It'll work fine with a very well insulated freezer I think. The freezer needs to over-cool during the day and then still retain a safe temperature when the PV stops working at night. The new A++ freezer I bought late last year has something like an 8 hour safe time with no power, so is pretty close to what you would need.

One thing I'd like to try is building a cold room into the house, and then fitting the freezer inside that. Even a traditional larder, built in to a north facing wall with external ventilation would probably help improve the efficiency of the freezer.

JSH, My plan includes a larder on the north side of the house with stone shelves, no insulation, ventilation etc. Regarding the freezer in the larder the problem is the compressor would heat up the larder!!! so i plan (in theory) to well insulate a chest freezer but move the compressor into the kitchen next door (within the insulation envelope) so the heat produced would help heat the kitchen. I asked a freezer engineer if this was possible and said he could not see why not but wondered if the extra length of tubing may effect the efficiency. (He also said forget about any warranty on the freezer).

Since my original post I have also thought that it would not be rocket science to build a "high temp alarm" that could switch over to mains power in the event of not enough coolth!

Sorry, I should have been clearer, I meant install the freezer so the casing was in the larder/cool room with the rear sealed to external vents to stop it heating the larder space.

I'm not sure about there being problems with the refrigerant in freezers in cold areas, my old freezer sat outside in the unheated (detached) garage for years with no problems. At a guess it got below freezing out there from time to time during the winter. My mother has had her freezer in an outbuilding for years, similarly with no problems (apart from the outside going rusty).

<blockquote><cite>Posted By: Ed Davies</cite>

Yes, but that's an<em>old</em>freezer. My understanding is that it is the different refrigerants in modern freezers which don't like cold temperatures.</blockquote>

Well, the one my mother has now is only around three years old, so will be using something like R134a or perhaps R600a, both of which are used in car air con systems that work OK down to very low temperatures. My new freezer uses R600a (isobutane) as do most new models I believe. Isobutane freezes at around -124 deg C, around the same as the older R22 (-117 deg C) so I really can't see why putting the warm-running condenser in a cooler atmosphere should do anything other than improve the COP of the freezer heat pump.

<blockquote><cite>Posted By: Ed Davies</cite>If you do have a freezer which can tolerate cool/cold, how about putting it in an enclosure in the input flow path for your MHRV?</blockquote>

Sounds a reasonable option for winter pre-heating, although the heat input would be modest (maybe an average of somewhere around 50 to 100 W?).

Inside a house, temperature averages about 10degC above its surroundings, and it needs insulation 100s of mm thick to keep it that way. But inside a freezer, temperaure averages about 30degC below its surroundings. Yet it only has (guessing) about 50mm of insulation.

Never understood the diffrence....

If you are hacking your freezer, how about insulating it all round with say 200mm of polystyrene or kingspan? (obviously not the electrics or the condensor coil), then it might keep cold without power through a few overcast days.

My previous freezer used 1 kWh/day (not measured the new one yet) - what size panel do you need to power that kind of load? Could be pricey...

Can you get 12V freezers for boats etc? might be easier to supply from PV than a 240v one with an inverter

The best I've seen use a bit under 200 kWh/year, so maybe around 550 Wh per day or so. This seems quite doable with a fairly modest PV set up.

As to why freezers only have such thin insulation I think it just comes down to marketing. Big freezers with a small internal compartment wouldn't sell well. If you work back to get the "heating" efficiency for a freezer you find they are pretty poor. My A++ rated one (235 kWh/year) uses around 27 W average to maintain a temperature difference of about 30 deg C for a sealed volume of around 1/2 m³.

For comparison, my new build would need a heat input of about 1200 W for a 30 deg temperature differential, ignoring ventilation heat loss. This works out at about 2 W for 1/2 m³, so the house will be around 13.5 times better insulated than the freezer. Illustrates reasonably well how poor these things are.

Posted By: joe90Feel free to discuss and point out where this theory falls down, or not.

Just got in and not read all the other suggestions in detail, except about the phase change material (water is a very good phase change material, as are some fats, so probably full of that already ), but going to throw in my two pence worth anyway.

I did a little experiment with my fridge a while back and just putting some Celotex on the sides reduced the energy use by about 30%. So always worth insulating. But isn't it always the case, and if you can combine it with good airtightness all the better.
Airtightness on a freezer! Yes, get a chest one (but don't fall in it), open the lid slowly and as infrequently as possible (a fridge will often be opened several times in a short space of time which release the cold air and replaces it with warm air. What a waste.
So I cannot see a problem with a freezer that is at the lowest temperature it can get to, is well stocked and is only opened once or twice a day begin run with a PV system (unless it is a stand alone one and then it will need a battery for some storage (not hard to sort out but starts to get expensive and electricity is really very cheap). Keep it well defrosted, Ice is a good insulator and tend to form on the bit that is meant to be cold (that water really is a wonderful material, can even make tea from it).

Get a plug in power meter and see what current your freezer draws and what the kWh are when it is in a room at 25°C (a worse case). Data is always useful in these cases.

If you want to use the waste heat from the back (there will not be much with a well insulated freezer) then just box the 'radiator' in get, a fan and pipe it to where you want it. Much easier than moving a compressor, which may also work as part of the auto-de-frost.

Hi ST, yes I had planned on gluing 100mm eps or similar to all sides to aid efficiency and I like your plan on ducting heat into the house rather than moving the compressor.

With regard 12volt fridges, I used to have one in a caravan and they are very small, haved looked on tinternet and found 12volt freezers but small and $840.

http://www.survivalunlimited.com/sundanzer.htm

SunDanzer is a well-known brand. Don't know about UK availability, but it ain't rocket science.

Also try a search on fieldlines.com for examples of v efficient / PV-driven freezers in use.

Rgds

Damon

Off-gridders in the US used to used Danfoss fridges & freezers because they were much more efficient than standard ones and came in 12 & 24 V versions. Now, though, standard fridges and freezers are a lot more efficient and inverters, etc, are cheaper and more accessible so nobody bothers. Danfoss's web site doesn't seem to even show complete appliances of this sort any more.

Posted By: WillInAberdeenIf you are hacking your freezer, how about insulating it all round with say 200mm of polystyrene or kingspan? (obviously not the electrics or the condensor coil),

Posted By: SteamyCelotex on the sides

Posted By: joeI had planned on gluing 100mm eps or similar to all sides to aid efficiency

Just realised that both the dew point and the ice point will be within the insulation layer... need to seal it all up vapour-tight outside, or ice will spall the insulation off the freezer.