Electric-Plus-Hydrogen (EPH) Systems
Electric-Plus-Hydrogen (EPH) systems augment the energy storage supplied by batteries, and make sense when a vehicle's daily energy use exceeds the amount of energy that can be supplied economically by batteries alone. EPH systems using hydrogen fuel cells completely eliminate greenhouse gases (carbon monoxide and carbon dioxide), oxides of nitrogen (NOx), and other pollutants, including particulate matter (PM). Systems using hydrogen internal combustion engine (HICE) generators produce trace amounts of (NOx), but eliminate all other pollutants, including greenhouse gases and PM.
Relatively small fuel cells or HICE generators can supply significant amounts of energy if operated continuously throughout a vehicle's duty cycle. This can greatly reduce the size and cost of the vehicle battery pack. TransPower can help you compare the capital and life-cycle costs of a pure battery-electric system with those of an EPH system. If we determine that the cost of the hydrogen system and hydrogen refueling is offset by the benefits of using a smaller, less expensive battery pack, we will provide the best EPH system for your vehicle and application. Following are brief descriptions of some of the EPH system and infrastructure options we can help you implement, with the assistance of our experienced team members.
Hydrogen Fuel Cells
Hydrogen fuel cells provide a promising zero-emission option for extending the range and duty-cycle of battery-electric vehicles. A fuel cell is an electrochemical device that combines hydrogen fuel and oxygen from the air to produce electricity, heat and water. Fuel cells operate without combustion, so they are virtually pollution free. Since the fuel is converted directly to electricity, a fuel cell can operate at much higher efficiencies than internal combustion engines, extracting more electricity from the same amount of fuel. The fuel cell itself has no moving parts - making it a quiet and reliable source of power. [Source: UTC Fuel Cells].
In an Electric-Plus-Hydrogen (EPH) configuration, the electric power generated by a small fuel cell is used to augment the battery pack in supplying power to the electric drive motor. The fuel cell is fueled by compressed hydrogen gas, usually stored in high-pressure tanks carried by the vehicle. To refuel, the vehicle is driven to a specialized hydrogen refueling station, capable of dispensing high-pressure hydrogen into the vehicle’s tanks.
Thor hybrid fuel cell bus deployed by ISE in 2002-03.
TransPower partner ISE Corporation is the world leader in combining batteries with fuel cells to power heavy-duty vehicles. ISE introduced the concept of a battery-fuel cell hybrid bus to the U.S. with its experimental Thor fuel cell bus, which was operated by SunLine Transit and several other California transit agencies in 2002-03. In 2004-05, ISE developed a fuel cell hybrid system for larger transit buses, using high-energy nickel-sodium chloride battery technology. Capping a decade of steady progress perfecting this technology, ISE recently delivered 20 fuel cell hybrid buses to BC Transit in British Columbia, ushering in a new zero-emission transportation service inaugurated during the 2010 Winter Olympic Games.
The ISE fuel cell hybrids developed for BC Transit combine hydrogen fuel cells with lithium battery packs, in a configuration that can be easily adapted to a variety of other bus and truck applications. As battery costs decline, smaller fuel cells can be employed, reducing the cost of the auxiliary hydrogen system (AHS) and the amount of hydrogen fuel required during operation. With ISE's active support, Transpower can provide fuel cell-based EPH solutions customized for many heavy-duty vehicle uses.
Hydrogen Internal Combustion Engine (HICE) Generators
Hydrogen internal combustion engine (HICE) generators are an alternative to fuel cells. HICE generators can approach the zero emission performance of fuel cells by using modified internal combustion engines combined with electric generators.
Ford and BMW are among current manufacturers of hydrogen-burning engines.
Gaseous hydrogen is injected into the engine, which then burns the hydrogen fuel much as it would burn gasoline for combustion. The engine is fueled by compressed hydrogen gas, in the same manner as a fuel cell.
HICE systems that use hydrogen engines to mechanically drive vehicles have been demonstrated in small numbers of concept cars, such as the BMW 745h. The feasibility of using a hydrogen engine in conjunction with an electric generator was demonstrated in a prototype hybrid HICE transit bus by ISE in 2004, and by Quantum Fuel Systems in versions of the Toyota Prius modified to run on a combination of hydrogen and battery power. ISE and Quantum are both Transpower partners.
Hydrogen Storage Systems
Hydrogen can be stored as a gas or a liquid, or even as a solid in the form of a "hydride" – a metal or metal alloy that has the capacity to absorb hydrogen at a certain pressure and temperature.
|Quantum hydrogen fuel tanks mounted on top of fuel cell bus.
For hydrogen vehicle applications, hydrogen is usually stored as a gas, because this is the technologically simplest storage method. The most prevalent forms of hydrogen production – steam reforming of methane (natural gas) and electrolysis – both release hydrogen in gaseous form. The hydrogen gas can then be stored at room temperature by pumping it into a pressurized tank. However, gaseous hydrogen storage has one key disadvantage – the tanks required are very large and heavy. As the lightest of all elements, hydrogen has a density of only 0.07 grams per cubic centimeter, less than 1/10th the density of gasoline. Therefore, for a hydrogen-fueled vehicle to achieve the same operating range as a gasoline or diesel-fueled vehicle, with the same size fuel tanks, the hydrogen must be compressed to at least 5,000 psi. Storage of hydrogen at this pressure requires either metal tanks, which are very heavy, or composite tanks, which are very expensive. Specialized transfer devices and plumbing are also required to transfer high-pressure hydrogen from stationary tanks to vehicles, and from vehicle tanks to the hydrogen-burning engine or fuel cell. Future advances in storage tank technology will enable hydrogen storage at pressures of 10,000 psi or above. These advances will enable increases in vehicle operating range and reduce the size of tanks required to store a given amount of hydrogen at a refueling facility.
|Hera hydrogen storage cannisters for low-power applications.
Storage of hydrogen as a liquid avoids the issues of high-pressure gas storage, but requires the hydrogen to be super-cooled to below its boiling point – minus 423 degrees F (-252.8° C). Liquefaction systems for cooling gaseous hydrogen to liquid form are expensive and energy intensive. Energy equaling 30-40% of that in the fuel is presently required for liquefaction. Storage of the liquid hydrogen then requires specially insulated tanks to prevent the hydrogen from boiling off. Even in well-insulated tanks, hydrogen can be expected to boil off at a rate of 1.7% per day. The combination of the energy losses from liquefaction and the storage losses from boil-off make liquid hydrogen impractical for most vehicle applications today. However, future improvements in the efficiency of liquefaction and in tank insulation technology may eventually make liquid hydrogen a more attractive option. The main advantage of liquid hydrogen storage is that much larger quantities of hydrogen can be stored in tanks of a given size, as compared with storage of hydrogen as a compressed gas.
| Hydrogen fuel tanks mounted on top of fuel cell bus.
Metal hydrides show promise for eliminating many of the issues relating to gaseous and liquid hydrogen storage, but the technology is in its early stages and has not yet been demonstrated in a form that is practical for widespread vehicle application. A metal hydride is formed when gaseous H2 molecules dissociate into individual hydrogen atoms and bond with metal atoms in the storage alloy. Removing heat drives this absorption process, while adding heat reverses the chemical reaction, causing the hydrogen atoms to reform as H2 molecules inside the storage vessel. Two key issues with hydrides are the high weight of the metal hydride compared to the amount of hydrogen stored, and the high temperatures required to release usable hydrogen gas from the hydride. For example, NaAlH4 – even though it is one of the most promising of the hydrides – contains only 4% hydrogen by weight and has a release temperature of 150° C.
Hydrogen Delivery Systems
For hydrogen to be utilized in a vehicle application, hydrogen must be delivered to locations where the vehicles can be conveniently refueled, and equipment must be available at these sites for the simple, rapid refueling of the vehicles. Hydrogen can be purchased in bulk and delivered to the refueling sites via tanker trucks (also known as "tube trailers"), or can be produced on site. Hydrogen pipelines may also be used to bring hydrogen to refueling sites.
Currently, the easiest way to deliver hydrogen is by tube trailer from bulk hydrogen manufacturers. The largest U.S. hydrogen manufacturers are Air Products, Praxair, Air Liquide, and the BOC Group, which collectively operate about 80 hydrogen-producing plants in the U.S. The majority of hydrogen used in the U.S. is currently delivered by pipeline, but pipeline access is presently limited to a few regions, primarily in California, Texas, Louisiana, and Indiana. Customers without pipeline access typically take delivery from these suppliers via trucks, which transport the hydrogen either in liquid form for vaporization on site, or in gaseous form via tube trailers. The easiest way for a hydrogen vehicle operator to receive hydrogen is in gaseous form via tube trailers that are left on site, as no permanent on-site infrastructure is required for vaporization or storage. However, an on-site compressor and refueling equipment are required to refill high-pressure tanks on the vehicles to be refueled.
While hydrogen transportation via truck allows the use of hydrogen with the lowest up-front investment, it is not practical or cost-effective for long term use or in larger hydrogen vehicle fleets. In the absence of a national hydrogen pipeline infrastructure, the best near term hydrogen delivery option is on-site hydrogen production. In the U.S., 95% of hydrogen is currently produced using steam reforming, a thermal process in which hydrogen is extracted from natural gas or other light hydrocarbons by reacting the hydrocarbon with steam. Small-scale reformers can be installed at refueling sites, which can then use natural gas delivered via pipeline to produce hydrogen. This is presently the most cost-effective and widely used method used by hydrogen vehicle fleet operators who want to produce hydrogen on site. However, reforming still requires use of fossil fuels, and as natural gas prices increase, so does the cost of reformed hydrogen.
A promising alternative to reforming for on-site hydrogen delivery is electrolysis, a process in which hydrogen is extracted from water. Electrolysis techniques include water electrolysis, which uses a catalyst and membrane to split water into molecules of hydrogen and oxygen, in a process that is essentially the opposite of how a PEM fuel cell converts hydrogen into electric power. Hydrogenics is a leading supplier of electrolysis systems, having provided hydrogen infrastructure in more than 60% of hydrogen vehicle fueling projects in North America. A similar electrolysis process is used by Proton Energy Systems, a subsidiary of Distributed Energy Systems Corp., in its electrolysis units. Electrolysis units are limited in capacity and remain fairly expensive, but larger, more cost-effective electrolysis systems are likely to become available within the next few years. Some of these may use innovative technologies such as a patented technology developed by Hydrogen Power, Inc. that uses aluminum as a catalyst to inexpensively separate hydrogen from water.