Mains Power and Time Required for Charging EVs and PHVs

ushoys · Tar Sands Joined: Jun 17, 2007 Posts: 28

I apologize if this subject has already been discussed in detail in a thread of its own, but I am new to this and have only just started thinking about the amount of electrical power and time required to re-charge the batteries of electric-powered vehicles.

Gasoline has an energy content of about 160 MJ/gallon (US). So if an average-size/loaded passenger car gets 25 mpg, that is about 6 MJ per mile consumed by the engine. With a gasoline engine thermal efficiency of 20%, that means 1.3 MJ of energy is used to actually move the vehicle one mile.

Assuming a 90% battery charging efficiency and a 90% efficient electric motor/drivetrain, that would mean a charging electrical power consumption of about 500 Watts for one hour for each mile of driving. The maximum practical electrical power supply to a battery charger via conventional 240V household wiring/outlet is about 2 KW. One hour of charging at 2 KW would be worth about 4 miles so charging at this power consumption for 12 hours would be worth about 50 miles per vehicle.

The average year-round electrical power consumption of a family home in the US is currently between 1.5 and 2 KW. So each vehicle charging at this rate for half of each day would increase the total electrical energy consumption of each household by 25%, or a 50% increase for 2 vehicles. There are 110 million households in the US so the amount of additional power required to charge 50 miles for 2 vehicles per household, is 500 GW for half the day, every day. That is about 50% of the total current US generating capacity. Most of this additional load would be during the night when cars are parked, so there would not only be a dramatic increase in electrical energy consumption but also a significant change in characteristic of daily power demand, with peak demand likely occurring during the night instead of the day, as now.

There is talk elsewhere on these boards of charging 70 KWHr car battery systems in 10 minutes at special charging stations. However, this would require about 420 KW of power per battery system. To allow this kind of power flow would require extremely high voltages and/or currents which, in-turn, would require high voltage insulation / large conductors that could not be easily connected to a vehicle system in a temporary fashion.

lper100km

Yes, it has been discussed a few times, particularly in relation to the claims made by high performance EV auto makers.

At the present state of battery technology, it would seem that a charge capacity of around 40kWh would be a reasonable max installed size for an EV. This would provide a balance between battery weight, volume and range at highway speeds. Regardless of that, you are quite right to point out that whatever battery size is installed it has to be capable of being charged from a home power outlet. It would be poor management to have to continually charge from complete discharge every time, but charging power losses mean that the overall efficiency of an EV is reduced somewhat below the road performance claims. Quick charging might be accomplished with the use of storage capacitors, but of course, these need to be charged up to provide the required level of energy. The electrics of such a system would probably be beyond the ability of most people to handle and safety issues abound.

The whole concept of the EV will succeed or fail by the ability to develop sufficiently capable batteries and transmission. The rest is styling, hype and marketing. Since the concept in general will be reliant on home charging over several hours, EVs seem destined to provide basic local driving capability. This is not to write them off at all, just a recognition of their potential place in the future of transportation. If they are to be affordable, they will also likely be more utilitarian than the vehicles the motoring public presently enjoys.

Starvid

ekaggata · Heavy Crude Joined: Apr 12, 2006 Posts: 126

Very interesting comments.
The practical issues are not complete dealbreakers - yes, we need enormous additional supply of electrical power, yes, there are safety issues, system complexity issues etc. They are not technologically insurmountable.

All these problems are very difficult. But what really gives me pause is the sheer enormity of the infrastructure build required. Cables as thick as wrists - or bodybuilders' biceps Smile

- just producing enough physical stuff to make any of this work. It boggles the mind.
You get the same feeling thinking about photovoltaics, about wind turbines, about nuke plants - whatever it happens to be.
I don't have a concrete point to make here .. no pun intended Smile

ushoys · Tar Sands Joined: Jun 17, 2007 Posts: 28

"I work at a restaurant at times, and the power consumption of the kitchen appliances is really astounding..... The main fuse is something like 700-800 amps and the vast majority of those 0.2 megawatts are used in the kitchen...... Anyway, the main wire is as thick as my wrist IIRC. That is, about the same as the gasoline hose at a service station. It shouldn't be hard to use for people at the service station".

Possibly, but 0.42 MW is twice the size of your kitchen feed and those appliances are permanently-wired installations, not quick-connects as they would have to be a battery-charging facility that is advertised as "full charge in 10 mins".

SteinarN · Tar Sands Joined: Sep 20, 2007 Posts: 95 Location: Norway

To deliver 0.42 MW at 230V requires 1056A from a 3-phase system. It is utterly impossible to install such a power delivery system in a normal house. Thats an industrial power supply!

Something more likely could be 400V 63A 3-phase which gives 400x63x1,73=43,6kW. That 70kWh battery would require a charge time of roughly 1h 50m with such a power supply. Hovewer that still would require major upgrade to the electric instalation in the house as well as the cables and delivery system in to the house.

ushoys · Tar Sands Joined: Jun 17, 2007 Posts: 28

"To deliver 0.42 MW at 230V requires 1056A from a 3-phase system. It is utterly impossible to install such a power delivery system in a normal house. Thats an industrial power supply!

Something more likely could be 400V 63A 3-phase which gives 400x63x1,73=43,6kW. That 70kWh battery would require a charge time of roughly 1h 50m with such a power supply. Hovewer that still would require major upgrade to the electric instalation in the house as well as the cables and delivery system in to the house"

Yes, industrial. Nobody was suggesting a 0.42 MW, 10 minute charging capability could possibly be installed domestically. It would only be at commercial battery charging stations, similar to gas stations. Even then it would be very difficult to get that power to vehicles safely and quickly.

The limit for a house charger without any upgrade to existing wiring would be about 2 KW per receptacle. That would take 12 hours to recoup 50 miles per vehicle. Making provisions for even your modest 44 KW at 400 Volts would require a revolutionary change in domestic electrical wiring and service.

Starvid

2 kW for domestic charging seems a little low. I charge my electric scooter at 1 kW.

Just make sure you don't run the stove at the same time as you charge! Razz

In reality power electronics would deal with that. They would register how many amps were available at any given moment and send them into the charger. So at night you could well be charging at 25 A, or more if you have 3-phase.

That is, if you hadn't programmed the car only to be charged according to some other parameters. Like, only when power is less than X cents, except that the car should be charged up to at least 50 % at any time no matter the cost, discharge power to the grid when price is Y or more but stop doing this at 25 % charge etc.
_________________
Peak oil is not an energy crisis. It is a liquid fuel crisis.

Starvid

Starvid

ushoys · Tar Sands Joined: Jun 17, 2007 Posts: 28

"What do you think is the danger? It's not like it's idiots slobbing around with large amount of flammable liquids".

True. It amazes me that there aren't more nasty accidents with people pumping gas into their cars. But my thinking was that to transmit this high electrical power from the charging station to battery sets, it would have to be at high voltages and currents though temporary and quickly connectable/disconnectable arrangements - with attendant risks of danger to people and equipment. But I'm sure nothing noted here is insurmountable.

Good point about the charcteristic of electrical power production to feed large battery charging loads. As I noted in my original post, one would assume that most of this charging activity would take place when most cars are not being driven - i.e. at night. That would allow existing Base Load generating plants to operate at full or higher load round the clock instead of instead of dropping down at night. Currently the South Eastern US electric utility I work for has a peak load of about 34 GW at 4 pm on a hot summer day, dropping to a low of 22 GW at night. That 12 GW swing has to be taken care of by very uneconomical generation. That is typically by adding expensive natural gas-fueled gas turbine and combined cycle peaking and intermediate service units during the day and inefficiently part-loading coal plants at night. Having more load at night would help smooth the daily load profile and allow for better electrical generation plants and policies.

Frank

Actually I think some of your assumptions might be questioned: you could easily pump twice that power into batteries (16+ amps at 240 VAC). I do this routinely in my EV conversion. The average commute in this country is 16 miles one-way. Only about 15% of energy in gasoline goes to move the vehicle. These are minor differences but when taken together have a compound impact in your calculations.

The EV list just had a discussion about A.C. requirements for charging along this line. For an "average" vehicle using golf cart batteries, something like 400 A.C. watt-hours/mile seemed to fall out of the discussion. More efficient batteries, greater use of composites, better aerodynanics, slower speeds, charging at work, etc. can reduce overall power demand and energy requirements but there's no doubt in my mind that electricity demand will climb.

On the other hand there is so much inefficiency in our consumption habits that I believe the typical suburban McMansion could easily save 25%. That can help pay for driving energy.