David B. Snyder

Solar Arrays for Direct-Drive Electric Propulsion: Arcing at High Voltages

The results from an experimental investigation to assess arcing during operation of high voltage solar arrays in a plasma environment are presented. The experiments were part of an effort to develop systems that would allow safe operation of Hall-Effect Thrustefls) in direct-drive mode. Arc discharges are generated when the array is biased negative with respect to the plasma. If sustained for long periods of time between adjacent solar cells, arcs may severely damage a solar array, thus significantly shortening its lifetime. Most often sustained arcs are triggered by plasma produced during short-duration discharge arcs (approximately 20 microseconds). These trigger arcs are sparked between the semiconducting cell and the covering dielectric. Both trigger and sustained (greater than 1 millisecond) arcs have been captured during the tests. Current and voltage waveforms associated with the different arc events are presented. The test results have defined operational limits (thresholds) for the various array concepts studied that minimize the likelihood of damage from sustained arcs. Experimental trends regarding the effect of the solar array substrate on arc duration are also presented.

Advanced solar cell and array technology for NASA deep space missions

A recent study by the NASA Glenn Research Center assessed the feasibility of using photovoltaics (PV) to power spacecraft for outer planetary, deep space missions. While the majority of spacecraft have relied on photovoltaics for primary power, the drastic reduction in solar intensity as the spacecraft moves farther from the sun has either limited the power available (severely curtailing scientific operations) or necessitated the use of nuclear systems. A desire by NASA and the scientific community to explore various bodies in the outer solar system and conduct “long-term” operations using smaller, “lower-cost” spacecraft has renewed interest in exploring the feasibility of using photovoltaics for missions to Jupiter, Saturn and beyond. With recent advances in solar cell performance and continuing development in lightweight, high power solar array technology, the study determined that photovoltaics is indeed a viable option for many of these missions.

Solar Arrays for Direct-Drive Electric Propulsion: Electron Collection at High Voltages

Solar array technologies that can empower electric thrusters in direct drive mode may provide significant mission benefits by reducing power processing, system complexity, weight, and cost over conventional systems. In direct-drive systems the solar arrays will operate at high voltages and must perform safely in the surrounding plasma environment, with minimal loss of performance due to parasitic current collection. For example, current state-of-the-art Hail effect thrusters in the kilowatt class require applied voltages of 300 V or higher. Results from experiments and modeling of electron current collection at high bias voltages (300-500 V) are presented. The experiments employed two sample solar array coupon technologies. A hollow cathode was used to emulate the induced environment around the solar arrays far from the Hall thruster and in nearby regions populated by charge-exchange plasma (10 1 2 -10 1 3 m - 3 and 0.5-1 eV). The measurements show that tens to hundreds of seconds are required before the collected current relaxes to a quasi-steady value. Comparisons with results from numerical calculations suggest that changes of the secondary electron yield properties of the dielectric materials may account for the observed current collection trends.

Low intensity low temperature (LILT) measurements of state-of-the-art triple junction solar cells for space missions

NASA has launched missions to the outer planets over the last 40 years, each mission increasing in complexity and power. Most of these earlier missions relied on nuclear power sources due to the lower amount of sunlight requiring prohibitively huge solar arrays. Newer missions include Rosetta, Dawn, and Juno are powered using solar energy [1,2]. Today, a shortage of plutonium coupled with improved solar cell efficiency and array structure design has led NASA to take another look at photovoltaics for planetary missions. Triple Junction (GaP/InGaAs/Ge) solar cells now exceed 28% AM0 1 sun efficiency and will soon surpass 30%, with newer IMM technology pushing 33% [3]. These cells are qualified for earth orbit missions but outer planetary missions require additional testing beyond this intensity and temperature range. This paper presents a summary of the performance of triple junctions solar cells under temperatures and intensities for the outer planetary missions, adding to previous work [4–7].

Ozone correction for AM0 calibrated solar cells for the aircraft method

The aircraft solar cell calibration method has provided cells calibrated to space conditions for 37 years. However, it is susceptible to systematic errors due to ozone concentrations in the stratosphere. The present correction procedure applies a 1% increase to the measured I/sub sc/ values. High band-gap cells are more sensitive to ozone absorbed wavelengths (0.4 to 0.8 /spl mu/m) so it becomes important to reassess the correction technique. This paper evaluates the ozone correction to be 1+O3/spl times/Fo, where O3 is the total ozone along the optical path, and Fo is 29.8/spl times/10/sup -6//du for a silicon solar cell, 42.6/spl times/10/sup -6//du for a GaAs cell and 57.2/spl times/10/sup -6//d.u. for a InGaP cell. These correction factors work best to correct data points obtained during the flight rather than as a correction to the final result.

Solar cell short circuit current errors and uncertainties during high altitude calibrations

High altitude balloon based facilities can make solar cell calibration measurements above 99.5% of the atmosphere to use for adjusting laboratory solar simulators. While close to on-orbit illumination, the small attenuation to the spectra may result in under measurements of solar cell parameters. Variations of stratospheric weather, may produce flight-to-flight measurement variations. To support the NSCAP effort, this work quantifies some of the effects on solar cell short circuit current (Isc) measurements on triple junction sub-cells. This work looks at several types of high altitude methods, direct high altitude measurements near 120 kft, and lower stratospheric Langley plots from aircraft. It also looks at Langley extrapolation from altitudes above most of the ozone, for potential small balloon payloads. A convolution of the sub-cell spectral response with the standard solar spectrum modified by several absorption processes is used to determine the relative change from AM0, Isc/Isc(AM0). Rayleigh scattering, molecular scattering from uniformly mixed gases, Ozone, and water vapor, are included in this analysis. A range of atmospheric pressures are examined, from 0.05 to 0.25 Atm to cover the range of atmospheric altitudes where solar cell calibrations are performed. Generally these errors and uncertainties are less than 0.2%.

A Newton-Raphson method approach to adjusting multi-source solar simulators

NASA Glenn Research Center has been using an inhouse designed X25 based multi-source solar simulator since 2003. The simulator is set up for triple junction solar cells prior to measurements by adjusting the three sources to produce the correct short circuit current, Isc, 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 Isc for small source changes on each sub-cell. A matrix, A, is produced. This is normalized to unit changes in the sources so that A×Δs= Δisc. 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, Δs = A-1×Δisc 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 Isc values. Even though the source lamps may degrade over time, the initial matrix evaluation is 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.

Thermal balance testing for advanced, lightweight solar array designs

The traditional modular building block for space solar arrays consists of solar cells mounted to an aluminum honeycomb panel with carbon composite facesheets. This design not only provides the structural rigidity for the array, but is critical for heat dissipation during operation in the space. The honeycomb provides consistent thermal conduction and emissive properties throughout the panel, and good thermal conduction from the solar cell to the panel all combine to produce a well-proven highly reliable mechanism for radiative heat transfer to space that is relatively simple to model, and therefore provide uniform solar cell operating temperatures. New, lightweight solar array designs are currently under development that differ from these traditional honeycomb panels. These new designs offer significant improvements in array specific power (watts per kilogram) and stowed volume. However, due to the unique, lightweight design of the blanket structure (interconnected solar cell attachment and support interface), these advanced array designs offer a significant challenge with regard to heat dissipation and accurate thermal modeling of the solar array. Recent modifications to facilities at the NASA Glenn Research Center provide thermal balance testing of new solar array designs to determine cell operating temperatures under various space environments.

ER-2 high altitude solar cell calibration flights

Evaluation of space photovoltaics using ground-based simulators requires primary standard cells which have been characterized in a space or near-space environment. Due to the high cost inherent in testing cells in space, most primary standards are tested on high altitude fixed wing aircraft or balloons. The ER-2 test platform is the latest system developed by the Glenn Research Center (GRC) for near-space photovoltaic characterization. This system offers several improvements over GRCs current Learjet platform including higher altitude, larger testing area, onboard spectrometers, and longer flight season. The ER-2 system was developed by GRC in cooperation with NASAs Armstrong Flight Research Center (AFRC) as well as partners at the Naval Research Laboratory and Air Force Research Laboratory. The system was designed and built between June and September of 2014, with the integration and first flights taking place at AFRCs Palmdale facility in October of 2014. Three flights were made testing cells from GRC as well as commercial industry partners. Cell performance data was successfully collected on all three flights as well as solar spectra. The data was processed using a Langley extrapolation method, and performance results showed a less than half a percent variation between flights, and less than a percent variation from GRCs current Learjet test platform.

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. AM0. This extrapolation, in principle, can have better than 0.1% error. The launch costs of such a payload are significantly less the 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.