Heating a Greenhouse With Compost: Pt 3

Part 3 Posted 5/12/2012.  Click here for the full sized original post.

“Patterns and Perceptions; Autumn Leaf Mould Energy Potential (Part 3 – Heating a Greenhouse with an External Compost Bed:- Design Tweaks, Internal Set Up and Initial Temperature Findings); Growth Limiting Factors.”

First I’ll whizz through the set up now inside the greenhouse, before giving some early temperature readings. (This will turn into my longest ever post, so either cut to the snowdrop pic, or don’t say you weren’t warned! And Carolyn and Michael ….. Look what ripples have developed from the pebble you dropped on the other side of the pond….)

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Fiona always reckoned we should have had mains electricity installed when we had the greenhouse put up. And of course she was right, but at least we had the line of the water to follow to get the armoured cable out to the greenhouse before the freezing temperatures arrived.

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Inside, I’ve mounted one axial 20 Watt fan vertically and joined it up to the top of the reactor spiral pipe where it enters the greenhouse, and then taken an outflow on into the inner insulated zone for another metre or so.

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The other end of the spiralled reactor pipe is also located in the inner insulated zone. ( see my previous post for external design). So in theory air should be circulated and warmed as it passes through the external compost bed.

Since I’ve had to install electricity I’ve also added another 20 watt axial fan (along with the pipe for ‘The Reactor’ Spiral, this is the only new purchase for the project)  at the apex of the greenhouse. This connects to the vertical black pipe at the greenhouse gable end, and then underground to a perforated soil pipe and rubble heat store beneath the central path. Originally the idea was to move warm air into this heat store with a battery operated fan, charged from a separate solar panel. Now the mains supply makes it much simpler.

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Both these fans are run through timers for now. So here’s the first issue. Run them continuously or intermittently? The apex fan is easy. At this time of the year it only makes sense to run it for a couple of hours around midday, when temperatures in sunny weather may reach high teens or even low ’20′s (Degrees C). Ideally, and for the future, I shall look into a temperature operated switch to control this function. This should avoid the crazy scenario of having the auto Bayliss roof vents opening, even slightly on a sunny day and allowing hot air to exit our Cedar framed Woodpecker Joinery built greenhouse.

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The timing for the lower fan is more tricky. After a bit of fiddling I’ve settled on having it running continuously overnight and from late afternoon to mid morning. In between these 2 times I have it on/off for about 20 minutes alternately (the minimum setting on this cheap timer control).

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So both of these fans together will run for less than 400 watt hours per 24 hours, and some of this daytime use will be covered by our own PV generated electricity. So the actual fan running costs with our Economy 7 Good Energy tariff will be only about 4 p per day. Of course apart from hopefully moving heat around, the other great benefit is that the fans create air movement in an otherwise enclosed potentially dank and damp environment. A huge plus for healthy plants in reducing fungal or mould issues.

I finished last time with a compost temperature of 2.5 degrees C the day after filling up the compost bed. Through the first week this temperature, taken at the top of the bed under the insulated cover, climbed promisingly into the 30′s but I was still only getting about 6 degrees C at the exit of the pipe into the inner zone. Why so low? What a failure. Ever impatient, I pulled the top of the heap apart and discovered that whereas the compost was indeed pretty uniformly warm, the central ‘Reactor’ core filled with 2 litre water bottles was only 13 degrees C.

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….. Notice the steam, and the white structures on the surface to the left are newly forming mushrooms…..

So rather than creating a Heat Store in the water bottles (which was what I’d thought would happen), I’d actually created a Cool Zone next to the inner half of the air carrying spiral pipe. This was a pretty basic design flaw!

Fortunately after removing all the bottles, I discovered that the inner core was wide enough to accommodate vertically two 40 litre plastic whitewash tubs, and after years of white washing our buildings we have plenty of these to spare. One on top of the other, with some simple wire handles added to aid lifting in and out of ‘The Reactor’ . So after drilling a few holes in their sides for aeration I now had an easily removable compost containing core to the reactor. I filled the tubs with already hot compost from the top of the pile and lowered them into place.

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On lifting the bed’s insulated covering lids I’d also noticed that as with my earlier trial with Big Bags, the overall compost level had settled a fair bit. So I topped up the heap with 6 tubs of shredded leaf material and 1 of fresh poultry coop manure. Some pee was added for good measure over the compost surface as a Nitrogen source and for added moisture, and also onto the top whitewash tub.

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As yet no compost was falling from the raised base of the heap, and for now I’m adopting a wait and see approach here. A quick vigorous wiggle of my aeration tubes, and back on with the covers. I also decided to insulated with bubble wrap both the entry and exit pipes into the inner zone inside the greenhouse. And waited.

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…. The base of the compost heap, where little material has so far fallen through. But after 2 weeks white fungal mycelia are obvious as shown below, between the metal bars of the grids ….

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Within 24 hours of removing the bottles and switching to compost filled tubs, I could see that I was on the right track. The exit air into the greenhouse from ‘The Reactor’ had now risen  up to 13 degrees C, and during the week it peaked at 19 degrees C, before dropping slightly after 7 days to 17 degrees C again.

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I’d already planned on a weekly top up to the bed as being an easy to plan routine, so on Sunday  2 weeks after set up, I emptied the core plastic tubs onto one side of the heap (which by now had cooled a bit to 32 degrees C), and refilled them again with some of this top of the heap hot compost.

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Then I again made up the whole heap with almost exactly the same amount of material as last week…i.e. 0.75 tub of poultry coop manure, 5 tubs of leaf material, and 1 tub of wet loosely torn cardboard. With some more pee (which had also been added on 2 other occasions through the week…..well it seems a shame to waste it….).

2 days later and the top of the compost bed has gone just over 40 degrees C, and within hours of changing the tub compost for ‘fresh’ material, the inner zone pipe was again reading 19 degrees C. After all the effort that’s gone into this, it’s a considerable relief to this blogger! ( Thanks again for Fiona for producing this early graph) :

Greenhouse Reactor Temperatures ‘C annotated

A maximum/minimum thermometer arrived 3 days ago so that for all the interested (??) readers, in due course I can create another graph of how the system is performing over the medium to long term.

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It’s been placed towards the end of the inner zone furthest from the warm air inflow. Inevitably this will be cooler than the area close to the warm air inlet, and so far its registered an apparently stable range of minimum of 8 degrees C, and a maximum of 12 degrees C. I also suspect that my digital thermometer may be reading at least a couple of degrees on the cold side since a few days after set up, when the greenhouse in the morning looked like this from the outside…

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….and this on the inside…..

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….I’d got a night time reading on the ground close by of ….

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….when the forecast was minus 5 degrees C only. Inside, at dawn the inner zone was still free of any evidence of frost or sub zero temperatures.

To add a bit of spice to the challenge, a couple of Lemon bushes (Meyer’s Lemon and Four Seasons variety) arrived just as I got the system working. These are to be my ‘Canaries in the Mine’, in case modern thermometer equipment does let me down.

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Healthy looking plants from Cross Common Nursery in Cornwall, with the Meyer’s Lemon with flower buds just about to open. I was assured that they were hardy to 3 degrees C, and could cope with the odd touch of zero temperatures. We shall see. But I’d love to be able to grow a Welsh Lemon to match our Welsh Nectarines in due course. And in addition the hope is that we can over winter in their pots a very few tender shrubs, like Ageratina ligustrina, shown below…

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And even more importantly start our tomatoes and peppers off earlier, so that we get some fruit before late July/August. Finally I hope to grow a few early vegetables under cover to avoid ‘The Hungry Gap’ which down here this year lasted into July! So apart from Pak Choi, WinterLettuce and some Salad Kale, (which I’d started off before the idea for ‘The Reactor’ had even germinated), I’ve also sown some Beetroot, Early Carrots and Oregon Sugar Pod mange-tout peas. In all 3 cases I’ve pre-germinated the seed inside, until the first roots are well formed, and then planted them up. In addition I’m trialling a system for pre-germinating small seed like carrots on wet kitchen towel, covered with black plastic, in the warm inglenook area by our stove. The reason being that handling such small seed when wet is almost impossible….. they stick to your fingers, and anything else for that matter….

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So now I just lift the moist kitchen towel off the tray when the seed have germinated as above, place on the soil surface and cover with compost. So you can see that this whole palava and experiment does have a utilitarian aspect, and isn’t just an excuse for me to practice some pseudo science! (Actually I’m intrigued by the potential of a similar compost bed to heat other structures, or to be used as a link with an air source heat pump…)

But all of this push to raise temperatures through the winter to a point where plant growth might be able to continue, albeit slowly, got me thinking about the other factors which might limit growth in this environment. Nutrients and moisture won’t be an issue I think, but light almost certainly will be. Last week, like much of the UK, we had persistent heavy rain….(153 mm in 7 days), and the PV readings confirm an appropriately gloomy scene. Barely 11 KW Hours in a week (the equivalent of about half a sunny day in April!) So I figured that adding a bit of extra light might be worth trying.

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Researching greenhouse lighting took me to the fascinating commercial site of Philips Green Power red and blue LED side strip lights,  recommended by several commercial tomato growers for year round production, including one from winter light deprived Finland. But sadly I couldn’t quickly find a UK distributor for what is no doubt way beyond our budget.

But what I did inadvertently drift into during this search was the murky world of UK hydroponics culture, clearly aimed at all those closet cannabis growers, where the fact that LED lights emit little measurable heat was promoted as a huge bonus. So back to the outbuildings where I knew I had 2 possible redundant light source options. Firstly a now rarely used Moth Light, which I had actually acquired from a local back street hydroponics shop in Bristol, years ago.

” So you’re going to use it for catching moths are you?”….. (Pull the other one)….

and when I returned to buy 2 more for friends who I’d introduced to the joys of ‘Mothing’…..

“So it’s for a friend to use to catch Moths, is it?”…… (Yeah, Yeah)

But this Moth Light was a bit delicate, and high wattage, so in the end I opted for my old Light Box designed to fend off SAD (Seasonal Affected Disorder) which afflicted me in earlier years, before I was able to spend more time outside. And so with two 55 watt Osram daylight fluorescent tubes, I now have this coming on through a timer for 3 early morning hours ending at dawn, and another 2 hour boost around lunchtime, at the Northern end of the greenhouse. And as a bonus it does of course emit a little extra warmth into the inner zone. So this adds about another 4.5 p to the daily spend!( So we’re now up to about 8.5 p per day.)

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But a few other interesting technical pieces and thoughts have surfaced from all this activity:

  • There have been real advantages in having a wooden framed and half timber clad structure. Fixings are easy to make, and the structure has a better in built insulation than all glass, or aluminium framing. In addition my insulation of the lower timber clad section with Celotex off cuts has had a huge benefit in raising light levels through greater reflection of what little light is available at this time of the year, from the foil outer layer of the Celotex sheet.
  • At other times of the year, Carbon Dioxide can be the limiting factor for plant growth in greenhouses. Sometimes by mid morning, vigorous plants will have ‘consumed’ all the CO2 in the greenhouse air during photosynthesis. After all there is only 0.037% atmospheric CO2, and even though the world is seriously concerned about it’s rising level, growth will slow or cease if it’s exhausted in an enclosed and unventilated greenhouse. Click here for a fascinating Video on how the largest commercial British Greenhouse Tomato grower has overcome this issue. And how amazing is it that with so little CO2 in the atmosphere, a mature Oak tree weighing about 14 tons, and containing about 8 tons of dry matter contains how much Carbon? 50% of the total dry mass, or 4 tons! So you can see why there are big concerns about global deforestation and releasing all this carbon back into the atmosphere very quickly.
  • There are sites for calculating  estimates of (conventional) heating requirements for greenhouses. (There are of course a huge number of variables affecting such estimates). Using the calculator on The Hartley Botanic greenhouse website, I arrived at a very rough required wattage of about 3 KW for the whole of our 14 foot by 8 foot greenhouse, or about 1 KW for my inner zone (10 foot by 7 foot), to protect to an inner minimum of plus 3 degrees C if the external temperatures fell to minus 15  degrees C – which is about the worst we can expect. This wattage, if supplied as electricity, would cost about 45 p per hour (3KW), or 15 p per hour (1KW) respectively  (obviously less if you have a night time tariff) … to put my endeavours, and costings into some kind of relative context.
  • Using water filled bottles around the inner perimeter of the greenhouse, between the insulated wooden cladding, and the insulated inner zone, will provide an additional thermal buffer should temperatures get really low outside, thanks to the marvellous, and for me difficult to grasp, concept of latent heat of fusion released when a liquid freezes.

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…..This is one of our Tomcot apricots, which requires winter chilling to allow fruit buds to  develop, so it’s outside the warmed Greenhouse Inner Zone. I might return to the subject of ‘chilling hours’ at a later date. (Inner zone polycarbonate sheet to the left, and Celotex insulation on the outer greenhouse wooden cladding to the right)….

Basically as water cools it’s temperature will fall in a linear fashion until it starts to freeze. But the actual act of freezing releases a considerable amount of  ‘latent heat’ as the water changes from liquid to solid. Click here for a clearer explanation than I can manage! How much heat is released ? Well…..144 BTU per pound of water. But what’s a BTU? Well 1 watt is 3.41 BTU/hour. So a 2 litre or 2Kg water bottle will give out 633 BTU during freezing or 184.6 Watt Hours. So 100 such bottles equates to 18.4 KW hours, (or the equivalent of running a 2 KW heater for about 9 hours, which would cost about £2.70). Now this latent heat WON’T raisethe surrounding greenhouse air temperature, but it should help delay the air temperature falling much below freezing until all this bottled water has frozen…and in turn protect the slightly warmer insulated inner zone from ever falling below zero…… or at least that’s my theory and I’m sticking to it!

  • If the weekly top up maintains at the current level, it will obviously be sensible to have filled back up stores of mulched leaves, or soaked cardboard to ease a speedy top up, and be able to cope with lying snow, or frozen conditions for a few weeks. I reckon perhaps 3 or 4 more Big Bags should nearly get us to March. And having filled another 2 already I don’t anticipate any shortage of material…the lawnmower is equally adept at mulching most herbaceous foliage which normally dies back and becomes soggy around now, and its particularly effective at dealing with Hellebore leaves. And returning to the example of the Oak, a single Oak tree will annually produce about 250 Kg dry weight of leaves per year.

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….. Lawnmower mulched leaves awaiting ‘The Reactor’…

  • With already filled bags and tubs to hand, the whole process of topping up the bed, aerating and changing ‘The Reactor’ core fuel took less than a quarter of an hour.
  • I’ve yet to address the issue of whether material will need raking out from the base, or whether it will naturally or through worm action fall out. Already there is significant bottom fungal mycelium growth, and indeed top mushroom formation after just 2 weeks, as I’ve shown above. I also may be being lulled into a false sense that this process will just chug along in a similar and predictable vein to the progress so far. Many folk find compost heaps cool irreversibly with time. Will mine be any different, with its regular top ups, and slight compost movement ??
  • The idea of chopped leaves as a ‘fuel’ for the ‘Reactor’ is intriguing to me. I looked up the calorific value of dry logs for burning as a fuel, and the general figure is about 18 Mega Joules (MJ) per Kg dry Weight. But I then found that dry Miscanthus X giganteus stems ( a 9 foot tall ‘Elephant Grass’  crop increasingly grown in the UK for burning in Biomass power stations) is also about 17 MJ perKG dry weight.  So virtually the same as the wood. And finally found that in China a study into the calorific value of Mangrove leaves gave an almost identical range of calorific values of 19.6 – 21.5 MJ per Kg dry weight. Composting is in some ways similar to burning in that much of this potential calorific value is released, as the composting or burning process progresses and carbon dioxide is released. Clearly composting is much more drawn out than burning, and stops a bit short of ash production. Or I hope it does, since spontaneous combustion of compost heaps that are too dry can occur, which would be a disaster next to a wooden framed greenhouse! Indeed in this application one really doesn’t want to produce finished compost in 18 days, with temperatures up to 65 degrees C (Click here for this method). A slower, slightly cooler approach is called for, to give an extended period of heat generation. But this is where my understanding of Physics is poor. A Joule or Mega Joule (a million Joules) is a unit of energy. A Watt or Kilo Watt( a thousand watts) is a unit of Power, which by definition is a unit of energy used per unit of time. So 1,000 joules per second is a 1,000 watt seconds, or a KiloWatt second. So what power does a Kg of dry wood release, if say it burned in an hour? The Maths would be 18 (MJ/Kg) divided by 60 (minutes) divided by 60 (seconds), which would give you a figure of 4722 watts per hour….or the more familiar 4.77 KW Hour( I think!). Turning this thought process around, I calculate that if Hartley Greenhouses reckon I would need about 1 KW/Hour to heat my inner zone, what weight of Miscanthus, at 17 MJ/Kg would that require, over a 7 day week? My maths was…..1 Kg would heat for 4.72 hours, so 24/4.72 = 5.o8 Kg would be needed for 24 hours, or (7 x 5.08)  about 35 Kg to heat the greenhouse for a week. By burning! But by composting ?? Interestingly weighing the full buckets of moist cardboard, and moist leaves needed to replenish the compost bed once a week, gave me figures of 7 Kg for the leaves (x 5 tubs), 8.4 Kg for the cardboard (1 tub), and about 7 Kg (1 tub) for the chicken coop manure. So a total of 35 Kg + 8.4 Kg + 7 Kg = 50.4 Kg moist compostable material to heat the greenhouse for a week. Which looks like about 800 Kg material for a 16 week winter. Quite a lot of plant debris. But easily available in a modest mature and tidied up autumnal garden. Now obviously this isn’t heating to the same Hartley temperature criteria of minus 15 outside and plus 3 inside. But unless I’ve gone seriously awry with my thinking or maths this figure is not a million miles from the amount required to release the energy by burning the material. Now of course the material added was as a top up to a much bigger overall mass, but I reckon for me it confirms the simple point that all this carbon rich plant material sitting around in the environment at the end of the growing season, waiting to quietly decompose and release the carbon back into the atmosphere, and the trapped photosynthetic derived energy which it still contains, can really be viewed as having significant ‘fuel’ potential. For now, (thank goodness!) I rest my case, until knowledgeable readers rip me apart over my simplistic logic…..

And finally a few more garden pictures including, to cheer the spirits, the first Snowdrop photo of the year.  I think this variety is called ‘Three Ships’, as in the Christmas Carol line ” I saw Three Ships come sailing in, On Christmas Day in the Morning”……..

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Thanks for reading this mega post. Sorry it was so long! And do browse around the rest of the Blog Pages….


  1. Hello Julian,

    You have been busy! Some great results. The next few months could be very interesting.

    I suppose that theoretically total decomposition to the basic building blocks for plant growth is possible and the reactor will produce very little residue provided that a balance can be achieved. The presence of the fungal mycelia must be a good sign as i think they are responsible for the initial breakdown of the lignaceous material. The reason why an ordinary heap sometimes dies could be that the temp gets too hot and removing heat from the centre continuosly helps to keep the temp in the region where the greatest variety of organisms responsible for decay work most efficiently. I wonder what the optimum temp for this type of decomposition is?

    Just a thought -How necessary is cardboard? Would shredded twigs or wood chips be a better scource. Does card board contain any fungicides. What other chemicals are present which may be concentrated by the decomp process which may ultimately slow it down or stop the reaction. All the available info says compost needs a mix of carbon to nitrogen to produce a good friable product with the carbon /lignin providing bulk. In this case you want to produce heat with compost as a possible bye product and the ideal is a process which is self perpetuating ie material in at top, heat out and as little as possible out of the bottom so is the percieved mix best for this process?


    Fingers crossed that the red line stays between 15 and 20 degrees.

  2. Good morning Dave,
    Thanks for the comments, which as always are very interesting. And you’ve raised a couple of points which in my typical rush ahead approach I hadn’t thought about re the cardboard. i.e. fungicides, etc. I’d just worried about any possible foreign fungal spores having come in on the cardboard, as a potential issue. I’ve no idea if the cardboard gets chemically treated in any way. Another thing to look up on, though I’m avoiding any cardboard with colour printing on it. But the 4 reasons I opted to use a bit of it were :- firstly we have a ready supply from a shop in town. Secondly by leaving it out in the rain its easy to tear it up small. Thirdly a lot is corrugated so I reckon that it helps to introduce air pockets into the mix, and fourthly I’ve always found lots of worms inside the corrugations when I’ve put it on our old compost heap, so reckon that they must like it. Do they go for the glue? I also figured that its essentially very fine wood fibres, so if it goes in wet, it should rot down fairly quickly. Time will tell. Also I’m trying to do this with materials that most gardeners might be able to access easily. I reckon that wood chips or shredded twigs would be equally suitable, but you would need a good shredder to generate them….. which is an issue for me ….(at the moment….). If this thing works all winter, then I can see that bonfires of brash might be outlawed in future as just too wasteful!
    Last night Minus 10 again on the bank and condensation inside the outer zone, and I think after our chat yesterday, you’re right about the mushroom. It’s in the Ink Cap group and may either be Coprinus macrocephalus (Uncommon, year round, dung heaps/rotting straw) or the very similar looking Coprinus lagopus (Occasional,summer to autumn, leaf litter soil in shady woods). Whichever it is, it obviously likes the sauna like conditions of ‘The Reactor’ to grow so quickly,

    • This morning lit wood burner at 05.30 and ch came on at 06.30. Pump had to run for 80 mins non stop before house was up to 18 degrees and burned approx 7kg wood = 24.5kwh Coldest night /morning so far?

      How much energy in a bale of haylage current price about £20!


  3. Dave,
    Yep pretty chilly, but next weekend is looking a lot colder, so keep your axe handy….. I guess if the haylage bale is half a ton or so, then quite a bit. Had to collect a new stove baffle plate today ( this time we’ve got 2 years out of it), and I had a chance to suss out stainless steel flexible flu liner pipe in their yard. I think if I were starting afresh ( or designing Reactor MK2) then the extra money for a metal liner would be worthwhile. I’m sure you’d get more efficicent heat transfer into the air passing through the middle of the spiral. And none of those pesky little holes to be filled!
    You’ll need a canoe in the morning if this rain keeps up! BW, Julian

    • Julian

      You can get the poly drainage pipe in various sizes without any holes in (apart from both ends!) just wondering if a matrix of smaller pipes spread through the heap would give better transfer. i guess there is a trade of between transfer and size of fan to push air through matrix. Also important to ensure the heap cannot bridge.

      Last year there was a lot in the gardening press about people getting some illness from compost – due to inhaling spores I think but your system does not allow air transfer directly from compost into greenhouse so should be all right.


  4. Hello Dave,
    Another very good point about the disease issue. As you know I’m prone to chest issues (part of the territory down here, with humidity and spores) so I had already decided to don my respirator ( bought late in the day after a few year’s wood turning) in future when I handle/disturb the heap or even the collected mulch, since it will need doing on a regular basis. It seems to me that if the ‘rare’ Coprinus megalocephalus has colonised my heap in just 2 weeks, then there will be fungal spore issues, or even the potentially more serious and rare Legionella longbeachae, which was isolated from a few Scottish gardeners earlier this year, apparently after just doing a google search. Copy paste the following for the link :http://www.dailymail.co.uk/health/article-2151107/Gardeners-warned-wash-hands-using-compost-rare-strain-Legionnaires-disease-infects-Scotland.html. I’ll put a bit more about this issue and warning in my next post, and I think take some more pics when I top up the heap this weekend….Mind you when did you hear a car manufacturer raise the issue of lung/respiratory problems with car air conditioning systems? Or Wood Burning Stove manufacturers raise the risk from inhaling toxic fumes when you open the stove doors? Or garden journalists about the hazards of using Glyphosate? It’s a jungle out there….
    It must be much safer to stay inside and watch TV….

  5. As you undoubtedly suspected I would, I checked your maths to see if I could catch you out, but I regret I have to report that your calculations are more or less correct! Congratulations!

    The physics is a little more suspect, although the general drift remains correct. You start, promisingly enough with the definitions of energy and power, but then get a little confused with the units. You say “1,000 joules per second is a 1,000 watt seconds, or a KiloWatt second”, whereas 1,000 Joules/sec is 1,000 Watts, or 1 KW. (Joules are units of energy, whereas Watts are units of power, or energy per unit time)

    It follows that if burning 1 kg of wood produces 18MJ of energy, then releasing that in 1hr (power) would be equivalent to a power output of 18*1,000,000/60/60 = 5,000 Watts or 5kW. (You say 4.722 kWh. A kWh is a measure of energy, not power!). Your 4.722 kW was based on Miscanthus rather than wood, but I get your drift!!!

    If you need 1kW to heat your inner zone(power), then it follows that to heat it for a week would require 1X24X7 = 168kWh (energy). This is the same as 168X60minX60sec/1000 = 604.8 MJ (energy).

    You get 17MJ of energy from 1 kg of Micanthus, so you need 604.8/17 = 35.6kg of Micanthus for a week’s heating. Same result, but a little more scientifically rigorous!

  6. On the subject of the cooling water bottles in your inner sanctum, you gently upbraid your readership with not having spotted the flaw in the design and I am suitably abashed. In mitigation, though, I feel I should point out that your analysis in the previous blog must remain correct, namely, “The 2 litre plastic pop/water bottles are chosen to act as an additional means of holding heat as a thermal store inside the reactor core, to help to smooth out any temperature fluctuations when the fan is switched on….”

    Clearly, from what you now say, they had the effect of conducting and convecting heat away from the coil, rather than smoothing out temperature fluctuations. This missing energy had to go somewhere, probably into the environment above and below the bottles. Another solution might therefore have been to insulate heavily above and below the bottles. However, using the extra heat source of compost in the space once occupied by the bottles is an elegant solution.

    My only problem is that since you now have bottles on the outside of the inner zone, and despite your arguments about utilising latent heat to stabilise the temperature, with which, as you know, I agree, are you not running the risk of conducting and convecting heat away from the inner zone in the same way as with the bottles in the core?

    By the way, in his comment above, Dave suggests the use of metal conductors running through the compost and into the coil (possibly through the pre-existing holes?). I agree this would be an excellent way of conducting the heat from the compost, which is itself a reasonably good insulator, as is the plastic pipe itself, to the recirculating air, reducing the temperature differential, and removing any risk of killing the composting process through over-heating. Copper would be the material of choice, failing which, aluminium. These “heat pipes” are regularly used in industry and even in PCs to conduct heat away from the CPU. This would however complicate the process of refreshing the composting material, and extracting spent material. Still, if you have a few thick and redundant copper wires hanging around in that shed of yours?

  7. KTB,
    As always thanks for the comments. And indeed for reading the darned thing in the first place. It’s great to have this sort of intelligent critique from you and Dave when I stray into areas, where I have only the vaguest notion of what I’m talking about! And indeed thanks for sorting out my confusion over energy and power…..though I’m still a bit confused about the idea of a joule/second (watt) hour.( as in KW hour)…seems a kind of physics, or time, tautology to me. Ah well. Perhaps a good job I ventured into biological sciences, rather than physical sciences or engineering…..
    I’m glad that my back of an envelope maths almost passes scrutiny as well. Its also interesting that your completed estimate of energy required (168 KWH ) to heat just the inner zone for 1 week to the rather extreme temperature illustration chosen, would, over a 16 week winter period amount to 2688 KWhours, or roughly 66% of the total existing hobbit house and garden annual electricity consumption! Pretty expensive for a few lemons, I’d say!
    Re the plastic bottles remaining cooler than expected, my own take on this, (to me it was clearly a very surprising finding), was that the air filled plastic pipe was itself too much of an insulator ( with its moving cooler air for most of a 24 hour period) from the heat of the external surrounding compost. So that not much heat ever made it to the bottles So I’m sure that a MK2 version would definitely run to trialling the more expensive option of a better heat conducting metal pipe, which should in turn result in significantly greater heat extraction from the heap.
    But the really interesting issue you raise is whether my bottles in the outer zone will draw heat from the inner zone, rather than act as a buffer. I don’t know how one could attempt to verify or challenge this, since I guess that they will always be at a lower temperature than the inner zone, although separated by a good twin wall polycarbonate sheet. I shall await a really freezing night with interest,

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