A research group in China has proposed a near-zero-energy retrofit strategy for traditional dwellings on the western Sichuan Plateau, combining rooftop PV with an air-source heat pump.

The researchers said the region’s high altitude, low ambient temperatures and strong solar radiation make it suitable for the approach, while its traditional homes typically suffer from high energy consumption and inadequate thermal comfort during the winter heating season.

“These buildings embody historical and cultural values and constitute an important component of rural architectural heritage,” the scientists said. “Most of these dwellings adopt stone–wood structural systems with thick walls but insufficient thermal insulation performance, resulting in poor indoor thermal environments during winter. Heating is generally provided by burning firewood or coal, which not only exhibits low energy efficiency but also causes air pollution and ecological pressure.”

The researchers selected a traditional dwelling in Zhibo Village, Maerkang, as a case-study building. The three-story stone-and-wood house has a total floor area of approximately 399 m², a height of 9.6 meters, 400-mm-thick walls and a slate-covered roof. Its window-to-wall ratio is 11% on the southern facade and 8% on the western facade.

Maerkang has a high-altitude continental monsoon climate, with long, cold winters and more than 120 frost days per year. Average January temperatures range from -6 C to -3 C, while nighttime temperatures can fall below -10 C. The area receives more than 2,000 hours of sunshine and approximately 3,700 MJ/m² of solar radiation per year.

The researchers recreated the case-study building in AutoCAD and SketchUp before importing the 3D model into Sware to simulate its indoor thermal environment and heating demand. They set the indoor target temperature at 16 C during the heating season and the ventilation rate at 0.5 air changes per hour. They then conducted hourly simulations to calculate annual energy consumption and winter heating loads.

The scientists converted the calculated heating load into the electricity required by a wall-mounted, low-temperature air-source heat pump. They assumed a seasonal coefficient of performance (COP) of 2.54, with the heat pump capable of supplying water at temperatures above 45 C when the ambient temperature is -15 C.

They then entered the calculated electricity demand into PVsyst to optimize the PV array’s orientation, tilt angle and size. The proposed system features 61 m² of 745 W heterojunction (HJT) bifacial, double-glass solar modules.

The simulations showed total heating demand of 15,670.9 kWh over the winter heating season, corresponding to electricity consumption of 6,162.2 kWh for the heat pump.

The researchers found that a 40-degree tilt angle was optimal for achieving zero-energy heating. They determined that the optimal rooftop PV installation area was 61 m², equivalent to 15.2% of the building’s total roof area. “By utilizing 210 mm heterojunction (HJT) bifacial double-glass modules with a conversion efficiency of up to 24%, an installation area of only 61 m² is required. This accounts for just 95.2% of the total available roof area, demonstrating that near-zero energy operation can be achieved without excessive modification to the traditional building envelope,” they emphasized.

“The total cost of the proposed system (including modules and installation) is approximately CNY 29,800 ($4,437), which represents only 19.2% of the local rural household’s annual disposable income,” the researchers added. “With an investment payback period of approximately 14 years (based on local electricity prices of CNY 0.52/kWh and annual savings of CNY 3,624), the scheme proves to be financially accessible for residents.”

The article “Innovative photovoltaic heating for ethnic traditional dwellings towards near-zero energy heating on Tibetan plateau” was presented in Case Studies in Thermal Engineering. Researchers from China’s Southwest Minzu University, Sichuan University, and Tongji University have participated in the research. “The results of this study provide useful methodological guidance and preliminary design references for the promotion of near-zero energy heating in similar rural contexts, while acknowledging that site-specific adjustments and further validation are necessary before broad application,” the academics concluded.