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