Suggestions on keeping the cook happy
Smooth Cruiser
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Post by Smooth Cruiser » Fri Oct 01, 2010 7:38 am

There are two reasons why 240V is better than 12V in this context, and why the fridge manufacturer has chosen to have the fridge choose 240V as the first preference if it is availbale.

The first is what you touch on in your last paragraph - consumer preference. In many cases for people camping or boatng 12V is power that they have to provide via solar, wind or generators, but 240V is essentialy "free" to them as the user - ie a powered site in a caravan park or shore power in a marina. Of course this isn't true if you are producing your own 240V off 12V through an inverter. In this particular scenario the best choice would probably be to keep the fridge running off 12V (more on this in a second as to why it may or may not be best).

The second reason is power losses in the wires. Say the fridge is 75W, with 12V supply this will draw around 6A of current. If the same fridge had a 24V supply it would only draw 3A. The same fridge again with 240V supply would draw around 0.4A (nbt linear as one is AC and one is DC).

The power loss along a piece of wire is equal to I^2*R where I is the current. So - the higher the voltage the smaller the current and the smaller the losses along the wires. Without getting bogged in the detailed calculations the power lost in a normal bit of wire feeding your fridge at 12V would be around 225 times higher in the 12V scenario vs the 240V scenario, because the current in the wires is higher. Now in a fridge with short cables this isn't going to make a huge difference - but the 240V would still be noticeably more efficient.

This is exactrly the same reason that grid power is transformed to as high a voltage as possible to go long distances - why transmission lines are 11kV or 33kV instead of 240V - the losses in a 33kV wire vs 240V wire are around 20000 times less to carry the same power. High voltage always wins for reducing losses - again the same reason it is better to set up a solar system as 48V rather than 12V.

Going back to running 240V off your inverter though - the reduction in cable losses running at 240V vs 12V are most likely more than offset by the inefficiency of the inverter itself. So in this situation where you are always running off your own power but sometimes have your inverter on - it would most likely be best to manually switch the fridge to always use the 12V inlet - this would see hte least losses. But then when you have free power (shore power) go back to 240V as the better option.

Hope this helps.

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Post by mahnamahna » Fri Oct 01, 2010 5:52 pm

Thanks Smooth brilliant explaination, I am sure the other not electrickery readers here will agree, that is about as good as it gets.

1 more question then, regarding setting up solar as 48v, is the saving of sending power from the panels to batteries from lack of loss in transmission greater than the loss from converting from 48v to 12v for each appliance assuming the usual set up position of solar panels on the cabin roof, batteries under the saloon seating and appliances where they would usually be (ie at the helm, at the galley, water pumps, starters for the motors, etc)?

Smooth Cruiser
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Post by Smooth Cruiser » Sat Oct 02, 2010 5:28 pm

mahnamahna - not sure whether you are talking about running the battery bank at 48V or 12V?

I assume you are meaning run the solar array at 48V and then step down to 12V in the solar controller and have the batteries and the rest of the boat at 12V.

Assuming a 300W solar array with an 8m feed wire.

With a 12V system (assuming solar voltage of 16V) the power losses in the solar panel supply wire are around 15W, with a 48V system the wire losses are around 1W. So there is a fair difference. But with the 48V system you may need a more expensive solar controller capable of stepping voltage down from a max of around 72V.

So the 48V system "loses" 14W less. Without knowing the specs of the solar controller it is hard to say whether the step down losses in the controller will be lower than 14W. Given that even the 12V system is still stepping down voltage and given that the controller is still a fiarly simple DC/DC step down, I think this is a fair assumption. The only losses will be heat generated in the transformer and judging from the relatively low amount of heat these things put out I would thnik the losses are fairly low so 48V would be the way to go purely to minimise losses - but of course the best solution needs to be also take into consideration the cost of the panels and the controller. If the wire run is longer the losses would be more and the argument may become more compelling.

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Post by Dreamtime » Mon Oct 04, 2010 8:08 am

Hey all,

I have delt with these guys before and have found them realy good. (I got a kegerator from them) ... 4cf2150d62

This one looks OK :D
Scotty & René

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Post by northerncat » Tue Oct 05, 2010 5:51 am

bear in mind cable cost can be up there for heavy 12 volt loads


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