polcat_gw

Raised floor

polcat
11 years ago

IÂm new here and just started construction of 12x10 HFGH. IÂm working on a solar set-up because this is out in the wilderness with no power. I built a raised floor (about 8Â off the insulated ground and built beams to hold pavers. I put in 4x4 posts and the beams rest on these to support the weight of the pavers. IÂm installing a trombe wall with 3" x 6Â pipes that will collect and blow hot air under the pavers to warm them during the day and release heat at night.

Can anyone think of reasons this might not work or have suggestion on how to improve it?

IÂll try to attach pictures...thanks.

http://photos.gardenweb.com/garden/galleries/2009/11/raised_greenhouse_floor.html

Here is a link that might be useful: Raised floor

Comments (21)

  • polcat
    Original Author
    11 years ago

    Here is the trombe wall under construction

    Here is a link that might be useful: Solar wall

  • idaho_gardener
    11 years ago

    You might think about getting some plastic barrels to hold water. And maybe a system that uses convection to heat the water using sunlight.

    Which wilderness are you living in?

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  • polcat
    Original Author
    11 years ago

    Idaho gardener, yes I was thinking about water. The barrels would probably be too heavy for the raised floor. But I have about 8 inches clear under the floor I thought maybe some 6 inch pipe running the entire width every 12 inches filled with water. That would be about 100 feet of pipe or 146 gallons...might work?

    My wilderness is Northern Arizona, about 7500' elevation. Last week when I finally got the GH together the temp inside the GH was 27 in the morning and 118(f) by noon!! Lots of work to do:-)

  • hex2006
    11 years ago

    Hi Polcat
    Thats a good amount of temperature swing :)
    I think you`ll need more mass than just the pavers to tame it. The 3" tubes may become a limiting factor in the transfer of heat unless you have plenty of fan power to overcome the resistance. It would be much easier on the fan if you could take air directly from the greenhouse :)

  • eaglesgarden
    11 years ago

    I think you might want to look into some phase change materials. These take up less space than water barrels and would add much less weight as well. More importantly though, they can "soak up" more heat energy faster than the water barrels would be capable of, and can store vast amounts of energy compared to merely warming water.

    Phase change is the way to go. (There is also some great information out there regarding Subterranean Heating and Cooling Systems (SHCS), but I think it is too late in the process for you to do that.)

  • hex2006
    11 years ago

    If the pavers are resting on the timber joists, you could excavate between the joists and replace the pavers afterwards.
    You`d have to uproot the insulation in the process but it would perform much better than an 8" airspace with 20 cubic feet of pavers as mass and you gain the advantage of phase change.
    By my reckoning the pavers heat capacity equates to ~540 btu/degF,the excavation may add another ~6624 btu/degF capacity for total of ~7164 btu/degF.
    In broad terms it could absorb or release 2000w of heat for each deg F gained or lost from the mass. For comparison, the pavers alone would account for 158w of the 2000w total :)

  • polcat
    Original Author
    11 years ago

    Hi Hex2006,

    Thanks for the input. Agreed the pavers wonÂt have enough mass to tame the temp swing. IÂll probably add a water reservoir (6" pipes) under the pavers also. Still, I see supplemental heating/cooling & exhaust in my future:)

    IÂve been working on solving the fan/pipe problem. Since I donÂt know what IÂm doing my first thought is mounting a 3" muffin fan to the top of each pipe that draws air from inside the GH at the top. They are rated at 3CFM and draw .14 amps (perfect for my solar/battery). IÂm hoping with resistance, IÂll get a slow enough flow to heat the pavers and the water pipes under the pavers???

    Open to any and all suggestions!

  • polcat
    Original Author
    11 years ago

    Hi eaglesgarden,
    I did read a lot about the SHCS, I guess I made some poor decisions. But the article I read talked about hundreds of feet of pipe, tons of gravel and temp diffentials to take advantage dew point etc. Seemed like a litlle overkill?

    Phase change does sound appealling from a weight and heat storage capacity. Are there any types currently in widespread use? Meaning cheap:-)

  • hex2006
    11 years ago

    Hi Polcat
    To heat the pavers (or any thermal mass) the most important thing is the amount of time the entire greenhouse air volume is in contact with the mass. To achieve this goal you need to use a high cfm rather than a low one.
    If the masses volume and temperature differential aren`t limiting factors you could go as high as one greenhouse airchange every minute and still not manage to store all the heat available from the air :)

    My greenhouse has 154sqft floor area and 900 cubic feet volume. I use 172ft of tubing buried in clay soil..no gravel ;)

  • eaglesgarden
    11 years ago

    polcat,

    The GHs they were working on were (I believe) far bigger than yours. With that in mind, you would need much less tubing, than they would for a SHCS. (I believe their rule of thumb was 6 feet of length for every 4 sqft of floor space.) I could be wrong about that ratio, though. The point being, they are suggesting using the water vapor that is in the GH normally to be your phase change material.

    You could retrofit your floor this way:
    Remove the pavers, and dig out (manually, since you probably can't any equipment in there) approximately 2.5 feet of trench space. You can lay your tubing in the trenches, and backfill with whatever soil you have (moisten it as you backfill a little to aid in compaction and heat absorption.....there are some articles about wet soil as a heat sink as well!). You want about 1 foot of seperation between levels of tubes, so backfilling and digging trenches down 2.5 feet would enable you to get 3 levels of tubes. Each level would be approximately the length (or width) of your GH, depending on the direction of your joists. The tubes would all meet at a plastic barrel and a fan there would be used to move the air of your GH through the tubing. This can both cool and heat your GH. Depending on the size of your GH how many barrels and fans you would need. (And as hex said, your fan would need to have sufficient cfm to circulate the volume of air approximately 5 times per hour (every 12 minutes). You probably will want that level when the fan is at medium, as well, rather than full power, so that the fan doesn't burn out.

    Other than that, you could put a water tank under your floor, and circulate the air through that! lol

  • hex2006
    11 years ago

    Hi Eaglesgarden
    The origonal shcs data suggested 5x an hour but the recommendation now is to use a lot more :)
    The 5x was geared to keeping the air speed in the tubes down to 2-4ft/sec but new data has proven its not the airspeed but the amount of time the "entire" greenhouse air volume spends underground.
    More airchanges/hr results in a longer duration underground. With fan speed control I can use upto 45x.
    Here are a few test results from my shcs, the soil temperature was 55.4F
    14 ac/hr Inlet: 82.4F Outlet: 55.4F (2.28sec 27F drop)
    27 ac/hr Inlet: 86F Outlet: 59F (1.14sec 27F drop)
    45 ac/hr Inlet: 86F Outlet: 62.6F (0.67sec 23.4F drop)
    Notice at 14x the air still drops to the soil mass temperature, 5x would transfer a lot less heat overall :)
    The energy transfer increases with the cfm, 14x: 1.72kw/hr, 27x: 3.45kw/hr, 45x: 5kw/hr.

    Phase change is responsible for a very large percentage even at high flow rates. Notice how it takes just 0.67 seconds to drop the air by 23.4F at 45x?
    There simply isn`t enough time in the tube for air to lose the heat by conduction given the tube area and temperature difference. Its mostly due to the phase change component.

  • tsmith2579
    11 years ago

    Go to the link. It has lots of info about greenhouses. Also, cut an paste this URL to get the cooperative extsion services in your state. It is a free service and they can help you.
    http://www.csrees.usda.gov/Extension/index.html

    Here is a link that might be useful: UGA Greenhouse Booklet

  • hex2006
    11 years ago

    Polcat
    As you are off-grid you can use a solar panel/battery combo to power the fans. There is a 13ft x 9ft (690ft3) greenhouse in scotland currently using 30w of solar panels to power the shcs fans. Arizona probably gets more winter sun than scotland does so you wouldnt have a problem charging batteries.

  • eaglesgarden
    11 years ago

    Thanks for the update hex!

    How many solar panels does it take to power the fans for this greenhouse mentioned in scotland? How much does that cost?

  • hex2006
    11 years ago

    The solar panels (29w in total) were collected as and when one came up cheap enough i believe. They charge a battery which powers three 24v fans to provide upto 16 airchanges an hour. The price of a new 30W pv panel is around $300 so its a fair investment.
    I have a 170w (677cfm) mains powered fan controlled by a differential thermostat and a speed controller. I`ll be replacing it with a more efficient 155w (870cfm) fan in the near future.

  • polcat
    Original Author
    11 years ago

    WOW! Thank you all for the great information.

    I have about 90W of panel with 900 amp hours of battery. If I use 12V fans, assuming 80% reserve on the batteries...I could run a 200W fan for 14 hours per day no problem. Home Depot sells a 500CFM solar powered vent fan with it's own panel for $217. That should be about 16 airchanges per hour by itself (10x12x10)??

    Time to start excavatin' some dirt!!

  • eaglesgarden
    11 years ago

    be sure to post pics as you do the work!

  • hex2006
    11 years ago

    Hi Polcat
    Its best to plan the tube lengths and layout before you start digging :) The shcs calculator on the Sunny John website is a good place to begin.
    There will be a certain amount of pressure loss dependant on the tubing layout. As the loss increases the fan will produce much less flow than the rated output of 500cfm.

  • polcat
    Original Author
    11 years ago

    Here is the latest picture. These are the INTAKE vents/fans at the top of the GH. This would replace the barrel plenums in the SHCS. The vents/fans feed the 3" pipes enclosed in the solar collector wall, I measured the air temp inside the pipes at over 170(f).

    I will post more as I excavate and attach the 3" pipe to the SHCS. Thanks for all the help!!!!

    Here is a link that might be useful: SHCS Vents/Fans

  • hex2006
    11 years ago

    That looks neat and tidy.
    I guess you`ll need to use 3 fans to get an even airflow through all the tubes. If some have more flow than others it won`t utilise all the mass and lower the overall efficiency.
    A plenum and a single fan is probably the most economical approach.

  • bwright
    11 years ago

    Your HFGH looks great. I didn't see any pics of the outside but I did notice the roof bracing inside. If you haven't already make sure you secure those roof panels. I ended up taping every edge with clear weather proof poly tape, cheap at the big box stores. As an added precaution and a great insulator I draped a clear solar pool cover up the sides and over the roof. You can get one large enough to do it in one piece and secure it with lenghts of aluminum angle screwed into the framework of the roof perimeter. As for heating never forget the good old standby, here I use propane because we can get lenghty spells of cloudy days that really cool off the thermal mass.