Andrew Kindler

LITHIUM BATTERIES FOR AEROSPACE APPLICATIONS: 2003 MARS EXPLORATION ROVER

Future NASA\ planetary exploration missions require batteries that can operate at extreme temperatures and with high specific energy and energy densities. Conventional aerospace rechargeable battery systems, such as Ni-Cd, Ni-H;! and Ag-Zn, are inadequate to meet these demands. Lithium ion rechargeable batteries are therefore being chosen as the baseline for these missions. The 2003 Mars Exploration Rover mission plans to deploy twin rovers onto Mars, with the objectives of understanding its geology, climate conditions and possibility of life on Mars. The spacecraft contain various batteries, i.e., primary batteries on the lander, thermal batteries on the back shell and rechargeable batteries on the Rovers. Significant among them are the Li ion rechargeable batteries, which are being utilized for the first time in a major NASA mission. The selection of the Li ion battery has been dictated by various factors, including mass and volume constraints, cycle life, and its ability to operate well at sub-zero temperatures (down to -30°C), at moderate rates. This paper describes the selection criteria, design and performance of the three battery systems on 2003 MER mission.

Recent advances in PEM liquid-feed direct methanol fuel cells

A direct methanol-air fuel cell operating at near atmospheric pressure, low-flow rate air, and at temperatures close to 60/spl deg/C would tremendously enlarge the scope of potential applications. While earlier studies have reported performance with oxygen, the present study focuses on characterizing the performance of a PEM liquid feed direct methanol-air cell consisting of components developed in house. These cells employ Pt-Ru catalyst in the anode, Pt at the cathode and Nafion 117 as the PEM. The effect of pressure, flow rate of air and temperature on cell performance has been studied. With air, the performance level is as high as 0.437 V at 300 mA/cm/sup 2/ (90/spl deg/C, 20 psig, and excess air flow) has been attained. Even more significant is the performance level at 60/spl deg/C, 1 atm and low flow rates of air (3-5 times stoichiometric), which is 0.4 V at 150 mA/cm/sup 2/. Individual electrode potentials for the methanol and air electrode have been separated and analyzed. Fuel crossover rates and the impact of fuel crossover on the performance of the air electrode have also been measured. The study identifies issues specific to the methanol-air fuel cell and provides a basis for improvement strategies.

Potentiostatic Depassivation of Lithium-Sulfur Dioxide Batteries on Mars Exploration Rovers

NASAs 2003 Mars Exploration Rovers, Spirit and Opportunity, have been performing exciting surface exploration studies for the past 3 years, providing conclusive evidence for the presence of past water on Mars. Although the rovers are being powered by Li-ion batteries and solar arrays, their critical entry, descent, and landing (EDL) maneuvers were successfully supported by primary lithium-sulfur dioxide batteries. These batteries exhibited voltage delay at the end of cruise, which necessitated a depassivation of these batteries prior to EDL. In the absence of conventional depassivation across a specified load, a new method of depassivation via potentiostatic discharge was employed for the mission. Several simulation tests were performed on cells, cell strings, and battery assemblies, at different potentiostatic voltages and durations to characterize the depassivation process. Effects of repassivation of the lithium anode subsequent to depassivation were also studied, mainly to establish the timeline necessary for depassivation prior to use during the EDL process. Finally, a phenomenological model was developed for the potentiostatic depassivation of Li-SO 2 cells, based on dielectric properties of the surface film on Li, which gave voltage predictions in quantitative agreement with the experimental data. The laboratory results obtained were subsequently corroborated by the in-flight data received from the spacecraft.

Direct methanol fuel cells-status, challenges and prospects

The status of direct methanol fuel cell technology with respect to power density, efficiency and integrated system operation have been summarized. The key challenge in improving power density is combining with operation at low air flow rates in order to maintain a water balance, and achieve attractive system mass and size. Improved catalysts and membranes with low methanol permeability are key to achieving these improvements. Challenges relating to miniature DMFC for battery replacement are discussed. Possibilities of reduction in catalyst and membrane cost suggest that premium power applications (100 W-5 kW) could be an early point of entry for DMFC into commercial markets.

Development and Prototype Testing of Low-Cost Lightweight Thin Film Solar Concentrator

A low-cost rigid foam-based concentrator technology development program was funded by the DOE SunShot Initiative to meet installed cost goals of

Commercialization of a direct methanol fuel cell system

75/m^2 vs. current costs of ∼

Lithium-sulfur dioxide batteries on Mars rovers

200–250/m^2. Phase 1 of the project focused on design trades and cost analyses leading to a cost-optimized self-powered autonomous tracking heliostat concept with a mirror surface area in the 100m^2 range. In Phase 2 30-year accelerated testing of the mirror modules based on ReflecTec film with 94% specular reflectivity bonded on composite foam substrate were initiated and completed in Phase 3. The tests with 15 coupons showed optical performance degradation of less than 5% in specular reflectance following 30-year equivalent UV testing and other abuse testing such as acid rain, bird dropping, thermal cycling, etc. A small scale prototype (3m×2m) heliostat design based on modular truss elements with removable mirror modules was developed in detail. In this phase components such as the dual-axis actuators were sized and selected based on wind load requirements and pointing accuracy demands were completed. Finite Element analyses for the mechanical structure with mirror modules were performed using three separate commercial codes — ANSYS, COMSOL and SolidWorks to validate the optical errors induced by wind loads on the structure up to 35 mph. Results indicated that the RMS deflections contributed to less than 0.4 mrad pointing error. Dynamic response of the heliostat indicated that the first 5 eigenmodes were in the 17–20 Hz range. The individual structure elements such as the trusses and c-rails were fabricated locally and assembled with the mirror facets in the lab for initial fit check and testing. The nine mirror facet surface errors were characterized using photogrammetry and verified using Reverse Hartmann techniques and showed to be in the order of 1 mrad or less. A three-level controller (main, gateway and heliostat) was architected and built. Tracking of the sun is done using NREL’s Sun Tracking Algorithm implemented in the gateway controller. Target-pointing vectors are calculated for each heliostat and conveyed wirelessly to the individual heliostat controllers for actuating the azimuth and elevation motors. The power subsystem consisting of solar panels and a battery provide 24V for the actuators and controller boards. The system was sized to provide adequate power for a period of 5hrs of operation when power is not available. Initial calibration will be performed with on-site camera tracking the sun’s image on a target located approximately 52m from the heliostat. Testing of the heliostat pointing under calm and windy conditions will be done to demonstrate overall performance that meet DOE targets of 4 mrad under 27 mph winds. Commercialization efforts are underway to transition the design to the commercial sector. The project is well on its way to approaching overall cost targets and current estimates are approximately S90–110/m^2 and lower costs can be achieved with alternates to the film we have identified.

Designing an autonomous power system for a stand-alone heliostat

This paper describes a major breakthrough in energy technology developed at the Jet Propulsion Laboratory that can be used in a wide variety of portable, remote and transportation applications without polluting the environment. The status, performance, and design considerations of the JPL non-polluting, Direct Methanol, Fuel Cell system for consumer equipment and transportation applications are reported herein. This new fuel cell technology utilizes the direct oxidation of a 3% aqueous liquid methanol solution as the fuel and air (O2) as the oxidant. The only products are CO2 and water. Therefore, because recharging can be accomplished by refueling with methanol, vehicles can enjoy unlimited range and extended use compared to battery operated devices requiring recharge time and power accessibility.

Low-Cost Lightweight Thin Film Solar Concentrators

NASA’s 2003 Mars Exploration Rover (MER) missions, Spirit and Opportunity, have been performing exciting surface exploration studies for the past six months. These two robotic missions were aimed at examining the presence of water and, thus, any evidence of life, and at understanding the geological conditions on Mars. These rovers have been successfully assisted by primary lithium-sulfur dioxide batteries during the critical entry, descent, and landing (EDL) maneuvers. These batteries were located on the petals of the lander, which, unlike the Mars Pathfinder mission, was designed only to carry the rover. The selection of the lithiumsulfur dioxide battery system for this application was based on its high specific energy and high rate discharge capability, combined with low heat evolution, as dictated by this application. Lithium-sulfur dioxide batteries exhibit voltage delay, which tends to increase at low discharge temperatures, especially after extended storage at warm temperatures. In the absence of a depassivation circuit, as provided on earlier missions, e.g., Galileo, we were required to depassivate the lander primary batteries in a unique manner. The batteries were brought onto a shunt-regulated bus set at pre-selected discharge voltages, thus affecting depassivation during constant discharge voltage. Several ground tests were performed, on cells, cell strings and battery assembly with five parallel strings, to identify optimum shunt voltages and durations of depassivation. We also examined the repassivation of lithium anodes, subsequent to depassivation. In this paper, we will describe these studies, in detail, as well as the depassivation of the lander flight batteries on both Spirit and Opportunity rovers prior to the EDL sequence and their performance during landing on Mars.

Direct methanol feed fuel cell and system

Concentrated Solar Power (CSP) offers a number of advantages over photovoltaic at large scale power generation, such as lower cost and the option of thermal energy storage. However, the costs of a power tower plant are dominated by heliostats, which are usually 50% of the entire facility. NASA5s Jet Propulsion Laboratory was awarded funding by the DOE SunShot Initiative to develop a low-cost, rigid foam-based solar concentrator to meet installed cost goals of S75/m2 versus current costs of