Coffee Joulies Review

Coffee Joulies!

     My Coffee Joulies arrived a couple of days ago, and I think my hot beverage consumption has tripled while I've been playing with my new toy.  So, what are they?

     The promise behind the Joulies is that they'll bring your coffee down to a pleasant temperature for drinking more quickly than sitting around waiting for it to cool, and that they'll then hold it at a "nice to drink" temperature for longer.  Sounds good right?  Having your coffee (or tea) not too hot to start with, but staying warm for longer.

     So do they work?  The good news is yes, they do.  After making a hot drink, drop in all 5 Joulies and it'll cool down much quicker.  (I tend not to have milk in my hot drinks, so I'm used to having to wait a while for them to cool before being drinkable.)  They also do, as advertised, extend the time your drink will stay hot for.  Giving you a little longer to drink it.  Little is the key word here though - don't think that Joulies are going to be a replacement for your thermos.  I haven't got a pair of thermometers handy for doing any objective testing, but the effect is obvious enough subjectively.  It's just more subtle than I'd like.  That's when using them at a Joulie to coffee ratio that's around twice of that mentioned in the brochure, so it's not exactly an overwhelming effect.

     How do they work?  Well, the brochure and website talk a lot about Phase Change Materials.  Or in the simplest of terms, things that melt.  When a solid reaches its melting temperature, it absorbs an extra kick of heat energy to push it from solid to liquid.  This is the latent heat of the material, or the heat of fusion.  When it cools down enough to solidify again it will release that same amount of heat energy.  This is the reason why the Joulies are able to both cool down your drink, and then heat it later on.  While maybe not the best analogy when we're talking about a food product, liquid sweat evaporating off your skin as steam to cool you down is making use of a similar process.

     The Joulies themselves are shaped like large (About 5cm long) stainless steel coffee beans, filled with a "Plant-based Phase Change Material".  I'm guessing that the "Plant-based Phase Change Material" might be carnauba wax, used in everything from coating smarties to polishing your car.  Although it's got a melting temperature of ~80 Deg C, which is quite a bit higher than the 60 Deg C quoted as the phase change point of the material inside the Joulies.  If you want to cool your coffee to 60 Deg C and hold it there though, it makes sense to have a material which absorbs and releases heat above that temperature.  It could be beeswax though, that could be considered plant-based I suppose, and it has a melting point of around 60 Deg C.  Or it could be another plant based wax with a melting point closer to 60 Deg C.  I'm just speculating, and pointing out that the material inside the Joulies isn't especially rare.

     So the theory behind them is sound, why is the performance so ho-hum?  Well, honestly I'm not totally sure - the physics look fairly sound:

     I'm guessing that the phase change part of the Joulies can absorb around 150kJ/kg, and the 5 of mine weigh in at ~150g altogether.  That means that due to the phase change component alone they can store around ~22kJ of energy to release back to your coffee.  Your coffee cup probably holds ~250g of coffee.  It takes a little over 4kJ to heat a kilogram of water one degree, which means that it takes around 1kJ of energy to raise your entire coffee by 1 degree.  This is starting to sound pretty good right?  The Joulies should be able to raise the temperature of my coffee by 20 degrees?  Assuming that the coffee cools by around 10 degrees every five minutes, the Joulies should give me an extra 10 minutes of drinking time.  The effect to me seems, subjectively, around half of that at best.  You could probably chalk this up to inefficiencies in the heat transfer process - for example the coffee is shedding heat faster than the Joulies can replace it, meaning that the coffee drops to an unpleasant temperature before the Joulies have given up all their heat energy.

     Then there's the downsides of using the Joulies.  They take up a noticeable amount of room in your cup, meaning less beverage fits.  It's something else to wash.  There's the worry they'll slide forward and chip your teeth when trying to get the last gulp.  You'll look like a bit of a tosser if you use them outside of home.

     It's a shame that the upside of the Joulies is so small, because it's a product I really wanted to like.  I love the concept, but they need to fill them with something that has around 2-3 times the latent heat before I'd consider them as anything more than a novelty.  Mind you, given some of the coffee snobbery that goes around, the small effect they do have is probably going to be enough to sell a lot of them to a very select market.

     For me personally, even though I own some, I can't see them being used very often.

Joulies with Aussie 20c piece for comparison.