High-Performance, Lightweight, Easy-to-Fabricate Heat Exchanger
power generation and storage
High-Performance, Lightweight, Easy-to-Fabricate Heat Exchanger (NPO-TOPS-59)
A key enabling technology for next-generation thermoelectric generators and other energy recovery systems
Overview
Researchers at NASA's Jet Propulsion Laboratory (JPL) have developed an advanced heat exchanger that addresses problems typically associated with metal-based systems. Unlike typical metal heat exchangers, which suffer from high thermal expansion and high density, JPL's innovation offers several improved properties. Its lightweight, high heat flux design offers a low coefficient of thermal expansion (CTE)—and therefore low expansion characteristics—and reduces the pressure drop during heat transfer. The design can handle high-temperature gases (up to 650 °C). This technology has undergone successful preliminary testing. Developed for an aircraft exhaust energy recovery application, the system meets the requirements of high-temperature, high-specific-power thermoelectric generators and other energy recovery systems for industrial, automotive, military, and space applications.
The Technology
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.
Benefits
- High performance: This technology offers high-efficiency thermal energy recovery with a low drop in exhaust flow pressure.
- Lightweight, yet high power: The innovative design results in its leveraging a high ratio of thermal conductivity to density.
- Easy to manufacture: The graphite material used in JPL's heat exchanger is widely available and reasonably easy to use in and compatible with standard machining and fabrication processes.
- Robust: As a graphite-based heat exchanger, JPL's technology has a low coefficient of thermal expansion and good structural strength.
- Flexible: The current prototype of JPL's innovation is easy to reconfigure into other designs using similar materials.
Applications
- Thermoelectric generators
- Thermal energy recovery in industrial processes (e.g., oil/gas, steel, glass, aluminum, cement, paper)
- Exhaust energy recovery systems for automobiles, aircraft, etc.
- Hypersonic engine and aircraft cooling
- Potentially extreme chemical and thermal environments on Venus
Technology Details
power generation and storage
NPO-TOPS-59
NPO-50566-1
Hendricks TJ, et al. Design and testing of high-performance mini-channel graphite heat exchangers in thermoelectric energy recovery systems. Proceedings of the American Society of Mechanical Engineers (ASME) 2017 International Mechanical Engineering Congress and Exposition. Nov. 3-9, 2017. Tampa, FL. Paper #IMECE2017-72411.
Similar Results
Passive Fuel Cell Surface Power System (PaCeSS)
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.
Thin Film Sensor for Ultra High-Temp Measurement
The thin film sensor’s principal advantage lies in its potential to take high frequency temperature measurements from the surface of a reentering spacecraft while simultaneously withstanding the high temperature and oxidizing environment encountered. This data provides engineers with operational phase measurements used to refine the spacecraft’s operational envelope and track flight hardware behavior in addition to providing high frequency temperature measurements that can inform the physics of a boundary layer.
Mismatches in coefficients of thermal expansion (CTE) are expected in TPS-based sensor applications because the metallic materials used for temperature sensing have thermal expansion rates that differ from the rates of the substrate and coating materials in the TPS. At high temperatures during reentry, this mismatch in CTE can create a significant strain differential between the metallic sensor, sensor leads, and the materials to which the sensor and leads are bonded.
High frequency response temperature measurements on the surface of entry spacecraft are not currently possible above ~700 F with existing measurement capabilities. This shortcoming is primarily due to the need for robust sensor behavior at temperatures of several thousand degrees F. The sensor design of this technology preserves the integrity of sensor components while enhancing its high temperature functionality.
The thin film temperature sensor has a technology readiness level (TRL) 5 (Component and/or breadboard validation in relevant environment) and is now available for patent licensing. Please note that NASA does not manufacture products itself for commercial sale.
Premixed, High-Pressure, Multi-Fuel Burner
NASA Glenn's fully premixed burner design accomplishes the rapid mixing of the fuel and air flows while simultaneously providing backside impingement cooling to the burner face. This novel burner technology has been demonstrated to operate on hydrogen-air mixtures at pressures up to 30 bar, and at equivalence ratios (Phi) ranging from 0.15 to 5.0, but typically at equivalence ratios below 0.6 or above 2.0 for extended periods of time. It has also been demonstrated to work well with hydrogen-carbon monoxide fuel mixtures in a 1:1 mixture (by volume). The design provides a uniform zone of combustion products and temperatures, and is able to achieve complete and rapid mixing of the reactant gases over a distance as short as 5 mm, with the combustion products attaining a fully-reacted state within about 10 mm downstream of the burner face. Effectiveness of the mixing is not dependent on the use of hydrogen gas, therefore the system works well for other gaseous fuels such as methane, propane, or natural gas, in a fully premixed mode.
The design of the Glenn's burner is simple and straightforward to manufacture using conventional techniques. The modular design of the burner lends itself to scalability for larger power output applications. This burner is simple to operate and is robust for use in an industrial setting such as low-emissions stationary gas turbine engine, or for aircraft gas turbine engines.
Additively Manufactured Oscillating Heat Pipe for High Performance Cooling in High Temperature Applications
The advent of additive manufacturing makes available new and innovative integrated thermal management systems, including integrating an oscillating Heat Pipe (OHP) into the leading edge of a hypersonic vehicle for rapid dissipation of large quantities of heat. OHPs have interconnected capillary channels filled with a working fluid that forms a train of liquid plugs and vapor bubbles to facilitate rapid heat transfer. Multiple additive manufacturing techniques may be used, including powder bed fusion, binder jetting, metal material extrusion, directed energy deposit, sheet lamination, ultrasonic, and electrochemical techniques. These high performance OHPs can be made with materials such as Refractory High Entropy Alloys (RHEAs) that can withstand high temperature applications. The structure of the OHP can be integrated into the constructed leading edge. The benefits include a heat transport capacity of 10 to 100 times greater than before. Integrated OHPs avoid the bends or welds in traditional heat pipes, especially at the locations where the highest thermal stresses might cause thermal-structural failure of a leading edge. Alternating the diameters of the OHP channels alleviate start-up issues typically found in liquid metal oscillating heat pipe designs in high temperature applications by aiding in the instigation of a circulating flow due to multiple forces acting upon the working fluid.
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.
