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User talk:KVDP/AT CAD Team/AT solar power tower

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BRASH ?[edit]

The first 2 (steam engine) systems could possible be made more efficient by adding an air compressor in the regeneration cycle, see

Steam engine efficiencies[edit]

-- From old talk page Andy Dingley ( )

Having looked at the Multiple expansion steam engine, I was wondering how much efficiency can be attained with using this engine (ie can the efficiencies of the engine (parts) be combined; ie 15% + 15% + 15%, ...) ? I'm also wondering about the same thing with other engines; ie I'm pretty sure the efficiencies of ie an Internal Combustion engine + a Stirling engine can be combined, but can ie a Steam engine + a Stirling engine or a Stirling engine and a Stirling engine be combined (both working solely on heat, whereas ie the internal combustion engine also works on the force of deflagration) ? If it can be combined, this would also imply that the engines are generally underdimensioned; ie that it can not extract enough heat from the available heat.

In addition, have there been any new advances to increase the efficiency from steam engines; from the article at it atleast does seem so, ie the AM and E Pritchard car seem to have been able to run 22-23 miles per us gallon; thus efficiency would be higher than 15% no ?, perhaps that these advances can be added to a compound steam engine to increase the efficiency ? —Preceding unsigned comment added by (talk) 12:00, 24 August 2010 (UTC)

Thanks in advance, (talk) 08:37, 19 August 2010 (UTC)

This is a difficult question, involving the definition of "system boundaries" in a thermodynamic system - always difficult. The efficiencies here (triple expansion engine) can, and have been, added (although exactly equal values sound dubious). This doesn't mean that other efficiencies can be added in the same way. As an illustration of how complicated it is, using that high-pressure cylinder alone (i.e. venting the intermediate pressure to air instead) would increase the efficiency of that first HP cylinder alone.
The problem is that you can only add efficiencies trivially like this if each cylinder is an isolated system. A particular problem here is "back pressure" - the HP cylinder can be more efficient operating to an exhaust pressure of a low atmospheric pressure, rather than maybe four or five times that, into an IP cylinder. Yet there's no way to achieve the efficiency of the two cylinders combined, or for the HP cylinder when used compounded like that to approach its simple efficiency. In particular, a third stage of expansion in a cylinder is pretty much only possible when working into a condenser, which gives an even lower exhaust pressure than atmospheric. (If the condenser failed on a voyage, a ship often re-configured such engines into two-stage compounds instead).
If you measure the efficiency of a cylinder, you've measured its efficiency in one circumstance. If you change that circumstance, perhaps by either adding or removing a later compounding stage, you will change that efficiency. The "15% + 15% + 15%" description might describe the sharing of relative power generation across the cylinders, but it's risky to see any of them as "Lego brick" units that will each extract 15% of available thermal energy and may be plugged together in any combination.
One of the few engines that can be added to the exhaust of another is a Stirling engine. This is because its input is purely heat, not pressure, and so a "downstream" Stirling doesn't have serious effect on an "upstream" engine. There's also the well-known ability of the low-enthalpy Stirling engine to run on very small temperature differences (i.e. an exhaust that's already cool). Be careful though, as most of these (the "coffee cup engines") work by also being very low power, i.e. low-mass displacer, low-friction seals etc. and they're impractical as a prime mover.
You need to understand enthalpy before going further. I suggest finding a book on steam turbines and reading that. The best one I have is a training manual for ship's engineers, not a thermodynamics book. It's far more readable than the physics texts. Steam turbines, because these were the leading edge of development at the time the theory was really understood. Although there are good texts on compound piston engines too, these are few and far between ("Power from Steam" being about the best). It would also be useful to look at Stumpf and uniflow engines.
I also need to find the time to expand and fix turbo-compound engine article some time, particularly for the Saurer work on highly supercharged diesel engines in the 1930s. These came about because it's possible to supercharge a diesel almost without limit, until eventually it becomes more difficult to extract the power from the expanding gas than it is to generate more gas power. A moving piston can't expand the gas enough to expand the necessary energy from it. Saurer's solution was to add a power turbine to the exhaust and to use this to also help drive the output shaft. Eventually this leads to the turbocharger - if you're adding power by supercharging with a centrifugal compressor, and you're having to use a turbine to extract enough of this power, then it's simpler to separate their mechanical coupling and use one to drive the other. Andy Dingley (talk) 12:41, 24 August 2010 (UTC)