A test facility was designed, and built to provide a place where photovoltaic systems may be assembled and electrically configured, to evaluate system performance and characteristics. The facility consists of a solar cell array of an initial 10-kW peak power rating, test hardware for several alternate methods of power conditioning, a variety of loads, an electrical energy storage system, and an instrumentation and data acquisition system

Forestieri, A. F.

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ERDA/NASA/1022-77/ 19
NASA TM-73787
THE ERDA/LeRC
PHOTOVOLTAIC SYSTEMS
TEST FACILITY
(NASA-TM-737P7)	 THP ERCA/LeRC PHOTOVOLTAIC	 11,78-15059
SYSTEMS TEST FAC111 TY (NASA)	 12 p
HC A02/MF A01	 CSCL 14B
Unclas
G3/09 55548
Americo F. Forestieri
National Aeronautics and Space Administration
Lewis Research Center
Cieveiand, Ghro 44135
i'
Work performed for
ENERGY RESEARCH AND DEVELOPMENT ADMINISTRATION
Division of Solar Energy
National Photovoltaic Conversion Program	 M^
Under interagency Agreement E(49-26)-1022
TECHNICAL PAPER to be presented at the
1977 Photovoltaics Solar Energy Conference
Commission of The European Communities
LuxembOUrg, Septei nber 27-30, 1977
I	 Nu ?. Govsrnmrn r Accession No, 3	 Recipient's Catalog No
NASA  TM-73787 1
—^^
C	 Lt,e and Sub1.Ue 115	 Report	 [)ate
THF. ERDA/Le RC PHOTOVOLTAIC SYSTEMS TEST FACILITY September 1977
6	 Performing Organstation Code
1	 Authorls! 8	 Perfor.	 ,,lq Organ,tabon Report No
Americo F. Forestlerl E-9352
10	 Work unit No
9	 Perfor-ning Organization Name and Add,ess
National Aeronautics and Space Admintstration
11	 Contract  or Gram No
Lewis Research Center
Cleveland, Ohio 44135 13,	 Type of Re,,r,rt and Period covered
Technical Memorandum
_
12	 Sponsor.ng Agency Name and Address
Energy Research and Development Administration ---
14 Sponsoring Agency f mw heport No.Division of Solar Energy
Washington. L.C.	 20545 ERDA/NASA/1022-77/19
15	 Supplementary Notes
Prepared under Interagency Agreement E(49-26)-1022.
16	 Abstract
The ERDA/LeRC Photovoltaic Systems Test Facility is drsigned, built, and operated by the
Lewis Research Center as a national facility to serve the needs of the ERDA National Photovol-
taic Conversion Program.	 The objective of the facility is to provide a place where photovoltaic
systems may be assembled and electrically configured, to evaluate system performance and
characteristics,	 The facility consists of a solar cell array of an initial 10-kW peak power
rating, test hardware for several alternate methods of power conditioning, a variety of loads,
an electrical energy storage system, and an instrumentation and data acquisition system.
17.	 Key Words (Suggested by Author($)) 18	 Distribution Statement
Photovoltaic systems Unclassified - tmlimited
Solar cell systems STAR Category 44
>" RDA Category UC-63
19	 Security Gassr f . (of this reporll 20	 Secunty Ctassrt. (01 this payei 21.	 No. of Pages 22	 Price*
Unclassified Unclassified
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For sale by the National Technical Information Service. Spnnpfield. Vnpin a 22161
THE ERDA/LeRC PHOTOVOLTAIC SYSTEMS TEST FACILITY*
A. F. FORESTIERI
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
Summary
The ERDA/LeRC Photovoltaic Systems Test Facility is designed, built,
and operated by the Lewis Research Center as a national facility to serve
the needs of the ERDA National Photovoltaic Conversion Program. The objec-
tive of the facility is to provide a place where photovoltaic systems may
be assembled and electrically configured, to evaluate system performance
and characteristics.
The facility consists of a solar cell array of an initial 10-kid peak
power rating, test hardware for several alternate methods of power condi-
tioning, a variety of loads, an electrical energy Etorage system, and an
instrumentation and data acquisition system.
*This work was supported by the Energy Research and Development
Administration.
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1. INTRODUCTION
The ERDA/LeRC Photovoltaic Systems Test Facility (STF) located at the
Lewis Research Center provides a vital support function to the overall ERDA
National Solar Photovoltaic Program. As a "breadboard" system, it allows
preliminary investigation and checkout of components, subsystems and com-
plete photovcltaic systems before installation in actual service. The STF
also permits the assessment of candidate photovoltaic systems and design
enn^Ppts, and can be used to determine optimum system configuraticns and
operating modes.
This report describes the STF and its capabilities. More detail can
be found in reference 1. Information obtained as a result of recent facil-
ity operation will also be described.
2. FACILITY DESCRIPTION
The elements of a photovoltaic power system are shown in figure 1.
The ERDA/LeRC Photovoltaic Systems Test Facility (STF) includeP all of
these elements as well as an instrumentation and data acquisition system.
Figure 2 is an aerial photograph of the STF which will be of some help in
the following description.
Solar Cell Array. - Figure 3 is a photograph of the STF array. The
array field consists of 240 4-foot by 8-foot solar cell panels mounted on
variable angle (5°-85°) support frames. There are eight rows of 30 panels
each. Each Lolar cell panel can be equipped with modules of nearly any
design. The present array utilizes modules from three manufacturers in-
stalled in the first two rows. These two rows have a peak power rating
(at 1000 W/in2 , 280
 C) of 10 M The remaining six rows (30 kW) when
equipped with modules will bring the array capability to 40 kW peak.
The modules may be interconnected electically in vi,rious configura-
tions. At present, the configuration is six series strings of manufac-
turer ;t, 19 series strings of manufacturer Y and 46 strings of manufac-
turer Z. This configuration pi7oduces an array output voltage (at maximum
power point) of about 170 volts. Sinc_- the series strings are not all
identical, there is a power loss due to mismatch estimated to he about 4
percent. Four wires per panel have been installed to the control room to
p_ovide flexibility in array electrical configuration.
Panel wiring is routed along, wiring trays to a terminal cabinet at the
end of each row. From thrre the wiring is routed underground through 350-
2	 ORIGrNAI. PAGE, la
OF POOR QUALITY
0
foot ducts to the control room. Each series string is connected to a suiar
array bulbar in the control room through a switch, an isolation diode and
a fuse. The output voltage of the solar array can be changed by reconfigu-
ring the array interconnection in the field or in the control room. The
array can also be electrically split into several independent arrays.
Energy Storage Eg21pment. - Many photovoltaic power systems require
on-site energy storage. The STF has 48 kWh of lead-acid storage tatteries
•	 for this purpose. The battery cells are of a deep-discharge-cycle-duty
type, with lead-antimony positive plate grids and calcium alloy negative
plate grids. The battery is housed in a battery shed immediately adjacent
to the control room. The battery shed is provided with forced air venti-
lation and a safety alarm which is triggered by loss of ventilation.
Plastic acid-resistant trays are also provided to contain accidental acid
spills. Room is available in the shed to expand the on-site storage to as
'	 high as 500 kWh.
The battery may be charged either from the solar array or from the
electric utility grid. Each charging method requires separate charging
equipment. For the STF as it is presently configured, the battery is
charged by a commercial battery charger operating from the utility power
sourcE c,r from a shunt battery charge controller when charged by the solar
array.
Power Conditioning Equipment. - Although the STF is a flexible power
system and can be interfaced with virtually any type of power conditioning
equipment, the initial test systems were assembled with a single-phase
5-kVA self-commutated inverter and a single-phase 8 kW line-commutated in-
verter which is transformer-coupled to a load bank and to the local elec-	 r
tric utility distribution network. For both types of inverters it was
found necessary to connect a capacitor across the array output (or inverter
input) for proper operation. In addition the line commutated inverter re-
quires a series inductance to increase the performance to acceptable
levels (2).
Electric Utility Distribution Network and Loads. - The interface with
the electric utility is achieved through proper matching transformers and
circuit I- eakers. Various loads, which are used as part of the systems
tests, can be supplied either by the solar array and/or the utility. With
the line commutated inverter in use the load sharing is accomplished as
described beicu. When the solar array can supply more power than is called
3
For by the load the excess power is fed into the utility grid. When there
is insufficient power from the solar array the load accepts what iu avail-
able from the array and makes up the difference from the utility network
automatically.
"n the cane of the self-commutated inverter, parallel operation as
described above is not possible. In this case the load is supplied either
by the array or the utility and this is accomplished through autom<<tic
switches. A self-commutated inverter that would permit sharing the loads
with the utility is under development.
Instrumentation and Data Acquisition System. - The Instrumentation
and Data Acquisition System (IDAS) includes transducers for measuring elec-
trical parameters (voltage, current, power, reactive power), temperature,
weather parameters (wind Epeed, wind direction, air temperature, humidity),
and insolation. The initial IDAS included a data logger interfaced with
the LeRC IBM-360 computer. Recently, a comes ned microprocessor mini-
computer data acquisition and display system has been installed. It will
complement the initial system by providing on-line results during STF
operation. A portion of the initial IDAS can be seen in figure 4. This
r	 equipment is installed in the STF control room housed in Lwo trailers
'	 approximately 350 feet east of the solar array field.
3. SAFETY CONSIDERATIONS
General LeRC safety procedures with regard to both ac and do circuits
of comparable current and voltage are applied to the STF. The solar array
field is regarded as a do substation and is enclosed on all four sic-
an 8-foot-high chain-link fence. The steel solar cell panel mounting
frames are connected to array field ground through a buried copper cable
network and ground rods. Frame grounding is for limiting; damage from
lightning and for personnel safety. Negative terminals of the series
strings of solar cell modules are connected to the array field ground.
A more detailed description of these connections, including photographs
and circuit diagrams, is given in reference 1.
As discussed earlier each series string of the array is connected to
the solar array busbar through disconnect switches. These switches are
opened and tagged with a warning when work is being done on panels within
the series strings. A master switch to isolate all array do power from
thu control room side of the busbar is also provided. This switch is
i_
i
opened and tagged when work is being done on the do lines within the con-
trol room.
Safety procedures for maintenance and inspection of the solar array
have been set up. The preferred method is to do all of this work at night
when the array is not producing power. An alternate method for daylight
has also been approved. This includes isolating the portion of the array
to be worked on with the switches mentioned above and° ,irther by covering
the panels with special reinforced black plastic, held in place by elastic
cord. Finally she isolated panels are short circuited at the outlut ter-
,	 minals.
4. STF INFORMATION
Since the STF has been installed a number of factors affecting the
solar cell array such as wind, ambient temperature, snow and dirt were in-
vestigated. Also, radiated electromagnetic interferrence and power losses
were determined (3).
The temperature of the sclar array depends on the way the solar cell
modules are packaged by the manufacturer and the way they are motmted in
the STF. The temperature of one type of module with an epoxy-glass sub-
strate which was mounted on plywood with 1/8" standoffs was typically about
300 C above ambient temperature. .Another with x finned aluminum back ran
about '100 C above ambient temperature. These measurements were made with
in-olation of about i000 W/m ` and wind velocity of about 8 miles per hour.
Snow was found to adhere better to the epoxy-glass substrate modules
than to the aluminum substrate modules even though both types used a sili-
cone rubber encapsulant. The array for all of these observations was mairi-
tained at an angle of 41 0 to the horizontal.
The effect of dirt on the solar cell modules was very dependent on the
construction of the module cover. After about three months exposure in the
STF the power loss due to dirt on the modules was about 8%. With washing,
about five or six percentage points of this loss could be recovered. A
more complete report on this subject can be found in reference 4.
Although the effect of environmental factors on the STF are of primary
interest it is also necessary to investigate the effect of the STF on the
environment. For example, radiated electromagnetic interference (EMI),
which could affect communication, was investigated as a function of array
size and system components operating. Preliminary results (5) show no EMI
0
I
i
1
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5
6problems were detected in operating either type of inverter with the solar
array. The EMI levels measured were well below the limits established by
the Federal Communications Commission for spu.ious and harmonic emissions.
J
REFERENCES
Forestieri, A. et al.: ERDA/Lewis Research Center Photovoltaic Systems
Test Facility. NASA TM X-73641, 1977.
2. Forestieri, A.: Svstems Test Facility (STF) Status. Proceedings of the
ERDA Semiannual Solar Photovoltaic Program Review Meeting, Silicon Tech-
nology Programs Branch, Williamsburg, Virginia, August 23-25, 1977. To
be published.
J. Forestieri, A.: ERDA/LeRC Systems Test Facilit y . Proceedings of the
ERDA Semiannual. Solar Photovoltaic Ptogram. ONF-770112, 1977, pp. 277-
303.
Forestieri, Americo F.; and Anagnostou, Evelyn: Real-Time and Accel-
erated Outdoor Endurance Testing of Solar Cells. NASA T11-73743, 1977.
D. Johnson, J. A.; Herk2, F. P., Jr.; and Knapp, W. D.: Photovoltaic Sys-
tem Test Facility Electromagnetic Interference Measurements. NASA TM
X-73640, 1.977.
ELECTRIC
UTILITY
DISTRIBUTION
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 ARRAY N 
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	 EQUIPMENT
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Figure 1. - Elements of a Photovoltaic Power System
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FIGURE 2. - ERDA/LERC PHOTOVOLTAIC SY'..!LMS TEST FACILITY (AERIAL VIEW)
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The ERDA/LeRC photovoltaic systems test facility

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