Subject: Fresnel Dream
Michael Foster's post also brings up another possibility for a
combined Solar-Stirling system - adding synergy - which concept is
sounding better the more I think about it. Maybe after some Java,
the bright idea will defocus, however.
As long ago as 1963, the predecessor to this concept was being
investigated.
www.solar1.mech.unsw.edu.au/glm/papers/CLFR-Geelong99V6.PDF
[side note] They were talking about the "oil shock" back then - 40
years ago - proving once again the doubly-apt aphorism that there
is "nothing new under the sun"....
To paraphrase: There has been a long history of development of
linear solar thermal power systems. The linear system is simpler,
more adaptable to the Fresnel and can possibly be used in
conjunction with the Sandia Solar Sterling (SSS) as will be
suggested, after this brief history.
The first parabolic trough collector was demonstrated in the late
1860's in France ! however, it was not until 1913 that useful
mechanical power was produced from solar energy in a 41 kW pumping
system installed in Egypt by Shuman. This system consisted of five
north/south axis parabolic trough concentrators with an aperture
width of 4 m and an aperture area of 1255 m2.
Shuman's system worked successfully for a number of years.
Further development of solar powered engines did not occur for
another 50 years due to the availability of cheap oil from the
1920's. The most successful solar thermal development has been the
linear SEGS plant installed by LUZ International Inc (Kearney et
al. 1985, Jaffe et al. 1987). These plants use single-axis
parabolic trough collectors that track the sun with a north/south
axis of rotation. The concentration ratio was 25:1 and the
absorber was a vacuum insulated flow tube that carried heat
transfer oil. This technology is now being developed by Pilkington
(Pilkington 1996), and work is proceeding on direct steam
generation in the solar array in order to reduce system costs.
OK now forget this part work as being no more than pre-history.
There is much more in the offing! Of course adding a Fresnel onto
the trough aperture is a "natural" progression, but here is the
added possibility of synergy. There is a natural fit here with the
trough and the SSS
First, in the trough you circulate a molten salt as your working
fluid rather than water Why? OK here is where it gets a little
complicated. Basically the idea is that at one site, the operator
would have an array of SSS for daytime use. They would be water
cooled rather than air cooled. Also at the site is a larger array
of Fresnel troughs. They are of the less-expensive linear troughs
design covered with Fresnels. They would be used both day and
night !!
During day, the troughs would be used to heat a molten salt which
is stored in insulated cisterns underground. At night the troughs
would be used to cool stored water, also held in other nearby
cisterns - which had been used to cool the SSS arrays during the
previous day's run. This hot water has been slightly cooled
already by a 10 hour stay underground (earth heat-sink) and at
night is cooled in the troughs through a reverse blackbody
process, given a lower working temperature and added Carnot
efficiency.
At night the SSS engine - the Stirling engine-generators which are
the most expensive part of the total system by far -- perhaps 75%
of the total cost - would normally sit idle without the present
synergism. The new plan is to use them (albeit at reduced
capacity) by pumping into the "hot" end, the molten salt which was
heated the previous day in the solar troughs. The mirrors are
superfluous at night.
This extended usage would mean adding some amount of plumbing onto
the SSS, but the payoff is that you effectively double your
operating time... so that even if the output is reduced during
night use, it is basically using the most expensive component -
the Stirling engines for free.
The Fresnel troughs are an inexpensive way to heat a storable
medium (molten salt) so that the site operator can get nearly 24
hour use from the SSS which would normally be limited to half of
that. There is another benefit in that morning startup would be
much more rapid and without metal-stress - since the SSS has been
used at night with the molten salt.
This is a hasty compilation of thoughts and may not be clear, but
it seemingly deserves to be tossed around further. Maybe there is
"something new under the sun" ?
Jones
