Elastocaloric Apparatus

[Enthalpy]

Hello everyone and everybody!

The elastocaloric effect is one more fascinating behaviour of NiTi alloy and other materials including rubber. NiTi, whose crystal can change between martensitic and austenitic easily and often reversibly, is known as a shape memory alloy. But as the heat of formation of the crystals differ, the transition absorbs or releases heat, here when provoqued by mechanical stress.
Refrigeration and Niti on Wiki

Research tries to build fridges, air conditioners and heat pumps using this effect. NiTi operates around adequate temperatures, survives many cycles, offers some 30K swing, and promises efficiency at least when exploiting its full swing. It can become a big thing - or not.

The patents I've seen let a cam and a roller pull thin wires directly, example
data.epo.org
and I propose changes that are improvements hopefully.

Guiding a translation between a roller and an elastocaloric element isn't simple. Side forces can let rub and wear, but balls are more difficult to integrate. Translation guidance needs also to stop the roll movements.

I suggest a lever instead. It's guided just by an axle, where bearings integrate easily. A lever enables different amplitudes at the cam and the elastocaloric element. It can also have an angle to give more design freedom, for instance for a cam acting in the radial direction. As inspired by oil pumps, a rounded lever avoids side movements of the elastocaloric element is more caring with it.

Marc Schaefer, aka Enthalpy

Edited September 21, 2019 by Enthalpy

[Enthalpy]

The endurance of elastocaloric elements is promising and gives hope for fridges and others. But the figures I saw don't suffice to operate for 30 years, less so if selling 10⁷ units.

I don't know where nor how the elements break. Under tensile strain, common materials use to break at the ends, where fastenings concentrate the stress. Solutions are know which could apply here too, others would be specific to elastocaloric materials.

I suppose active wires can have thicker ends to reduce the stress there, with smooth transitions. Musical strings are ground on a local support, and so could the active elements be made thinner except at the ends. A special rolling mill too could reduce the section locally. A forge can supposedly start from a wire and make its ends thicker. Material can also be added at the ends, electrolytically or with molten metal - if any useful, because the active material deforms hugely, so the added material would experience abnormal strain where it's thin.

Easier than wires, active bands can keep wider ends when cut. Punching cuts almost arbitrary shapes, laser tools too and they can leave the material as new, water jets are an alternative, and chemical etching creates smooth rounded edges naturally. Bands can be stacked.

Parts exposed to cyclic stress last longer if ground finely and parallel to the stress direction, even polished. This should apply to elastocaloric elements too and combines well with local thinning.

Elastocaloric materials can expand much between both states. If one state can be stabilized locally, so the ends are stiff like a normal material, the strain will be small there and let live the fastenings for long. Maybe local heat treatment, deformation, alloying achieves that.

The elements may endure constant end stress better than strain. It's a matter of dispersion and of varying ambient temperature.

Marc Schaefer, aka Enthalpy

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Cyclic traction doesn't help endurance, as it widens existing micro-notches. Compression would be better for that but only thick short elements resist buckling, which demands big forces. Shear is an option, as torsion creates much stress from a small force.

A tube experiences a shear more constant than a rod and can lead a fluid. A helical spring is one good means to achieve a uniform torsion, it packs material in a small volume, and design parameters adjust the reasonable force widely. Helical springs serve for NiTi actuators, I hope they fit elastocaloric elements too.

Marc Schaefer, aka Enthalpy

Edited September 21, 2019 by Enthalpy