The Tesla, Chevy Volt and Nissan’s Leaf have now been available in the market for a few years. These cars are designed to be chargeable by plugging in to the electric grid at your home. Using electricity to power a vehicle is currently about 1/3 the cost of using gasoline ( which of course may change over time ) The Volt is a hybrid – meaning that it carries a small gas powered electric generator to provide a backup for the batteries and extend the car’s range from about 40 miles to 300 miles. The Leaf has no backup, but has about a 70-100 mile range.
There is a very significant problem with electric cars – recharging them takes a huge amount of energy . The Volt will require about 8kwh and the Leaf more like 20kwh to fully charge. House electrical systems are simply not designed to deliver the required amount of energy to recharge these batteries in a reasonable time. A standard 115V outlet delivers only 1.5kW of power, meaning that charging these vehicles takes anywhere from 5 to 13 hours on a standard plug. The vehicle manufacturers have provided a higher voltage “level 2” charge capability which enables the use of a residential 220V circuit normally used to service electric clothes dryers or kitchen stoves. This supports a charge time of between 1 and 3 hours. However, very few homes have this kind of power available in the garage, and if they do, it is likely dedicated to another appliance.
Consumers currently spend about 10-15 minutes at the gas station to refill their gas tanks. To provide a similar capability for electric cars, manufacturers have developed a “level 3” charge capability, which delivers far higher power levels to cars and intended for deployment at commercial locations. Level 3 charging systems are quoted to take less than 20 minutes to charge a car to 80%. Residential customers wishing to have Level 3 charging capability currently need to have the power company upgrade their electrical service to a commercial-grade high-power three phase 220 or 480 volts, which is very expensive, along with installing a power converter to provide the Level 3 voltages. Power companies have been somewhat reticent to support this as the local grid is not designed to support this level of energy draw.
The lack of a quick charge capability at the home creates significant inconvenience for consumers – if the vehicle charge is used up during the daily commute cycle, the car is unusable until the next morning, and further, there is no backup in case the owner forgets to charge it or there is a power outage at the house.
The three classes of charging capability are summarized in the following table.
|Level 1||120||12||1.44||60||Single||NEMA 5-15R|
|Level 2||208/240||32||6.7/7.7||60||Single||SAE J1772/3|
|Level 3||500||100/200||50/100||DC||N/A||J1772 Combo, CHAdeMO|
Is there a better way to enable fast charging at the home?
Electric cars would seem to be much more attractive if the 20 minute charge could be delivered at the home without re-wiring. Commutes could consume the battery power, and the car would still be available for evening activities and emergencies. Below are some concepts that would enable this capability
There is an opportunity to build a “quick charge” station that can use a standard 115V plug. The strategy here is to put a set of batteries in the garage that essentially trickle charge off of the 115V supply and build up 20-30kvh of energy. While the vehicle is in use, this pack is charging up off the electric grid.
The local battery pack would provide a level 3 interface to the car. Using this interface, the energy from the pack is quickly transferred to the batteries in the vehicle, and then the local charging station begins it’s own longer-term recharge cycle.
The advantages are that the car charges very quickly, in less than 1/2 to 3/4 hour. So that the car is usable for evening activities. In addition, the charging current is spread out over the whole day, which is of huge benefit to the utility’s power generation capability.
This concept becomes practical only when the level 3 charging interface is available on the cars, and when the cost of the battery system is about 10-20% the cost of the overall car. The key technology challenge is in developing batteries that cost less then about $200 per kwh of storage capability, which is just now emerging. Further, one needs to develop low cost electronics for power management to enable interfaces from the batteries to both the 115VAC utility and the 480VAC car
It is expected that there will be a joint marketing effort with the vehicle manufactures for these types of charging stations, as charging times will be a significant barrier, and this approach solves that problem
Alternative Approach #1a
There has been continued development of a concept called a “flow battery” in which electric charge is stored by modifying the valence of molecules in a conductive liquid. The advantage of this kind of storage is that the total storage is defined only by the size of the tanks that hold the liquid, and the power delivery is limited only by the size of the active membrane used to extract or inject electrons.
There are two architecture possibilities for this kind of charging system.
In the first, the flow battery is simply another form of storage used in the garage, with a level 3 interface to the car for quick charging. Although practical to implement now, the cost per kwh of storage capacity has prevented this approach from being practical.
In the second, the car battery itself is a flow cell, and just like a gasoline car, the “discharged” form of the storage media is drawn off the car, and the “charged” media replaces it. The disadvantage of this approach is that the car must then have sufficient storage capacity for both the charged and discharged media, and, as of now, the cost of these storage media have been too high.
As low cost storage media are developed, these approaches could end up being the best option, however, for the time being, a standard lead-acid is the lowest cost, highest density solution and is very practical to implement today
Alternative Approach #2
There is another source of energy that is available at the house – natural gas. The inherent energy in natural gas is very high, more than enough to power a 50-100kw electric generator. The simple solution – install an electric generator at your house running off of natural gas. The generator can be built to very efficiently provide both standard 110/220V as a house electric backup, and a Level-3 charger for the car. The cost for these types of units can be between $5,000 and $20,000 depending upon power delivery capability. Remarkably, the cost of generating electricity from natural gas is far less than the cost of buying electricity from the power company, approximately 2-3 cents per kwh vs. 8-12 cents per kww from the power company. It is potentially cheaper to simply run your entire house off of natural gas based electricity and drop your electrical service entirely. Natural gas rates are low, and expected to stay low for 20 years, so the investment in the generator can be justified.
Alternative Approach #3
Many folks that own an electric car see it as their “second” vehicle. The range and ease of refilling of the gas car is simply hard to beat.
The gasoline car has almost everything you need to charge your electric car. It has the ability to convert the chemical energy in gasoline into the kinetic energy of rotating tires, so all that is necessary is a way to convert that kinetic energy into electricity. A simple approach is to provide a set of “rollers” the car’s tires can spin to generate electricity. Using the conversion factor of 50kw = 67 horsepower, you can charge the batteries by running your gasoline car at about 4000RPM for 20 minutes. This technique is interesting in that the actual hardware needed to support this functionality is relatively simple and “off the shelf”. The actual generator can be a “power take off” or PTO generator, which is designed for farm tractors that have a direct user accessible coupling to the rotating axle. these are remarkably inexpensive, and coupled with an auto, can be a very fuel efficient and cost effective solution.
For more information contact steven <dot> warwick <at> innovationscommercialization <dot> com