Webmaster said:
So far we are only guessing at total system efficiencies....
...but we are getting some pretty good guesses. I've calculated overall system efficiency two different ways, and I've come up with numbers that line up pretty well.
First, at top-down estimate. I know what my oil consumption was in gallons per degree day back before I had wood, and I know the approximate efficiency of my oil burner (from manufacturer's specs). I also have two full seasons of heating solely with wood. From that, I can calculate that I'm getting about 15,000,000 BTU per cord.
Second, a bottoms-up estimate. This involves measuring the moisture content of the wood, weighing it, burning it, and measuring the heat delivered into each heat load (baseboards, hot tub, hot water, and storage). From that I calculate that my system efficiency is about 55% based on a theoretical fuel value of 8600 BTU/lb of bone dry wood. Given my mix of species, that works out to around 14,000,000 BTU/cord. I'd expect this number to be lower than the first, because there are heat losses from the system that end up heating the house anyway - heat radiated from the boiler, stovepipe, and chimney, for instance. The first number in effect gives me credit for those losses, while the second number does not.
These numbers tally very well with my brother's gasifier experience.
He had a very clean burning conventional boiler, and switched to a gasifier. He saw a 40% reduction in the amount of wood required to generate the same amount of heat. He's even more obsessed than I am - weighs all his wood and keeps a journal.
Based on a fair amount of hard data, I think a properly operated gasifier uses about half the wood of the best non-gasifier designs. I also expect that there's a lot of variation, and we're still working on exactly what 'properly operated' means.