LPG Chest Freezer Heat element temps?

[errant]

Hi fellas. 1st post here. I know some basics of refrigeration and how it works but I would like some more technical knowledge for a project I'd like to try. I need to know what temperature the heat element in a chest Freezer needs to be heated to for it to work.

I was looking at using a two way Gas/Electric Chest freezer (or straight gas or straight elec) as my fridge using a controller I made to turn it off when it hits whatever temp I set it to (probly 5 deg C). I use the controller to regulate a home brew fridge at the moment.

I want to do this to reduce the power consumption as grid power will not available and do not want to have to buy too many solar panels to run it.

Then I thought, why can't I heat the element with hot water from my solar oven which will get the water to boil? The water manifold heats the water quite fast too (I'd probly use glycol as I'm getting 120 deg C inside the oven.). That way it would draw virtually no power during sunshine hours, the hot water would replace the gas as heat source. I would cycle the water with a small 12v pump, switching it as necessary. Then at night, the electric kicks in.

Anyone got any ideas in regards to this or info on the temps required? Cheers!

[errant]

Upon further thought and research, I'll use an electric absorption freezer and use the aqua-ammonia in the mainfold/oven straight from the freezer instead of water to transfer the heat and use the electric heating element after the solar oven/heat collector in the system to reduce the amount of elec energy required. I'll design another solar oven/heat collector just for this project.

Thoughts?

[Graham]

I do not think you will get it hot enough although it seems that heat pipes are moving some pretty high temps. One would not normally not bother with a temp controller on an absorbtion cooler/freezer as there is a danger of vapour locks if the controller short cycles. Buy big batteries if you want to run it all night. We use parrafin (kerosine) as a heat source if gas is not available. Have you thought of bio gas? With regard to the desired temp what you essentialy are trying to do is boil water in the same fashion as a coffee percolator and this acts as a pump to assist in the circulation. Good luck and keep us posted.

[errant]

Thanks for your input Graham. As mentioned above, I'm using the controller on a home brew fermentation fridge at the moment and to maintain the temp at 10 deg C (ambient is 30C) the bar fridge (106W/0.73A/240VAC) is on for 6 mins per hour (was a boring hour timing that!). Am I right in my thinking that a chest freezer would be even more efficient in this respect due to the extra insulation and cooling area?

If I am, I was thinking I won't need much of a battery bank to run it or panels to charge the system, perhaps only 200W per day.

Ideally, I would convert the freezer to 12V, removing the need for an Inverter, is that possible too?

I wouldn't even need to use the solar/absorbtion system then. I'd just have to make a 12V temp controller, get a panel and a big battery (probly out of a forklift).

I did consider Biogas as the digester would be fairly easy to make, it would seem, but I'm looking for simplicity to maintain with as few mech/elec parts as possible.

Quite an interesting dilemma that will need much more thought! I'm open to all suggestions! Thanks.

[errant]

Opps! I meant 6min/30mins, so 12 minutes per hour for the Bar fridge! I don't see an edit button anywhere!

[spimps]

Worked for electrolux on absorption in the 70's electric heaters started at 120watts,some margfinally smaller for caravan fridges.A two stage heater was introduced late 70's that was 30watt and 120 watt so it kept ticking over when temp was reached on the 30watt and demand via a thermostat switched the 120w on.
LPG flame temp would be 1000c but significant heat is lost through the flue,electric heaters which fit snugly up the tube are more efficient as there is negligeble heat loss through the flue so you can probably get away with half that temp.
There were 12volt heaters for caravan fridges for travelling,less wattage than 240v if I remember correctly,haven't got any left but http://www.leisureshopdirect.com have some.

Dumped the tech stuff a while ago

Make sure they are plum level for best performance.

[errant]

I found this by a bloke here in Australia, a chest fridge that uses 100W per day!

(remove the space, won't let me post links)
http ://mtbest.net/chest_fridge.html

I see that I'll need to make a zero standby power temp controller like his to achieve this.

I still don't want to use an inverter though, can I convert a 240VAC chest freezer to 12VDC? What Compressor would be best? Cheers!

[spimps]

Thought you were using an absorption type freezer.Don't know of any 12 volt compressors that have suffucient capacity,there are small ones on leisure boats ect.No problem as far as converting electrics if the right capacity can be found and if you are able to remove old refrigerant, fit compresor and re charge.
http://www.e-zkold.com/conversion.htm

Might be of interest.

[lowcool]

chest freezer size would depend on breast size,danfoss may have one?

[Graham]

Nick UK found this link. www.globalcooling.com/products.html It is another option. Small hermetic compressors are manufactured and are a lot more efficient than an electric element. A dual system is another option, solar absorbtion during the day and electric comp at night. I heard that the WHO has mortuaries in Zimbabwe that run like this (never seen one so can't tell you how it works. I have also heard of an australian who rigged up a car air con to a large windmill, the evaporator was in a box large enough to handle a case of beer. He had a reverse clutch that used 12v to disengage the drive, when the rmp was sufficient he cut the power and engaged the comp. From what I understand it need a lot of supervision. But for cold beer who cares LOL

[errant]

Thanks for the ideas fellas.
I haven't bought the freezer yet spimps, gotta sort out what I want/need yet heh.
Yeah, a marine place is probly the go for something like this, I'll have to price it first though. I'l check out Danfoss to see if they make a 12V unit suitable too.