Wikipedia:Reference desk/Archives/Science/2022 August 14

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August 14

Solar-powering a freezer: how much anti-freeze water to keep contents frozen overnight

I was thinking about energy storage and solar power and how maybe phase change inside a freezer could allow a freezer to operate soley on solar energy. I'm not entirely sure that it's worth the trouble in practice (I think it is since lithium batteries are expensive) but I was wondering whether anyone could verify my calculation. The first result on Google suggested a typical consumption for a freezer is 0.6 kWh per day and I presume solar power to be available 8 hours per day so the freezer would need to store 0.4 kWh in frozen water, neglecting heat pump efficiency. If 0.4 kWh = 1.44 MJ and 1.44 MJ / 6.02 kJ/mol = 239 mol then I calculate 4.3 litres of water are needed. If ethylene glycol is added to lower the freezing temperature of the water, does that also significantly affect the enthalpy of fusion of the water? The enthalpy of fusion of ethylene glycol is greater than that of water but I don't know how that's affected when they mix. 78.148.95.111 (talk) 12:28, 14 August 2022 (UTC)

0.6 kWh/day seems a bit poor to me, but it depends on the size of your freezer.

You neglect heat pump efficiency. With a coefficient of performance of 1, the electric energy used is equal to the heat removed from the freezer, for which your calculation is correct, but most freezers are better (around 4), so you may need a bit more ice. On the other hand, if you keep the door closed while it's switched off, you'll need less. The freezer specs may mention the time they typically remain frozen after a power failure. It seems that many can handle around 24 hours, provided you keep the door closed and the environment is not too hot.

Don't count on 8 hours of sunshine per day. In summer you may get more (here, it's often over 14 hours), but in winter, less (sometimes in November 8 hours for the entire month). Of course, in winter you may need less energy. Solar panels provide some electricity under an overcast sky, but it's negligible. Adding something to the water to lower the freezing point is a good idea, as it allows keeping the freezer well below freezing, which you probably want.

In the past, people kept their cold storage rooms cold the entire summer using blocks of ice. PiusImpavidus (talk) 20:00, 14 August 2022 (UTC)

Why not insulate your freezer properly, overcool it during the day, let it warm up to -18 deg C overnight? Greglocock (talk) 09:42, 15 August 2022 (UTC)

Is this a real-world application or are you just doing theoretical calculations? You brought up batteries; if cost is the driving factor, and the weight is not a big deal, lead-acid batteries win on cost-to-power ratio. This is why ICE vehicles use them, and why off-grid power applications generally do, until you get up to utility-scale stuff where other factors come into play. Lithium-ion batteries are used where capacity and low weight become the overwhelming factors, such as in portable electronics and a lot of battery electric vehicles, though many of the latter still go with NiMHs because they're cheaper with still competitive qualities, and also have fewer safety issues. Also, with the presence of solar energy, using the sun's heat may make an absorption refrigerator competitive. These are used in RVs because they can run directly from a heat source, such as fuel combustion. As with solar water heating this avoids the substantial losses you incur from the solar panel (commodity panels have only around 20% percent efficiency). --47.147.118.55 (talk) 08:00, 16 August 2022 (UTC)