DESCRIPTION: The required high electrical power efficiency for the closed cycle cryocoolers needed to deploy 4 degree kelvin (K) superconducting  radiofrequency (RF) receivers, 3K optical absorption energy sensors, and future <1K quantum computers on power sensitive platforms depends on saturating the heat capacity of the exhaust gas with energy from the warmer just compressed gas. Thus the heat exchangers (called recuperators and regenerators for DC and AC flow systems respectively) must efficiently conductively transfer heat perpendicular to the gas flow, while simultaneously not conducting it along the gas flow. Materials with highly anisotropic and large thermal conduction are needed to minimize weight/volume. Affordability requires development of simple, safe manufacturing techniques with etched or machined, hermetically sealed gas flow channels.

The current state of the art in recuperators consists of stacks of silicon wafers corrosively etched into the flow channels and maximal heat exchange surface area. Silicon is isotropic thermally, thermal insulating layers must be inserted between each pair of wafers, and the wafers must be relatively thick and heavy in order for etch processing and assembly breakage to be acceptable. Thus Si recuperators tend to be heavy and large to achieve adequate thermal performance and consequentially expensive to launch.

Regenerators generally consist of layers of porous materials, often stacked grids or packed powders, which may be (effectively) sintered to ensure they will not settle under the impact of vibration (launch!) and gravity. Flow channel optimization is currently quite immaturely understood or not independently controlled.

Proposers are expected to focus on solutions appropriate to the temperature range well below 20K where current cooler technology is weakest and proposals extendable to below 1K will be rated especially favorably. Solutions that allow a common manufacturing technique to be applied in a sequence of different temperature range layers will be preferred over techniques requiring different manufacturing/assembly techniques for different thermal zones. Only proposals appropriate for fully closed cycle coolers will be viewed as responsive.
 

PHASE I: The original proposals should define a specific set of materials and construction method(s) to be explored during the phase 1 base award. Determine feasibility for the development of low SWaP-C Cryogenic Heat Exchangers based on Highly Anisotropic Materials.  Demonstrate feasibility through experimental tests to define critical parameters not already available in the research literature, not just numerical simulations.  By the end of the Phase I effort, predictions of the ultimate performance of the heat exchanger should be possible Phase II awards will be determined on the technical approach and performance capabilities of the heat exchanger as demonstrated in the Phase I feasibility tests documented in the final report. The initial Phase II proposal should also discuss the remaining technical risk items and include a plan to reduce them in Phase II. The Phase I option period should further refine any remaining materials selection issues, e.g. by addressing desirable chemical alterations for adjacent thermal zones, or experimentation with alternative sample production techniques.
 

PHASE II: Phase II should fully develop and demonstrate an improved performance heat exchanger prototype. For a recuperator, the metric might quantify how perfectly the input and exhaust gas matches in temperature at each point along the flow axis, times the weight * volume product for a 20 to 4K gradient to be spanned. During the Phase II base period, subassemblies and fabrication techniques should be separately demonstrated and iterated for performance. Any necessary materials growth/fabrication issues should be resolved adequately to allow the effort to proceed. Phase II should culminate in the construction and test of a complete heat exchanger sufficient to allow potential users to evaluate the utility of the technology. Note that the production of vibration by the coolant flow must be minimized.
 

PHASE III DUAL USE APPLICATIONS: During Phase III, the prototype heat exchanger developed during the Phase II effort would be reiterated and integrated into a full 4K cooler (or colder) cryo-cooler demonstration unit that will be evaluated independently as a cooler in a government lab and then  incorporated in a trial 4K sensor system unit. Collaboration of the heat exchanger vendor with a cooler company is highly desirable by this stage since to be commercially successful the heat exchanger must be merged into complete coolers and those merged into systems with sensors/processors requiring cryogenic cooling. This may be ESM systems on military platforms, earth observing space satellites like WindSat, medical instrumentation, or earth based quantum computers used in data centers. The specific product developed here will have utility primarily as an enabler of the utility of the materials science/physics driven phenomena observable only at/ below 4K. This includes many known quantum phenomena and most of the methods of quantum computing, long held as the ultimate replacement for Si computers now that Moore's law is ending. 4K conventional computing is also being pursued by the US government for use as major servers due to their projected superior energy efficiency. Given the improved low noise floor that cryogenics provides naturally, improved medical sensors, such as more accurate brain mapping systems, are also likely. However, the methods of using layered materials to produce heat exchangers developed here will have general utility at higher temperatures where the need for refrigeration is large on commercial scales, from flash freezers and LN2 temperature baths for tire recycling to above room temperature cooling of telecom power amplifiers, motors and turbines.
 

REFERENCES:

1. Advances in Cryogenic Engineering, Volume 21, K. Timmerhaus, Springer Verlag, 1975 and Volume 43, edited by Peter Kittel, 2013.
 

2. Advanced Low Temperature Thermoelectric Materials for Cryogenic Power Generation Project, NASA, 2015, retrieved from https://data.nasa.gov/dataset/Advanced-Low-Temperature-Thermoelectric-Materials-/iqd9-8a9w/about
 

3. A High-Performance Thermoelectric Material for Low-Temperature Applications; retrieved from http://chemgroups.northwestern.edu/kanatzidis/Reprints/CsBi4Te6_science.pdf
 

4. An Ultra-Compact Laminar-Flow Cryogenic Heat Exchanger, retrieved from dotynmr.com/download/pubs/1992_ACE_Doty_UCLFHE.pdf
 

5. Cryoginics ’98 IIR International Conference Prague, Czech Republic, May 12 – 15, 1998, retrieved from www.isibrno.cz/cryogenics98/
 

6. Thermal analysis of cyclic cryogenic regenerators, International Journal of Heat and Mass Transfer, Vol 17, p 37-49 (2974).
 

7. Regenerator Materials – regenerator plot chart, retrieved from cryogenics.nist.gov/MPropsMAY/RegeneratorMaterials/RegenPlot.htm
 

8. Cryocooler, retrieved from https://en.wikipedia.org/wiki/Cryocooler