One prediction on electric cars

[GoldDigger]

Replaceable battery packs are obviously an option and could effectively refuel a vehicle maybe even as quickly as replenishing a liquid fuel tank.
It's something I've seen suggested elsewhere.

Tesla has actually implemented that at a limited number of corporate owned charging stations. In their model it is assumed that on the way back from your extended trip you will stop and get your original battery swapped back in again. Otherwise the warranty and other issues could get out of hand very quickly.
And, of course, since only one model from one manufacturer in involved, standardization becomes very easy.

 

[mivey]

It isn't.
My position is that a watt is an instantaneous unit. That what it is. So watt needs no qualification.It is just a watt.
Of course you can use a qualifier like average to get average power over a period.

You treat watt values and power values differently??? Why?

With dimensional units or values you seemed to take the same stance. You have stated that "kW is an instantaneous value. Time or duration doesn't come into it." The presence of three time dimensions instead of two in the units associated with the value does not mean time or duration does not come into it.

Our meters usually report average watts, not instantaneous watts, especially for AC systems where we would average over a cycle or more. The idea that we report an average watt value but want to call it an instantaneous value seems odd to me. I could understand you trying to make a point that the average value is a scalar instead of a vector but to say it is instantaneous makes no sense.

Why take the position that the default for watt must mean an instantaneous value? Consider that in physics "watt" is an average value and by using the unqualified "watt" one usually means an average value (unless the context is a clear qualifier). It is when referencing power at an instant in time that we would add the "instantaneous" qualifier to get "instantaneous watts".

Even with balanced, steady-state three phase where the power is constant for the system as a whole (the time-varying components cancel but a constant component remains when you sum the three instantaneous power functions), we still have to consider what happens with the individual phases and that the power is cyclical in nature or we will not be able explain the actions in the individual phase conductors and how the physical arrangement can make them change.

I see that saying watt values are instantaneous and that time doesn't come into is opinion rather than fact. I can agree with that persepctuve from a limited viewpoint (like with constant DC values).



As for facts supporting both average and instantaneous values, consider the following returned via a simple search (that mirror the countless others you can find):

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html#c2

Concerning reactive power from Pender's EE principles reference:

The name "wattless component" is sometimes used for reactive component, since the average watts corresponding to this component of current or p.d. is zero; the instantaneous watts corresponding to this component, however, are not zero. Hence the adjective "wattless" is misleading.

and for a local other than myself we have from our very own winnie: http://forums.mikeholt.com/showthread.php?t=94657&p=780615#post780615


Anyway, I hope my position is clear enough and I fail to see the value in saying that the kW value used in utility billing is instantaneous.

Can't think of anything else to say at this point other than I think I disagree with what I think you were saying but I still doubt I'm completely clear on what you meant.

 

[mivey]

enter service station and robotics to pop the batteries out and input new batteries?

That is interesting and I have not thought about that. Kind of like the propane bottle swap service.

 

[Besoeker]

You treat watt values and power values differently??? Why?

I don't. The watt is the unit of power

With dimensional units or values you seemed to take the same stance. You have stated that "kW is an instantaneous value.

Yes. Because it is.

Our meters usually report average watts,

My domestic meter records kWh. It's an integrated value.

especially for AC systems where we would average over a cycle or more. The idea that we report an average watt value but want to call it an instantaneous value seems odd to me.

You're right. That's exactly why I wouldn't do it.

Why take the position that the default for watt must mean an instantaneous value?

Quite simply, because that's what it is.

I see that saying watt values are instantaneous and that time doesn't come into is opinion rather than fact.

It isn't opinion. It's just what it is.

.As for facts supporting both average and instantaneous values, consider the following returned via a simple search (that mirror the countless others you can find):

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html#c2
From your link

" the instantaneous electric power in an AC circuit is given by P=VI where V and I are the instantaneous voltage and current."
" the average must be obtained by integration."

Anyway, I hope my position is clear enough and I fail to see the value in saying that the kW value used in utility billing is instantaneous.

And I didn't. I didn't mention billing.

Can't think of anything else to say at this point other than I think I disagree with what I think you were saying but I still doubt I'm completely clear on what you meant.

Quite simply that a watt is a watt. That's all.
I accept that, in AC circuits particularly, an average value is taken and stated without that qualification. It isn't normally necessary to do so. In the same way you will give a voltage as say, 240Vac. You and I both know it means RMS even though that is not, as a rule, explicitly noted. They are useful conveniences. Conventions if you like. Common usage.

We do that in life. With a lot of things. If I ask our stationery custodian, a pretty lady as it happens, for a pen, I'll get a ball-point*. Paper will be A4. No more detail required.

But a watt is still a watt.

Here's a pretty picture I made for here when power factor was being discussed:



*In my younger day they were called a Biro after the inventor, L?szl? J?zsef B?r?.
Another bit of trivia for the pub quiz...........

 

[gar]

140926-0750 EDT

I agree with Besoeker that a watt is an instantaneous unit, voltage, and current are also instantaneous units of measurement.

An ordinary AC wattmeter does not provide an instantaneous power measurement, but rather a short time average measurement. The same for AC voltmeters and ammeters.

An ordinary DC voltmeter does not read the instantaneous DC voltage, but rather a short time average DC voltage.

An oscilloscope provides an instantaneous measurement of a signal input within its bandwidth, but some sort of average measurement for signals that exceed its bandwidth. This same statement applies to ordinary AC and DC instruments.

An ordinary moving coil DC meter can approximate a ballastic galvanometer over some limited range. It can be used to estimate the duration of a short pulse with certain background information.

When I started this thread I never expected it to go off on these tangents.

The Tesla prediction is interesting. Will it prove true is quite debatable.

Locally I get second hand information that electric cars are not being sold, but are primarily being leased. But it is clear that if government regulations further increase mileage requirements that more electrification is required. Regeneration and operation of the internal combustion engine near peak efficiency become important.

A tremendous amount of input energy in local driving can be saved with some electrification of cars.

At our local green fair earlier this summer Ford had a display showing their hybrid system. This uses a mechanical system to combine two electric motor-generators and the internal combustion engine to run the IC engine near a optimum point when on and provide a variable output shaft speed and charging or discharging of the battery. The motor-generators are really synchronous motors with permanent magnet fields. There is a lot of power capability in a small package.

.

 

[ggunn]

This is a silly argument, IMO. In AC power the fact that in theoretical terms a Watt is an instantaneous quantity is a useless distinction; 120 times a second the voltage crosses zero so the voltage current product is zero at those times no matter what. In order to determine how many Watts a load is consuming you have to average consumption over a finite interval of time for it to mean anything in the real world.

 

[gar]

140926-1307 EDT

From http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=2&pid=2&aid=7
The power capability of North America was 1,253,306 kW in 2011 or1.2 * 10^9 W.

From http://cuos.engin.umich.edu/ the power ouput of their high power laser is around 10^22 W/sq-cm. The spot size is small so that does not tell me the actual instantaneous power but it is large.

The distinction between instantaneous, short time average, and long term average power is very important.

Look at the light output of an ordinary incandescent light bulb with 60 Hz excitation. You will see a 120 Hz modulation of the light intensity. Greater with a 757 pilot light, less with a 100 W bulb, and much less with a 10,000 W bulb.

I once built a garage light control system with a 15 Hz modulation of the headlights.

Compare an internal combustion engine with a DC motor. Instantaneous peak power output of an IC engine is about equal to its maximum continuous power output. The peak power capability of a DC motor, excluding inertia, is possibly in the range of 10 to 100 times its continuous power capability.

There is an important difference between instantaneous and average power.

Why does a circuit breaker trip on transformer inrush? Because there is a difference between the instantaneous and average capability of the breaker. The average power dissipated in the breaker from the inrush peak is much lower than is the instantaneous power. The average power used to launch a rocket ship would never get it off of the ground unless the instantaneous power was sufficient to overcome the downward force of gravity.

.

 

[FionaZuppa]

http://en.wikipedia.org/wiki/Electric_power

power is energy, no way around it
W =J/s
J = N m
its work

and hmmm, 1J = 1/3600th of 1 Wh, interesting linkage huh.

joule
jo?ol,joul/
noun
noun: joule; plural noun: joules; symbol: J

• the SI unit of work or energy, equal to the work done by a force of one newton when its point of application moves one meter in the direction of action of the force, equivalent to one 3600th of a watt-hour.

 

[GoldDigger]

http://en.wikipedia.org/wiki/Electric_power

power is energy, no way around it
W =J/s
J = N m
its work

NO!
What you are saying is the same as saying that MilesPerHour is distance just because it has miles in the list of units.
Or that acceleration (m/sec²) is also distance. It is not.
Or that force (N) is also energy since it is Joules/meter. They are related, no more than that.
Power is a time rate of energy transfer, and does not even make sense for any situation in which energy is not going anywhere nor changing form.
Typically in a system of mechanical units there are only three independent units, most often chosen to be time, mass and distance. All other units are derived from combining those three.

Then there are a few oddball units, like temperature, which can actually be derived from the other three, but generally nobody but a physicist has any idea how, so we use arbitrary units like degrees Celsius or Fahrenheit instead.

Besoeker: Habeo.

 

[Besoeker]

power is energy, no way around it

Sorry. That's just plain wrong.
From your link:

W =J/s

Sinks your contention at a stroke.

 

[jumper]

http://en.wikipedia.org/wiki/Electric_power

power is energy, no way around it

Um, your own wiki article states that it is different.

Electric power is the rate at which electric energy is transferred by an electric circuit.

 

[rbalex]

power is energy, no way around it

If you want to cite something authoritative about SI units you should probably go to the source: The Bureau International des Poids et Mesures (International Bureau of Weights and Measures).

Table 3 will be especially enlightening. I linked to it but you can "drill down" from the home page (English version upper right hand) if you care to via / "practical information" / the "SI" menu (top of page) / "SI derived units" / "Units with special names and symbols; units that incorporate special names and symbols: see radian, steradian, hertz, newton, pascal, joule, watt, coulomb, volt, farad, ohm, siemens, weber, tesla, henry, degree Celsius, lumen, lux, becquerel, gray, sievert, katal" / Table 3.

Interestingly enough neither energy (Joule) nor power (Watt) are fundamental units, but power is indeed the rate of energy.

 

[jumper]

If you want to cite something authoritative about SI units you should probably go to the source: The Bureau International des Poids et Mesures (International Bureau of Weights and Measures).

Table 3 will be especially enlightening. I linked to it but you can "drill down" from the home page (English version upper right hand) if you care to via / "practical information" / the "SI" menu (top of page) / "SI derived units" / "Units with special names and symbols; units that incorporate special names and symbols: see radian, steradian, hertz, newton, pascal, joule, watt, coulomb, volt, farad, ohm, siemens, weber, tesla, henry, degree Celsius, lumen, lux, becquerel, gray, sievert, katal" / Table 3.

Interestingly enough neither energy (Joule) nor power (Watt) are fundamental units, but power is indeed the rate of energy.

Nice link. Thanks.

 

[Besoeker]

If you want to cite something authoritative about SI units you should probably go to the source: The Bureau International des Poids et Mesures (International Bureau of Weights and Measures).

Table 3 will be especially enlightening. I linked to it but you can "drill down" from the home page (English version upper right hand) if you care to via / "practical information" / the "SI" menu (top of page) / "SI derived units" / "Units with special names and symbols; units that incorporate special names and symbols: see radian, steradian, hertz, newton, pascal, joule, watt, coulomb, volt, farad, ohm, siemens, weber, tesla, henry, degree Celsius, lumen, lux, becquerel, gray, sievert, katal" / Table 3.

Interestingly enough neither energy (Joule) nor power (Watt) are fundamental units, but power is indeed the rate of energy.

Being a Brit, I routinely use SI.
Being an ancient Brit, I was brought up with Imperial.
I went through CGS, MKS, and lastly SI.

I'm still conversant with Imperial. Length/distance for example, gives you miles, yards, feet and inches. And some of the more obscure like links, chains,furlongs. I know that a mile is 1760 yards. Or 5280 feet. Know but not a need to know. And lots of other things in a similar vein.

But, in business we use entirely SI.
What I like, is the simplicity of way the units hang together and how the need for conversions is largely avoided.
Just multiples and sub-multiples in many cases.
One kilometer is 1000 metres, a nanometre is 1*10^-9metres.

All a bit off topic except that electric cars have batteries and motors usually rated in SI units.
Neither the volt nor the amp has an Imperial equivalent. So you are obliged to use SI for some things anyway.

 

[K8MHZ]

The hertz is used only for periodic phenomena, and the becquerel is used only for stochastic processes in activity referred to a radionuclide.

I did not know that.

 

[mivey]

The watt is the unit of power

Yes, we all know that. What was being discussed at that point was the values the utility measures and bills.

With dimensional units or values you seemed to take the same stance. You have stated that "kW is an instantaneous value

Yes. Because it is.

We bill for an average kW value, not an instantaneous kW value.

My domestic meter records kWh. It's an integrated value.

In addition to the energy billing I know you are aware of demand billing, which uses an integrated value of kW.

I see that saying watt values are instantaneous and that time doesn't come into is opinion rather than fact.

It isn't opinion. It's just what it is.

Despite the evidence I have posted to the contrary. If you have any evidence to support that all watt values are instantaneous values then please post it.

I fail to see the value in saying that the kW value used in utility billing is instantaneous.

And I didn't. I didn't mention billing.

But your response was to a post that was discussing that very thing so that dog won't hunt my friend. Besides, in response to my statement about your opinion that "watt values are instantaneous" you just finished saying "It isn't opinion. It's just what it is.". You say it in one sentence then say you don't say that in another. Why the zig-zag? Are you really trying to explain what you mean or be cute? I don't mind some Tom-foolery from time to time but come on now.

Quite simply that a watt is a watt. That's all.
...
But a watt is still a watt

OK, it's a watt but I have provided links that show there are both instantaneous and average values of watts.

Perhaps you are making some statement about a unit definition or something you consider to be important. All that is great but regardless of a standard unit definition or whatever, we can talk about values that may have path or time dependencies that can't be ignored. Evidently you see some value in focusing on the units or something but I am just not picking up the value of what you are putting down.

 

[mivey]

When I started this thread I never expected it to go off on these tangents.

But not shocked I'm sure.

Locally I get second hand information that electric cars are not being sold, but are primarily being leased.

I've seen two type. The one with all-electric laments over the charging costs. The one with the hybrid is quite happy to pay more and is convinced he will save money over time but even if he doesn't he is having fun with the experiment.

 

[GoldDigger]

As I see it, for power you either have an instantaneous value or the time average of an instantaneous value.
The thing that makes energy (at least transferred energy) different is that it cannot ever be an instantaneous value. Any energy transfer will take a finite (although sometimes very short) period of time.
Note that I specifically qualified this as a transfer of energy. The kinetic energy or potential energy of a body or a system, among other examples, can have an instantaneous value without any problems.

 

[mivey]

In AC power the fact that in theoretical terms a Watt is an instantaneous quantity is a useless distinction; 120 times a second the voltage crosses zero so the voltage current product is zero at those times no matter what. In order to determine how many Watts a load is consuming you have to average consumption over a finite interval of time for it to mean anything in the real world.

That's what I'm thinking. I'm sure Bes has some valid point he is making and I hate to miss an opportunity to learn but I'm just not there.

 

[mivey]

power is energy, no way around it
W =J/s
J = N m
its work

Methinks your typing got out of hand again. Perhaps you need to re-think what you meant to say as what you typed is wrong.