Digitated Energy Storage Devices (DESDs)

The desirability of storing energy in a low cost device with a high energy density, a low weight, long life and that have the ability to absorb and discharge this energy quickly, such as is required in hybrid vehicles for the absorption of the energy wasted in braking and discharging energy quickly as needed for spurts of acceleration, has been the subject of much R& D over the past couple of decades. These are tasks that lead acid batteries do not do well, advanced batteries, NIH and lithium batteries, do better do much better in terms of weight, energy density, and lifetime, but they are much more expensive than the heavy and bulky lead acid batteries that they would replace. Costs of advanced batteries will come down as the scale of production is increased, but not as low as Pb-acid batteries because of the cost of materials. Capacitors charge and discharge rapidly but to date they have not been able to handle the energy needs of automobiles in a reasonable size or cost.

The newest technology being developed to fill this need, that I have heard of, is that being done by Researchers at The University of Arizona who are developing an ultracapacitor technology based on DESDs (Dictated Energy Storage Devices) built on Nani-scale structures that could be used in hybrid vehicles to improve their performance.

DESDs have a very high capacitance-to-volume ratio that's more than 10,000 times larger than a conventional parallel-plate capacitor of the same size. This makes for a device with large capacitance in a small package.

The UA researchers construct DESD capacitors by using commercially available porous membranes as template platforms. The membranes have a pore diameter ranging from 15 nanometers to 1 micron and a hole density of 10 million to 100 trillion pores per square centimeter.

To form the capacitors, the membrane pores are filled with copper to create a large copper surface area in a small space.

This is important because the ability to store electric charge is proportional to the surface area of a capacitor's plates. The honeycomb of conductors formed in the nano-meter-sized membrane pores has a much larger surface area and ability to store electricity than a conductor with just the surface area of the membrane alone.

In addition to making hybrid vehicles more efficient, DESDs also could make them more environmentally friendly because DESDs don't wear out like batteries and would last for the life of the vehicle and beyond.

Limitations

"The limiting factor right now is the low voltage (less than 5 volts) that can be imposed on the DESDs," Professor Olgierd Palusinski, who is leading the effort, said. The voltage limit is caused by the small space between conductors in the membrane. At higher voltages, electricity will spark between the conductors, causing loss of charge in the same way that the static charge on your body will discharge to a doorknob during dry weather.

This voltage limitation can be bypassed by connecting the DESDs in series, with the voltage capacity increasing in direct proportion to their number. Unfortunately, connecting them in series lowers the overall capacitance of the array, lowering the amount of electricity it can store. "But this reduction in capacitance can be compensated by connecting several DESD arrays in parallel," Palusinski explained. The capacitance of devices adds when they are connected in parallel.

Palusinski added. "We are getting close to the commercial development stage, but still need to do additional studies."

Other Technologies

There are at least three other technologies, other than advanced batteries, being developed to meet some of the shortcomings of the lead-acid battery:

Firefly Energy is developing carbon-graphite foam-based a lead-acid battery technology that it claims can deliver a unique combination of high performance, extremely low weight and low cost, all in a battery which utilizes the best aspects of lead acid chemistry while overcoming the corrosive drawbacks of this same chemistry. This product technology delivers to battery markets a performance associated with advanced battery chemistries, but for one-fifth the cost, and can be both manufactured as well as recycled within the existing lead acid battery industry’s vast infrastructure.They claim to be on a pace to see the initial manufacturing of its batteries by late 2007 for use in 2008 Husqvarna lawn and garden equipment.

Axion Battery is developing a lead-acid battery-supercapacitor hybrid that uses negative electrodes made of microporous activated carbon with very high surface area. The result is a battery-supercapacitor hybrid, the e3 Supercell, that is claimed to use at least 70% less lead, offers faster recharge rates, higher power output and longer cycle-life; and can be manufactured in thousands of existing plants around the world. Axion is now producing prototype e3 Supercells in small quantities, although no plans for commercial production have been announced. Their batteries, though relatively inexpensive, and have a higher life than standard Pb-acid batteries, they do not offer the high energy density and are not as low weight as advanced batteries.

EEStore is the developer of the capacitor based electrical energy storage unit (EESU). The EESU is projected to offer up to 10x the energy density (volumetric and gravimetric) of lead-acid batteries at the same cost. In addition, the EESU is projected to store up to 1.5 to 2.5 times the energy of Li-Ion batteries at 12 to 25% of the cost. It claims to remain on track to begin shipping production 15 kilowatt-hour EESU to ZENN Motor Company in 2007 for use in their electric vehicles.

Thanks to jcwinnie at After Gutenberg for the tip on DESDs