Efficiency grows from the ground up

Proterra puts its prototype composite body, battery-dominant hybrid-electric transit bus to the test this year

By David Hubbard

The Proterra battery-dominant alternative-fuel transit bus represents a giant step toward the objective to replace standard diesel-electric hybrid combinations. The energy technologies at work in this most recent iteration in zero-emission come from the concept development company, a concept development company headquartered in Golden, CO.

Proterra unveiled its HFC35 at the October 2008 APTA Expo, in San Diego, CA. The vehicle is a 35-ft composite-body bus that incorporates hydrogen fuel cell or all-electric battery power propulsion that is rechargeable from a plug-in system. This unit employed two Hydrogenics 16 kW hydrogen fuel cell APUs and a UQM PowerPhase 150 electric propulsion system.

The bus can accommodate a variety of auxiliary power units (APUs) that include diesel, gasoline and CNG housed in a unique removable drive assembly cradle.

Dale Hill founded Proterra in 2004 with the help of his principal colleagues who together designed, engineered and led the manufacture of the 36 EcoMark I, 45-ft CNG-fueled hybrid-electric shuttle buses built expressly for the Denver 16th Street Mall. That fleet has since logged over three million miles and carried over 165 million passengers, making it the world’s largest and most successful alternative fueled hybrid-electric vehicle transit bus fleet.


The Proterra team, formerly Mobile Energy Solutions, worked in the ATP program the Federal Transit Administration (FTA) funded in the 1990s, which opened the door to composite technology and the NABI 45-ft CompoBus now making a comeback in Los Angeles. Other agencies involved in the project include the California Air Resource Board (CARB) and the Center for Transportation and the Environment.

Proterra is developing the HFC35 with funding from FTA National Fuel Cell Bus Program. Hill says it is the first-ever ground-up design of a lightweight, energy-efficient, alternative-fuel hybrid transit bus. The 35-ft bus features a cab-forward design on European styling for maximum seating to match the average 40-ft vehicle. The all-composite body is 34-ft9-in in length and seats 37 passengers with two ADA wheelchair spots by the two doors.

“This vehicle truly represents a major leap forward in public transit,” he says. “Instead of retrofitting a conventional diesel bus, we have a revolutionary ground-up design.”

Claiming to have started small but thinking big, Proterra is a partner in the FTA Electric Drive Strategic Plan (EDSP), the future of transportation as the Administration sees it. Hill says the goal is to put a $1 million zero-emission transit vehicle into operation by 2012 well before the 2030 mandate under the EDSP.

“At this point we are not even close,” says Proterra marketing director Joshua Goldman.  “Presently most commercially available fuel cell battery-powered buses are barely approaching $2 million. Our fuel cell vehicle is currently in the $1.5 million – $1.75 million range depending on options. All battery, fast charge electric buses are coming down significantly in cost and adaptability for the transit industry. Proterra sees this as a viable path and we are working diligently toward that $1 million goal.”

While it remains to be seen if everyday commercial use of hydrogen fuel cell buses is  truly financially attainable, Proterra is pioneering battery-electric technology it thinks has a chance to actually overtake fuel cells as a viable zero-emissions technology.

“By 2012 we would like very much to see an all electric vehicle equivalent to diesel in performance,” says Goldman. “We want it to cost the same or less than a diesel-powered bus with the benefit of zero emissions.”


The TerraVolt fast-charging energy storage system featured in the HFC35 has the ability to solely power a full battery-electric transit bus. The energy storage system is good for 10,000 total discharge cycles (estimated to last the life of the bus).

The TerreVolt features proprietary Li-ion battery pack from Altairnano, a highly efficient battery management and energy system. The batteries mount into the floor cavity for convenient access.

Goldman says the TerraVolt system recharges through a Proterra rapid charge station in only minutes at route layovers, an overnight plug-in or during operation from any form of APU. The fuel cell hybrid has a range of 250 miles before recharging.

The energy and vehicle management system features a vehicle control computer, remote programming and an advanced driver’s station with action prompts.

The system records information to gauge the greatest efficiencies for future travel on repetitive routes. GPS gating initiates operating modes based on location, altitude and other environmental conditions to run in quiet mode, zero emission or all-battery as circumstances dictate.

The future

Looking the future as an all-electric vehicle, Proterra is geared the bus toward the advent of a public recharging system, in which energy from the grid charges a stationary recharge system for use when the bus pulls up.

“It takes a significant amount of electric energy to recharge the bus batteries in under 10 minutes,” says Goldman. “Using energy directly from the grid cause excessive energy draw from that specific section, and easily require significant upgrades to a city’s infrastructure. With the capability to solely power a full battery-electric transit bus, it is fully rechargeable from a rapid charge station at route layovers, an overnight plug-in or during operation from any form of APU.

In service the TerreVolt system recharges in relation to normal transit duties. To hold the gross vehicle weight to a minimum, Proterra designed a 25-mile vehicle to run approximately two hours at a stretch on an urban route, as opposed to a heavier battery system designed to hold a charge for 150 miles.

Proterra says all-electric propulsion eases the energy drain hauling around the weight of diesel fuel on board. One gallon of diesel weighs seven pounds with an energy equivalent of 33kwt hrs. Batteries of equivalent energy weigh between 500-1,000 lbs, with efficiencies on the electric bus between 15 and 18 mpg.

Coupled with the composite body construction, the lightweight electric bus is an extremely efficient vehicle.

Goldman says the fast blue box  recharge technology strategically placed through a city’s transit bus system could easily supplant the maze of overhead wires used for light rail and trolleys, creating greater efficiencies for much less money.

Beginning this year, the prototype on display at APTA Expo will begin an FTA-sponsored nationwide trial that starts in Columbia, SC. The City of Burbank, CA, also will serve as a national test market for the new hydrogen fuel cell plug-in hybrid bus, running on various routes within the BurbankBus network. BR