Justin Jones wrote:For my purposes, heat and char production are of primary importance. My electricity needs are relatively small and can be met by a stationary bicycle generator, though I understand there's a lot of sweat involved for even small yields, so a cogeneration system is definitely more desirable.
How much heat do you need (i.e. northern climate with harsh winters)? Is space cooling necessary during summer months? What about food storage - a freezer is probably the best thing for long term food storage, and that's a substantial electrical load. It seems you'll be using a laptop computer at the very least - well, what about internet connection? This will require additional electricity. Try to get a realistic estimate on the electrical loads you expect to need, and do not low ball there - if anything, be conservative in the estimates. I don't expect even the most modest off grid homes to get by on less than 3 KWh daily.
In my opinion, solar panels are the most practical means to generate electricity in a remote setting, at least in most settings. If a solar array and battery system can meet your electrical needs most of the time, then it makes practical sense to keep a tank of propane to run a small backup generator (assuming infrequent use - that is, only when required to bulk charge the battery and protect from a dangerously low state of charge). I suggest propane because it stores indefinitely vs gasoline or diesel fuel, and I see your electrical demands as very modest. A small Honda generator is a good choice for a very small system (like an EU1000).
Now, let's assume you want to get away from commercial fuels entirely and use wood for electricity generation. In that case, there are three options (well, only two really). A wood gas engine system, a charcoal engine system, or a small piston steam engine.
1. Wood Gas Engine System - The best example of which I know is the system developed by Ken Boak of the UK. See description here: http://www.powercubes.com/listers.html
. Mr. Boak has the system set up to run the engine for part of each day on wood chips. Heat exchangers on the system pick up the heat from the cylinder cooling water and the engine exhaust to heat a large store of water in an insulated vessel. The heat is transmitted to the home using a hydronic heating system (pumping hot water into the home for heating applications). The down side of this is primarily the fuel processing required. A second down side is the necessity to operate the system at a fairly high rate to keep temperatures in the gasifier high enough to generate a clean fuel gas. On this last point, Mr. Boak uses most of the electricity generated in electric space heaters just to load down the engine and help keep the gasifier temps up. It's possible to run a smaller system, but the smaller the system the more fuel processing is required - sort of a catch 22.
This system seems overkill for your application. Also, the fuel processing required to run a system like this for primary heat would be daunting. A better configuration for making use of a wood gas engine system would be to use traditional firewood for heating applications, then process only enough wood fuel as required for use in a small backup generator system - sort of like making small wood chips or chunks for a small engine wood gasifier in lieu of storing propane. However, you would need solar/wind/hydro for primary electricity.
2. Charcoal Engine System - This process entails processing wood to charcoal, but capturing the heat from the process and storing in a thermal mass such as water. The heat can then be used for heating applications, and the charcoal can be stored for use in small engines as required. I don't know of a viable system in operation, at least not on a residential scale, but it's clear that it's possible. Note that charcoal can run very small engines more easily and more cleanly than wood.
3. Piston Steam Engine - Personally, I believe this has the most promise. However, it's not practical due to the lack of hardware. There are decent small engine expanders available that are very durable, but one would have to engineer a steam generator with control system and an efficient small wood gasifying furnace to operate unattended - possible, but not for the faint of heart. The benefit of this system is primarily the ability to use wood with very little processing, and the ease in heat recovery (most of the heat is available at the steam condenser). It can also be very quiet and operate at a low output for long periods. Steam engines have also earned a reputation for longevity.
In my opinion, I say stick with PV/wind/hydro with battery system, and use a small propane fueled backup generator as required. Use wood only for heating applications.