Paul Ciszek said:
That is my point, exactly. So, has this Brayton cycle been tried with
solar heated engines? Does it work any better than the closed
Stiriling engine?
The Brayton cycle is the classic cycle of gas turbines (intake, compression,
heating at constant pressure, expansion, exhaust). But I haven't seen it
used in solar installations. The 'trick' would be to get the air to flow
through the reflector's focus and gain a lot of heat in the very short time
it is there (to get much power, you need a healthy flow rate). But that's
no different than any other open cycle setup.
The closed Stirling can have the air pressurized so that even during the
'low pressure' phase, the air is much denser than STP. The denser air can
absorb heat from the heat exchanger surface better/faster, meaning it can
absorb more of the energy applied to it.
In the Brayton though, you're limited as to how high a pressure the
compression stage can go. Higher compression ratios mean higher temperature
going *into* the heating section from the compressor. Not a problem with
combustion versions, but with fixed temperature heating plates that means
less heat energy can be transferred into the air in the heating section, so
less total power.
And rigging the Brayton's exhaust so it helps heat the compressor outlet
could be a tricky engineering problem. The Stirling does this with it's
recuperator.
daestrom