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    • CommentAuthorcaspen
    • CommentTimeMay 18th 2014
    We have PVs generating electricity. If we invest in a Watson, we can use an optiplug which will turn on gadgets that are unaffected when turned off because insufficient electricity is being generated eg a dehydrator or washing machine.

    Do you know if we could use it to run an additional freezer? The freezer would be turning off whenever there was no electricity generated. But if we didn't open the freezer when it was off, eg in the evenings and at night, would the food inside be OK until the freezer turned on in the next generating cycle. It's likely to be the excess fruit and veg from the garden.

    Many thanks for your experience / knowledge.
    • CommentAuthortony
    • CommentTimeMay 18th 2014
    risky! you could finish up losing the contents. the freezer would be struggling after 10 hours and warming up too much, then if it was a very cloudy day the next day :shamed:

    some kind of bypass switch that would switch on at -16 would solve it and then use the one you are planning to use to run it at -20 that would save a reasonable amount of money.
    • CommentAuthormarkocosic
    • CommentTimeMay 19th 2014
    Frig it via the thermostat? Force it to run when the PV is doing it's thing rather than preventing it from running when the PV isn't.

    The problem you'll run into is complexity. A piece of shit G to A rated old freezer that's better off scrapped than running is easy to frig. (mechanical thermostat) Modern A+++ equipment that's worth having is hard to frig. (inverter driven digital thermostat and so forth)

    You might get lucky and find an A+ or A++ that still has on/off contacts to decide when to run. Wattson runs a contactor to bypass the OE thermostat and force the freezer to run; fit a second thermostat to stop it by -30C* or so?

    *ask a refrigeration guy for a sensible number.

    Exporting the electricity and making jam from the fruit also works. :wink:
    • CommentTimeMay 19th 2014
    The starting current for a fridge is pretty high, can the Watson cope with it.

    I cut the energy use of my fridge by 30% by just adding some insulation to the sides.
    • CommentAuthorEd Davies
    • CommentTimeMay 19th 2014
    Posted By: tonysome kind of bypass switch that would switch on at -16 would solve it and then use the one you are planning to use to run it at -20 that would save a reasonable amount of money.

    Seems a good plan to me. Less temperature margin for power cuts, though.

    Not overly convinced by A++++++ fridges. Seems to me that the extra PV to run a more moderate one (A+ or so) can be cheaper than the extra cost of the fridge, bearing in mind that the fridge usefully acts as a heating element throughout the heating season. In the summer your extra PV runs it anyway, though you lose the CO₂ benefit of exporting the electricity.
    • CommentTimeMay 19th 2014 edited
    Posted By: SteamyTeaI cut the energy use of my fridge by 30% by just adding some insulation to the sides.
    Would that work on
    Posted By: markocosicA piece of shit G to A rated old chest freezer
    • CommentAuthorGarethC
    • CommentTimeMay 19th 2014
    Sorry for a tangential question, but why don't fridges have drawers the way freezers do (or just doors for each shelf)? Seems like an easy and cheap way to reduce the amount of cold air falling out when you open the door, but I've never seen it.
    • CommentTimeMay 19th 2014 edited
    Posted By: fostertomWould that work on
    Not sure if it would make it an A, but would help.
    Part of the problem is that in my instance, the fridge energy use is pretty low anyway.
    I seem to remember that our old friend Damon dug out seem info about how the rating system works, moving average I seem to remember.

    Posted By: GarethCbut why don't fridges have drawers the way freezers do
    Not the air you need to keep cool, its the goods inside. There is very little air in a fridge, which has a low SHC when compared to a few cans of beer. So probably not worth the inconvenience.
    • CommentAuthorEd Davies
    • CommentTimeMay 19th 2014
    Posted By: SteamyTea: “There is very little air in a fridge, which has a low SHC when compared to a few cans of beer.”

    Indeed, the effect of air getting into the fridge is not large but to the very limited extent it does matter the effect of condensation of water vapour (and freezing it in a freezer) is nearly as important as simple cooling.

    Number sources can be found in this blog post: http://edavies.me.uk/2013/12/dehumifiers/ , particularly the table in the other place linked from there.

    Room air at 20 °C, 60% RH would contain 9 g/kg of water vapour. Cooled to 5 °C, 100% RH it would contain 5 g/kg so 4 g/kg would need to be condensed out. At around room temperature water has a latent heat of vaporization of about 2.4 MJ/kg so 2.4 kJ/g or with 4 g/kg of water vapour 9.6 kJ/kg of air.

    For comparison, the SHC of air is around 1 kJ/kg so cooling it from 20 °C to 5 °C needs 15 kJ/kg so a total of 24.6 kJ/kg. A litre of air has a mass of just under 1.3 g so it'd be about 30 J/l.

    A heat pump with a cooling COP of 2 (equivalent to a heating COP of 3) consuming 150 watts would extract 300 W (or 300 J/s) so get rid of that in a tenth of a second.
    • CommentAuthormarkocosic
    • CommentTimeMay 19th 2014
    The A+++ fridge freezers are worth the money. All the chiller is in the freezing compartment. It super-freezes the freezer part to get good long runtimes on the compressor, then every time you open/close the door it quickly re-distributes and re-chills the air in the fridge bit by exchanging with the freezer part. Thought has been given to humidity control in veggie section and temperatures of the meat/fish/dairy sections, such that food lasts significantly longer than it does in an older fridge. How much of that is the A+++ vs decent temperature/humidity control I don't know, but the two usually go hand in hand and reduced food spoilage more than pays for the delta cost of the fridge freezer when new.

    fostertom - not even close. If the fridge/freezer isn't A-rated weigh it in for scrap now as it's better for the environment than keeping it. We're talking £3000 in electricity to run it for 20 years vs £600 type money here!
    • CommentAuthorcaspen
    • CommentTimeMay 21st 2014
    Thank you all for your comments. Lots to absorb before we make a decision.
    • CommentAuthorJonti
    • CommentTimeMay 22nd 2014
    Posted By: SteamyTea
    I cut the energy use of my fridge by 30% by just adding some insulation to the sides.


    did you just insulate the sides and door/front or did you also do the back?

    • CommentTimeMay 22nd 2014
    Just the sides. Luckily for me the gap I had allowed for nearly 4 inches of insulation.

    • CommentAuthorrhamdu
    • CommentTimeMay 28th 2014
    Posted By: GarethCwhy don't fridges have drawers

    Ikea used to make a below-worktop fridge with drawers. Perhaps they still do. It was very expensive and I guess they never achieved economies of scale.

    One other thought. If we are going to apply demand management to fridges, thermal mass becomes important. So don't drink all the beer.
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