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It is of course desirable to insulate houses so that they stay warm for longer in cold weather. Doing this really well is relatively easy for new builds, but for old houses one rapidly reaches the point where it isn't practical to improve on. And that's still not good enough for really cold spells.
I
propose a new method of external protection, which effectively aims to increase the temperature of the immediate external environment. This means there is a lower thermal difference over the house's insulation, which in turn means that less energy needs to be used to maintain the interior at any particular higher temperature.
As liquids freeze, they release hidden energy (often known as 'latent' heat). For a solid to melt, this energy has to be resupplied.
For water, this energy is 334 kilojoules per kilogram.
This large amount of thermal energy required is why one often sees solid and liquid water together; the water doesn't just instantly freeze or melt.
For comparison, the specific heat - the energy required to raise the temperature of liquid water by one degree Celcius - is 4.2 kilojoules per kilogram.
So, the point is, that one could potentially store energy in a liquid, and use it to buffer a region from a drop in temperature.
I have considered three form-factors for such a buffer. Both would need to be situated around (i.e. not attached to, but nearby) the house to be protected, and particularly up-wind.
1) A spray system. Water is released into the environment in the form of small droplets. These have a large surface area, and so can freeze to ice quickly. Provided they settle within a collection area this will efficiently warm sub-zero air.
The potential drawbacks of this method are the risk of the spray freezing up, or the spray particles escaping due to high winds (i.e. the ice blowing onto your house is undesirable).
2) Thin 'buckets'. These would provide a more passive system, at a cost of a slower buffering rate per unit volume. When mostly or completely frozen, the buckets would need to be emptied of ice and refilled. Vertical racks of these could be positioned as expedient.
For a bespoke system, I imagine them as being metal-walled, with a lever which opens them up so the ice slab can be removed. An improvised home-built system could probably just use any empty plastic containers which are to hand.
3) Laminar flow surface. Essentially a combination of the above two methods. Water is allowed to flow over a surface exposed to the cold air, building up a layer of ice.
This might have less issues with clogging of the outlet than a spray and fluid escape. Design of the surface and water flow rate may need careful regulation to ensure an even buildup however. Ice would need to be smashed off for removal.
In all cases, the ice produced could be collected and used for refrigeration in an ice-house over summer.
In places where water is plentiful (like the UK), this system would mean that old houses would not need massive refurbishment merely to deal with a few days a year of sub-zero temperatures.
'reverse phase change roof'
_27reverse_20phase_20change_20roof_27
a phase change building material to keep building cool [xaviergisz, Oct 12 2010]
Snow harvesting infrastructure
Snow_20harvesting_20infrastructure
The ice produced may usefully be fed into this. [Loris, Oct 13 2010]
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//the ice blowing onto your house is undesirable.\\ |
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Why? I should think a layer of ice on your entire house gives excellent insulation. |
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//I should think a layer of ice on your entire house gives excellent insulation.// |
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I suppose it may do under some circumstances. Snow is a good insulator, but this isn't snow, so probably isn't.
By bringing it to the house, it would bring along the coldness which was supposed to be trapped out the way. It would introduce moisture when it melts. It's heavy, so if it builds up it'll damage stuff. And make it hard to open doors or walk on paths.
And be unpleasant to walk through while still airborne
Much better to set the particle size such that it gets sequestered out of the way. |
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//Provided they settle within a collection area this will efficiently warm sub-zero air.// |
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How much warmth are we talking about? Just enough to bring the room above zero? |
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useful if you wanted to keep a room at 0C... not so much for anything else. |
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//How much warmth are we talking about? Just enough to bring the room above zero?// |
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This will warm the _external_ air to a maximum of zero degrees Celsius. (Well, technically ... you could use something with a higher freezing temperature. But in this external, area-effect form-factor, I think it has to use something cheap and plentiful.) |
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//useful if you wanted to keep a room at 0C... not so much for anything else.// |
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I realise now that I have neglected to mention some important physics. (Idea edited.) |
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Heat loss is proportional to the difference in temperature. This means that if we can increase the _external_ temperature by just a few degrees, this will have a big effect on the energy required to maintain the _internal_ temperature.
So you could maintain your house nice and toasty with a boiler which couldn't cope with an external temperature of, say, -15 degrees C or so. |
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If the air is cold enough to freeze the water, surely you'd be spending energy keeping the water from freezing in the first place? |
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You may as well cover the whole house in a jacket of water to buffer the temperature - in fact a specially mixed fluid which had a phase change at room temperature would be pretty good. |
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yahbut it might be easier to keep your house at 20C indoors if it's surrounded by -20C air than if it's surrounded by 0C water. |
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How do you propose to keep the ice at the correct temperature? It will get a lot colder very quickly if the outside temperature drops suddenly, like when it gets dark. |
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//If the air is cold enough to freeze the water, surely you'd be spending energy keeping the water from freezing in the first place?// |
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When the water freezes, it's doing its job. The proposal assumes you have access to copious water. Throughout the cold spell, you allow more and more water to freeze, and store the ice somewhere out of the way. It will be useful when it's not so cold, for refrigeration purposes.
So no, you don't need to spend energy preventing the water from freezing. |
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//You may as well cover the whole house in a jacket of water to buffer the temperature - in fact a specially mixed fluid which had a phase change at room temperature would be pretty good.// |
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Indeed. However, setting that up would presumably be more hassle than totally covering the house in a thick layer of external insulation. Which is what I was trying to avoid. |
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//How do you propose to keep the ice at the correct temperature? It will get a lot colder very quickly if the outside temperature drops suddenly, like when it gets dark.// |
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I don't propose holding the water/ice at any temperature. The idea is to let water freeze, warming the air with the latent heat.
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To see whether this idea is practical, it would be nice to have some numbers.
I can think of two ways of working out how much water would be required. One would use black-body radiation to work out the heat loss of open ground to space per metre squared. The other is to assume constant conditions for incoming air. Here is the latter: |
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Air has a density of 1.3kg/m^3 at 0degC; let us assume that we're talking about temperatures close to (but below) zero degrees. The specific heat capacity is approx 1kJ/kg.
The energy released by freezing 1kg of water can warm 334 kg of air by 1 degree. This fills 257m^3.
Which means 4kg freezing water will warm 1 km of air per meter-squared per degree. |
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To warm a 10km/hour wind for 10 hours, we'd need 400 kg water per metre squared, per degree.
That may seem like a lot - well, it is, for a single house in isolation. But on a larger scale it might become practical.
One could warm the wind ahead of a city, for example, with a large fountain sourced from a lake. This would yield a more pleasant environment downwind, and significantly reduce heating costs. On this industrial scale the ice generated could be efficiently recovered, stored and used, too. |
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What about the water needed to buffer temperature in still air conditions? An online blackbody spectral calculator suggests that the heat loss at 273 degrees Kelvin (i.e. 0 degrees C) is 283 W/m^2 (at emissivity of 0.9, which is similar to many building materials). If this were correct, each meter square would lose 283 joules per second. Which would mean freezing about a kilo of water per metre square every 20 minutes.
This seems to be in the right ball-park - if as night falls the ambient temperature is 0 degrees, after 12 hours of darkness 36kg of ice would be predicted per m^2 of a lake's surface; approximately a 4 cm depth of ice. This excludes insulation effects of the ice itself and re-radiation from the surrounding environment - so it's on the high side, but it doesn't seem to be ridiculously far off. |
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As I understand it, the house would be surrounded
by a high wall of ice - what would that do to the
daylighting and more importantly, the view? I
visited
Crested Butte, CO in the winter and snow was up
to
the eaves, offering some kind of microclimate, but
all
of the windows looked on to little caves of melting
snow with some weak light coming down from
above.
Also, my climate in Devon rarely has freezing
temperatures long enough to warrant hoisting all
those buckets onto a fence. |
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Where do you get the liquid water from? |
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and how much energy is expended in pumping water and moving ice versus how much it would take to just install electrical heating elements around the outside of the house and warm the air by a degree or so that way? |
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//As I understand it, the house would be surrounded by a high wall of ice - what would that do to the daylighting and more importantly, the view?... Also, my climate in Devon rarely has freezing temperatures long enough to warrant hoisting all those buckets onto a fence.// |
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The 'wall' could be several meters away from the house. Also, it wouldn't necessarily be a 'solid' structure. More of a fence or perhaps a set of shelves in practice. Nevertheless, it would spoil any view somewhat. At least you would only need to leave it up for a month or two per year.
You're also correct that it wouldn't be worth lifting buckets up high, so a manual setup would be limited to about shoulder-height. Taller structures refilled by pipework would still be feasible. |
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Fortunately for you, the climate in Devon, being both in the south (of the UK) and with a prevailing wind from the sea means you don't have to worry too much about freezing temperatures. |
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//Where do you get the liquid water from?// |
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Disregarding the tense of that question, water is virtually free in the UK. Granted, a lot would be needed for this scheme to work, but it falls from the sky most days for half the year, round here, so it could be locally sourced, if necessary.
Incidentally, it occurred to me that a large storage tank could hold both stored water and the used ice. It would need careful design to avoid overfilling, rupture, or freezing of the outlet, and extraction of ice - but in terms of efficient use of space that's got to be optimal. |
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Yes [Loris], but those days, there is not much call for keeping the temperature outside your house a little below 0C. |
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// there is not much call for keeping the temperature outside your house a little below 0C.// |
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Many people didn't enjoy the few really cold days and -15 degree nights the UK had at the start of this year, so that isn't even true.
People do also tend want to keep their homes warm, which is what this idea helps with. |
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What you have to appreciate is that this idea is intended to help cover a few atypically cold days, without the difficulty (practical impossibility) of retrofitting a large thickness of insulation and establishing full draft exclusion in old buildings. |
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And the ice generated would be useful in summer, too - it's win-win. |
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Over the last couple of weeks there's been a bit of a cold spell in the UK, which I've used to perform an experiment. |
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I've been seeing how much ice I could generate in my back garden, using containers formed mostly from the bottoms of disposable plastic milk containers.
This has been an interesting experience, and I've created quite a lot of ice. I've been removing ice from each container in the morning and again at night, occasionally multiple times. Obviously the colder it is, the thicker the ice - but the more often you remove it, the more you get (because the ice insulates the water as it builds up). |
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So this process with expedient containers is encouraging. A well designed system to make the process more efficient could yield a lot more ice (which would buffer the temperature better, too). Something like 10 tons of ice is probably the maximum which could reasonably be stored near most houses - which for visualisation would fill a cube of about 2.2 meters if packed solid. |
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I've tried to determine a useful quantity of ice; one use would be maintaining a refrigerator. Free-standing electric fridges don't have a great deal of insulation, and also I suspect that their efficiency of cooling isn't very high. Nevertheless, suppose we need as much heat energy to be acquired as an electric fridge uses: |
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Values of fridge energy usage vary widely - unsurprising since there are many variables, particularly fridge size and efficiency. The one I chose is 0.66kWh per day. This is equivalent to 0.66*365*3,600,000=870,000,000 joules/year.
To sink this amount of heat energy, we need 870,000,000/334,000=2600kg of ice (2.6 tons).
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That's well below my proposed maximum - so even if I'm a bit out, and an allowance is made for loss due to imperfect insulation of the store, it seems doable. There is even potential to store over a year's supply; useful to cover a mild winter. |
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