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Shape Memory Alloy Heat Engine

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Design, model and demonstrate a shape memory alloy engine with efficiency approaching 50% of Carnot's limit that converts the waste thermal energy with low temperature gradients to power applications on an aviation platform. 

DESCRIPTION: Structural health monitoring sensors (acoustic, optical, piezoresistive, etc.) are currently either powered by batteries or directly through a central power supply. This creates a challenge in their long term deployment as batteries will have to be changed periodically and in mobility as wiring becomes cumbersome. In an ideal scenario, low power consuming sensors could be developed, for structural health monitoring and feedback control on aviation platforms, which can be directly powered through energy source available in the environment. One such energy source is low-temperature gradients. Although low-grade heat (hot-side temperature <50oC) is available in variety of places, current thermal energy harvesting technologies, primarily thermoelectric generators, have low efficiency at these low temperature gradients to be cost-effective. Shape memory alloy (SMA) engine provides transformative pathway to harness the abundant low temperature gradients, thereby, opening the possibility to implement self-powered sensors in variety of scenarios. This technology can provide options to increase the flight time and safety of missions in the expanded battlefield within contested air space in accordance with the Army Modernization plan for future vertical lift platforms. Thermoelectric generator (TEG) is the most widely used heat recovery system that competes with the SMA engine, since both convert thermal energy into electrical power. However, recent results indicate that SMA technology can efficiently work at very low thermal gradients (10K gradient at hot-side temperature of 323K) by chosing alloys with low phase transformation (PT) temperatures and a small hysteresis. At these low temperature gradients, TEGs are not efficient. A variety ofdesigns have been suggested in literature for SMA heat engines since the discovery of these alloys. However, most heat engine prototypes suffer from drawbacks including (i) necessity of large separation between the hot and cold sources, (ii) remarkable degradation in the efficiency as the heat engine runs for longer times (high fatigue), and (iii) low power conversion efficiency. All these past approaches are based on TiNi SMA wires. In order to overcome the known drawbacks, new prototype of SMA engines are required that utilizes the optimized material composition, and thermal transport through air and fluid. The prime goal of this program is to design, model and demonstrate SMA engine with high efficiency approaching 50% of Carnot’s limit. The new SMA engine design should provide large power density and efficiency at small temperature gradients, and demonstrate ultra-high life-time by using ternary or quaternary SMAs. The design focus should be on developing a small system that easily converts waste thermal energy of all forms (conduction, convection and radiation) into electrical energy. Thermal energy assessment on the aviation platforms (aircraft, helicopter, UAVs, docking, etc.) should be included in the analysis. 

PHASE I: Identify SMA material composition that demonstrates PT around room temperature and exhibits low hysteresis and high fatigue life.; (b) Design and model the SMA engine architecture that demonstrates power density greater than 50W/kg and efficiency approaching 50% of Carnot’s limit; (c) Develop system-level mathematical model that combines all the parameters representing materials, mechanical friction, frequency of operation, thermal transport, and mode of heat transfer; (d) Demonstrate feasibility of the newly designed SMA engine concept. Identify key requirements for validating the self-powered structural health monitoring sensor node using 10oC temperature gradients, potential challenges, accuracy, limitations and approach for Phase II demonstration. Reliability and durability analysis on SMA engine should be included in the report. Evaluation criterion for Phase I report will include feasibility of the SMA engine design to meet weight, size, efficiency, power density and reliability requirements. 

PHASE II: (i) Characterize the electrical, mechanical and thermal properties of the SMA composition over the wide range of temperatures. Demonstrate SMA composition with phase transformation occurring below 40oC while exhibiting achievable strain above 3.5%. (ii) Demonstrate SMA material manufacturing capability using thin film technology (magnetron sputtering, photolithography, wet etching, rapid thermal annealing, surface finishing) that can be scaled and deployed for manufacturing textured wires of lengths larger than 8 inches. (iii) Develop a fully functional and packaged SMA heat engine with power density greater than 65W/kg that can operate below hot-side temperature of 40oC and ambient cold-side around 25oC. (iv) Perform comparative analysis of the new SMA engine with respect to the traditional thermal generators in terms of efficiency, weight and size. (v) Demonstrate continuous powering of wireless structural health monitoring sensors through SMA engine. (vi) Perform reliability analysis through accelerated field tests under the realistic platform conditions. 

PHASE III: Demonstrate the gain achieved in terms of lower weight and size (at least 25%) through the use of new SMA composition and device architecture. Develop potential transition partners including Army, other DoD agencies, and U.S. industrial sector for transitioning developed power source. 

REFERENCES: 

1: A. D. Johnson - US Patent 4,055,955 (1977)

2:  J. J. Pachter - US Patent 4,150,544 (1979)

3:  D. J. Sandoval - US Patent 4,010,612 (1977)

4:  F. E. Wang - US Patent 4,275,561 (1981).

5:  D. Avirovik, A. Kumar, R. J. Bodnar and S. Priya, "Remote light energy harvesting and actuation using shape memory alloy-piezoelectric hybrid transducer," Smart Materials and Structures, 22, 052001 (2103).

6:  Y. Sato, N. Yoshida, Y. Tanabe, H. Fujita and N. Ooiwa, "Characteristics of a new power generation system with application of a Shape Memory Alloy Engine," Electrical Engineering in Japan, 165, 8-15 (2008).

7:  Chluba, C.

8:  Ge, W.

9:  Lima de Miranda, R.

10:  Strobel, J.

11:  Kienle, L.

12:  Quandt, E.

13:  Wuttig, M. "Shape memory alloys. Ultralow-fatigue shape memory alloy films," Science, 348, 1004-1007 (2015).

KEYWORDS: Thermal Power, Heat Engine, Wireless Sensors, Structural Health Monitoring 

CONTACT(S): 

MF Mohan Sanghadasa 

(256) 876-3342 

mfmohan.sanghadasa.civ@mail.mil 

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