Breville HotCup: Thermodynamic Reflections

Friends, you may recall my long-standing fascination with boiling water efficiently: see for example:

• Kettle versus Quooker (2023)
• A Watched Pan (2022)
• Gas or Electric Kettle? (2012)

So ‘boiling water’ was a topic on which I thought I had written my last word. But visiting some sophisticated neighbours, I saw that they had a Breville HotCup – a kettle that held a reservoir of water, but which then dispensed just a single cup of boiled water at the push of a button. Wow!

Remember that in a conventional kettle one almost always boils too much water, thus wasting energy. And in a Quooker one keeps several litres of water at 100 °C so it is ready when you require hot water. Could the HotCup be the clever device that boils exactly the right amount of water just when you need it, without wasting energy on ‘standby’?

Well, after reflecting on the future rubbish I was creating, I bought one and tested it. It is a perfectly pleasant item, and does indeed dispense individual cups of water quickly – and since this is how I generally consume tea – I must confess to being pleased.

But after assessing its performance from an energy efficiency standpoint, I found myself disappointed. At best, it is ~ 80% efficient, but at its worst it is only 25% efficient! It took me some time to work out how it could be so bad, but I did eventually figure it out. Allow me to explain.

What is a Breville HotCup?

There a number of HotCup models, but the key idea behind them all is that it is a kettle which holds a reservoir of water, and then boils and dispenses just a single ‘cup-full’ – variable between 150 ml and 320 ml – at a time.

The video below shows the HotCup in operation along with the equipment I used to make measurements.

Measurements

I couldn’t see immediately how the device worked, but I set out to measure its performance using the standard techniques of ‘kitchen calorimetry’.

• I weighed the HotCup when empty and then filled it with about 1.5 lites of water. I then weighed the amount of water dispensed (g).
• I measured the temperature of the water reservoir, and the maximum temperature of the dispensed water (°C).
• I recorded the electricity used on a plug-in electricity meter (in kWh).
• I timed the boiling process using the timer on my phone (s).

From the mass of water dispensed and its rise in temperature, I could work out how much heat energy had been given to the water. I could then compare this with the measured amount of electrical energy consumed. Comparing these two figures I could work out the efficiency with which the consumed energy had been converted into hot water.

Remembering the Golden Rule of Experimental Physics, I repeated the experiment multiple times to assess more or less what was going on. Then when I had practiced a couple times, I made one set of readings with the HotCup set to dispense small cups of water (~150 ml) and one with it set to dispense large cups of water (~330 ml). For each setting I repeated the measurements until the reservoir appeared to be empty. The results are shown below.

Click on image for a larger version. Results for successive SMALL cups of water dispensed. Top Left: The temperature of the reservoir was observed to rise as cups of water were dispensed reaching nearly 80 °C. Top Right: The time taken to dispense a cup of water decreased from about 50 s to about 20 s. Bottom Left: The maximum temperature recorded in the cup into which the water was dispensed. Bottom Right: The estimated efficiency of water heating. The average efficiency is only 25%.

Click on image for a larger version. Results for successive LARGE cups of water dispensed. Top Left: The temperature of the reservoir was observed to rise as cups of water were dispensed. Top Right: The time taken to dispense a cup of water was about 40 s. Bottom Left: The maximum temperature recorded in the cup into which the water was dispensed. Bottom Right: The estimated efficiency of water heating. The average efficiency is about 80%.

Conclusions

Having observed the device in operation and measured its performance, I think I can now see how it works.

I think that the HotCup always boils the same amount of water in a boiling chamber – a mini kettle-within-the-kettle. The device uses the pressure built up within the chamber to push out the boiled fluid, and then discharges the unused hot liquid back into the reservoir.

By analysing the inefficiency of the device as a function of the amount of water dispensed, I estimated the volume of the boiling chamber to be approximately 400 ml.

Click on image for a larger version. Plotting the inefficiency as a function of dispensed volume, I estimate that the ‘boiling chamber’ within the HotCup has a volume of approximately 400 ml.

The TOP LEFT graphs in the two panels above show that the reservoir temperature rises after each cupful has been dispensed. It is clear that this rise is larger for the small cup (150 ml) dispensation in which most of the hot water (400 ml – 150 ml = 250 ml) is put into the reservoir. I made a model of this – shown as a dotted red line – and this seems to roughly describe the data.

Thermodynamic Reflections

Friends, I have had this device in my house now for a couple of months, and since my wife and I generally boil the kettle for individual cups of tea, it is quite convenient.

But as a calorimetric thermodynamicist, I must confess that after making these measurements I was at first disappointed. But on reflection, the performance is actually not so bad.

The inefficiency is the exactly the same as if one used 400 ml of water in a kettle to prepare a single beverage. This volume is close to or below the minimum fill level for many kettles. And so although these results look bad, they are probably no worse than using a kettle.

However, if my wife and I both wanted to drink tea at the same time, or if I wanted to boil larger volumes of water for cooking, a conventional kettle would be more efficient.