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Wolford, David S.

Publications and source records attributed to Wolford, David S..

8 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

NASA Glenn Research Center Solar Cell Experiment Onboard the International Space Station

Accurate air mass zero (AM0) measurement is essential for the evaluation of new photovoltaic (PV) technology for space solar cells. The NASA Glenn Research Center (GRC) has flown an experiment designed to measure the electrical performance of several solar cells onboard NASA Goddard Space Flight Center's (GSFC) Robotic Refueling Missions (RRM) Task Board 4 (TB4) on the exterior of the International Space Station (ISS). Four industry and government partners provided advanced PV devices for measurement and orbital environment testing. The experiment was positioned on the exterior of the station for approximately eight months, and was completely self-contained, providing its own power and internal data storage. Several new cell technologies including four-junction (4J) Inverted Metamorphic Multi-junction (IMM) cells were evaluated and the results will be compared to ground-based measurement methods.

Solar Cell Calibration Facility↗
NASA NTRS2015

On-Orbit Measurement of Next Generation Space Solar Cell Technology on the International Space Station

Measurement is essential for the evaluation of new photovoltaic (PV) technology for space solar cells. NASA Glenn Research Center (GRC) is in the process of measuring several solar cells in a supplemental experiment on NASA Goddard Space Flight Center's (GSFC) Robotic Refueling Mission's (RRM) Task Board 4 (TB4). Four industry and government partners have provided advanced PV devices for measurement and orbital environment testing. The experiment will be on-orbit for approximately 18 months. It is completely self-contained and will provide its own power and internal data storage. Several new cell technologies including four- junction (4J) Inverted Metamorphic Multijunction (IMM) cells will be evaluated and the results compared to ground-based measurements.

solar cell↗
NASA NTRS2014

On-Orbit Measurement of Next Generation Space Solar Cell Technology on the International Space Station

On-orbit measurements of new photovoltaic (PV) technologies for space power are an essential step in the development and qualification of advanced solar cells. NASA Glenn Research Center will fly and measure several solar cells attached to NASA Goddards Robotic Refueling Mission (RRM), expected to be launched in 2014. Industry and government partners have provided advanced PV devices for evaluation of performance and environmental durability. The experiment is completely self-contained, providing its own power and internal data storage. Several new cell technologies including Inverted Metamorphic Multi-junction and four-junction cells will be tested.

robotic refueling mission↗
NASA NTRS2012

A Newton-Raphson Method Approach to Adjusting Multi-Source Solar Simulators

NASA Glenn Research Center has been using an in house designed X25 based multi-source solar simulator since 2003. The simulator is set up for triple junction solar cells prior to measurements b y adjusting the three sources to produce the correct short circuit current, lsc, in each of three AM0 calibrated sub-cells. The past practice has been to adjust one source on one sub-cell at a time, iterating until all the sub-cells have the calibrated Isc. The new approach is to create a matrix of measured lsc for small source changes on each sub-cell. A matrix, A, is produced. This is normalized to unit changes in the sources so that Ax(delta)s = (delta)isc. This matrix can now be inverted and used with the known Isc differences from the AM0 calibrated values to indicate changes in the source settings, (delta)s = A ·'x.(delta)isc This approach is still an iterative one, but all sources are changed during each iteration step. It typically takes four to six steps to converge on the calibrated lsc values. Even though the source lamps may degrade over time, the initial matrix evaluation i s not performed each time, since measurement matrix needs to be only approximate. Because an iterative approach is used the method will still continue to be valid. This method may become more important as state-of-the-art solar cell junction responses overlap the sources of the simulator. Also, as the number of cell junctions and sources increase, this method should remain applicable.

Snyder, David B.↗
NASA NTRS2012

A Low Cost Weather Balloon Borne Solar Cell Calibration Payload

Calibration of standard sets of solar cell sub-cells is an important step to laboratory verification of on-orbit performance of new solar cell technologies. This paper, looks at the potential capabilities of a lightweight weather balloon payload for solar cell calibration. A 1500 gr latex weather balloon can lift a 2.7 kg payload to over 100,000 ft altitude, above 99% of the atmosphere. Data taken between atmospheric pressures of about 30 to 15 mbar may be extrapolated via the Langley Plot method to 0 mbar, i.e. AMO. This extrapolation, in principle, can have better than 0.1 % error. The launch costs of such a payload arc significantly less than the much larger, higher altitude balloons, or the manned flight facility. The low cost enables a risk tolerant approach to payload development. Demonstration of 1% standard deviation flight-to-flight variation is the goal of this project. This paper describes the initial concept of solar cell calibration payload, and reports initial test flight results. .

Snyder, David B.↗
NASA NTRS2007

GaAs Photovoltaics on Polycrystalline Ge Substrates

High efficiency III-V multijunction solar cells deposited on metal foil or even polymer substrates can provide tremendous advantages in mass and stowage, particularly for planetary missions. As a first step towards that goal, poly-crystalline p/i/n GaAs solar cells are under development on polycrystalline Ge substrates. Organo Metallic Vapor Phase Epitaxy (OMVPE) parameters for pre-growth bake, nucleation and deposition have been examined. Single junction p/i/n GaAs photovoltaic devices, incorporating InGaP front and back window layers, have been grown and processed. Device performance has shown a dependence upon the thickness of a GaAs buffer layer deposited between the Ge substrate and the active device structure. A thick (2 m) GaAs buffer provides for both increased average device performance as well as reduced sensitivity to variations in grain size and orientation. Illumination under IR light (lambda > 1 micron), the cells showed a Voc, demonstrating the presence of an unintended photoactive junction at the GaAs/Ge interface. The presence of this junction limited the efficiency to approx.13% (estimated with an anti-refection coating) due to the current mismatch and lack of tunnel junction interconnect.

Wilt, David M.↗
NASA NTRS2000

Rare Earth Optical Temperature Sensor

A new optical temperature sensor suitable for high temperatures (greater than 1700 K) and harsh environments is introduced. The key component of the sensor is the rare earth material contained at the end of a sensor that is in contact with the sample being measured. The measured narrow wavelength band emission from the rare earth is used to deduce the sample temperature. A simplified relation between the temperature and measured radiation was verified experimentally. The upper temperature limit of the sensor is determined by material limits to be approximately 2000 C. The lower limit, determined by the minimum detectable radiation, is found to be approximately 700 K. At high temperatures 1 K resolution is predicted. Also, millisecond response times are calculated.

Chubb, Donald L.↗