Passive Fuel Cell Surface Power System (PaCeSS)
Mechanical and Fluid Systems
Passive Fuel Cell Surface Power System (PaCeSS) (MSC-TOPS-138)
Concept to enable passive phase change cooling of fuel cells
Overview
NASA Johnson Space Center is developing a passive thermal management cooling system for a fuel cell. The cooling system uses a radiator comprising a shape memory alloy (SMA) actuator, and a thermosyphon-based system for heat transport that can eliminate the need of moving parts. Relying upon partial gravity for operation, the innovation, called “PaCeSS” for short, is highly promising for power generation and storage in lunar and planetary surface operations, and may be compelling for terrestrial industries interested in passive thermal management.
Current fuel cells require active thermal management components such as pumps to push liquid coolant through a fuel cell stack and electronic control valves to operate radiator panels. These components have moving parts presenting several disadvantages: they limit the operational lifetime and reliability of a fuel cell system, and are a source of parasitic power draw because they consume energy from the fuel cell stack to power their operations.
PaCeSS could eliminate the wear-and-tear of moving parts and reduce complexity associated with fuel cell system thermal management. It is a system-level solution that spans heat acquisition, heat transport, and heat rejection by incorporating a two-phase thermosyphon for passively removing heat from a fuel cell stack through a transport path. It then uses an SMA mechanism to actuate a radiator panel that can vary the heat rejection rate into space.
The Technology
NASA’s envisioned Lunar and Martian surface operations will require constant and reliable power systems. Traditional power architectures, including solar cells and batteries, cannot be solely relied upon due to the lengthy lunar nights and challenging thermal environments. How-ever, fuel cells, including primary fuel cells and regenerative fuel cells, represent a promising means for energy generation and storage on planetary and lunar surfaces.
PaCeSS could further improve mission flexibility by significantly enhanc-ing reliability and longevity with fully passive fuel cell power generation capability. Test systems have been built to validate the performance characteristics of various PaCeSS technology elements, and many of the component materials have already been characterized. Some of these novel technology elements already demonstrated include a two-phase thermosyphon operation in fuel cell conditions, a passive shape memory actuator operation using two-phase water, and a shape memory alloy radiator turndown.
Although the current design of the shape memory alloy actuated rad-iator system is dependent on partial gravity and space-like environments where heat rejection is performed primarily via radiation, there may be ways of using the same basic system for controlling fuel cell temper-ature via convective heat rejection for terrestrial applications. Addition-ally, other elements of this concept could be commercialized terrestri-ally, including the thermosyphon heat transport mechanism, a multi-purpose vapor chamber, and a thermal management system that uses water by-product as the thermal management medium.
The Passive Fuel Cell Surface Power System is at a technology readiness level (TRL) 3 (analytical and experimental critical function and/or characteristic proof of concept) and is now available for patent licensing. Please note that NASA does not manufacture products itself for commercial sale.
Benefits
- Improves reliability
- Extends operating life
- Reduces modes of failure
- Generates its own cooling medium
- Reduces mass
- Reduces complexity of control systems
- Improves output efficiency
- Eliminates parasitic power draw
Applications
- Intended for lunar/planetary applications with at least partial gravity and little to no atmosphere
- Primary fuel cell surface power
- Regenerable fuel cell surface energy storage
- Electrolyzer/unitized reversible fuel cells
Technology Details
Mechanical and Fluid Systems
MSC-TOPS-138
MSC-27638-1
Similar Results
Adaptive Thermal Management System
Efficient thermal management has long been an issue in both commercial systems and in the extreme environments of space. In space exploration and habitation, significant challenges are experienced in providing fluid support systems such as cryogenic storage, life support, and habitats; or thermal control systems for launch vehicle protection, environmental heat management, or electronic instruments. Furthermore, these systems operate in dynamic, transient modes and often under extremes of temperature or pressure. The current technical requirements associated with the thermal management of these systems result in control issues as well as significant life-cycle costs.
To combat these issues, the Adaptive Thermal Management System (ATMS) was developed to help provide the capability for tanks, structural walls, or composite substrate materials to switch functionality (conductive or insulative) depending on environmental conditions or applied stimuli. As a result, the ATMS provides the ability to adapt between both heating and cooling modes within a single system. For example, shape memory alloy (SMA) elements are used to actuate at certain design temperatures to create a conductive bridge between two metal plates allowing broad-area heat rejection from the hotter surface. Upon cooling to the lower design set-point, the SMA elements return to their original shapes, thereby breaking the conductive path and returning the system to its overall insulative state.
This technology has the potential to be applied to any system that would have the need for a self-regulating thermal management system that allows for heat transfer from one side to another.
Ball Valve Assembly Yields Linear Flowrate Control
Prototype thermal control valves for the next generation spacesuit were challenged in maintaining precise thermal control, so engineers created a design that functions like a traditional ball valve but added tapered-valley contours to the ball that yields a variable orifice which is more predictable at controlling flow.
The key differences between the TCBV and traditional v-channel ball valves are that this technology has one inlet and two outlets allowing the split-flow of fluids whereas traditional v-channel valves only have one inlet and one outlet. Additionally, traditional v-channel ball valves don’t enable the full flow rate of a given system while this technology does.
The ball valve is held in place within the TCBV using two PTFE seats compressed by spring-loaded side plates. The hole in the middle of the ball valve and adjoining tapered valleys mate with the PTFE seats to create varying sized orifices depending on valve position. Specially designed O-ring seals surrounding the ball valve assembly allow the seats to move within the pocket while preventing internal leakage.
In this technology’s spacesuit application, coolant is fed to the ported ball valve where the coolant is apportioned to each valve housing exit either primarily feeding the cooling and ventilation garment or the bypass circuit back to the spacesuit’s thermal cooling system. The apportionment is determined by the astronaut’s manual valve adjustment or automatically by the suit.
How to Train Shape Memory Alloys
Glenn researchers have optimized how shape memory alloys (SMAs) are trained by reconceptualizing the entire stabilization process. Whereas prior techniques stabilize SMAs during thermal cycling, under conditions of fixed stress (known as the isobaric response), what Glenn's innovators have done instead is to use mechanical cycling under conditions of fixed temperature (the isothermal response) to achieve stabilization rapidly and efficiently. This novel method uses the isobaric response to establish the stabilization point under conditions identical to those that will be used during service. Once the stabilization point is known, a set of isothermal mechanical cycling experiments is then performed using different levels of applied stress. Each of these mechanical cycling experiments is left to run until the strain response has stabilized. When the stress levels required to achieve stabilization under isothermal conditions are known, they can be used to train the material in a fraction of the time that would be required to train the material using only thermal cycling. As the strain state has been achieved isothermally, the material can be switched back under isobaric conditions, and will remain stabilized during service. In short, Glenn's method of training can be completed in a matter of minutes rather than in days or even weeks, and so SMAs become much more practical to use in a wide range of applications.
Thermal Management for Aircraft Propulsion Systems
Aircraft thermal management systems typically comprise over half the mass associated with full electric power propulsion systems, with significant negative impact on fuel efficiency. In addition, the traditional method of using jet fuel to cool aircraft generators does not provide enough cooling for use in flight-weight cryogenic systems. Lastly, the much higher bus voltages required for flight-weight systems (4.5 kV vs. 270 V) introduce additional spark-ignition hazards associated with alternative cryogenic cooling fuels, including liquid methane or liquid hydrogen. The Glenn flight-weight thermal management system addresses all of these problems by using the considerable waste heat energy from turbogenerators to create a pressure wave thermoacoustically. This wave can then be delivered quietly and efficiently via routed ductwork to hollow pulse-tube coolers located near any component in the aircraft that requires cooling. The tubes can be fabricated in any length and can be curved to fit any space. This technology also allows waste heat energy to be used in at least four ways: 1) the waste heat energy can drive a thermoacoustics-based ambient or cryogenic heat pump; 2) it can be channeled directly into a thermoacoustic engine that generates power; 3) it can convectively preheat the fuel/ or air supplied to the aircraft engine; 4) it can drive a pulse-tube generator providing power. The delivered thermoacoustic power can provide cabin cooling as well as ambient/cryogenic cooling of converter, cables, and motors. In addition, this power can be converted to local electric power through the use of a transducer (such as a linear alternator) or piezoelectrics. Further, the efficient thermal management system enables the size, mass, and resultant cost of the radiating fins to be reduced. Glenn's system offers an efficient method of cooling next-generation flight-weight electric aircraft with significant benefits for fuel efficiency and safety.
High-Performance, Lightweight, Easy-to-Fabricate Heat Exchanger
Researchers at JPL have developed, built, and tested an innovative heat exchanger that offers reduced thermal expansion, increased structural strength, low pressure drop, and improved thermal performance while lowering the weight associated with typical heat exchangers. This innovation would benefit the commercial thermoelectric generator, aircraft, and industrial processing (i.e., glass, steel, petrochemical, cement, aluminum) industries by improving energy management/efficiency, reducing carbon dioxide emissions, and increasing system durability due to the reduced stress from thermal expansion.
The Problem
Thermoelectric generator systems require high-performance hot-side and cold-side heat exchangers to provide the temperature differential needed to transfer thermal energy while withstanding temperatures up to 650 °C. Because the hot-side heat exchangers must have a high heat flux, they are often made of metals such as stainless steel or Inconel® alloys. Although these materials can operate at high temperatures, resist corrosion, and are chemically stable, they also have several drawbacks: (1) Their lower thermal conductivity negatively affects their thermal performance. (2) Their higher thermal expansion leads to stresses that compromise system structural integrity. (3) Their high mass/volume reduces the power density of generator systems into which they are integrated. As a result, they are difficult to integrate into viable energy recovery systems. They also make the systems unreliable, non-durable, and susceptible to failures caused by thermal-structural expansion.
The Solution
JPL researchers chose to replace the metal in traditional heat exchangers with graphite, which offers an improved conductivity-to-density ratio in thermal applications as well as a low coefficient of thermal expansion. In addition, they used a mini-channel design to further increase thermal performance. Combining more advanced materials with the innovative thermal design has yielded significant improvements in performance. For example, a 200-cm3, 128-g version of JPL's exchanger successfully transported 1,100 W from exhaust at nearly 550 °C with approximately 20 W/cm2 thermal flux and a pressure drop of only 0.066 psi.
JPL's technology combines lightweight, high-strength graphite material with a mini-channel design that offers high thermal performance. Further development and testing are underway.
Inconel is a registered trademark of Special Materials Corporation.
