Compost Heater

Good Morning,

Firstly, I enjoy gardening but don't like spending money on fancy things, I prefer to make them from recycled materials I can find. My greenhouse is made of scrap clear corrugated sheets heated and reshaped in the sun into panels, then lined with bubble wrap for insulation for context.

Yesterday while collecting the grass off the lawn It dawned on me the heat that is generated when grass is in a pile, I live in Scotland so heat and fertile compost is like gardening gold and grass is one of the best growers up here!

My theory for this was that if I can siphon the heat as its generated I can make a space heater, not a new concept, using the simple rule that heat rises.

So I thought, surely I can take advantage of this process for more then just the end result. I routed out a 225 litre compost bin and sat it opposite my greenhouse. I collected 20m of 80mm drainage pipe and, with considerable effort, managed to coil it inside the compost bin on a gradual incline. The grass was fairly warm to the touch already but it was raining heavily at the time so no sun light was warming it up. Where the pipe exited the container I wrapped it in bubble wrap for insulation and to stop air escaping through the holes, and fed the other end through the wall of my greenhouse into the insulated area. The bin was filled with 20cm of green grass at the bottom, then a mix of leaves and grass going from green to brown, all pressed together around the pipe. The idea being the pipe supplies the air flow for the bacteria and also the heat transfer.

Initially it just made the greenhouse smell of grass but I thought, its just been put in, ill give it time to start working again. I came back 3 hours later, and the pipe in the greenhouse was putting out a decent jet of hot air, about 30 degrees or so! The pipe was also condensing the moisture from the hot air too.

I left it overnight and early the next morning the inside had condensation inside and was a good 5 degrees warmer. So the heater worked but the heat retention was too low and the delivery too slow to get it hot inside.

It also occurred to me that as the material inside the computer wasn't that large it probably wont take THAT long to rot down if that temperature is being maintained so efficiently. So I set about planning a larger one for an enclosure next to it that was 3m by 5m and 1.5m high. The premise being I fill it to 1m in depth and insulate the top.

My question to the more experienced community is this, will the heat transfer pipe control the heat from a larger unit enough to prevent it getting too hot and setting on fire. Im currently thinking of having 30cm of material on the floor, lay the initial pipe coil so that the center of any one composting area is no more then 30cm from a pipe or wall, inclining the pipe gradually as it goes round and having a second layer 30cm above the bottom one, then adding a final 30cm of material on top.

It will take about a month to fill the enclosure I reckon as the grass needs to grow! but I hope if would last a good 6 months over winter!

 

Why not make a hot bed in the greenhouse like the Victorians did and they still do at Heligan. Where you put a pile of manure in the greenhouse let it get hot, turn it over to moderate and spread the heat put 6 inches of soil on the top and plant into that, you could put a cold frame type structure around it to get really warm.
To improve delivery why not install a solar powered fan, something like those found in a PC would work.

 

My garden is on a steep slope, the gradient of the ground is about 30 degrees consistently down to a river. To utilize the ground I made a large decking platform on the slope concreted into the bedrock. The greenhouse is situated on the corner of this decking where its closest to the ground, sheltered by some trees to protect it from snow. So the greenhouse has a wooden floor with flatbed plastic soil containers for beds about 6 inches deep. It for growing fruits and other none root produce. Its also a 3mx4m enclosure as thats the max I could make with what I had after building the decking.

I use the hotbed method in my polytunnel which is larger but with the greenhouse I dont have the area to work with inside so Im having to make an external system, I also dont have access to manure so only have greens and browns to work with.

The greenhouse uses grow lights for sunlight but it could do with more heat inside for growing less native plants. Im determined to grow tomatoes and peppers this year since Im locked inside anyway!

I dont think my compost can reach critical mass for temperature but ive seen some horror stories of grass, hay, manure etc bursting into flames when they get too large, not sure where the threshhold is tho.

I was going to look at getting a fan for improving the heat exchange but after looking this morning it doesnt need it, its a clear draft of hot air coming out of the pipe and the other end is sucking cold air in.

Ive started revamping the enclosure for the larger system and have JUST enough pipe left over to lay it, if I recycle the smaller version too, just need to make sure I can get inside it to actually fill it haha!

 

I tried a few years ago, with limited success, to do the same thing but with a water system. My theory being a coiled water pipe in the compost and circled around the greenhouse and as I said, limited success. It worked more as a soil warming affair, rather than stove house

 

Water was my original idea too but thermodynamically water takes a lot more heat to warm up, as we know from the power requirements from a kettle! I suspected water wouldnt heat up enough for convection to occure. Air seems to work very well, although it will be dependent on outside temperatures as well, since its raising the intake air, so in summer it will be hot, in winter probably cool.