If a hydrogen-oxygen fuel cell is designed to operate also in reverse as an electro-lyzer, then electricity can be used to convert the water back into hydrogen and oxygen. This dual-function system known as a regenerative fuel cell (also called uni-tized regenerative fuel cell, URFC), is lighter than a separate electrolyzer and generator and is an excellent energy source in situations where weight is a concern.
Scientists at AeroVironment of Monrovia, California and NASA developed a propeller-driven aircraft called Helios, to be used for high-altitude surveillance, communications, and atmospheric testing. Helios was a $15 million dollar, solar-electric project. The unmanned aircraft had a wingspan of 75 m and was described by some as more like a flying wing than a conventional plane. In 2001, in a test flight, Helios reached an altitude of almost 29.4 km, an altitude considered by NASA to be a record for a propeller-powered, winged aircraft. Helios was designed for atmospheric science and imaging missions, as well as relaying telecommunications up to 30 km. The 5-kW prototype was powered by solar cells during the day and by fuel cells at night (a URFC device). Regretfully, in another test flight in 2003, Helios crashed.
For automotive applications, the Livermore National Laboratory and the Hamilton Standard Division of United Technologies have studied URFCs in great detail and found that, compared with battery-powered systems, the URFC is lighter and provides a driving range comparable to that of gasoline-powered vehicles. Over the life of a vehicle, the URFC was found to be more cost-effective because it does not require replacement. In the electrolysis (charging) mode, electrical power from a residential or commercial charging station supplies energy to produce hydrogen by electrolyzing water. The URFC-powered motor car can also recoup hydrogen and oxygen when the driver brakes or descends a hill. This regenerative braking feature increases the vehicle's range by about 10%, and could replenish a low-pressure (about 14 atm) oxygen tank, the size of a football.
In the fuel-cell (discharge) mode, stored hydrogen is combined with air to generate electrical power. The URFC can also be supercharged by operating from an oxygen tank instead of atmospheric oxygen to accommodate peak power demands such as entering a freeway. Supercharging allows the driver to accelerate the vehicle at a rate comparable to that of a vehicle powered by an ICE.
The URFC, in a motor car, must produce ten times the power of the Helios aircraft prototype, or about 50 kW. A car idling requires just a few kilowatts, highway cruising about 10 kW, and hill climbing about 40 kW, but acceleration onto a highway or passing another vehicle demands short bursts of 60 to 100 kW. For this, the URFC's supercharging feature supplies the additional power. A URFC-pow-ered motor car must be able to store hydrogen fuel on board, but existing tank systems are relatively heavy, reducing the car's efficiency or range. Under the Partnership for a New Generation of Vehicles - a government-industry consortium dedicated to developing vehicles with very low fuel consumption - the Ford Corporation provided funding to Lawrence Livermore National Laboratory, EDO Corporation, and Aero Tec Laboratories to develop a lightweight hydrogen storage tank (a pressure vessel). The team combined a carbon-fiber tank with a laminated, metalized, polymeric bladder (much like the ones that hold beverages sold in boxes) to produce a hydrogen pressure vessel that was lighter and less expensive than conventional hydrogen tanks. Equally important, its performance factor - a function of burst pressure, internal volume, and tank weight - was about 30% higher than that of comparable carbon-fiber hydrogen storage tanks. In tests where cars with pressurized carbon-fiber storage tanks were dropped from heights or crashed at high speeds, the cars generally were demolished while the tanks still held all of their pressure - an effective indicator of tank safety. Unlike other hydrogen-fueled vehicles in which refueling needs depend entirely on commercial suppliers, the URFC-powered vehicle carries most of its hydrogen infrastructure on board. Unfortunately, even a highly efficient URFC-powered vehicle needs periodic refueling, and until a network of commercial hydrogen suppliers is developed, an overnight recharge of a small motor car at home would generate enough energy for a driving range of about 240 km (150 miles), exceeding the range of present-day electrical vehicles. With the infrastructure in place, a 5-min fill up of a 350 atm (5000 psi) hydrogen tank would give a range of 580 km (360 miles). The commercial development of the URFC for use in automobiles is, however, at least five to ten years away.
Utilities are also looking at large-scale energy storage systems employing regenerative fuel cells. The proposed systems store or release electrical power through an electrochemical reaction between two liquid electrolytes such as sodium bromide and sodium polysulfide stored in tanks. Inside the cell, the two electrolytes are separated by a thin, ion-selective membrane. Inside this big rechargeable battery-like device, when subjected to current during charging, bromine is produced at the anode. The bromide ions in the electrolyte combine with bromine to give perbromide ions. In the discharge cycle, perbromide is converted to bromide ions, producing at the same time electric energy . Systems based on vanadium salts or zinc/bromine are also being developed and are commercially available [103, 104]. Such regenerative fuel cell storage systems are expected to store up to a whopping 500 MW of energy for up to 12 h .
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