The motivation behind the study stems from the accelerating phaseout of conventional hydrofluorocarbon refrigerants under international agreements such as EU regulation 517/2014, which restricts the use of fluids with high global warming potential. Isobutane and LPG have emerged as popular retrofit candidates for the phased-out R12 and R134a working fluids because their thermophysical properties closely resemble those of the legacy refrigerants, they deliver improved latent heat transfer, they are compatible with the mineral and synthetic oils already used in compressor lubrication, and flammability risks can be managed with established control methods. Because experimental trials with flammable hydrocarbons are costly, time-consuming, and prone to human error, the researchers turned to numerical modeling, which can deliver accuracy comparable to physical testing while eliminating many practical constraints.

The test rig at the heart of the study is a compact domestic refrigerator with a 120-watt compressor, an air-cooled condenser, a 2-meter capillary tube, and a plate-type evaporator. During the experimental phase, the team gathered data using pressure gauges, K-type thermocouples, a refrigerant flowmeter, and a wattmeter, averaging results across three repeated tests for each combination of refrigerant and condenser length to ensure repeatability. These averaged experimental values were then fed into CYCLE_D-HX, which models each subcomponent of the vapor compression cycle, including the compressor, condenser, capillary tube, and evaporator, and evaluates performance based on user-defined specifications such as refrigerant type, cycle configuration, and heat exchanger temperature profiles.

The modeling framework draws on well-established heat transfer theory. For the air-cooled condenser, the researchers applied the log mean temperature difference method, computing the bare-tube area, fin area, and flow areas from the condenser geometry, then deriving the air-side convective heat transfer coefficient using the Kays and London correlation for free convection across tube banks. The refrigerant-side coefficient during condensation was calculated from mixture Reynolds number correlations that account for the coexistence of liquid and vapor phases along the tube. The overall heat transfer coefficient was then assembled as the reciprocal sum of the individual thermal resistances, incorporating the convective coefficients on both sides and the conduction resistance of the aluminum tube wall, whose thermal conductivity is 237 watts per meter-kelvin. Pressure drop in the condenser was modeled using modified Chawla correlations for two-phase flow, capturing the frictional losses that arise as the refrigerant changes phase from vapor to liquid.

One of the most practically significant findings concerns pull-down time, the interval required for the evaporator chamber to cool from ambient temperature, set at 32 degrees Celsius under the ISO-8187 standard for this refrigerator class, down to the target temperature of minus 12 degrees Celsius. When the system ran on R600a with the 5-meter condenser, the cabinet reached minus 11 degrees Celsius within the 240-minute pull-down window, satisfying the ISO requirement. With the shorter 2.5-meter condenser, R600a achieved only minus 5 degrees Celsius in the same period. LPG performed less impressively on this metric, reaching minus 3 degrees Celsius with the short condenser and minus 9 degrees Celsius with the long one. The researchers attribute R600a’s superior cooling performance to its nature as a single-component, refined fluid that compresses easily and absorbs more latent heat during expansion, whereas the multi-component LPG mixture compresses less readily and absorbs less latent heat.

The coefficient of performance, the ratio of useful cooling delivered to the electrical energy consumed by the compressor, told a different story. At the 2.5-meter condenser length, COP ranged from 1.73 to 2.57 for LPG and from 1.68 to 2.42 for R600a, with the highest value of 2.57 achieved by LPG. At 5 meters, COP ranged from 1.43 to 2.26 for LPG and 1.34 to 2.20 for R600a, with the lowest value of 1.34 recorded for R600a. On average, LPG delivered a COP approximately 4.42 percent higher than R600a at the shorter condenser length and about 4.07 percent higher at the longer length. The COP declined over time for both refrigerants, reflecting the changing operating conditions as the cabinet cooled toward its setpoint.

Power consumption patterns reinforced LPG’s efficiency advantage. With the 2.5-meter condenser, LPG consumed between 1.56 and 2.32 kilowatts during operation, roughly 16.02 percent less on average than R600a, which ranged from 1.65 to 2.37 kilowatts. At the 5-meter length, the gap narrowed considerably, with LPG consuming about 3.75 percent less than R600a, whose consumption ranged from 1.82 to 2.98 kilowatts. The researchers note that power consumption increased both with time and with condenser length, driven primarily by the rising mass flow rate of refrigerant through the compressor. The lowest power draw of 1.56 kilowatts was recorded for LPG with the short condenser, making that configuration the most energy-frugal option in the study.

Heat rejection and heat transfer characteristics favored R600a. The average heat rejection rate with R600a exceeded that of LPG by 1.23 percent at the 2.5-meter condenser and by 1.85 percent at the 5-meter length, with R600a values ranging from 5.65 to 6.98 kilowatts depending on configuration. The overall heat transfer coefficient followed a similar pattern, running 0.98 percent higher for R600a at the short length and 1.39 percent higher at the long length, with values spanning 0.668 to 0.822 kilowatts per square meter-degree Celsius. Both coefficients increased with condenser length, indicating that longer condensers transfer heat more effectively. The researchers point out that R600a’s lower global warming potential makes it a preferred choice for modern systems despite LPG’s efficiency edge in other metrics.

Pressure drop inside the condenser tubes, which arises from both friction and the phase change from vapor to liquid, also differentiated the two refrigerants. R600a exhibited an average pressure drop 7.46 percent greater than LPG at the 2.5-meter length and 6.50 percent greater at 5 meters, with values ranging from 21.0 to 47.4 kilopascals for R600a and 20.1 to 43.4 kilopascals for LPG. Pressure drop increased with condenser length for both fluids, and the researchers emphasize that high pressure drops degrade system performance and component reliability, while low pressure drops prolong equipment life. The thermophysical properties of the refrigerants, including density, latent heat, and kinematic viscosity, strongly influence this behavior, which explains why the LPG mixture consistently produced the lowest pressure losses.

Taken together, the results present a nuanced picture with no single dominant configuration. The system performed best with LPG and a 2.5-meter condenser in terms of coefficient of performance and power consumption, while R600a with a 5-meter condenser excelled in cooling capacity, achieving the lowest cabinet temperature, and in heat rejection from the condenser. The findings suggest that refrigerant selection and condenser geometry must be optimized jointly rather than in isolation, and that the ideal choice depends on whether the design priority is energy savings, rapid pull-down, or thermal performance. As climate pressures intensify and regulations tighten around high global warming potential fluids, studies of this kind provide a computational foundation for designing the next generation of energy-efficient, environmentally responsible domestic refrigerators.

Subject of Research: Thermodynamic modeling of domestic refrigeration using hydrocarbon refrigerants with varying condenser lengths

Article Title: Thermodynamic modeling and performance assessment of a domestic refrigeration system using selected hydrocarbon refrigerants with varying condenser lengths

Article References: Oyedepo, S. O., Adelekan, D. S., Adenugba, O. E., Oyebanji, J. A., Babarinde, T. O., Kilanko, O., Banjo, S. O., & Nwaokocha, C. N. (2026). Thermodynamic modeling and performance assessment of a domestic refrigeration system using selected hydrocarbon refrigerants with varying condenser lengths. Frontiers in Thermal Engineering, 6, Article 1794072. https://doi.org/10.3389/fther.2026.1794072

Image Credits: AI Generated

DOI: 10.3389/fther.2026.1794072

Keywords: domestic refrigerator, hydrocarbon refrigerants, R600a, LPG, condenser length, coefficient of performance, CYCLE_D-HX, thermodynamic modeling, heat transfer, energy efficiency, vapor compression, global warming potential

The study, led by Ihsan M. Khudhur of Northern Technical University with colleagues Omer K. Ahmed, Omar Rafae Alomar and Sameer Algburi, was conducted outdoors in Kirkuk, a hot, semi-arid city at roughly 35.5 degrees north latitude that experiences punishing summer irradiance and distinct seasonal swings. The team built a direct-expansion photovoltaic/thermal heat pump prototype in which a one-square-meter, 150-watt photovoltaic module doubles as the evaporator of a small vapor-compression heat pump. A roll-bond aluminium plate bonded to the back of the module allows refrigerant to evaporate directly at the panel surface, absorbing heat that would otherwise accumulate in the silicon cells. On the condenser side, a water-cooled heat exchanger upgrades that recovered heat into useful hot water, so the same incident sunlight yields both electricity and thermal energy from a single footprint.

The passive half of the cooling strategy is unusually low-tech. The researchers dispersed titanium dioxide nanoparticles into linseed oil, a renewable and biodegradable drying oil, at a ratio of 15 percent nanoparticles to 85 percent oil, homogenized the mixture with magnetic stirring and ultrasonic dispersion, and applied it to the glass cover of the panel at three nominal thicknesses: 100, 200 and 300 nanometers. Screening with a solar cell analyzer under 1000 watts per square meter of halogen illumination showed the thinnest layer performed best, raising the conversion efficiency of coated mini-cells from 3.44 percent uncoated to 4.41 percent at 100 nanometers, with improved short-circuit current density and fill factor. Thicker coatings actually degraded performance, likely by scattering or absorbing useful light, so the 100-nanometer layer was carried forward into the field trials.

With both cooling mechanisms in place, the team ran a full seasonal comparison across four representative months: January, March, August and October. For each season they tested three refrigerants in the same hardware: R134a, the legacy hydrofluorocarbon baseline with high global warming potential; R600a, an isobutane hydrocarbon with negligible climate impact; and R1234yf, a low-GWP hydrofluoroolefin increasingly used in automotive and stationary systems. The same refrigerant charge mass was used for all three fluids to keep the comparison consistent, an intentional design choice the authors note may understate each fluid’s optimal potential. Measurements were taken on three clear-sky days per condition between 07:00 and 17:00 using calibrated irradiance meters, K-type thermocouples, an infrared thermometer, and current-voltage tracing, with standard deviations reported for repeatability.

The temperature results are the headline. In August, when an uncoated baseline panel reached a scorching 69 degrees Celsius at 14:00, the coating alone brought the surface down to 62 degrees, the heat pump alone to 39 degrees, and the combined active-passive system to 35 degrees, a 34-degree mitigation. In winter the combined approach reduced peak surface temperature from 46 to 29 degrees Celsius; in spring the drop was from 46 to 27 degrees; and in autumn from 48 to 24 degrees. Intriguingly, the combination outperformed the sum of its parts: the passive coating delivered 6 to 10 degrees of relief while the heat pump alone provided 19 to 22 degrees, yet together they produced nonlinear gains that exceed what either mechanism achieves independently.

Those temperature reductions translated directly into electrical performance. Compared with the uncoated baseline panel, the heat pump alone raised electrical efficiency by roughly 19 to 24 percent across the seasons, while the heat pump plus coating pushed gains to 24 to 29 percent. The best seasonal configurations show the pattern clearly: electrical efficiency climbed from 10.21 to 10.60 percent in winter with R1234yf, from 10.48 to 10.75 percent in spring with R600a, from 8.86 to 9.25 percent in summer with R600a, and from 9.87 to 10.18 percent in autumn with R600a. The authors argue that the coating acts as a thermal buffer that stabilizes the glass-to-cell interface and dampens morning transients, allowing the semiconductor junction to operate closer to its ideal fill factor and open-circuit voltage, while the heat pump removes heat from behind the panel and eliminates localized hot spots.

The heat pump’s coefficient of performance followed a daily rhythm, rising through the morning, peaking near solar noon when irradiance was greatest, and falling toward evening. Winter favored R1234yf, which posted a maximum COP of 2.65 at 13:00 under 760 watts per square meter of irradiance, with a daytime average of about 1.73. But from spring onward the hydrocarbon R600a dominated: a peak COP of 3.78 in March, 4.27 in August under 882 watts per square meter, and 3.85 in October. This seasonal flip is the study’s most practically consequential finding. It suggests that no single refrigerant is optimal year-round, and that system designers in hot, high-irradiance climates should match refrigerant selection to the season, favoring R1234yf in cold months and R600a for the bulk of the year.

Overall efficiency, defined as the sum of electrical and thermal efficiencies, reached its highest values in the transitional seasons with R600a: 73.44 percent uncoated rising to 76.59 percent coated in spring, and 75.27 percent rising to 77.32 percent coated in autumn. Even in winter, the best case with R1234yf improved from 62.97 to 64.93 percent with the coating. The authors frame this as more than an incremental gain: the coating does not merely assist the heat pump, it enables it, presenting a thermally stable evaporator surface that reduces the compressor’s workload per unit of heat extracted, so electrical and thermal outputs improve together rather than trading off against each other.

The team is candid about limitations. The campaign covered a single location with three test days per season, refrigerant charges were not optimized per fluid, and the long-term durability of an organic linseed oil matrix under ultraviolet exposure, oxidation, dust accumulation and thermal cycling remains unproven. Still, the engineering appeal is clear: the system uses commercially available components and a cheap, easily applied bio-nano coating requiring no manufacturing changes, offering a practical template for solar heating and cooling in the sweltering regions where photovoltaic losses bite hardest.

Subject of Research: Experimental enhancement of a direct-expansion photovoltaic/thermal heat pump system using a TiO2-linseed oil nanocoating and alternative refrigerants across four seasons

Article Title: Performance enhancement of photovoltaic/thermal systems with heat pump units using alternative refrigerants and nanoparticle TiO 2 with linseed oil cooling coating

Article References: Khudhur, I., Ahmed, O. K., Alomar, O. R., & Algburi, S. (2026). Performance enhancement of photovoltaic/thermal systems with heat pump units using alternative refrigerants and nanoparticle TiO2 with linseed oil cooling coating. Results in Engineering, 32, Article 112728. https://doi.org/10.1016/j.rineng.2026.112728

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.112728

Keywords: photovoltaic thermal, heat pump, TiO2 nanocoating, linseed oil, R600a, R1234yf, R134a, coefficient of performance, solar cooling, renewable energy, seasonal performance, passive cooling