Thinking about composting and micro generation

Before I go on, let me address the alternative fuel Nazis. Yes, what I am proposing is not ultimately sustainable in the short term. We need stop gaps. It took us hundreds of years to develop our addiction to fossil fuel. It may take longer to get off it.

Most cities today get their power from far away. It seems we lose at least 30% due to line loss. See this link for an explanation of line loss. That author says up to 30%. I'm going with more because of the local connections, transformers, overloaded wires, etc.

Most houses receive a good supply of natural gas that could be used to generate power at the source. This would bey definition be 30% more efficient or better. Efficient small internal combustion engines are available now. Small gas turbines are available for quantities appropriate to office buildings. Sterling engines have been a twinkle in the eyes of many eco-friendly energy generation buffs for a long time. Small diesel engines are also there and can be run on natural gas.

Technology is changing as well. These small, efficient internal combustion engines were non-existent 20 years ago. Rising fuel prices and EPA regulation have pushed that technology along. I suspect if there was a demand for really, really efficient micro generation, it might advance even more.

The bonus here is that a natural gas fueled residential micro generator could be partially or even fully powered by methane recovered from composting. The size and efficiency of the composting operation is the only limit.

Additionally, heat recovered from the composter can be used to directly heat the home or heat water to a level suitable for showers and baths. The composter could be used to preheat water for dish washing or other sanitary uses.

Now let me address the "no way will it pay back" zealots. Return on investment and total cost of ownership has to do with time and technology. We should have all learned by now that durable products have good payback over time. We should have also learned that over time, technology improves everything. Also, over time, old commodities that were once plentiful and cheap become constrained and expensive. Oil prices are headed north of $100.00 / barrel again. The reasons, in the long term, are persistent. The biggest reason, economic development in the "3rd world", has just begun to surface. Pay back is about to become a back of the cocktail napkin equation.

My probable design

How Big ???
A guy named Jean Pain did this in 1980 with a 200 ton composting pile. He supplied all the heating and power for his home and ran his truck with the methane. These guys were getting 1000 btu / hr per ton of active compost to be used for heating. Their pile was 220 tons.

The material density is highly variable obviously. Jean Pain said it was 40lbs / cu ft. wet. That's about the density of douglas fir lumber. As a check 10 tons is then 500 cu. ft.


If I get 1000 btu / hr then well have 240,000 btu / day.

Our furnace is 66,000 btu. So the composter could theoretically compensate for it running for 3.5 hours. That seems like way more than it runs. Need an hour meter. Anyway, 10 tons of compost could heat the house. Further research seems to be needed.

Composting time seems to depend on how much decomposition you are after. The New England project seems to have gone for 5 months. Pain seems to have gone up to 18 months. With a continuous top charger, bottom unloader this will equate to a rate of "flow" through the unit. Finding the right rate is going to be a lot of trial & error.

So let's say four containers 5' x 5' x 5'. This would be low enough for Teri to throw stuff into. We might want to make it a series of 10 units 2' x 5' x 5'. Or we could make it tall & skinny. The neighbors might have issues with that, dunno. Modular would allow for an incremental build as well. It's going to take time to do, and time to charge with material. Getting one unit up at a time probably makes some sense.

As I hinted at above, I'm thinking about more of a continuous feed design. I think a top loader with a 45 deg ramped bottom. This will allow us to charge and unload the compost incrementally. Since our green waste production is not going to be in huge amounts like a farm would have.

I'm initially thinking that concrete block construction covered with rigid foam insulation and stucco or siding over that is the way to go. We are trying to conserve heat. I was struck that neither of the projects I found insulated the pile.

I would accomplish heat recovery with copper or stainless pipes running vertically in the stack. Then they'd run under ground into the house.

Heat storage is always interesting on these things. I think a heat exchanger is in here someplace. Using primary water in the recovery system is a bad idea. First we probably want it treated to prevent corrosion & allow for best pump life. (may want to think about that, a leak could kill the pile). Second a break could get compost into the house water supply. Not good. Also a heat exchanger would allow both the hot water system and the forced air system to use the energy.

I think a tank full of water and rocks is probably a good storage medium and could double as the heat exchange vessel. Above ground is probably good for starters until we get this all dialed in. I'll probably have to make it out of fiberglass or find a good used / surplus tank. Need to cover it with a shed to make it look nice. I'll have to do some research to figure out the in flows and out flows of energy.

The storage tank(s) could be modular as well.

So, here's the big mystery to me. The composting process generates methane. Neither process recovers that. Pain uses a digester, presumably with manure or compost, to get gas. The New England process says they could gain efficiencies by recovering the gas, but they don't do it in that project. Dunno yet.

Okay .... well, that's a lot to think about. We'll see if it still makes sense after thought and more research.