Scientists from DHA Suffa University in Karachi, Pakistan, jointly with researchers from King Abdulaziz University in Saudi Arabia have tested a new design for a paraffin-based thermal energy storage system intended for solar collectors. By modifying the internal geometry of the heat exchanger, the scientists managed to accelerate heat accumulation by 1.5 times and substantially increase its efficiency, potentially making solar energy more suitable for heating and hot-water systems.
The new design is based on the use of a phase transition, for instance, the melting of paraffin. During this process, the material absorbs a large amount of energy with virtually no change in temperature. This makes it possible to create compact and stable heat accumulators. However, paraffin has a major shortcoming in that its thermal conductivity is low: heat from hot-water pipes slowly diffuses into the material, causing melting to take a long time; during the reverse process, heat returns to the system in an uneven manner.
To solve this problem, the scientists modified the internal geometry of the heat accumulator. Instead of standard straight fins, they used a combination of M-shaped and rectangular plates. Placing M-shaped elements on coolant pipes not only increases the heat exchange area but also creates localized vortex flows in the molten material, enhancing convection. Rectangular fins attached to the tank walls enable a more uniform heat distribution across the entire volume, including remote areas.
An experimental unit was created to test this solution. It consisted of a steel tank filled with RT50 paraffin (melting point of about 50°C) and a system of five hot-water pipes. One version of the unit used smooth pipes and the other featured pipes with combined fins. Computer simulations of the thermal processes were conducted at the same time.
The tests showed a noticeable effect. While in the classic configuration without fins, it took paraffin about 67 minutes to melt completely, this process was reduced to 44 minutes in the modernized configuration. The experiment confirmed the calculations: taking heat losses into account, complete melting took 51 minutes, which was still 16 minutes faster than the original version. The reverse process (solidification) took 48 minutes.
Crucially, this is not just a matter of speed. Analysis showed that roughly 94% of the accumulated energy comes from the latent heat of phase transition, which ensures high storage density. While the portion of energy that can be converted into useful work remains low, the system itself becomes much more efficient for thermal applications. The addition of fins has increased the overall energy reserve by almost 55%.
The key effect is achieved through the combination of two heat transfer mechanisms. Thermal conductivity is dominant at the initial stage, which is why the increased surface area is crucial. As the paraffin melts, convection (fluid flow that accelerates heat transfer) begins. The M-shaped fins enhance this effect by creating microvortices.
The researchers note that this design could be further developed by, for instance, using more complex biomimetic fin shapes or adding nanoparticles to the material to enhance thermal conductivity. Nevertheless, it has already been shown that even relatively simple changes in geometry can significantly accelerate the operation of heat accumulators and improve their efficiency, which is important for expanding applications of solar energy.
