> You are overestimating the effectiveness of a real-world Stirling.

I most certainly am not :-).
Realistic stirlings run Th/Tc of 2:1 to 2.5:1 for a 50% to 60% Carnot 
efficincy and enthusiastic ones run 3:1 or more for a 66%+ Carnot 
efficiency. Good supplies of non-existeum help lomgevity in such cases. I 
allowed 20% end to end efficiency.

        >> Make that say 5 kg to allow 20% storage
        >> to output (easily in a Stirling's
        >> capability).

The electrical to mechanical conversion can be quite good - say 80% +, so 
with a 2:1 Th/Tc that needs 50% of Carnot (0.5 x 0.8 x 0.5 = 0.2). Doable in 
serious applications. Higher hot temperatures will help this at the expense 
of 'engineering difficulties'.

> Neither the Stirling nor any device could convert the heat energy of a
> fuel with 100% efficiency, even in theory.  This is a result of the
> consequences of entropy on thermal rules.  Stirling is not an exception.
> Stirling can achieve very high thermal efficiency (in theory), which is
> less than the energy contained in the fuel.

Aye.
E_theoretical max = E_Carnot = (Thot-Tcold)/Thot.

> In reality, practical Stirling engines do not see extraordinarily high
> efficiency.  They still can in theory but there's materials and cost
> limitations.

20% of fuel energy is entirely doable for serious designs.

> Their remarkable property right now is being able to do
> work off of small temperature differentials, thus being able to use
> "waste heat" that is essentially free when available.  Look how much a
> car wastes!

LTD (low temperature differential) Stirlings have their niches but are 
unlikelt to be overly useful in gross energy applications (such as the one 
being discussed.).Thermal "energy harvesting" is a very low efficincy niche 
area due to the Carnot efficiency limit as above.

A cute auto waste heat user, based on the Carnot cycle, is the "Vuilleumier 
machine". Gargoyle knows. NO compression or pistons per se - just dual 
displacers.


   R



>> Hydrogen gives you about 30 kWh/kg, so for 25 kWh at 100% efficiency you
>> need ~1 kg

>> Make that say 5 kg to allow 20% storage to output (easily in a Stirling's
>> capability).

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