Panel solar cookers occupy a distinctive niche among solar cooking technologies. Unlike parabolic concentrators with their curved dishes or box cookers with insulated chambers, panel cookers use flat reflective sheets arranged around a cooking vessel. They are cheap, lightweight, easy to assemble and highly portable, which makes them attractive for decentralized deployment without specialized tooling or infrastructure. Yet their adoption has been held back by comparatively low thermal efficiency. Conventional designs typically deliver standardized cooking power of 50 to 120 watts, with stagnation temperatures of 130 to 150 degrees Celsius under favorable irradiance. Previous research has largely focused on refining reflector geometry to squeeze more sunlight onto the pot, but as optical interception approaches practical limits, further reflector refinement alone appears unable to overcome the fundamental thermal shortcomings of these systems.

The Tunisian team set out to test a different hypothesis: that the dominant bottleneck in panel cookers is not optical interception but receiver-side thermal resistance, the heat that escapes through convection and radiation from the cooking vessel before it can reach the food. To investigate, they built two identical hexa-panel solar cookers, each with six trapezoidal reflector panels made of 3-millimeter polished aluminum, forming a hexagonal aperture of 0.84 square meters. A black-painted copper pot with a capacity of 4.6 liters sat at the center, enclosed in a transparent plexiglass shell that traps warm air around the vessel through the greenhouse effect. The cookers were mounted on a steel frame with manual dual-axis tracking, and the two prototypes were run side by side to allow strictly simultaneous paired comparisons under identical weather conditions.

The experimental campaign, conducted between May and September 2025, was unusually rigorous. Twenty distinct test conditions were evaluated across four pot heights ranging from 0 to 9 centimeters above the cooker base, three lid types, and four external fin arrangements, with each configuration repeated multiple times on independent days. The researchers also modified the standard ASAE S580.1 testing procedure in three deliberate ways: they cut the data acquisition interval from 600 seconds to 60 seconds to reduce averaging errors under fluctuating irradiance, lowered the load ratio from 7 to 4 kilograms per square meter of aperture to reflect realistic cooking practice, and extended the valid ambient temperature range to 40 degrees Celsius to suit Kairouan’s summer conditions. Prior work has shown that the standard 600-second interval can overestimate cooking power by up to 16.2 percent under variable skies.

The first variable examined was pot position, and the results revealed a clear geometric optimum. In no-load stagnation tests, the absorber temperature and the first figure of merit, a ratio combining optical efficiency and heat loss, peaked when the vessel sat 3 centimeters above the cooker base, reaching an average stagnation temperature of 217.37 degrees Celsius with the plexiglass enclosure. Under load tests with 3.5 kilograms of water, the same position delivered the highest standardized cooking power of 138.95 watts, an average thermal efficiency of 14.3 percent, and the fastest heating time of about 104 minutes. The researchers interpret this as a balance point: low positions sit below the convergence zone of reflected rays from the six panels, while higher positions expose more of the pot to ambient convective losses. Statistical analysis confirmed the position effect was highly significant, although the difference between the two lowest positions was too small to be statistically distinguished.

The lid experiments produced perhaps the most counterintuitive finding. Although copper conducts heat roughly 8,000 times better than air and vastly better than glass, the transparent glass lid outperformed the opaque copper lid decisively, raising standardized cooking power by 12.5 percent. The reason lies in how panel cookers deliver energy: a significant fraction of incident sunlight strikes the pot lid directly. A transparent lid lets that radiation pass through and be absorbed by the blackened interior surfaces of the pot, while an opaque lid absorbs the energy at its outer surface, where much of it dissipates to the surroundings through radiation and convection before conducting down to the water. The second-law analysis made this vivid: the opaque lid produced the highest entropy generation and exergy destruction of all configurations tested, showing that optical transmittance at the receiver matters more thermodynamically than material conductivity.

That said, finned lids offered a practical compromise. When the researchers fitted hollow cylindrical copper fins extending into the cooking fluid on the opaque lid, the configuration recovered roughly 74 percent of the performance gap between the glass and plain opaque versions. This is significant for deployment contexts where tempered glass lids may be unavailable, fragile or cost-prohibitive. Meanwhile, adding fins to the outside of the pot produced a steady, monotonic improvement across every configuration tested. One small fin of 1-centimeter height lifted cooking power by 4 percent; two fins of 2-centimeter height, riveted radially around the pot, pushed standardized cooking power to 152.775 watts, an overall gain of 12.3 percent, and raised thermal efficiency to 15 percent, 15.4 percent above the conventional pot. Heating time dropped from 112 minutes to just 96 minutes.

The exergy analysis, which quantifies the thermodynamic quality of the energy conversion rather than just its quantity, reinforced these findings with unusual depth. Using the Gouy-Stodola theorem to link cumulative entropy generation to exergy destruction, the team showed that the two-fin, 2-centimeter configuration achieved the lowest entropy generation, 1,152.37 joules per kelvin, and the lowest exergy destruction, 349.35 kilojoules, of all seven receiver configurations. Remarkably, the statistical analysis revealed that fin number, not fin height, was the dominant geometric parameter: doubling the number of fins reduced entropy generation more than doubling their height. The overall second-law gains were substantial, with the optimized configuration cutting exergy destruction by nearly 25 percent relative to the worst-performing opaque-lid case, confirming that the improvements reflect genuine reductions in thermodynamic irreversibility rather than artefacts of increased heat input.

The integrated economic and environmental assessment, the fourth E in the study’s 4E framework alongside energy, exergy and economics, suggests these gains translate into real-world value. The capital cost of the cooker ranged from about 134 to 146 dollars depending on configuration, yet the fin-assisted design reduced the lifetime cost per meal by 17.2 percent compared with the least efficient version and shortened the payback period to 0.861 years against LPG fuel savings. A sensitivity analysis testing pessimistic assumptions, a shorter 7-year lifespan, only 230 cooking days per year and a 12 percent interest rate, still kept the payback period below 14 months. Environmentally, the optimized configuration achieved 53.7 percent higher cumulative avoided carbon dioxide emissions over its 10-year life than the least efficient design, earning an estimated carbon credit of about 27 dollars, with net lifetime mitigation of 1.87 tons of carbon dioxide.

The study does carry caveats the authors acknowledge candidly. The mechanistic explanation of the optimal pot height rests on geometric arguments rather than direct optical flux mapping, and the relative contributions of enhanced convection versus additional solar interception by the fins remain unseparated. Fins also introduce practical trade-offs: more crevices to clean, potential coating degradation under prolonged ultraviolet exposure, and additional rivet points that could corrode over time. All experiments were conducted at a single site with one cooker geometry. Still, the broader message stands out clearly. In the loss-dominated regime where panel cookers operate, receiver-side thermal resistance is a genuine and addressable bottleneck, and attacking it with a few centimeters of copper and a sheet of glass can match the gains of far more complex reflector engineering, at a cost of a few dollars per modification.

Subject of Research: Receiver-side heat-transfer enhancement and 4E performance evaluation of a panel solar cooker

Article Title: Passive receiver-side enhancement of a panel solar cooker: Experimental quantification of fin-assisted heat transfer and integrated 4E performance assessment

Article References: Mankai, S., Ouled Saad, F., Madiouli, J., Chemkhi, S., Shigidi, I., & Sghaier, J. (2026). Passive receiver-side enhancement of a panel solar cooker: Experimental quantification of fin-assisted heat transfer and integrated 4E performance assessment. Results in Engineering, 32, Article 113539. https://doi.org/10.1016/j.rineng.2026.113539

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113539

Keywords: solar cooker, panel cooker, heat transfer, fins, exergy analysis, entropy generation, cooking power, renewable energy, techno-economic analysis, carbon mitigation, Tunisia, ASAE S580.1

The research team, led by Dixit Arya together with Nisha Netam and Anuj Kumar Shukla, simulated the melting of a phase change material inside a closed rectangular enclosure fitted with conducting fins, comparing configurations with one, two, and three fins while holding the total fin area constant. Crucially, they ran the simulations not only at normal Earth gravity but also at five and ten times standard gravitational acceleration, conditions representative of aerial flight maneuvers and rotating machinery. The central insight is deceptively simple but consequential: gravity does not merely press down on the material, it fundamentally rewrites the physics of melting, and the fin geometry that works best at one gravity may underperform at another.

To understand why, one must appreciate the role of buoyancy-driven convection in latent heat thermal energy storage. When a phase change material melts next to a heated wall, the liquid layer that forms does not sit still. Warmer liquid is less dense, so it rises, while cooler liquid descends, creating circulating currents that ferry heat away from the melting front far more efficiently than conduction alone. The strength of this convective stirring is captured by the Rayleigh number, a dimensionless quantity that scales directly with gravitational acceleration. Multiply gravity by five or ten, and the Rayleigh number climbs accordingly, intensifying the internal flow and accelerating the melt. The simulations quantified this effect dramatically: maximum velocity magnitudes within the liquid increased by a factor of three to five under hypergravity compared with the baseline case.

The headline result concerns melting time. Under five times normal gravity, the finned configurations melted the material up to 47.8 percent faster than an unfinned enclosure, and at ten times gravity the improvement reached 48.2 percent. In practical terms, a thermal storage unit that would take minutes to fully charge at Earth gravity could accomplish the same task in roughly half the time during aggressive flight maneuvers. For aerospace engineers, this is more than a curiosity. Thermal management systems on high-speed aircraft and spacecraft must absorb transient heat loads quickly, and a storage medium that charges faster under acceleration effectively buys safety margin precisely when thermal stress is greatest.

Yet the study’s most provocative finding concerns fin placement and number, and here the conventional wisdom takes a beating. Engineers often assume that more fins mean better heat transfer, since each fin extends the heated surface into the material. The simulations tell a subtler story. A single, longer fin positioned in the lower region of the enclosure delivered the best performance among all cases examined. The reason lies in fluid dynamics: the lower fin sits where buoyant plumes originate, and its length allows it to channel warm liquid upward along its surface, feeding and reinforcing the convective circulation. Multiple shorter fins, by contrast, fragment the liquid domain into smaller cells, and fins placed in the upper region can act as barriers that suppress the very fluid motion that hypergravity amplifies. In effect, poorly placed fins strangle the convection that gravity is trying to supercharge.

This trade-off between conduction enhancement and convection suppression is the intellectual heart of the paper. Fins excel at conducting heat into solid regions that convection cannot reach, but every fin is also an obstacle in the flow path. Under hypergravity, where convective currents are the dominant heat-transfer mechanism, the penalty for obstructing flow grows correspondingly. The researchers’ comparative analysis of single-, double-, and triple-fin arrangements demonstrates that the optimal design is not the one with the most metal but the one that cooperates with the buoyant flow field. A long lower fin guides the flow; a cluster of short upper fins fights it.

The energy accounting reveals further nuances that any system designer must confront. Because fins displace a small volume of phase change material, the total energy stored in the finned enclosures decreased slightly, by between 0.38 and 1.32 percent, relative to the unfinned case. That loss is modest, but the gains in charging speed are not: the mean power delivered during melting increased by up to 90.8 percent in the best configurations. In other words, fins transform the storage unit from a slow, capacious reservoir into a fast, responsive one. However, the study also reports a reduction in energy storage density of roughly 7.5 percent, a metric that matters where every kilogram and every liter counts, as it always does in aerospace applications. The trade-off between storage capacity and heat-transfer performance is therefore not eliminated by hypergravity but reshaped by it.

These results arrive amid a growing body of research on phase change behavior across the gravity spectrum. Previous experimental work has documented accelerated melting under hypergravity in pin-fin heat sinks for airborne electronics, while other studies have explored the opposite extreme, where microgravity nearly eliminates buoyant convection and melting stalls into a slow, conduction-dominated process. Numerical investigations have examined metal foam composites, topologically optimized fins, and even magnetic field coupling under variable gravity. The new study contributes a systematic comparison of fin number and placement at constant total fin area, a controlled variable that isolates the geometric question from the material question. Its findings complement earlier work showing that fin length arrangements and enclosure inclination strongly influence melting at normal gravity, and extend that framework into a regime where the Rayleigh number is an order of magnitude larger.

The practical implications reach beyond aircraft. Rotating systems, from centrifugal thermal storage units to machinery on space stations with artificial gravity, all impose effective accelerations on their thermal storage media. Battery thermal management, a pressing concern for electric aviation, has already seen proposals combining phase change materials with fin structures under variable gravity environments. The new results suggest that such systems should be designed with the gravity vector in mind: orienting fins to nurture, rather than obstruct, buoyant circulation could yield charging-rate improvements approaching a factor of two, while naive fin layouts might deliver far less than expected. The finding that a single well-placed fin outperforms multiple shorter ones could also simplify manufacturing and reduce mass, a rare case where the better design is also the lighter one.

As with any numerical study, the results rest on modeling assumptions, and the authors note that the data underlying their findings are available upon reasonable request. Experimental validation under sustained hypergravity, which requires centrifuge facilities or parabolic flight campaigns, remains the necessary next step before these design rules enter flight hardware. Still, the study offers a clear and actionable principle for engineers racing to manage ever-growing heat loads in fast-moving machines: in the world of latent heat storage, gravity is not an obstacle to be engineered around but a fluid-dynamic resource to be harnessed, and the humble fin, placed with an eye on the flow, is the lever that unlocks it.

Subject of Research: Fin-enhanced melting of phase change materials for latent heat thermal energy storage under hypergravity conditions

Article Title: Impact of Multiple Fins on the Melting Characteristics of PCM in a Closed Enclosure Under Hypergravity Conditions

Article References: Arya, D., Netam, N., & Shukla, A. K. (2026). Impact of Multiple Fins on the Melting Characteristics of PCM in a Closed Enclosure Under Hypergravity Conditions. International Journal of Aeronautical and Space Sciences. https://doi.org/10.1007/s42405-026-01240-5

Image Credits: AI Generated

DOI: 10.1007/s42405-026-01240-5

Keywords: phase change material, hypergravity, latent heat thermal energy storage, fins, buoyancy-driven convection, Rayleigh number, thermal management, aerospace, melting time, energy storage density, CFD simulation, heat transfer