Well, this is going to be a lengthy post. I think I need the additional words to make a clear argument. Please read carefully and critically as the information is likely to challenge your beliefs.
I believe that a small piston steam engine has the potential to be superior to a wood gas engine system for combined heat and power in the off-grid/remote residential setting. I realize that this option is severely hobbled due to a lack of hardware, and just as important a general lack of knowledge (even engineers often don’t have a firm grasp on the principles, but the lack of real hands-on experience is even more damning). Also, please note that a conventional piston steam engine is not going to cut it. The thermal efficiency of these systems is generally too low to be practical. However, certain things can be done to dramatically increase the efficiency of a piston steam engine system.
The main problem I see with a wood gas engine system in this setting stems from the fact that a modest off grid home does not require electricity generated at a high rate. A wood gas engine system can generate electrical power with impressive efficiency. However, this is not useful when much of the energy is lost in batteries and inverters. More important, the losses associated with operating a constant speed generator at part load is significant. An engine capable of 20% thermal efficiency near full load might see 10% at 25% load, and small wood gas engine systems suitable for residential scale combined heat and power are generally only about 15% efficient at best. The best compromise I’ve seen is the system maintained by Ken Boak. The configuration he uses does a great job of minimizing the losses while recovering most of the heat for useful purposes. Most interesting in his case is the way he loads the engine down to pull air through the gasifier at a sufficient rate and to see high engine efficiency. He does this by operating electric space heaters off the generator head. While effective, I don’t consider this to be an efficient use of electricity in the off grid setting. The generator head provides ac power while also charging the batteries, and heat from the engine cooling system and engine exhaust is stored in water. When the engine is shutdown during most of each day, then the battery provides electricity with an inverter and the store of heated water provides heat.
It seems to me that a slow moving piston steam engine that operates at a low and near constant output for long periods could be superior. The engine can operate at much lower speeds for superior longevity, there would be a lot more heat captured from the system (proportionally speaking), and the gasifier could be a lot simpler. The engine can be used to drive an efficient permanent magnet alternator for charging a modest battery (the ability to maintain a smaller battery here is an advantage), and ac electricity can be provided with a good inverter. Matching the alternator output more closely to actual loads in the home will minimize battery losses.
Again, the problems with this alternative were mentioned in the first paragraph. My point is this: why not find a way around these problems?
It’s possible for a piston steam engine to see a thermal efficiency equal to and superior to a wood gas engine system. There are several configurations that can do this, but I tend to favor the compounded piston engine with reheat and heat regeneration as this approach does not require extremely high steam temperatures. A small engine with this general design constructed during the late 1920’s saw a net thermal efficiency approaching 30%, and this was with peak steam temperatures under 900F as I recall. Peak steam pressure was well under 1000 psig. It’s possible for a two cylinder compound piston steam engine with reheat and heat regeneration to see a thermal efficiency of 15% with steam pressure under 500 psig and with a peak steam temperature under 600F (need special attention to minimize thermal losses here with good heat exchangers and good thermal insulation). The reason this configuration works so well is that adding steam reheat between the cylinders allows for increasing the steam temperature as the steam expands through the system. Increasing the steam temperature increases the steam pressure in direct proportion, and this increases engine power. Since the steam is already a superheated vapor while expanding through the engine, then most of the heat added to the steam with this reheat process is converted to additional engine work. The low pressure steam finally exhausted from the low pressure cylinder is highly superheated, and this heat can be regenerated back into the system with air heating (both the air supplied to the gasifier, and the air used to support combustion). Furthermore, since this process increases furnace temperatures (which is not required for a limited steam temperature), then a lot of excess air can be used, and this provides a greater mass of air for full heat regeneration. So, most of the heat added with reheat is converted to additional engine work, yet a lot of this heat can be returned to the system. Looking at the system in this way makes the most sense to me, and I believe that most people with a mechanical mind can see the benefit when presented this way.
The reason I am mentioning this is that it’s clear from reading this forum that there are many people here with the fabrication skills to make something like this. Myself, I have good ideas and a solid foundation in physics and engineering principles, but I don’t have the tools nor the fabrication experience to do this in a cost effective way. I believe a slow moving piston steam engine optimized for continuous operation at 1 KW output for extended periods with special emphasis on heat recovery for useful purposes, and with a net thermal efficiency on the order of 15% would be superior to any small wood gas engine system in the residential combined heat and power setting. I believe it will provide what’s needed with a lot less fuel consumption. Think about it.
