Applications for Pico Products
Stratospheric Temperature Experiment with Phase Change Material
Introduction
This experiment is designed to show the implications when adding a latent heat storage medium to water. The latent heat storage is via a phase change material (PCM) which has energy storage properties above that of “sensible heating” (water, thermal bricks, and concrete).
The PCM will melt when its designed critical temperature has been reached, and now becomes a storage device. When the energy around the PCM falls below that of the charging energy (one degree below set designed melting point) the PCM will begin to return that stored energy until the stored energy as heat is depleted and the PCM will return to a solid state awaiting a recharge cycle. This cycle can happen thousands of times before the PCM falls below the required standard.
This research is for the domestic heating systems in present use around the UK.
Equipment used
Data Logger
A TH-03 temperature to PC data logger with three precision resistive temperature sensors was used to measure the temperature in three places within a vessel holding pre–warmed water at 38.5 °C.
Measurements were taken at 1 second intervals over a 10 hour period.
Test Vessel
Test vessel was a 6 mm glass Demi-jon with a volume of 5 litres. A cork stopper was used with a hole for the sensor wires to pass through. Insulation was bubble–wrap 10 mm deep over the full surface of the vessel.
The demijohn is housed within an insulated and temperature controlled container with an internal volume of 15 litres — the temperature control was switched off and the box was used as more insulation.
PCM
The type of PCM used was Climcel 24 with a 24 °C melting point.
5 sachets of 292 g of the PCM were wrapped around the top third of the demijohn, and then insulated with bubble–wrap. The total weight of the PCM was 1.46 Kg.
Results
Two experiments were performed one with PCM the other identical to the first but with no PCM.
PCM start temperature: 34.39 °C
Water temperature top: 38.8 °C
Water temperature bottom: 34.56 °C
Figure 1 below shows the results collected:
Figure 1: A graph of the effects of PCM in water
Analysis
It is evident from the graph that the PCM latent storage material has made a difference to the water temperature at the top and the bottom of the vessel and temperatures levels have been maintained for longer. After 10, hours a noticeable difference had occurred in the upper and lower temperature zones.
Also noted is the linear rate of decent by both tests but in the PCM test the difference between upper and lower measured water temperature levels was reduced by 1 °C. This might not seem a vast improvement with respect to the final temperature but the ratio of PCM to water is only 1:5, so with increased PCM and also more efficient use of the PCM in the tank, efficiencies will increase.
The duration of the tests, 10 hours will have to be extended because the PCM did not reach it’s critical temperature of 24 °C.
Conclusion
Future applications for PCMs using solar heating panels in domestic heating systems would be more viable in climates such as the UK with varying amounts of solar radiation, and would also increase normal domestic heating efficiencies to allow less grid supplied electricity to be used and in turn reducing the carbon dioxide output.
For example. keeping the top third of a domestic water tank covered with PCM will allow hotter water to be drawn off. That is, the PCM will allow half a tank of hotter water to be drawn, rather than a full tank of water at a lower temperature.
Cost implications are obviously an issue as energy per kilowatt hour will be cheap for the next few years though the oil reserves are declining and this will increase the price of energy from fossil fuel making this and any solar system an alternative option.
PCMs have one disadvantage in that the efficiencies are increased with mass so space and structural integrity will be a consideration when calculating its worth.
