Advances in Capacitor Materials
Electrochemical capacitors, or supercapacitors, have gained intense interest as an alternative to traditional energy storage devices. Applications for supercapacitors range from plug-in hybrid electric vehicles (PHEVs) to backup power sources. While the power density of supercapacitors surpasses that of batteries, commercially available batteries have a significantly higher specific energy density.
NASA’s Ames Research Center in Moffett Field, CA, has developed electrode composite materials that combine graphene with a metal oxide nanocomposite of manganese oxide and cobalt oxide that result in exceptionally high energy density and power density. Another Ames technology for creating supercapacitors combines electrochemical capacitor devices and batteries to create an energy storage medium. This not only allows adequate transport between capacitor electrodes, but also suppresses electrical shorting between electrodes, has a relatively low interface resistance between each electrode and any substance that physically separates the electrodes, and has reduced capacitance.
Nanostructured materials are used to improve supercapacitor performance. These materials offer a high surface area and usable porosity for a given volume and mass, both of which are highly desirable for supercapacitor operation. The supercapacitors have applications in the energy industry, consumer electronics, power delivery, and hybrid vehicles.
Marshall Space Flight Center in Alabama has developed a solid-state ultracapacitor using a novel nanocomposite dielectric material. The design is based on the internal barrier layer capacitance (IBLC) concept, and it uses dielectric and metallic conductive ink formulations. Processing methods provide for unique dielectric properties at the grain level. Nanoscale raw material powders are tailored using a variety of techniques, and then formulated into a special ink. This dielectric ink is used with metallic conductive ink to print a capacitor layer structure into any design necessary to meet a range of technical requirements. The innovation is intended to replace current range safety batteries that NASA uses to power the systems that destroy off-course space vehicles, but also has applications in regenerative braking systems for vehicles, batteries for hybrid and electric cars, smart grid and renewable energy, and medical devices.
NASA has also been in the forefront of creating innovative battery management technologies. Johnson Space Center’s battery charge equalizer provides individual cell charging in multi-cell battery strings using a minimum number of transformers. By effectively keeping all the cells in a multi-cell string at the same charge state, it maximizes the battery's life and performance. Designed to augment a simple, high-current charger that supplies overall battery system energy, the system achieves equalization without wasting energy or creating excess heat. It complements existing high-voltage chargers and instrumentation systems, and offers safe and low-cost management for lithium-ion (Li-ion) batteries used in electric vehicles and other next-generation renewable energy applications.
The system equalizes battery strings by selectively charging cells that need it. The technology maintains battery state-of-charge to improve battery life and performance. In addition, the technology provides fail-safe operation and built-in electrical isolation for the main charge circuit, further improving the safety of high-voltage Li-ion batteries. The equalizer can charge an individual cell bank at the same time the main battery charger is charging the high-voltage battery system.
Johnson researchers also developed a battery management system (BMS) that monitors and balances the charge of individual battery cells in series, and provides fault detection of individual cells in parallel within a battery pack of hundreds of cells. The technology is comprised of a simple and reliable circuit that detects a single bad cell within a battery pack of hundreds of cells, and monitors and balances the charge of individual cells in series. The BMS is cost effective, and can enhance safety and extend the life of critical battery systems used in electric vehicles and other next-generation renewable energy applications.
Glenn Research Center’s masterless, distributed, digital-charge control for multiple battery cells in a series significantly improves fault tolerance. Existing digital charge control systems for batteries are susceptible to single-fault failure; Glenn’s controller uses a single charge controller per cell and links them via an isolated communication bus such as a controller area network (CAN). Because the charge controllers are linked in a masterless way, the failure of one or more does not impact the remaining functional controllers. Each cell's charge controller monitors critical parameters of the cell, such as voltage, temperature, and current. For cell balancing, each charge controller periodically and independently transmits its cell voltage and stores the received cell voltage from other cells in an array. The controller is suited for battery energy storage systems that need cell-level charge control for safety.
Powering Next-Generation Spacecraft
It might sound surprising, but there are currently only two practical options for providing a long-term source of electrical power for exploring space: the light of the Sun or heat from a nuclear source such as a radioisotope. Solar power is an excellent way to generate electricity for most Earth-orbiting spacecraft, and for certain missions to the Moon and beyond that offer sufficient sunlight and natural heat. However, many potential NASA missions would visit some of the harshest, darkest, coldest locations in the solar system, and these missions could be impossible or extremely limited without the use of nuclear power.
Radioisotope power systems (RPS) are a type of nuclear energy technology that uses heat to produce electric power for spacecraft systems and science instruments. That heat is produced by the natural radioactive decay of plutonium-238. Radioisotope power is used only when it will enable or significantly enhance the ability of a mission to meet its science goals.
RPS are not new to the U.S. space program; they have made historic contributions for more than 50 years. RPS have enabled NASA’s exploration of the solar system since the Apollo era of the late 1960s. They are compact, rugged, and provide reliable power in harsh environments where solar arrays are not practical.
RPS offer the key advantage of operating continuously over long-duration space missions, largely independent of changes in sunlight, temperature, charged particle radiation, or surface conditions like thick clouds or dust. In addition, some of the excess heat produced by some RPS can be used to enable spacecraft instruments and onboard systems to continue to operate effectively in extremely cold environments.
A uniquely capable source of power is the radioisotope thermoelectric generator (RTG), a nuclear battery that reliably converts heat into electricity. RTGs work by converting heat from the natural decay of radioisotope materials into electricity. RTGs consist of two major elements: a heat source that contains plutonium-238 dioxide, and a set of solid-state thermocouples that converts the plutonium’s heat energy to electricity. The thermocouples in RTGs use heat from the natural radioactive decay of plutonium-238 to heat the hot junction of the thermocouple, and use the cold of outer space to produce a low temperature at the cold junction of the thermocouple.
RTGs have enabled NASA to explore the solar system for many years. Apollo missions to the Moon, Viking missions to Mars, and the Pioneer, Voyager, Galileo, and Cassini missions all used RTGs.
NASA and the Department of Energy developed a new generation of power systems that can be used on space missions. The new RTG, called the Multi- Mission Radioisotope Thermoelectric Generator (MMRTG), was designed to operate on planetary bodies with atmospheres such as that on Mars, as well as in the vacuum of space. The MMRTG is a more flexible, modular design capable of meeting the needs of a wider variety of missions, since it generates electrical power in smaller increments — slightly above 100 Watts. The design goals for the MMRTG include ensuring safety, optimizing power levels over a minimum lifetime of 14 years, and minimizing weight.
The MMRTG is designed to use a heat source composed of eight General Purpose Heat Source (GPHS) modules. The MMRTG contains a total of 10.6 pounds of plutonium dioxide that initially provides approximately 2000 Watts of thermal power and 120 Watts of electrical power. The thermoelectric materials have demonstrated extended lifetime and performance, and are the same as those used for the two Viking spacecraft that landed on Mars in 1976.
The MMRTG also powered the Mars Science Laboratory (Curiosity) mission to Mars, and will power the next generation of Mars rovers, the Mars 2020 rover. The power system does not require sunlight, permitting spacecraft to land at more diverse locations regardless of season, time of day, or latitude. Because of the number of moving parts, rovers and other mobile explorers require more power than landers. If rovers are solar-powered, they must land and operate within a fairly narrow latitude band near the equator where enough sunlight shines to provide adequate electricity.
MMRTGs will enable an operating lifespan on Mars’ surface of a full Martian year (687 Earth days; a little less than two Earth years) over a wide latitude range. That means it opens up more regions of Mars to exploration, giving mission planners more choices in selecting landing sites that have characteristics related to Mars’ potential as a habitat for life. The MMRTG is also crucial for Curiosity’s thermal stability. Waste heat from the unit is circulated throughout the rover system to keep instruments, computers, mechanical devices, and communications systems within their operating temperature ranges. This system-wide thermal control does not draw on the rover’s electrical power, and precludes the need for radioisotope heater units for spot heating.