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Heating and cooling: Energy efficient food storage

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    As we all know, fridges comprise a significant component of most homes' electricity consumption and so I'm wondering about ways to make the storage of food more energy efficient.

    The first thing that springs to mind is whether the pipe on the back of the fridge should really be buried 2m underground where the temperature is about 8 degrees C - this should significantly improve the fridge's efficiency during summer. But during winter, perhaps the fridge provides a useful amount of space heating. But, then again, you don't need space heating 24/7.

    It sounds like the ideal would be to have the "external" pipes of the fridge buried 2m underground right next to your GSHP pipes. How does that sound???

    Also, I'm yet to see a fridge with a really large amount of insulation around the edges (most fridges seem to have about 40mm of PUR). I wonder if you could just glue foil-face PUR/PIR to the outside of the fridge?!?
    • CommentAuthorbillt
    • CommentTimeJul 27th 2010
     
    Posted By: dan_aka_jackAs we all know, fridges comprise a significant component of most homes' electricity consumption and so I'm wondering about ways to make the storage of food more energy efficient.


    Really? An A rated larder fridge, even a big one, should use less then 180kwhr per year, or less than 0.5 units a day. As the average household consumption is about 12 units a day it is hardly significant. A freezer will use getting on for twice that amount of electricity.

    The radiator on the back of a fridge is designed to exchange heat to the air. Burying it is unlikely to improve its performance.
    • CommentAuthorRobinB
    • CommentTimeJul 27th 2010
     
    I disagree with billt. To my mind it IS a significant component and increasingly so in greener homes - freezers obviously even more so than fridges. Avoiding built-ins or anything else that hinders airflow down the back of the fridge helps, as do shapes with higher volume to surface area and decent insulation.

    I'm going to do a few experiments with additional insulation once we're in the new house (more space) before possibly replacing it with the bigger, posher, cold drink dispensing one that some of the family hanker after. Not sure if external chilled water dispenser is good (saves opening the door) or bad on balance. Any views?
  4.  
    Posted By: billtReally? An A rated larder fridge, even a big one, should use less then 180kwhr per year, or less than 0.5 units a day.


    Our largish A-rated fridge-freezer uses about 280kWhr per year; a bit less than a tenth of our total electricity consumption.

    Posted By: billtThe radiator on the back of a fridge is designed to exchange heat to the air. Burying it is unlikely to improve its performance.


    As I understand it, heat pumps like the heat differential to be as small as possible. So my thinking is that the heat pump in the fridge will be more efficient if it has to achieve 5 degrees C internally from 8 degrees externally (the ground temp 2 meters down) rather than 25 degrees externally (the air temp in our kitchen on a hot summer day).

    Ground source heat pumps work OK with their pipes buried underground (although you do need very long pipe runs, of course).
    • CommentAuthorMartinH
    • CommentTimeJul 27th 2010
     
    Older fridges can indeed use a major chunk of household consumption - I think a 300l "C" rated upright freezer uses 750kWh per year. We have a 320l A++ rated Vestfrost chest freezer which uses 201 kWh per year and a 350l A+ rated fridge that uses 142 kWh. Most of these items have a 10+ year life, so the energy (and cost) savings can be significant, and energy is unlikely to get cheaper in the future....
    • CommentAuthorbrig001
    • CommentTimeJul 27th 2010
     
    One of mine from a while ago: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=3476&Focus=58685#Comment_58685
    I intend to put a pantry type cupboard in our new kitchen: insulate a normal kitchen unit, add seals to the door and ventilate the back to outside on a north facing wall. Should keep cool(ish) for free.
    • CommentAuthorDamonHD
    • CommentTimeJul 27th 2010
     
    Our newish A+ fridge/freezer uses a little over 1kWh/day, 20% of our consumption, so is annoyingly significant.

    http://www.earth.org.uk/note-on-Siemens-KG34NA10GB-upright-fridge-freezer-REVIEW.html

    Rgds

    Damon
    • CommentAuthordjh
    • CommentTimeJul 27th 2010
     
    Improving fridges (and finding the best ones) is an interesting topic. There've been useful discussions on both the Navi and AECB forums.

    With regard to putting the radiator somewhere cooler - yes this is a good idea in principle but - you can't do it with many modern appliances because they are designed to have it in a heated room and won't work properly if it's put somewhere cold (the refrigerant doesn't vaporise properly). So be careful about what you try it with.

    With regard to adding extra insulation - yes this is a good idea in principle but - be careful because some appliances have heat exchangers in odd places and you can't just cover those.

    Amory Lovins at the Rocky Mountain Institute has done some interesting work in this area.

    What about a good old-fashioned larder instead of a fridge? (or as well as)
  9.  
    Many thanks for the replies.

    Posted By: djhWhat about a good old-fashioned larder instead of a fridge? (or as well as)


    Yes, we should certainly consider that. We're veggie so a lot of the stuff we currently cool to +4 could probably survive perfectly well in a lada.

    In our house we don't have space for a separate room but perhaps we could achieve a "lada-like" effect by insulating a kitchen cupboard very well and then having a small "radiator" in the back. Water would be pumped through this radiator and then into - say - 20m of pipe buried 2 meters underground in a shaded part of the garden. The end result would be a space cooled to about 8 degrees C (which might well be perfectly adequate for salad, veg, cheese, eggs, bread etc), whilst only requiring a small water pump.

    Can you buy small heat-pumps off the shelf for making your own fridge?!
    • CommentAuthorMartinH
    • CommentTimeJul 27th 2010
     
    Our A++ rated freezer is a case in point - the heat exchanger is built into the external walls, which get noticeably warm. Always struck me as an an odd way to construct them wrt efficiency having a -18C to +35C gradient across the insulation. That said - the heat exchangers are not as vulnerable as the "radiator" type hung on the back. However, you can generally add extra insulation to all the other walls of this type of freezer
    • CommentAuthorjoe90
    • CommentTimeJul 27th 2010
     
    On a previous thread for this subject I mused whether it was better to put a fridge in a cool larder (which would then be heated by the Heat pump) or put it back in the kitchen (where the heat pump would work harder to cool the contents. I know it would void any guarentee but I have concluded it would be best to put the fridge in a cool larder but run the pipes through a wall and have the heat pump in the kitchen. Best of both worlds????

    The larder in my new build would be North facing, single brick walls, outside of house insulation envelope, stone shelves, stone floor, air bricks to outside.
  12.  
    Posted By: dan_aka_jackIt sounds like the ideal would be to have the "external" pipes of the fridge buried 2m underground right next to your GSHP pipes. How does that sound???


    This is flawed reasoning, at least, for a temperate climate like the UK where some heating is required pretty much at any time of year. A passivhaus can work mainly through the "incidental gains" that modern living provides: heat from lights, computers, TVs, fridges etc. When one understands how a fridge works, it is easy to see that it doesn't consume as much energy as one might imagine. The "waste" heat that a fridge has to dissipate comes from two sources: (1) leakage from its environment through the insulation to the inside and (2) heat in the products that are to be chilled by the fridge. Minimizing (1) is a good idea of course, but (2) is a "free" source of energy from the environment, usually from outside of your house. In this mode, the fridge is acting as a heatpump with a pretty decent COP - meaning that you're getting more heat out than you're putting in in electricity to tun it.

    If you dump the heat from the fridge into the ground, you will marginally improve the fridge's COP, but will vastly reduce the system COP since all the heat from (1) will be useless dumped into the ground and any advantage from (2) will be wasted.

    So, don't put a fridge into an uninsulated room - though if it's in an unheated room at least (1) is minimized. You definitely don't want to lose (2) though.

    Of course, in a hot climate, it's a different balance as, in the summer, the excess heat from the fridge just adds to the air conditioning load, but this would rarely be a problem in the UK I would think as most nights the temperature drops below the figure normally used to determine heating/cooling degree day data (18C).

    Paul in Montreal.
  13.  
    Posted By: Paul in MontrealThe "waste" heat that a fridge has to dissipate comes from two sources: (1) leakage from its environment through the insulation to the inside and (2) heat in the products that are to be chilled by the fridge. Minimizing (1) is a good idea of course, but (2) is a "free" source of energy from the environment, usually from outside of your house. In this mode, the fridge is acting as a heatpump with a pretty decent COP - meaning that you're getting more heat out than you're putting in in electricity to tun it.


    Although, of course, as soon as you take the chilled products out of the fridge to consume them you end up putting the heat back into them, either from your body, the air in your house, or from the cooker, which negates at least some of the apparent gain.
  14.  
    Hi Paul,

    Many thanks for the detailed reply.

    In our house, we probably only run the heating for about a third of the year. So for 2/3rds of the year, the heat output from the back of the fridge isn't benefiting us - it really is waste.

    Also, my "ideal" system would be to run the "warm side" pipes from the fridge literally right next to the "cold side" pipes for the home's GSHP system, buried 2m underground. The idea being that in summer (when the GSHP system isn't running), the fridge will be marginally adding to the heat stored in the ground (which'll be useful in winter). And in winter, the fridge will be pretty directly providing heat for the GSHP system.
  15.  
    Posted By: dan_aka_jackIn our house, we probably only run the heating for about a third of the year. So for 2/3rds of the year, the heat output from the back of the fridge isn't benefiting us - it really is waste.
    I would think that for part of that 2/3rds of the year the reason you don't need to run heating is that you have sufficient incidental gains, the fridge being one of them. I doubt the heat you'd put in the ground in summer would have any long term storage effect as the pipes aren't buried very deep plus the rate of heat output of the fridge is really quite low - if it's only using 0.5kWh per day, that really isn't much energy. In winter, you'd lose some of the heat to the environment so you'd actually get a better bang for the buck by just having the heat directly put into your house from the fridge. It would be interesting to do a full analysis of the energy flows, but I suspect that, overall, you'd be better off with the fridge inside - in anycase, the fridge would be taking heat out of your house and putting it in the ground, effectively increasing the heatload on your house so, unless you could recover that with 100% efficiency, you'd be worse off than just having it inside the insulated envelope.

    Posted By: MarkBennettAlthough, of course, as soon as you take the chilled products out of the fridge to consume them you end up putting the heat back into them, either from your body, the air in your house, or from the cooker, which negates at least some of the apparent gain.
    Good point! I wonder if eating chilled products has a slimming effect due to the body needing to provide the energy to warm them back up to body temperature? I can see it now: "The Ice Diet"!!

    Paul in Montreal.
    •  
      CommentAuthorfostertom
    • CommentTimeJul 27th 2010
     
    Similarly drying clothes and logs indoors, and importing cold water into WC cisterns - big heat absorbers. However, food shd be considered a housewarming fuel - as long as you stay there for a gd while after eating it.
  17.  
    Posted By: Paul in Montreal
    Posted By: MarkBennettAlthough, of course, as soon as you take the chilled products out of the fridge to consume them you end up putting the heat back into them, either from your body, the air in your house, or from the cooker, which negates at least some of the apparent gain.
    Good point! I wonder if eating chilled products has a slimming effect due to the body needing to provide the energy to warm them back up to body temperature? I can see it now: "The Ice Diet"!!

    Paul in Montreal.


    http://health.howstuffworks.com/wellness/diet-fitness/weight-loss/question447.htm

    Need I say more?
    • CommentAuthorjohndes
    • CommentTimeJul 27th 2010
     
    Maybe its time the old fashioned larder made a comeback in the design of new homes. Insulated internal walls to cavity standard with a non insulated 100mm wall to the external elevation.
    • CommentAuthorSteamyTea
    • CommentTimeJul 28th 2010 edited
     
    Defrosting a fridge/freezer can have a significant impact on the running costs, ice insulated well. I mentioned on another thread about switching a fridge off for a period of time, this will not reduce energy use overall, but could reduce running cost/CO2 use. It will rely on a well insulated fridge and some sort of over temperature override, but nothing too tricky.
    If you think how you actually use a fridge, there seems to be a flurry of door opening in the morning (turn it off then) and again in the evening (turn it off again then). During these times of use a lot of the energy will be used just cooling the replaced air, the food and drink will not have enough time to react to the increased air temperature so should not spoil. One way around this may be to make sure your fridge is as full as possible, canned drinks maybe. That way you are reducing the volume of air that can be replaced and replacing it with something that has a higher SHC, so stabilising the temperature (but needing more energy overall to initially cool).
    Alternatively get a small fridge that is used for items that spoil quickly i.e. milk, meat etc, and then use tinned or dried food that has no temperature storage issues.

    Another interesting point I heard on here is that if your fridge is installed over an area of underfloor heating you will have serious problems, same applies to fitting a fridge into a cabinet, there will be lack of airflow, reducing the COP somewhat, must be true of fitting one next to a hot oven/Aga. I know the back of my fridge gets up to 50 C, tempting to dry clothes on but then it would overheat in no time. Somewhere I have some fridge data that made interesting viewing, the temperature swings where about 23 C in the freezer compartment, shall try and dig it out sometime.

    Found the link to that other discussion:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4185&page=1#Item_29
  20.  
    Posted By: Paul in MontrealI would think that for part of that 2/3rds of the year the reason you don't need to run heating is that you have sufficient incidental gains, the fridge being one of them.


    OK. Let's be really generous and say that for 1/4 of the year (when the air temp is cool but not so cold that we need the heating), the heat output from the fridge is what's allowing us to remain warm enough to mean that we don't need the heating.

    But that still leaves half of the year when it's plenty warm enough to mean that we'll be comfortable without either central heating or the fridge. For that half of the year, the fridge/freezer has to achieve a sizeable temperature difference: perhaps as large as from +30 air temp down to -20 inside the freezer. If we just consider the summer months, the fridge/freezer should run considerably more efficiently with the "hot" side of the heat pump buried underground (assuming we're using a refrigerant which is happy to run at relatively low temps).

    The question then becomes: will the increase in fridge efficiency during the summer months be offset by the loss of the fridge's heat output in the winter months (although, remember, we're not completely losing that heat because the GSHP system will get some benefit from that heat).

    Also: I take the point that if we had a Passivhaus then yes, incidental heat gains are useful. But we have a Victorian end-of-terrace house. So far we've insulated and draught proofed the livingroom but it'll take years to insulate and draught proof the entire building fabric; and even when we finish the house will still be a long way shy of PassivHaus standards; so much so that I expect it'll still make sense to programme the heating to only heat rooms which are being used. We're not gutting the building so we wont be able to make it particularly airtight (for example, we're very unlikely to lift the floor in the kitchen; we'll probably just externally insulate the kitchen walls; so the kitchen will always be a bit draughty).

    We only use the kitchen for perhaps 90 minutes each day so the 24/7 heat from the fridge will mostly be of no use to us.

    My hunch is still that - for our house - the increase in the fridge's COP achieved during summer months by burying the "hot" pipes of the fridge will more that offset the loss of the he fridge's heat output in the winter months. If only we had a decent physical model which could experiment with these things without actually having to build them.
  21.  
    Posted By: dan_aka_jack
    Also: I take the point that if we had a Passivhaus then yes, incidental heat gains are useful. But we have a Victorian end-of-terrace house.
    They're useful in all buildings, just that in a passivhaus they may be sufficient to obviate the need for heating altogether.

    Posted By: dan_aka_jackFor that half of the year, the fridge/freezer has to achieve a sizeable temperature difference: perhaps as large as from +30 air temp down to -20 inside the freezer.
    You're in the UK - there's no way you have half the year with an air temperature of 30C inside your house. Anytime the outside temperature is less than 18C (which is a lot more than half the year in the UK) the fridge is "helping".

    Posted By: dan_aka_jackWe only use the kitchen for perhaps 90 minutes each day so the 24/7 heat from the fridge will mostly be of no use to us.
    Keeping your house a a relative constant temperature and humidity level is actually a good thing for its contents - the more airtight and insulated it is, the easier this is to achieve without heating/cooling.


    Posted By: dan_aka_jackMy hunch is still that - for our house - the increase in the fridge's COP achieved during summer months by burying the "hot" pipes of the fridge will more that offset the loss of the he fridge's heat output in the winter months. If only we had a decent physical model which could experiment with these things without actually having to build them.
    I doubt it. As someone pointed out, their fridge was using 0.5kWh per day which is what, 5p worth of electricity? The cost of adding the pipes to the garden, extra refrigerant to fill those pipes, the lowering of efficiency due to the extra pumping losses of the now large refrigeration loop, all to improve the COP by a small margin?

    Tangentially, I used a similar logical deconstruction to save myself from installing a drain water heat recovery unit. As I only spend $100 a year on hot water, the most I could save would be about $30 (the unit only works on the shower) - factoring in the cost of the unit and the labour for installation meant that it would be decades before I broke even.

    I'd recommend focusing your efforts where it will have an effect, and the better your insulation and airtightness, the more the "waste" heat from your fridge will be a benefit.

    Paul in Montreal.
  22.  
    Posted By: Paul in Montreal
    I'd recommend focusing your efforts where it will have an effect, and the better your insulation and airtightness, the more the "waste" heat from your fridge will be a benefit.


    Agreed.

    Although one point that I don't think has been raised yet: not all the heat from the back of the fridge originates from either the room or the contents of the fridge. No heat pump works at 100% efficiency (thermodynamics says it can't). Some of that heat is just from friction (and that heat comes at a high carbon cost - it's as bad if not worse than resistive heating).

    According to wikipedia, the compressor's dissipated work is reflected in the COP: http://en.wikipedia.org/wiki/Heat_pump#Efficiency

    Does anyone know how supermarkets maximise the COP of their refrigeration systems?
  23.  
    Solar-thermal powered fridge (kind of): http://en.wikipedia.org/wiki/Einstein_refrigerator
  24.  
    Posted By: dan_aka_jackSome of that heat is just from friction (and that heat comes at a high carbon cost - it's as bad if not worse than resistive heating).
    It can't be worse than resistive heating but you're correct, every Watt that the fridge consumes appears in the "waste" heat stream - so if your fridge consumes 200W when running, it's the same as if it was a 200W resistance heater, at least as far as energy usage is concerned.

    Posted By: dan_aka_jackDoes anyone know how supermarkets maximise the COP of their refrigeration systems?
    Yes, they used water-based chillers that lower the condenser temperature - this maximizes the COP. BTW, you had it backwards when you worried that burying the coils from your fridge would upset the refrigerant - the coil on the back of a fridge is the condenser and the cooler it is, the more effective it is. Some supermarkets then use this heat for the hot air blowers at the front door!

    Paul in Montreal.
    • CommentAuthordjh
    • CommentTimeJul 28th 2010
     
    Posted By: dan_aka_jackCan you buy small heat-pumps off the shelf for making your own fridge?!

    Yes. You can certainly buy them for the marine market and I expect you can get them cheaper elsewhere!
  26.  
    One random thought: if you're designing a house with whole-house MVHR, could you place an MVHR extraction vent immediately above/behind the fridge. The thinking being that you'll keep the fridge's rear radiator nice and cool by pulling air past the radiator (hence maximising your fridge's COP) but your MVHR unit will recover some of the heat produced by the fridge.

    Heck, even in existing homes without whole-house MVHR, perhaps it would still be possible to mechanically ventilate the back of the fridge to keep the rear as cool as possible whilst also hanging onto the heat produced by the fridge.

    Posted By: Paul in Montreal
    Posted By: dan_aka_jackSome of that heat is just from friction (and that heat comes at a high carbon cost - it's as bad if not worse than resistive heating).
    It can't be worse than resistive heating


    My thinking was: resistive heating, for all its sins, does convert almost 100% of the electrical energy entering the device to heat (i.e. it has a COP of 1.0; an electric heater drawing 1kW of electrical power will produce 1kW of heat).

    Is it not conceivable that a device could have a COP of less than 1? Or maybe it's not? For example, say a electric car's electric motor draws 10kW and does 9kW of mechanical work then doesn't the motor produce heat at only 1kW? i.e. it has a COP of 0.1? Or have I got my thermodynamics wrong?!

    BTW, you had it backwards when you worried that burying the coils from your fridge would upset the refrigerant - the coil on the back of a fridge is the condenser and the cooler it is, the more effective it is.


    Ah yes, I was wondering about that. But how does what you say square with what djh said:

    Posted By: djhWith regard to putting the radiator somewhere cooler - yes this is a good idea in principle but - you can't do it with many modern appliances because they are designed to have it in a heated room and won't work properly if it's put somewhere cold (the refrigerant doesn't vaporise properly). So be careful about what you try it with.


    I'm confused
    • CommentAuthorSteamyTea
    • CommentTimeJul 29th 2010
     
    Dan

    In a way you are right, the terminology changes, COP or Coefficient of Performance tends to be used for heat pumps and Efficiency for motors.

    Re a 10kW motor in a car doing 9kW of work then the efficiency would be 90% or 0.9, tend not to use the COP term here.
    Re a 1kW heater converting 100% (or nearly) to 1kW heat is correct, unless you measure from the energy stored in the fuel that made the electrical energy and transmission losses etc. This is where many people who claim that there is an 80% loss in the 'transmission' of electricity get their figure from, they take the delivered energy divided by the primary energy.

    All goes back to the Laws of Energy Conservation as opposed to conserving energy, which we tend to talk about on here, causes confusion.
  28.  
    Posted By: SteamyTeaRe a 10kW motor in a car doing 9kW of work then the efficiency would be 90% or 0.9, tend not to use the COP term here.


    Yes, indeed - sorry; I had my tongue slightly in cheek when using the term "COP" for an electric motor. As you say, no sane person would normally talk about the "COP" of a motor. I was kind of coming at it from the angle of someone who was only interested in the motor's heat output and the mechanical work produced by the motor was considered "waste". My thinking was: if you bought an electric motor to heat your home then you'd be a) an idiot but b) you might refer to it as a heater with a COP of 0.1

    And yeah, I agree, resistive heating sucks when considered from the perspective of fossil fuel burnt per unit of heat output.
    • CommentAuthorbrig001
    • CommentTimeJul 29th 2010
     
    Posted By: dan_aka_jackHeck, even in existing homes without whole-house MVHR, perhaps it would still be possible to mechanically ventilate the back of the fridge to keep the rear as cool as possible whilst also hanging onto the heat produced by the fridge.


    We did just that, but our fridge has the "design feature" of having the compressor and condenser (ie. all the hot bits) immediately below the freezer. Blowing air over these continuously with a couple of PC fans dropped the energy consumption even accounting for the power taken by the fans.
    Brian.
  30.  
    Posted By: brig001...our fridge has the "design feature" of having the compressor and condenser (ie. all the hot bits) immediately below the freezer. Blowing air over these continuously with a couple of PC fans dropped the energy consumption even accounting for the power taken by the fans.
    Brian.


    Excellent info, thanks. I might give that a go!

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