Researchers in Iraq have paired a bio-based titanium dioxide and linseed oil coating with a heat pump, reaching 77.32 per cent combined efficiency and cutting panel temperatures by up to 34C.
A research group in Iraq has built a direct-expansion photovoltaic-thermal heat pump with a bio-based coating, as reported by pv magazine. The coating is 100 nanometres thick. It is made of 15 per cent titanium dioxide nanoparticles and 85 per cent linseed oil by mass. Both ingredients are cheap and widely available.
The idea is to attack panel heat from two directions at once. Solar panels lose electrical efficiency as they warm up. A coating helps passively, while a heat pump pulls heat away actively. What is new here is how those two interact across a full year of conditions.
How the system is built
The test rig paired a 1.0064 square metre monocrystalline module rated at 150W with a 120W vapour-compression heat pump. A roll-bond aluminium plate measuring 1.30m by 0.40m sat behind the panel and acted as the evaporator. It drew heat out of the module, and a water-cooled condenser turned that heat into hot water.
The refrigerant circuit also held a compressor, filter-drier, sight glass and capillary tube. That tube had an inner diameter of 0.787mm and an outer diameter of 1.803mm. So the whole system relies on standard, off-the-shelf parts.
Three refrigerants, four seasons
The team tested R134a, R600a and R1234yf, each with and without the coating. The three differ sharply in climate impact and safety. R134a is a non-flammable hydrofluorocarbon with a relatively high global warming potential. R600a, or isobutane, has a low global warming potential but is highly flammable. R1234yf is a hydrofluoroolefin with a very low global warming potential and mild flammability.
Testing ran outdoors in Kirkuk, in northern Iraq, across four months of 2025: January, March, August and October. The researchers took hourly readings from 07:00 to 17:00 over three clear-sky days per condition. R600a performed best in spring, summer and autumn, while R1234yf led in winter.
What the numbers show
Adding the coating raised combined efficiency in every season and with every refrigerant. “The best seasonal cases were 64.93% overall efficiency in winter with R1234yf, and 76.59%, 71.51% and 77.32% in spring, summer and autumn with R600a, respectively,” the researchers said.
The heat pump’s coefficient of performance peaked near solar noon. It reached 2.65 in winter with R1234yf, then 3.78, 4.27 and 3.85 in spring, summer and autumn with R600a.
Panel temperatures fell sharply against the baseline. Peak surface temperature dropped by 17C in winter, 19C in spring, 34C in summer and 24C in autumn. Those cooler operating temperatures explain the gains in electrical output.
Why the low-tech part is interesting
Coatings and heat pumps are both established. What the team claims is a practical combination that needs no change to manufacturing. “These results indicate the potential practical feasibility of the proposed PVT-HP system by relying on commercially available components with a low-cost, easy-to-apply bio-nano coating without complex manufacturing modifications,” the group concluded.
They added that the gains are largest in hot, high-radiation conditions. Kirkuk is an obvious test bed for that. Whether the coating holds up over years of dust, heat and ultraviolet exposure is a question the paper does not answer.
The work appears in Results in Engineering, under the title “Performance enhancement of photovoltaic/thermal systems with heat pump units using alternative refrigerants and nanoparticle TiO2 with linseed oil cooling coating”. The authors come from Iraq’s Northern Technical University and Al-Kitab University.
