A fridge idea?

[MickeyB]

Calling all fridge people who are more enlightened than me.

I was just listening to a person on youtube who was adding 6 inches of R2 insulation to make an efficient fridge, and my mind wandered when he was laying up lots of fibreglass on the inside to make it neat. 

I wondered if it was possible to put glass there instead for a easy to clean and simple box.

Which then made me think about a second layer of glass on the outside, and pull a vacuum in between just like our double or triple insulated glass windows.  This would make a large fridge without the mess of the foam etc.

Anyone ever had this stupid idea before?  A quick google reveals that a triple glazed window has an R rating of 6.4!

Wouldn't be cheap I assume, but the savings on amps down the road could be huge.

Mike

[Zanshin]

I have been working on reducing the energy draw of my fridges for years. With only mediocre success. One of the issues is that there is no room to put extra insulation on most installations. I seal up the insides to prevent air leaks and also put a layer of polyurethane foam (the black stuff used in refrigeration) on the bottom and sides of my fridge and freezer and then a large sheet of the same (ugly) foam on top of the lids. The foam made a huge difference in energy consumption.

What I'm working on now is programming and sensing. With cheap temperature sensors (and a pressure/humidity one) I can tell if the door has been opened and also if items have been removed or put in. Using that information I can program the duty cycle of the compressor to optimize efficiency. If I also program using the supply voltage (which can tell if the engine or generator or solar panels are charging) then I can optimize even further - especially the freezer. If the freezer knows that the batteries are being charged, it can run the compressor to cool the contents down below what the "normal" setting is. But this is still a work-in-progress and I don't know if it will ever be a commercial product... but it is fun to tinker with.

Regard vacuum seals between layers I don't think that they will survive the constant flexing they would get aboard a sailboat. But there are some very impressive insulating sheets that will do as good a job with a lot less weight and volume and which are not hygroscopic... but they carry a very steep price.

[Maravilla]

Which types of foam are you referring to?  We are looking to get some additional insulation around our frig and sometimes up front expense is well worth it.

[Zanshin]

Normal refrigeration foam is probably the best to use - cheap, effective, pliable and relatively non-hygroscopic. There are various types of "aerogel" available which work incredibly well, but are quite expensive. Sort of like the AGM vs. Lithium battery arguments.... I'm not sure that the extra price up-front will be amortized over time. But a good aerogel insulation on a freezer and I bet that the ice-cream will always be rock-solid even in the tropics (that's the "holy grail" of refrigeration - -15°C).

[Maravilla]

Thank you

[rene460]

Hi Mike, you need the refrigeration foam, as it must have closed cells to prevent moisture passing through the foam.  When moisture reaches the inside, nearest the food compartment, the temperature is low enough that it condenses, and around a freezer it may even freeze.  Water expands on freezing and will break up the insulation.  It is not so important when insulating to keep heat in as the moisture tends to move out.  Blocks of foam insulation are cheaper than foam in a can that froths up and sets in place, but are better in larger flat areas.  They also must still be sealed around the edges to keep out the moisture.  A barrier of metal foil, if properly sealed around the edges, also works as a vapour barrier.  But those blocks can be very difficult to properly installed around most boat fridges, where foam can still be expanded in place.  The main thing is not to put too much in a closed place, as it will expand anyway, and can easily break something.  So a bit of a mix as the space dictates.  The thickness does not have to be uniform, every square metre or even fraction of a square metre, looses heat according to the thickness, so even partial extra insulation can help.  But there is diminishing returns as you need to double the thickness to halve the heat transmission.  So main reason for uneven thickness is for convenience in installation, or to fill awkward places where things can fall and be hard to retrieve.

As I see it, the trouble with glass, as already mentioned, is its lack of flexibility.  Difficult to instal in confined places.  And it still must be sealed around the edge.

But the fridge is invariably the highest or close to highest load on our electrical systems, so if you are having trouble keeping batteries charged, reducing heat gain into the fridge is a good place to start, as it all has to be transported out, requiring the compressor to work hard for longer.

rene460

[MickeyB]

Many thanks all for the input.  My fridge is actually nice and cold the entire time, and solar keeps it topped up nicely.

It was one of those mad moments I suffer everynow and then - safe to say this idea can be torn down.  Was worth the idea though :-)

Thanks all

Mike

[rene460]

Hi Zanshin, interesting that you are working on optimising the fridge running cycle on voltage as well as the other strategies you mentioned.

On our previous boat, we were having trouble with keeping the battery up.  But hey, it was only a light weight 24 ft boat, and we did have a fridge.  Only 11 litre capacity car fridge, which I had mounted in a large plastic box with the air end out for free air flow, and filled the gap with foam.  We found that we had to turn if off at night, so needless to say we tried to minimise opening the lid at night.  Then we would be frustrated during the day to notice the sun shining and the thermostat switching off the fridge.  It was a bit of a pain to keep adjusting the thermostat, and it was still not really satisfactory.

Then one day the thermostat failed.  As a quick fix, I bypassed it, and we got into the routine of plugging in the fridge when the sun shone, getting as cold as we could without freezing anything then unplugged it again.  And we made sure we got it good and cold by late afternoon   It got us through without food spoilage through our longest trips which at six weeks with no chance of shore power, are quite a challenge on such a small boat, not only due to lack of power.  But I like your microprocessor solution.  I think you will be very pleased with the result, apart from, or perhaps in addition to the continual challenge of tweaking it a bit to make one more improvement.

The other thing we did was to always have a couple of freezer bricks close to the plate as a bit of a cold storage reservoir.  I believe the same idea is the basis of the eutectic fridge systems.

rene460

[rene460]

Mike your post came in while I was typing.  Not a crazy idea, insulation is cheaper and much lighter than extra battery capacity, and does not require extra charging capacity, so always an easy boost to the power balance.  And you have probably noticed how many forum members are having trouble keeping batteries charged under the load of the devices they want to run.  But if the fridge is already well insulated then the benefit will be minimal.  I have a mate that did detailed calculations, he claimed that four inches was about optimum for maximum benefit if that is of any help.  I suspect he might be a little conservative, and three inches would probably do for most of us.  But a freezer will perhaps benefit from more.

However the main thing is that you can keep the fridge cold, with minimal battery worry.  And if you can’t see any obvious deficiency in your fridge insulation, it is probably not worth doing anything about it.  There are usually too many other jobs requiring attention on a boat, even polishing the decks.

rene460