Passive solar heating

11 years ago

I am planning on buidling a temporary 11x 12 ft hoophouse to put over an already built 4x8 raised bed. I am very interested in passive solar heat for this. My last frost date is around april 1 and I would like to plant beans around feb 1- feb 15 in the greenhouse. I have seen the wall o water on the internet and my question is this- could I use several of those together, side by side and wrap the walls of my raised bed to give them more heat? i know this is not the way they are used- that they usually sit on top of the soil over tomato plants. Theoretically, though, do you think it might work to add a few degrees at night? I am also going to use floating row covers inside the hoophouse as well. Thanks! Kim

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Comments (13)

cole_robbie
11 years ago
The problem with passive solar is that the heat tends to not last throughout the night. It takes a big mass of water to hold heat all night. I put a link below to the water bags that FarmTek sells. You can make your own, but if I were you I'd copy their designs.

I'd also have a back-up source of heat if at all possible, like an electric soil heating cable or a small propane heater. But as soon as you start spending money, you will want to grow a more monetarily valuable crop like tomatoes or cukes to recoup your investment. Beans are typically not grown in greenhouses for that reason.

Here is a link that might be useful: FarmTek water bags
curlygirl
11 years ago
You might want to look into phase change materials. This blog talks about using PCMs in a greenhouse:

http://greenhousefashions.blogspot.com/

The benefit of PCMs is that they can absorb a ton more heat than water in much less space. The amount of water you would need to bank enough heat would almost fill the greenhouse. What little space you'd have for plants you could not really use because you don't want the plants shading the water.

I am currently designing a greenhouse that we are building this coming spring. We are primarily using a Subterranean Heating and Cooling System which sucks the hot, moist air and sends it underground where it condenses and releases its heat. You can cool your greenhouse all summer long this way and use the stored heat in the winter to heat your greenhouse. This system is used successfully with hoop houses so you might want to look into this as well. Our challenge is that we have a high water table which prevents us from putting in an appropriate-sized heat sink so we are looking into doing a hybrid between PCMs and SHCS.

Best of luck!
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Um....just a retired chemist stumbling across this thread. The Glauber's salt should be used in SOLID form, not as a solution. The salt itself is just sodium sulfate with 10 waters as part of the solid crystal structure. Upon heating, the crystal structure breaks down, liberating the water molecules to form a very concentrated solution of sodium sulfate. When the temperature drops, the salt tends to recrystallize, releasing heat. I don't know about the practicability of scaling the process up to a greenhouse wall scale (I've got a simple PVC free-stander, so it's out for me...), but the salt is certainly not "toxic" in the usual sense (remember, ingesting too much water will kill you!) and it would not attack PVC pipes. Just based on supposition and observation in my own greenhouse (which gets both too hot and too cold), I would be most concerned about the kinetics of the process, i.e., how quickly would unstirred salt be able to make the initial phase change to liquid. I would not expect this to have much of an effect on the high temps of the greenhouse. The other process, that is the recrystallizing, has a tendency to occur rather quickly once it gets going, meaning all of the heat is going to be released over a short period of time, although admittedly it will take it time to radiate away from sizable samples of the salt as might be stored in PVC pipes. The only other issue that might be a problem is the potential for the liquefied salt to remain in that state even when the temperature drops, barring some kind of agitation (forming a supersaturated solution). Probably this could be forestalled by not using pure-grade salt but rather a (cheaper) technical grade. Or, you could just add a little dirt to each pipe! Edit: looks like there is some reasonably current research into the practical problems associated with use of Glauber's salt for thermal storage: http://intraweb.stockton.edu/eyos/energy_studies/content/docs/FINAL_PAPERS/4B-4.pdf This post was edited by steve_in_los_osos on Mon, Dec 3, 12 at 11:23
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A 100% passive solar home is a very complicated matter. I believe it can be done, but not easily, and perhaps not affordably either. The building would need to be very efficient at capturing solar energy, but it would also have to be very good at storing this energy. Instantly we have a design conflict, all those places that allow heat in during the day will start to leak heat backwards when the sun is gone and the outside air temperature falls. Things can be done to correct this, but the complexity starts to increase dramatically. Also, to store heat you need mass. Concrete or brick are popular, but they're bulky and expensive... Well, expensive is relative, I'll conceed it may just be me who is looking at cubic foot of concrete and wishing it were cheaper! Such a house which collects lots of solar energy, stores it well and has enough thermal mass to remain warm overnight can be designed and built at some expense, but that's not the full selection of problems facing the 100% passive solar house, it also has to perform well throughout the whole season. The traits that make it work so well in winter may cause it to work too well in summer and overheat terribly. Personally, I have discarded the idea of the 100% passive solar house due to it's complexities, expense and design limitations. But I have not given up on the idea of the 100% solar heated house. Passive solar is still a very adaptable technology and can provide a substantial amount of free heating, but it has limitations and should only be used up to the point where it is doing what it should - providing free heat for a sensible cost. Going extreme and trying to squeeze that last 20% or 30% of your heating needs from it by adding vast quantities of expensive glazing, bulky thermal mass and inconveniant manual systems such as heat retaining curtains and solar shades that must be ajusted frequently is irrational. Active solar technology, such as solar air heating pannels can provide the remainder for a much lower cost and avoid most all the design issues. Active solar also has the advantage of being able to turn it off when not needed, a conveniance of fossil fuels that has allowed a lot of badly designed buildings to remain comfortable all year round. It could certainly help compensate for any small design flaws in a passive solar home. I suspect the only way to make something perfect is to design around any faults it has.
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Yes, we did it to our 1830s house. The primary reason was the same one you are considering. We went on a major conservation rampage, shortly before utility rates doubled since our home had some major heating issues. Our porch needed replaced and it seemed like the perfect time to install a greenhouse in its stead. Ours is 12 x 24, has a brick floor and is heated with a gas fireplace. We also have a quiet, commercial ventilation fan at its apex to exhaust hot air, and shade it in summertime. Inside the greenhouse are three (what used to be) exterior doors into the house, and a window. We leave them open to the greenhouse year round with no problem. Ours is not directly south facing, so we do not get the larger solar gain in winter, but do get it from the west wall in the afternoon. On cold sunny days, we also get it from overhead. The south and east exposures are against the house, as it sits in an ell shaped area. We get a lot more sun in spring and fall, of course, and it has to get pretty cold outside before the heat comes on. What it does for us, it to provide a windbreak and a dead air space in winter, and that in itself is an excellent insulator. The kitchen stays a lot warmer now because of it, and it's very pleasant in winter, to have a fountain running out there, and ferns hanging outside the kitchen window. Because we are constantly in and out in all seasons, tending the g'houses, gardens and poultry, it also makes a respectable mud room to drop the boots and coats. We sometimes sit out there in winter to read, or have our morning cuppa. When we were doing the energy conservation thing, we got central heat (hot water radiators), new windows and did a lot of caulking and such. It cut our energy consumption for heating almost in half. May I ask what zone you're in? It would make a difference in how practical using a g'house would be, and how you will need to outfit it. If ours isn't shaded in summer, it gets hotter than Hades in there. That is something to consider.
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cole_robbie
11 years ago
PCM is neat stuff.

That blog linked to this company selling PCM-filled PVC pipe lengths:
http://rgees.com/products_pcm-pipe.php

They don't list prices. Do you have any idea how much they cost? Is it feasible to make your own?

Here is an informative page I found from an Indian company hat manufactures PCMs:
http://pcmenergy.com/products/doctorsays.htm
curlygirl
11 years ago
I have read that you can and it is really inexpensive (if you are using Glauber's Salt) but you need to mix in other ingredients in order for it to work over time. Below is a link to a guy who mixed his own PCMs and used it (along with other strategies) to heat and cool his house:

http://www.allanstime.com/SolarHome/index.html

One thing you need to be careful of is handling the material during a time when it is optimal humidity because that, too, could affect performance.

I don't know about prices. I have stayed away from Indian companies because of shipping but I have been in touch with Rgees (they are in North Carolina). You can buy samples for $25 each but the minimum order is $150. After a certain volume, the price goes down considerably. They can also sell their PCMs in bulk and you can fill containers yourself but again, you have to be careful of humidity. Once Rgees comes up with an estimate of PCMs for my prospective greenhouse design I'll update you on price.
Azjohn
11 years ago
I am in the process of building a new greenhouse and need some help. I am planning on using barrels of water for heat storage, and would like to know if you can get too many?

I have a small greenhouse that works very well with just water for storage, but it is against my house and has an upstairs porch as its ceiling.

This one will be larger and freestanding, so I'm wondering if the barrels will be sufficient.

I had really wanted to try the SHCS system, but was discourged when I could find no uptodate info, and besides I am actually going to convert an existing building, and I would have had to remove it to dig the underground portion of the system.

I am not familiar with with the players on this forum, but have been very impressed with the expertize on the "Container Gardening" forum. I am hoping someone can help, as I have a number of ideas for enhancing the system, but would like some help.

Thanks in advance, I would really appreciate help.

John
KimHat
Original Author
11 years ago
Curly Girl: Thank you for the information. Do you happen to know the mass of PCMs per square foot or volume of space. Also, is PVC the only material you can safely store it in, or can I put PCMs in a simple plastic bag or something. Also, the PCM websites all have temperatures, what temp should I get for a hoophouse? Thanks! Kim
cole_robbie
11 years ago
azjohn - I'd say that water barrels would not be enough heat by themselves...except you're zone 8 and in Arizona, I presume, so I doubt you need much heat. What are the outside day and nighttime temps that you expect?

The only problem with too much water is usually running out of space. For it to hold too much heat, you'd have to be having extremely warm temps in the first place, and probably would not be greenhouse growing anyway.
cole_robbie
11 years ago
And thanks for the link, curlygirl. With some rough estimates using the math they give for a greenhouse sample, I would need a couple thousand pounds of the stuff to make a significant difference with just the sun as a heat source.

However, I am using a wood-burning forced air furnace right now, and I would probably be best off to use small amounts of PCM within and around my current ducting, so I won't give up on the idea just yet.
curlygirl
11 years ago
I just heard back some encouraging news from Rgees. They recommend my putting encapsulated PCMs within the heat sink of the SHCS -along with the soil and/or gravel. That way, I can have the dehumidifying benefits of the SHCS along with the extra heat storage of the PCMs. Please note that SHCS is a phase change system -it is just that my high water table has me pursuing this other avenue (if my heat sink was flooded, it would take away my heat and -if this happened enough times- would fill my tubing with dirt).

AZjohn -I know what you mean about SHCS! Have you been looking at Sunny John's site? He used to have great forums where people could discuss their plans and challenges. I used to read them all the time when I was renting and could only dream and research. Now that I have a house and could use the forums to ask for feedback on my particular challenges, the forums are gone! However, I have some good news. SHCS is alive and well! Check it out:

http://www.roperld.com/science/ymcasolargreenhouse.htm
http://www.roperld.com/science/SGHBackyardSHCS.htm
http://barrettstudio.wordpress.com/2011/12/22/the-practically-zero-energy-year-round-greenhouse/
http://permaculturenews.org/2010/08/16/clever-rocky-mountain-greenhouses-give-major-season-extension/

KimHat- I know you can store PCMs in other materials such as metal tubes but I am having trouble finding that information right now. I would just go on the greenhouse fashions blog and ask questions in the comments. The blogger, Nina Reinhart, works for Rgees and is interested in getting PCMs into greenhouses. I think primarily because it would help their company expand into a new market but also because PCMs are absolutely perfect for greenhouses. The large temperature swings, the limited space, the high cost of heating and cooling, etc. Anything you can do to reduce temperature swings passively is something that will pay you back every year. I say all this without knowing what they actually cost but I think if you are in the planning stages of your greenhouse then it would make sense to think about how to incorporate them into your design. You can add them later but for me, aesthetics are pretty important so I am working them into the design now.

Also, there is another PCM product made by another company called Bio-Based PCM which you can integrate nicely into your house and it actually acts as a moisture seal too:

http://www.treehugger.com/green-architecture/bio-based-phase-changing-material-adds-instant-thermal-mass.html

Cole Robbie (love the name, by the way!)- where do you live? What are you growing in your greenhouse?
cole_robbie
11 years ago
Thanks. I'm in Illinois, zone 6. My greenhouse season is in the spring. I raise bedding plants, tomatoes, and cucumbers.

PCMs in a greenhouse should be in the floor or in the benches used to hold plants. PCM paver stones for a walkway would be great. I have a gravel floor right now. For something with tons of mass, it does not hold enough heat to make more than a few degrees difference at night. Even with my wood stove furnace going full blast (it's in a nearby shed, not the poly house itself), the best I can do is about 15 degrees over the outside temp. My double inflated layer of 6-mil poly has a very low r-value, 1.3 I think. Plus, I have roll-up sides, which are hard to seal around, especially on windy nights.

For next year, I am going to try to heat water or ideally PCM with the hot air from the wood stove. Radiant floor heat with hot water through PEX is the best setup I have seen.
KellyBell
11 years ago
Please forgive my ignorance as i am a grandmother and a medicine woman trying to care for my little lake side community and only educated enough to take advantage of the expertise of those with the education(LOL) but I have had these subjects of thermal mass and PCM's swimming in my head for weeks in the planning of a (Passive)4 season greenhouse here in Oklahoma...
honestly heating is not my problem as it is dec 1st and 72 degrees outside today. we are in a record drought and the summers are reaching 114 for more days than i care to remember. I live out in the country where our power is never reliable as we are always the last to be reached in a crises( after every thunderstorm)..LOL and as of this date i haven't the money for a solar set up...I need to "COOL" my green house... Im thinking of digging down 4 feet (a walipini style. Ive read that the ground temp is a little over 50 degrees at 4 ft deep( not including any solar warmth.. I'm wondering if You might have any advice about my plan...Please make it simple...LOL

Oh and BTW has anyone considered a encapsulated soy wax?..isnt this the Phase change material that is being considered for so many new building materials now? wall board and bubble cloth?
Ive seen something like this put in pvc tubes, caped on the end and applied to the north wall for temp consistency in one application and now they are making this available for a pretty penny Im sure....is this worth trying to do myself? any help or advice would be so appreciated!..
KellyBell
11 years ago
these are the sites i have found....

http://large.stanford.edu/courses/2010/ph240/reinhardt1/

http://www.fact-sheets.com/home-garden/energy/drywall/
slicer68
11 years ago
! would like to make contact with anybody who has
installed a solar under ground water tube heating
system in their greehouse.

12/11/12
wayne
317 852 9232

Passive solar heating

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