As hydrofluorocarbon-134 is being phased out due to its adverse impacts on the global climate, the refrigerant is becoming difficult to acquire and the Department of Defense (DoD) has a need to transition to more widely available refrigerants with lower global warming potential (GWP). The leading alternative for vehicle air conditioning (A/C), hydrofluoroolefin-1234yf, is deemed safe for commercial automotive use despite being mildly flammable. This refrigerant is, however, unacceptable for military use due to the risk of sustained burning. While development of other non-flammable refrigerants and blends may provide a short-term path to solve DoD’s refrigerant problem, a long-term solution is transitioning away from vapor-compression (VC) to solid-state, non-VC technologies such as elastocaloric cooling using Nitinol (nickel-titanium alloys with shape memory or super elastic behavior). The latter approach is potentially more efficient, more robust, and has zero GWP, thus providing a solution that will reduce the energy use and GWP of military cooling.

The goal of this project is to develop and demonstrate deployable, compact, high-power-density elastocaloric A/C for DoD cooling applications capable of replacing VC systems and addressing concerns associated with high GWP refrigerants currently in use. The work will focus on achieving three specific objectives that will advance the technology for DoD applications. Those are:

Objective 1. Optimize Nitinol alloys for military A/C applications to reduce stress/strain hysteresis and fatigue and maximize caloric performance.

Objective 2. Optimize and demonstrate active regenerators to meet military vehicles' temperature span and lifetime requirements for A/C.

Objective 3. Demonstrate feasibility of a full cooling system that meets or exceeds current VC system performance for efficiency and size of A/C in military vehicles.

Ames Laboratory of the U.S. Department of Energy is teaming up with ATI Specialty Alloys and Components and G.RAU Inc. to demonstrate next generation regenerative elastocaloric heat pump technologies for DoD cooling applications. When converted to practice, elastocaloric cooling has the potential to improve cooling efficiency by ~30% over VC because elastocaloric effects can be generated with very high efficiencies. Simultaneously, elastocaloric cooling completely eliminates risks of refrigerant flammability and leakage and improves system robustness by using a solid refrigerant and water-based cooling fluids without any high-pressure lines. The project team will use modeling and laboratory experiments to address the main research and development needs for elastocaloric cooling, that is, reducing fatigue and enhancing longevity of Nitinol-based solid refrigerants, boosting both volumetric and gravimetric cooling power densities of elastocaloric systems, and adapting overall system design for military A/C applications.

Deployment of solid-state, zero-GWP alternatives to VC will i) provide cooling performance and efficiency better than VC, ii) eliminate greenhouse gas emissions from military cooling systems, and iii) eradicate the risks associated with conventional refrigerants such as flammability, toxicity, and potential health hazards. This project, to optimize Nitinol-based materials, design and demonstrate active elastocaloric regenerators, and develop elastocaloric systems for military vehicle A/C applications, will move the technology well beyond the current state of the art. The proof-of-concept system developed during this project will provide the basis for transitioning military cooling away from VC, with its environmental impact and limited refrigerant availability, to efficient, robust, and environmentally-benign elastocaloric cooling technology.