parabolic mirror question

[Florian Jakob]

Hi,
if I use a parabolic mirror to concentrate sunlight, how much of the
energy can be concentratet? 100% or less?

thanks, Florian

 

[Eeyore]

In reality, there's no perfect reflection. So it's always less than 100%.

Perhaps in the region of 98% with a good mirror.

Graham

 

[[email protected]]

Perhaps in the region of 98% with a good mirror.

That seems very unlikely outdoors.

Nick

 

[Eeyore]

The best, most expensive, mirrors that are able to withstand weather
only reflect about 97%. They do make better mirrors but they're only
used for things like lasers. More common materials are a little less
reflective.

This isn't terribly important because mirrors are relatively cheap
and making them larger to compensate for a little loss isn't all
that expensive.

You do realize that ordinary PV cells aren't built to withstand the
temperatures such a concentrator will produce, don't you? You need
special cells that are more expensive and even then you'll need to
cool them or their efficiency plummets.

Or intercept the infa red etc with a filter. I heard somewhere that water worked
well in that respect.

Graham

 

[Bill Kaszeta / Photovoltaic Resources]

Hi,
if I use a parabolic mirror to concentrate sunlight, how much of the
energy can be concentratet? 100% or less?

thanks, Florian

The concentrator can only focus the direct beam solar irradiance, about
20 % of the solar irradiance on a non-concentrating solar panel is contributed
by the blue sky, reflectance from clouds, ground reflectance (on to tilted
panels), etc. and is lost to focusing systems.

You also have to consider the tracking accuracy, the higher the concentration,
the more inportant the accuracy of aiming at the sun.
Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
[email protected]

 

[Al]

| Hi,
| if I use a parabolic mirror to concentrate sunlight, how much of the
| energy can be concentratet? 100% or less?
|
| thanks, Florian

Yes and, more to the point, definitely not more than 100%

 

[DavidM]

That seems very unlikely outdoors.

Here is a home made parabolic reflector;
http://cockeyed.com/incredible/solardish/dish01.shtml

It was not built for efficiency, more a burning device. It gives some
idea of the difficulties involved with harnessing the power, never mind
achieving anything close to 100%

Not sure why they have not attempted to measure the energy yet. We
measured the energy from one burning peanut in kiddies Biology lessons.
It would be a simple task using 1kg of water in a black can with a
thermometer.