How Does Thermal Mass inside a hoop house really work?

Ok I have to admit I don't really understand the concept of thermal mass in passive solar design. Most of the books and videos on the topic say that adding large volumes of water - like 55 gallon drums painted black - will store heat that will be released as the temperature inside the greenhouse drops thus keeping the space warmer. I have no problem understanding how the sun can warm the drums and cause them to heat up. And warm barrels releasing heat to the cooler air is also easy to understand.

But designers also talk about solar collectors which are basically insulated boxes painted black and glazed on one side. These are supposed to get super hot when the sun shines.

My question is: So wouldn't the addition of lots of black surface area inside your greenhouse cause the interior temperature to rise more during the day than if they weren't there? In other words, how come the black barrels don't turn your hoop house into an oven?

Any physicists out there care to take a crack at answering this one?

During the summer, my greenhouse is like an oven, I leave it wide open to ventilate all that heat. During the winter, the auto-ventilation system still opens itself on sunny days. So yup. Greenhouses are essentially ovens.

My greenhouse has about 10 square meters of surface exposed to the sun, so during a sunny day during the winter, it is receiving about 1200 calories of sunlight per second. Plants absorb about 75% of solar radiation. Bare soil absorbs about 83%. So that's not much different than a perfect black body, so I'll ignore the difference in the analysis that follows.

The mass of the air in my greenhouse is approximately 10,000 grams. So if there were no other mass in the greenhouse and all the sunlight was being captured by some perfectly black paint, the air temperature would be rising about 0.12 C per second, 7 C per minute, or  420 C per hour. Yikes!!! That's an oven.

Supposing that we then put two  55 gallon barrels of water in the greenhouse. They have a mass of 416,000 grams. Then the temperature would be expected to rise 0.003 C per second, or 0.2 C per minute, or 12 C per hour. That approximates what I observe in the real world when the sun starts heating the tables, plants, potting soil, etc...

A wise architect once said, insulate the outside with the thermal mass inside. Then heat the mass fast or slow, still it will last,,,

Thomas - One of the laws of thermodynamicss is that heat always moves toward cold.  The heat generated during the day will be absorbed by the cold of the thermal mass.  When the sun goes down the precess is reversed and the heat stored in the mass then moves back out into the cooler air of the greenhouse, helping to mediate the temperature.

Thanks for the replies. I'm still a little fogged here. Let me restate the question:

I have a standard cattle panel hoop house measuring 6 feet by 15 feet. (90 sq ft) Winter and summer, on sunny days the interior temperature is a full thirty degrees warmer than outside. If I add four fifty gallon drums of water, (200 gallons) to the interior will that mass affect the interior temperature of the hoop house when the sun shines. And if so, how?

In other words, if I simply add thermal mass to the interior of a green house can I expect to see any difference in the temperature differential between Outside and inside on sunny days?

Thanks.

Thomas Vincent wrote:Thanks for the replies. I'm still a little fogged here. Let me restate the question:

I have a standard cattle panel hoop house measuring 6 feet by 15 feet. (90 sq ft) Winter and summer, on sunny days the interior temperature is a full thirty degrees warmer than outside. If I add four fifty gallon drums of water, (200 gallons) to the interior will that mass affect the interior temperature of the hoop house when the sun shines. And if so, how?

In other words, if I simply add thermal mass to the interior of a green house can I expect to see any difference in the temperature differential between Outside and inside on sunny days?

Thanks.

Krofter stated it correctly. When the sun heats up the greenhouse during the day, the barrels will absorb some of the heat so the greenhouse will not heat up as quickly as it would have without the barrels. The temp in the greenhouse will be warmer than outside but not as much warmer as it would be without the barrels. Because it takes more heat to heat water than air, it will not get warm as fast and the max temp in the greenhouse will not get as high as it would have. The same will happen at night when it cools down. The temp outside will drop. The temp in the greenhouse will drop but not as fast as it would without the barrels of water because as the temp in the greenhouse drops, the barrels will start giving the heat back. The temp in the greenhouse will not get as low as it would get if you didn't have the barrels. This is over-simplified and at extreme temps, the water volume won't be enough to hold the temp.

Todd,

Thanks, that was very helpful.

One geeky question and then I'm done. When the barrels of water absorb heat on sunny days, is it only radiant heat from the sun - heat that would otherwise add to the temperature of the space - or are the barrels only absorbing heat from the room itself? (By convection?)

Thanks to everyone.

Thomas Vincent wrote:Todd,

Thanks, that was very helpful.

One geeky question and then I'm done. When the barrels of water absorb heat on sunny days, is it only radiant heat from the sun - heat that would otherwise add to the temperature of the space - or are the barrels only absorbing heat from the room itself? (By convection?)

Thanks to everyone.

The barrels absorb heat from the room any time the temp of the room is higher than the temp of the barrel, just as they give off heat any time the room is cooler than the barrels.

Thomas Vincent wrote: When the barrels of water absorb heat on sunny days, is it only radiant heat from the sun - heat that would otherwise add to the temperature of the space - or are the barrels only absorbing heat from the room itself? (By convection?)

Hi Thomas, the heat enters the barrels from both radiant heat transfer (sunlight hitting the drum) and from conduction (warm air touching the cool barrel).  I believe the third method of heat transfer (convection) is basically the air moving around, for instance a leak in the greenhouse wall would convect cold air in at night and warm air out during the day.  I could be explaining that last one poorly...

In your question I take it that you're thinking radiant energy that hits the barrel would otherwise hit the ground of the greenhouse and it's a wash?  That could be but the ability of a surface to efficiently transfer radiant energy depends on the surface.  Imagine a metal drum, a plastic drum and a light colored wood barrel sitting outside in the sun in Arizona in the summer.  Which one would burn your butt if you sat on it?  They all experience the same air temp (conduction)but the metal absorbs (and releases) that radiant energy better.  So if the floor of the greenhouse absorbs radiant energy as well as a barrel, it would be a wash.

One thing some crafty folks do is situate the barrels where the sunlight will hit them in the winter but not in the summer.  In winter you get the radiant heat and conductive heat gain.  In summer you don't get the radiant gain so you don't heat the barrels as much.

A subtlety is that at night the barrels are radiantly heating the great outdoors in addition to the greenhouse interior.  That's why on a cold winter night when you stand two feet from a window it feels cold on your face.  That's thermal energy leaving your face to heat the outdoors.

I think when it comes to winter growing, you really need to look at the practicality of it, especially on the small home scale. This is how I look at it: What kind of harvest, if any, can I really get without running a 5' run of insulated ducting from my house heater to a greenhouse? What's the cost effectiveness of heating, in essence, 1 room that constantly loses heat, but only gets some warm air when the thermostat room activates the heater? And I'm still not gonna have a jungle out there. lol You're probably talking about a room, that if I'm lucky can keep at 45o-55o degrees at best during winter. What grows well at that temp?  And that's a conventional solution, which I would guesstamate would cost me and extra $30 bucks a month on the heating bill. I've been working with "thermal mass" experiments to see if I could produce enough heat to actually grow a sustainable crop in -5 to 20 degree F, and to get right to the point: "Thermal Mass" heating is the lowest common denominator for trying to heat ANYTHING! It can keep a room above freezing, but heating essentially a rock is a waste of your energy source, and yes this includes your fires energy. It's just completely inefficient. Water barrels take up valuable space in small green houses, are a pain to move when full, for what they do. Painting surfaces black is great for winter, but what about summer when that space is approaching 160o degrees? LOL.. Also indoor growing and the bugs... There is also the consideration of fresh air being introduced. I think in essence my conclusion is that there's no way in hell you're going to in grow winter and summer crops without: 1) the building being specifically designed for it (efficiency) using modern materials, and supplemented with some permie tech to offset cost. And yeah, aint talking about no Harbor Freight green house... lol I'm currently working on a way to increase a fire energy source specifically for green houses, but I haven't seen anyone grow anything but scrub brush in winter without pro-pane heater heat. The questions you're asking, people have been racking their brains over for a long while. My honest belief is that a very small fire can be used, however that source of energy needs to be magnified by 10. One can spend a lot of money on perm tech, but there are some serious limitations cost verses what you can actually grow at home on a small scale I believe. Just an opinion.

re: I think if you're going to do some solar, go all the way. 6 large panels-coverter-maybe some batteries and you can run 1 or 2 of those small 6x10 electric room  heaters ($12 bucks walmart), maybe a dish heater, however with fans is probably better depending on the size of your green house. It would also allow you to run some fans in summer. If you want to just test the "thermal mass" heating concept: take a heat rock, you know the ones, for pet lizards, put it in a 10 gallon fish tank, and put it in your 40o to 50o garage with a couple of thermometers. (suction cuppys) 1 inside and 1 on the outside. I think you'll see just how inefficient "thermal mass" is. The rocket stove, and the "conductive" metal is what actually heats the air in the space. And that's what you want to do: heat the air. Not put the energy from the fire or energy source in to non-conductive materials like cob, concrete, rock, etc, which only have about a 1 to 1.5 efficiency. (ie. it takes 3 hours to heat up, then cools down in 3 hours) Not to mention those non-conductive materials "radiate" jack and shiznit when jack left town. lol.. Anyways, Good Luck to ya!

It might help to think of added thermal mass as a thermal version of a mechanical flywheel. (Spinning wheels and treadle sewing machines use the flywheel effect to smooth their operation.) Any mechanical device that includes a flywheel requires some extra effort (energy input) to increase its speed but comparatively less energy to maintain the speed. And when the energy input is stopped, the flywheel slows gradually, keeping the device running while slowing. For a greenhouse, raising its temperature a degree takes more energy if it includes extra mass, so it warms more slowly. Conversely, when the sun quits shining on it, it will cool slowly (not rapidly like deserts do). The solar collector box is desert-like, so it can reach much higher temperature on a sunny day but quickly cools at night. That's a terrible greenhouse. Note though that the air at the top of an unventilated greenhouse and well above the significant mass will get hotter (by convection), but your plants are likely in the lower and cooler zone.

I have built two Harbor Freight greenhouses. I loved the first so much that when I moved and couldn't take it with me, I immediately built another. What a greenhouse does for me, is that it allows season shifting. Without added heat, it gives me a frost-free growing season extension of about 8-10 weeks in spring and fall. The shelving, soil, and water in the greenhouse store heat during the day, and release it at night, thus minimizing freezing temperatures at night. On a longer scale, it stores heat during sunny weather, and releases it during storms. So what that means in practice, is that the greens that are producing food for me in May in the open field, are producing in March in the greenhouse.

Thanks to one an all for your replies. They were all helpful in elevating my understanding of thermal mass, what it can do and what it cannot.

The following is a reprint of my response to another hoophouse forum here on permies.

I have completed adding a second layer of plastic on my cattle panel hoop house. Added a total of 190 gallons of water (milk jugs and 3 55 gal drums.) all painted black. Finished it off with pallet counters covered in terra-cotta tiles. (See pictures)

At the end of a miserable cold and rainy week here in the Pacific NW with an average daily outdoor temp of 45 degrees F., the average daily interior temp was 51.8!
Even better, the interior temp At sunrise was always at least two or three degrees higher than outside. This is all I was really after so I consider this experiment to be a success.

Not sure how much of this improvement was due to the extra plastic (insulation) and how much due to the added water (thermal mass), because I added both at the same time, but I think I can state pretty conclusively that double walled hoop houses with significant amounts of thermal mass can create a better growing environment in winter in the Maritime Northwest, without the use of fossil fuels, than occurs outdoors.

Now I just have to tackle that pesky "lack of sunlight in winter" problem! 😄

PS I will be posting full results on this hoophouse along with my experiments making biochar at my blog: tinkersblessing.com

University of Missouri did this as an experiment once.  

* Barrel color didn't seem to matter.
* Barrel material (metal vs plastic) didn't seem to matter.
* It takes a LOT of water.  In their setup, the barrels were, I think 3 high (9 feet) on the north wall.  North wall was well insulated, as were the end walls.  Greenhouse was something like 12 or 15 feet wide.

So they had 165 gallons of water for 24 to 30 square feet of growing area. Ballpark 5 gallons per square foot.

I had a tiny green house 8x8 no insulation, built of old windows, and hence leaky.  

I had 3 55 gallon barrels, and a dozen 5 gallon pails, so in effect 4 barrels of water.  That left about 48 square feet of growing area.  Ballpark 4 gallons per square foot.

I set up alarms on my phone.  8 p.m. shut the greenhouse.  8 a.m. open the green house.  

I had a remote thermometer in the greenhouse that I could read from the house, and another one outside.  By morning the temp inside the greenhouse was typically 3 C warmer than outside.

Ok.  Single pane.  R1.  Go to R2, and I could probably get 6 C warmer.  Increase the weather sealing bring that up to 10 C.

For us 10 C would give us anohter month at each end of the growing season.  Here that's a big deal.  Right now our growing season is only aobut 110 days long.

It also means we can grow tomatoes.  Good tomato years are rare here.  Not enough heat.   Eggplants and peppers just sulk outside.

So it's worth doing.


***

A larger hoop house keeps heat better.  You get about an extra degree for each meter of height.  This is a combination of having more air to cool at night, and having a higher ratio of space to edge.

A double plastic house with the sheets kept separate by a blower works better than a glass house.   You can get away with 3-4 mil plastic for the inside, but should use 6 mil for the outside layer.

You can buy/make half arches.  If you have a large building (shop, house, barn) you can use it for the north wall.  Insulate that wall as needed.  Surplus heat during the day can heat the building.  This works well for a shop.

The history of greenhouse thermal mass is kind of ironic. We began with nothing but thermal mass, in the 1600's northern Europe called them fruit walls:



Then we discovered that adding some expensive glazing really helps:



Then we became spoiled by abundant energy, energy to waste (not just in the energy to make glass but also to heat the greenhouse) and then went here:



Now we have forgotten where we began and don't quite understand how thermal mass works.

Great article on fruit walls and Chinese greenhouses.

.

We just built our greenhouse this year.  It still need windows and a door.  We did have a LOT of broken rock/cement block from an old wall/fence at my son's house.  So I lined the edges inside with said rock to serve as thermal mass, thinking it would work as the barrels.  Filled the center with topsoil and compost (going to plant direct to ground and it is urban hard packed clay underneath).  What are your thoughts?

May I add some refinements?  The thermal mass does indeed act as a "flywheel".  But there are significant issue associated with the efficiency of moving the heat in and out of the mass.  At one point in the past I used multiple 1 Gal jugs, laid on their sides, (so that the heat could flow from one to another by conduction).  This worked to a degree, but the heat in the greenhouse was rising faster than the heat sink could absorb it by radiation and conduction from the surrounding air, so the water would warm up only a few degrees in the course of the day.  I was able to roughly double the amount of heat captured by hanging a sheet of plastic in front of the jugs, and blowing air sucked from the peak of the greenhouse through the wall of water.  (Essentially I was adding extra heat in the form of heated air, a very crude air-to-water heat exchanger.)  I eventually abandoned this because the plastic jugs deteriorated and leaked after a year or so.

I then moved to bulk storage.  And here there is another element introduced:  the water in your 55 Gal drums, (and my large (450 L) fiberglass tanks) will stratify.  That is the heated water rises to the top, and when it reaches the temperature of the surroundings will not absorb any more heat.  (Feel your drums some day when the sun is shining on them - you will find that the bottom portion remains cold - obviously not storing heat).  This effect becomes relevant whenever the water volume exceeds somewhere around 25 Gal., and gets worse as the volume increases.  (Parenthetically, this is why the smaller jugs work better - the stratification is confined to small distances, and the heat in the top of one jug gets conducted into the bottom of the jug above it.)  What to do?  I don't have a solution for the 55 Gal drums. but my large tanks are kept more uniform by the use of small pumps sitting in the bottom.  Actually there is an additional element here - I pump the water from the bottom of the tank through used car radiators in the peak of the greenhouse, to capture the heat from the air, before returning it to the tank.  One of the pumps failed recently, affording an unplanned experiment.  The water in the tank with circulation through the radiator, on Oct 28, 2017, reached a peak temp of 31 C, while the uncirculated tank reached 23 C .  (Oblique comment: the $15 pumps from China on eBay aren't worth it.  They die after a few months. Spring for higher quality up front).  Conclusion: heat delivery takes place over a rather short time frame;  heat capture needs to try to follow as closely as possible the rise in available calories.  And there are ways to optimise this.

Good post David.  There is an underlying ideal at work here even if people are not conscious of it, that is Active vs Passive. Especially in a perma-culture setting Passive is always first choice and Active has a bad image of being overly complicated.

But active, as you know, can yield huge performance benefits.  I think that a "properly" designed thermal mass storage system would have an active deposit system as you have engineered, and also an active withdrawal system which would then become computer controllable. There is no efficiency in heating a space when it doesn't need it. If it is indeed a storage system the mass should also be insulated to extend beyond only day to night cycles but also to compensate for rainy day periods.  

All of that is also applicable to rocket mass heaters which would benefit from becoming Pro-Active.

There are degrees of complexity in "Active".  How complex is perceived as appropriate for a "Permie" is probably variable, but I do agree that there is a bit of a bad rap in many people's minds for any form of active controls.  In practice, my system does have some technology.  The pumps to pump the water through the radiators (my "heat exchangers") are controlled by a differential thermostat, so that they run only when the temperature of the air, (in the peak), is higher than the temperature of the water.  It was apparent that the heat was coming out of the water in the tanks reasonably efficiently, as the temp in the tanks drops as much as 20 degrees C overnight.  But I have gilded this lily a little - I am pumping the water through black plastic piles buried  8" down in the growing beds, 24 hours a day, (at least when the cheap pumps are still working).    I did this on the principle that heating the plants' toes made more sense that trying to heat the entire volume of air in the greenhouse.  But it had an unexpected effect - it added the soil in the growing beds to the thermal mass.  The temperature in the tank with a functioning soil pump runs about 2 degrees lower than the other one, and the soil temperature in the the "circulated" beds, runs 2 degrees higher than the ones without circulating water.  In addition to the pumps, I have 4 large computer fans which draw the air from the peak through the radiators.  All these  pumps, fans and controllers run off a 90 Watt P-V panel with a small battery storage (actually the battery from my ride-on mower) which carries the  soil pumps through the night.
Obviously, since I am able to quote these temperatures, I also have a monitoring system capable of storing a log of data drawn from, (in my case), 4 separate channels..  (Mine currently polls the temp sensors every 10 minutes).  
And that is the extent of my "active" technology.
Does movable thermal curtain insulation count as "active"? (That is my next "refinement", after I get, and install, some new pumps, which hopefully will last a little longer.)

Great Links Tom, very informative. I've been puzzling over how to grow figs and actually get a good crop, looks like the fruit walls are the solution. I had no idea they were so effective, or had such a long history

bob day wrote:Great Links Tom, very informative. I've been puzzling over how to grow figs and actually get a good crop, looks like the fruit walls are the solution. I had no idea they were so effective, or had such a long history

Thanks. The article doesn't say, but by the pictures I think the reason they are so effective is that the fruit trees are trained to grow right against the wall, cuddling to keep warm.

David Maxwell wrote:...  The pumps to pump the water through the radiators (my "heat exchangers") are controlled by a differential thermostat, so that they run only when the temperature of the air, (in the peak), is higher than the temperature of the water.  It was apparent that the heat was coming out of the water in the tanks reasonably efficiently, as the temp in the tanks drops as much as 20 degrees C overnight.  But I have gilded this lily a little - I am pumping the water through black plastic piles buried  8" down in the growing beds, 24 hours a day, (at least when the cheap pumps are still working).    I did this on the principle that heating the plants' toes made more sense that trying to heat the entire volume of air in the greenhouse.  But it had an unexpected effect - it added the soil in the growing beds to the thermal mass.  The temperature in the tank with a functioning soil pump runs about 2 degrees lower than the other one, and the soil temperature in the the "circulated" beds, runs 2 degrees higher than the ones without circulating water.  In addition to the pumps, I have 4 large computer fans which draw the air from the peak through the radiators.  All these  pumps, fans and controllers run off a 90 Watt P-V panel with a small battery storage (actually the battery from my ride-on mower) which carries the  soil pumps through the night. Obviously, since I am able to quote these temperatures, I also have a monitoring system capable of storing a log of data drawn from, (in my case), 4 separate channels..  (Mine currently polls the temp sensors every 10 minutes).

NICE! The heated beds is an awesome idea. I'm not a biologist but I think soil temperature sends signals to the plant to tell it how and when to grow. It at least is vital in seed starting and I know high soil temps tell the plant to go dormant until it cools off a bit. You have designed a "task heating" system (like "task lighting"). Heat only where and when you need it, not heat always everywhere. Only through active systems can you achieve this ideal.

Now that you have the (active) components in place you could add a compost pile into the system, extracting heat from the decomposition and giving off some co2 to make happier plants. and once again serendipitously adding some more thermal mass. Because you have an active system the compost pile need not be in the green house, just nearby connected by two pipes and a vent duct.

David Maxwell wrote: Does movable thermal curtain insulation count as "active"? (That is my next "refinement", after I get, and install, some new pumps, which hopefully will last a little longer.)

Well we had been talking about active/passive in heat transfer systems. Movable insulation is more of an adaptive system and yes it would be an active adaptive system. An example of a passive adaptive system would be deciduous trees on the south side of your house, they adapt from shade in the summer to sun in the winter.

The Chinese consider roll-up thermal curtains as essential. See that article I posted above. What I think the ideal would be is insulated shutters of some sort that when open become light-gathering reflectors.  Stand on the north shore of a frozen lake and you get twice as much sunlight - direct from the sun and seemingly just as much reflected off the lake.

I read many years ago about movable insulation with ping-pong balls filling large cavities between sheets of glazing. He moved them in and out with a blower and duct work. It at least would be a show for guests - as soon as the sun sets these balls start filling-up the windows.  

I need to read through the entire thread, but the only thing I see wrong with the first analysis (suggesting a thermal rise of 420C/hr, is that this ignores the heat radiated out the north facing wall and  absorbed by the ground within the greenhouse.  If you take into account that you are likely to find it is only rising 1/10'th that or something.  Still, to hot for plants.  I do find it interesting that adding the extra thermal mass brings it in line with what you see.  Maybe adding the heat lost through the ground and north walls will help make the initial calc more realistic.