Technical investigation

Illustrating two causes of inefficiency in electric kettles sold in the 2020s

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Figure 1. Water still boiling in a 2021 electric kettle, five seconds or more after the concealed heating element has been switched off. Under the control of the kettle's automatic cut-out, this water has now been continuously boiling for about 23 seconds.

This article shows with photographs two causes of inefficiency in an electric kettle bought in 2021. These are:

  1. An automatic cut-out that continues to push heat into the water after it has started boiling, so that heat is lost vaporising water.

  2. A significant heat reservoir within the kettle. The heat in this reservoir is left behind and wasted when the water is poured out. It may also be lost vaporising water after the power has been turned off, as shown in Figure 1.

It then reports a simple experiment to quantify the heat lost in the kettle's heat reservoir.

Although the kettle shown here is typical of many kettles, not all kettles are the same. Some of them have adjustable target temperatures for the automatic cut-out and do not have to boil the water. Some of them have the heating element in the water rather than concealed in the base.

The kettle shown here uses a bimetallic strip and relies on steam passing down a plastic duct on the back of the kettle to heat the strip. Its heating element is concealed in its base, where there is evidently a significant heat reservoir.

The photographs in Figures 2-9 are a sequence showing what happens when this kettle is left to switch itself off automatically. The stopwatch next to the kettle is intended to show the relative time of each photograph. (It was not started exactly when the heating started). The glass walls of the kettle show the water boiling. The ring of blue LED light around the base of this kettle goes on and off with the heating element, so each photograph also shows whether the heating element is on or off.

They show that this kettle kept the power on (about 2.1 kW as measured by a plug-in power meter) for more than 15 seconds with the water already boiling and that even after the automatic cut-out the water continued to boil from the kettle's stored heat.

Quantifying the waste heat left in the kettle

After the photographs here I made an experiment to estimate the heat left behind in this kettle.

The kettle was heated to near boiling, turned off, and emptied immediately. Then a known quantity of cold water at a known temperature was poured in as quickly as possible, in order that it would absorb the heat left in the kettle's hidden heat reservoir. This new water was left in the kettle for 90 seconds to absorb heat and then its temperature measured again.

In more detail, the temperature of this water was measured, both before and after it was in the kettle, by putting it into a vacuum flask and shaking the flask to ensure an even temperature and then making the measurement. Room temperature was about 19 °C, slightly below the initial temperature of the new water, meaning that no heat could come into the water from the environment.

Results

Initial temperature of cold water in vacuum flask = 21.2 °C

Temperature of water back in vacuum flask after 90 seconds in the just-used kettle = 32.9 °C

Mass of water determined by weighing = 630g

From these figures it is possible to calculate the waste heat energy that has been absorbed by the new water assuming a specific heat of water of 4,180 J/(K kg) as follows:

Waste heat = (32.9 - 21.2) x 0.630 x 4180 = ~31 kJ

To put this in perspective, the theoretical amount of energy that would be required to heat the kettle's minimum fill of water (600g) from room temperature (say 20 °C) to 100 °C would be:

Heat = (100 - 20) x 0.6 x 4180 = ~200.6 kJ

So ignoring other losses, to boil water on its minimum fill this kettle would waste in heating itself energy equal to at least 31/200.6 or ~15% of the heat that actually went into the water.

This is must be less than the true figure as the heat left in the kettle must be more than the amount actually absorbed by the new water, for several reasons.

  1. By the laws of thermodynamics, it would never be possible to extract all the heat left behind in the kettle. The best you can hope for is that the temperature of the water becomes equal to the temperature of the kettle's heat reservoir. If the thermal capacity of the heat reservoir is small relative to the thermal capacity of the water, that would mean that most of the heat is then in the water. Further investigation is needed to measure the thermal capacity of this hidden reservoir, which this simple experiment does not attempt.

  2. Heat is constantly lost to the environment while the new cold water is absorbing heat from the kettle.

  3. The choice of 90 seconds to absorb the heat was arbitrary and may not have been long enough to absorb as much as possible. Further work would be required to find the optimum time, given the previous point.

Figure 2. The kettle just before the water started to boil. The water surface is flat, although many small bubbles are rising from the base of the kettle. In this model, the ring of blue light visible round the base here indicates that the heating element is on. The stopwatch shows 2 minutes 1 second. This stopwatch was started at an arbitrary time and its purpose is to show the relative times of different photographs. It does not represent the total time since the start of heating.
Figure 3. The water is just starting to boil, with the stopwatch showing 2 minutes 5 seconds. This point will be used as a reference in the following descriptions.
Figure 4. With the stopwatch showing 2 minutes 9 seconds, around 4 seconds after it started to boil, the water is now boiling vigorously with large waves on the surface.
Figure 5. With the stopwatch showing 2 minutes 20 seconds - 15 seconds after the water started to boil - the kettle is still steaming ahead on full power (so to speak).
Figure 6. With the stopwatch now showing 2 minutes 23 seconds - about 18 seconds after the water started to boil - this photograph shows that the blue ring of light around the base of the kettle has finally disappeared, meaning its automatic cut-out has switched off its heating element. This story of waste heat is not over yet though, as the water continues to boil.
Figure 7. With the stopwatch now showing 2 minutes 32 seconds - about 18 seconds after the water started to boil and 9 seconds after the first photograph with power to the heating element cut off - the kettle is still boiling the water. (Figure 1 is a photograph 4 seconds before this.)
Figure 8. With the stopwatch now showing 2 minutes 41 seconds - 18 seconds after the first photograph with the heating element off - there are still steam bubbles rising from part of the base, meaning that kettle is still emitting heat into the water from its hidden heat reservoir, at a temperature above boiling point,

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