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Re: Solar Energy



Stephen Lajoie <lajoie@eskimo.com> wrote:

>How do you propose to save energy over a six month period?

How about a 16' solar closet? Some 55 gallon drums full of water inside
a cube, surrounded by 2' of insulation. The insulation would cost about
$1/ft^2 and the drums are free, sometimes, sometimes $15 each. The box
itself might cost 50 cents/ft^2 of all-around surface.

A cubical solar closet, L feet on a side, has a time constant of about
L^2 days. A 16' solar closet would take about 288 days to cool to 70 F,
if it sat outside at a constant temperature of 32 F, with no sun.

A 3' cube full of water surrounded by 6" of insulation has an RC time
constant of 432 hours, or 18 days. So if it starts out at, say 117 F,
and sits outside at 32 F, it will cool to 70 F according to the equation
70 = 32 + (117-32) exp(-t/18), where t turns out to be 14.5 days. 

An 8' solar closet with 18 55 gallon drums full of water, and 1' of
insulation, would cool from 145 F to 70 F in 46 days, sitting outside
in the same cold, cloudy weather.

>How do you plan to move all this energy about?

Moving heat is difficult. One can move air efficiently through large
ducts over short distances, say 20', and hot water over longer distances,
using air-water heat exchangers, but that is more expensive.

>For the UMPTEENTH time... There was NO allowances for reliability...

I wonder, when you say "reliability," Steve, are you talking about
the fact that some days are cloudy, or that solar equipment breaks? 

>There was NO allowance for clouds, fog, dust...

Clouds and fog are covered in NREL's solar data book, which gives averages
and extremes for amounts of sun and outdoor temperatures. Cloudy days
reduce solar input by about 50% where I live. This is fairly predictable,
over long times. I've read that dust on glass, even very dusty looking
glass, reduces the sun's intensity by at most 6%.

>I mention the need for storage, and someone says that they can save 90% of the
>energy put into a battery. Yeah, sure.

That was me, altho you are misparaphrasing. Having studied batteries and
patented one battery system, I recall that lead-acid battery charging is
about 90% efficient, ie if you do it right, you have to put 10% more energy
into a battery than what you can extract. That's for the battery itself.
That doesn't include the efficiency of the charging circuit.

>Over a period of three months?

Who said anything about 3 months? Of course batteries discharge over time,
altho not much if they are kept cold. But if they are cold, they have
limited capacity, until they are warmed up to room temperature, at which
point their capacity returns. Cold batteries are also difficult to charge,
and they last a lot longer than warm ones.

>Just the temperature difference between 80 F and 0 F (summer to winter) will
>reduce that down to 40%,

I wonder where you got that number? I wasn't aware that just temperature
cycling had any effect on battery capacity or state of charge, in itself.
Warm batteries do self-discharge more rapidly, but you seem to be saying 
something about temperature cycling...

>I think that most people that don't live in third world countries will 
>not accept the poor reliability that such small scale solar systems would 
>provide.

Again it seem strange to talk about the sun as being "unreliable." It is,
of course, in the short term, but over longer periods of time, measured
in weeks, it is very reliable. So if a solar system doesn't depend on
getting sun every day, like clockwork, eg if it stores heat for a few weeks
or months, it too can be very predictable and reliable. And if it runs out
of heat once in a while, that is not a disaster. That event may be part of
a design that makes sense, economically.

>...your numbers about the 
>cost of solar power are highly deceptive. Hidden is the cost of reliability.

Where is it then, exactly? What is the cost for a given level of reliability,
and what are the consequences of a "failure"? Buying some power from someone
else? Using up some reserve? Missing 8 hours of work? An airplane crash?

>I think it is important for people to know that solar power is not the 
>answer that some people make it out to be.

I think it's important for people to realize that there is more than
one form of solar power, when they read sentences like the one above.
Or perhaps the sentence might be phrased less misleadingly.

>In the small scale, yes, you can cheat out of paying your fair share of
>the power grid cost...

My electric company, which is not known for its benevolence, charges about
$8 per month to be connected to the grid, even if I consume no electricity.
Is this too little? Perhaps they should raise the minimum rate...

Nick