The stakes are considerable. Global energy demand is projected to rise by roughly 48 percent by 2040, and heating and cooling remain stubbornly dependent on fossil fuels. Renewable energy currently supplies only about 23 percent of the heating and cooling sector, a share that lags far behind the rapid gains made in renewable electricity. Heat pumps, which move heat rather than generate it, are widely seen as the most promising bridge technology, but their efficiency depends intimately on the temperature of the reservoir they tap. Air source heat pumps extract thermal energy from ambient air, which is easy to install but fluctuates wildly with the seasons. Ground source systems bury a heat exchanger in the soil, where temperatures remain nearly constant year-round, but they demand drilling, land, and higher upfront capital.

The research team, led by Mustafa Jaradat with Omar Alsotary, Ohoud Aljaloudi, and Madhar Haddad, anchored their analysis in real measurements rather than generic design assumptions. They drilled a ten-meter borehole at the Amman site, lined it with PVC pipe, and lowered a copper-tube heat exchanger into the ground. A UNI-T UT330-USB data logger, rated to ±0.5 °C accuracy and protected against water and dust ingress, recorded ground temperature continuously from December 2021 to December 2022. Over that full annual cycle, the soil temperature at the borehole base stayed within a remarkably narrow band of 21.7 to 22.8 °C, confirming the thermal stability that makes ground source systems attractive in the first place.

To probe how much heat the ground could actually deliver, the team circulated tap water through a two-meter copper coil, manually rolled into a spiral and suspended ten meters below the surface. Water entered the ground loop at roughly 13 to 15 °C and emerged at up to 18.7 °C, a gain of about 3.7 °C that the researchers used in a sensible-heat energy balance to calculate the ground-side heat transfer rate. A short circulation test showed the soil temperature dropping from 22.1 to 21.7 °C as heat was extracted, corresponding to a daily degradation rate of 0.088 °C per day. Crucially, the authors caution that this short-cycle rate describes only the instantaneous response of the coil during active circulation; the full-year record shows the undisturbed ground swings far less than a naive extrapolation would suggest.

On the modeling side, the team built a first-order thermodynamic model of a vapor-compression heat pump cycle, defining four key states: saturated vapor at the compressor inlet, superheated vapor at the compressor outlet, saturated liquid at the condenser outlet, and a saturated mixture after the expansion valve. The model held the indoor setpoint at 24 °C and 38 percent relative humidity, with the compressor outlet fixed at 70 °C and 1866 kPa for the R410A case. A compressor isentropic efficiency of 0.75 was applied uniformly across all three refrigerants to keep the comparison fair. The expansion valve was treated as an isenthalpic throttling device, analyzed through the Joule-Thomson coefficient, which governs how much the refrigerant cools as its pressure drops.

Three refrigerants were put through this framework: R410A, the workhorse of modern residential heat pumps with a global warming potential of 2088; R134A, an older benchmark refrigerant with a GWP of 1430; and R513A, a lower-GWP blend at 631 that is positioned as a near-drop-in alternative to R410A. The inclusion of R513A is what gives the study its contemporary edge, because international agreements such as the Kigali Amendment to the Montreal Protocol and the EU F-Gas Regulation are phasing down high-GWP hydrofluorocarbons. The question is whether a greener refrigerant can match the thermodynamic performance of the incumbent, or whether efficiency and climate impact remain locked in a trade-off.

The answer, at least for the air source configuration, is that R410A still wins on raw efficiency. Modeled against the full October-to-March weather record for Amman, the R410A air source heat pump achieved an average coefficient of performance of 4.97, with hourly values ranging from 4.22 to 6.20. R513A came in at an average COP of 3.49, ranging from 2.97 to 4.50, while R134A trailed at 3.13 on average. Hourly compressor power told the same story: R410A consumed the least, between 0.79 and 1.05 kWh, R513A sat in the middle at 0.82 to 1.27 kWh, and R134A demanded the most at 0.91 to 1.44 kWh. R410A also absorbed the most heat at the evaporator, peaking at 4.14 kWh, compared with 2.86 kWh for R513A and 2.79 kWh for R134A.

The ground source system, by contrast, drew its evaporator heat from a buffer tank of water held at a steady 18 °C, fed by the buried heat exchanger. The model assumed the refrigerant left the evaporator as saturated vapor at 15 °C and 1250.8 kPa, conditions tied directly to the measured buffer-tank temperature rather than an assumed design margin. Scaled so that its delivered heating capacity met the same one-ton target used to size the air source unit, the ground source heat pump achieved a COP of 4.79, delivering 3.51 kW of heat with a compressor power of 0.73 kW and an evaporator load of 2.78 kW. Its annual electricity consumption came to approximately 3203 kWh, lower than any of the three air source configurations, which ranged from 4095 kWh for R410A to 5283 kWh for R134A.

But the authors are careful not to declare an unconditional winner. The ground source system’s lower absolute consumption partly reflects its smaller delivered capacity, 3.51 kW versus roughly 4.65 kW for the air source unit, rather than efficiency alone. Per unit of heat actually delivered, the air source heat pump running R410A retains a marginally higher COP of 4.97 compared with the ground source system’s 4.79. The ground source advantage lies elsewhere: its hour-to-hour energy consumption is remarkably steady because the underground temperature barely moves, whereas the air source system’s output fluctuates with every cold snap. The researchers also quantified their measurement uncertainty, noting that the ±0.5 °C sensor accuracy propagates to roughly ±19 percent on the 3.7 °C temperature difference used in the ground-side heat calculation, a band too narrow to explain the 28 percent gap in annual electricity between the two systems.

The study’s broader message is that refrigerant selection and heat source selection are not independent choices. R410A delivers the strongest thermodynamic performance in both architectures but carries the heaviest climate burden, while R513A offers a meaningful reduction in global warming potential at a measurable cost in efficiency. R134A, the legacy option, performs worst on both counts in this comparison. For homeowners and policymakers, the practical takeaway is that the optimal configuration depends on priorities: if minimizing electricity per unit of heat is the goal, the air source heat pump with R410A leads among the modeled options; if steady output and lower absolute consumption at a smaller capacity are preferred, and if drilling costs and land availability permit, the ground source system with R410A remains a defensible choice. As heat pump adoption accelerates worldwide, studies like this one, grounded in a full year of site-specific measurements rather than generic assumptions, will be essential for guiding that transition in the climates where it matters most.

Subject of Research: Comparative experimental and modeling analysis of air source and ground source heat pumps using R410A, R134A, and R513A refrigerants

Article Title: Experimental and comparative analysis of air source and ground source heat pumps: refrigerant performance and energy efficiency

Article References: Jaradat, M., Alsotary, O., Aljaloudi, O., & Haddad, M. (2026). Experimental and comparative analysis of air source and ground source heat pumps: refrigerant performance and energy efficiency. Results in Engineering, 32, Article 113582. https://doi.org/10.1016/j.rineng.2026.113582

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113582

Keywords: heat pumps, ground source heat pump, air source heat pump, R410A, R513A, R134A, coefficient of performance, refrigerants, geothermal energy, energy efficiency, Amman, heating and cooling

The team prepared the nanorefrigerant by first dispersing aluminium oxide nanoparticles in HFE-7000 through mechanical stirring and ultrasonication, then adding Tween-80 as a surfactant to stabilize the suspension. The two-step approach produced a nanorefrigerant that remained stable for 72 hours without visible sedimentation, a critical benchmark for practical refrigerant applications.

Testing followed American Society of Heating, Refrigerating and Air-Conditioning Engineers standards for refrigerator performance measurement, covering evaporator and condenser temperatures, compressor power consumption, and cooling capacity. The researchers varied nanoparticle loading from 0.01 to 0.1 percent by volume and measured performance across a range of operating conditions.

At 0.1 percent loading, the coefficient of performance rose by 18.4 percent compared with pure HFE-7000, while compressor energy consumption fell by 15.2 percent. Evaporator air temperature dropped by 6.8 percent, and cooling capacity increased by 12.4 percent, confirming that the nanoparticle suspension enhanced heat transfer on both the evaporator and condenser sides of the system.

The improvements stem from the higher thermal conductivity of the nanorefrigerant, which rose by 21.4 percent at 0.1 percent loading, and from changes in boiling behavior inside the evaporator. The nanoparticles promote nucleate boiling, increasing bubble frequency and improving the heat transfer coefficient during phase change.

Performance gains peaked at 0.1 percent loading and declined at higher concentrations. At 0.2 percent, the coefficient of performance fell below the peak as particle agglomeration increased fluid viscosity and raised pumping losses inside the compressor. The researchers identified this concentration as the optimal balance between heat transfer enhancement and flow resistance.

Tween-80 proved essential to the nanorefrigerant’s stability. Without the surfactant, nanoparticles settled within hours, degrading heat transfer performance. With it, the suspension remained uniformly dispersed, allowing the nanoparticles to circulate continuously through the refrigeration loop and maintain consistent performance across the test duration.

The study also examined compressor discharge temperature, which decreased by 4.2 percent with the nanorefrigerant, reducing thermal stress on the compressor and potentially extending its service life. Lower discharge temperatures also suggest reduced refrigerant decomposition risk inside the compressor.

The researchers conclude that aluminium oxide nanoparticles in HFE-7000 with Tween-80 offer a practical route to more energy-efficient domestic refrigeration, provided nanoparticle loading remains near the 0.1 percent optimum and surfactant stabilization is maintained throughout the system’s service life.

The nanoparticles at the heart of this study were produced through a citrate sol-gel route, a wet-chemistry technique favored for producing oxide powders with tight control over particle size and purity. Aluminium nitrate nonahydrate and citric acid, each at 0.6 molar concentration, were dissolved in distilled water and combined drop by drop while stirring at 80 degrees Celsius for eight hours. Raising the temperature to 200 degrees Celsius for three hours drove off the solvent and produced a fluffy polymeric citrate precursor, which was then ground and calcined in a muffle furnace at 800 degrees Celsius for three hours. The calcination step is what converts the organic-metallic precursor into the final crystalline oxide, and the furnace temperature strongly influences the resulting crystal structure and particle size.

Characterization confirmed the quality of the resulting powder. Powder X-ray diffraction verified high crystallinity, while field emission scanning electron microscopy and high-resolution transmission electron microscopy revealed a spherical morphology with an average particle diameter of roughly 22.5 nanometers. Energy-dispersive X-ray spectroscopy confirmed the elemental composition, and Brunauer-Emmett-Teller analysis measured a specific surface area of 26.2 square meters per gram with a mesoporous structure. That combination of small particle size and high surface area matters for refrigeration applications because heat transfer between the particles and the surrounding fluid scales with exposed surface, so finer particles with porous structures can exchange thermal energy more effectively per unit mass than coarser powders.

The choice of HFE-7000 as the base fluid reflects both safety and environmental considerations. Hydrofluoroethers of this class are odorless, non-flammable, and non-explosive, and they carry low global warming potential together with zero ozone depletion potential, positioning them as alternatives to conventional hydrofluorocarbon refrigerants that face tightening regulation. HFE-7000 also has a low boiling point and a low latent heat of phase change, which the researchers note reduces the heat input needed to induce boiling and limits heat losses in the test apparatus. These properties make the fluid well suited to bench-scale vapor compression rigs, though its thermodynamic characteristics differ from the refrigerants used in most commercial household units.

Dispersion stability is the central practical challenge for any nanorefrigerant. Nanoparticles suspended in a liquid experience strong van der Waals attractions that drive them to clump and settle, and refrigerant fluids, being notably low-polarity, offer little natural resistance to this agglomeration. The two-stage sonication protocol used here addresses the problem directly: an initial 60-minute ultrasonic bath breaks apart pre-existing agglomerates, and a second 30-minute sonication after Tween-80 addition distributes the surfactant evenly so it can coat the particle surfaces and provide steric stabilization. Surfactants of this type adsorb on oxide surfaces and create a protective barrier that keeps particles separated, which is why the treated suspensions resisted sedimentation over the observation period while untreated mixtures degraded quickly.

The concentration range explored in the work spans five formulations, and the behavior across that range illustrates a well-known trade-off in nanofluid engineering. At low loadings, added particles increase the effective thermal conductivity of the working fluid and provide additional nucleation sites for boiling, both of which improve heat exchange in the evaporator and condenser. As loading rises, however, particle-particle interactions increase the fluid’s viscosity, which raises the work the compressor must do to circulate the refrigerant and can promote agglomeration that negates the thermal benefits. The study identified an optimal window between 1.2 and 1.4 percent by weight, beyond which these penalties begin to outweigh the gains. The precise optimum in any given system depends on the balance between conductivity enhancement, boiling dynamics, and viscous losses, which vary with compressor design and operating temperatures.

The performance metrics reported in the study connect directly to the physics of the vapor compression cycle. An improved coefficient of performance means the system delivers more cooling per unit of electrical input, which translates into lower household energy consumption for the same refrigeration duty. Reduced discharge pressure and a lower compression ratio ease the mechanical load on the compressor, the component that typically dominates both the energy budget and the failure statistics of domestic refrigerators. Enhanced subcooling of the liquid refrigerant before it enters the expansion device increases the fraction of the evaporator length available for useful phase-change heat absorption, squeezing more cooling capacity from the same hardware.

Environmental context strengthens the case for this line of research. Domestic refrigerators run continuously, and their cumulative electricity demand is substantial on national grids, so even single-digit percentage efficiency improvements aggregate into meaningful energy and emissions savings when multiplied across hundreds of millions of appliances. Pairing a low-global-warming-potential base fluid with efficiency-boosting nanoparticles addresses both sides of the environmental ledger: the refrigerant itself contributes less to climate forcing if released, and the improved efficiency reduces the indirect emissions associated with electricity generation.

Several questions remain before such nanorefrigerants can move from laboratory rigs to commercial products. Long-term stability must be demonstrated over months and years of continuous circulation, not merely the hours observed in bench testing, since surfactants can degrade or desorb over time and particles can slowly deposit on heat exchanger surfaces or inside compressor components. The interaction between nanoparticles and compressor lubricants, the potential for particle abrasion of moving parts, and the manufacturability of consistently dosed nanorefrigerant charges all require further study. Cost considerations also matter, since the synthesis route described here is laboratory-scale and scaling citrate sol-gel production to industrial volumes would need its own economic analysis. Nevertheless, the demonstration that a carefully characterized, surfactant-stabilized alumina suspension can improve coefficient of performance while reducing compressor stress marks a concrete step toward refrigeration systems that cool more effectively while consuming less energy.

Subject of Research: Nanorefrigerant research

Article Title: Synthesis, characterization, and performance evaluation of Al2O3 nanoparticles in HFE7000 refrigerant for domestic refrigeration

Article References: Digra, M., & Singh, J. (2025). Synthesis, characterization, and performance evaluation of Al2O3 nanoparticles in HFE7000 refrigerant for domestic refrigeration. Frontiers in Thermal Engineering, 5, Article 1682295. https://doi.org/10.3389/fther.2025.1682295

Image Credits: AI Generated

DOI: 10.3389/fther.2025.1682295

Keywords: nanorefrigerant, HFE-7000, aluminium oxide nanoparticles, domestic refrigeration, coefficient of performance, Tween-80, thermal conductivity, energy efficiency, vapour compression system, nanofluid stability, sustainable cooling, heat transfer enhancement

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