Running a freezer intermittantly musings...

[Tim+]

Currently looking an an interesting electricity tariff that has just 3 ?on peak? hours per days, all the rest are charged at a variable off-peak rate.

We?re soon becoming an EV owning household so our bills will go up. Charging the car can be easily done outside the peak rates. Meanwhile we have an old fridge/freezer we use in the summer months at a beer/wine store and the contents of that won?t come to significant harm if I turn it off for three hours a day.

We also have a small four star rated chest freezer that only gets used as an ?overflow? from our kitchen freezer and it would be easy to use it only for relatively ?non-critical? foodstuffs.

How badly would food storage be affected by turning a chest freezer off for

3 hours a day (assuming we avoided opening it during those hours)?

I?ve got a temperature probe in the freezer at the moment and will have a test tomorrow to see how much the temperature rises over 3 hours.

Tim

[Robin]

Depends on how full it is but approximately not at all. Most freezers these days will last a day if half full, longer if full, without harm. Your manual may tell you. Failing that a (probably playing safe) guide from UK Power Networks is 15 to 24 hours

[Theo]

Agile Octopus?

It's probably OK, but you'd need to confirm if that's the case on a hot day. The things that make sense when it's 20C might not when it's 35C. Also bear in mind the temperature does affect food lifetime - if your freezer is 3* and that means -20C, regularly running it at -5C might only be

1*, meaning you can't keep food for as long - even if it's not actually melted.

What I'd do is put a wireless temperature sensor in the freezer[1] and hook that up to something that will turn on the power if the temperature were to get above say -10C. I wouldn't be surprised if there's a gadget to do that.

Theo

[1] My current setup is these Zigbee sensors: wifi controlled socket/power monitor and Home Assistant on a Raspberry Pi with a Zigbee dongle. There are people using Agile Octopus with Home Assistant too - might help charge the EV at the right time

[Tim+]

Yep.

Um, -18 according to this website.

Well, if my thermometer is correct, my freezer is only -15 so it would seem it?s not performing to spec. Be interesting to see what happens without power for 3 hours. I?m not too worried if it goes up to -10. I?m sure I?ve eaten food that been stored in out motorhome freezer that probably doesn?t get anywhere near that! If it gets much warmer maybe think about gluing some extra insulation on (or would it make more sense to put it

*inside*?). We don?t need all the volume.

Will have a shufty...

Tim

[Dave Liquorice]

What climate class is this freezer and is it located in the right place

for that class?

For a decent energy effciency class freezer (A or above) it's reckoned that up to 12 hours without power is safe. The instructions may have something to say about that.

[polygonum_on_google]

sy to use it only

Some freezers are supplied with freezer blocks expressly intended to increa se the time they can keep cold in a power outage. If it doesn't manage at p resent, maybe something as simple as putting some freezer blocks in would e xtend the time sufficiently?

[Theo]

That's a good thought. Even simple plastic milk bottles filled with water would be a good way to build thermal mass.

Theo

[Nick Odell]

I will be very interested to learn the results of your temperature probe experiment. My only experience has been of power cuts of up to twelve hours or so. By placing a total ban on opening the freezer door during the power cut the contents have always remained solidly frozen. Anything over 12 hours I do not know: we have generally panicked at that stage and taken the contents around to a freezer-owning friend who has not been affected by the outage.

Nick

[Tim+]

Think I could manage that. ;-)

Tim

[Chris Hogg]

When we lived out in the country away from instant access to shops, we would keep several bottles of milk in the freezer. Be aware that they do swell up as the milk/water freezes and they can split. OK while they're still frozen, but not so good when they thaw. This was more likely to happen with the big, 4-pint bottles because they're thinner than the 2-pint ones.

[Jim GM4 DHJ ...]

I'm cheap but you are a cheapskate ...

[Robin]

Some go a step beyond that by adding salt to the water so, if the temperature rises close to 0, the bottles melt before the food and "sacrifice" their latent heat to protect the food. I've never seen evidence of how well it works in practice but cheap salt is, err, cheap.

[Jim GM4 DHJ ...]

watch out now timmy you don't want the wife to defrost do you.....

[Jim GM4 DHJ ...]

I doubt it

[Spike]

Not a good idea.

With the water in ice form, it has a specific heat capacity of only 2.1 kJ/kg to give up to keep the freezer contents cool.

The real bonus of the ice-water bottles comes when the internal temperature of the freezer reaches 0 degC, at which point the comparatively massive Latent Heat of Melting cuts in at 334kJ/kg, essentially holding the freezer contents at that temperature until all the ice has melted.

Use this huge the Latent Heat of Melting by employing a liquid that is frozen at the normal freezer internal temperature, say -18degC, but which melts at perhaps -15 degC, using that Latent Heat of Melting to keep the freezer contents at that temperature.

A NaCl salt solution of 220g/litre freezes at -15degC, and a mixture of propylene glycol and water in a ratio of 30/70 glycol/water freezes at

-16.6 degC. Both of these would use that lovely ice-to-water transition to keep the freezer at that temperature until it has all liquefied - which would seem very unlikely indeed in 3 hours.

If you want to use this approach to your problem, I suggest using small bottles of solution, say 4 x 500ml, instead of 1 x 2litre bottle, as the surface area of the former is greater and will give better heat transfer as a result.

[Jeff Layman]

I'm obviously missing something here but can't see how it will make any difference if you turn it off for 3, 6, 10 hours or whatever. If you want to take a litre of water from 10 degrees above zero to -20 you will need a specific amount of energy to do it. If you stop supplying that energy halfway through the freezing process, then assuming no heat is gained in the interim, when you start the freezing process again you will need exactly the same amount of energy to continue the freezing process as you would have if you hadn't stopped halfway through.

Now look at that in the same light as what you are proposing for your freezer. If, while it was running continuously. it takes 1kW hour over

24 hours to keep the contents frozen, what makes you think it won't use the same amount of energy /over the 24 hours/ if you turn the supply off for 3 hours in the middle of that 24 hour period? It will just use it in 21 hours rather than 24.

[The Natural Philosopher]

I had assumed he would run it on cheap overnight tariff

[Bob Eager]

You are missing something. He is looking at an energy tariff where 3 hours of the day are much more expensive.

The same amount of energy - yes. The same cost - no.

[Tim+]

Of course total energy consumption will be the same, but running cost depends on the relative cheapness of the non-peak electricity supply. If I can shift the consumption away from the peak rate charges it will hopefully reduce the cost of running the freezer.

Tim

[Jeff Layman]

Of course! Somehow that didn't register. :-(

I had a look at the Agile Octopus site, but couldn't see /exactly/ what time they were using for the peak. They state 4 - 7pm at the top of their "what does it cost" page, then at the "How does it work" heading they mention 4 - 8pm, then a bit lower at the "Select a profile..." bit: For a "Family home" they mention 3 - 6pm For "Working 9 to 5" it states "with a sharp peak of energy use around

7pm, that tails off slower and later into the evening." For "Night chargers" they don't actually state a time but just mention shifting most of the energy use to the early hours.

Perhaps it's there somewhere. Or do they give a short range (maybe 3 -

8pm) and you can choose your three hours in that range. It just intrigued me, as I hadn't heard of the high short-term peak charge previously.