⚡ electrical engineering

Comparative Thermal Performance of Double Helical Coil and Finned Tube Heat Exchangers in PCM-Based Solar Water Heating

This study experimentally and numerically demonstrates that integrating a double helical copper coil into a paraffin wax-based latent heat storage system significantly outperforms both finned tube and conventional configurations in solar water heating under Gulf climatic conditions, achieving higher outlet temperatures and extended thermal retention times.

Original authors: Belal ALemour, Omar Badran

Published 2026-08-11

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Belal ALemour, Omar Badran

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Sun's Unreliable Gift and the Thermal Sponge

Imagine the sun as a generous but unpredictable friend who shows up to your party with a giant bucket of hot water. It's fantastic while it's there, but the moment the sun dips below the horizon or a cloud rolls by, the party cools down fast. This is the central challenge of solar water heating: the energy is abundant, but it arrives in fits and starts, leaving us with cold showers when we need hot ones most. To fix this, scientists use a clever trick involving "Phase Change Materials" (PCMs). Think of a PCM like a thermal sponge. Unlike a regular sponge that just soaks up water, a thermal sponge soaks up heat. When it gets hot, it doesn't just get warmer; it changes its state (like ice turning to water), absorbing a massive amount of energy without changing its temperature much. When the sun goes down, the sponge slowly squeezes that heat back out, keeping your water warm long after the sun has gone to sleep. The catch? These sponges are often made of materials like paraffin wax, which are great at holding heat but terrible at letting it move through them quickly. It's like trying to fill a room with water using a single, tiny straw; the process is agonizingly slow.

The Great Heat Race: Coils vs. Fins

This paper sets out to solve that "slow straw" problem by testing two different ways to speed up the heat transfer in a solar water heater. The researchers built a small-scale solar system and filled the storage tank with paraffin wax. Then, they ran a race between two different heat-exchange designs to see which one could charge the "thermal sponge" faster and keep the water hot longer.

The first contender was a double helical copper coil. Imagine two copper pipes twisted together like a double-stranded DNA spiral, winding their way through the wax. The second contender was an aluminum finned tube, which looks like a central pipe with flat metal fins sticking out all around it, similar to the cooling fins on a motorcycle engine. They also tested a standard tank with no special tricks to see how much of a difference these designs actually made.

The results were clear. The double helical copper coil was the champion of the race. It managed to heat the water to a maximum of 71.2 °C, which was hotter than the finned tube's 66.3 °C. More importantly, it kept the water hot enough for a shower (above 45 °C) for 6.8 hours, compared to just 3.4 hours for the standard tank. Even after the sun went down, the coil system kept the water warm (above 40 °C) for 8.1 hours, while the finned tube managed 7.2 hours.

Why did the spiral coil win? The researchers used computer simulations to peek inside the tank and see what was happening. They found that the spiral shape acted like a giant, winding highway for heat, spreading energy evenly throughout the wax. The finned tube was also good—it was much better than doing nothing—but the heat tended to get stuck in the areas right next to the fins, leaving some parts of the wax melting slower. The coil, with its larger surface area and spiral design, created a more uniform temperature, melting the wax faster and storing more energy.

The team didn't just guess these numbers; they built a real prototype and ran the tests for three days, then double-checked their findings with detailed computer models. The models matched the real-world experiments with high accuracy, showing less than a 10% error margin. This gives them strong confidence that the double helical copper coil is a superior way to make solar water heaters work better, especially in hot climates like the Gulf region, ensuring that hot water is available even when the sun isn't.

Technical Summary: Comparative Thermal Performance of Double Helical Coil and Finned Tube Heat Exchangers in PCM-Based Solar Water Heating

Problem Statement
Solar water heating systems face a fundamental limitation due to the intermittent nature of solar radiation, which restricts the continuous availability of hot water, particularly during nighttime or periods of low irradiance. While Phase Change Materials (PCMs), such as paraffin wax, offer a solution through high-density latent heat storage at nearly constant temperatures, their practical application is hindered by inherently low thermal conductivity. This limitation results in prolonged charging (melting) and discharging (solidification) processes. Although various heat transfer enhancement techniques exist, there is a lack of direct, comprehensive comparisons between different internal heat exchanger configurations (specifically double helical coils versus finned tubes) under identical operating conditions, supported by both experimental validation and rigorous numerical modeling.

Methodology
The study employs an integrated experimental and numerical approach to evaluate thermal performance under climatic conditions representative of the Gulf region (Bahrain).

Experimental Setup: A laboratory-scale prototype was fabricated comprising a flat-plate solar collector (0.60 m²), a 60 L storage tank containing 20 kg of paraffin wax, and a circulation system driven by a 24 V DC pump (0.015 kg/s). Three distinct configurations were tested under identical conditions:
1. A conventional storage tank without PCM.
2. A PCM tank equipped with a double helical copper coil (two concentric coils, 12 turns, inner diameter 120 mm, outer 250 mm).
3. A PCM tank equipped with an aluminum finned tube (central tube with 12 radial fins, 120 mm height).
  Temperature data was collected via six calibrated sensors monitoring the collector inlet/outlet, tank stratification (top, middle, bottom), and the PCM core.
Numerical Simulation: Three-dimensional Computational Fluid Dynamics (CFD) simulations were conducted using ANSYS Workbench. ANSYS Fluent utilized the enthalpy–porosity method to model transient fluid flow, heat transfer, and PCM phase change. ANSYS Thermal was used for steady-state thermal finite element analysis of the collector. A mesh independence study confirmed that a grid of approximately 2.1 million elements provided sufficient accuracy.
Validation: The numerical model was validated against experimental data, with a maximum prediction error required to be below 10%. Statistical metrics including Mean Absolute Error (MAE), Root Mean Square Error (RMSE), and the coefficient of determination ( $R^2$ ) were used to assess model reliability.

Key Results
The study yielded the following comparative findings:

Thermal Performance: The double helical copper coil configuration demonstrated the superior thermal performance among all tested cases. It achieved a maximum outlet water temperature of 71.2 °C, extended the duration of hot water availability (above 45 °C) to 6.8 hours, and increased thermal retention time (above 40 °C) to 8.1 hours.
Finned Tube Performance: The aluminum finned tube also significantly outperformed the conventional tank, reaching a maximum outlet temperature of 66.3 °C and extending thermal retention to 7.2 hours. However, its performance was slightly lower than the double helical coil, attributed to heat transfer being concentrated primarily around the radial fins rather than distributed uniformly.
Temperature Distribution and Phase Change: CFD results indicated that the double helical coil produced a more uniform temperature distribution and higher heat flux throughout the PCM volume. The spiral geometry facilitated simultaneous melting across the storage volume. In contrast, the finned tube exhibited localized high-temperature regions, with heat propagation between fins relying more heavily on conduction through the PCM.
Model Accuracy: The numerical model showed excellent agreement with experimental measurements. The Mean Absolute Percentage Error (MAPE) was 2.96% for the double helical coil and 3.41% for the finned tube, with $R^2$ values exceeding 0.98 for both enhanced configurations, confirming the model's validity.

Significance and Claims
The authors claim that this study provides a comprehensive assessment of two practical heat transfer enhancement techniques for paraffin wax thermal energy storage. By integrating laboratory experiments, transient CFD simulations, steady-state thermal analysis, and mesh independence verification within a unified framework, the research addresses the gap of limited direct comparisons in existing literature.

The primary significance of the work lies in demonstrating that increasing the internal heat transfer area is an effective strategy to overcome the low thermal conductivity of paraffin wax. The study concludes that the double helical copper coil offers the most effective solution for enhancing latent heat storage performance, providing a reliable framework for designing efficient solar water heating systems specifically optimized for Gulf climatic conditions. The validated numerical model is presented as a robust tool for future design and optimization of PCM-based systems.

The Sun's Unreliable Gift and the Thermal Sponge

The Great Heat Race: Coils vs. Fins

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