There was a pretty successful Kickstarter a while back for a product ("Coffee Joulies") that was exactly that, but for your coffee mug. Little bean-shapes made of stainless steel, filled with material that changed phase at 140F.

There's also Ember [1] (also a kickstarter I think) that just made a mug with a heating element and controller to keep your beverage at a specific temperature. Using materials with specific phase change temperatures sounds like an innovative solution, but it's also grossly engineering a problem that can be solved better by $10 worth of components.

[1] https://ember.com/products/ceramic-mug

Is having a processor and electronics and software and sensors, which, despite the cost of components of $10, requires electricity. Is that really that much better than just building a better thermos?

( These things are amazing; if you haven't experienced one yet I recommend picking one up from REI: https://www.yeti.com/drinkware/rambler-20-oz-tumbler/YRAM20.... )

A battery powered mug with an app isn't really any simpler of a solution than fancy ice cubes.

Vacuum mugs already advertise hours of heat retention. Most hot drinks aren't going to taste better after sitting for hours and hours anyway.

I don't know, personally I would absolutely hate having to remember to keep my mug charged - although it's a damn cool product, just not for me.

I'm not sure I can agree about a phase-change solution being overengineered, however; while the materials would be more complex, the design itself should be far simpler than an electronic mug, and exactly the same as a vacuum sealed one.

I'd also be really worried about toxicity and materials that changed phase at that temp... Or well any temp. Can it leech through the walls? I wouldn't be Keen on dropping molten steel in my coffee either.

It's isolated components of vegetable oil:

http://discovermagazine.com/2013/september/21-running-hot-an...

Not really terrifying.

Seems clear that the stainless steel is to keep the material out of your liquids. It is not molten! Keep in mind you regularly put stainless steel in your mouth and lick it.

I’d say this is a reasonable use.

It was pretty successful, but the product did not work: https://marco.org/2011/08/10/coffee-joulies-review

And also the Temperfect coffee mug (https://joeveo.com/), which builds the material into the coffee mug lining.

On-demand water heaters are really only feasible if you have natural gas or propane heat in your house. The electric ones are quite feeble and expensive to operate. This is a good idea. My intuition though is that the additional up-front costs of this type of setup would be enough of a barrier that adoption would be slow.

My brother uses an on-demand electric water heater he built himself (he is an ee) and it works just fine. Also, it's less expensive to operate than a tank-type heater, as there is no cooloff loss.

I tend to think cool off loss is much less of a problem than people think. The insulation around those heaters is incredible.

I once turned off my water heater before leaving on vacation, and when I got back after a week the water in the tank was still probably 10 to 15 degrees F warmer than room temperature.

Also in a lot of cases (cool climate, electric resistance heat, water heater inside conditioned space) that heat isn't really lost, it's just offsetting heat that would have come from something like an electric baseboard heater.

I had a bad water heater in my apartment that kept turning off. It would take me 2 or 3 days to notice that there wasn't any hot water because I could usually get a few hot showers out of it before I noticed a problem.

I have an on demand 10kW water heater, it's my shower.

There are actually companies manufacturing these things but mainly to increase heat-storage capacity of water tanks for solar heating elements. They use thiosulfates (the stuff in hand warmers) or paraffin wax.