N221-083 TITLE: Variable Conductance Thermal Management Technology

OUSD (R&E) MODERNIZATION PRIORITY: Quantum Science

TECHNOLOGY AREA(S): Sensors

The technology within this topic is restricted under the International Traffic in Arms Regulation (ITAR), 22 CFR Parts 120-130, which controls the export and import of defense-related material and services, including export of sensitive technical data, or the Export Administration Regulation (EAR), 15 CFR Parts 730-774, which controls dual use items. Offerors must disclose any proposed use of foreign nationals (FNs), their country(ies) of origin, the type of visa or work permit possessed, and the statement of work (SOW) tasks intended for accomplishment by the FN(s) in accordance with the Announcement. Offerors are advised foreign nationals proposed to perform on this topic may be restricted due to the technical data under US Export Control Laws.

OBJECTIVE: Develop a technology that dynamically adjusts the thermal conductivity between a sensor and its environment to assist in maintaining a stable temperature with minimal power draw. The technology should be compact, robust, and easily adaptable to a variety of sensor shapes, sizes, and internal heat loads.

DESCRIPTION: For a wide variety of sensing systems, a stable temperature is crucial to minimize systematic errors in the measurement output; however, a fieldable sensor is often expected to operate through extreme environmental temperature swings that often exceed 100ºC. To combat this temperature swing, a sensor is stabilized by a thermal management solution. Reference 1 contains a summary of a variety of such solutions. Of these, the most common options for the high-precision inertial sensors of primary interest are thermoelectric coolers or simple heaters to actively control the temperature. But while the efficiency of these options are good when the sensor temperature setpoint and the ambient temperature are similar (deltaT ˜0ºC), the power draw becomes large when those two temperatures differ significantly (deltaT»1ºC).

Variable conductance heat pipes (VCHP) suggest an interesting possibility. The thermal conductance of these devices passively changes depending on the temperature of the environment. With a careful selection of materials and dimensions, the VCHP can provide an extremely small thermal conductance when the environment is much colder than the sensor, and a high conductance when the environment and sensor are similar in temperature [Ref. 2]. As a direct consequence of this variable conductance, the demand on the active portion of the thermal stabilization is reduced, resulting in a much lower overall power draw. The promise of this variable conductance has been successfully demonstrated in designs intended for lunar landers and rovers immersed in the large lunar daytime/nighttime temperature swings [Ref. 2] And yet VCHPs are not a universal solution. In particular, they are rigid devices that require highly customized designs depending on the sensor size and shape.

The proposed variable conductance solution (VCS) will comprise an alternative material or technology that has the benefits of a variable conductance, but is more easily adaptable to unusual shapes, including a combination of flat and curved surfaces. Table 1 outlines three model environments and a model sensor. The VCS will act as the interface between the sensor and environment, limiting the temperature swing of the sensor to the specified range even as the environment’s temperature varies much more widely.

Table 1. Proposed Variable Conductance Solution (VCS) Scenarios

	Scenario 1	Scenario 2	Scenario 3
Environment temperature range	-10ºC to 40ºC	20ºC – 85ºC	-40ºC to 85ºC
Nominal sensor temperature setpoint	55ºC	50ºC	25ºC
Sensor temperature deviation from nominal setpoint over environmental temperature range after application of the VCS*	± 5 ºC
Sensor steady state heat load	Threshold: 1W – 10W Objective: 1W – 50W
Sensor Shape	Must include at least one curved and one flat surface (specifically, the sensor shape could be a cylinder or hemisphere).
Sensor Volume	Threshold: < 25L Objective: < 2.5L
Additional volume allotted to the VCS	< 10% sensor volume
VCS power draw	< 1W

*In a fieldable device, a secondary active temperature stabilization system will provide the final stabilization < 1ºC.

In these configurations, a range of model sensor heat loads and volumes are provided. A single instance of the VCS does not need to accommodate all of these loads and sizes simultaneously. Instead, the system can be designed for a single heat load and a single shape; however, proposed solutions must be sufficiently flexible in design and fabrication to be easily adaptable to other shapes and heat loads.

PHASE I: Perform a design and materials study to assess the feasibility of the selected technology and its ability to meet the goals of one of the scenarios in Table 1. For the chosen scenario, the study will include:

• An estimate and justification of the dependence of the thermal conductivity on the environmental temperature.
• An estimate and justification of the range of sensor heat loads the system can accommodate while still meeting temperature stability specification.
• An evaluation of the technology’s SWaP that would be built for Phase II.
• A discussion of the fabrication process including an assessment of risks and risk mitigation strategies.
• A discussion of the technology’s compatibility with the other two scenarios not selected.
• A discussion of the technology’s rate of adjustment
• A discussion of the technology’s radius of curvature limitations.
• A discussion of the technology’s compatibility with shock/vibration mounts.
• A discussion of the technology’s sensitivity with respect to orientation to gravity.
• A discussion of the technology’s ability to be adapted to different sensor shapes, including smaller sensor sizes, concave curves, and tight radii of curvature.

The Phase I Option, if exercised, will include the initial design specifications and capabilities description to build prototype solutions in Phase II, as well as a proof-of-principle demonstration of thermal conductivity variation of the proposed technology.

 

PHASE II: Fabricate and test three (3) prototypes of the design developed in Phase I, ? one for each sensor shape. A mock sensor can be constructed from a simple shell with an internal heat load and sufficient thermal sensors to capture potential thermal gradients. The completed prototypes will undergo testing at a minimum of five temperatures that completely span the specified environmental temperature range. the technology’s thermal conductivity and power draw will be reported. At each temperature, the mock sensor’s steady state temperature and settling time constant will be reported for sensor heat loads that span the range specified. The final report will include a discussion of potential near-term and long-term development efforts that would improve the technology’s performance, SWAP, and/or ease of fabrication. It will also include an evaluation of the cost of fabrication and how that might be reduced in the future. The prototypes should be delivered by the end of Phase II.

PHASE III DUAL USE APPLICATIONS: Continue development to assist the Government in applying the VCS to a fieldable, thermally stabilized inertial sensor. As thermal management is also an important consideration for high-precision sensors outside of military applications, development should continue to assist interested commercial parties. Geological resource exploration and monitoring require ruggedized sensors that would benefit from this technology. It could also be applied in medical systems as those often have stringent thermal and power draw requirements. More generally, thermal management is an important consideration in areas as wide ranging as solar cells, rechargeable batteries for electric vehicles, and data centers.

REFERENCES:

1. Lupu, A.G., et al. "A review of solar photovoltaic systems cooling technologies." IOP Conference Series: Materials Science and Engineering. Vol. 444. No. 8. IOP Publishing, 2018. https://iopscience.iop.org/article/10.1088/1757-899X/444/8/082016/pdf.
2. Anderson, William, et al. "Variable conductance heat pipes for variable thermal links." 42nd International Conference on Environmental Systems. 2012. https://arc.aiaa.org/doi/pdf/10.2514/6.2012-3541.

KEYWORDS: Thermal management; power draw; environmental temperature; sensors; inertial sensors; thermal conductance; materials