EEStor Advanced Capacitor Best Use In A Hybrid Car

11 comments

June 24, 11:43 AM

by John Ryden, Global Warming Examiner

 

 

Zenn Electric Car

The amount of energy that can be stored in a capacitor is determined by the formula:

E = (C * V²) / 2

E = Energy in Joules

C = Capacitance in Farads

V = Voltage

Being able to increase the voltage improves the storage capacity exponentially as the voltage increases. For example, conventional capacitors operating at 2.7 volts would require 45 million farads of capacitance to hold the equivalent of one gallon of gasoline (12.7 KWH). At 400 volts (a number I picked for comparison purposes only), a capacitor would only need 560 farads of capacitance. (EEStor actually hopes to get 3.5KV which would only require 7.3 farads.)

The production of these new capacitors has been delayed; my guess being that they are running into engineering problems trying to bring this technology from the lab to commercial scale production. When they are produced, Zenn Motor Company is planning on using them to power their all-electric vehicle. [I actually saw a Zenn all-electric car on the way to work today. Was in the city and I assume this car was running on lead-acid batteries.]

Assuming that supplies of this new capacitor may be limited, or that the capacitor may not be as inexpensive as EEStor expects, the best value-add for this technology will be for use in gasoline electric hybrid vehicles. A gas electric hybrid vehicle will typically get between 20% and 30% better fuel economy than a comparable gas only vehicle. It gains this advantage by using an energy storage device like a battery or capacitor, to smooth out the power flow from the engine and to recover the energy from braking. An engine that doesn’t need to provide bursts of power for accelerating can be smaller and can be tuned to only run in its most efficient power range. You don’t need a lot of capacity to perform this type of energy buffering. My guess is that about 300 Watt Hours (wh) would be sufficient. That’s the energy equivalent of about 3 ounces of gasoline or enough power to drive a 40 mpg car about 1 mile. This obviously would not work as a plug-in hybrid, but the technology could turn a 30 mpg car into a 40 mpg car.

Why not just use a 300 WH battery? The problem with a battery of that size is that it probably would not be able to produce sufficient current to drive the car. A battery has limitations on how fast it can charge and discharge. Because a battery changes chemical composition, it can degrade after too many cycles and start to loose its ability to hold a charge. A battery can also be very sensitive to temperature. You need a large size battery to be able to provide enough current to provide acceptable acceleration.

None of these should be problems for a capacitor. Because it is a solid state device, it should charge and discharge very fast. Because there is no chemical change, it should have a virtually unlimited number of cycles and should not be very sensitive to temperature. That means the capacitor buffering device can be relatively small.

A capacitor could also be used with a battery pack for plug-in electric hybrids (if battery storage is cheaper per wh). The battery in this car would not have to be as capable because it would only be used to recharge the capacitor. Charging the capacitor could be done at any power level, much in the same way a camera flash uses a capacitor to build up a charge from the battery. The battery in a camera can not produce enough current to provide the burst of power for the flash so it builds up current in a capacitor that is discharged to produce the flash. In this mode the battery provides an electrical assist to the car in a similar manner that the battery in a Toyota Prius provides a mechanical assist to the car.

If the capacitor (or capacitor/battery combination) becomes slightly larger, say 1.6 Kwh of capacity, then a plug-in hybrid becomes practical. At 1.6 Kwh, a 40 mpg car will be able to travel 5 miles on a charge. If all the trips are 5 miles or less then the car would never have to burn any gasoline. If longer distances are traveled the effective mpg is still enhanced. Based on average commuting distances (10% numbers are my estimated breakdown of a single number for comparative purposes) then 52 mpg could be achieved compared to 40 mpg for a non-plug-in hybrid. [This chart is only to show the greatly increased fuel economy based on even a small storage device and doesn’t reflect any actual car that I know about.] The key point here is that the first small 300 Kwh storage device adds more value per device than each incremental storage device added.

If you could charge the car at intermediate points, then fuel economy would improve even more. The capacitor has the advantage of being able to store its charge very quickly. I could easily picture grocery stores, offering free, quick charges to their customers. Just like selling milk below cost to get shoppers in the store. 1.6 Kwh of electricity would be only 16 cents at 10 cents per Kwh.

Notice that the most benefit from the energy storage device is from the first 300 wh of storage. This improves the car’s mpg from 30 to 40 (assuming a 30% improvement) by going to a hybrid. The next 970 wh of storage improves it another 30% to 52 mpg on average. If EEStor’s device turns out to be more expensive than they are anticipating, it would still have a tremendous value add to hybrid car technology as a small device. You would achieve overall better fleet fuel economy for 211 cars each with 300 wh devices than one all-electric car with one 64 Kwh storage device. General Motors could use this device in their Chevy Volt.

This is the type of technological development that might qualify for the type of program that John McCain suggested should earn a $300 million prize. I would suggest that the government might offer tax credits on the purchase of hybrid electric, plug-in hybrid electric and pure electric cars as a way of stimulating demand. (John McCain also suggested a $5,000 tax credit to US Automakers for every zero-carbon emission car they sell.) Our automakers are going to need help if we expect them to make the financial investments in new, low carbon emission vehicles. Tax Credits might do more good than just offering a one-time prize.

I wish EEStor the best of luck in getting their new storage device into production. This is technology that could fundamentally change how cars are designed, significantly reduce our dependence on foreign oil, and improve global warming.

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