Biomass · Stirling
The Stirling engine burns fuel outside itself, which means it will run on wood, pellets, gas or anything else that makes heat, without soot or ash ever touching a working surface. Sealed versions run for tens of thousands of hours without being opened.
It is also the technology on this hub with the longest record of commercial disappointment, and understanding why is more useful than any specification.
How it works
A fixed quantity of gas is sealed inside the engine and never leaves. Air will do; helium and hydrogen do better because they move heat faster. The machine shuttles that gas between a hot end and a cold end. Heated, it expands and drives a piston. Cooled, it contracts and is drawn back. Maintain a temperature difference across the two ends and the motion continues.
Because the fire is outside the sealed volume, the working gas never meets fuel, ash or exhaust. Nothing carbons up. The oil is not contaminated by combustion products. This is the fundamental advantage over any engine that burns its fuel internally, and it is why the Stirling is the natural candidate for wood.
The component that makes the cycle worth building is the regenerator: a porous matrix sitting between hot and cold ends that absorbs heat from gas travelling one way and gives it back to gas travelling the other. Without it, that heat would be thrown away twice per cycle. With it, the engine reaches efficiencies that justify the hardware.
Stage one
Gas at the hot end takes in heat and pushes the power piston. This is where work comes out.
Stage two
Gas moves to the cold end through the matrix, leaving much of its heat stored there for later.
Stage three
Cold gas is compressed, which takes far less work than compressing it hot. Heat is rejected here.
Stage four
Gas returns through the matrix and collects the heat it left behind, arriving hot without new fuel.
Two architectures
Every Stirling engine you will encounter falls into one of two families, and the difference decides whether the machine is something you maintain or something you install and leave.
Pistons are tied together by mechanical parts: crankshafts, connecting rods, sometimes a swashplate. The output is a rotating shaft, which then drives an alternator the ordinary way.
This is the architecture of the demonstration engines, the bench models, and most of what a person can realistically build or repair themselves.
The cost is that a linkage needs seals where it passes out of the pressurised volume. Those seals slide, and sliding seals wear and leak working gas. Continuous thermal cycling is hard on them. Maintenance is a standing commitment.
No linkage at all. The power piston and the displacer are held by tuned springs and gas bearings, and they oscillate in resonance without touching anything. Magnets in the moving piston pass through a fixed coil, so the engine generates alternating current directly with no separate alternator.
Nothing crosses the pressure boundary, so the whole engine can be welded shut. That is what makes long unattended runs possible.
The cost is that you cannot open it, cannot service it, and cannot make one. It is a sealed component bought as a unit, and this is the architecture behind essentially every residential micro-CHP attempt.
The governing fact
The residential units developed for the European market were rated at roughly one kilowatt of electricity, alongside heat output measured in several kilowatts and in some designs several tens of kilowatts[1]. One unit of electricity for five, ten or more units of heat.
Everything else about this technology follows from that ratio. The machine is a heating appliance with a generator attached, not a generator with waste heat to spare. It only runs when the household needs heat, so it only produces electricity when the household needs heat. In a mild week it sits idle. In summer it is furniture.
Read charitably, this is a fine fit for a cold-climate property that heats continuously for six months and wants power in exactly those months, when solar is at its weakest. Read honestly, it means the annual electrical yield is bounded by the heating season and not by the engine.
The efficiency question, answered carefully
Two numbers get quoted about Stirling CHP and they measure different things. Electrical efficiency, the share of fuel energy arriving as electricity, is modest and sits in roughly the same band as other small engines. Total system efficiency, counting the heat as useful output, is high, because almost all the fuel energy ends up as either power or heat and the heat is wanted.
The high figure is real and it is also close to meaningless as a comparison against solar or wind, which are not producing heat you would otherwise have bought. Compare like with like: against a stove you already own, the Stirling adds electricity. Against a generator, it makes far less power per unit of fuel.
A peer-reviewed review of Stirling CHP concludes that relatively low on-site operational efficiencies, alongside considerable investment costs, are the main issues holding the technology back[2]. On-site is the operative word. Measured performance in installed systems has repeatedly fallen short of design figures.
The commercial record
This is the part of the subject that most writing skips, and it is the part that decides whether you can act on any of the above.
The most heavily backed residential Stirling programme was developed under British Gas across roughly seven years, using free-piston technology pioneered in the United States, and was partnered with a domestic heating manufacturer to bring a wall-mounted unit combining engine and condensing boiler to market. Trade coverage at the time of its closure reported that performance had been broadly in line with expectations, but that unit cost could not be brought down to a workable level and no viable business plan could be constructed. After a year of seeking a buyer, the venture was shut down[1].
That pattern repeats across the sector. The review literature lists a series of ventures in micro and small-scale Stirling CHP, a striking number of which appear under two names because they were reconstituted after failing[2]. Engines continue to be manufactured, and the strongest current applications are remote and industrial rather than residential.
What this means practically
Where it fits
A Stirling system makes sense on a property that heats continuously through a long cold season, already burns wood or pellets, wants a modest continuous electrical supply in exactly those months, and can absorb a substantial capital cost against a long payback. That is a real description of some properties and not of most.
The small fans that sit on a stovetop and turn without wiring are Stirling engines, and they are a genuine demonstration of the principle at work. They are not generators. They convert heat into rotation to move air, and there is no electrical output to take away. Useful, and a different thing entirely.
If the goal is a small amount of electricity from a stove already burning, thermoelectric modules deliver less power for far less money and complexity, with nothing that can seize. Compare the two honestly before assuming more output is worth the difference.
Whatever the source, the sizing question is the same one the rest of this domain asks. The Power Needs Calculator works out what your household actually draws, and a one kilowatt continuous supply covers more of an ordinary load list than people expect, provided the heat is wanted at the same time.
Common mistakes
A figure above eighty percent counts the heat as useful output. It is a true number about a heating appliance and it says almost nothing about how much electricity you get.
The engine runs when the house needs heat. Multiply the rating by heating hours, not by 8,760, and the annual figure falls by more than half in most climates.
It is welded shut by design, which is the same reason it runs so long unattended. Serviceability and longevity are the same trade made in opposite directions.
This sector has more discontinued products than live ones, and the web pages outlive the ventures. Confirm current supply and support directly before any plan depends on a specific unit.
The engineered systems pair the engine with a burner designed for it. A stove whose output swings with every reload is a hard heat source, and the coupling is the difficult part rather than an afterthought.
A sealed engine with no parts supply has a service life bounded by the company that made it. On a twenty year horizon that is a real risk and belongs in the comparison.
Next
Both pathways covered so far throw off far more heat than electricity, and both are only worth the money if that heat lands somewhere useful. That is a subject in its own right, and it changes the arithmetic on every system in this hub rather than just these two.
The combined heat and power guide is next in the sequence and is being written now.
Sources
Ratings on this page describe units developed for the European residential market and are included to show the characteristic heat-to-power ratio, not as specifications for anything currently sold. Product availability in this sector changes and many published pages outlast the ventures behind them, so confirm current supply, support and parts directly before planning around a specific machine. No figure here is taken from a company selling the equipment it describes.
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