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- Path: sparky!uunet!ogicse!plains!news.u.washington.edu!milton.u.washington.edu!whit
- From: whit@milton.u.washington.edu (John Whitmore)
- Newsgroups: sci.physics
- Subject: Re: Solar Concentration limit ?
- Message-ID: <1992Jul25.050806.2371@u.washington.edu>
- Date: 25 Jul 92 05:08:06 GMT
- Article-I.D.: u.1992Jul25.050806.2371
- References: <24427@sophia.inria.fr>
- Sender: news@u.washington.edu (USENET News System)
- Organization: University of Washington, Seattle
- Lines: 35
-
- > Let us consider a black surface exposed to the solar radiation (say f.i.
- >F=1000 W/m2).
-
- > Without concentration it reaches an equilibrium temperature calculated by :
- > 4
- > sigma*T = F (about 100 deg. Celsius)
- >
- > In a perfect concentrator, with a given concentration factor f, we have :
- > 4
- > sigma*T = f * F
- >
- >For a given (great but non infinite) value of f, we can then obtain very great
- >calues of T (we know that temp. > 3000 K are "easilly" obtained) but the
- >problem is that in the formula herover it is theoretically possible to obtain
- >for T a value of f.i. 7000 K, i.e. a temperature wich is over the sun
- >temperature.
-
- >We know on another hand that this is strcitly impossible due to the 2nd
- >principle of thermodynamics.
-
- If one looks at the object and image of a simple lens system,
- the brightness (watts/sq.cm/steradian) is the same. Concentrating
- by making a smaller image area increases the steradian measure of
- the incident light cone. Conversely, concentrating by reducing
- the solid angle of incident light results in a larger image area.
-
- This implies that an imaging optical system consisting of simple
- lenses doesn't break the second law of thermodynamics with respect
- to radiant heat flow. I suspect that generalizing to non-imaging
- optics is possible, but the more puzzling case is nonlinear optical
- effects (like holograms). I think the second law of thermodynamics
- is rather easier to apply than a full analysis of possible optical
- concentrators!
-
- John Whitmore
-