Carnot efficiency in solar power systems is not as important as dollar efficiency. Flat plate solar thermal can easily provide 100C temps which can be cheaply stored for long periods of time. A lower carnot efficiency does lead to the need for larger equipment for each watt of output. Wind turbines are a good example of a large device with poor thermal efficiency but excellent dollar efficiency.

Sigh. I had no desire to repeat a thermo 1 lecture at this late stage in my life, but on thinking about it, decided that it might be good to do so as background information for those who might be misled by the above two entries. (Sorry, EP, all of the below is elementary and you need go no further. Have a good vacation.)

"Low temperature solar thermal" bears no internal contradictions (oxymoron- a congregation of contradictory words). Elementary thermodynamics declares that any temperature difference between heat source and sink allows a reversible heat engine receiving heat from the high temperature source to produce power at an efficiency equal to the temperature difference between source and sink divided by the high temperature (Carnot efficiency).

Thus a flat plate solar receiver, delivering heat at say 200C (473K), might allow a reversible engine rejecting heat to atmosphere at 50C (323K), an efficiency of 150/473 = 31%

But, alas, we all know that there is no such thing as areversible heat engine in real life, except perhaps one running infinitely slowly, producing no power.

Experience shows that real vapor cycles (steam, organics, etc operating on the Rankine cycle) might deliver an efficiency perhaps 60% of Carnot, or higher if all components are highly efficient. Thus a real vapor machine operating as above might have an efficiency somewhat above 15%, which must be further attenuated by other component efficiencies such as those of the absorber, alternator, electronic convereter and all the rest of reality.

This brings up a source of common errors. Efficiency is unfortunately, variously defined and often misused and/or confused. The carnot efficiency above refers to the CYCLE efficiency of an ideal machine. The COMPONENT efficiency, is quite different; it is defined as the ratio of component performance relative to an ideal machine performance. In the case of an expander in an ordinary vapor cycle, its efficiency may be defined as its power output divided by the ideal power output (mass flow rate times isentropic enthalpy drop from receiver pressure to rejector pressure). This may range from zero ( a throttling valve) to very high, approaching 100%.

In a reversible thermal machine, all components must have 100% efficiency, even though the ideal cycle efficiency might be say, a mere 30%.

What I was getting at in my remark above, is that IF there is a very good vapor expander available, THEN relatively low temperature vapor cycle systems might give good overall performance, and one should always consider this possiblilty when judging relative merits of the several solar power opportunities.

Disclosure- I am not a fan of PV- reasons given above. PV gets far too much money, and I and my poverty-ridden fellow thermal machine enthusiasts, get little or none. Drat!. Nevertheless, grieviously burdened by futile PV envy tho I might be, I shall now return to my nap.