Trying silicone tubing as an insulator for a home-made solar kettle

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Fig 1. A translucent silicone tube on the 22mm copper pipe of the solar kettle. This view of the closed end of the pipe also shows its solder ring end stop and the supporting stainless steel threaded rod. The silicone is intended to reduce heat loss but it also reduces the optical efficiency. This silicone tube has been on the kettle heating water to near boiling point multiple times, with no sign of deterioration.
Fig 2. Plots of temperature against time in water heated in the solar kettle in the south of England on 1 December 2022, a day with little wind and ambient temperature around 11°C. The heating was done alternately with and without the silicone sleeve shown in Figure 1. Each curve has been positioned on the x axis so that it passes through the the point (100 s, 35 °C). For the curve 6 this was done by extrapolation, for the others by interpolation. GMT time for each curve and the approximate elevation of the sun is shown. Temperature was measured by a KT-type thermocouple in the heated water.

Here are some experimental results from testing the translucent silicone tube shown in Figure 1 as an insulator on the heat collector of a home-made parabolic trough solar kettle.

This kettle was described in an earlier article. It heats water in the 22mm horizontal copper pipe shown in Figure 1, which lies along the focal line of the trough. The parabolic trough is 1m long and 80cm across from rim to rim.

The silicone tube rests on the copper pipe itself without any support, meaning that it is in contact with the pipe. During heating, it glows bright white like a fluorescent light tube. It feels warm but not extremely hot.

The tube is slightly larger diameter than the copper pipe and has to be stretched to get it over the 22mm solder-ring end cap.

Difficulties in evaluating its performance include random variations in sun strength, caused by clouds and haze, and changes the amount of wind. As an attempt at an experimental control for this, the kettle's performance was measured alternately with and without the silicone tube over several hours on the same day. (A better control would be a second kettle heating at the same time and place).

Thursday 1 December 2022 was a day with most of the sky visibly clear, little wind and an ambient temperature measured around 11°C. Water was heated in the kettle while a temperature was recorded by hand using a KT-type thermocouple thermometer with resolution of 0.1°C. The bare thermocouple, immersed in the water being heated, was pushed approximately halfway down the length of the collector pipe. Care was taken not to disturb it during heating, since experience shows that the temperature along the pipe is not uniform. Between heatings the probe was moved when refilling the kettle.

The resulting measurements are shown in Figure 2. With the exception of the final curve 8, the copper pipe when bare heated the water faster than it did when inside the silicone tube. The final curve 8 was started at approximately 14:19 GMT when the elevation of the sun was already less than 13° and falling more rapidly than earlier.

Conclusions

The silicone tube has undergone tens of heat cycles of the kettle with no visible deterioration. (To qualify this, the kettle has always had water in the pipe and not been allowed to boil dry or left to stagnate.)

In conditions of low wind, the silicone tube reduces the efficiency of the kettle. Further work is needed to determine whether it could help in windy conditions, when heat loss from the bare tube would be higher.

Supplementary data

The raw data for each of the eight curves shown in Figure 2 is available. It consists of the following files with the following SHA256 hashes at the following locations on this website.

2b64c147876ba1614ab96ee30e4a27ae30e64f25b301ed6ace5324d31174e4c1  /download/h1.1dec2022.csv
86bf8b3cdcbc2cc36ca758213f5e7b966197a8ec01e2c2e487091396ec5b1559  /download/h2.1dec2022.csv
c5eb9f8cbcbbc29e761e2db61b371276e49bffbf028ac7d2d45b8c327584a59e  /download/h3.1dec2022.csv
1e1a44636fb2797962ec9e1dc014d4c9b7cb2d10a73b32c6969f5fc46f0635f6  /download/h4.1dec2022.csv
672781531b962c207e9806647b585b1306ea8ed87bc9b9b6c5f7a4680f16b6d2  /download/h5.1dec2022.csv
0bdd14b695bd86fd14018b1fd4b82529e2c1ace451d36c9675ec0a36c92dd894  /download/h6.1dec2022.csv
0791d6925ef3d57e61681353ed9cd3b1f3ba669f55961119360a74f4581b95ee  /download/h7.1dec2022.csv
2ebfb1d56912b6153eea2c6497a24fbe2dec3b906ac1e64d24668967be43d1f1  /download/h8.1dec2022.csv

These files are in CSV (comma-separated variable) format, meaning that each data point is represented by one line in the file, with fields on the line separated by commas. The fields for each data point are:

minutes,seconds,temperature

The minutes and seconds are wall-clock time (approximately GMT). When the minutes exceed 59 they roll over to zero. Hours were not recorded but it can be safely assumed that if the minutes on a line are less than the minutes on the previous line then the wall-clock hour has incremented by exactly one.

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