Small Cabin

# Posted: 5 Feb 2016 18:21
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Geez, that is very much like something I was musing about/wishing for in another thread, except I wondered if it could be partly underground for more protection from freezing. I'll be very interested in the comments, suggestions and verdicts from the really experienced and knowledgeable minds here!!!

HiLonesome
Member

# Posted: 5 Feb 2016 18:47
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Yes, I need to test how deep I can bury this, I thought the cement portions could be buried. Or the cement could be even higher and the whole thing buried deeper, but that would be at the expense of the size of the solar collector.

# Posted: 7 Feb 2016 15:40
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HiLonesome, you have pricked my memory of my study of passive solar technology I did about 45 years ago as I was planning on building a large home from stone and concrete on an exposed hilltop in upstate NY. I personally think your plan will work. It is an ideal application for off-grid use. Here are some thoughts for you to consider:

How reliable is the sun during the winter months? Do you ever receive several days in a row (2 or more) of freezing temps AND complete or nearly complete cloud cover? The water will act as a heat sink which may keep it from freezing on cloudy days provided it has had absorbed sufficient heat during sunny days. Superinsulating the shelter would definitely help the water retain heat, especially if you construct the shelter with a removable insulated cover for the glass wall that you can install during periods of no sun and at night to avoid heat loss through the glass wall. Adding other heat retaining materials (heat sinks like stone, concrete, containers with a water-antifreeze mix, etc.) within the enclosure would also help by releasing heat as the space inside cools. Also, insulate the floor material. I would consider using frost-protected foundation technology (Google it) to preclude heat loss through the floor and have a thick mass of concrete or stone on which the tanks rests to help in heat adsorption. If you do all of this, you may find that you may have to take measures to keep the water from getting too hot. So, yes, ventilation, particularly during warm weather and a lot of sun, will be needed. Add air intake vent(s down low and air exit outlet(s) at the highest point of the structure, above the glass wall in your diagram, the vents to be opened when necessary to prevent the water from getting too hot. I would place the lower vents on the wall opposite of the upper vents.

A few final thoughts: If, after all this, your passive solar heat sink is not enough to keep the water from freezing during extreme cold periods, a small electric heater and a small generator to power it to raise the temp inside the structure should be all that is needed, all the more reason to have other heat sinks inside the structure. Keep in mind that as you use the water, the water remaining in the tank has less heat retained than when the container is full. Don't forget your local community college for a source of experts for assistance in designing a structure to accomplish your need.

Good luck!

# Posted: 7 Feb 2016 17:32
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adakseabee
I'm so glad you weighed in. I think HiLonesome has hit on something really valuable and, while I can't build myself, my cabin builder is a green builder and would LOVE to see/do a structure like this, I believe. Since HiLonesome is in WA and I'll be in OR, the situation and plans would be similar.

HiLonesome
Member

# Posted: 7 Feb 2016 23:20
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Thanks adakseabee these are good thoughts. I had wondered about heat loss through the floor but at a certain depth (below the frost line) it's a net heat gain. I'm not sure how deep I can get. I was proposing air recyclicing in the interior with no external air intake, did you mean an external intake on the opposite wall? Also I will not be there enough to manage a cover during cloudy periods. We definitely have full clouds for several days at a time at this location.

Does anybody know what happens to a poly tank when the water freezes when it is, say, half full? Would it burst or just expand upward as ice? Thanks for the help.

Just
Member

# Posted: 7 Feb 2016 23:35
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Quoting: HiLonesome

just asking for your opinion!

Ours has been in continuous uses since 78 . Our temps are similar to yours . we get very little collectable heat from Nov, 20 to Jan. 20 days are too short and dark here . I would agree you would be wise to have a back up heater for the cold dark days .
We do not store any heat we gain in the glazed space we pump it under our home with a warm air fan and store it in that enclosed insulated space in 800 concrete blocks . The theory is they will give up there heat over night and be reheated the next day .
On sunny days it is amazing how much heat will be produced .
Paint everything on the inside dull black . we often get up to 85*on clear sub 0 days if only for 2 or 3 hours on short winter days. good luck, good project ,I'd do it again

Excolite glazing

# Posted: 8 Feb 2016 00:19
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Quoting: HiLonesome

did you mean an external intake on the opposite wall?

Yes, an air inlet (low and in the back) and air outlet (high and at the top in front above the glass assuming the sloping roof design in your diagram), for natural air flow through the space when the glass front is exposed to the sun to help regulate the temperature within the enclosure to avoid it getting too high. The point is, you want to avoid generating steam from boiling water. I don't know how efficient your system will be. It should be designed to generate and retain sufficient heat to avoid the water inside the tank from freezing during times when it is overcast and the temp outside is below freezing, yet avoid the water temp from getting too high at any time of the year. Once the ambient outside air temperature remains above freezing I would expect that either the glass can be covered or the roof overhang far enough to keep the glass shaded. In either of these cases you will still get heat gain in the enclosed structure, therefore, vents top and bottom will be necessary to keep the temp inside from making the water too warm for the planned use. That is why I suggest you contact your community college for assistance in designing a passive solar system that can work year round at your latitude. Certainly, do your due diligence and scour the Internet and local library. Your local librarian may be able to obtain books on passive solar techniques. I'm certain this has been done by others and their stories will prove invaluable for their lessons learned.

This makes me want to go out and build structure designed to do just what I have been describing so I can give you and others hard numbers. I am afraid that the regulation of temperature will be subject to experimentation in order to determine what works for you. But the principals do not change. You just need to regulate the temperature of the water to avoid it either freezing or getting too hot for your planned use and certainly not so hot it boils.

This can work if properly designed.

# Posted: 8 Feb 2016 01:27
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How long (and at what temp) will the water stay "good," as in free of odor and other nastiness? In short, if one averages, say, 20 gallons used per day and uses about a 600-gallon tank, would the water be safe for a month, even in the summer when the stored water temperature would be warmer?

# Posted: 8 Feb 2016 14:10
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Julie, it is difficult to answer "What if ..." questions such as the one you pose. If the water is properly treated to potable standard and the container does not allow light to reach the water stored inside, the water should remain potable for the duration you desire, if not indefinately. However, if the water is not treated, of course it will not be potable. There are different chemicals that can be used to make water potable, depending upon the source and method of treatment I take it you will be putting collected rainwater into a cistern buried underground? The water should be filtered prior to it going into the cistern, then disinfected (treated chemically is most common), even if it is not going to be used as potable water. i.e., used to wash dishes or taking a shower, washing hands, etc. Personally, I would not use stored untreated water for those uses. Water for flush toilets doesn't necessarily need to be disinfected, but may have an odor if it is not. Water can also be treated with ultraviolet light or membrane filtration, but the equipment necessary for those processes do not lend themselves for off-grid use.

A good starting point for research into treating water is found at this link:

http://www.harvesth2o.com/filtration_purification.shtml

Many other sources of information on this subject are available on the I-net. Just Google "Making Rainwater safe to drink" or "Making water safe to drink" and you will have enough information to make you feel as though you are drowning in information.

# Posted: 9 Feb 2016 01:09 - Edited by: Julie2Oregon
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adakseabee
Thank you! I appreciate the link! Yeah, I've found ready info on filtration but not on chemical treatment. Well, besides adding bleach, of course.

# Posted: 12 Feb 2016 11:44
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I like the general concept of this idea but I think that the heat transfer from the hot air through the wall of the tank will be very poor. It might actually work better to not use a hot air collector but instead just use double paned glass and let the sun shine directly onto the black painted side of the tank like a giant batch water heater. With the current design, I don't believe there is any chance at all of overheating a 500 gallon tank with just the heat transfer from the air.

You said you have expected winter highs in the 20s and 30s but what can you expect for a worst case low temperature scenario? Can it drop to single digits overnight followed by a snowy overcast day and another really cold night? This can be a problematic scenario for an uninsulated water tank and especially any pipes or valves.

I think a better but slightly more complicated solution would be to insulate the tank heavily and then setup a small system to heat the water directly. You could make this a passive thermosiphoning system if you want to avoid dependence on a pump. To do this you would put a solar water heating panel below the height of your tank and then connect it to a vertical copper coil which is inside your tank by using through-wall fittings. Here's a simple diagram I found online:

http://www.localwise.nl/wp-content/uploads/2013/11/image3320.png

It would be mandatory to fill the loop with nontoxic 50/50 propylene glycol/water mix for frost protection to -20 otherwise the pipes in the panel will burst when it gets really cold. A thermal expansion tank in the loop would also be good to have as a precaution in case your glycol mix boils.

You wouldn't need a very big panel at all since you're not really trying to actually make hot water, you just want to keep it frost-free. A simple small homemade panel with bent S tubing would probably work fine. You could probably mount it near vertical for winter use at your latitude to keep the snow from covering t when you are away.

A closed loop glycol system like this is pretty common in cold climate solar water heating systems but typically they put the panel on a house roof and then you have to use a pump and a differential temperature controller and have power for those. This adds significant complications to the system and more things to go wrong when your not around your cabin. I like the idea of using a simple passive thermosiphon system instead because there is less to go wrong with an unattended cabin system.

# Posted: 12 Feb 2016 20:43
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I do not believe it was HiLonesome's intent, nor mine, to heat the water within the tank by hot air. The whole purpose of the glass front of the container box with the water tank inside the box was to allow solar radiation to reach the side of the black-colored wall of the water container. I addressed the insulation aspect, but not in the sense of heating air to heat water. It was to avoid loss of heat, either through the walls, roof, or floor at low temperatures and when the sun was not shining on the tank through the glass. The purpose of the vents was simply to keep the air in the box from getting too hot when the sun was shining and the glass was either covered or left uncovered.

The intent was to make a simple system, one that is simple to operate and without the complication of piping to heat the water through a solar collector. In the system that HiLonesome illustrated, the box with the one glass wall is the solar collector. The water and any dense material such as stone or concrete onto which the sun can shine inside the box, in addition to the side of the water tank, acts as a heat sink.

HiLonesome
Member

# Posted: 14 Feb 2016 00:08
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Actually my drawing might not show it very well but I did mean that the solar collector is an air heater that heats up the shed through air convection. The collector is two layers with the front being glass. The tank itself is in the dark. You can look up designs that use different materials to collect the heat within the enclosed collector. Window screen, pop cans, and perforated soffit panels are most common. Based on the air temps being reported with the home made rigs it would not be possible to overheat the water. Yes there might be times when ambient temps are single digit with cloudy weather. I'm not sure how hot the water in the tank would get in winter or how long the thermal mass could prevent freezing. Thanks for everyone's input. It might be it just has to be built to answer the questions.

# Posted: 14 Feb 2016 10:18
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Sounds like you might have a similar climate to me. I'm in the Shenandoah Valley of Virginia about zone 7a. The lowest winter temps usually don't go below 0F but it did happen last year and some people had pipes burst in places where they never had problems before. One aquaintance has a large (maybe 500 or 1000 gallons) outdoor above ground uninsulated water tank for his tiny off-grid house and although he had built a heavily insulated box around the pipe connection at the bottom of the tank it froze solid and broke.

Last night it was 7F and that may have been our lowest temp for this winter.

I do like the idea of keeping it as simple as possible and just heating air if possible in order to avoid complications with an unattended system. The tradeoff with having a big wall of glass is that there is greater heat loss when the sun isn't shining. Another design possibility would be to put the air heating panel completely below the tank. The advantage of putting the panel below the room with both the output and intake in the floor would be that it would be self-dampening and you could heavily insulate the south wall along with the other walls.

# Posted: 14 Feb 2016 11:46 - Edited by: slgerber
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Well I tried using SketchUp to make a little model but the version I was using seemed very buggy and I got frustrated and reverted to pencil and paper!

This drawing is a cutaway of the east side. The solar air heating panel slopes and faces south naturally. The main limitation of this design is that you need a building site that drops off sharply to the south or else you need to elevate the building somehow in order to leave room below the floor level for the panel. By the way, there's no reason you couldn't set something like this up to heat your cabin as well.

solar_cistern_heater.jpg

neckless
Member

# Posted: 15 Feb 2016 16:35
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if u are useing a tank and gravity feed to house, why not just put in ground and isulate the top...that never frezze...as for how long water will sit around, my 300gal tank last foe three weeks or more , i add a small pinch of bleach, 10 parts per millon is all u need, as long its not in sun,,,