VincentG wrote: ↑Sun Sep 24, 2023 4:18 pm
The following is an excerpt taken from
Stirling Engine Design Manual by William R. Martini
Second Edition January 1983
Prepared by NASA for U.S. DEPARTMENT OF ENERGY
Conservation and Renewable Energy
Office of Vehicle and Engine R&D
As the gas is transferred at zero total volume change from the cold
space to the hot space the pressure rises. This pressure rise results in a
temperature increase in the gas due to adiabatic compression.
Therefore, at the end of the transfer process the mixed mean gas temperature
in the hot space will be higher than 900 K. Point 3 is calculated for all the
gas to be exactly 900 K. Adiabatic expansion then takes place. Then by the
same process as just described, the transfer of the expanded gas back into the
cold space results in a lower gas temperature than 300 K at the end of this
stroke. The computational process must be carried through for a few cycles
until this process repeats accurately enough. This effect will be discussed
further in Section 5.1.6.
Unfortunately, it was not explained further in any certain terms(at least in my eyes). It makes rational sense to me. The challenge then is to put this effect to good use.
This "hysteresis" is usually considered a "bad" thing. An probably inevitable heat loss due to adiabatic compression.
In a "normal" Stirling engine with both a hot chamber or side and a cold chamber or side, the "extra" heat generated from "heat of compression" causes an elevation in temperature throughout the working fluid including in the cold side, which is viewed as detrimental. Heating up the cold side is not good. That extra heat will have to be removed or will degrade the temperature differential.
However, IMO, a Stirling engine already mostly resolves this situation by shifting all the gas to the hot side 90° prior to this high compression temperature rise. The "extra" heat is just added, or in effect "pumped" over to the hot side.
Likewise, the displacer again shifts during expansion cooling so the "extra" cold pulls heat away from the "sink" just cooling it further.
A thermoacoustic or thermal lag type engine, as I think has been demonstrated by recent experiments, has no "sink" to lose the "extra" heat to. During compression ALL the gas is pushed back into the hot chamber.
Another dilemma, as far as trying to utilize this hysteresis effect is, IMO, Stirling engines, contrary to popular opinion, do not run on a temperature difference. A Stirling engine runs on heat.
You have a gas at some initial ambient temperature, expand it with heat to do "work". Essentially that is all. The so called "compression" is really a "contraction" which is a natural consequence of the conversion of the heat into work.
So in actuality, the "hysteresis" is a myth. How can you have this "heat of compression" resulting from contraction? The "analysis" based on so-called "ideal" gas behavior is mostly a mathematical fiction in all likelihood.
So how to utilize an effect that doesn't actually exist?
Not to say that heat of compression and cooling from expansion is not real generally, it's how a heat pump operates.
I think that generally, Stirling engines already take advantage of this effect to a greater or lesser extent as described above.
It helps, I think, to have an idea of what is actually going on so as to more clearly determine ways to maximize the advantage.
The usual approach is to escort "waste" heat through or over to the "sink" for disposal as quickly as possible. To me that seems counterproductive. Thermodynamics "LAW" insists upon it as mandatory.
Pushing heat back towards the heat source does not really seem like the best possible solution to me either. To one degree or another you have a heat engine/heat pump at least partly working against itself.
I'm not looking for 100% efficiency or "perpetual motion". I'd be content with a practical, working, power producing heat engine of just about any kind.
Ultimately, I'm tending towards the conclusion that the reason these engines are not already available at every corner hardware store has more to do with politics and economics than any actual inadequacy in the engines. They are already very practical and very efficient, and have been for over a century, and have only been improving. NASA has already sent some off beyond the solar system somewhere, I think. They can run continuously for 30 years without maintenance. Such high efficiency Stirling engines COULD BE, or could have been mass produced at a low cost for decades already. The technology has been proven over and over and over again.
INFINIA SOLAR Stirling engines were ready for "automotive-scale manufacturing" ten years ago.
http://santamarta-florez.blogspot.com/2 ... ipcard&m=1
Where are they now?
Before INFINIA there was Stirling Engine Technologies.
https://youtu.be/_rl1H-53Mks?si=o4bwF6bHFwuJygtQ
Before that was Advanco
https://youtu.be/vGdT9w4ubLc?si=lrX5udiLaogDH7uQ
As far as I know, about the only one of these that hasn't been turned into scrap metal is down in my workshop. One of these, but without the dish.
https://youtu.be/VEpq-WCTOrM?si=hzkvXRWWXW8fNqL7
Still not sure what if anything I might be able to do with it, without any dish, control panel or software to operate it.
https://youtu.be/6-OlbCAVBdo?si=yFpVEb837UQB_R4P
How much more advanced could this technology get?
I have a feeling our efforts to improve our little tin can engines must be rather amusing.