Experiments in creating a wearable temperature regulator

Motivation #

I worked in Shanghai for a few months in the summer of 2017. The weather there is tropical-ish, so the temperatures can quickly get to more than 35°C and it can be quite humid. At the time, I was doing an internship at the Solar Energy Institute in Jiao Tong University.

Some of the experiments my colleagues and I involved dangerous chemicals, which means they were performed under a fume hood. The issue was that the fume hood would also suck all the cold air produced by the air conditioning, and full body protective gear (with double or triple gloves) meant that it would get hot quickly.

After hearing my colleagues say "太热 !" many times, I thought this was a good opportunity to play with an old idea : what if, instead of cooling the whole room, we could cool only the people inside ? This would basically be a miniature wearable refrigerator. I had seen a similar project called Wristify, now developed by EMBR labs.

Sourcing the components #

There are two main technologies to cool objects : the vapour-compression cycle used in air conditioning units and fridges, and the thermoelectric effect, also called Peltier effect. The Peltier effect works by injecting a current through a special semiconductor junction to force a transfer of heat from one side to another. Peltier modules are small, simple and lightweight, making them ideal for space constrained applications like cooled cameras sensors, computers and so on. The major downside is the poor efficiency of the system (they are basically 10x less efficient than conventional vapour-compression cycle units).

The idea was simple : make a sort of wristwatch with a Peltier module attached. This would draw the heat from the wrist, which would cool the body thanks to circulation of blood. However, there is a catch : the efficiency of Peltier modules is very low, and, besides, the heat extracted from the body (to reduce its temperature) has to be evacuated somehow : For 1W of heat extracted from the body, the Peltier will heat up by another 1W, which means the system has to evacuate 2W.

The solution I chose was to connect the wristwatch to a small bag containing a battery and heat exchanger.

Wristify's approach is to use short thermal pulses, according to its patents. This means that the wristwatch dissipates the heat passively, drastically restricting its cooling power. This still makes for a very sleek product, though !

Sourcing the parts was extremely easy, being in China. For a European engineer, witnessing the sheer power of AliBaBa/TaoBao is simply shocking. I would order some exotic part in the evening and see it sitting on my desk at work in the morning. The amount of choice was astounding and I could even talk to factory representatives !

I bought some cheap PC water cooling equipment which included the necessary water pump, reservoir, tubing and heat exchanger. I also bought a few 40x40mm Peltier modules, a 12V battery, a PWM controller with a potentiometer, a switch, a wristband and some aluminium. I also bought a bathroom pouch, which looked sturdy but could let the air in, so that the heat exchanger would work properly.

Assembly and tests #

For assembling the parts, I had access to a very nice hackerspace in Shanghai (https://www.makertour.fr/workshops/xinchejian-hackerspace#presentation).

When the first parts arrived, I assembled them to check if everything would fit.

This dry run was useful : the bag I chose proved not to be sturdy enough. I quickly ordered another one, with rather original graphics on it.

I punched holes in the more rigid bag to add controls :

• The DPDT switch was used to reverse the flow of current, allowing to select between cooling and heating
• The potentiometer of the PWM controller to vary the amount of cooling/heating
• A general SPST switch to turn on and off the system

I ran a few tests and the system worked rather well. I did not run it for too long, being unsure about the safety of the whole "cooling the wrist will cool the body". The sensations were that of plunging my feet in a tub of ice cold water or on a hot water bottle. At full power (around 20W), the system felt surprisingly powerful.

Some closing thoughts #

The irony is that the system was only ready when summer was gone in Shanghai and the weather was back to nice breezes.

I had obviously plans for a much better version, with a wristband module a lot more integrated and a quick release mechanism, a custom bag, etc. This would make for a nice V2 when I decide to invest the time into it.

I also bought a copper braid made for jumping automotive batteries, to replace the current bracelet made of stainless steel (a poor conductor of heat). I did not test the new copper braid with the system, but it was surprisingly beautiful and I ended up making a fashionable belt out of it to offer to my sister-in-law...

I later learned that driving a Peltier with a PWM signal (with a high frequency) was very inefficient. Indeed, this application paper from TI shows that for their test setup, using a constant current instead of a PWM current allowed the cold side to be 8°C colder. If I were to make a serious version of this project, I would probably design a custom PCB to avoid this issue.

I was obviously very afraid of having the water leak and cause a short circuit with the battery. Given the large size of the battery, this would have been quite dangerous, even with the built in battery management system. I took special care in securing the tubes, and I ran my firsts tests with the battery disconnected (using a power adapter or a lab power supply instead). Finally before taking the plane back home, I removed all the water from the system, and disconnected everything.