On Wed, 5 Jul 2006 17:11:54 +0000 (UTC), Aaron Coolidge
wrote:

Ron Rosenfeld wrote:
: What are the design considerations for converting a heating element that
: has been designed to function on 120VAC to 12VDC?

: For example, I have a TANIS system in my a/c which draws about 375W @
: 120VAC.

You wouldn't convert it. You'd just hook it up to ~125 VDC. It will work
great. You may have trouble if it has an electronic thermostat (SCR/TRIAC)
because these require the reversals of the AC voltage to stop conducting
(you can turn on a TRIAC with DC across it, but you have to interrupt
the current flowing to shut it off).

Ok, I see what you have in mind.

So instead of a 12V DC PV system and 12V batteries, we would set up a
120VDC system and batteries. Depending on the batteries, some fancy wiring
would be needed also to do the trickle charging for the a/c battery.

I suspect the costs of setting up a low wattage 120VDC system would be
excessive compared with the 120VAC system.

I did find a 4.5W 12V thin film solar panel for $45. You'd need 10 of them
in series and they only have a 2 year warrantee. That's $450 without
putting together a 120VDC battery bank.

Sunwize makes a 5W panel available for $68.60 with a twenty year warrantee,
which is closer to the "industry-standard". You'd still need ten of them
for $686.00.

Then for batteries, you'd need to size them, too, appropriately. I guess
with the 60 cells a 120VDC battery would require, I'd want to use a sealed
AGM or Gel Cell or some other maintenance free battery bank. Then you'd
have to check the rating at the discharge rate. If the heater draws 1A for
four hours, and you have a deep discharge battery that you can draw down
50%, you'd need to figure the battery capacity at that current draw.
(Battery capacity changes depending on current draw). I see a 12AH 12V AGM
battery for $15.95. At 0°F and drawing 1.12A, it has a capacity of 8.16AH.
A four hour heating run would draw it down to 50% capacity. There's
another $159.50.

Since he's not running the heater every day, (I'm assuming once/week), the
array should replenish the batteries after a few days of sunshine.

Of course, when you get into PV systems with a nominal voltage of greater
than 50V, there are a variety of NEC code requirements to be complied with.
They are safety related but, if he's at a public airfield, there may be
inspection fees and a requirement for a licensed electrician.

I still think the 12VDC / Inverter system will be less expensive than the
120VDC system when all is said and done. It may or may not be cheaper than
running a line to a nearby hangar and paying for the electricity, depending
on those costs.

For an AC system you could save money by using larger panels. ($/watt
decreases as watts increase). I see a 12V 30Watt panel that would meet the
requirements for $178.20. A Trojan 27TMX 12V 105AH (20hr rate) battery is
$118.25 and you'd only need one of them.

So for panel and battery we're at just under $300 for battery and panel. I
see 300W inverters available for as low as $27 but I'd guess $100 would
purchase a more reasonable unit. Add mounts, wiring, etc. and we're
probably close to the $500 I originally guesstimated for the system, versus
probably double that for the 120VDC system.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)