ASK THE EXPERTS: Steam Electricity

I live in an off-grid cottage and my biggest energy expense is for propane, which I use for space heating in winter. Because my farm has more firewood than I can possibly use, I’m planning to install a wood-fired, water-heating stove to eliminate the need for propane. I would like to find a way to convert wood to electrical energy as a backup, so I don’t need to rent a generator and can run electricity to other dwellings and buildings on the farm.

I know this technology was around long ago, because steam locomotives used it to generate electricity for their running lights. Only 100 to 150 psi of steam was needed. A small boiler/turbo generator could be installed for backup or full-time power as needed, with no dependency on outside energy sources.

Are there any companies that make small boiler turbo generators (5–20 kW) that could be run on wood?

Clyde Koral • via e-mail

You’ll likely be out of luck trying to buy a commercial, home-scale steam turbine generator that burns wood, for two very good reasons—safety and practicality.

Water expands to 1,700 times its original volume when heated to steam, and each gallon of water in a boiler carries the potential energy of a stick of dynamite! Boilers for steam turbines and steam engines must be monitored constantly, especially when burning fuels like wood where the energy density varies from one chunk to the next. Even a 10-minute trip to the fridge for a soda and sandwich is too long to leave a woodfired steam system unattended.

We have a wood-burning steam engine here at our off-grid shop, spinning a 2 kW alternator, but it’s there for fun, not to depend on for backup electricity. Someone has to monitor, stoke, and water the boiler all day long, thus getting little work done in the shop, making steam backup power impractical for us. Steam power is fascinating, though, and there are many science-fair-sized steam turbine models on Internet video sites, or you could join a steam-engine enthusiasts club for help in restoring an antique or building your own.

There are other options besides steam for making electricity from wood, but all are complicated and expensive do-it-yourself projects at the 5 to 20 kW scale you want:

• Stirling cycle engine: These heat-powered machines are quite safe, but very pricey. Plans for machining and building your own are available, but few actual products larger than toy model fans for your wood heater exist. Plus, they have a reputation for early failure.
• Thermoelectric cells: Also common in wood heater fans, these use the Peltier- Seebeck effect to make DC power directly from heat. Modules of 25 to 100 watts are very expensive, and past products have suffered reliability problems from overheating.
• Wood gasification: This technology uses heat and chemical reactions to break down wood into flammable gases for burning in a standard internal combustion engine. Tens of thousands of vehicles were retrofitted with gasifiers in Europe and Asia during 1940s wartime gasoline shortages. You can buy a parts kit today to build your own, and plans abound. However, wood gas is not a “hit the switch and forget it” sort of fuel, and deadly carbon monoxide is one of the gases it produces and burns. You can’t just throw logs into your gasifier; charcoal, sawdust, or very small chunks of wood are required. Gasification is a very advanced do-it-yourself project, but is probably your best bet if you choose to continue your quest.

Making electricity with firewood is a difficult way to go, and requires lots of time, money, advanced skills, and imperturbable enthusiasm. If I had a huge surplus of wood as you describe, I’d harvest sustainably, sell the extra wood, and invest the proceeds in greater energy efficiency for my home and more solar-electric modules for my roof.

Dan Fink • www.otherpower.com