Here's a commercially made Organic Rankine Cycle generator.
http://www.freepower.co.uk/FP120%20SpecDimensions.pdfGross power output 130 kW
Required heat input 742 kW
Inlet temp 280 deg C (553 degK)
Outlet temp 136 degC (409 degK)
Carnot limit = 1-(409/553) = 26%
Actual efficiency = 130/742 = 17.5%
But don't forget the rather inconvenient 632 kW of cooling also required...
These guys used to have a 6kW machine as well (more suited to domestic generation, operated at 180 degC input) but it took 70 kW of heat input for 6 kW electrical output - 8.6% efficiency. It's no longer on their website but was last year.
Assuming that these guys are reasonably good at their jobs and better than me trying to cobble together bits and bobs out of old car parts, perhaps 10% is the best I could aspire to for an ORC system that operates at realistic domestic temperatures.
Now, even the best evac tubes (
http://www.solarenergy.ch/factsheets/scf927en.pdf) are about 60% efficient (gross area solar irradiation to heat output) at 0 degrees temperature difference and this decreases as you get hotter. If the water temp is 200 degC inside and the air temp is 30 to 40 degC the efficiency will be below 50%.
10% * 50% is already 5% without thinking about pipe heat losses, inverter efficiency, heat storage, where all that cooling is going to come from (or excess heat is going to go to).
I absolutely agree that if the energy source is free and the heat is there anyway, it doesn't matter. I would strongly encourage tinkering an be delighted to hear stories.
However tubes and ORC engines are not free, so I wouldn't advise anyone to take out a second mortgage and cover their roofs with tubes in the hope of becoming a domestic power station and getting rich off feed-in tariffs.
