The enormous weight of lead-acid batteries and limited range of electric cars illustrate the importance of energy density in energy storage technologies.
Researchers at the US Naval Undersea Warfare Center Division have developed a semi-fuel cell, which is a high energy density source for underwater vehicle applications with energy densities approaching 6 to 7 times that of silver-zinc batteries. The new electrochemical system is based on a magnesium anode, a seawater/catholyte electrolyte and an electrocatalyst of palladium and iridium catalyzed on carbon paper.
To put this in perspective compare some of other battery technologies currently in use:
Long a mainstay for undersea vehicle programs, the lead acid battery has been used because of its low cost, known performance, reliability and reasonable cycle life. Its principal disadvantages are low specific energy (30 Wh/kg) and energy density (65 Wh/litre), loss of capacity at low temperatures and the production of hydrogen gas during charges as well as high rate discharges. Nickel cadmium batteries have a specific energy (30 Wh/kg) and energy density (75 Wh/litre) that are comparable to lead acid. Their cost, performance, reliability and cycle life are also comparable. Unlike lead acid batteries, however, cold temperatures do not degrade their performance significantly. A major limitation of the nickel cadmium battery is memory effect, requiring more stringent battery management.
Until recently, the silver-zinc battery has been the battery of choice for long range missions. Silver-zinc batteries are available off-the-shelf and have a higher specific energy (130 Wh/kg) and density (240 Wh/litre) than most other commonly available secondary batteries. High cost, limited cycle and shelf life, and a long recharging process reduce its overall attraction. While at normal discharge rates, 40 to 50 cycles can be expected from the battery, this reduces to 10 or 15 at high discharge rates. Cycle life is also reduced if the battery is discharged below 80% of rated capacity and thus, a 20% reserve is required at the end of the mission. Silver-zinc batteries have been used extensively in AUVs and their performance is reliable and documented. Their high cost and short life have, however, prompted consideration of alternative technologies.
Note that the high cost and short life-time of the silver-zinc battery has restricted its use to specialty applications such as underwater vehicles. Its not clear what the lifetime would be for the Navy semi-fuel cell. Still, its energy density greatly surpasses than of any type of battery that turned up on some Google searches.
|Share |||Randall Parker, 2002 December 26 12:21 PM Energy Tech|