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Moderating temperature in hoop house  RSS feed

 
Posts: 54
Location: Southern Michigan
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Good morning all, I have a small hoop house 8’x14’x7’ at the peak. It’s relatively well sealed from air leaks but, naturally, the temperature drops ultra fast at night. I’m just trying to figure out how to keep it from dropping below about 40 degrees so my seedlings will be safe. The last 2 years I ran a space heater on freezing nights but it’s not a very good option.
  I have some stuff laying around that I’m wondering about using. I have lots of 2” rigid foam panels that I had to take out of the barn (the damn chickens started eating it). I am thinking about putting these on the north side of the structure but do I add them inside or outside?
I also have 2 55 gallon barrels I could fill and use to replace my current work bench area.
Another idea I had might be stupid- could I pull an old quilt over the whole structure?  It’s hoops are quite strong and I thought that might help stabilize temps.
Thanks for the ideas everyone!
 
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Hi Grace, I'd say all three will help.  I'd probably put the insulation inside but what's most important is to have a decent connection between it and the poly.  For instance if it's 1" away from the poly, air will travel between them and the heat will escape with it.  So sealing the edges of the styrofoam to the hoop poly will likely help it perform better.

Fill those barrels for sure.  Painting them matte black and keeping them where the sun will hit them will help even more.  If they're against the North wall, insulating the North side of them will also help.  It focuses their heat radiation towards your plants instead of towards the wall.

A quilt would be awesome.  Not sure if it will work so good if it gets wet.  Maybe cover it with a tarp if it's going to rain.  You could try to rig up a pipe on the downhill side of it so you can roll it up the hoop during the day instead of having to remove it entirely and fling it back up the next night.
 
Grace Gierucki
Posts: 54
Location: Southern Michigan
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Thank you! I’ll try all three and report back. I just got my new plastic installed and I think there is enough slack that I can slide the panels between the frame and the plastic and then I’ll backfill from the outside with the old junk straw that I’m breaking down.
I have an old tarp we used to use for the pool that should work well for keeping my blankets dry. Putting a pole on one side is a good idea, it should also help hold it down in the breeze.
  Thanks again, happy growing! 
 
pollinator
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You might google "Chinese Greenhouse".  It's similar to a hoop house except the north, east and west walls are solid and either insulated or thermal mass (sometimes the north side is a large dirt mound), they often include a thermal blanket that rolls down over the glazing.  The blanket is often just made of straw.  An old quilt would work I'm sure.



Bubble wrap is frequently used with greenhouses and has the advantage that you don't have to roll it out of the way during the daytime.  They make clear pool covers out of a heavy duty bubble wrap, some even come with 5-6 year warranties against degrading in sunlight.

The foam board would definitely work, whether to put it inside or outside would depend on which tends to be dryer.  Any moisture that permeates the foam will reduce it's insulating properties somewhat.
 
pollinator
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You have a good handle on it, Grace. I will expand on some points.

If you keep square bales of straw, you could drop a row or three stacked around the perimeter.

If you use stone or gravel or anything else that counts as thermal mass, you could drop tarps over straw on your pathways and drop that thermal mass three inches or so thick. Your thermal mass pathways would act like the water-filled barrels, though it would be best if they were at least partially sunlit during the days.

Incidentally, the water-filled barrels might be the best option available. I would paint them matte black, as dark and not-shiny as possible, to make the most of all incoming sunlight, and insulate them from the north side.

I would position any space heater such that the barrels trap any heat escaping to the north.

You might also consider row composting inside your greenhouse, should you have the materials available. If you made a row pile of hot compost along the inside perimeter of the greenhouse, you would get heat from that as well.

I think, at the end of the day, if you take however many of the steps mentioned in these posts that you think will work for your situation, and then supplement it with something like a propane heater with a thermostat that would turn on within a reasonable margin of 40 degrees, you will burn much less propane, and the plants will still benefit from the added carbon dioxide.

-CK
 
Grace Gierucki
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Location: Southern Michigan
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Peter, someday I’d like an earth sheltered growing space but for now this is all I have available. It sure is fun to look for ideas.
 
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Grace, THanks for sharing your post, I enjoyed your thoughts and the ideas replied.

Perhaps a double layer of plastic with a place to circulate (blow) air between the layers would maximize the thermal gain from some of the other ideas.
It makes the covering of the greenhouse into to a sorta like big bubble wrap insulation blanket.
Search Google for: Poly inflation blowers
 
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I added a second layer of poly to my small hoop house this winter and it worked quite well. Check out my blog post if you are interested in details.
https://tinkersblessing.com/2017/11/17/double-wall-hoop-house-results/

T. Vincent
 
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Hola all,

I had good results with a semi-temporary/ semi-permanent (zoning & building regs!) hoop house/ sunspace (30' x 12' x 9') attached to the south side of my cement block house in Albuquerque.  It's designed to be able to reconfigure on short notice onto two 12'by10' 'accessory structure footprints', attached or detached, or completely removed (due to boundary setbacks), should a neighbour raise an objection or the city throw the book at me. 

I use recycled/ recyclable painter's plastic in frost season & shade fabric in hot season.  Both can be rolled up or/ and down and are clamped on.  I never got as far as strapping down the plastic, and perhaps for the best because we have one or two nights a year with 80 mph perfectly-aligned canyon winds that will the take the plastic whether or not anything else is attached.  I also use row cover under that.  With the thermal mass of the wall plus the extra buffer at plant level, I almost got a breeding crop of cool season Chilean aji through the winter, if only one of those windstorms hadn't also taken us to six below! 

Typical winter nights are in the teens, sometimes single digits, with most days well above freezing.  Often overheating is a problem, unless I open the house windows near each end and using fans get a circulation going.  In that case, I have to damp down the woodstove as the house quickly get to 70-80 with that wonderful musty-soil greenhouse smell. 

The only thing I'll change in the future is baseboards which won't rot or attract termites, a perpendicular/ axial tube to keep the ribs evenly spaced (not trivial to affix due to rubbing-on-plastic problem), and a quick-release/ -adjust criss-cross bungee-like long strap system. 

Best, Patrik
 
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I went with a second hand wood stove in my similar sized GH. An electric space heater just costs too much to run IMHO. Adding another layer of poly or bubble wrap will help, the barrels full of water will help as well, but depending on how cold the outside temps get at night I would want something I knoew was going to keep all my hard work warm till the sun came up in the morning.  I typically open my GH about the second week in April, but our nights still get below freezing.  I found the woodstove on Craigslist and installed it in one corner.  So far after about five years of use I have never lost a single plant due to them getting too cold and I have had a few nights get into the low teens.
 
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Re: https://permies.com/t/82794/Moderating-temperature-hoop-house

Grace Gierucki writes from Southern Michigan:

>I have a small hoop house 8’x14’x7’ at the peak.

Say 8’x16’x8’, in rounder numbers.

>It’s relatively well sealed from air leaks but, naturally, the temperature drops ultra fast at night.

With no thermal mass, it would quickly drop to about 30 F at night, where I live near Phila. Southern Michigan is colder and cloudier, with a 20.9 vs 30 F average temp and 750 vs 1000 Btu/ft^2 of sun on an average January day in Lansing, based on NREL’s Blue Book.

>I’m just trying to figure out how to keep it from dropping below about 40 degrees...I have lots of 2” rigid foam panels...

R8? Rectangular? How about R19 fiberglass batts between two layers of north greenhouse polyethylene film?

That would make the greenhouse thermal conductance about 8’x16’/R2 = 64 Btu/h-F for 2 layers of south glazing + 12’x16’/R20 = 10 for the roof + 6’x16’/R20 = 5 for the lower north wall + 2x7’x12’/R20 = 8 for the endwalls, totaling 87 Btu/h-F.

Where I live, a south wall receives 1000 Btu/ft^2 of sun on a 30 F January day, in about 6 hours. With 80% solar transmission, the 128 ft^2 south wall would transmit 128ft^2x0.8x1000Btu/ft^2/6h = 17067 Btu/h, raising the house temp to 30+17067/87 = 226 F for 6 hours/day before it drops back to 30 F, theoretically.

With LOTS of thermal mass and mass surface, the south wall would transmit 102,400 Btu of sun on an average January day, ie 102400/24h = 4267 Btu/h, making the greenhouse a constant 30 + 4267/87 = 69 F.

>I also have 2 55 gallon barrels I could fill and use to replace my current work bench area.

Each drum would have about 500 Btu/F of thermal mass and 40 ft^2 of mass surface and 60 Btu/h-F of air-water surface conductance with a thermal equivalent circuit like this, viewed in a fixed font (Tg is the house air temperature):

  17067 Btu/h Tg
     --       |      1/87
|---|—>|------*------vvv--- 30 F
     --       |
              |
              <
              < 1/120  (How do we post in a fixed font?)
              <
              |
              |
             ---
             --- 1000
              |
              |
              - 

with a Thevenin equivalent circuit like this:

       1/87
    ----www----
   |           |
   |           |
   |           <
   | Vt        < 1/120       Vt = 30 + 17067/87 = 226 F.
  ---          <
   -           |
   |           |
   |          ---
   -          --- 1000 Btu/F
               |
               |
               - 

which is equivalent to:

       1/50
    ----www------- T
   |           |
   | 226 F     |
  ---         ---
   -          --- 1000 Btu/F
   |           |
   |           |
   -           -
            
with water temp T and a 1000/50 = 20 hour RC time constant.

If T = Td just before dusk and T = Tn just before dawn, 18 hours later,

Tn = 30 + (Td-30)e^-(18/20) = 17.8 + 0.407Td, and

Td = 226 + (Tn-226)e^-(6/20) = 58.6 + 0.741Tn, then

Tn = 17.8 + 0.407(58.6+0.741Tn) = 41.7 + 0.302Tn, so

Tn = 41.7/(1-0.302) = 59.7 F.

Just before dawn, (59.7-30)50 = 1484 Btu/h flows out of the drums, warming the house air to 30 + 1484/87 = 47.1 F.

Just before dusk, Td = 58.6 + 0.741Tn = 102.8 F. And the greenhouse air will be hotter: (226-102./(1/50) = 6160 Btu/h flows into the drums, adding 6160/120 = 51 F to the drum temp to make 154 F air, if I did that right. Too hot.

Suppose we limit the drum temp to 80 F max by opening a greenhouse vent whenever the greenhouse air temperature reaches 80 F using an unpowered vent opener or a thermostat and a 24V 2W motorized damper actuator or a fan. The vent needs to dump about 17067 – (80-30)87 = 12717 Btu/h. with an A ft^2 open area for upper and lower vents and an 8’ height difference, 16.6A = sqrt 8 (80-30)^1.5 = 12717 makes A = 0.77 ft^2, eg a 1 ft^2 vent. Ventilation will also desirably reduce wintertime humidity.

With water temp T = 80 just before dusk and T = Tn just before dawn, 18 hours later,

Tn = 30 + (80-30)e^-(18/20) = 50.3 F.

Just before dawn, (50.3-30)50 = 1016 Btu/h flows out of the drums, warming the house air to 30 + 1016/87 = 41.7 F.
 
nick pine
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Two layers of greenhouse polyethylene film over a half-cylindrical 8’x16’ x8’-tall frame would make the curved greenhouse south wall/roof area 4’Pi16’ = 200 ft^2 with a 200ft^2/R2 = 100 Btu/h-F thermal conductance. With R20 insulation, the north wall conductance would be about 200ft^2/R20 = 10 Btu/h-F. R20 endwalls would add Pi8’^2/R20 = 10, making the total greenhouse conductance 120 Btu/h-F.

Where I live near Philadelphia, 1000 Btu/ft^2 of sun falls on a south wall over an average 30 F January day in about 6 hours. With 80% solar transmission, the 128 ft^2 south wall projection would transmit 128ft^2x0.8x1000Btu/ft^2/6h = 17067 Btu/h. With no thermal mass, the greenhouse air temp would be about 30+17067/120 = 172 F for 6 hours/day before it suddenly fell back to 30 F, theoretically.

LOTS of thermal mass and mass surface and 102400/24h = 4267 Btu/h on an average January day would make the greenhouse a constant 30 + 4267/120 = 66 F, 24 hours per day.

Each of 2 55 gallon water-filled drums would have about 450 Btu/F of thermal mass and 25 ft^2 of mass surface and 1.5x25 = 40 Btu/h-F of air-water surface conductance with a thermal equivalent circuit which makes more sense if viewed in a fixed font like Courier New after downloading. Tg is the greenhouse air temperature:

  17067 Btu/h  Tg
     --        |     1/120 F-h/Btu
|---|—>|-------*------vvv--- 30 F
     --        |
               <
               < 1/80
               <
               |
              ---
              --- 900 Btu/F
               |
               - 

with this Thevenin equivalent circuit https://www.allaboutcircuits.com/textbook/direct-current/chpt-10/thevenins-theorem/ :

       1/120
    ----www------------ Tg
   |           |
   |           <
   | Vt        < 1/80       Vt = 30 + 17067/120 = 172 F.
  ---          <
   -           |                
   |          ---         
   |          --- 900 Btu/F    
   |           |                
   -           -  
                                            
which is equivalent to: 
  
       1/48 = 1/(1/120+1/80)                              
    ----www----                 
   |           |
  --- 172     ---               
   -          --- 900 Btu/F   
   |           |                
   -           -             


with a 900/48 = 19 hour RC time constant.


137 F  |         Tg peak air temp, which

       |                 falls quickly at dusk.
       |     Tg
       |         Tg
       |  Tg       
  84   |      T T T peak water temp, which does not.   
       |Tg  T      T       
       |  T      Tg    T
       | T                 T
       |Tg               Tg   T            
       |Tg                       T
       |T                           T
       |Tg                        Tg    T
       |T                                   T
  54    ------------------------------------------
                 6h                         24h


If T = Td just before dusk and T = Tn just before dawn, 18 hours later,

Tn = 30 + (Td-30)e^-(18/19) = 18.4 + 0.388Td, and
Td = 172 + (Tn-172)e^-(6/19) = 46.6 + 0.729Tn, then
Tn = 18.4 + 0.388(46.6+0.729Tn) = 36.5 + 0.283Tn, so
Tn = 36.5/(1-0.283) = 50.9 F.

Just before dawn, (50.9-30)48 = 1004 Btu/h flows out of the drums, warming the greenhouse air to 30 + 1004/120 = 38.4 F.

Just before dusk, Td = 46.6 + 0.729Tn = 83.7 F. The greenhouse air would be hotter: (172-83.7)48 = 4238 Btu/h flows into the drums, adding 4238/80 = 53 F to the drum temp to make 137 F air, if I did that right.  

We could limit the air temp to about 80 F max by opening a greenhouse vent whenever the indoor air rises to 80 F using an unpowered vent opener, or a thermostat and a 24V 2W motorized damper actuator or a fan. The vent needs to dump about 17067 – (80-30)120 = 11067 Btu/h. With an A ft^2 open area for upper and lower vents and an 8’ height difference, A = 11067/(16.6sqrt((80-30)^1.5) = 0.67 ft^2 min, eg 1 ft^2 vents, which could vent up to 16600 Btu/h at 332 cfm and 80 F.

Ventilation can also reduce wintertime humidity. Air at 80 F (540 R) and 60% RH (an upper limit to avoid mold) has a water vapor pressure Pw = 0.6e^(17.863-9621/540) = 0.628 “Hg. The humidity ratio wa = 0.62198/(29.921/Pw-1) = 0.0133 pounds of water per pound of dry air. NREL says the average outdoor humidity ratio w = 0.0024 in January in Philadelphia (although the current online Blue Book is missing Phila data.) A 100 cfm vent airflow would remove 60m/hx100x0.075lb/ft^3(0.0133-0.0024) = 4.9 lb/h of water vapor from the greenhouse. Some people  say that green plants can evaporate 1 lb of water per day per square foot of greenhouse floor space.

With 4 drums:

       1/120
    ----www------------ Ta
   |           |
   |           <
   | Vt        < 1/160       Vt = 30 + 17067/120 = 172 F.
  ---          <
   -           |
   |          ---

   |          --- 1800 Btu/F
   |           |  
   -           -


which is equivalent to:

       1/69
    ----www----
   |           |
  --- 172 F   ---
   -          --- 1800 Btu/F
   |           |
   -           -

with an 1800/69 = 26 hour RC time constant.

If T = Td just before dusk and T = Tn just before dawn,

Tn = 30 + (Td-30)e^-(18/26) = 15.0 + 0.500Td, and
Td = 172 + (Tn-172)e^-(6/26) = 35.4 + 0.794Tn, then
Tn = 15.0 + 0.500(35.4+0.794Tn) = 32.7 + 0.397Tn, so
Tn = 32.7/(1-0.397) = 54.2 F.


Just before dawn, (54.2-30)69 = 1670 Btu/h flows out of the drums, warming the house air to 30 + 1670/120 = 43.9 F. More drums help. We put 200 55 gallon water drums as plant pallet supports into a 20’x96’ single cover greenhouse in PA, and it never froze in wintertime.

The drum heat would last longer if the greenhouse air were kept at a constant 40 F all night instead of gradually cooling from the peak to the minimum temp, but that would require some sort of control.

With more controls, 8 drums in an R20 4x4x8’ insulated box with an R1 90% transparent south wall could store more heat, ie 3600(132-44) = 317K Btu, vs 8 cooler drums exposed to greenhouse air storing 3600(58-44) = 50K Btu. To reduce heat loss at night, a 500 cfm high temp 23 watt fan, eg https://m.grainger.com/mobile/product/4WT44?cm_mmc=PPC:+Google+PLA&s_kwcid=AL!2966!3!166593068073!!!g!102431315157!&ef_id=WGOu3wAAAH9I1Syw:20180705181606:s could circulate warm air through an R20 partition during the day, with no airflow at night. The 4’x8’ vertical partition could separate the south wall and the vertical drums in a single 2x4 layer under a 4’x8’ bench would have an equivalent circuit like this during the day:

  3888 Btu/h     1/500 fan  R20/96ft^2 box           
     --        
|---|—>|----------vvv---------vvv--- 80 F
     --       |          |
       R1/32  |          <
80 F ---vvv---           < 1/320
                         <
                         |  T
                        ---
                        --- 3600 Btu/F
                         |
                         - 

which is equivalent to this:


          1/32  1/500       1/4.8
    -------www---vvv---------vvv---- 80 F
   |                     |
   |                     <
   | Vt                  < 1/320       Vt = 80 + 3888/32 = 202 F.
  ---                    <
   -                     |                
   |                    ---         
   |                    --- 3600 Btu/F    
   |                     |                
   -                     -  

and this:

       1/34.8
    -----vvv-----
   |             |     
  --- 185       ---
   -            --- 3600               

   |             |   
   -             -  

with a daytime time constant RC = 3600Btu/F/34.8Btu/h-F = 103 hours.

RC = 3600(20/128+1/320) = 574 hours at night:

                           R20/128ft^2 box
                          -----vvv----- 40F           
                         |
                         <
                         < 1/320
                         <
                         | T
                        ---
                        --- 3600 Btu/F
                         |
                         -

Tn = 40 + (Td-40)e^-(18/574) = 1.2 + 0.969Td, and
Td = 185 + (Tn-185)e^-(6/103) = 10.5 + 0.943Tn, then
Tn = 1.2 + 0.969(10.5+0.943Tn) = 11.4 + 0.914Tn, so
Tn = 11.4/(1-0.914) = 132.3 F.
Td = 10.5+0.943x132.3 = 135.2 F.

If we insulate the entire greenhouse (including the south wall) with R20 soap bubble foam at night and fill the 6 triangular spaces between 2 drums and a 2’x4’x4’ tall R20 box with an R1 transparent south wall with 90% solar transmission, the 6 spaces (totaling 2’x4’- 2Pi(1’^2) = 1.72 ft^2 could hold 8 4”x3’ vertical PVC perforated pipes (to reduce airflow resistance) surrounded by about 300 pounds of 2” diameter rocks, which would add about 48 Btu/F to the drum thermal capacitance and 108 Btu/h-F to the drum thermal conductance, with an equivalent circuit like this, during the day:

  1944 Btu/h     1/500 fan  R20/48ft^2 box           
     --        
|---|—>|----------vvv---------vvv--- 80 F
     --       |          |
       R1/16  |          <
80 F ---vvv---           < 1/189
                         <
                         |  T
                        ---
                        --- 948 Btu/F
                         |
                         - 

which is equivalent to this:


          1/16  1/500       1/2.4
    -------www---vvv---------vvv---- 80 F
   |                     |
   |                     <
   | Vt                  < 1/189       Vt = 80 + 1944/16 = 202 F.
  ---                    <
   -                     |                
   |                    ---         
   |                    --- 948 Btu/F    
   |                     |                
   -                     -  

and this:

       1/16.4
    -----vvv-----
   |             |     
  --- 186       ---
   -            --- 948               

   |             |   
   -             -  

with a daytime time constant RC = 948Btu/F/16.4Btu/h-F = 58 hours.

RC = 948(20/64+1/189) = 301 hours at night:

                           R20/64ft^2 box
                          -----vvv----- 40F           
                         |
                         <
                         < 1/189
                         <
                         | T
                        ---
                        --- 948 Btu/F
                         |
                         -

Tn = 40 + (Td-40)e^-(18/301) = 2.3 + 0.942Td,
Td = 186 + (Tn-186)e^-(6/5 = 18.3 + 0.902Tn, then
Td = 18.3 + 0.902(2.3+0.942Td) = 20.4 + 0.850Td, so
Td = 20.4/(1-0.850) = 136 F.
Tn = 2.3+0.942x136 = 130 F.

Foaming the greenhouse at night lowers the heat requirement to (40-30)30 = 300 Btu/h. The rocks lower the min usable water temp to 300/189 = 42 F. The 2 130 F drums can keep the greenhouse air 40 F for 948(130-42)/(24hx300) = 11 30 F cloudy days in a row.

If cloudy days are like coin flips, a greenhouse that can store enough heat to keep itself 40 F for N 30 F cloudy days in a row would have a Solar Heating Fraction (SHF) of 1-2^-N, eg 0.5 for 1 cloudy day, 0.75 for 2, 1-2^-3 = 0.875 for 3, and so on. For instance, the 8 drums in the box would store 317K Btu. Keeping the greenhouse 40 F for a day requires 24h(40-30)120 = 28.8K Btu, so N = 317K/28.8K = 11 days, with an SHF = 1-2^-11 = 0.9995, ie 99.95%, ie solar-heated in all but 4 hours per year of 12 months of January weather.

In summary:                                      
                                         min
# of   C      RC      Twp    Twm   dT    usable
drums  Btu/F  hours   peak   min   F     temp   SHF

0      0      0       172    30    142   -      0
2      900    19      87     51    36    55     0
2+foam 948    58/301  136    130   6     42     0.9995
4      1800   26      78     54    24    48     0.23
8      3600   41      75     58    16    44     0.70
8-box  3600   103/574 135    132   3     44     0.9995
oo     oo     oo      66     66    0     40     1

Nick
 
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That's some nifty math Nick!  Now what would happen if instead of 55 gallon drums of water, you had a 55 gallon drum of glycol?  It phase changes at 63 degrees which could do some interestingly beneficial things.  Getting heat into the drum could require a circulating heat transfer liquid. 
 
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Mike Jay... what would happen if instead of 55 gallon drums of water, you had a 55 gallon drum of glycol?  It phase changes at 63 degrees which could do some interestingly beneficial things.  Getting heat into the drum could require a circulating heat transfer liquid.  [/quote wrote:

This, which melts at 27.7 C? https://www.sciencedirect.com/science/article/pii/S0927024811000365 Seems complicated and expensive, compared to water.

Nick

 
Mike Jay
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Sorry, I meant glycerin (by product of making biodiesel).  Wikipedia
 
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Remember that the phase change energy of water is one of the best substances.  I am not finding it online quickly but guessing that you will probably need 10Xs as much of most other substance to store the same amount of heat as water.
 
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Water's phase change is great, unfortunately it occurs at a temperature that isn't conducive to most greenhouse plants (32F).  Glycerine isn't as good but it's not that much worse.  My google-fu is letting me down at the moment but I believe it was 2/3's as good as water.  And it occurs at a much better temperature (63F).
 
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