Trying bubble wrap as an insulator for a home-made solar kettle

by Stephen Hewitt | Published 8 January 2023

Here are some experimental results from testing the plastic bubble wrap shown in Figure 1 as an insulator on the heat collector of a home-made parabolic trough solar kettle under conditions of low wind. An earlier article reported similar investigations using a silicone tube as an insulator.

Procedure

Water was heated in the kettle with and without bubble wrap on the copper pipe and the increase in temperature against time recorded in each case, and later compared. Some more heatings with the silicone tube on the same days are also reported here.

The plastic bubble wrap came from a used padded envelope. It was attached to the copper pipe using a small amount of clear sticky tape. Three separate pieces of bubble wrap were used, each wrapped around the pipe in a spiral with the bubbles facing the pipe, and intended to be two layers deep. The three cylinders formed in this way were butted up to each other along the pipe, with the result that the pipe was covered between its supporting clips.

The testing was performed in the same way as for the silicone tube. To measure temperature a KT-type thermocouple was immersed in the water being heated, approximately halfway down the copper pipe. Care was taken not to disturb it during the measurement period. Temperature (and time) was recorded at intervals while the kettle heated water.

Results are presented here recorded on 7 December, 9 December and 18 November 2022, all days without much wind.

As an experimental control, on 7 and 9 December, heating with the bare copper pipe was also recorded, both before and after the the bubble wrap. On two of the days heating with the silicone tube was also recorded. On 18 November heating was done only with bubble wrap.

Results

The results are shown in Figure 2. In every case, the heating with bubble wrap (and silicone tube) was slower than the slowest heating with the bare pipe on the same day. For 18 November, when no heating was done with a bare pipe, the bubble wrap curves have similar slopes to the ones recorded on 9 December.

The bubble wrap underwent multiple heat cycles of the kettle with no noticeable deterioration as already found for the silicone tube. (Again, to qualify this, the kettle always had water in the pipe and was never allowed to boil dry.)

Discussion

Figure 2 shows that on 7 December, with the exception of the first two curves, the heating with the bare pipe became progressively slower with time elapsed since solar noon. The silicone and bubble wrap curves are slower than the slowest bare pipe curve, which started after them at 14:23 GMT.

It seems likely that some random variation in sun strength occurred as there were observed cloud on this day. For example the kink in curve 12 (bubble wrap 13:41) made by the point at 70°C corresponds to a a small cloud on the sun noted at the time. On the other hand, this curve's overall similarity to the second bubble wrap curve recorded immediately afterwards shows consistency. But this was not the only cloud or suspicion of cloud or haze.

Figure 3 shows further analysis of the results for 7 December. The rate of temperature increase plotted at each temperature is the average rate in the interval since the previous measurement, obtained by dividing the temperature difference by the time since the last measurement.

The approximate equivalent net heating power shown on the right hand scale has been calculated in a similar way as in the first article, on the assumption that the copper pipe and the water only within the 1m length of the reflector are increasing in temperature, and water and copper are at a uniform temperature and ignoring effects caused by changes in water density.

This might be an underestimate because the pipe is actually about 10% longer than 1m (and it is full of water). On the other hand it might be an overestimate because from previous measurements it seems possible that the middle of the pipe, where the temperature probe is positioned, is usually at a higher temperature than the ends, especially the open end. Since the water was at a uniform temperature when it went into the pipe and this would mean at least sometimes the temperature in the middle must have been increasing at a higher rate.

Figure 3 shows that above 90°C, the rate of heating with the bare pipe has fallen to levels comparable with the bubble wrap at similar times of day. This suggests the possibility that at 90°C the heat loss of the bare pipe is high enough that in comparison the better insulation of the bubble wrap starts to compensate for worse optical efficiency.

It seems that the bubble wrap reduces the overall efficiency of the kettle in conditions of low wind However the uncertainties of clouds and sun strength mean more tests would be preferable for a more sure conclusion.

Supplementary data

The raw data for each of the 14 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.

e49bd067626900c243ea1c1e302f52795e994dad8ab08077d01b11adf32dffbd  /download/1.bare-11.36-9dec2022.csv
bb7c7feb87a4a075c647c8f32a0c94bc0531df6d2ef8989eb5567427c38ddb35  /download/2.bare-11.49-9dec2022.csv
2e31acc2dc62fe544f85b47a6f6b2825ddfa488f113d5d5894ee5728f080e2ce  /download/3.silicone-12.08-9dec2022.csv
60789676f8908a4b1a07b6afe869ab85f13e3722c4a2269afc0763eacf78c3f0  /download/4.bubble-12.37-9dec2022.csv
0d36646a2f7d83a36e4e3b21e72a56cff9773472edf7b0a31c33f06abd738ba4  /download/5.bare-12.59-9dec2022.csv
04d462e4c08a69cc8f938b4299f2bd6c31bde4c91a41f45f791ffe3faab7a37a  /download/6.bubble-12.30-18nov2022.csv
e900a44d2c9db10b14663b92b72713af7d4ab5b5c03d730f48d61223213312e0  /download/7.bubble-13.00-18nov2022.csv
887ae45d0993fc8de942e3b89e4372ab4b7064da9ac42a8e08bee1c15257312a  /download/8.bare-12.29-7dec2022.csv
da23e453750561a422bc3cc6397e595574818587443933f813e4474eca21833d  /download/9.bare-12.42-7dec2022.csv
e67ec92fa7c4ce5b3e606739e612416cca2f268580501c7100f14b791617578c  /download/10.silicone-12.57-7dec2022.csv
7b217bb044c3579e216343b2f88c0988874172d4d3f3a722acfcfdc7d0fcb646  /download/11.bare-13.19-7dec2022.csv
135335fe9821a6f5d09b258937d2109a5b017cd785ad1229de2c57d48247c272  /download/12.bubble-13.41-7dec2022.csv
c796ac1e337e1c9e1aab9e1da6aa90ffaecfcac2c330510e1bd8c2059357e124  /download/13.bubble-14.01-7dec2022.csv
8350f7ce82abd88a147e446a46c7f559bbcd1486a0cd0f33423bdb7a25b286ff  /download/14.bare-14.23-7dec2022.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.