A question for passive storage afficionados

sand_mueller(z 7a, oklahoma)January 24, 2014

So we had zero degrees this morning. today it is sunny and 42 degrees. Sunday it will be 66 degrees and sunny. If it were any warmer today, for the health of my plants I would have to ventilate...on Sunday I'll have lots of windows open. Why would I want to hold heat inside the gh hoping that my solar devices will soak it up while leaving it a decent temp for my crops...spinach doesn't like 85 or 90 degrees. For me the crop comes first and I have to keep good temps both high and low?

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curlygirl(5-6 Massachusetts)

These conditions are exactly why Subterranean Heating and Cooling Storage systems are used. Rather than ventilate the heat of the day to the outside, the system pulls that air down underground into a heat sink where a forced dew point occurs which releases the heat into the heat sink. Cooler, drier air comes out the other end, conditioning the spaces, much like a refrigerator. The heat sink stores a ton of heat which can be used at night to heat the greenhouse. This is not considered a "passive" system so perhaps I am not answering your question. I think for truly passive greenhouses, the design uses many strategies to mitigate temperature swings (southern exposure, pit design, etc) and then the excess is ventilated. If that is not enough to maintain temperatures year round, then I think the greenhouses are simply not used in the winter.

SHCS link:

PCMs in greenhouse blog that shows how it mitigates the temperature swings (this greenhouse is not entirely passive either because it isolates the PCMs and sends air through fans over them):

    Bookmark   January 24, 2014 at 4:47PM
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sand_mueller(z 7a, oklahoma)

I do believe that your underground storage must have merit. Its not possible for me to build that in now, but I may move the greenhouse and build it in later. I think orientation is overrated. I have an east facing greenhouse tucked under a fence row of deciduous trees on the west. Excess heat in spring and fall here is a bigger problem for me than total winter efficiency. My only contrarian point is that in order to grow the citrus I want I have to be able to heat the air when frigid air arrives. Digging a structure into the ground is my favorite design.

    Bookmark   January 27, 2014 at 12:15PM
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I think you've hit on the main question/dilemma of passive heat storage. Without some active movement of heat, it is hard to store all the heat while it is available (and the same later for getting it back out).

There are a few things you can do, such as use a liquid storage medium, which will develop currents to mix itself. And you can design your heat storage with a lot of surface area relative to the mass, so that it can transfer heat in/out more quickly. (The hanging, flat black plastic water bags are one design that uses both these ideas).

But there are other engineering problems associated with these solutions. Namely the high surface area storage tends to be optimized for quick short term effects. It might get you thru a cold night after a sunny day, but will not be very helpful if the next day is cold and cloudy too. (Not enough thermal mass).

Ultimately, you want a large thermal mass and a means to push/pull heat to/from it faster than can be done by passive transfer alone. At least that is what you want if you are intending to avoid or minimize paying for extra heat in some form.

The other design ideas you mention will definitely help. But if you really want to minimize external heat input, you will probably have to go to some active heat moving of some sort (fans, etc), just to be able to capture and reclaim more of that daytime heat, rather than venting it off to the outside. And multiple solutions may be helpful too (adding some short term passive storage, along with longterm storage and other cold growing techniques (like interior high tunnels, heated ground, etc).

    Bookmark   January 27, 2014 at 5:30PM
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sand_mueller(z 7a, oklahoma)

Steve and Curly Girl, these are my own design for passive storage. I use them liberally in greenhouse and cold frames. My main interest is in having something that gains and loses in a hurry, in part because Oklahoma goes so rapidly from sun and too hot to bitter, dark cold.

    Bookmark   February 5, 2014 at 12:18PM
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Another problem with passive mass, for people using expensive heat, it can make your heat run more, especially on a cloudy mornings. On a sunny day, the extra energy required to heat up the mass is free from the sun. But with no sun, if you're trying to heat the air to a higher temp than what the mass cooled to last night, that's the scenario where mass works against you.

    Bookmark   February 6, 2014 at 9:45PM
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curlygirl(5-6 Massachusetts)

Agree with Cole Robbie. I have a lot of thermal mass right now which (for now) is working against heat but once my greenhouse SHCS gets charged for the first time this summer, along with the thermal mass above, I think it will work for me. We have highly insulated walls and high performance windows keeping the heat in but because the thermal mass has yet to be charged by the sun (several sunny days in the early fall) it is just an enormous heat sink sucking up the meager heat we do get from the sun. Still, the temperatures are in the 60s most days and has yet to go below the 40s at night.

Sand Mueller -the dark water jugs are a great cost effective measure but I think you will get better results from PCMs. Water is technically a phase change material but it does not absorb heat rapidly enough for what you are looking for and a PCM that is mixed for your temperature range will absorb a lot more than water in less space. I think I read that PCMs absorb 10 times the amount of water.

    Bookmark   February 7, 2014 at 6:25PM
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While what Cole Robbie says is true under some circumstances, I would say that you need to look at how your GH is temp cycling to find out if that is really a concern for you.

If you've ever skied into a remote log cabin in the winter, and built that first fire, you've had first hand experience on how thermal mass can keep you cold. (It takes forever for the wood fire to get all the mass of the cabin up to a decent temp for people. But once its up to temp that same thermal mass, now warm, will help keep the place warm if you're not around to feed the fire for a while.)

Same in the GH. If you are letting the GH temps (and the thermal mass temps) fall way below the normal operating temp, then it is going to take longer, and use more BTUs to get them both up to operating temp again.

BUT (and it's a big but) if you have heat which takes over once the thermal mass no longer can keep keep the GH at operating temp (that is when the thermal mass is at that operating temp or just below it), and the heater holds the thermal mass and GH at that temp, then you are NOT heat up the thermal mass. This is of course the ideal situation, where you use what is available in your thermal mass storage, and once it gives out another heat source takes over. And while it probably can't really work perfectly this way all the time, I think it can work this way quite a bit of the time. (extreme cold snaps and when the power/fuel goes out for the heater are the exceptions that come to mind)

Another "complicating" factor is if your thermal mass is totally or partially not connected to the GH (say where you use fans to pull/push the heat to/from the GH). In this case you can let the thermal mass get colder than the normal operating temps (to try and save plant in there until your heater can come on again), but not have to heat the thermal mass back up again right away. A SHCS is one such system. You can "overdraw" heat from the subsoil, but not reheat it right away, and wait for the next sunny day to turn the SHCS fans back on. There are other active heat storage systems that also would allow this. Of course water in containers in the GH would not work this way.

I think what it comes down to is the overall input of heat during the winter for your area and your GH. Are you in a typically sunny area, where you can likely count on regular sunny days to recharge the thermal mass? If so, an appropriate sized active thermal mass system can significantly reduce your fuel bills and/or let you run your GH longer in the spring/fall. If you are in an area that has long spells with no sun in the winter, then even an active thermal mass system may not buy you as much heat, and passive thermal mass may not do much good.

Just my thoughts/experiences.

    Bookmark   February 7, 2014 at 7:41PM
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