bcfromfl

What's all this about heat sinks?

bcfromfl
14 years ago

OK, Ill bite.

IÂm in the design stages for a better greenhouse, and know the problems associated with heat gain. I like to think IÂm pretty open-minded, and open to all suggestions with regard to heating and cooling devices. I certainly donÂt have any education in physics or thermal dynamics, but I must admit from my backyard efforts with a Rube Goldberg greenhouse that I think all this talk about heat sinks is a direction in futility. Am I missing something?

IÂve followed all the threads, and there seem to be some common denominators. Please chime in with your thoughts and/or comments.

- Some discuss the merits of a heat sink/thermal mass with only a few gallons of water. Even in a small gh, any significant benefit would require hundreds, maybe thousands of gallons.

- Some of the containers cited are plastic, which wonÂt transfer thermal differentials efficiently. Even a steel drum would be maddeningly slow.

- Thermal mass heat for nighttime augmentation seems to be discussed in the same breath with heat sinks for daytime cooling. In order for thermal mass to be able to store heat or cold, especially passively, there must be several things present. Two important factors are heat calories, and temperature differential. Thermal mass for heat works well, because there is so much horsepower from sunlight that a significant mass of water can be heated fairly quickly with just a few hours of sunlight. LetÂs say that the ambient temperature of a barrel of water is 60F, and the sunlight hits the barrel, heating the outside to (at least) 85 or 90F. Therefore, thereÂs a differential of 25 or 30F with tons of heat calories. The heat stored will radiate out slowly as the gh temps drop overnight.

Now, letÂs say that that same barrel has cooled down to 60 through the night hours, and the greenhouse starts heating up again in the morning. The temperature differential in the morning hours is the smallest, maybe a few degrees, so no benefit in cooling (although none necessary). As the sun hits the greenhouse, and the barrels, it quickly overwhelms the system and the barrels heat up quickly, reaching some equilibrium based upon the thermal conductivity of the barrel, the temperature differential before the sun gets lower in the sky, etc., etc.

I suppose some would argue that the water absorbs calories that otherwise would have raised the inside temps even more, but I maintain that a greenhouse will reach some arbitrary temperature during the day anyway, then start radiating that temperature differential out through the walls and roof depending upon the R-value.

The real problem, I think, is that the thermal storage device would heat up close to its maximum temperature by, say, noon or so, just when the benefit of a thermal sink is needed. With a water temp of around 75-80F by that time, and a gh temp of possibly a few degrees more than that, there is not as much temperature differential to benefit the cooling side of things. YouÂd need a water temperature of near freezing to be enough to overwhelm the heating capacity of sunlight inside the gh.

Also, I think there is a difference in thermal dynamics between the temperature gain by sunlight pouring calories into an object, and a potential cooling effect because of air molecules randomly hitting a relatively cool surface. I think this can be explained by the efficiency differences between radiant and convected heat.

IÂm sure thereÂs a Newtonian Law here that says that energy can neither be lost nor gained (or something like that), so the mean result is neutral. But somehow in practice this just doesnÂt seem to apply satisfactorily with respect to passive thermal mass devices in greenhouses. I can see under-floor, active systems with pumps, pipes, etc., but not passive ones.

Thoughts? Comments?

Comments (44)

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    I won't bite.....

    ""I think all this talk about heat sinks is a direction in futility.""

    OK you have read all our ramblings and debates and formed your opinion.

    Now, as you are interested in the subject I suggest you look for sites (and I don't mean websites, but some of the places have websites) of historical interest in say Europe. You are looking for castles, manor houses, nobleman's residences.

    Any place that has/had a garden a couple of hundred years ago.

    Look out for "walled gardens", "orangery", and "reflecting pool" references.

    Fascinating reading. Imagine incorporating some of that into your greenhouse.

    Back to the present day.....

    Look up information on "solar structures" as in places to live. Storing solar energy and releasing it at night, i.e. moderating the day highs and night time lows by using thermal mass. However most humans need a lot less light than a plant.

    Combine the gardening methods of 200 years ago with the modern methods of building an energy efficient solar heated (and dare I say it solar cooled!) home and you are well on the way to saving a few bucks on both heating and cooling a greenhouse.

    Also you are zone 8 FL.

    I am zone 4b IA.

    Bear that in mind when you see "talk about heat sinks is a direction in futility"

    It may be futile in FL. In Iowa, as I have spent the last five years getting it right, it is the way to go to produce tens of thousands of plants a season using very little added energy (electricity)

    I intend getting the "thermal mass" in there tomorrow, and starting planting Sunday!

    As a side note to stressbaby, and without diverting the thread, I solved my water problem. I now have tons and TONS of nice, fresh, clean water just piled up in my yard waiting to be scooped up and stuffed in trash cans! Thermal mass, and PURE thermal mass.....

    :)

  • stressbaby
    14 years ago
    last modified: 6 years ago

    Chris, you mean the thermal mass that other people call "poor man's fertilizer?" :D

    I think I understand what you are saying, bcfromfl. Let's say your barrels are 90F at 3pm. The heat would be much more useful if it could be trapped in the barrels until midnight, then released. But in a passive system, those barrels could be down to 85F when the GH is 80F, 75F when the GH is 70F, and then by the time you reach the minimum temp when you would want the heat, most of the heat isn't there. The same thing happens in reverse in the morning. I'm obviously just making up these numbers, but I think that some time back, someone posted actual barrel temp readings to support this notion.

    Someone, maybe it was Chris, suggested two or three ways of thinking about thermal mass. One was passive...the barrels in the GH. Another was active management, and this may sort of address your concerns. Active management is the pumps that move the water through a system on a timer or the fancy "barrel-in-a-PVC tent" plan for which Nathan provided a link some time back. These just seem like ways to address the inefficiencies you mention.

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  • bcfromfl
    Original Author
    14 years ago
    last modified: 6 years ago

    I apologize if my post came across as argumentative or with lack of respect, as that was not my intent.

    Passive thermal mass for heating WILL work, but the space required for it to work well is difficult to justify for small hobby greenhouses, in my opinion. I have a book on this very subject, that has all the BTU calculations worked out, but the storage is well beyond what most have suggested in this forum. If you don't plan to use the spaces under benches for growing plants, then it becomes more attractive as an option, I suppose. Active units greatly enhance the efficiency, and I'm planning an under-floor unit in this regard.

    I haven't been able thus far to locate any of the references about walled gardens, orangeries, or reflecting pools, but my guess is that those were used for supplemental heating, not cooling. Correct me if I'm wrong! My difficulty with this topic is with the passive cooling side of things, not so much with the heating.

    I think it's helpful to have these sorts of discussions, because for someone building a greenhouse, it could save all sorts of trouble and expense if they didn't have the best design worked out. Greenhouses are expensive, and nothing worse than spending weeks working on something and not have it work like it's supposed to!

    Bruce C.

  • birdwidow
    14 years ago
    last modified: 6 years ago

    I too have wondered about trying to use water as a heat sink and while my 12 X 16 GH isn't tiny, it isn't that large either and I really want to be able to use the space under my benches for material storage and maximize growing and work space.

    We just came through a reasonably mild winter for the Chicago area, several nasty ice and snow storms notwithstanding, and it's going to get cold again before the last frosts of March, but even when it did drop down to sub-zero for days at a time, between a solar pool cover and a high effeciency gas furnace, it held it's heat quite well with very little energy use.

    So for me at least, it comes down to saving at best $100- $150.00 a year on fuel, or losing a lot of space in what is after all, a hobby GH, built purely for my pleasure.

    So perhaps that may be the real basis for any decision: what are you willing/able to spend to maximize your pleasure in and with your GH?

    BTW, however: I made a mistake with the pool cover. It worked as expected, but was just too much to get onto the GH and we are not looking forward to wrestling it off. I'll be going to a more permanent, interior insulation this fall. Darned if I know what we will do with it, so any suggestions would be welcomed.

  • stressbaby
    14 years ago
    last modified: 6 years ago

    I don't think you came across as argumentative, you raise good points.

    The space issue is significant. Personally, I prefer to use my space for plants. That is why, compared to Chris, I have relatively little in the way of thermal mass in my GH. I also have way higher heating bills. My choice. I just prefer to use my space in other ways. No problem, right?

  • orchiddude
    14 years ago
    last modified: 6 years ago

    I dont think all the thermal mass in the world is going to heat a greenhouse. Well, it might a few degrees. Nobodys doing it. I wish there was someone that had a system set up in a zone 7 or lower that showed this working.

    I would be the first one to buy solar panels but the cost out weights the return over the next 10 years, plus the maintenance.

    I really think the hybrid car is going to fall into the same catagory. It cost to much, plus maintenance equals the same as the gas expense of a different car.

    Just some ideas.

  • bcfromfl
    Original Author
    14 years ago
    last modified: 6 years ago

    As I was outside today working in the yard, it occurred that my observations in the Fakahatchee Strand might have some relevance here. For those of you who aren't familiar with it, it's a subtropical swamp within the Everglades in south Florida. I used to spend quite a bit of time in there, looking for orchids to photograph. The water is everywhere, and the ephiphytic orchids grow on the trees which have their lower trunks and roots submerged.

    On one trip to photograph the ghost orchids in bloom, I also brought my thermometer/hygrometer. I wanted to see what the typical summer conditions were like, so I could better match my own conditions for my orchid collection. The photo below was taken in mid-July:

    {{gwi:156584}}

    So why is this important? It illustrates the effect of a giant mass on a microclimate. The surrounding Everglades on this day were perhaps at 90F, humidity about 85%. The shade from the canopy of trees accounts for the lower temperature in the swamp (not the water), which also traps the evaporation from the water increasing the relative humidity. But that's not really why the orchids can live in there and nowhere else. The importance of the water comes into play in the wintertime, when the water moderates the cool nighttime temps. The heat rising from the water, or mud later in the winter as things dry out, keeps temps from falling below 50F or so -- even though the open Everglades may drop to the low 40s or even high 30s. The canopy helps trap some of the warmth and keeps it from escaping so quickly. There's so much mass there that it keeps pouring out 50+ degree temps all night long.

    From WWII until the late 1950's or so the swamp was heavily culled of its giant cypress trees, and when they came crashing down it brought a lot of the canopy with it. Conditions began changing in there as a result, and only fairly recently have they begun improving.

    So, the mass has more benefit on the heat side of the equation, because heat rises. The water in the swamp isn't really "solar" mass, because very little sunlight actually reaches the water to warm it. But the overwhelming storage capacity of the water, even at a relatively low temperatures, is more than enough to maintain the subtopical environment through the tough winter months. I used to know what the water temp is, but I've forgotten. Probably around 65F in the summer, 58F in the winter.

    This doesn't perfectly match what a solar mass design would be in a greenhouse, of course, but maybe the principles involved are helpful.

    I'm starting to collect data in my backyard for baseline figures to use when I build a solar mass system in the future. I have a temperature probe in the dirt floor of my greenhouse, and it measures 58F after a cold night when temps in the gh dip into the 40s. It will climb to 60-61F during the day, when temps inside rocket into the high 80s/low 90s. Even with 110 square feet of flooring, there is no apparent effect on keeping the inside temps cooler.

    For those of you who are interested in seeing the results of an active solar mass greenhouse in action, check out the link below.

    Bruce C.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    orchiddude,

    "dont think all the thermal mass in the world is going to heat a greenhouse. Well, it might a few degrees. Nobodys doing it. I wish there was someone that had a system set up in a zone 7 or lower that showed this working."

    You are joking right?

    Do you want me to create a website showing how it is set up, to detail the how to, and how not to, the what to plant when and what not to even attempt?

    O.K. I will bite!

    I just bought a load of seeds today. My structure is not set up yet so this is a perfect time to start a photo journal of the attempt to produce 10,000 plus bedding plants and veggie starts in a 24x14 greenhouse in zone 4b by using as little extra energy (electricity) as I can.

    As you may know from the Christmas wish list thread I have my Kill-a-watt all set up and ready. Hey it is about time I actually contributed something to this forum.

    This is going to be fun......

    bcfromfl,

    I went out into the swamp that is my back yard, 15 inches of snow cover is slowly dissapearing. I would like to measure the soil temp at 8 inches but I only have a drill bit 6in long for my cordless drill!!!

    :)

  • orchiddude
    14 years ago
    last modified: 6 years ago

    The problem I see with us greenhousers is it depends on what temperature you are trying to get at night. You might want 33°F and I might want 55°F. I have never seen anyone using barrels of water to create a 55°F night temp. I understand that it might help with 2 or 3 degrees here or there but for the room it takes to create that heat, its almost not worth it. You have to have a few tons of water plus a place to keep it. I hear it, I just dont see my friend down the road doing it, I dont see the guy 50 miles away doing it, and I dont see the huge commercial guy 3 states over doing it. :-(

    Now lets flip over to the other hand, so to speak. I do know that something is happening when I pack my greenhouse full of plants, and I mean pack, with just a little room to walk. I use less gas and seem to keep more heat just from the plants being in there. I am guessing the plants holding the heat is the cause of this, but for me to fill the place up with water, I just dont see the return on investment.

    PS....the best way to save gas is with the help of mother nature. This winter, I only spent $200 total. Thats down $100 from last year. Mother nature was on my side this year.

    Spring is here in AL.....I am pulling cycads out today.

  • stanc
    14 years ago
    last modified: 6 years ago

    Hi,
    I just set-up a weather guard green house here in zone5.
    You can find them at DGI worldwide.
    As you know heating at night can be a problem.
    I have a min/max guage in it and it got to 101 yesterday and about 28 last night due to heat loss.
    What type of heat sink are you using?
    How big is it ?
    The greenhouse is 8x8x6.5 high
    The area is about 232
    I just want to heat it to about 50 at night.
    So what would be best for a heat sink method.

    Thanks
    Stanc

  • orchiddude
    14 years ago
    last modified: 6 years ago

    It aint going to happen Stanc....if you find a way, to heat free to 50 at night in your zone, let me know. We will patent it and make MILLIONS

  • birdwidow
    14 years ago
    last modified: 6 years ago

    I still believe the real factor is heat vs. space, costs, and the temps we each want to maintain for our own purposes, which differ.

    Some of us are mostly into early spring sowing and shut down their GH's for winter and others keep year round hothouses. Factor in the different GH's and climates, and it's obvious that one size or heat system, passive or otherwise- does NOT fit all.

    My GH is to be as much a tropical fish room as for seedlings and plants, so I either need to heat it to a steady, reliable 78 deg., or use a lot of extra electric tank heaters and then, if an ice storm took down the electric lines in the middle of the night, I'd be scrambling to get kero and/or propane heaters going. No thanks.

    I have a small portable generator that will easily power the furnace fan, the air pumps and the filters, and as long as the Nat. Gas is running, and it never fails, my GH, and thereby the tank water will stay warm, filtered and the fish will get plenty of air, and that's all they really need.

    I'm trapped by one of the most voracious electric utility companies in the world and given a choice between bills from Comm. Ed or NiGas, I'll take the gas. So for me, heavy insulation and a very large, high effeciency Nat. Gas furnace was the most sensible plan.

    But works for me in the Chicago area, for MY purposes may well not for someone else in another climate, who uses their GH for a different purpose.

  • bluepine
    14 years ago
    last modified: 6 years ago

    I've been toying with the idea of using the ground underneath the greenhouse as the heatsink and using active solar heating. This isn't new but in the past year or so it has at least become "thinkable" for my area using evacuated solar hot water panels and pumping the liquid through tubing under the greenhouse. Because of some new players in the market the price for a 30 tube evac solar array has dropped from around $3000 to $700 in just the past couple years. For my latitude I would get an everage of 20,000 BTU's/day with 30 tubes in the middle of december.

  • bcfromfl
    Original Author
    14 years ago
    last modified: 6 years ago

    bluepine -- you might already know this, but you can't use solar mass as you planned both ways. Either you insulate the excavation to store heat, or you improvise a method to transfer heat to the ground (not insulated) so that it can be drawn away.

    When you use the term, "heat sink", that implies a cooling device. But what you are describing is only for heat storage.

    BTW, you can get the same BTUs out of a solar pool heater for about $150-$200. Or, you can make one yourself using the same basic design for around half that.

    Bruce C.

  • garyfla_gw
    14 years ago
    last modified: 6 years ago

    Bruce
    Im in Palm Beach county so heat is far more of a problem than in your area even in winter.
    I decided to go with an open lattice for the roof and heat the pools during cold spells. Have a total of 1500 gallons of water that is heated with standard aquarium heaters. Guessed at 1500 watts in 3 locations and they had no problem keeping the water at 70 though the low was only 37 for two hours.The air even directly above the large pool dropped into the fifties though.
    I suspect the system will not work for extended cold spells or lower temps but will provide a frost proof area. At least I hope so. lol Have WAY too much stuff to bring into the house for that storm of the century that happens every 5 years or so. gary

  • birdwidow
    14 years ago
    last modified: 6 years ago

    Not a critique- just an observation from an old lady who is painfully aware of how quickly a spring again becomes winter, but reading some of these posts reminds me of tech geeks who spend so much time fiddling with and fine tuning the controls on their sound systems, they have little or no time to enjoy the music.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    birdwidow,

    Interesting observation.

    ""tech geeks who spend so much time fiddling with and fine tuning the controls on their sound systems, they have little or no time to enjoy the music."

    Have you ever thought that some of us get a great deal of pleasure from messing with wood, plastic, trash cans, thermostats, pumps, fans, timers, wood stoves, windmills, truck batteries, you name it, we mess with it!

    The music is when we wake up in the morning, look at our fancy min/max thermostats and see that the outside low was 22F and the greenhouse low was 34F and we yell.....

    YES! YESS!! YESSS!!! (and high five the cat because only the cat understands these things)

    The fact there are plants in there is just a cover, otherwise people think we are crazy.

    Me and the cat know the greenhouse is just somewhere to sit in the sun and play with pumps and timers and fans and other electrical stuff......

    The plants are our cover story.

    :)

    P.S the cat is not really interested in the fact the temperature did not get below freezing. Catnip plants are very frost hardy.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    Back on the topic.

    ""- Some discuss the merits of a heat sink/thermal mass with only a few gallons of water. Even in a small gh, any significant benefit would require hundreds, maybe thousands of gallons.""

    The smallest amount of water in the smallest greenhouse that actually works is the "Wall O' Water" it does the job.

    ""- Some of the containers cited are plastic, which wont transfer thermal differentials efficiently. Even a steel drum would be maddeningly slow.""

    In a passive system the heat flow both ways is really slow no mater what the container is made from.

    ""- Thermal mass heat for nighttime augmentation seems to be discussed in the same breath with heat sinks for daytime cooling.""

    Yes you are correct. If you are activly cooling during the day and activly heating at night then adding thermal mass will reduce your energy costs both night and day.

    ""a significant mass of water can be heated fairly quickly with just a few hours of sunlight. Lets say that the ambient temperature of a barrel of water is 60F, and the sunlight hits the barrel, heating the outside to (at least) 85 or 90F. Therefore, theres a differential of 25 or 30F with tons of heat calories. The heat stored will radiate out slowly as the gh temps drop overnight""

    The tragic fact is that in a passive system no where near the amount of potential heat energy that enters a greenhouse on a sunny day heats the barrels.

    ""Now, lets say that that same barrel has cooled down to 60 through the night hours, and the greenhouse starts heating up again in the morning.""

    Because of the differential and the slow transfer of heat then if the water is at 60F in the morning your plants survived. (don't ask... If my water temp at the top of my trash cans is 55F at sunset then I can get through a night outside of 26F. If the trashcans are not 55F or the expected low is lower than 26F then Mr Heater gets turned on)

    ""The real problem, I think, is that the thermal storage device would heat up close to its maximum temperature by, say, noon or so, just when the benefit of a thermal sink is needed. With a water temp of around 75-80F by that time, and a gh temp of possibly a few degrees more than that, there is not as much temperature differential to benefit the cooling side of things. Youd need a water temperature of near freezing to be enough to overwhelm the heating capacity of sunlight inside the gh.""

    You have defined the problem exactly!

    Once your heat sink is hot then your fans kick in. BUT! you have stored several hundred/thousands of BTUs in the heatsink that may save you BTUs that night.

    Using a thermal mass for cooling is not what a greenhouse is. Using a thermal mass for extra heat at night is what it is all about. Store heat during the day and release it at night. You can do that without moving the thermal mass.

    As you stated, ""Youd need a water temperature of near freezing to be enough to overwhelm the heating capacity of sunlight inside the gh."" Yes, that is correct. During the night you have to cool your thermal mass as cold as you can to get ready for the next day. However you cannot do that in the greenhouse. There are plants in there....

    I see us yelling "thermal mass" and "fans" at the posters in here zone 6 and below who have a daytime heat problem and are at this time of year probably heating at night.

    I do not think I ever advised anyone in FL to get trash cans.....

  • bcfromfl
    Original Author
    14 years ago
    last modified: 6 years ago

    I'm beginning to wonder if most of the comments with respect to "heat sinks" throughout the rest of the forum were actually referring to solar mass, because many of the (good and welcome!) thoughts here seem to be about heating, and not cooling. Heating is important to me too, as this part of Florida gets surprisingly cold, fairly often. I'm not even as warm as South Carolina! I've mentioned heating and solar mass in this thread too, but only as a means to help explain the differences and factors influencing each.

    But by far the biggest issue with me, and with most others, is cooling, and that's what I'm exploring with my design. I'm fairly certain at this point that I'm going to have to build an air chiller, but I hope some will chime in with other possibilities that have measurable results.

    Speaking strictly passive systems, the biggest difference between heating and cooling is the way the storage devices become charged, and then release their charge. It's not as cut-and-dried as this, as both types of heat transfer come into play with heating and cooling. But basically, solar mass for heating is overwhelmingly influenced by radiant energy, and cooling mostly convection. Radiant is much, much more efficient because of the penetrating effect of the heat while charging (exciting molecules in three dimensions), and the fact that the radiant heat can discharge over a moderate distance by heating objects within proximity without traveling across the open air space. Passive cooling doesn't work this way -- it depends upon direct contact with the storage device (a two-dimensional surface) by air.

    The only thing I can think of where the "heat sink" can come into play in a system also utilizing solar mass, is absorbing calories that otherwise would have heated the airspace inside the greenhouse. But as I mentioned before, I think the heating of a greenhouse in the sun happens so quickly, that any benefit of this principle would be insignificant, occurs at the wrong time of day (morning), and is quickly overwhelmed by the heat gain.

    If the moderating effect of a storage mass was perfect, the temperature ranges of the storage device(s) would remain constant. This is not the case. The average ambient temperature will quickly rise above the starting point, in just a matter of days even in a large system, proving that the passive radiant energy is more efficient, and influential, than convection. (Active convection is more powerful than active radiant, but more energy-intensive.) This helps nighttime heating, but makes daytime cooling even less significant.

    I hope you all will keep posting, but I'd like to hear more about the cooling-side of the equation!

    Thanks!

    Bruce C.

  • bcfromfl
    Original Author
    14 years ago
    last modified: 6 years ago

    After reading and re-reading several threads on the forum, as well as the posts here, I think it's become apparent that some terms are being interchanged. That's lead to some confusion here in this thread specifically.

    "Heat sink" means heat is drawn out of, or away from, an object or air space. For those with experience in electronics, a heat sink is attached to components that are hot, and looks like metal fins.

    "Solar mass" or "thermal mass" refers to a device that stores solar heat, for augmenting/replacing conventional heating sources. The device can be an object or defined pit used to modify temperatures elsewhere, or a tank within the confines of the greenhouse. Thermal mass can be water, earth, rock, metal, or other material with storage capabilities.

    "Passive" is a system that operates simply with no mechanical influence. "Active" involves pumps, fans, etc.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    bcfromfl,

    Heat sinks, yes a heat sink in electronics is a way to move heat from a very small area, the "chip" (in some cases a very small area that may be a few square millemeters.) to the housing of the device ( a relativly bigger thermal mass) which is connected to an even biger external mass ( the "heat sink") which is then cooled by a fan, radiation, convection into the case the device is in and thereby the heat is transferred to the room the equipment is in. Then the room is designed to remove heat, maybe to the rest of the building. or the room is so large that the extra heat that the device produces is just lost in the vastness of that area.

    In electronics the idea is move the heat away and get rid of it. That is an electronic heat sink.

    OK, greenhouses.

    We need light, well the plants need the light, but sometimes we don't need the heat that comes with the light.

    In that case the best thing we can do is make sure that all the light that does not fall on the plant is reflected back out. Paint everything in there white. Make sure everything in there reflects back short wave IR and does not convert it to long wave IR.

    Fans are good, with an all reflective greenhouse interior (and I mean benches and pots etc) enough airflow should enable you to keep your greenhouse just a few degrees above the outside air temp on even the sunniest day.

    You want lower than the outside air temp? Then you start to need things like swamp coolers and wet walls and misters.

    Active cooling is a lot harder than passive heating in my opinion. In a situation where we need all the heat we can get then the large thermal mass in the same place as we need the heat stored is great. In a situation where you need cooling you need to dump the heat somewhere.

    In my case I use the greenhouse for plants 3-4 months a year. Heating would be too expensive and cooling it would be worse.

    If I got everything sorted out I could cool the greenhouse using the water I irrigate with during the summer, but as there is nothing in it then why bother.

    Next project is to use the irrigation water (52F) to cool my home before it is sent to the field.

  • mokevinb
    14 years ago
    last modified: 6 years ago

    Chris brought up an idea that might work, but it might have to be an active or partially active system. It is, you can use an outside means of a heat sink/solar mass to cool and heat your GH. What I have in mind is what we use here in the midwest to heat and cool our homes. By now most of you have heard about geothermal heat pumps. The principal is simple. You use the ambient temp of the soil to both heat and cool your home! What you would do is run a series of tubes beneath the flooring of your GH to act as both a source of radiating heat and removing heat. The tubes are filled with water which is pumped through the system. (Here in the midwest we have to either use an ethylene glycol solution or a saline solution to prevent winter freeze up.) In turn the solution is ran out into another series of tubes buried below the frost/heat level of your soil. What will now happen is this, in the winter your system brings warmer water into the GH from the underground system and in the summer it removes heat from the GH and disperses back into the ground. To add even more capacity you might want to add a chilling tower for the summer months, and some sort of solar heating system for the winter months. Just my thoughts. I know; half baked, Rube Goldberg, definitely water on the brain!

  • birdwidow
    14 years ago
    last modified: 6 years ago

    Moke: Throughout history, so-called, half-baked notions have resulted in all of the great innovations in design, engineering, and construction. Think about the unknown Roman engineer, who first IMAGINED an arch. Then, the brilliant engineer who designed the Pantheon, 1,300 years before the first European ever thought to figure out WHY that seemingly "unsupported" dome hadn't collapsed.

    Think about what Roebling IMAGINED. (Better yet, walk it, and let its glory feed your soul) It too, "couldn't be done". Nothing new and innovative is ever CREATED until someone IMAGINES it.

    But the Rube Goldberg appellation is applied to designs that are over complicated, and until technology will allow a simple and financially feasible installation of geothermal heating and cooling for low to average budget garden hobby GH's, I think we will have no choice but to rely on foundation and wall insulation, and furnaces and/or water storage in winter, and vents, fans, misters and shade cloths in summer.

    But you just keep the "half-baked" notions coming. I have posted more than a few myself, and not all have turned out so "half" after all. LOL!

  • mokevinb
    14 years ago
    last modified: 6 years ago

    Very true Birdwidow, but Bruce is down there in FL where his demands for heating are a lot less than ours, but the delicate tropicals he might be growing are very cold sensitive. I know there are many orchid growers here in the midwest, and upper midwest, but most of them have resigned themselves to having adequate supplemental heating to keep their units properly warmed. For them the issue of geothermal mass and heat sinks are pretty much superflous falderol. Something they might think about, but which might not be as effective as it could be in a warmer environ. I agree with you that vents, misters, heaters, and insulation are much more practical solutions. Although I wonder if it would be possible for a GH to be built, in the midwest or upper midwest, that would be practically energy free! Minimumal amounts of input with maximum amount of gain through use of solar, geothermal, thermal mass and insulation.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    mokevinb,

    I had to look up "falderol"

    OUCH!!!!

    ""Noun: falderol

    1. Ornamental objects of no great value
      - folderal [non-standard], frill, gimcrackery, gimcrack, nonsense, trumpery [archaic], falderal""

    I think some people on here think the solar, thermal mass, heatsink stuff cannot solve their heating/cooling problems so they discard it totally.

    The idea is, you do all the solar, heatsink, thermal mass stuff to your greenhouse and then supplement it with energy to heat and cool it to your specifications.

    ""I agree with you that vents, misters, heaters, and insulation are much more practical solutions""

    That is fine if energy costs are no problem. Throw energy at your greenhouse at night to warm it, then as soon as the sun comes up throw more energy at it to cool it.

    If you are using energy at night to heat your greenhouse and during the day you are using energy to cool it then there is room for improvement.

    ""Although I wonder if it would be possible for a GH to be built, in the midwest or upper midwest, that would be practically energy free! Minimumal amounts of input with maximum amount of gain through use of solar, geothermal, thermal mass and insulation.""

    Yep! I have one, but I don't grow orchids. I just grow what my structure is capable of growing at certain times of the year.

    Do what I do as regards insulation, solar gain, thermal storage and all the rest of the geeky no heater stuff... then add a heater.

    Your heating costs should be lower.

  • birdwidow
    14 years ago
    last modified: 6 years ago

    Sometimes, indeed, very often; one must look back, to see forward.

    Question: What did they do to heat/cool production GH's before we had electic heaters or gas furnaces, or even electric fans? Answer: They dug GH foundations down to below the frost line, lined the walls with brick and/or stones and put glass roofs over them; often whitewashed in summer.

    Now imagine that same basic design concept, utilizing the latest in insulating materials, thermopane glass and/or polycarb and while you are at it, think about how the Romans heated their baths.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    birdwidow,

    None of all this discussion can really help those of us who built a greenhouse and are trying to spend as little hard earned money to run it as we can.

    What this last five years or so of owning one, and researching all I can about greenhouses in cold climates, has made obvious to me is,

    If you build a greenhouse you incorporate all the energy saving designs into the build. It is not really cost effective to retrofit many energy saving ideas into an existing structure.

    The fact that energy is so cheap (unreasonably so) means the added cost to an existing structure never pays you back.

    Anyone on here building a new greenhouse can take advantage of our discussions.

    My next greenhouse will not look like, or waste energy, the same way as the thing in the back yard I have now does.

    Attention potential new greenhouse owners!

    Look how they did it 100yrs ago, then adapt what they managed to do then with the materials we have today.

    (if you are fluent in several dialects of Chinese then look what they are doing now with the same things they have had available for a couple of thousand years.)

    :)

  • mokevinb
    14 years ago
    last modified: 6 years ago

    Chris; you, Birdwidow and I are all three on the same page! There are things that can and should be done in the design and construction stages that can reduce energy consumption and make your GH, if not energy free, at least energy conservative. The big problem I see are so many people want to have a GH, and they want it so much, they send that order off to some company for a one size fits all GH which is not suitable for thier climate or that is little more than just a plastic hut with minimual R value or solar gain. That is where knowing what will work in your zone is to your advantage.
    As you, I and Birdwidow all three live here in the midwest, we have to agree that R value is very important. If our structure is unable to retain the heat it gains, then we must supplement the heat at certain times of the day and year, or we must overcome the build up of it at others. This is where things like thermalpane glass, low E films, and tightness of the structure all become critical. Birdwidow mentions the use of earth contact and thermal mass. These two crucial, and often overlooked solutions, are not incorporated in many modern designs. But dollar for dollar will give more back than what they will cost in the construction phase. Also, an individual building his own unit, can use recycled materials in this area of construction which will further reduce his cost and help the environment even more.
    As to the issue of use. Personally I don't just want to have a GH to start plants in, I want it to perform several functions all year long. When you consider the amount of time and money it takes to construct and maintain a GH, I can not see any advantage to putting one up if you only plan on using it for a few weeks in the late winter and a few weeks in the fall. The Hobbiest GH Gardner must realize that if he isn't willing to be energy conservative, than he might as well just buy his plants from a commercial source, and figure that he is helping the environment more by not building a GH that is a plastic hut with a mega btu heater in it.

  • PonderGuy
    14 years ago
    last modified: 6 years ago

    Here's a link to an greenhouse description with some interesting ideas.

  • PRO
    Nell Jean
    14 years ago
    last modified: 6 years ago

    Simply put, I think using whatever Rube Goldberg inventions we can devise makes the same sense as putting on a sweater and socks when it's cold in the house rather than increasing the thermostat several degrees.

    I don't care to get into the laws of physics or whether the methods that work for me in the climate in which I live are universal, but there are sensible solutions for using inexpensive methods to conserve energy. On the other hand, I have no illusions that barrels of water will keep a constant GH temperature the year around.

    My needs are not your needs. My little HFGH is just what I'd hoped for.

  • chris_in_iowa
    14 years ago
    last modified: 6 years ago

    foxesearth,

    I have no illusions that barrels of water will keep a constant GH temperature the year around either.

    That is why I do not grow plants in there all year. As to ""putting on a sweater and socks when it's cold in the house "" you are correct, (in fact that is what I do. in winter the thermostat is set at 63F and in summer it is set at 80ish, in my home) Talking plant wise some plants can put on sweaters and socks when the temps are below what they would like, to see it in action see the winter sowing forum. Some plants cannot, tomatoes, peppers, etc.

    You work with what you have, I know how my structure will perform and I use it accordingly. I also know how much it will cost me and the equipment I need to maintain a constant 65F in there all year long. I don't do it because to be honest growing plants and nurturing them for years is not what I get pleasure from.

    My greenhouse produces bedding plants and veggie starts, that is its only growing function. Also in the fall I do use it to ripen tomatoes and peppers that I deliberatly plant in containers so I can put them in the greenhouse in the fall.

    Traditionally on the night of thanksgiving I leave the greenhouse door open to kill off eveything left in there. Otherwise I would be stuck with the dilema, do I heat it for a week, then a week more?

    Which brings me around to the post from mokevinb,

    ""As to the issue of use. Personally I don't just want to have a GH to start plants in, I want it to perform several functions all year long. When you consider the amount of time and money it takes to construct and maintain a GH, I can not see any advantage to putting one up if you only plan on using it for a few weeks in the late winter and a few weeks in the fall.""

    I totally agree! I use mine all year, the fact that there is green things in there for 8-10 weeks in late winter/spring and again for 6 weeks in the fall does not mean I am not using it all year.

    On a sunny day in mid winter there is no better place to sit and have a morning coffee, read a book, have lunch, play on the net, (ummm yes my greenhouse has wireless internet access capability...) strip down and re-build a tiller engine. No plants in there though.

    In summer no plants in there. TOO HOT!. But on a rainy day in July what better place to go to have coffee, read a book, have lunch...

    My structure is in use all the time. But not growing plants.

  • birdwidow
    14 years ago
    last modified: 6 years ago

    It would seem that we are all on the same page, just reading slightly different books. I can't imagine ever using more than 25% of my GH for plants. The rest will be fish tanks. We each have our own interests.

  • mokevinb
    14 years ago
    last modified: 6 years ago

    For me, my perfect use will be as Aquaponics, but the cost of year round conventional heating is not economically feasible or desireable. If I am raising fish for my use and a small direct marketing plan, as well as veggies via hydroponics primarily for direct marketing, then just doing it for 6 to 9 months of the year is not economically feasible. Depending upon the type of fish and produce I choose, I will need to make a year round committment. I know that Birdwidow has the same committment to her tropical fish. She needs to have a space capable of year round use, with reasonable costs of maintainance. Your non-season use of the GH as work and recreational space is great! Your facility is not just sitting there waiting for the next period of activity, like an expensive sleeping giant. You are getting both practical use and enjoyment from it, but for those wishing to use the facility on a year round basis, as a GH, they must use design elements and systems that will provide them with the greatest returns for the most economical costs.

    Today I went to my local lumberyard to check on the cost of installing low E glass in the wooden sashes I removed from my rental house during a recent remodel. The cost for install the glass in a double hung 32 x 53 window will cost me a little under $100 per unit! At that price I can get new windows for just a little more, and will not have to build window frames for the sash and have the additional cost of the materials for them! From what I learned I can go with double strength regular glass for about half the price of the low E, but I will not be getting the benefit of lowering the transmission of energy through the glass. So once again, the economic side of cost versus returns is illustrated.

  • birdwidow
    14 years ago
    last modified: 6 years ago

    Moke: Just a thought, but before you believe you have found your answer, look deeper. It's amazing what's out there and you can do all your looking, and get even more ideas with some websearching.

    For instance: Have you priced solid vinyl framed, thermo windows lately? For a GH, vinyl is better and a lot cheaper than wood. Think too, about how few windows in a GH really have to be operable. Per sq. ft., non-operable windows are the cheapest. Also, awning windows are cheaper and better suited for use as side vents than single or double hung, and there too, cheaper.

    If you don't mind hand operation, at least on the side windows (i.e. side vents), you could leave in the crank mechanisims on awning windows and they will come with screens; yet another "extra" for a GH. For short side vents placed low, to admit cooler air from the ground, the awning style windows sold for basements are hard to beat for price.

    Or, if you really want to go with single pane and make storm panels for them, which, if you have the time, isn't all that big a deal: for the cheapest non-operable windows of all, check out barn sashes, now also available in vinyl.

  • oakhill (zone 9A, Calif.)
    14 years ago
    last modified: 6 years ago

    "I have never seen anyone use barrels of water to create a 55 deg. F night temp." While supplying that amount of heat is probably not feasible, it is possible to provide significant amounts of heat to reduce gas/propane/electricity use using water filled containers in the greenhouse. In my 3 year old structure, the solar heated water provides several hours of heat each night. Over a month and full winter periods, this results in significant energy savings.

    A sample calculation:

    1 BTU = 1 lb. water raised 1o F.
    60 o F to 80 o F solar heat gain = 20 BTU per lb. water

    10- five gallon water bottles = 400 lbs. X 20 = 8000 BTU
    50- five gallon water bottles = 2000 lbs. X 20 = 40,000 BTU

    This absorbed heat is then available for release at night. Using the example of 40,000 BTU, this is equivalent to 40 cubic feet of natural gas or 0.43 gallons of propane per night.

  • mokevinb
    14 years ago
    last modified: 6 years ago

    Birdwidow, You are right, that was what I am talking about doing. Using fixed units for the majority of the glazing and operable ones to facilitate ventilation.

    I've pretty much given up on the idea of using the old sash for anything other than windows in out buildings that I want to build. I have decided that I do want to use the old style partial earth contact design, with glazing from the sill, up. I want to use insulation against the foundation from the ground to frostline, and am going to install tubing beneath the floor for heating. As to the heat source I am considering a combination of a ground loop geothermal system with a solar collector. I know that in the winter our coldest temps come at night so I am planning on installing a superinsulated water storage tank and propane fired hot water heater to raise the temp when necessary. In the summer, I am thinking that I might be able to use the same system to draw heat away from the GH, just have the loop bypassed to the solar collectors and the hot water heater turned off. If I were to go with Aquaponics in this GH I might need the heated water year round for my fish, as some breeds (Tilapia and others) require higher temps to encourage growth.

  • nathanhurst
    14 years ago
    last modified: 6 years ago

    I have been using a 4tonne cooling mass all summer and have computer collected data that shows that I am cooling the greenhouse during the day and heating it at night with a COP of around 30 (1W of control electricity gives 30W of thermal energy). People are always amazed when they walk into my greenhouse on a hot summer's day and the temperature is down 10C.

    The theory that everything reaches a steady state is correct, except it forgets the time constant involved. It takes half an hour of peak sun to heat my water 1C, an hour with shadecloth. 8 hours of effective sun heats my water 8C, and releases 130kbtu as it cools again that night.

    Another effect is the evaporative cooling of the water itself - the water sheds heat at a tremendous rate once it exceeds the wetbulb temperature significantly. This is effectively evaporative cooling, but you can separate the evaporation from the cooling.

    (My cooling mass also holds fish, and warming the water makes the fish grow faster)

  • mokevinb
    14 years ago
    last modified: 6 years ago

    Thanks for the information Nathan. I know your weather down under has a tendenancy towards being much higher than what I have here. In the winter though, it is not uncommon for us to have night time lows in the negative degrees.

  • paradoxg
    14 years ago
    last modified: 6 years ago

    so this is what have here on long island, a 20x48 double poly covered hoophouse, on any given sunny day, it will get well over 100 degrees inside, even if its below freezing outside, even with a 24 inch fan, its quite difficult to keep the GH withhin 10 degrees of outside, the way around that Im told from a friend with a GH building is shadecloth, as it stands, on any given morning, by about 10:30 or 11am, the fan kicks on, and starts exhausting excess heat, (I have it set at 80 degrees)

    I have a coal stoker stove for heat that idles all day long, and starts kickin up after the sun goes down, Im told the max the stove puts out is 90k BTU, tonight its mild out, probably in the 50s, the GH is 65, however much coal I burn, times the BTU value is the heat demand for that given night. once the GH temp starts falling below 65 degrees, Im buring coal to keep it at 65, the longer it takes to fall below that temp, the further into the night I can go before Im paying to heat the house.

    on the flip side, the longer into the morning I can go before the fans kick on, the longer I can go before Im paying to cool it,

    the actual numbers dont really matter, fact is, I pay to heat it at night, I pay to cool it during the day, the longer into the night or day I can go before I have to do one or the other, the less I spend to do it, so with a heat-bank, I could absorb some of the surplus heat during the day, which would release its heat value as the GH cooled off in the evening at a slower rate.

    my plan is to build a heat storage container running the length of the greenhouse, exact numbers I dont know, but Im figuring on laying down on the floor 2 inch styro insulation, on top of which Ill frame a box maybe 16 inches high, running the length of the GH, inside that box, Ill lay 4 inch ducting, and some sort of poly pipe for water to flow through, and fill it in with sand. on top of which, I will place my benchtops, so maybe an 8ft wide box running 38 feet in the center of the greenhouse. in the morning, as the GH starts warming up, Ill use duct fans to blow the warm GH air through the pipes in the insulated sand bed, this small fan will continue doing that for as long as the GH temp is warmer than the sand temp, the exhaust fans will continue to work as they already do. the water loop will be plumbed through the coal stove to extract the stray heat created during the day, and store in the sand as well.
    in the evening, this heat bank wll start to radiate heat as the surrounding temp drops below the temp of the bank, although it probably wont provide anywhere near the heat required, it will provide an additional heat source, as well as a means of harvesting some of that exhausted heat from the daytime sun.

    I have no idea how much heat I can store that way, or how long it will take for the temp of the heatbank to drop to the ambient temp, what I do know is that it has to be of some help, so Im gonna give it a shot, if nothing else, Ill have an elevated structure to support my benchtops, heres hoping.

  • nathanhurst
    14 years ago
    last modified: 6 years ago

    It doesn't matter how cold or hot it gets, the important point is that I can offset my heating and cooling costs by stockpiling warmth. Does your greenhouse get too hot in the day and too cold at night? Then an active thermal mass can help. I'm transfering 5kW of heat with my small swing in temperature. Heat transfer is roughly linear with change in temperature, so a bigger swing means things are more efficient. My greenhouse is also not built to the high spec required in colder climates, I'm sure that with better air seals (no inch wide gaps around windows) my system would work even better. By using an active system, rather than just lots of milk containers, the heat transfer only occurs when it is helpful, allowing the lower thermal mass of the greenhouse to be heated directly by a burner and saving considerable energy(money).

    A quick refresher: 1kg of water, changing temperature by 1 degree C involves 1kcal (4.2kJ) of energy. Multiply up to match your needs. The bigger the temperature swing, the less water for a given energy store.

    If you get the issue 100 of ReNew (http://www.ata.org.au/publications?page_id=15) there is an article by yours truly on the results of using a car radiator for moving heat in and out of an aquaponics fish tank. I think the results speak for themselves.

    My system is so effective that I can grow tropical plants without any heater backup, even though night time temps drop to 0C. And it uses but a whiff of energy to do it. Furthermore, the idea obviously scales bigger, can be implemented by anyone and can be built entirely from waste parts.

    Paradoxg: Water is a much better storage medium than sand.

  • juliyaanthony
    11 years ago
    last modified: 6 years ago

    I really want this information to set up my new home furniture.It will help me solve my issue.

  • KellyBell
    8 years ago
    last modified: 6 years ago

    nathanhurst what zone are you in and does this make your system more efficient?....why hanv'nt i seen more people talk about digging down a little 2-4 ft to keep the GH cooler.. animals do this... Is there anyone out there with statistics on their experience with this? also has anyone tried PCMs?.. of either a salt or paraffin?...this is being studied and marketed as we speak I know here has got to be a smart person out there that has figured this out( and is willing to share this info rather than go into business and sell the info?

  • paradoxpoet
    6 years ago
    last modified: 6 years ago

    Man that's a lot to read. New here and just wanted to say; you beat that horse way dead. I have an IBC tote aquaponics system (thanks tote ponics) I started this year and I wanted to keep it going through the winter with as little cost as possible. So I started a geodesic frame to cover it (thanks desert domes). There is a vast amount of info on all these systems seperatly on line but I see two of you in here are running the same types of aquaponics in greenhouse systems that I am starting. I assume that the amount of water in our systems will itself be all the heatsink we need. If I am wrong let me know. My frame is from 10' EMT and there is an eight foot ladder in the frame also. I am going to use 6 mil plastic from Home Depot this winter and upgrade next year when the wife leaves me alone about money. Let me know what you think.

  • cole_robbie
    6 years ago
    last modified: 6 years ago

    You're throwing away money if you don't buy actual greenhouse plastic with UV inhibitors.