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Seasonal Heat Storage in Sand

Store the heat of summer, for winter, in a sand-filled bunker
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An insulated pit full of sand located under a house, heated during the summer using solar energy, heats the house in the winter.

Solar energy is concentrated onto one end of a heat pipe. The other end of the heat pipe is embedded in the heat storage medium (sand). So the sand gets hot. Heat is extracted from the storage by adjusting a movable panel of insulation over the pit. The lid is closed during the summer, and opened up when heat is needed.

Sand is a cheap & safe medium for heat storage and can withstand very high temperatures. 'Special' sand (alumina aka aluminum oxide aka bauxite) has a much higher heat capacity than regular sand - almost as good as water.

The pit would be insulated by foam glass, which is can be buried and can withstand high temperatures. Water-based heat pipes can transfer heat up to 300C or so... which is a convenient temperature at which to collect solar energy.

Some problems might be that the collectors could not be very far above the storage pit, the reflectors might be inconvenient or unsightly, and the heat pipe might transfer heat in the wrong direction.

afinehowdoyoudo, Nov 28 2008

Geothermal Heat Pump http://en.wikipedia...eothermal_heat_pump
Great idea - and the way I read it, you've augmented it with a passive solar component . [zen_tom, Nov 28 2008]

Seasonal Heat Storage on Wikipedia http://en.wikipedia...sonal_thermal_store
[afinehowdoyoudo, Nov 29 2008]

Heat capacity of some constituents of bauxite http://www.minsocam...n/AM76/AM76_445.pdf
[afinehowdoyoudo, Dec 03 2008]

Cenicom pointfocus.com/pdf/090-Cenicom-Plant.pdf
A power generation scheme using many similar components - this would be simpler and lower-temperature [afinehowdoyoudo, Dec 03 2008]

A study of candidate materials for seasonal heat storage http://www.ecn.nl/p...x?nr=ECN-RX--06-017
All of these are dissociation/asociation reactions, not sensible heat storage. The best, magnesium sulphate, has equivalent heat storage per volume as aluminum oxide heated to ~900C (!) [afinehowdoyoudo, Dec 25 2008]


       sand? why sand. so many better materials.
WcW, Nov 28 2008

       Weird. I had exactly the same sort of idea while travelling on a freezing circle line train on saturday. People moan about it's being too hot on the tube in summer, and the biggest difficulty with air con all the way down there is getting rid of the heat. Can't we just keep that heat until winter?
theNakedApiarist, Nov 28 2008

       This is an example of sensible heat storage. 'Sensible' means that heat transfer causes a change in temperature of the material.. makes sense, eh? Compare this to 'latent' heat storage, where the material stays at the same temperature but changes phase e.g. solid -> liquid. The heat is 'latent' or 'hidden' in the phase change. There are some interesting materials for latent heat storage, but thats not part of this idea. So if you want to store sensible heat, there is temperature change (dT) in the medium. More dT, more heat. Another factor is the heat capacity of the material: how much heat it stores for each degree of temperature rise. And of course, more material stores more heat. So in equation form,   

       Sensible Heat (Q) = Heat Capacity (Cv) * Volume (V) * Temperature change (dT)   

       A convenient measure of heat energy is the GigaJoule (GJ). 1 GJ is about 1 kiloWatt for 278 hours (about 12 days). Thats $27 of electricity at local rates, more elsewhere. So to get through an average winter (around here) in my small well-insulated house, I want about 40 GJ.   

       So, heres some likely candidates for sensible heat storage with their heat capacity per degree C (per cubic meter m^3) and useful temperature range. The lower limit on useful temperature range is about 20C because heat stored below room temperature is unavailable.   

       Water: 4.18 MJ/C*m^3 20 - 90 C   

       Rocks or sand: 1.8 MJ/C*m^3 20 - 900 C (or higher)   

       Aluminum oxide 3.0 MJ/C*m^3 20 - 2000C   

       Iron: 3.9 MJ/C*m^3 20 - 1100 C   

       Water stores the most per degree C, but the useful range is limited by the boiling point at atmospheric pressure. Those other materials could (at least half-bakedly) be heated far more, hence more storage per unit volume. Also, an insulated pit to store very hot sand will be easier, cheaper and more reliable than an insulated tank to store very hot water.   

       The limiting factor on the max temperature of the heat stash then becomes concentrating and transferring solar energy to the sand pit. I considered some schemes where the sand is transported by conveyor to a solar furnace atop the house, and drops down into the pit after being heated to a glowing-red-heat. Safer and more practical is to use a water-based heat pipe, which is good up to about 300C, which also happens to be a useful collection temperature for reasonably low-tech solar concentrators.   

       So for my application I would need to build an insulated pit of about 50 m^3 ... a cube of 3.7 m (12 feet) per side. On the solar collection side I would need about 10 m^2, with a concentration of about 10X..
afinehowdoyoudo, Nov 29 2008

       [rcarty], I think 'climate change' is more accurate than 'global warming'. Its a pretty mild winter here now, but last year was a ball-buster. Besides, once we get world government they'll regulate on all us carbon-spewers.
afinehowdoyoudo, Nov 29 2008

       I would think that the latent heat that could be stored in a tank of water/ice would be more useful than the sensible heat that could be stored in sand. Heating the sand above ambient would require using energy; one may as well use that energy for other things or store it in a more useful form. A tank of water with lots of ice in it would be a useful heat sink for an air conditioner in the summer (improving its efficiency substantially if the ice itself didn't cause problems); when outside temperatures were below freezing, pumping heat from the water (freezing it) would be the most efficient way of heating the house.
supercat, Dec 01 2008

       Tell me more about why 300°C is a convenient collection temperature, if you please.
Texticle, Dec 01 2008

       I expect you are grossly underestimating the amount of heat storage required. It is not uncommon in the HVAC industry to store energy as ice or clod water created during the "off peak" utility times for use during the "on peak" utility times. A commercial structure can easily go through 1 million gallons of cold water in an afternoon (due to phase change, less ice would be required). Storing enough energy for a season seems unlikely. I expect if this worked well, it would not be so cold in the desert at night.
cblunds, Dec 01 2008

       One could easily adapt this idea to regions that contain active volcanoes by building houses over pits of lava, then shielding them from the lava heat with insulated lids. Spent lava couldbe regenerated using heat pipes, or lava could be delivered when available using lava trucks insulated with foam glass.   

       Wait - with the heat pipes my anno is redundant with the idea. No heat pipes. Just lava. The foam glass trucks would turn like cement mixers and have a picture of the goddess Pele on the side.
bungston, Dec 01 2008

       Smells like geothermal.
Spacecoyote, Dec 02 2008

       Nice idea bungston - the lava-powered home could well be the home of the future - want a hot bath? Simply fill bath with cold water, and then drop a lump of lava in it to heat. Cup of tea? Some lava please. One lump or two?
zen_tom, Dec 02 2008

       I must admit, a benefit of hot sand is that it has less of a tendency than lava to set up in the pipes.
bungston, Dec 02 2008

       The big issue with actually raising or lowering the temperature of any significant mass of material is transferring the heat to it. Thermal penetration into soil (or sand) can be measured in inches. Either a material with a high heat capacity and a high thermal transmission rate (not a real common combination, although aluminum oxide might cut it) or a very extensive heat transfer network (water pipe grid or the like). The amount of energy required to pump the thermal fluid for the latter would probably be significant, and might exceed that required for the aforementioned heat pumps with a simple single loop.
MechE, Dec 02 2008

       I think the real trick will be getting something that can retain a season's worth of heat.
ye_river_xiv, Dec 02 2008

       [UnaBubba], thanks for pointing out the difference between bauxite and aluminum oxide.. I was being sloppy. Turns out the heat capacity of raw bauxite ( a mixture of various hydroxides of aluminum) is still pretty good.. about 3.0 MJ per degree C per cubic meter. So using raw ore might be a more economical approach. See the paper by Hemingway et al (linky)   

       [MechE], you have to dig down about 15 feet to get to earth that does not change temperature seasonally.. so I don't know where your //..inches..// come from, unless you mean a hundred or so of them. & the thermal conductivity of aluminum oxide and hydroxides is very high, around 30 W/ m*K.. i.e. about 20 times the conductivity of regular rocks and sand. Also, using a heat pipe to transfer heat into the mass could provide extra distribution & surface area.   

       [Texticle], 300C is a convenient temperature to gather solar energy because a non-tracking line-focus concentrator (e.g. parabolic trough) can easily reach that temperature. To get much higher temperatures, a point-focus concentrator is required.   

       [ye_river_xiv] and all those other folks who question being able to store a season's worth of heat... well, how much you need varies greatly. If you have a drafty old mansion in northern Manitoba, you might need a TeraJoule to get through a winter. In southern California, a few GJ is probably enough for a moderate house. I calculate my needs to be approx 50 GJ, and I stand (or shiver) by my calculations.
afinehowdoyoudo, Dec 03 2008

       Yes! Lava! We could even use it in the lamps.
afinehowdoyoudo, Dec 03 2008

       [Iron] I was thinking in terms of significant temperature change, specifically the frost line, which is rarely more than a couple of feet under the worst conditions. Ground loop heat pumps are frequently buried no more than 3-4 feet below the surface (for the horizontal loop type). Please note, however that the more common 12-20 feet you see quoted is for moist soil, the presumably dry sand you would want under your house is going to be significantly less conductive. I agree that aluminum oxide might solve the problem, if it is not to expensive (since I estimate you need about 198 tonnes of the stuff [3.96 g/cm^3*1000000 cm^3/m^3*50m^3/1000000 g/tonne). My concern with heat pipes is the energy required to pump through them, a large grid to cover a 50 m^3 volume will have a significant head loss and require a fair sized pump.
MechE, Dec 04 2008

       my back-burner design just uses 2 heavily insulated tanks, predictably one with hot water, the other with cold water(/alcohol compound) the hot one gets recharged during the summer, the cold one during the winter, for year-round heating and cooling purposes (including refrigerator and domestic hot water, etc).   

       More expensive than current energy costs, though and not particularly small, either (sorry, can't find the numbers I came up with exactly). Even with mass production, you'd still probably be looking at the cost of a couple inground swimming pools. Mind you my design, for simplicity's sake doesn't include heat-pumps, the hot tank is heated to 80C, the cold one cooled to -20C.   

       But pound for pound I think water's still the best bet.
FlyingToaster, Dec 26 2008

       What about mud, mixing the sand and water? You get the benefit of both worlds. Most of your mass is none evaporative, and locally available in desert climates. when turned to icy mud, you still get the 97 kw in a cube during phase change. Same goes for higher boiling point hopefully never reached. The energy would be mostly stored in the sand.
pashute, Jul 01 2010

       latent heat of water is greater than that of everything else, and the temperature range is compatible with the outside environment which means least amount of thermal leakage.
FlyingToaster, Jul 02 2010


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