An advanced point focusing solar technology demonstrated potential for near term commercialization as a renewable energy technology. The design features combine to produce a highly efficient, low cost, safe, adaptable, durable system which is simple to manufacture, install and maintain

Borton, D.

Rice, M.

Rogers, R.

Rogers, W.

English

NASA Technical Reports Server

FRESMEL CONCENTRATING COLLECTCR 
Power Kinetics, Inc. 
William Rogers 
David Borton 
Mark Rice 
Robert Rogers 
I NT RODUCT I ON 
During the oil emhqrgo of 1971-74, the Northeastern part of 
our countrv w ~ s  hown to be particularly vulnerable to shortaqes 
of fossil fuels which, for the most part, were coming from 
overseas sources. Other enerqy technologies had to be found. To 
displace fuels in many applications, though, alternative energy 
sources had to be able to deliver high a~alitv energy reliably. 
Therefore, even though the direct sunlight available in the 
Northeast nay only total one half that available in the sunniest 
region of our country, there appeared to be a real potential for 
cost effective solar hardware even seven years ago. The energy 
user who cocld divers' - - into : l  ternative enerqy sources cou1.d 
rec'sce the impact of . en foe price increases snci also reduce 
thc risks of having to shutdown operations because of a lack of 
sufficient heat, process steam or conventional coolinq. 
The two msjor hurd1.e~ wc had to overcome before we could 
begin an extensive effort to pro2uce active alternative energy 
equipment were: 
1. Tn prwic?e solar energy even during the har. I coi+ 
weathzr for m i c h  the riortheast is infamous, and 
2. To provide tbjs alternative energy at a price 
competitive with traditional fuels. 
!-:it!? energy consuaption increasing warldwide we he1 ieved that, 
in a reasonable amount oF time, prices of traditional fuels 
would increase sufficiently to make focused solar enerqy a 
viable arternative. 
Sxcentrating tbe sun allows heat losses to k a  minimized 
once khe energy has been captured. Therefore, even sunlight 
during the winter months could hp utilized. With the sun's 
energy being reflected from 864 Fauare feet of mirrored surface 
onto a few square feet of heat transfer material, subzero 
teaperac *Jres become less of a factor in \~sef\11 energy 
production. 
Although focusing the sun overcame our first perceives 
h u r 2 l . e  without difficull ty, it tended to anpl ify the effects of 
the seccnd hurdle. Any complexity acl?ed to solar energy 
equipment increases the already large fro~t-end costs associated 
with equipment which gathers signilicant quantities 2f low 
density energy. Cur research efforts 1~1;er the last seven years,. 
17i 
https://ntrs.nasa.gov/search.jsp?R=19810022001 2020-03-21T09:46:32+00:00Z
for the most part, were directed tawards the need to develop 
mechanical and proceclural methods for reducinq harfiware costs. 
See figure 1. 
IiARDWARE DESIGN 
Major goals which directed our efforts in engineering cost 
effective designs for concentrating solar energy were: 
1. The minimization of the overall weight of the solar 
energy collection equipment,while utilizing 
inexpensive materials; 
2. The simp1ificat;on of components snd optimization of 
the number of different parts along with the 
manufacturing procePures need& to produce them; 
3. The embo(?iment of desiqns which can be readily 
shipped, rapidlv assembled and optically aligned, 
easily tester? and quickly repa ' .  e2 by available 
Snhor; an(' 
4. The incorporation of features and co~ponents which 
auqmcnt reliable, safe and durable opcr,=?tion. 
Kjn3mizin2 the wciqht - of - tbe col lector ~rescribcd t h c  
implementation of two concepts: 
1. The c~istribution of forces from wind an? qravitv loadinq 
o n  the eq~lipmcnt, an2 
2. The use of a Frcsne! conccpt. 
Distributing the forces of wind and qravjtv over many parts 
a?lows 1ightweig;it components to be a6cquate for bcarinq the six 
tons af force anticipated from a 90 mph wind. The Fresnel 
concept is complem~ntary to the concept of distributed .loar:ing. 
Eight thin one foot square mirror tiles treated for outdoor use 
have been supported by liqhtweight aluminum stressed-skin 
supy?rt pane's w i l i c h  are pivoteci c?n their centers of gravity to 
pron,oe the motion necessary for elevatjon tracking. Using the 
Frcsncl mirror concept and 6istrihutcd loadinq permits \;in<' to 
pass throuqh the collectcr structure when the mirrored columns 
arc positioned to "feather" in the wind like open Venetian 
blinds. The mall surface area of each column allows common 
materials and construction techniques to meet the demands on 
these parts for stability and durability. Consequently, material 
:wight is mi~lmized and the corresponding cost associated with 
naterial quantitv avoided. 
T!lc simplification of cornponene and their materia! 
-- -
a qufacturing pr.)cesses was aided h v  several iterations of 
i: :ign, and construction of several. senera tjons of prototype 
ei, iprncrt. Our current c?csigns t i s ~  larqe numbers of identical 
p a r t s .  Because the demands Cor stren~th in any one of these 
parts is small, exotic raterials are avoided. Durinq the 
installation of equipment at a site, special erection equipment 
is usually unnecessary c?ue to the manageable size of individual 
parts. We founc? these choices in desig~ promoting our goals for 
reducing the overall installed cost of equipment. 
T h e  embodiment of practical aspects of design which provide 
-
the p:t:ker, shipper,Tite erection crew and operator with items 
whjch make their jobs easy, promotes acceptance of the 
tectinolocjy and enhanmsits ccst/henefit ratio. 
We have found that by incorporatinq operational schemes, 
such as keepina the reflector surface upsicie down except during 
operation, limits rpflector oxposure to- dust, ice, snow and 
vandals and ~ n h ?  .zes safety. Upon loss of power or occurence of 
other stow parameters, the unit returns the mirrors to this 
inverted ~ s i t i o n  "over the topn so that the intense focused 
radiation at no time comes below the receiver. The design o -  
other components and software subroutines incorporates this kind 
of failsafe orientation. We have found that "add onn safety 
packages are seldom as reliable, and have an undesirable "add 
ano1 cost. 
Although developing the objectives for our qoals demanded 
more common sense than any other rcsourcc, the technical 
capa5il i ties of Rcbnsselaer Polytechnic Institute, the 
organization withia which wc performed our research, were 
essential to every staqe of finalizing and testing component 
designs. With the right combination of simp1 icity and 
conplexity, we believe we have achieved a c7esiqn for collecting 
solar energy whjch is compatible with the special needs of our 
region of this country. 
SYSTEG TESTING 
Base? on the preliminary work and receiver heat transfer 
analysis, two receiver designs were selected for manufacture a ~ d  
testing. The first was a conically wound copper monotube boiler 
with 30 degree cone half angle, and the second, a steam unit 
heater emp!oyina steel cubes with aluminum fins. (Sce Figs. 2 6 
3 )  
Solar energy input was determined bv an Eppley normal 
incidence pyrheliometer with a 5 1/2 deqree aperture which had 
been recently calibrated by the Atmospheric Sciences Research 
Center in Albany, N.Y. This was coupled to a strip chart 
recorder which provided a record of instantaneous insolation 
readings. Integrated values corresponding to the discrete time 
periods for collector output measurements are utilized to 
calculate collector efficiencies throughou' the day. 
Cut?ut was determined by measurement of the quantity of 
water converted to steam and the pressure of the saturated steam 
transferred to the RPI steam system. System efficiencS- figures 
include losses ri:lm 120 feet of insulated steam line. Water flow 
was calculated by two metho8s: 1) by a Badqer Recordall 
Flowmeter and 2) by measurement of lost weiqht from the boiler 
feed tank. The test fluid loop is illustrated in Figure 4 . Note 
that steam condensate is returned to the boiler feed tank f:rom 
the steam trap. In the test of the fin tube boiler, t h e  
variation of efficiencies to some extent are a function of water 
source. Tbat is, part of the time water is fed directly to the 
boiler from the city water supply at 60F. When sufficient 
condensate accunulated in the feed tank, the water source was 
switched to the feed tank at >150F. 
The results of performance testing of these boilers are 
presentecq in Fiqures 5 and 6 . The fin tuhe boiler exhibited an 
average daily efficiency of 57%. The conical mnnntube boiler had 
an average daily efficiency of 68% an8 a peak efficiencv cf 79%. 
The graph of the test results indicates the dependence of 
efficiency on solar conc?itions. The collector has an effective 
anert~re much less than the pyrheliometer. Thus the 
pyrheliomcter accepts a greater amount of circumsolar radiation. 
Significant improvements in perf(~rmance can he expected 
wtrcn the department store mirror tiles are replaced h v  thin low 
iron glass mirrors with 10% better reflectivity. Also, the 
forming of the curves of the reflector columns to more precise 
tolerances are now possih1.e which will result in an 
additionalimprovement in performance.The fin tube boiler ha? 
very wide fins between and in front of the fluid tubes, which 
contributed to enhanced convective losses. The: use of copper 
fins would improve the pcrformance of this type of receiver. 
CONCLUSION 
This advance : point focusincj sol ar techno!oqy !>as 
demonstrated potential f c r  near term commerciaLization as an 
effective renewable energy technoloqy. The unique design 
feattires combine to produce a hiqhly-efficient, low cost, safe, 
adaptable, durable system which 7 simple to manufacture, 
install an6 maintain. 

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Fresnel concentrating collector

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