# Efficiently freezing an ice pack once per 48 hours

Question: What are some good electricity-efficient ways to cool an ice pack down to ~5 degrees C once every 48 hours?

Background: I am in a warm climate and connected to the grid in a US city. I don't freeze anything else regularly.

Ideas:

• I think the most efficient method is to share a freezer, e.g. find someone with unused freezer capacity and stick my ice pack in there. But I am interested in other methods as well, and how they compare to the sharing method.

• I do have a high-efficiency freezer with 70L capacity. Maybe this would be the most efficient, even with nothing else inside other than the ice pack?

• I know there are those portable thermoelectric coolers, but I'm not sure how efficient those would be, or how well they would function at high ambient temperature.

Very similar to this question, only lower capacity: How to get a small volume of freezer capacity with high efficiency and low cost?

Some years ago I read of an off-grid farmstead in Australia that used a conventional chest freezer as a refrigerator. The way he did it was to use a probe and temperature controlled relay to turn the freezer on and off.

He added 2" of closed cell XPS polyurethane sheet to the front, ends and top, but not the back, as the radiant grid was under the skin there.

My recollection that his average power usage was absurdly small.

Ok: Found a different blog about the same conversion:

http://www.aselfsufficientlife.com/chest-freezer-to-fridge-conversion-the-most-energy-efficient-fridge-ever.html

He didn't bother with extra insulation. He reports 0.15 to 0.18 kWh/day.

Ok. It's not clear if you need a freezer, or if you need a way to make ice packs to keep your food cold. If the latter, then this is an easy conversion for your mini-freezer. If the former, you can set the temperature lower, and operate it as a freezer.

Not mentioned in the article is that a closer to full cooler is thermally more stable. The mass of all the cold stuff helps the new warm stuff from warming it up much.

General notes on efficiency:

• Chest units are more efficient than uprights because cold air doesn't rush out of them and fall on the floor when you open them.
• Small units use less energy than large units, but efficiency/liter is actually lower because the surface is larger compared to the volume.
• More recent units are more efficient than old units.

With reasonable amounts of thermal mass in the unit, this is an ideal solar application. A 300 W solar array will generate about a kWh/day after conversions. (1 kW => about 1000 kWh/year ranging from about 700-800 in Germany to about 1400 in southern Arizona) You should have battery storage for about 3-5 kWh This will also power a laptop, and charge your phone, and allow some LED lighting.

• Thanks! I currently have the thermostat-controlled new top-loader, but no insulation mods, solar power or additional thermal mass. Awesome ideas, I will work on them! Nov 9, 2020 at 17:52