[Stoves] Green Steam Engine

[Crispin Pemberton-Pigott]

Dear Kevin

 

Interesting perspectives.

 

I was involved in a very brief attempt to build a small steam engine in
Swaziland with a guy who had been trained 'old school' and knew how to
design one. A heat engine is not necessarily as inefficient as a hot air
engine, however. Because the working fluid can be something other than
steam, and because the heat released during condensation does not happen
with an expending and contracting gas, there are some pretty big
efficiencies available. Stirling engines (which are supposedly hot air
engines) can be run with compressed Helium and get high efficiencies.

 

A thermoacoustic refrigerator is a type of Stirling engine. The ones
designed by Los Alamos labs these days are 45% efficient at 150 kW. They use
helium as a working fluid at about 15 bars I think. That is higher
efficiency than a diesel engine, but it moves heat, not generates shaft
rotation.

 

It is however an externally fired 'heat engine' (with a resonant frequency
of about 40 Hz).

 

Regards

Crispin

 

 

 

Dear Crispin

 

Sadly, many "Boiler and Steam Regulations" are an unfavourable collection of
Bureaucracy, Conservatism, and Obsolescence." The cruel fact is that many
boilers have exploded in the past killing many people, due to incompetence,
poor design, lack of attention, poor controls, etc. 'the Laws of the Day"
were made to "plug the holes." A good steam plant is a great way to head
building complexes, and only low pressures are needed. Steam Power Cycles
typically use the Rankine Cycle, and it demands high temperatures and high
pressures for efficiency.Most steam Power Plants operating at below 500 PSIG
are considered obsolete. A "Steam power Plant" running on 15 psig steam (2
bar) can generate shaft power, but will be dreadfully inefficient. A 4 bar
plant (60 psig) is much better, but still well within the "Dreadful
Efficiency Class"

 

One cruel reality of steam is that it is a great concept, but surprisingly
complex in practise. Another cruel reality about steam is that there are a
lot of encumbering laws and Regulations associated with it. One can find
ways to evade the regulations, and this is OK, until there is an accident.
That's when the Boiler Inspector has a Field Day. 

 

As a General Concept.... "There is no way that one can sensibly generate
shaft power with small steam engines, because of dreadful system efficiency.
However, there may indeed be sensible "small steam power" generating
systems, providing there is a sensible use for low grade waste heat from
engine exhaust. For example, a small steam engine system for driving air
circulating fans in a Lumber Kiln, or a Greenhouse.

 

Hot air engines would have approximately the same engine efficiency as would
a steam engine having steam entering the engine at a temperature similar to
the temperature of the hot air entering the "Hot air engine system."

 

Best wishes,

 

Kevin

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