A combined photovoltaic/thermal collector has been built and is now being tested. Initial tests have concentrated on evaluating the thermal efficiency of the collector before and after the silicon cells are mounted. With likely improvements in bonding between cells and receiver and in the absorptivity of the cells, thermal efficiencies greater than 50% can be expected for the combined receiver operating at 100 C

Samara, G. A.

English

NASA Technical Reports Server

IiN!EZRATED PHOTOVOLTAIC-T€EIL SOIAR 
mm CONVERSION SYSTEMS 
George A. Samara 
Sandia Laboretories, Albuquerque, New Mexica 87; .-5 
(Internally Funded) 
/ 
PriAcipal Investigators: D .  G.  Schueler 
R. P. Stromberg 
Person in Charge: 0 .  A .  Samara 
presented a t  
National Solar Photovoltaic Pz :-am Review Mesting 
July 22-25 (197’5) LOG Angeles, California 
297 
https://ntrs.nasa.gov/search.jsp?R=19760005395 2020-03-22T17:56:57+00:00Z
AIsTRAm 
One potential. solution t o  the economic viabi l i ty  of solar enepgy is the 
application of t o t a l  e n e m  concepts t o  solar energy cmversian systems,  he 
t o t a l  energy conmpt e I V i S i 0 n S  providing; a major-fraction of the energy requSre- 
ments of residential and Ught commercial buildings frcrsn solar energy. Specifically, 
electricity,  a i r  conditioning, space heerti.ng; anB hot water w o u l d  he provided. 
b i l i t y  of a combined photovoltaic-thermal solar t o t a l  energy fjysteai, and ( i i )  t o  
develop the hardware which can provide the proper ra t io  of electr ical  and thermal 
energy for  residential and ccnanercialapplications a t  a cost competitive wi th  
alternate energy sources. 
The approach adczted is based on solar concentration ( in i t ia l ly  40-50 sans). 
S;;xnlight is concentrated M photovoltaic cell:' (currently single crystal silicon) 
which absorb the l ight and convert SOBE of the solar energy into electricity.  
The reminder of the absorbed energy, which is  converted into heat i n  the cell ,  
is extracted as useful thermal energy by a circulating fluid.  
that the  dollar values of the electr ical  and thermal energies produced by the 
combined system are nearly equivalent fo r  the applications envisioned here, the 
cost-effectiveness of the combined collector system is essentially doubled com- 
pared t o  photovoltaic concentration systems which do not matte use of the excess 
heat. 
The ov*rall objective! of th5s Sandia progmm is ( i )  t o  demonstrate the ferrsi- 
Since w e  f i nd  
The program consists of four closely coordinated tasks: (1) Systems Analysis, 
(2) Silicon Cell Development fo r  High Illumination, High Temperature Operatian, 
(3) Development of the Combined Photovoltaic-Thermal Collector, and (4) Systems 
Demonstrations. Work on the first three tasks is well underway. The basic plan 
for the program is t o  bcild on existing broad technology bases and t o  concentrate 
on developing an overall technical and economic view of the combined to t a l  energy 
systems. 
computer simulation of both solar c e l l  and overall system configuration and per- 
formance, i n  p a d l l e l  efforts which seek to  define c r i t i ca l  or important design 
paramet s. 
balance between ( i )  the solar input power, ( i i )  the power load t o  the system, 
(iii) the energy storage requirements, and (iv) '-.he available conventional power 
sources. I n  addition, since both solar input powe-s and load power requirements 
Are time varying functions, dynamic systems modeling tech;  nivn* 
determine the optimal system configuration and enerr_/ str 
GO support load demands for a l l  available time hir:,o. .c 
I n  the early phases Qf this program we have emphasized modeling and 
The successful utilization of solar energy depends on achieving the proper 
x e c  ?d t o  
,& necessary 
wer and load 
data fo r  my given app1ic.-tion. 
solar energy systems computer analysis code (SOLSYS). We are employing this 
code t o  assess the technical and economic feasibi l i ty  of the  combined ,Jstem 
relative t o  various uti l ization scenarios. I n i t i a l  studies have dealt w i t h  
configuring individual residences using Albuquerque solar and load data. 
Touards this end Sandia has developed a large 
The Silicon Cell Development task has emphasized the redesign of the  c e l l  
t o  yield high efficiency a t  the operating conditions of the combined system 
(40-50 suns and temperatures t o  .., 100°C). A Sandia-developed numerical semicon- 
ductor device analysis code which yields one-dimensional numerical solutions 
of the carrier transport equations throughout the thickness of the c e l l  i s  
being used and has proved t o  be an indispensable aid towards achieving the  
desired goal. Analyses have been carried out on a number of c e l l  designs a t  
solar  intensities ranging from 1 t o  140 suns and temperatures from 27 t o  20OOC. 
A design has been selected and is being experimentally tested which yields E!$ 
conversion efficiency a t  50 suns and 100°C. The modifications needed, i n  com- 
parison wi th  the conventional cel l ,  include lowering the substrate r e s i s t i v i t y  
by an order of magnitude t o  .U 0.3 P-cm, reducing the junction depth from 0.5 
20 -3 t o  0.2 (rm, decreasing the diffused surface layer concer.tration t o  ~ Y . X l 0  cm , 
and optimizing the top surface metallization. 
b u i l t  modified ce l l s  have thus f a r  given excellent confirmation of the code 
results. 
Eqerimental  r e s u l t s  on Sandia- 
A combined photovoltaic-thermal collector has been b u i l t  and i s  now being 
tested. I n i t i a l  t es t s  have concentrated on evaluating the thermal efficiency 
of the collector before and af te r  the silicon ce l l s  are mounted. With l ikely 
improvements in bondi-ig between ce l l s  and receiver and in  the absorptivity of 
the cells,  thermal efficiencies > 50$ can be expected fo r  :he combined receiver 
operating a t  loO"C. 
Since we are s t i l l  i n  the  early stages of t h i s  program it i s  expected that 
the analytical and experimental work under the various tasks w i l l  contime fo r  
the near f u t u r e .  
and other loads; the modified c e l l  work w i l l  empkiasize optimizing the top- 
surface metallization; and the  combined collector work w i l l  concentrate on 
optimizing both electr ical  and thermal efficiencies and gaining operational 
experience under f ie ld  conditions. 
development model w i l l  be formulated. 
The systems analysis w i l l  be expanded t o  include other c i t i e s  
Plans for a complete integrated system 
The attache3 figures elaborate on and further i l lus t ra te  certain aspects 
and accomplishments of the program. 
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SOUR ENERGY SYSTEMS CONIPAl?lSoN 
FIGURE 1 (top) A solar energy system us- solar-thermal conversion 
and the& cascading. 
A solar energy system using combined photmltaic  and 
thermal convewion. 
(bottom) 
S O U R  COUECTOR 
F-2. Anintegrated 
photovoltaic- 
thendl solar 
concentrating 
collector. 
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SUMMARY 
e Sandia's Solar Energy Systems Computer Analysis Code (SOLSYS) 
is  Being Utilized to Assess the Technical and Economic Feasibility 
of Combined Photovoltaic Systems Relative to Various Utilization 
Scenarios. 
Combined Systems Using Concentrators and High Efficiency ( 2 10%) 
Silicon Cells can Provide Residences (or Communities) with a 
Reasonable Balance of Electrical and Thermal Em rgy. 
0 The Device Analysis Code has been Used to Redesign the Silicon 
Cell for High Temperature, High Illumination Operation. 
0 A Design Yielding 1B Efficiency at 50 Suns and lO0C has been 
Selected. 
0 Modified Cells have been Fabricated and Tested. Results have 
Confirmed Code Predictions. 
A Combined Collector has been Built and is being Tested. 
Thermal Efficiencies '50% at 50 Suns and IOOOC can be 
expected. 
307 


Integrated photovoltaic-thermal solar energy conversion systems

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