alt.energy.renewable

Erdemal wrote:
> Attempt to set efficiency from sun light to electricity
> using a parabolic reflector and a 'stirling engine':
>
> - an aluminium foil seems to be able to reflect more than
> 90% of sun radiations (1)(2)

I did a quick websearch to see what the Kramer Junction
solar thermal plants were using and found this document
at NREL.
/csp/troughnet/pdfs/

It looks like they get 93% reflectance on their reflectors but
they're considerably more expensive than aluminum foil.

> - that energy should enter the hot source of the thermal
> engine with little loss : some thermal solar panels have
> an efficiency of over 80%(6). Usual glass absorb very
> little of sun energy (0).

Solar thermal collectors get less efficient the higher the temperature
difference inside and outside the collector. 80% efficiency is typical
for a solar air heater. I've heard around 50% for average flat plate
solar water heaters. These solar thermal collectors only run about 60%
even with the vacuum, anti-reflective coatings and cermet selective
coating.

> Stirling engines are said to come close to Carnot efficiency
> So what ? 30% efficiency when sun shines ?

I found this article which quotes their efficiency at about 15%
/wiki/Parabolic_trough

This would make the steam turbine about 27% efficient, which isn't
bad at all. This makes the entire efficiency add up to something
like...
100% (sunlight) x .93 (mirror) x .60 (collector) x .27 (turbine)

I suspect there are also losses in the pipes leading from the
collectors and that the turbines themselves are a bit closer
to 30% than 27%.

Just a note to those who feel it's a burden to produce 'back up'
generating capacity for those times the sun doesn't shine. The
Kramer Junction plant burns natural gas to power the turbines
when needed. This means it is its own backup and there is no
extra cost to maintain a separate backup generator.

Anthony