New Greenhouse build with thermal mass storage; air-to-water heat exchange system

Here is an update on my greenhouse.  I got the cage installed over the tanks, sheet metal over the cage, plastic tubes for tank access, and the greenhouse frame is almost complete.  I found a lot of polystyrene sheets and roof shingles for a great deal so I'll use that in the walls.  I'll be installing cedar fence picket lap siding on all exterior walls.  I plan to leave the poly plastic on the glazing surface while I work on the rest of the frame and the inside.  
I'm thinking of installing automatic vent openers to open two 28 x 30 window panes at the top of the glazing surface.  I'm still not sure if this will be enough ventilation in the summer heat.  I guess I could add a shade cloth in the summer to help reduce some heat.

more photos

In terms of ventilation during te summer, I found that passive ventilation was inadequate, no matter how big.  I now use a salvaged hot-air furnace blower to suck the excess heat out.
But there is an angle here which I have only recently thought through.  The question has an underlying assumption that the greenhouse is to be used year-round.  There are people who actually use the greenhouse only fall, winter, and spring, (and design the slope of the glazing to maximise heat capture with the sun low on the horizon), and then actually close the greenhouse tightly in summer to super-heat the soil, killing any and all insect pests etc.  (What this does to the fungal soil networks upon which many plants, (but not brasiccas) depend, I do noty know...)

David Maxwell wrote:In terms of ventilation during te summer, I found that passive ventilation was inadequate, no matter how big.  I now use a salvaged hot-air furnace blower to suck the excess heat out.
But there is an angle here which I have only recently thought through.  The question has an underlying assumption that the greenhouse is to be used year-round.  There are people who actually use the greenhouse only fall, winter, and spring, (and design the slope of the glazing to maximise heat capture with the sun low on the horizon), and then actually close the greenhouse tightly in summer to super-heat the soil, killing any and all insect pests etc.  (What this does to the fungal soil networks upon which many plants, (but not brasiccas) depend, I do noty know...)

Thanks David.  I may shut down in the summer if heat is unmanageable.  The soil cooking idea sounds interesting, I wonder if I could just use the greenhouse as a giant composter in the summer.  
I'll think i'll try to exhaust hot air out of the greenhouse using the blower from the heat exchange system. Hopefully winter glazing angle + shade cloth + blower exhausting out of the upper glazing panes will be sufficient. If not, oh well!  

I'm planning on doing something similar with ten totes in a row, but above ground and
boxed in on all sides with 5" freezer panels I'd salvaged @ 50 cents sq ft via craigs list.

My question is; I'm trying to find simplest cheapest plumbing for connecting all their 2"
outlets together. Like to find a simple screw on "T" that'd accept push on hose or any
other easy combo requiring fewest parts & adapters that won't stick out significantly.

Any suggestions appreciated, everything I've seen so far looks more convoluted than it
oughta be.

Shane, Google IBC caps and you'll get all kinds of options.

Jeffrey Sullivan wrote:Shane, Google IBC caps and you'll get all kinds of options.

Thanks, Jeffrey, been scouring photos at google for "IBC fittings",
like to find a T, like this from England could work, if available here...

shane connor wrote:I'm planning on doing something similar with ten totes in a row, but above ground and
boxed in on all sides with 5" freezer panels I'd salvaged @ 50 cents sq ft via craigs list.

My question is; I'm trying to find simplest cheapest plumbing for connecting all their 2"
outlets together. Like to find a simple screw on "T" that'd accept push on hose or any
other easy combo requiring fewest parts & adapters that won't stick out significantly.

Any suggestions appreciated, everything I've seen so far looks more convoluted than it
oughta be.

Shane,
That's awesome to hear your connecting 10 together!  What size greenhouse are you heating?

I don't know if it would be more convenient to make all the fluid connections out the top fill cap.  For me it was since I buried them, and I also didn't want to deal with the complex connection at the drain valve.  I found a fill cap with two 2" NPT ports. I purchased them here: Lexington Container Company
In my design, one port will have a pvc tube running down to the bottom of the tank and then plumbed to the 2nd tank to join the 2 tanks.  
Another option for using the top cap would be to buy pvc bulkhead fittings and drilling holes into a solid top cap that has no existing port hole.  I realize you may not fit 2 bulkheads next to each other on the fill cap, depending on the size pipe you're using.
You could also use a uni-seal fitting in the top caps like this: UNISEAL  They work with standard PVC pipe sizes and they claim up to 65 psi in static conditions.

I'm interested to hear more about your project!

 

Dan,

I'd posted about the project a couple days ago here...
https://permies.com/t/63868/Considerations-cooling-GH-hot-desert

I'm wanting to use H2O to draw heat out of closed CO2 system year round for
hot desert location, or rather for as much of each day as I can before surrendering
to excessive heat gain and opening it up to natural ventilation.

Looking at plumbing them all together at bottom and drawing water from there
to radiator in GH and having one manifold tube spanning all their tops for return.

Figure with common plumbing at bottom, they'll be self leveling and if I find one
or two getting all the action I'll close down their spigots a bit to even out some.

More about also incorporating evaporative cooling there, too, and night sky radiative
cooling of that water mass and also redirecting radiator in & out airflow to ambient
to cool that water anytime night air is cooler than the water, all at that link above, too.

Plan is subject to revision as reality reveals itself

- Shane

PS - On track to figuring out my original IBC question, 2" female to Tee will get'r'done.

I made more progress on the greenhouse.  Got the fan, heat exchanger, temperature controller and some of the ducting installed. 8" Max HO 932 CFM fan, 16" x 16" badger heat exchanger.  I'm having issues with the water portion of the system though.  

First I tried installing the submersible pump out of the tank, inline next to the heat exchanger.  When I attempted to fill the system with water up to the top of the heat exchanger I noticed significant pressure on the tanks, bulging the tops quite a bit.  Probably should have done the math to anticipate this!  Then I decided to install the pump inside one of the tanks so I don't need to fill the system above the top of the tanks. (the photo doesn't show the current water line setup, I was just testing the system on one tank which worked great) Currently I have the pump in tank "A" pushing water through heat exchanger then dumping into tank "B".  To join tank A to B, I am using uniseal fittings through the sidewalls of the tanks, about 3" down from the top of the tanks.  The tanks are about 3" apart at the holes. Well the water is leaking out from one of the uniseal fittings! I took it out, cleaned up the hole, reinstalled and it's still leaking.  I'm thinking of installing bulkhead fittings with seals instead of the uniseals, but the bulkhead fittings I currently have are too long. I could flip one bulkhead fitting around and put the nut on the inside of the tank and that might do it.  The further along I get the more I'm thinking it might be a bad idea to dump 20" of soil over the tanks, making tank access much harder.  I may just use large planters instead.

 

A little more progress.  Installed lower glazing (double paned glass), shingle roof, and lap siding on rear wall using recycled cedar fence. I decided the upper glazing that's about 30" x 137" will be made using hinged windows on actuators in order to vent. I plan to work on it intermittently through the summer and hope to have it done this fall!

thanks for sharing Dan! I'm also in 5b in Maine. Building a cheep cold frame for this winter, but would like to explore something more efficient like your design. Please keep us updated! Thanks again!

I haven't made much progress lately due to family life and focusing on the plants outside the greenhouse.  I'm going to try hard to get it sealed up by next month so I can use it by this winter.  

Michael Jay Anthony wrote:thanks for sharing Dan! I'm also in 5b in Maine. Building a cheep cold frame for this winter, but would like to explore something more efficient like your design. Please keep us updated! Thanks again!

Sorry, just found this thread and wanted to provide important warnings.  The initial thread indicates the greenhouse build is in the area of Denver, CO.  I saw several references to clay in the build.  I have around 10 years geotechnical experience in the Colorado area, primarily in the Denver metro vicinity.  I also have decades of experience in reducing risk from natural hazards. The primary clay types locally are comprised of Illite/Montmorillinite, which are subject to potential massive expansion with addition of water.  I have seen steel i-beams supporting large building walls crushed by that expansion.  The buried water tanks should be open for inspection all around to ensure no leakage is reaching the clay.  I would suggest any future construction for buried tanks include a sealed floor and sump area that can be regularly pumped out with crushed stone (generally pea gravel) around the tanks to allow any leakage to move to the base and sump.  Failure to keep water away from the underlying clay will likely lead to crushing of the tanks, leading to leakage and even more expansion, which will endanger the overlying structure's physical status.

The other threads were quite interesting as I am looking to construct a similar albeit larger system in Upstate NY (currently experiencing subzero Fahrenheit temperatures for an extended period).  I have considered similar construction for several years.  One design incorporated reflective panels that could be closed during storms or cold nights.  The reflective insides were designed to reflect additional solar radiation into the primary greenhouse glazing, if needed.  In semi-arid areas of the country, with wildfire hazards, placing reflective exterior panels might reduce potential for ignition during close passage of a wildfire.  Past research by the National Institute of Standards and Technology (NIST) on what glazing best protected from infrared (heat) infiltration by wildfire where a fire at ~130 feet could autoignite wood identified that plain aluminum foil over windows reduced heat introduction better than triple glazed units.  I realize that the greenhouse will be the least of local concerns during a wildfire, but this method could also protect a home.

I also noted some discussion regarding warm soil temperatures versus air.  My great grandfather had a cold frame on the south side of his home in the teens and twenties with only single pane glazing.  He would fork substantial amounts of fresh horse manure into the bed during the fall and then shovel dirt back in.  The composting horse manure increased the soil temperatures sufficiently that he enjoyed broccoli and other brassica family vegetables into the new year.  Warm soil is a major aide to plant survival during cold air temperatures.  With the foam panels available today, it would seem simple and inexpensive to insulate a growing area with placement of panels on the glazing area at night and during storms to maintain growing temperatures inside, especially using multiple heat sources such as composting, thermal mass and even confined animals such as rabbits or chickens (larger also possible, but also more expensive to build for).  Have you thought of incorporating a chicken coop or animal shelter into the north wall?  With the proper setup, it would be relatively simple to move waste bedding into a compost area on the north floor of the greenhouse to provide a secondary heat source and future source of incredible fertilizer.  I hope someone finds these tips useful.

Thanks for the input Richard. What a coincidence, I haven't worked on the greenhouse for a few months and you ended up posting this while I was working on it last weekend!  
With respect to water leakage, I decided to leave the tanks accessible and plant in large planters instead of filling the entire base with soil.  The only penetrations in the tanks are at the top so I'm not too concerned about a large amount of water leaking out. When it rains or snows I notice the ground in the greenhouse does get wet so I was thinking of ways to reduce this.

I think reflective panels over the glazing is a great idea.  When you look at how much heat is lost through the glazing and consider the increased # of hours of darkness in the winter it really makes sense.  I think that Reflectix material is promising since it has reflective coating on both sides with insulation in the center.  My issue is that I'd want to make it automated and I know the simplest automated system will still be a lot of work.  I like the idea of reflective panels that fold in.  I came across a greenhouse with that feature on the internet, I believe it was an effort by a college and they did an extensive study on capturing and conserving heat. I will try to find it. If I build a reflective/insulating barrier I'm leaning towards a rolling system like a roll up garage door.

You may want to see if your library has this older book:

Movable insulation : a guide to reducing heating and cooling losses through the windows in your home by William Langdon.

It has a number of ideas for houses and greenhouses.  It changed my mind about how to do my insulating.  I'm designing my greenhouse around using Reflectix style insulation between the trusses.  I originally was going to do a garage door style unfurlment but now I'm going to try more of a roll up style (like in the picture).  That way the Reflectix isn't sliding and wearing down its edges on a track.

I figure I'll have the insulation clamped to the peak of the greenhouse.  Each chunk of insulation will be wrapped around its own piece of pipe that will fit on cleats attached to my trusses (pitched at a decent angle).  I'll have another piece of pipe or EMT up there to wind up or play out the strings that roll up each roll of insulation.  That pipe will connect to a bike sprocket and chain on the wall so I can operate it by hand morning and evening.  Once I'm happy with how it works, I'll rig up a motor and timer to do it automatically.

Thanks for the book reference. I will check it out.  
I've seen external roll up systems with straw here: http://www.lowtechmagazine.com/2015/12/reinventing-the-greenhouse.html
So you plan to have one roll up style system on the inside in between each truss? I assume you'll need a ledge on the trusses for the insulation to roll along as it is extended/retracted. I was thinking of making one single roller 11 feet wide that contacts the backside of the truss, but there are definitely drawbacks to this, like sagging in the middle and how to keep the ends somewhat sealed.   I like your concept, it keeps the insulation closer to the glass and leaves more space for plants.

From what I have read, the main heat loss in small-scale greenhouses occurs at the edges of the curtain, which needs to be sealed.  So, multiple smaller curtains suffer from the problem of multiple edges. Hence a single wide blanket is preferable..  
How best to seal the edges?  Neatest way I have seen is to fasten magnetic tape to the sides of the curtain, and build ledges along the outer walls to support the edges, also equipped wth magnetic tape.  The two magnetic tapes will automatically align themselves, avoiding the issue of the roll getting off-centre.  The long section of curtain is supported by wire cable stretched parallel with the joists, (metal clothesline has been suggested).  The designs I have seen urge the incorporation of  weights to maintain an equal tension on the wires so they don't sag as the temperature rises, but I am not convinced an 8 ft. length of wire is going to change in length that much.  Have any of you any experience here?

David Maxwell wrote:From what I have read, the main heat loss in small-scale greenhouses occurs at the edges of the curtain, which needs to be sealed.  So, multiple smaller curtains suffer from the problem of multiple edges. Hence a single wide blanket is preferable..  
How best to seal the edges?  Neatest way I have seen is to fasten magnetic tape to the sides of the curtain, and build ledges along the outer walls to support the edges, also equipped wth magnetic tape.  The two magnetic tapes will automatically align themselves, avoiding the issue of the roll getting off-centre.  The long section of curtain is supported by wire cable stretched parallel with the joists, (metal clothesline has been suggested).  The designs I have seen urge the incorporation of  weights to maintain an equal tension on the wires so they don't sag as the temperature rises, but I am not convinced an 8 ft. length of wire is going to change in length that much.  Have any of you any experience here?

Thanks David!  Magnetic strips sound great.  A friend told me his father uses them with insulating curtains in his mountain home and I was intrigued but never asked more about them, and I wasn't sure if they were homemade curtains.  My past experiences with adhesive magnetic strips have always been failures so I've had a general bias against them, but after you mentioning them now I'm interested in looking into them further.  I like that they solve both the alignment and sealing issues at the same time.  
 If you have anymore info on them, or a link to a build with magnetic curtains please share.  

I only just found this thread so this is possibly too late...

Did you run some math regarding thermal uptake of the water mass vs thermal loss of the greenhouse? It usually works out that you can't efficiently transfer enough heat from a hot greenhouse to the water during the day to completely offset the thermal loss by the greenhouse at night. Remember that not all days are cloudless...

What happens is that the system "runs down" to equilibrium over about a week or so. Here is a video explaining what goes on:

Hi Dan, yes I looked at the Chinese greenhouses with external rolled insulation.  I love the idea but I'm not sure how to automate them and not face issues with snow/ice.  Bonus tip: If I was going to do an exterior blanket, I would seriously look into using concrete curing blankets as the material.  It's designed for the elements and is insulating.

Yes, my plan is to add wider ledges to my trusses for the insulation to roll down.  I agree that one wide piece would have much less of an edge sealing issue.  On the other hand, this way I can hang stuff from the trusses without it blocking the insulation.  Plus my greenhouse will be 40' wide...

The ideas of magnets and wires are also covered in that book  I hope that the reflectix is stiff enough to span but if not I can run some fishing line horizontally across the bottoms of the trusses before I put on the ledges.  I like the magnets but I'm not sure I trust them to adhere and handle being rolled up repeatedly.  I might just put them at the very bottom so that when I start rolling the insulation back up it helps the "roll" get started.

FYI, my trusses will be curved.

Nick Kitchener wrote:I only just found this thread so this is possibly too late...

Did you run some math regarding thermal uptake of the water mass vs thermal loss of the greenhouse? It usually works out that you can't efficiently transfer enough heat from a hot greenhouse to the water during the day to completely offset the thermal loss by the greenhouse at night. Remember that not all days are cloudless...

What happens is that the system "runs down" to equilibrium over about a week or so. Here is a video explaining what goes on:

Nick,

Thanks for the link. It's all going to come down to how the system is designed(insulation,size of thermal mass,etc.), the external temps and sun exposure, and the temps you are trying to achieve in the greenhouse.  I'm banking on Colorado's 300 days of sun to help it work. I agree, if we get numerous cloudy days in a row I'll probably need some backup heat.  I'm basing the design off of a very similar greenhouse by Russell Benoy, the link is in my original post.  He detailed his build and results with extreme detail.  
Also, if you factor in an insulating curtain over the glazing this reduces the heat loss dramatically. I posted some calculations on heat loss for my system with and without a curtain in this thread. I predict that without a curtain the temps will occasionally get down too low in the winter, but with a curtain it should work all year.

finished side walls and side glass windows. Purchased Solexx to make the upper glazing panels that will open with hinges.

Sorry for the late comment.  
Dan, your math looks right.
My analysis is indicating that the water tank shouldn't be insulated because it's better to have the added thermal mass of the soil and groundwater if any.  

I was surprised to discover that thermal diffusion in still-water or wet soil is actually *lower* than dry soil!  So in theory having wet soil is better than dry soil because it actually *decreases* the rate at which heat diffuses away, although only if the water isn't flowing through the soil in the particular time-frame of interest.  

You may have read about the so called “umbrella” insulation method that insulates the soil horizontally on each side of the building.  I figure this does create a longer thermal path from the thermal mass to the winter air.  Ideally this length would be increased to the 6-month thermal-penetration depth, that seems to be in the range of 13 ft for organic soil to 20-30 ft for mineral soil.  

As a side note, it is said that the “umbrella” helps by deflecting rainwater away from the thermal mass.  This seems true only if water is percolating *through* the mass, and not simply going around it or going into it and staying there.  I'm guessing that with a high water table, rainwater doesn't flow through the ground so much as it runs over the ground and into drains, streams and rivers.  If true then having the “umbrella” insulation wouldn't be so significant.  Anyway, at first you might think (like I did) that water increases soil conductivity so this must be bad for thermal storage.  While water does increase conductivity of soil, it increases volumetric heat-capacity even more, so as a result, thermal diffusivity is apparently *decreased*! (assuming the water isn't flowing in that time-frame).  

Sure, I'd say use the “automotive radiators from junkyards that have electric fans built into them” if the price is right, but only if it's a good value.  Be prepared for radiator leaks, and try to prevent corrosion.  “12 volts” is also good for solar cells.  According to Dan's math, it will flow heat at a rate of 10,000 BTU/hr (daytime avg.) in this lower-temperature difference application (more like 20-50 kBTU/hr in a car with a higher temperature difference).  

It's common for convection to be the bottleneck in heat transfer.

You can see in the builditsolar link that even with this kind of system there is still a daily swing of 40 deg.F.   To reduce this to 20 deg.F would require some combination of more heat exchanger area, more thermal storage, more insulation, more venting, more shading and/or low-e glass.  

Whoa, I just noticed this thread spilled onto page 2.

Does Solexx cost $1.50/sq.ft?  I'm wondering is that the best option when, in comparison for the same price corrugated Polycarbonate Roof Panel is $1.28/ft^2 at the big-box store and 2 layers of poly is $0.22/sq.ft. ?

That flexible magnetic tape is usually too weak.  I think rare-earth magnets on steel are where it's at.  I'm wondering about a round magnet with a bearing used as a wheel and rolling along a track.  

Mike,

Thanks for the info.  Sorry for taking so long, I've been pretty busy.  I just paid $2 per sq ft for 5mm solexx at a local garden store.  It is pretty costly if you're on a budget. I only used it on the upper section where I wanted to be able to cut it and also keep the window lightweight so it can be opened as a vent.  I was just at a Home and Garden show and spoke to a greenhouse builder who said he uses an Italian made triple wall polycarbonate that has a 2.5R value and 80% light transmission. I did not ask him how much it costs. His company is Heirluum Greenhouse, he said he has a website but I can't find it.

I just visited a friend's 12' diameter geodesic greenhouse the other day and was blown away! It's passive, I think 800 gallons. He has kale, strawberries and a few other veggies doing really well.  I like that the water tank is tall and doesn't take up much room.  I think it was $10k. Part of me is wishing I would have made a simple system like this but I'm way too far to turn back now.  I included a few pics.

I had friends over last weekend and made some significant progress on the greenhouse and getting it sealed up. We got the door installed and sealed, most of the interior panels on to cover the foam and installed the upper solexx vent window(screwed down until I need ventilation near summer) Without the system running I recorded temps on a partly sunny day at 36 outside/ 117 inside!  Yesterday I attempted to run the system and discovered 3 burst spots on the 180 degree bends on the heat exchanger.  I'm guessing I forgot to blow the water out of it and it froze and burst.  Obviously using water creates a problem here so I'm thinking about a solution. I really like using water because its cheap and if I have to empty the tanks I can just water my lawn with it.  I'm wondering if I can insulate the heat exchanger to protect it.  When the system is running it should never get below freezing inside but there's definitely a risk if the water pump or air blower fails.  

Mike Phillipps wrote:Sorry for the late comment.  
Dan, your math looks right.
My analysis is indicating that the water tank shouldn't be insulated because it's better to have the added thermal mass of the soil and groundwater if any.  

I was surprised to discover that thermal diffusion in still-water or wet soil is actually *lower* than dry soil!  So in theory having wet soil is better than dry soil because it actually *decreases* the rate at which heat diffuses away, although only if the water isn't flowing through the soil in the particular time-frame of interest.  

Mike,
The soil temperature is pretty cold in the winter, so if I didn't insulate around the tanks I assumed I would lose a lot of heat to the soil. Water temps are going to be in the 80s and soil temp is between 30s and 40s in the winter.  Does that make sense?

I just made some progress on the temperature controller!  I wanted to control the system beyond just running it when the temperature goes above 85F or below 40F since the water temperature may get too warm to cool the greenhouse or too cold to heat it.  I assembled an Arduino Uno, 2 temperature sensors and a relay.

Currently the logic operates as follows:
Turn on air blower and water pump:  IF greenhouse temp >85F and (greenhouse temp - water temp) >15 degrees
Turn on air blower and water pump:  IF greenhouse temp <50F AND (water temp - greenhouse temp) < 15 degrees

The last thing to do would be to add an operation to open a vent or turn on an exhaust fan if the temps get too hot, maybe around 90F

The greenhouse is running nicely now that I have the Arduino functioning as the temp controller.  I need to open the door part way during the day to remove some heat.  In the morning when the temps are in the mid 20's F the greenhouse is in the mid to high 40s, so it is maintaining a decent temperature overnight.  I added some pepper plants and a basil plant from my indoor garden to see how plants like it in here.  The next step will be to add an electric actuator to the upper window, parts are on the way!

Does anybody have suggestions for a simple humidifier? It gets pretty dry inside during the day.  I'm not looking to connect to house plumbing, I'm thinking of something with a 5-10 gallon reservoir.

Add more plants and you'll have more humidity!

You could put solar water heaters at the top behind the solex, connecting them to the water tanks.
Could be a viable heat sink.  
A car radiator series as air to water heat exchange might also be a possibility.

I realised that I never posted a follow-up with my experiences.  When I was posting previously I had only my two tanks of water, heated by air-to-water heat exchangers (car radiators), controlled by a differential thermostat.  Since then I added black plastic pipe buried about 8" deep in the grow beds with the water from the storage tanks circulated through them continuously.  I did not anticipate how effective this would be.  The temp in the water storage does not increase nearly as much through a sunny day as it it used to, but nor does it drop down at night the way it did.  I reckon I have effectively more than doubled my heat storage capacity.  The soil used to get down to freezing on really cold nights, (didn't seem to bother the greens...); now the soil remains consistently ~ 10 degrees C +/-. (Lowest soil temp so far this winter 8 degrees).  Air temps still drop to just under 0 C on really cold nights.  I reckon the only way to modify this is to add movable thermal curtains of some sort over the glazing.

Glad you did David Maxwell.  I see was confusing one of your posts with the Dan Miano's photos.

Dan,
Any updates on your IBC totes and how effective they heat and cool your greenhouse?  I live in the southern USA and have been thinking about doing something similar.  
I am thinking of a full diffused light (9' by 20') greenhouse with sunlight coming through the roof and all the walls.  I also have a high water table and expect the temperature to stay around 55F all year round.
The lowest temperatures maybe 9 degrees for a couple of nights in winter and a couple of months of 95F during summer.

Dennis Bangham wrote:Dan,
Any updates on your IBC totes and how effective they heat and cool your greenhouse?  I live in the southern USA and have been thinking about doing something similar.  
I am thinking of a full diffused light (' by 20') greenhouse with sunlight coming through the roof and all the walls.  I also have a high water table and expect the temperature to stay around 55F all year round.
The lowest temperatures maybe 9 degrees for a couple of nights in winter and a couple of months of 95F during summer.

Dennis,

The system works really well at night. It keeps the nighttime temp at least 20 degrees F above the outside temperature.  If it is sunny, the daytime temp can be as much as 50 degrees hotter inside.  I’m pretty sure it is so high because, 1) I insulated all non-glazing walls with 2.5” foam 2) I don’t have enough ventilation, I need to open the door in the afternoon, and 3) the glazing is glass vs a diffusing material that would let in less solar radiation.
If I did it over again I am almost certain I would have used the passive open water tank that sits at the back of the greenhouse, optimizing the shape to take up minimal floor space.  In my post I added pics of a geodesic dome greenhouse with this system.
I am also working on a different glazing design in my head (maybe not needed in southern climate). Instead of double paned glass I’m thinking a single layer of woven poly film and some simple insulating shutters/curtains that open and close behind the poly film.  My logic is: minimal glazing cost, maximizes solar radiation while diffusing light, insulating shutter/curtain provides better nighttime insulation than any glazing.  The trick is of course designing simple insulating curtains.  

I really like the woven poly film that I have been using, it’s much less expensive than any polycarbonate,  impact resistant and long lasting. However, the insulating factor is definitely less than double wall poly and the light transmission is about the same, so I think the main advantage is cost.

Hi, I see that Charli has built a greenhouse with an underground water heat sink and many other people are looking at the same idea.
I live in the South of England and have started a Victorian style lean-to greenhouse project in which I'm hoping to incorporate a heat sink to prevent winter freezing, grow a peach tree and extend the growing season.

The greenhouse would be circa 5m x 3m x3m at its highest point. The concrete foundations are in place for a double skinned rear and side wall which I may insulate. The side wall will be insulated with a compost heater and I wanted to utilise compost for inside heat too, not yet sure how.

A 1m x1m x 0.8m deep hole has been dug out for the heat sink.  Clay and flint prevented us from going further as did the fact that I shouldn't have really undermined the foundations, so need to fill it back up asap. The intention was to use solid mass (pipes/fan/solar heater etc) but I'm now wondering if I should use water?

Does anyone have any experience of the efficacy of a water based heat sink system as opposed to solid mass, taking into account costs of set up?  Any tips much appreciated.

Water has over twice the heat storage capacity of stone per given volume. Here is a link on heat capacity of earth materials:  https://www.yourhome.gov.au/passive-design/thermal-mass


Here is a greenhouse company that uses above ground water tanks as a passive heat sink:  https://youtube.com/c/GrowingSpacesGreenhouses  

I toured a 12 foot version of this dome and was amazed. Outside: 20°F with 8 inches of snow on the ground, Inside: about 65°F

Not as simple as this.  Indeed, water is more efficient at heat storage per unit mass than rock.  BUT 1) A major limitation is getting the heat both into and out of the storage, at least in terms of air temperature. (Using the soil in the grow beds as thermal mass, and circulating heated water from the water storage, through buried pipes, (as I did, albeit not deliberately planned), provides good control of soil temperatures.  (And lettuce in particular is remarkably tolerant of air temperatures significantly below freezing - it goes limp and looks terrible until the air warms back up, when it comes right back to crisp, bright life.)  2) One needs an awful lot of water to store the heat - I read a figure something like 3 to 5 Gal of water per square foot of area, at the time I was building my own greenhouse, (but cannot put my hands on the reference today).  I was unable to provide anywhere near this, (above ground), without encroaching on growing space excessively.   3) Large volumes of water rapidly stratify heat, reducing significantly the efficiency of heat storage, unless acively stirred.  4) Much more to the point, GAHT heat storage, (Ground to Air Heat Transfer, with perforated pipes buried in coarse rock, through which air is blown) is based on an entirely different principle:  the heat in the air is stored by condensation of moisture into water, on the cool rock surfaces, and released by evaporation.  The energy stored and released by the phase change is roughly 100 times greater than raising the temperature of liquid water 1 degree.

I am in zone 5b, on the east coast of Canada. I have something like 1Gal of water/ sq ft - quite inadequate.  I have added active heat capture, by blowing the heated air in the greenhouse through  salvaged  car radiators, with small solar pumps to circulate the water, (on differential thermostat).  But when I added circulation of the water from my storage tanks through buried pipes in the grow beds, I realised an approx. doubling of heat storage.

Thinking about the various options based on observation of various design trials and thermodynamics with the keep it simple principle, here are some considerations in choosing a method of collecting and storing heat.  

(1) reduce the number of steps and components.   Direct radiant heat absorption and reradiation simplest but may occupy space for plants.  Direct water absorption with circulation to storage and end use one extra step but greater flexibility for storage and use.  Would work well circulated through compost then soil and then into reservoir.  Demonstrated by Edible Acres.  I had thought of solar panel pumping to roof absorber which would empty to reservoir at night.    
(2) Air is lighter and easier to circulate but harder to heat and cool while holding little actual thermal difference.  The transfer of heat from air to water and back to air in the original post proved to be complex involving 2 step conversion and to circulation inputs.  Circulating air to rock storage is one step with additional phase change possibility without additional resource as mentioned in previous post.
(3) Excess and deficiency of both air and radiant  plus soil temperature exist in greenhouses therefore having both in storage would be beneficial.  
(4) Design should not be based on some ideal but what you can practically accomplish in your circumstances,

has anyone considered a shallow pond/trough as the thermal mass in the front of the greenhouse?  Would save all the digging and soil stability concerns of the IBC totes

a 2' tall 3'x12' footprint timber-framed pond running the length of the greenhouse, along the front knee-wall.  EPDM liner to contain the water.  That's 525 gallons of water; about the same as 2 IBC totes.  You could grow water plants directly in the trough, or cover with a cap to make growing beds so you don't sacrifice any space

same air-to-water-to-air heat exchanger setup, to get the heat into and out of the water as needed.

Davis Tyler wrote:has anyone considered a shallow pond/trough as the thermal mass in the front of the greenhouse?  Would save all the digging and soil stability concerns of the IBC totes.

Demonstrated that it transfers heat from the ground.  Circulates through storage tanks and aquaculture troughs or sprinklers.