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Sand for Battery - Heat Water Options
#1 Posted : Tuesday, September 09, 2008 12:45:55 PM
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I've looked into thermal storage for my solar tough project.
(my goal is to heat using solar even at -40F)

Unfortunately most thermal storage is either HUGE or requires meltable items like Paraffin wax.

The commercial thermal brick heater units work only because they use electric and can heat to 1000's of degrees.

The closest I found to a workable Day/Night plan for my place was 250 gallons of Paraffin wax, but that would be expensive.
#2 Posted : Wednesday, September 10, 2008 6:55:48 PM
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I never heard of using wax.  Have any links so I can read up on it?  For my system I am looking around 2500 gallons.  I never thought of it being like a geothermal project so I am back to doing more research.  I think I am on the right track but still unsure just how effiecient it would be and I guess I just won't know unless I do some experimenting.  Any ideas let me know...  Thanks
#3 Posted : Wednesday, September 10, 2008 10:48:25 PM
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In designing your system there are three primary concerns.  These are amount of energy stored (energy density), rate of energy loss (can you store it until needed) and can you get the energy into/out of the storage medium (transfer efficiency.  Sand can be heated to a higher temperature than water but can either a wood stove or the solar water heater provide these higher temperatures in suficient quantity to heat the sand that hot?  If not then using sand is not indicated as volume for volume water stores more energy compared to sand at the same temperature.  If you can supply enough energy for the higher temperatures can you readily transfer it into the sand?  This will require a more comprehensive heat transfer system as transfering heat to a granulated solid isn't as easy as transfering it to a liquid.  Again for a home system this is unlikely.  Finally, to keep the heat until it's used you need to prevent leakage.  Easily done for both systems, insulate.  High temp dry sand use roxul, won't burn but will compress so needs a on-combustable support structure (steel).  Water, styrofoam sheets glued to the sides/bottom of the cistern and a floating lid.  You give dimensions of 5x5x15 feet. reduce it buy the thickness of the insulation and calculate the volume.  Look at your energy bill from last year to find the units it uses (BTU's) for example then calculate how many of these units wouldbe stored in your system, (I'll leave that to you as part of the fun of creating such a system).  Then using last years heating bills figure out the btu's needed per degree day for your house and that will allow you to figure out how many days heat storage you have at different outdoor temperatures.  Have fun.
It can be done!
#4 Posted : Thursday, September 11, 2008 8:40:47 AM
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Thanks for the time and thought.  I have been heading in this direction for some time.  On my wood stove I added gauges and monitor the temp in all stages looking for efficientcy.  I tend to over burn a bit at 550 *F  in the fire box and my air jacket runs about 450 *F averge.  The insulated plentuem on top around 225-250 *F and by the time it pipes to the NG Furnace ductwork 120-150 Max *F.

As for the NG I use I don't have a calculation accuarate yet but the bill is 35.00 dollars a month.  I want to just make all of my hot water and blow out the piolet light for a while to see how I do for a year than maybe turn it off.

As for volume thats easy enough approx 375 square foor storage.  As for sealing, hummm never thought of the heat sealant.  Good point.  The whole thing is wrapped in concrete made to hold water.  I knew transfering heat from a liquid to a solid was harder thats why I thought of adding water to just cover the sand and use it as a conductor.  GeoThermal wells operate much the same way with liquids and solids and they do just fine and in fact are some of the most efficient ways to go.  All I am doing is controlling the confined area to 375 sq feet and super heating it with a smaller system.  Making the insulated cover was a good idea, hadn't thought that thru yet.  I knew with a wood floor 5 feet above the usable 15 foor of heated sand I was going to need a vapor barrier and some how ventilation.  By insulation top that sits on top of the sand I could vent much easier with out much loss of storage. 

The last part of you post leaves me wondering.  I guess I can figure how many units of gas I used per month but what do I do from there?  I used about 154 Units of Gas for one year.  Not too bad but I want it lower....  Since I put in the wood furnace to my other NG furnace the gas company has changed my meter twice in the last 6 years and last fall they call me and asked me if I knew if there has been a leak and was going to change it again. 

Anyway do you have a formula I could use to help me.  As for the Kansas solar index I think it is 5.4 hours per day usable.  Summer the system may not heat as hot but still heat water hot enough I would think.  With the formula I know approx heat of water off wood stove will be around 225-500 *F and the solar system (your guess is better than mine) what it would produce.  I have been looking and I know all systems vary but how hat does a solar water heating system get, should be around 225*F on a kansas summer day right?  Thanks for the time.



#5 Posted : Thursday, September 11, 2008 8:31:30 PM
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Ah, water in sand.  Waste of effort, using the water to transfer heat to the sand means a maximum temperature of, drumroll.... 100 celsius, the boiling point of water.  As sand only makes sense if you attain higher temperatures an all water system is indicated.

Degree days are a measure of how much energy you need to heat.  For example the standard is to heat a home to  68 F.  Lets say for today, the average temperature over the whole day is 67 F (this info for your area should be available from the local weather people).  This translates to 1 degree day of heating.  Based on house design, leakage, insulation etc each house will require a different amount of heat for a degree day.  My natural gas heating bill actually tells me the degree days for the billing period and how much energy I used so I can get that calculation easily.  Thus I can calculate BTU's per Degree day (you might use different units such as therms, calories, kilocalories, celsius temperatures.... etc but that's just a conversion).  Now for an example (the numbers are not real but show the process) December the degree days might be 27, an average temperature of 41 Farenheit, and my calculations have shown me I need 10,000 BTU's per degree day so I would need 27 * 10000 = 270000 BTU's per day to keep my house at an average temperature of 68 F.  A BTU raises the temperature of 1 lb of water 1 degree Farenheit.  375 cubic (not square) feet of water is (375cf x 7.48USG/cf x 8.33lb/USG) = 23366 BTU's per degree.  Let's say I maintain a storage temperature of 190 F and can extract heat until the system is down to 90 F.  I thus have 100 degrees of storage so my cistern will store 2336600 BTU's.  Thus I have about 8 to 9 days of heat stored.  Does this warrent a solar system for the summer months, not if you buy it but possibly if you DIY it.

The tricky part comes next, figuring how much heat you can get from solar, the efficiency of putting heat into storage from wood, the efficiency of storing it (rate of loss) and the efficiency of getting it back out.  My "gut feeling", if you insulate the cistern it can be used well, no pun intended, in an inexpensive setup, if you don't insulate it will suck heat out of your house and not be a net benefit.

It can be done!
#6 Posted : Thursday, September 11, 2008 8:31:30 PM
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Posts: 134,494


I started reading Mother’s Earth News almost a year ago. Since then I have bought the CD set of past issues. I find myself still going through them filling in the gaps between issues that come in the mail today.

Since my quest for knowledge in solar water heaters. I have been reading and taking notes on all kinds of designs. I plan to build one but I haven’t rushed into it. As I gather supplies (all freebies so far) I have more questions and new ideas on how to design the system for me.

Right now I am looking for a way to combine my woodstove (winter months) and solar water heat (summer months) into the same system for little cost. Most of that is easy if I follow the dreams of past inventors featured in Mother Earth News and the past issues combining design and technique efficiency and bold originality. I use a Wood Furnace in the winter months and plumbed it to my NG Furnace which I use only as back-up and adding a plywood blank for a filter that can be easily removed (back pressure) makes it a Central Wood Heat Furnace and the hot air comes through the regular heat duct for each room. I thought of using it to preheat/heat my water in the winter just as the guy from Issue # 85 by making a plaster of paris block of copper tubing and mount close to the side of the stove for the collection of radiant heat. I have a typical old farm house with a cistern inside for water (now sits empty). Through the years changes were made and now we have city water and the well is gone. I know it leaks and won’t hold water but it can be resealed. I had the idea that I could use it to store heat. I think if I plumb it right I could store heated water from the wood furnace in the winter and solar heated water during the warmer Kansas months as a huge heat exchanger/battery. I thought if I just used a typical hot water tank system and with bad weather or some other problem the heated water wouldn’t last long.

Where I differ in my quest is I want to use my obsolete cistern (located in the basement corner) to store my heat, and what will give me the best possible out come. Since the cistern is not in use and has no other function I have thought of filling it in and knock down the side reinforced side walls and concrete over making the basement bigger. The cistern actually goes about 10 feet lower than the basement floor and about 15 foot actual usable height I have two ideas:

1. Using the cistern as my heat exchanger I create a bigger storage capacity. Several coils or grid structure in a 5x5x15 foot high cavity will take a few Kansas summers days to heat I am sure but once it is there I see reserves that will last longer than a few night time hours or a rainy day. Or will it cool off quicker than it can heat up? I understand pumps and thermostats help but just how long will water stay warm in a cavity of this size and wrapped in concrete under ground.

Which brings me to the Sand Battery…

2. If still using the old cistern as a heat exchanger with a grid system and actually filled it with sand as an insulator/conductor will this hold heat more efficiently? Perhaps adding just enough water to cover the sand will also increases the efficiency to conduct heat. I was wondering what Mother thinks about my sand battery for a heat battery?

By adding sand I think this will help hold the heat and carry it maybe longer than 3-5 days (remember this is to preheat water and depending on efficiency eliminate the NG water heater almost completely). By extending the efficiency retro fitting the cistern would be worth the trouble. Of coarse if there was problems it would be a nightmare.

I know there are risk in such a design, example: A leak in a line or rupture and that during installation would be most critical time for this to happen. But I also feel that if care is given and what ever is checked before buried in water or sand being such a large heat battery that the risk of over heating has been minimized. Also whatever is buried should be heavier constructed so that if let say a pressure problem was to happen it would find the weak link above ground level.

So I have you thinking now… I don’t have a cistern but what if I built a cement block sand battery next to my solar water heater or even build my solar water heater on my new heat battery above ground. Tell me what you think before I get the sand and in over my head.


Any suggestions are welcome.  Enginering ideas while on paper are welcome.  Just tell me what you think...





















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