In addition to fuel cell research and development, SERC installs and conducts research on solar electric (photovoltaic) energy systems. A prime example is the 9.2 kWp photovoltaic (PV) array at Humboldt State University’s Telonicher Marine Laboratory in Trinidad, CA. The array was installed in 1990 and is situated approximately 150 meters from the ocean in a cool, marine environment. It is an integral part of the Schatz Solar Hydrogen Project, an ongoing renewable hydrogen demonstration project. The primary function of the SERC array is to power an air compressor for the marine laboratory. Excess power produced by the array is shunted to a Teledyne Energy Systems ALTUS™ 20 electrolyzer to produce hydrogen fuel for a proton exchange membrane (PEM) fuel cell. Together, the PV array and the PEM fuel cell power the air compressor 24 hours a day using renewable solar energy.

The Schatz Solar Hydrogen Project and the 192-module PV array.

The PV array consists of 192 ARCO M75 PV modules, a single-crystal silicon module with a rated power of 48 W. The individual PV cells are laminated to tempered glass with ethylene vinyl acetate (EVA). During the study period reported below, the modules were configured in 12 independent subarrays consisting of 16 modules wired for 24 Volt DC operation (for the array's current configuration read "Renewables Renewed," our newsletter article describing the system upgrade in 2006/2007). The array is tilted permanently at an angle of 30 degrees to the horizontal. One module was damaged in 1996 and replaced with a module with similar size and performance characteristics, a Siemens SM50-H.

Pre-Service Analysis
Prior to initial installation in 1990, research engineer Jim Zoellick generated current-voltage (I-V) curves to describe the performance characteristics of each of the 192 modules. Environmental factors, namely temperature and incident radiation, affect the shape of the curve, so a module will have a particular I-V curve for a given set of environmental conditions. The current and voltage data can be adjusted to estimate a module’s performance under other conditions, such as the Standard Test Conditions (STC) at which manufacturers rate the power output of their PV modules. Standard Test Conditions are a cell temperature of 25ºC and 1000 W/m2 insolation.

The purpose of Zoellick’s study (see "Effects of Mismatch Losses in Photovoltaic Arrays,") was to investigate the effect of mismatch losses on power output from the array. The data indicated that mismatch losses were very small, averaging 0.1%, and that the actual field performance of the modules was lower than the nameplate rating. Although Zoellick et al. concluded that mismatch loss is an unnecessary consideration in designing arrays composed of undamaged, production-run modules of a single model number, the pre-service data provided a valuable baseline for future analysis of the array.

In-Service Analysis
Between September 2000 and June 2001, research engineers Antonio Reis and Nate Coleman generated I-V curves and performance parameters for the 191 original ARCO M75 PV modules to evaluate the changes in their performance after 11 years in the field. Each module was removed from the array and cleaned prior to testing. These new data were compared to the original 1990 data to determine the extent to which each module’s power production had degraded over time. A mean comparison of the individual module parameter estimates showed that maximum power output at Normal Operating Cell Temperature (NOCT) decreased to 38.13 W from 39.88 W, a change of 4.39%. The reduction in Pmax is primarily due to a decrease in current output, which is believed to be caused in part by the following visually observable physical defects:

1. mild discoloration or browning of the EVA encapsulant over every cell in the module, as shown above, left		2. delamination of the EVA encapsulant at the silicon cell-EVA interface, as shown above, middle		3. intense browning of the EVA above individual cells, presumably caused by localized hot spots, as shown above, right

Reis and Coleman presented the results of this study, "Comparison of PV Module Performance Before and After 11 Years of Field Exposure", at the 29th IEEE Photovoltaics Specialists Conference held in New Orleans in May 2002.

Solar Photovoltaic Module Quality Assurance Testing
Solar photovoltaic (PV) technology is moving from the margins into the mainstream in markets from North America and Europe to Africa, Asia, and Latin America. While the rapid growth of solar markets around the world is based largely on sales of high performance PV modules, the presence of low quality products in some markets creates cause for concern.

Faculty and students from SERC and HSU's Energy, Environment and Society Graduate Program have played a central role in efforts to ensure the quality of solar modules sold in the Kenya solar market. The work has involved laboratory testing at HSU as well as field testing of solar equipment in Kenya, and the results have been published in academic journals as well as the Kenya based Solarnet magazine. The project has made a very real difference, as sales of the lowest quality brands of solar modules in the Kenya market have declined sharply.

For additional information on this area of study read our newsletter article, “Building Institutions for Renewable Energy Markets.”

Glossary of Terms for PV Systems
DC (direct current): form of electricity characterized by non-varying, time-independent currents and voltages. Photovoltaic modules produce direct current when exposed to sunlight. Utility electricity supplied to households in most parts of the world is alternating current (AC).
Electrolyzer: device that uses electrical energy to force a nonspontaneous reaction to take place, producing chemical change by passing an electric current through a medium. In a renewable hydrogen system, renewable energy is used to power an electrolyzer to split water into hydrogen and oxygen gases.
Ethylene Vinyl Acetate (EVA): material used to encapsulate photovoltaic cells in manufacturing modules
Insolation: incident solar radiation; power or intensity of sunlight on a unit area of a surface. The magnitude of one full sun of insolation is 1000 W/m2.
I-V Curve: locus of operating points for a photovoltaic module under various loads and particular environmental conditions.
Mismatch losses: difference between the maximum output power available from an array and the sum of the maximum output powers for each of the modules in the array.
Normal Operating Cell Temperature (NOCT): environmental conditions under which manufacturers rate photovoltaic modules, cooresponding to 1000 W/m2 of insolation and a cell temperature of 47oC.
Open circuit voltage (Voc): voltage output from a photovoltaic module when no current is flowing.
Parallel connection: electrical connection in which like poles are connected together. Current is additive in a parallel connection, and voltage is constant.
Photoelectric effect: phenomenon whereby electrons are emitted from a surface upon exposure to, and absorption of, electromagnetic radiation that is above the threshold frequency particular to the surface.
Photovoltaic Array: group of photovoltaic modules wired in series and/or parallel to produce the desired voltage and current output.
Photovoltaic Cell: semiconductor material (usually made of specially treated silicon) that uses the photoelectric effect to generate electricity from light.
Photovoltaic module: group of photovoltaic cells that are wired in series and/or parallel to produce the desired voltage and current output and encapsulated in glass-fronted, framed sheets.
Pyranometer: instrument that measures insolation.
Series connection: electrical connection in which the negative pole is connected to the positive pole of the next device in the circuit so that current flows first through one device and then through another. Voltage is additive in a series connection, and current is constant.
Short-circuit current (Isc): current output from a photovoltaic module when the resistance of the circuit is zero (zero voltage output)
Standard Test Conditions (STC): environmental conditions under which manufacturers rate photovoltaic modules, cooresponding to 1000 W/m2 of insolation and a cell temperature of 25oC.
Subarray: part of a photovoltaic array with dedicated safety breakers and disconnect hardware.
Thermocouple: instrument that measures temperature based on a voltage created by the junction of two dissimilar metals.