# Heat made cold: 2.2 K in a tiny prototype.

A device with two shape-memory alloy films uses a 130 °C external source to drive a cooling cycle. At device level, the reported temperature span is 2.2 K and absolute cooling power is 2.09 mW.

## Heat can drive the cycle

The hot source actuates one film, while the elastocaloric cycle of another film creates a temperature difference. The external result is 2.2 K at device level and 3.3 W/g.

## Two films, two jobs

One film is the actuator; the other is the refrigerant. The chain is heat → phase change → strain → phase change → temperature difference.

## Larger does not mean the same test

The Joule-heated configuration reports 12.9 K at material level and 4.0 K at device level, at 86 °C. Those values stay separate from the external-source result.

## 2.2 K is a span

The calculation is 1.2 K − (−1.0 K) = 2.2 K. A 2.2 K interval has the same size as 2.2 °C, but it is not an absolute temperature of 2.2 °C.

## The promise stops at milliwatts

The prototype has a small active mass, milliwatt-scale absolute power, and a need for scale-up. Over 2,000 cycles appear in a separate film test, not as product certification.

## Prototype, not appliance

We can say that the 130 °C source drove 2.2 K cooling at device level. We cannot say that any waste heat will cool a home.

## Sources and recheck

- [Nature Energy: Heat-driven elastocaloric cooling with shape memory films](https://www.nature.com/articles/s41560-026-02122-6)

Recheck at a scale demonstration, or sooner if the paper is corrected.