[SGVLUG] Off-topic - for Tom - plug-in car that canpower yourhouse

[David Lawyer]

On Mon, Apr 16, 2007 at 09:20:09AM -0700, Emerson, Tom (*IC) wrote:
> > -----Original Message----- Of David Lawyer
> > On Fri, Apr 13, 2007 at 09:46:00AM -0700, Jeremy Leader wrote:
> > 
> > > So for example, I commute 20 miles to work, and 20 miles 
> > > back.  So I could
> > > just about do my daily commute for only the cost to 
> > > recharge the battery 
> > > each night [...]
> > 
> > Note that [snipping several really good sounding stats...]
> 
> > So is electric power for automobiles more energy 
> > efficient?  For using electricity the 30% efficiency is 
> > reduced by: 
> 
> [snipping more equally-valid seeming arguments]
> 
> > increase in rolling resistance [...]
> 
> Whups -- gonna call you on that.
> 
> The batteries will only increase the "rolling resistance" if they are
> actually touching the ground (and, umm, happened to be round and you
> were using them as tires...)  If you are suggesting that the increased
> *weight* will increase the rolling resistance -- yeah, probably true,
> but this is offset by the fact that hybrids and electrics are typically
> fitted with "LRR" tires -- Low Rolling Resistance -- which is
> accomplished by filling them to a higher tire pressure (like a bicycle
> tire -- something like 90 psi)

In comparing an electric car with a gasoline one, a fair comparison
would be for them to use the same type of tires.  The gasoline auto
could also use the LRR tires.  One drawback to LLR tires is that they
damage the pavement more.  I've heard the pavement crack when riding
over it in a bicycle and according to studies, trucks do the lion's
share of damage to pavements with their high pressure tires.
> 
> > [...] and aerodynamic drag due to the batteries.
> 
> <ding> number two: again, the batteries will only affect the
> aerodynamic profile of a car if they are OUTSIDE of the car (i.e.,
> the battery compartment somehow disrupts the airflow)  Hybrids and
> electrics are actually designed to have a lower CD than your average
> car (CD = Coefficient of Drag), so in this case I believe you are
> mistaken.

Batteries occupy volume and with the same degree of streamlining,
higher volume means more aerodynamic drag.  It's not a lot
however.

One point you didn't mention is that hybrids use more aluminum, etc.
than gasoline auto that tends to make them get better mileage.  It's
not necessarily more efficient if you factor the energy cost of making
the aluminum, etc.

So in comparing electric cars to gasoline cars, the cars need to be of
similar type construction for a fair comparison.  Otherwise one could
erroneously claim that a gasoline auto gets better mileage than a
diesel auto by making the gasoline auto with lightweight materials,
better streamlining, and high pressure tires, but not doing this for
the diesel auto.

			David Lawyer