engineer with Parker Hannifin’s hydraulic accumulator division.
It is the type of advancement that supports the use of accumulators in the weight-conscious environment of a vehicle. Parker Hannifin’s Runwise system weighs 20% as much as a steel counterpart, can be stored in tighter spaces, and resists corrosion. A 20 US gallon composite model -seen as a typical size for a one-way system -weighs 250 lbs, compared to a 25 US gallon steel version that weighs 1,250 lbs. It can also be charged and dissipated very quickly.
An added advantage is that hydropneumatic accumulator bladders can be easily repaired. But they do require care. The high pressures in most applications will require some attention to sizing and handling. For example, those working with bladder-style versions need to add the first 50 psi slowly until the bladder expands within the shell.
“These systems are out there today,” adds Guy Rini, chairman of the Technology and Maintenance Council task force looking at hybrid technologies. Three Tier 1 suppliers are working with hybrid hydraulic systems while there are more than a dozen working with hybrid electric systems. “On the capacitor, side I know of three systems that are on the road,” he adds.
With all the energy being directed into hybrid vehicle development, there are likely more to come.
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