Airport Power

David Lesher wrote:
"Kyle Boatright" writes:
I need a trickle of electrical power about a dozen times a year in my
hangar. I need it for my 125 watt preheater if I'm taking an early-
morning flight in the winter, and will use it for a .25A trickle charger
if I've let the aircraft sit for more than a week or two.

Well, you can handle the trickle with a solar array of some kind.

I agree. If you can mount the solar panel so that it has about the same
angle from horizontal as your latitude, and so that you can get to it
occasionally to wipe the dirt off of it, it will work a little better.

125W @ 8H = 1KWH
At 13.8 VDC; 72 Ah-H w/100% efficiency.
Say 150 Ah-H at real world inverter levels.

In a later post, David said he only needed to run the heater for about
4 hours. I haven't measured any of them, but I can believe that a
really cheap inverter might only be 50% efficient. Tripp Lite claims
that their 150 W inverter is about 89% efficient at full load [1], and
another site [2] gives about the same numbers.

So, to get 125 W of AC to the heater, the inverter will need about 140 W
of DC input, or about 12.2 A at 11.5 V. (This is an average; the battery
will start out at around 12.5 V and shouldn't be run much lower than
10.5 V.) This is 48.8 Ah at the four hour rate. Powersonic [3] claims
that their 60.0 Ah/20hr gel-cell battery will do 48.0 Ah at the 4 hour
rate. Flooded batteries often have a "reserve capacity" rating, which
is the number of minutes they will run a 25 A load. Since your load is
about half of this, you would look for one with a 120 minute or better
reserve capacity rating. Interstate [4] says their SRM-24 will support
a 15 amp load for 4.6 hours, so that would work.

But wait! All of those ratings are at "room" temperature - 20 C for the
gel-cell and 27 C for the flooded battery. The curves for the gel-cell
battery show that its capacity at 0 C is about 90% of the 20 C capacity,
and its capacity at -20 C is about 65% of the 20 C capacity. So, at 0 C
you need a gel-cell with a nominal rating of 66.7 Ah/20hr or better, and
at -20 C you need one with a nominal rating of 92.3 Ah/20hr or better.
The flooded battery doesn't have curves like this, but looking at the two
cranking amp ratings, the rating at -18 C is 80% of the rating at 0 C.
If you ass-u-me that it follows a similar curve as the gel cells, the
SRM-24 might still cut it at 0 C (3.9 hours), but for -20 C, you
probably need to move up to the SRM-29 (6.4 hours at 27 C, derated to
4.0 hours at -20 C).

None of these batteries can be taken lightly. The gel-cells will weigh
40 to 70 pounds and the flooded ones will weigh 50 to 60 pounds. If you
have a choice, buy a battery that has a built-in handle. This becomes a
consideration if you're taking the battery home to recharge it every time.
You could put it on the solar panel, but it'd take a long time to charge
back up at 0.25 A - something like 200 hours of full sun, or three weeks
straight if you live in Phoenix.

If you do use a battery and an inverter, keep the wiring as short as
possible, but especially the DC wiring. I would cut off the cigarette
lighter plug that comes with most small inverters and wire the inverter
directly to the battery terminals. Anything that you can do to keep the
battery warm would also help; a plastic battery box that isn't sitting
right on the floor would help. For the extension cord from the inverter
to the heater, use as heavy and as short a cord as you can. It's only
about 1 A at 120 V, but since this electricity is relatively expensive,
you don't want to waste it if you can help it.

Hmm, you could use Qnty 9-10 13.8v batteries in series; 4 Ah-H
Gel-Cells are not all that pricy. Looks to be ~~$15 each for 5Ah-H.
Then a kludge to parallel them when charging off the array.

This would work too; you don't lose anything in the inverter, but as you
noted, it may be harder to control. With one big battery and an
inverter, you can use a regular 120 V AC lamp timer to turn the heater
on when you want it. With this setup, you'd probably use something like
a spring-wound timer with a switch or relay rated for DC.

For ten batteries at an average voltage of 11.5 V each, you'd need
1.09 A from each battery for four hours, or 4.35 Ah/4hr. This is at
least a 7.0 Ah/20hr nominal gel-cell at 20 C or 0 C, or a 9.0 Ah/20hr
nominal at -20 C. These batteries together will all weigh 60-65 pounds,
but at least you can carry them two or three at a time instead of all
at once. It would still take a few weeks to charge all of these
batteries off of an 0.25 A solar panel.

Either gel-cell option will probably cost around $150 for the batteries;
the flooded battery option will probably cost around $80. The inverter
should be about $30 or so. There are chemistries like NiCd and NiMH that
have higher energy density, i.e. they don't hurt as much when you drop
them on your foot, but they also have much higher dollar density as well.

There are places that sell "solar kits" for cabins and other isolated
applications. They usually have the solar cell(s), charge controller,
mounting hardware, and some cable. Some of them include an inverter;
you get to buy your own batteries for most of them. Googling on "cabin
solar kit" or looking at the ads in "Home Power" magazine are good ways
to find systems like these.

Another possible source of power might be a wind generator, but this
probably wouldn't be too practical unless it's really, really windy
close to the ground; towers are expensive and you probably don't want to
stick it too high up in the air at the airport. Maybe you could put it
down low, by a busy taxiway, and convert prop wash into free juice...

If what you're really after is heat, the best answer is often to burn
the hydrocarbons fairly close to where you want the heat. That probably
wouldn't work very well in this situation, though. I had some other
creative ideas, like a weight on a rope that turned an alternator pulley,
but that turned out to require way too much weight.

Standard disclaimers apply; I don't get money or other considerations
from any of the companies mentioned.

Matt Roberds

[1] http://www.tripplite.com/products/pr...productID=2553
[2] http://www.solar-electric.com/solar_...r_electric.htm
[3] http://www.power-sonic.com/techman.pdf
[4] http://www.interstatebatteries.com/w...uct_marine.asp