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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjondavies
    • CommentTimeMay 9th 2016
     
    Hi
    We are planning to install a small leanto greenhouse (approx 3m x 2m) in front of a south facing garage. There is roof space for some solar thermal panels so I have been thinking of how this could help keep the greenhouse frost free by storing solar energy in the ground below the floor slab and then recovering some of it.

    I was thinking it might be fun and innovative to try something along the lines of the Earth Energybank concept (from zero carbon solutions) but at a lowish cost for a DIY system (we are trying to heat a greenhouse to avoid frost not a whole house). The idea would be to drill three or four 1.5m deep holes in the ground and then put pipes in to pump heat from a solar thermal panel into the ground during the summer and pump out to a radiator during the winter. Insulation would be laid and then a concrete screed over the area and a greenhouse erected. We may put a simple form of under floor heating in rather than a radiator.

    If I can achieve this for around £1,000 then it would be worth doing. I appreciate that performance can not be guaranteed but as long as I can keep the green house at a few degrees above freezing then I could live with that as an experiment. I am trying to assess whether I should go with one set of pipes and a heat exchanger or two sets of pipes and no heat exchanger (pump the solar thermal straight under ground). The heat exchanger seems to isolate different bits of the system better but I wonder whether I need it. The control systems for this may be a little complex. I plan to try and put temperature sensors in the holes to monitor the temperature of the earth.

    I attach a rough conceptual diagram of the two options. It would be great if I could pick the combined brains of this forum to see whether this idea or something slightly modified could fly. There may be simpler ways of heating but I like the idea of a low cost way of using energy stored in the earth.

    Kind regards

    Jon
      garage-slab-01.jpg
    • CommentAuthortony
    • CommentTimeMay 9th 2016
     
    The Victorians used to bury manure or organic material in a trench and as it rotted down the heat generated did the frost free job. Involves digging every year.
  1.  
    Do you have any idea how high the local water table is?

    If it's not too high then one consideration is to (part) bury the greenhouse!

    e.g.

    http://www.permaculture.co.uk/readers-solutions/year-round-growing-underground-greenhouses
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 9th 2016
     
    Why not make a simple ASHP from an old fridge.
    Basically make a simple heat exchanger than goes in the fridge, then a fan on the back of the fridge to waft the warm air around the greenhouse. The heat exchanger could just be a coiled up 4 inch flexible extractor pipe with a bathroom fan on it. Put a load of salt in the fridge as well and that will stop a lot of the icing happening (as it will absorb moisture).
    You may be able to run it off a couple of PV modules and a battery.

    Alternatively/as well as, double glaze the green house, maybe with Acrylic sheet, insulate as much as you can, so floor, walls, anything North, North East, North West facing as these don't get any direct light in winter. Could just be temporary sheets of celotex or similar. And don't forget the floor.

    A £1000 would buy around 6.6 MWh of electricity. So a fan heater and timer may be the simpler and cheaper option.
    • CommentAuthorjondavies
    • CommentTimeMay 9th 2016
     
    Thanks for the great suggestions...we are paving the greenhouse and we grow hydroponically so there is no option for manure. Also there is a fence in front of the area so if we dig down then we lose the sun (I like the idea if we had the right garden).

    Steamytea you are right about the simple option of a heater or an ASP but I am keen to experiment on storing the energy in the earth as I think it could possibly be used to retrofit to existing buildings with suspended floors in the future so it was more to trial the concept and learn something from it. The location cries out to use solar in some way!
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 9th 2016
     
    If you do the sums, you will find that with the small temperature differences, and the long storage times needed, you will store about nothing.

    You can test this if you want. Run your kitchen tap for a minute or so, then take the temperature. That will give you an idea of what the ground temperature is.
    Now, treat the earth as a semi infinite heat sink i.e. it can just about absorb all the energy you can put into it.
    Now work out just how much energy you can collect and how much you need to keep the green house above say 5°C.
    Take one from the other and I am sure you will end up with a negative number.
    • CommentAuthorjondavies
    • CommentTimeMay 9th 2016
     
    Steamytea
    You could be right but if that is the case why would zero carbon solutions be developing something along these lines for new build housing? Their system is with a ground source heat pump so I was trying to see whether I could take cost out for a lower performing system. Not sure whether I am going down the right route..the alternative would be to use a ground source heat pump but that strikes me as very pricey.
    • CommentAuthortony
    • CommentTimeMay 9th 2016
     
    You could use a muck heap or compost or even woo chip heap with a pie buried in it moving heat from there to your green house.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 9th 2016 edited
     
    Because they are using a GSHP, which we know works without any 'forced' storage.
    They use similar systems in Canada. They pump warmed water from the HP down the borehole in the summer/early autumn, then pump it out in the winter/spring.
    This causes a greater fluctuation in the ground temperature around the borehole, but still within the range that the HP works efficiently. It can be done at little extra cost too.
    Worth remembering that most of habitable Canada is at a lower latitude than the UK, and Toronto is about level with Madrid. This gives it one of the best solar resources in the world (intensity times hours of daylight).
    Also GSHP are really water source heat pumps. They often rely on the slow movement of ground water as that has a very large catchment area.

    If you have one of those infra-red thermometers, try heating a couple of bricks up in the oven to about 30°C. Then stick one in a wrapped up duvet, and the other leave on the kitchen top. Take temperature readings every 10 minutes and see what is happening (plot the results in Excel). It will highlight why insulation is so effective.

    I did my BSc in storing solar energy, conclusion was that yes you can store a tiny amount for a short time, and at relatively low temperature differences. To give you an idea of how useless the efficiency was, I was pumping in about 500W into a kg of granite (which has a lower SHC than air) and the 'room' was cooling within a few hours. The 'room' was a well insulated box that was about 1/4 of a cubic metre in size.

    Just insulate the floor with a good 200mm of polystyrene or similar.
    • CommentAuthorjondavies
    • CommentTimeMay 9th 2016 edited
     
    Tony
    We are going to have to sacrifice the compost bin for the greenhouse so we just don't have the space (or that large a garden to produce much now). The greenhouse will be a leanto against the garage wall (north) with fences on two sides (south and east) with access from the west.

    Steamy
    So you would reckon that it would only be worthwhile if we went for a GSHP? The arguments coming out of the zero carbon solutions are that the earth at 1.5m actually retains the heat quite well and for some months.I guess it could mainly be the GSHP rather than putting the solar energy in that makes the difference. However De Montford university's Institute of Energy and Sustainable Development (IESD) seem to think it has some merits and people like Vaillant are investing in it. In this system they are putting the GSHP under the building itself.

    See http://www.dmu.ac.uk/about-dmu/news/2015/november/academics-to-trial-green-energy-system-that-could-revolutionise-home-heating-even-in-older-homes.aspx
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 9th 2016
     
    I think it is the GSHP that is doing the work. The SHC of dry earth is not great (800J/kg.K) though you do get a fair amount of mass (2400-2800 kg/m^3), but the thermal losses are great (clay is between 0.15 and 2.5 W/m.K).

    Some people on here have tried this sort of thing out (not mentioning any name :wink:).
    •  
      CommentAuthorjoe90
    • CommentTimeMay 9th 2016
     
    I did see a TVs programme years ago where they dug a pit in the greenhouse and filled it with broken glass, put a plastic 2 inch pipe into it and up to within a couple of inches of the ridge, a small computer fan pulled the hot air down into the pit. The fan was turned off by a light operated switch ( when it got dark the fan stopped). The heat within the pit rose to stop frost within the greenhouse. I never saw a report on how efficient it was but sounds ok.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 9th 2016 edited
     
    How long was the pipe, if it was more than about a meter, I suspect that there was no air movement (would have to know the size of fan though).
    And why broken glass? it has a lower SHC than soil (670 J/kg.K), and a thermal conductivity of 0.96 W/m.K.
    • CommentAuthorEd Davies
    • CommentTimeMay 9th 2016
     
    Interseasonal heat storage only tends to work with very large volumes (to get the area/volume ratio down). Large as in serious civil engineering projects typically used for district heating systems for villages or towns. Anything smaller may have a small interseasonal component - some stored heat helps you through the worst bit of the winter but generally they mostly rely on collecting at least some heat as they go along - perhaps saving heat from one week to the next.

    3x2 metres or so is pretty small, particularly without insulation around it. I'd be sceptical of it doing much good at all.
    • CommentAuthormike7
    • CommentTimeMay 9th 2016
     
    Posted By: Ed DaviesInterseasonal heat storage only tends to work with very large volumes


    Seconded. Also, the prevention of frost is a very modest target compared with liveable house heating (I'm assuming you are somewhere where the winters are around UK average), so maybe something a lot simpler would suffice.

    The greenhouse itself will act somewhat as a solar panel. If you can carry off some of the warmth reaching the floor and store it just for a day or three, that might be enough. That could be by circulating water or air through buried pipes at different depths .. or... well, I'm wondering if increasing the conductivity of the ground below your slab would do the job as well, salting away your solar warmth, and when that has leaked away, thermally linking your slab to take better advantage of the relatively stable ground temperature a few feet down.

    Steel scrap is almost worthless at the moment, and a load of short lengths of rebar driven down into the ground could do that and save a lot of digging. Some sums are needed which my head is not up for at the moment, so I'll just leave it as a thought.
    • CommentAuthorgyrogear
    • CommentTimeMay 10th 2016
     
    If you've got the space inside the GH, why not use a drum or two of static water, that will heat up during the day and release its heat at night ? (Mebbe your hydroponic tanks could even fulfill this function... ?).

    gg
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 10th 2016 edited
     
    The water barrel idea is a good one.
    You could pipe your bath and shower waste though it, as a simple heat exchanger (water swirls around the faces of a pipe so has the maximum area). So maybe a simple waste tube surrounded by a larger main drain pipe that contains water.
    Does depend on your plumbing layout and water regs.
    You could, if you have some forced extraction do the same with waste air from the house. Maybe a single room MVHR unit though the wall into the lean too.
    • CommentAuthorEd Davies
    • CommentTimeMay 10th 2016
     
    The coldest nights tend to have clear skies so likely follow days with at least some sun so using a solar thermal panel to help warm a water in a barrel in the greenhouse might be useful. A day or two's storage at this scale makes a lot more sense than an attempt at interseasonal.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 10th 2016 edited
     
    Then have a fan heater set to 3°C as a 6 quid backup. Or even a cheap 500W security lamp.
    If you have ever seen the film Saving Grace, you will know what I mean:wink:
      Cornflakes.jpg
    • CommentAuthorskyewright
    • CommentTimeMay 10th 2016
     
    Posted By: joe90I did see a TVs programme years ago where they dug a pit in the greenhouse and filled it with broken glass...

    Sounds like "It's Not Easy Being Green"?

    https://en.wikipedia.org/wiki/It's_Not_Easy_Being_Green

    I've not watched it (no TV). but the 'book of the series' used to be on our library bus.

    As to it's effectiveness...
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 10th 2016
     
    He was down my way I think, not had a frost on my car this year at all.
    But if you went to the hassle of digging a pit and filling it with dung, why not collect the methane of it and burn it (via a CI engine and a generator).
    Would need to be a big pit mind, AD don't work too well in out climate on the small scale. It it that surface to volume ratio that Ed mentioned.
    • CommentAuthorskyewright
    • CommentTimeMay 10th 2016
     
    Posted By: SteamyTeaHe was down my way I think, not had a frost on my car this year at all.

    IIRC he (Dick Strawbridge) wasn't aiming at interseasonal storage anyway. more like evening out the diurnal temperature variation, with a bit of season extension as a bonus?
    •  
      CommentAuthordjh
    • CommentTimeMay 10th 2016
     
    If you want to stop a greenhouse freezing then I would first concentrate on insulating it. You can manually cover the windows with sheets of insulation of course, and many people use sheets of plastic bubble foam over the winter. But my favourite design is to use soap bubbles to insulate the greenhouse, combined with a water tank to store the bubbles and heat. Search for "soap bubble greenhouse" to find some examples.
    • CommentAuthorCharli
    • CommentTimeMay 10th 2016
     
    Glass heat sink storage (as described by joe90)- it was on the tv show 'it's not each being green' with Dick Strawbridge, I think. Reckoned it was only good for storing overnight heat, not seasonal.
    - http://www.reuk.co.uk/Solar-Greenhouse-Heat-Sink.htm

    This is a similar design using all the underground of a greenhouse:
    http://www.ceresgs.com/#!10-Dos-and-Donts-for-Designing-a-Ground-to-Air-Heat-Transfer-system/cf3k/55131f4c0cf2aa18115c5439
    This is what I am planning on doing for a frost-free greenhouse experiment (I'm allowed to experiment in the garden! Further experimenting in the house has been banned!). I'm excavating 1m deep under the greenhouse (that is 5m by 3m)- sticking some pipes and a fan in to transfer air, putting the greenhouse back on top (north wall and stem walls all insulated, roof is an old polycarbonate triple-layer conservatory roof, and windows are old double glazed units).
  2.  
    Posted By: CharliI'm excavating 1m deep under the greenhouse (that is 5m by 3m)- sticking some pipes and a fan in to transfer air, putting the greenhouse back on top (north wall and stem walls all insulated, roof is an old polycarbonate triple-layer conservatory roof, and windows are old double glazed units).

    If its a lean to - be careful about the foundations of the adjacent building!
    • CommentAuthorCharli
    • CommentTimeMay 10th 2016
     
    Peter_in_hungary- my greenhouse isn't a lean-to (and realistically I don't think I'll get down a meter... but I'd like to try!)
    • CommentAuthorjondavies
    • CommentTimeMay 10th 2016
     
    Thanks to everyone for all the creative ideas and discussion which have given me food for thought. Practically I could put a water barrel in the greenhouse which will absorb heat during the day and then release it at night. I liked the idea about heating this water to a higher temperature using solar thermal but would be slightly worried about legionella above 20C - generally they reckon you have to heat static water to 65C to be safe which would be a no-no. I do have a 5,000 litre tank for rainwater capture but I would certainly not want to take the risk on that.

    The attachment shows the space I have for the greenhouse. The lean to will be against a south facing garage wall. The garage is not connected to the house (it does have electrics) so using heat from the house is probably not realistic - the outlet for the MVHR system is on the furthest side of the house to the garage. You can see the compost bin that I am going to have to part with to put the greenhouse up!

    The floor slab will be well insulated whatever we do - I have a reputation as the insulation king to keep up - and the greenhouse will pick up any winter sun. I am thinking I should just admit defeat but the south facing garage roof just cries out to me for a solar solution. Maybe a solar panel with a car battery and a heater is the best I can do to keep the frost off at night- with a mains backup as Steamytea suggested to keep it above 3C.
      space for greenhouse.jpg
  3.  
    If your anywhere near Bolton your quite welcome to come and have a look at mine which we constructed for seedling germination when the price of oil went through the roof. It does work but you need to start with a good quality glasshouse with rubber seals. The germination bench was 45 gallon drums painted black with a slatted wooden bench above and plastic rigid cloches to cover the seed trays. A bubble wrap roller blind was used on the underside of the roof but only on nights when frost was predicted as the bubble wrap if used continually makes the seedlings go leggy.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 10th 2016 edited
     
    PV is a lot cheaper to install, maintain and operate that ST, just whack a kWp on the garage roof, a simple off grid inverter and a couple of 300 W fish tank heating element.
    Why do you need to worry about legionella, you going to shower in it? Just seal it up and let it be, it is only a store.
    • CommentAuthorgyrogear
    • CommentTimeMay 10th 2016 edited
     
    You could also insulate the inner side of the garage wall, then paint bricks BLACK, so as to store GH heat in that wall...
    Also put a solar thermal air heater on the garage roof, and duct the air into the GH to boost solar gain during the day... Then *several* water barels !

    You could also suspend a salvaged automobile radiator at GH apex, and spin the fan (on a timer) and circulate water through the rad with an aquarium pump, to a barrel, to boost HW.
    etc.
    gg
   
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