12 V 25W strip heater

[la7qz]

Hi

I'm in the process of building a fridge / freezer using the reverse cycle Stirling engine from a Coleman Stirling Cooler. According to the manufacturer, this unit can deep freeze a well insulated box up to 200 litres. We have six inches of insulation in a much smaller compartment.

Unlike the Coleman Cooler which uses termosyphon CO2, heat transfer will be via large heatsinks inside the box bolted to the cold side of the Stirling engine. Since this will be a deep freeze with a thermostat controlled fan bringing cold air into the fridge compartment, there will be a build up of frost on the heat sinks.

I need one or two 12V strip heaters to heat the heatsinks to melt away the frost. I'm thinking two strip heaters around 20 - 30 W should do it. They must be flat strip heaters without fins which can be bolted direct to the heat sinks. The object is to heat the heat sinks quickly while heating the freezer compartment as little as possible. Once average run time etc. has been figured out, the strip heaters will be run on a timer which will switch the strip heaters on once per night while switching the fridge off, then turn the fridge back on once the frost has been removed.

Any ideas on a source for the 12V strip heaters would be most welcome. The ones I have found online are all 120 or 240V.

Owen, Donna and Sparky

Yacht Magic

St Maarten

Netherlands Antilles

[Jon Miller]

Reptile heater mats might do the job?

Regards  

[stop4stuff]

i'd say summat's amis...

You want to waste energy heating a freezer compartment so the freezer fins don't ice up?... forget a heater (suitable wire at the right length could be a heater)... stick a fan in the freezer side (or turn the stat down)... or don't run the stirling quite as fast

[tanner0441]

Hi

If you want 12V heaters the 12V elements from caravan fridges. Depending on the model they vary from 90 to 130W at 12V. I know it is a bit more than you want but perhaps you could run it on a shorter cycle. They are also on Ebay.

Apart from the most recent electric only fridges, which use a compressor, all the gas/electric fridges run on the absorbsion cycle, boil a mixture of amonia and hydrogen gas with a heat source and the resulting seperation of the two absorb heat from the bit in the box that gets cold.

Hope this helps.

Brian

[wooferhound]

I was also going to suggest a 7w muffin fan.

[AbyssUnderground]

How about a peltier? They get rather toasty quite quickly and come in lots of sizes, and 12v. Not sure how it would fair up inside a fridge though since they transfer hot and cold to opposite sides...

[fungus]

I think some newer cars have heaters underneath the wing-mirrors? Nice small size which could work, same sort of thing as the ones on the back windows ..

Or just get a load of nichrome and coil it up, insulate

[jlt]

you could try getting some radiant ceiling wire it is insulated.so you could glue it directly to your fins.not sure how many feet you would need for 12 volts.

[RogerAS]

On this Stirling engine forum another person is attempting something similar.

http://www.stirlingengineforum.com/viewtopic.php?f=1&t=425

You might find help there.

[scottsAI]

La7qz,

Please post any pictures as you disassembled the cooler!

Some Stirling engines can be run backwards, hot/cold side swap places! (just what you want:-)

Is there a motor driving the Stirling or is it sealed?

If you can run it backwards do so to defrost, let rest couple minutes to drip down. Make sure you put in a drain. Restore normal operation.

Things to consider: Circulating sea water to hot side of Stirling? The colder the better. Does not take any more power than an air fan, less noise, not warming up compartment.

If you cruse in cold water than you may want the little extra heat from fridge.

Since your adding a defrost controller, monitor hot side of engine, goes above a limit, set an alarm, same on freezer. Think of anything else it can do?

Have fun,

Scott.

[la7qz]

Hi

There will of course be a fan circulating air over the heat sinks to remove heat from the freezer compartment. A fan inside a deep freeze compartment will not thaw out any frost. Running the Stiling engine slower will of course make the compartment warmer and make our icecream melt. There is no way to run the thermostatically controlled Free Piston Stirling Cooler slower other than selecting a different temperature setting.

Heating the cold bits periodically (on a timer) is a pretty standard way of defrosting freezers. The object is as I said to heat the heat sinks quickly while heating the air in the compartment as little as possible. There is also no way of running it backwards as has been suggested by others since it doesn't have a crank. Anyway, I don't think running a Stirling engine backwards will actually swap the hot and cold sides.

Owen

[scottsAI]

La7qz,

Guy built 65cc Rhombic Stirling engine:

http://www.youtube.com/watch?v=ssvHGUlFokA&feature=related

2:48 into video, he describes reversing (backwards) his Rhombic Stirling engine would swap the hot/cold ends. Then demonstrates it does.

Don't know what type of Stirling engine you have so can't tell if it's reversible.

Have fun,

Scott.

[(unknown)]

One or two WH25 resistors should suffice............

http://www.welwyn-tt.co.uk/pdf/datasheet/WH.PDF