Dennis J. Eichenberg

Glenn Research Center, Cleveland, Ohio

Development and Testing of a Prototype

Grid-Tied Photovoltaic Power System

NASA/TM—2009-216062

December 2009
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Dennis J. Eichenberg

Glenn Research Center, Cleveland, Ohio

Development and Testing of a Prototype

Grid-Tied Photovoltaic Power System

NASA/TM—2009-216062

December 2009

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2009-216062 1 

Development and Testing of a Prototype Grid-Tied 

Photovoltaic Power System 

Dennis J. Eichenberg 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Summary 

The NASA Glenn Research Center (GRC) has developed and tested a prototype 2 kW DC grid-tied 

photovoltaic (PV) power system at the Center. The PV system has generated in excess of 6700 kWh since 

operation commenced in July 2006. The PV system is providing power to the GRC grid for use by all. 

Operation of the prototype PV system has been completely trouble free. 

A grid-tied PV power system is connected directly to the utility distribution grid. Facility power can 

be obtained from the utility system as normal. The PV system is synchronized with the utility system to 

provide power for the facility, and excess power is provided to the utility. 

The project transfers space technology to terrestrial use via nontraditional partners. GRC personnel 

glean valuable experience with PV power systems that are directly applicable to various space power 

systems, and provide valuable space program test data. PV power systems help to reduce harmful 

emissions and reduce the Nation’s dependence on fossil fuels. Power generated by the PV system reduces 

the GRC utility demand, and the surplus power aids the community. 

Present global energy concerns reinforce the need for the development of alternative energy systems. 

Modern PV panels are readily available, reliable, efficient, and economical with a life expectancy of at 

least 25 years. Modern electronics has been the enabling technology behind grid-tied power systems, 

making them safe, reliable, efficient, and economical with a life expectancy of at least 25 years. Based 

upon the success of the prototype PV system, additional PV power system expansion at GRC is under 

consideration. 

The prototype grid-tied PV power system was successfully designed and developed which served to 

validate the basic principles described, and the theoretical work that was performed. The report concludes 

that grid-tied photovoltaic power systems are reliable, maintenance free, long life power systems, and are 

of significant value to NASA and the community. 

Introduction 

The NASA Glenn Research Center has a wealth of experience in photovoltaic power systems. The 

work was done under the Hybrid Power Management (HPM) Program. The GRC Electrical and 

Electromagnetics Branch initiated the HPM Program for the GRC Technology Transfer and Partnership 

Office. HPM is the innovative integration of diverse, state-of-the-art power devices in an optimal 

configuration for space and terrestrial applications. The appropriate application and control of the various 

power devices significantly improves overall system performance and efficiency. Applications include 

power generation, transportation systems, biotechnology systems, and space power systems. 

A PV power system can either be a stand alone, off-grid system, or a grid-tied system. An off-grid PV 

system provides local power only. A grid-tied PV system is connected directly to the utility distribution 

grid. Facility power can be obtained from the utility system as normal. The PV system is synchronized 

with the utility system. The PV system provides power for the facility, and excess power is provided to 

the utility. 

Photovoltaic power systems have tremendous potential. The 89 petawatts of sunlight reaching the 

Earth’s surface is approximately 6,000 times greater than the 15 terawatts of average power consumed by 

the human population. PV generation has the highest power density of renewable energies with a global 
NASA/TM—2009-216062 2 

mean of 170 W/m2 (Ref. 1). PV is entirely pollution free during use. PV system production end wastes 

and emissions are manageable utilizing existing pollution controls. End-of-use recycling technologies are 

under development to minimize negative environmental effects. PV systems can operate with minimal 

operator intervention or maintenance after initial setup. 

Grid-tied PV power systems provide many benefits. Operating costs of a PV power system are low 

compared to conventional power technologies. PV can displace the highest cost electricity during times of 

peak demand in most climatic regions, and thus reduce grid loading. Net Metering is often used, in which 

independent power producers, such as PV power systems, are connected to the utility grid via the 

customers’ main service panel and meter. When the PV power system is generating more power than 

required at that location, the excess power is provided to the utility grid. The customer pays the net of the 

power purchased when the on-site power demand is greater than the on-site power production and the 

excess power that is returned to the utility grid. 

Grid-tied PV power systems can be used locally thus reducing transmission/distribution losses. 

Transmission/distribution losses in the U.S. were approximately 7.2 percent in 1995 (Ref. 2), thus 

providing a potential for significant energy savings. 

Theoretical power generation has been determined for the prototype grid-tied 2 kW photovoltaic 

power system. The actual 2 kW PV power system installed at GRC served to validate the theoretical work 

that was performed. 

Analysis 

Predicted Power Generation 

The National Renewable Energy Laboratory has developed a calculator to determine the energy 

production of grid-tied PV power systems throughout the world. The calculator is based upon the work of 

David F. Mennicucci (Ref. 3). 

The calculator creates hour-by-hour performance simulations that provide power estimated monthly 

and annual energy production in kilowatts and energy value. Users can select a location and choose to use 

default values or their own system parameters, for size, electricity cost, array type, tilt angle, and azimuth 

angle. The calculator can provide hourly performance data for the selected location. 

The calculator uses typical meteorological year weather for the selected location, and determines the 

incident PV array solar radiation and the PV cell temperature for each hour of the year. The DC energy 

for each hour is calculated from the PV system DC rating and the incident solar radiation, and then 

corrected for the PV cell temperature. The AC energy for each hour is calculated by multiplying the DC 

energy by the overall DC-to-AC derating factor and adjusting for the inverter efficiency as a function of 

the load. Hourly values of AC energy are summed to determine monthly and annual AC energy 

production. 

A PV system is rated upon its nameplate DC power rating. This is determined by adding the PV panel 

power listed on the nameplates of the PV panels in watts (W), and dividing the sum by 1,000 to convert 

the rating to kilowatts (kW). PV panels are rated for standard test conditions of 1,000 W/msolar 

irradiance, and 25 °C PV panel temperature. 

The PV system AC rating is determined by multiplying the nameplate DC power rating by an overall 

DC-to-AC derating factor. The DC-to-AC derating factor account for losses from the DC nameplate 

power rating, and is the mathematical product of the derating factors for the components of the PV 

system. The derating factor components includes PV array nameplate DC rating accuracy, inverter and 

transformer losses, mismatch, diode and connection losses, DC wiring losses, AC wiring losses, PV array 

contamination, system availability, and shading. 

The derating factor for PV array contamination accounts for snow and other foreign matter on the 

surface of the PV array that prevents solar radiation from reaching the solar cells. PV array contamination 

is location and weather dependent. There are greater contamination losses in high-traffic, high pollution 

areas with infrequent rain. Snow reduces energy produced, and the severity is a function of the amount of 
NASA/TM—2009-216062 3 

snow and the duration that it remains on the PV array. Snow remains the longest when sub-freezing 

temperatures prevail, small PV array tilt angles prevent snow from sliding off, the PV array is closely 

integrated onto the roof, and the roof, or another structure in the vicinity, facilitates snow drift onto the 

array. 

The tilt angle for a fixed array is the angle from horizontal of the inclination of the PV array. Thus a 

tilt of 0° is a horizontal array, and a tilt of 90° is a vertical array. An array is normally tilted at the 

location’s latitude. This normally maximizes annual energy production. Increasing the tilt angle favors 

energy production in the winter, and decreasing the tilt angle favors energy production in the summer. 

The azimuth angle for a fixed array is the angle clockwise from true north that the PV array faces. An 

azimuth angle of 180° (south-facing) is normally used for locations in the northern hemisphere, and 0° 

(north-facing) for locations in the southern hemisphere. This normally maximizes energy production. For 

the northern hemisphere, increasing the azimuth angle favors afternoon energy production, and decreasing 

the azimuth angle favors morning energy production. For the southern hemisphere, decreasing the 

azimuth angle favors afternoon energy production, and increasing the azimuth angle favors morning 

energy production. 

The monthly and yearly energy production estimates are modeled using the selected PV system 

parameters and weather data that are typical or representative of long-term averages. Weather patterns 

vary from year to year, so the model is a better indicator of long-term performance than of performance 

for a particular month or year. PV performance is largely proportional to the solar radiation received, 

which may vary from the long-term average by 30 percent monthly and 10 percent yearly. For these 

variations, and the uncertainties associated with the weather data and model, future months and years may 

have actual PV performance that differs from the model. The variations may be as much as 40 percent for 

individual months, and up to 20 percent for individual years. Long-term performance over many years is 

expected to be accurate within 10 percent. 

There are other factors that affect model accuracy. Nearby buildings, objects, or other PV arrays and 

array structures that shade the PV array will cause a reduction in actual energy production from the 

model. Snow or other contamination of the PV array will cause an inconsistency with the model. There is 

a reduction in power generation over time that is not factored into the model. Aging is due to weathering 

of the PV array, and is typically 1 percent per year. 

The predicted AC energy for a grid-tied 2 kW DC PV power system located in Cleveland, Ohio is 

shown in Table 1. The assumptions made for this prediction is that the PV array is crystalline silicon, the 

PV array is fixed at a tilt of 41.4°, the PV array azimuth is 180.0°, and the DC-to-AC derating factor is 

0.770. The predicted AC energy is shown graphically in Figure 1. 

TABLE 1.—PREDICTED ENERGY FOR A GRID-TIED 

2 kW DC PHOTOVOLTAIC POWER SYSTEM 

IN CLEVELAND, OHIO 

Month Solar radiation, 

kWh/m2

/day 

AC energy, 

kWh 

January 2.49 122 

February 3.34 147 

March 4.09 196 

April 4.94 222 

May 5.48 243 

June 5.56 231 

July 5.55 237 

August 5.47 235 

September 4.90 208 

October 3.91 177 

November 2.18 94 

December 1.68 76 

Total annual 4.14 2189 

 
NASA/TM—2009-216062 4 

Figure 1.—Predicted Energy for a Grid-Tied 2 kW DC Photovoltaic Power System in Cleveland, Ohio. 

TABLE 2.—PREDICTED ANNUAL ENERGY FOR 

A GRID-TIED 2 kW DC PHOTOVOLTAIC 

POWER SYSTEM 

City Tilt, 

degrees 

Annual AC energy, 

kWh 

Cleveland 41.4 2189 

Tampa 28.0 2728 

Los Angeles 33.9 2939 

Phoenix 33.4 3234 

Las Vegas 36.1 3327 

Anchorage 61.2 1588 

Montreal 45.5 2201 

Berlin 52.5 1508 

Matsumoto 36.2 2368 

Moscow 55.8 1534 

Paris 48.7 1631 

The predicted annual AC energy produced from a grid-tied 2 kW DC PV power system for different 

areas throughout the world are shown in Table 2. The assumptions made for this prediction is that the PV 

array is crystalline silicon, the PV array tilt is fixed at the appropriate angle for the specific location, the 

PV array azimuth is 180.0°, and the DC-to-AC derating factor is 0.770. It is interesting to note from the 

world comparison that Cleveland is a viable location for PV production, with potentially significantly 

greater annual energy production than Berlin, Moscow or Paris for the same PV system properly oriented 

for the specific location. In addition, the potential annual energy production from the same PV system in 

Cleveland is not significantly less than such a system located in Tampa. 
NASA/TM—2009-216062 5 

Test Objectives 

The objective of testing the prototype 2 kW grid-tied photovoltaic power system is to validate the 

theoretical analyses and verify long-term system performance. This validation provides confidence in 

using the analyses for developing and optimizing conceptual grid-tied photovoltaic power system designs. 

Testing of the prototype 2 kW grid-tied photovoltaic power system was performed at the NASA 

Glenn Research Center. Of particular interest is the long term performance of the system. Power and 

energy were monitored on a regular basis. 

Test Hardware Description 

Prototype 2 kW Grid-Tied Photovoltaic Power System 

The prototype 2 kW grid-tied photovoltaic power system test hardware includes the photovoltaic 

panels, and the DC/AC inverter. The photovoltaic panels are shown in Figure 2 and the photovoltaic 

inverter is shown in Figure 3. The system block diagram is shown in Figure 4. System component 

specifications are included in the Appendix. 

The photovoltaic panels are semiconductor panels that convert energy from sunlight to DC electrical 

power. The panels are unbreakable and maintenance free. The system consists of ten, 200 W (at 27.1 V) 

panels connected in series. Each panel consists of 54 silicon poly-crystalline cells in series. The panel 

efficiency in providing power from the sunlight is rated at 15 percent. Each panel weighs 39 lb, and is 

58.5 by 38.6 in. The life expectancy of the PV panels is 25 years or greater. The support structure for the 

panels is a fixed mount to orient the panels southerly with a tilt of 41.4° from horizontal to align the 

panels for the latitude of Cleveland, Ohio. Solar trackers are available to increase the available power 

from the Sun. Under ideal conditions, a tracker can provide up to 50 percent additional daily power from 

a PV array. In temperate latitudes, a tracker can provide an annual 25 percent increase in power from a 

PV array. Numerous technologies are available for trackers, including electric drives and fluid drives. 

Trackers are mechanical devices, and thus have potential life and maintenance issues. To avoid life and 

maintenance issues, a fixed mount was used for the prototype 2 kW PV power system. 

The photovoltaic inverter converts the DC electrical power derived from the photovoltaic panels to 

single phase, 60 Hz, sinusoidal AC power synchronized to the AC utility power system. The inverter 

integrated into the prototype PV power system is rated at 3.3 kW, 208 VAC with an efficiency of 

94 percent. The inverter weighs 49 lb, and is 28.5 by 15.9 by 5.75 in. The life expectancy of the inverter 

is 25 years or greater. 

Figure 2.—Prototype 2 kW System Photovoltaic Panels. 
NASA/TM—2009-216062 6 

Figure 3.—Prototype 2 kW System Photovoltaic Inverter. 

Figure 4.—Prototype 2 kW Grid-Tied Photovoltaic System Block Diagram. 
NASA/TM—2009-216062 7 

The prototype PV system was designed per the National Fire Protection Association (NFPA) 70, the 

National Electrical Code (NEC). The system concurs with all articles of the NEC, and in particular 

addresses Article 690, Solar Photovoltaic Systems. In addition, the system was designed to be completely 

in concurrence with the NASA GRC Safety Manual. The PV panels meet the requirements of 

Underwriters Laboratories Standard for Safety UL-1703 concerning flat-plate PV modules and panels. 

The PV panels are fused to provide overcurrent protection. The PV panels, and the entire PV system, are 

bonded and grounded per the NEC. A lightning arrestor is included to protect the PV system from 

lightning strikes. The PV inverter includes an NEC compliant DC and AC disconnect switch to provide 

complete system isolation whenever the disconnect switch is opened. The inverter is configured to 

automatically open the inverter circuits whenever utility power is lost to prevent power from the PV 

system from feeding back into the utility power system, thus preventing a potential safety hazard. 

Instrumentation 

The prototype 2 kW grid-tied photovoltaic power system contains an internal instrumentation system. 

The photovoltaic inverter includes an integral backlit 2-line, 16-character, liquid crystal display (LCD). 

The display provides instantaneous inverter output power, daily and lifetime inverter output energy, PV 

array voltage and current, utility voltage and frequency, time online, and fault messages. 

Test Procedures 

The tests described in this report were conducted at the NASA Glenn Research Center in Cleveland, 

Ohio. The lifetime energy production of the prototype 2 kW grid-tied photovoltaic power system is 

monitored at the PV inverter. The measured monthly energy production is compared to the analytical 

monthly energy production. 

Test Results 

System Performance 

Table 3 summarizes the monthly and annual energy production results for the first year of use of the 

prototype 2 kW grid-tied photovoltaic power system, along with the analytical results. Figure 5 indicates 

the monthly energy production, and Figure 6 indicates the total energy production. 

TABLE 3.—PROTOTYPE 2 kW DC GRID-TIED 

PV POWER SYSTEM PERFORMANCE 

Month Predicted 

AC energy, 

kWh 

Measured 

AC energy,

kWh 

January 122 82 

February 147 94 

March 196 164 

April 222 251 

May 243 262 

June 231 274 

July 237 246 

August 235 247 

September 208 246 

October 177 174 

November 94 91 

December 76 62 

Total annual 2189 2193 
NASA/TM—2009-216062 8 

Figure 5.—Prototype 2 kW DC Grid-Tied PV Power System Monthly AC Energy. 

Figure 6.—Prototype 2 kW DC Grid-Tied PV Power System Total AC Energy. 
NASA/TM—2009-216062 9 

Discussion 

The objective of testing the prototype 2 kW grid-tied photovoltaic power system is to validate the 

theoretical analyses, and verify long-term system performance. The analytical model effectively predicts 

the annual AC energy produced by a grid-tied photovoltaic power system. 

The rather large monthly variation in energy production of the prototype 2 kW PV power system 

shown in Figure 5 indicates the large seasonal variation in sunlight in Cleveland, Ohio. A grid-tied PV 

power system is advantageous in regions, such as Cleveland, in which utility power is essential in times 

of limited sunlight to meet the power requirements of the load. The significant power produced by the PV 

power system in the summer months is valuable in meeting the high utility system power demand at that 

time. 

The monthly energy production of the prototype 2 kW PV power system shown in Figure 5 over the 

past four years indicates very consistent long-term system performance. No effort has been made to clean 

the PV panels, or to clear them of snow for these tests. Providing cleaning and snow clearing of the PV 

panels would improve PV system performance, but the test results obtained without cleaning or snow 

clearing indicate the excellent performance attainable from this system under no maintenance conditions. 

Consistent long-term PV system performance is obvious from the total PV system energy production 

shown in Figure 6. Long-term system performance has been excellent over the past four years with no 

system failures, no down time, and no required system maintenance. 

Concluding Remarks 

The NASA Glenn Research Center has successfully designed, developed, analyzed, and tested a 

prototype 2 kW DC grid-tied photovoltaic power system. The goals of the project include reducing the 

Nation’s dependence on fossil fuels, and reducing the production of harmful emissions. The objective of 

this work is to develop a viable prototype 2 kW DC grid-tied photovoltaic power system to validate the 

theoretical analyses, and to verify long-term system performance. 

Theoretical analyses have been performed successfully to predict the energy produced by a grid-tied 

PV system in Cleveland, Ohio. Empirical test results obtained from the experimental hardware 

successfully validated the basic principles described, and the theoretical work that was performed. Of 

particular value, are the analytical tools and capability successfully used for this project. Performance 

predictions can be made confidently for grid-tied PV systems of various scale. 

The prototype 2 kW DC grid-tied PV system has provided consistent long-term performance over the 

past four years with no system failures, no down time, and no required system maintenance. From the 

analyses and empirical test results, it is apparent that grid-tied PV systems are viable in Cleveland, Ohio. 

The report concludes that the implementation of grid-tied photovoltaic power systems can provide 

significant improvements in power system performance, reduce dependency on fossil fuels, and reduce 

the production of harmful emissions. The prototype system has been efficient, reliable, and maintenance 

free. The prototype grid-tied PV power system has been of great value to GRC and to the local 

community. 

NASA/TM—2009-216062 11 

Appendix—Equipment Under Test Summary Data Sheet 

Prototype 2 kW Photovoltaic System 

1.0 Photovoltaic Panels 

1.1 Type Silicon Poly-Crystalline 

1.2 Peak Power (Wp) 200 W 

1.3 Max Power Voltage (Vmp) 27.1 V 

1.4 Max Power Current (Imp) 7.4 A 

1.5 Open Circuit Voltage (Voc) 34 V 

1.6 Short Circuit Current (Isc) 7.8 A 

1.7 Short Circuit Temp Coefficient 5.6 mA/°C 

1.8 Open Circuit Voltage Coefficient –0.12 V/°C 

1.9 Max Power Temp Coefficient –0.5%/°C 

1.10 Max Series Fuse 15.0 A 

1.11 Normal Operating Cell Temp (NOCT) 45.0 °C (113 °F) 

1.12 Width 98.1 cm (38.6 in.) 

1.13 Length 148.5 cm (58.5 in.) 

1.14 Depth 5.5 cm (2.2 in.) 

1.15 Weight 17.7 kg (39.0 lb) 
NASA/TM—2009-216062 12 

2.0 Inverter 

2.1 Type Sine Wave 

2.2 Max AC Power Output 3300 W 

2.3 Nominal AC Output Voltage 208 VAC 

2.4 AC Output Voltage Range 183 to 228 VAC 

2.5 Nominal AC Frequency 60 Hz 

2.6 Max Continuous Output Current 18 A 

2.7 Max Total Harmonic Distortion 3% 

2.8 Power Factor >0.9 

2.9 DC Input Voltage Range 195 to 600 VDC 

2.10 Peak Power Tracking Voltage Range 195 to 550 VDC 

 2.11 Peak Inverter Efficiency 94.7% 

 2.12 Night Time Power Consumption 1 W 

 2.13 Output Overcurrent Protection 25 A 

 2.14 Operating Temperature Range –25 to 65 °C (–13 to 149 °F) 

2.15 Enclosure Type NEMA 3R (outdoor rated) 

2.16 Disconnect PV/Utility disconnect 

2.17 Cooling Convection 

2.18 Communications One RS 232 and two RJ45 ports 

2.19 Display Liquid Crystal Display 

2.20 Width 40.3 cm (15.9 in.) 

2.21 Height 75.5 cm (28.5 in.) 

2.22 Depth 14.6 cm (5.7 in.) 

2.23 Weight 22.2 kg (49.0 lb) 

 
NASA/TM—2009-216062 13 

References 

1. Smil, Vaclav, “Energy at the Crossroads: Global Perspectives and Uncertainties,” The MIT Press, 

November 1, 2003. 

2. Department of Energy, “U.S. Climate Change Technology Program: Technology Options for the Near 

and Long Term,” DOE/PI-0002, November, 2003. 

3. Mennicucci, David F., “Photovoltaic Array Simulation Models,” Solar Cells, 1986, pp. 383–392. 
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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The NASA Glenn Research Center (GRC) has developed and tested a prototype 2 kW DC grid-tied photovoltaic (PV) power system at the 

Center. The PV system has generated in excess of 6700 kWh since operation commenced in July 2006. The PV system is providing power 

to the GRC grid for use by all. Operation of the prototype PV system has been completely trouble free. A grid-tied PV power system is 

connected directly to the utility distribution grid. Facility power can be obtained from the utility system as normal. The PV system is 

synchronized with the utility system to provide power for the facility, and excess power is provided to the utility. The project transfers space 

technology to terrestrial use via nontraditional partners. GRC personnel glean valuable experience with PV power systems that are directly 

applicable to various space power systems, and provide valuable space program test data. PV power systems help to reduce harmful 

emissions and reduce the Nation’s dependence on fossil fuels. Power generated by the PV system reduces the GRC utility demand, and the 

surplus power aids the community. Present global energy concerns reinforce the need for the development of alternative energy systems. 

Modern PV panels are readily available, reliable, efficient, and economical with a life expectancy of at least 25 years. Modern electronics 

has been the enabling technology behind grid-tied power systems, making them safe, reliable, efficient, and economical with a life 

expectancy of at least 25 years. Based upon the success of the prototype PV system, additional PV power system expansion at GRC is under 

consideration. The prototype grid-tied PV power system was successfully designed and developed which served to validate the basic 

principles described, and the theoretical work that was performed. The report concludes that grid-tied photovoltaic power systems are 

reliable, maintenance free, long life power systems, and are of significant value to NASA and the community. 

15. SUBJECT TERMS

Photovoltaic conversion; Energy conversion; Electric power 

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