From: gary@ke4zv.uucp (Gary Coffman)
Newsgroups: sci.energy
Subject: Re: SOLAR efficiency
Date: 5 Oct 92 15:50:36 GMT
Article-I.D.: ke4zv.1992Oct5.155036.11517
Reply-To: gary@ke4zv.UUCP (Gary Coffman)
Organization: Gannett Technologies Group

In article <1992Oct4.122100.9126@gallant.apple.com> ems@gallant.apple.com (Mike Smith) writes:
>In article <1992Sep30.102738.14004@ke4zv.uucp> gary@ke4zv.UUCP (Gary Coffman) writes:
>>
>>Well, on the flip side, my central unit, with a SEER of 10.6 can
>>bring my brick house with white roof and tree shading back down to
>>68 degrees after a 90 degree day in about 10 minutes, using about
>>2 kWh in the process. Meanwhile your window unit has used about
>>7 kWh keeping the house cool all day. 
>
>Mighty precient of you to be able to see the duty cycle of my wall
>unit in the white space of my posting.  Care to fill in the turn off
>times for the two scenarios (start at noon vs start at 7pm).  And
>while your at it, tell me the BTU capacity of my wall unit...
>
>While it is fun to pull numbers out of the air, please don't presume
>that they have any validity ...

I figured that since you said "little wall unit"  you were referring
to a 5,000 BTU model, about the smallest available. The largest wall 
unit I've seen is 20,000 BTU.  If you have one of those, then multiply 
the electricity usage figures by about 3, the bigger units are more 
efficient.

>You have as much chance of telling me how many kWh I use as you have
>of telling me the species of trees in my yard ...

You've previously given the approximate square footage of your home,
you've mentioned the daytime temperature in the house without cooling,
you said "small wall unit", etc. I'm willing to bet I'm close on the
power consumption estimate of 7 kWh from noon to 7pm. If anything, I'm 
on the low side.

>>>The house IS a storage system...
>>
>>But not a very good one.
>
>Quite good enough.  It has a serveral hour lag relative to the sun.
>(That is, it takes several hours for the solar heat to reach the 
>interior)  Go back and re-read the statement about tons of stucco
>and drywall and ...

And mine's brick with drywall etc. Probably not much different in
thermal lag than yours, and 2100 square feet. When you start talking 
about foot thick masonry, *then* you're talking storage.

>>Yes I'm presuming exactly that. While intra-regional interties exist,
>>inter-regional ties aren't practical on the scale required. The losses
>>are too great for transcontinental tie lines without superconductors,
>>and the grid management problem would be incredible.
>
>A 1000 or so mile intertie exists in the Pacific Northwest, I think.  I'm
>also a bit surprised that in one breath you talk about the DC technology
>that is used to prevent grid management problems and in the next
>you assert that long interties can't exist due to these ... 

How far is it from Southern California to Georgia?  I make it about
2,800 miles. A thousand miles doesn't get you to Texas. EHV tielines
mitigate an otherwise intractable grid management problem, but they
don't eliminate it. The losses are also 2.8 times worse on a trans-
continental line than on a 1,000 mile tieline. To supply an entire
region's peaking load, the tie line would have to be very low resistance
or the IR drop becomes really excessive. Can that reasonably be done with 
aluminum, or even copper? 

With an 800,000 volt line, you need 1 ampere of capacity for every 800 
kW transferred. To send one nuke's worth of power, 1000 MW, requires 
the tie to carry 1250 amps. Copper has a resistance of 10,700 ohms per 
circular mil per 1000 feet. The usual rule of thumb is that it takes 
700 circular mils to carry 1 ampere of current continuously and safely. 
So we need a cable of 875,000 CM. That gives us a resistance per 1000 
feet of 11.44 ohms. That means we lose about 14.3 kV per thousand feet or 
the entire 800,000 volts in about 56 miles. That obviously won't do. 
So let's look at it from the other direction. Let's assume we can
tolerate a 10% loss on a 2,800 mile line. That means the total line
resistance can't exceed 64 ohms, or .00432 ohms per 1000 feet. That
would be a cable of 2.48 million CM, or about 1.6 inches diameter.
Well what do you know, you could do it after all if you installed
roughly 224 million pounds of copper line per 900 MW you wished to
transfer. What's copper going for per pound these days?

Gary
Newsgroups: sci.energy