Brian P. Dougherty

Versatile light-emitting-diode-based spectral response measurement system for photovoltaic device characterization.

An absolute differential spectral response measurement system for solar cells is presented. The system couples an array of light emitting diodes with an optical waveguide to provide large area illumination. Two unique yet complementary measurement methods were developed and tested with the same measurement apparatus. Good agreement was observed between the two methods based on testing of a variety of solar cells. The first method is a lock-in technique that can be performed over a broad pulse frequency range. The second method is based on synchronous multifrequency optical excitation and electrical detection. An innovative scheme for providing light bias during each measurement method is discussed.

Measured Performance of Building Integrated Photovoltaic Panels: Round 2

Architects, building designers, and building owners presently lack sufficient resources for thoroughly evaluating the economic impact of building integrated photovoltaics (BIPV). The National Institute of Standards and Technology (NIST) is addressing this deficiency by evaluating computer models used to predict the electrical performance of BIPV components. To facilitate this evaluation, NIST is collecting long-term BIPV performance data that can be compared against predicted values. The long-term data, in addition, provides insight into the relative merits of different building integrated applications, helps to identify performance differences between cell technologies, and reveals seasonal variations. This paper adds to the slowly growing database of longterm performance data on BIPV components. Results from monitoring eight different building-integrated panels over a twelve-month period are summarized. The panels are installed vertically, face true-south, and are an integral part of the building’s shell. The eight panels comprise the second set of panels evaluated at the NIST test facility. Cell technologies evaluated as part of this second round of testing include single crystalline silicon, polycrystalline silicon, and two thin film materials: tandem-junction amorphous silicon (2-a-Si) and copper-indium-diselenide (CIS). Two 2-a-Si panels and two CIS panels were monitored. For each pair of BIPV panels, one was insulated on its backside while the backside of the second panel was open to the indoor conditioned space. The panel with the backside thermal insulation experienced higher midday operating temperatures. The higher operating temperatures caused a greater dip in maximum power voltage. The maximum power current increased slightly for the 2-a-Si panel but remained virtually unchanged for the CIS panel. Three of the remaining four test specimens were custom-made panels having the same polycrystalline solar cells but different glazings. Two different polymer materials, Tefzel and Kynar, were tested along with 6 mm-thick, low-iron float glass. The two panels having the much thinner polymer front covers consistently outperformed the panel having the glass front. When compared on an annual basis, the energy production of each polymer-front panel was 8.5% higher than the glass-front panel. Comparison of panels of the same cell technology and comparisons between panels of different cell technologies are made on daily, monthly, and annual bases. Efficiency based on coverage area, which excludes the panel’s inactive border, is used for most “between” panel comparisons. Annual coverage-area conversion efficiencies for the vertically-installed BIPV panels range from a low of 4.6% for the 2-a-Si panels to a high of 12.2% for the two polycrystalline panels having the polymer front covers. The insulated single crystalline panel only slightly outperformed the insulated CIS panel, 10.1% versus 9.7%.Copyright

Parameters Affecting the Performance of a Residential-Scale Stationary Fuel Cell System

Researchers at the National Institute of Standards and Technology (NIST) have measured the performance of a residential fuel cell system when subjected to various environmental and load conditions. The system, which uses natural gas as its source fuel, is capable of generating electrical power at three nominal power levels (2.5 kW, 4.0 kW, and 5.0 kW) while providing thermal energy for user-supplied loads. Testing was conducted to determine the influence of ambient temperature, relative humidity, electrical load, and thermal load on system performance. Steady-state and transient tests were conducted. The steady-state tests were performed in accordance with the American Society of Mechanical Engineering (ASME) Fuel Cell Power Systems Performance Test Code (PTC-50) for fuel cell power systems. The results of the investigation are being used to develop a proposed rating procedure for residential fuel cell units.

Comparison of Predicted to Measured Photovoltaic Module Performance

Computer simulation models to accurately predict the electrical performance of photovoltaic modules are essential. Without such models, potential purchasers of photovoltaic systems have insufficient information to judge the relative merits and cost effectiveness of photovoltaic systems. The purpose of this paper is to compare the predictions of a simulation model, developed by Sandia National Laboratories, to measurements from photovoltaic modules installed in a vertical wall facade in Gaithersburg, MD. The photovoltaic modules were fabricated using monocrystalline, polycrystalline, tandem-junction amorphous, and copper-indium diselenide cells. Polycrystalline modules were constructed using three different glazing materials — 6 mm low-iron glass, 2 mm ethylene-tetrafluoroethylene copolymer (ETFE), and 2 mm polyvinylidene fluoride (PVDF). In order to only assess the simulation model’s ability to predict photovoltaic module performance, measured solar radiation data in the plane of the modules is initially used. Additional comparisons are made using horizontal radiation measurements. The ability of the model to accurately predict the temperature of the photovoltaic cells is investigated by comparing predicted energy production using measured versus predicted photovoltaic cell temperatures. The model was able to predict the measured annual energy production of the photovoltaic modules, with the exception of the tandem-junction amorphous modules, to within 6% using vertical irradiance measurements. The model overpredicted the annual energy production by approximately 14% for the tandem-junction amorphous panels. Using measured horizontal irradiance as input to the simulation model, the agreement between measured and predicted annual energy predictions varied between 1% and 8%, again with the exception of the tandem-junction amorphous silicon modules. The large difference between measured and predicted results for the tandem-junction modules is attributed to performance degradation. Power measurements of the tandem-junction amorphous modules at standard reporting conditions prior to and after exposure revealed a 12% decline. Supplying post-exposure module parameters to the model resulting in energy predictions within 5% of measured values.

Fast and reliable spectral response measurements of PV cells using light emitting diodes

We present a measurement system for absolute differential spectral responsivity of solar cells based on high-powered LED arrays coupled to an optical light guide capable of large area illumination. Two different measurement techniques were developed and tested with the same measurement apparatus on a variety of solar cells. The first method is an individual LED lock-in technique that can be performed over a broad frequency range. The second method is based on synchronous multi-frequency optical excitation, called the Fourier transform (FT) technique, using the LEDs and detection with a spectrum analyzer. A scheme for providing light bias using the LEDs during either measurement scheme is discussed.

Absolute spectral responsivity measurements of solar cells by a hybrid optical technique

An irradiance mode, absolute differential spectral response measurement system for solar cells is presented. The system is based on combining the monochromator-based approach of determining the power mode spectral responsivity of cells with an LED-based measurement to construct a curve representing the light-overfilled absolute spectral response of the entire cell. This curve can be used to predict the short-circuit current (I(sc)) of the cell under the AM 1.5 standard reference spectrum. The measurement system is SI-traceable via detectors with primary calibrations linked to the NIST absolute cryogenic radiometer. An uncertainty analysis of the methodology places the relative uncertainty of the calculated I(sc) at better than ±0.8%.

Experiences With Using Solar Photovoltaics to Heat Domestic Water

The solar photovoltaic (PV) industry continues to make progress in increasing the efficiency while reducing the manufacturing costs of PV cells. Economies of scale are being realized as manufacturers expand their production capabilities. Products are commercially available that integrate photovoltaic cells within building facade, fenestration, and roofing components. Legislation and incentive programs by government and commercial entities are supporting both reduced first costs and greater rates of return. The combination of factors support improved cost-effectiveness. As this trend continues, more options for using PV become possible. One such application is a stand-alone, PV-direct, solar water heating application. Solar water heating can be effectively accomplished by directly using the DC power production from solar photovoltaic modules. A simple controller having multiple power relays connects the PV modules with different combinations of in-tank resistive elements. The controller actively changes the resistive combination so that the photovoltaic modules generate power at or near their maximum output. The technology, which has been patented, is applicable to configurations that use a single water heater and to two water heaters that are piped in series. Prototypes using both tank configurations were monitored at four field sites. This paper emphasizes the single-tank application and the field results from installations in Maryland and Florida.

Large-area irradiance-mode spectral response measurements of solar cells by a light-emitting, diode-based integrating sphere source

An irradiance-mode absolute differential spectral response (SR) measurement system based on a light emitting diode (LED) array is described. The LEDs are coupled to an integrating sphere whose output irradiance is uniform to better than 2% over an area of 160 mm by 160 mm. SR measurements of solar cells when subject to diffuse irradiation, as provided by the integrating sphere, are compared with collimated irradiance SR measurements. Issues originating from the differences in angular response of the reference versus the test cells are also investigated. The SR curves of large-area cells with dimensions of up to 155 mm are measured and then used to calculate the cells short circuit current (I(sc)), if illuminated by a defined solar spectrum. The resulting values of I(sc) agree well with the values obtained from secondary measurements.

Model (At Least) Twice, Build Once: Experiences with the Design-Bid-Build Process for Solar Photovoltaic Arrays

Commercial-scale solar photovoltaic (PV) arrays were designed, constructed, and are now operational on the Gaithersburg, Maryland campus of the National Institute of Standards and Technology (NIST). A design–bid–build process was followed where the contractors used photovoltaic system modeling tools both during the initial design phase and during the postbid, prebuild phase. To help investigate the specific aspects of the contractors’ evolving designs, the authors conducted their own independent photovoltaic system modeling. This independent modeling helped identify design elements that could be improved and so aided efforts to maximize the annual renewable energy generation. An estimated 2.5% gain in annual energy generation is being realized as a result of this independent modeling effort. To provide context for the modeling work and the lessons learned, key events impacting the design–bid–build process are described. The installed systems are summarized and also contrasted with the proposed designs. The power generation at three sites are compared over two different 12-month intervals. [DOI: 10.1115/1.4036055]

PARAMETERS AFFECTING THE PERFORMANCE OF A RESIDENTIAL-SCALE STATIONARY FUEL CELL SYSTEM

Researchers at the National Institute of Standards and Technology (NIST) have measured the performance of a residential fuel cell system when subjected to various environmental and load conditions. The system, which uses natural gas as its source fuel, is capable of generating electrical power at three nominal power levels (2.5 kW, 4.0 kW, and 5.0 kW) while providing thermal energy for user-supplied loads. Testing was conducted to determine the influence of ambient temperature, relative humidity, electrical load, and thermal load on system performance. Steady-state and transient tests were conducted. The steady-state tests were performed in accordance with the American Society of Mechanical Engineering (ASME) Fuel Cell Power Systems Performance Test Code (PTC-50) for fuel cell power systems. The results of the investigation are being used to develop a proposed rating procedure for residential fuel cell units.