COMPOSTING HEAT – CAN I USE HEAT FROM HOTBIN COMPOSTING
Can I use the heat from the HOTBIN composting to heat my house, water or greenhouse?
Several customers have asked questions about heat recovery and using the heat from HOTBIN composting to heat water or heat the greenhouse. Here is a quick summary of the answers:
Can I recover the heat from the HOTBIN and use it to heat water? Yes, but three problems make this impractical.
1) An easy way to extract the heat would be to wrap a plastic or copper tube around the inside of the walls, circulate cold water through the pipe and the water inside the pipe heats up (just like a small immersion heater). This would collect the heat – but as soon as you remove heat from the waste, your waste will cool and stop hot composting – it will go cold and stay cold. Basically the HOTBIN will no longer be HOT!
2) You could extract the heat after it has left the bin (ie collect the steam from the valve) and then condense the hot steam via a heat exchanger. This is complex and the heat exchanger systems are expensive. It would only be worth it if the cost was offset by the energy saving.
3) A typical HOTBIN burning about 1-2Kg of waste a day equates to the heat output from a 30 to 60 watt bulb. A central heating systems typically is pumps out 2Kw (2000 watts) of heat. There is not enough heat to make an impact.
There are large community composting schemes that recover heat from compost heaps. They tend to have a large tonnage of waste – at least 10 times that of a normal house/garden. The theory and engineering of hot composting are covered in R. T. Haug, Practical Handbook of Compost Engineering. I don’t believe the heat output and cost of equipment to support ‘heat collection’ from a domestic composting bin is really a worthwhile option.
CAN I PASS THE HOT GASSES INTO THE HOUSE to HEAT IT? NO!
As above there is not enough heat energy to heat the house. But the bigger issue is health. The hot gas leaving the valve is carbon dioxide and hot water (ie steam). If you pipe this into the house, the water will condense on walls and within weeks you will have a black mildew mould all over the house. We need to breathe oxygen and hence need fresh air in the house. Pumping carbon dioxide in is a bad idea – although we doubt you could create a serious issue, elevated CO2 levels are to be avoided as they cause nausea and dizziness. (THE release of gases outside is different – they are quickly dispersed and diluted as soon as they leave the HOTBIN).
CAN I USE THE HOTBIN to HEAT THE GREENHOUSE? YES!
The heat is not enough to warm a greenhouse over winter, but there should be enough to keep the frost at bay. Also the gas leaving is a mix of steam and carbon dioxide – just what any plants inside the greenhouse need. With a few hours of daylight – the plants should have a slightly enriched growth environment.
PS: if you do want to move you HOTBIN into the greenhouse this Autumn – please ensure you empty the waste out first. Do not attempt to move your HOTBIN when full – the bin can weigh over 100Kgs. This is unsafe to move – think of your back, the recommended max lift is 20Kgs. You will undoubtedly damage the hot weld seal between the upper and lower halves too. This can only be repaired by an expensive trip back to the factory repair shop.