Innovation and design

Dissolved copper levels in a copper solar kettle

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Figure 1. Testing dissolved copper in water from a home-made solar kettle. The test strip on the left is a control with tap water that had not been in the kettle and the strip on the right is with tap water that had been in the kettle for about 90 minutes without heating.
Figure 2. An endoscope looking down the copper spout of the solar kettle in February 2024. The circular opening in the top of the photograph is the open end of the horizontal copper pipe of the kettle.
Figure 3. An endoscope view inside the copper pipe in March 2024. The nature of the apparent orange ridge running parallel to the axis of the pipe is unknown.

In May 2024 test sticks showed dissolved copper of around 0.2mg/L in tap water that had been in the copper pipe of the home-made solar kettle constructed in 2022, which was described in the earlier article A home-made solar kettle, its performance and its problems.

The tap water stood in the kettle without heating for around 90 minutes before being removed for testing.

As an experimental control, tap water that had not been in the kettle was also tested. The same glass jug of tap water was divided between the kettle and the control. The control showed less than 0.05mg/L, which is the lowest value above zero on the colour chart. The tap water was from a house with copper pipes. Without a blank of distilled water for comparison it is not possible to say whether there was any detectable difference from zero for the control.

Figure 1 shows the colours of both test sticks, which had been dipped into their respective samples simultaneously.

Similar results came from water heated in the kettle. On 2 June 2024 the experiment was repeated in the same way except that the sun was used to heat the water in the kettle to around 90°C, taking around 11 minutes. The water was then left in the kettle cooling for about 5 minutes.

To equalise the temperatures after this, the tap water control and the test sample were left overnight next to each other in separate glass bottles. Again the control showed less than 0.05mg/L and the water from the kettle around 0.2mg/L.

These test strips were “SenSafe John's Copper Check 0-2ppm” bought for £16-19 on the web from itseurope.co.uk, ITS Europe, The UK Centre for Homeland Security, Building 7, Chilmark, Salisbury, SP3 5DU. They were manufactured in the USA by Industrial Test Systems Inc.

The copper pipe had been used on the kettle since its construction in 2022. By June 2023 it had performed dozens, possibly hundreds, of water heatings to around 90°C and several times had accidentally been left in focus without water for short periods, almost certainly exceeding 100°C on those occasions. From June 2023 until these tests it had not performed many heatings (because of a lack of sunshine) and over the following winter it was stored indoors.

Only (very soft) tap water had gone into this kettle. However it was situated near the coast.

Figure 4. Another view inside the pipe in March 2024. In a few places like this one, the light-coloured deposits on the walls have a greenish tinge, possibly indicating the presence of copper salts.
Figure 5. Another view inside the pipe in March 2024. In some places, light-coloured lines like these are straight and run parallel to the axis of the pipe.
Figure 6. The far, closed end of the pipe as seen with an endoscope. The visible detritus may be shavings from the endoscope cable which were made where it was forced in a tight curve into the pipe entrance from the spout.

Figures 3 - 6 show the inside of the pipe in March 2024. These photographs were taken with an endoscope, which showed diffuse white coloured deposits (Figure 2) and in some places linear light-coloured streaks (Figure 5). In some places these line-like streaks ran parallel to the axis of the pipe.

The future design of a similar kettle might consider an olive ring compression joint to attach the end-stop rather than soldering. This end would then be removable for inspection. It is unknown whether periodic descaling might be necessary if hard tap water were used. If so a removable end might also help with this.

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