Seasonal performance comparison of four multi-function double-layer CdTe ventilated windows in different climates

Windows play a critical role in building thermal performance and energy efficiency. An outer PV + PCM double-layer ventilated window (OPVPCM-VW) integrating heating insulation with ventilation cooling was developed in our previous study. But direct exposure of PV + PCM to cold outdoor environments causes significant heat loss from daytime PV heat and nighttime PCM latent heat, severely undermining thermal efficiency in cold weather. Thus, an internally placed structure (IPVPCM-VW) is proposed, which employs a clear glass layer and an intermediate air gap to insulate the PV and PCM from heat loss to cold outdoor environments. Comparative analysis reveals a much lower thermal efficiency for OPVPCM-VW (12.03%) versus IPVPCM-VW (27.95%), attributed to the latter's enhanced thermal resistance to the outdoors. Although OPVPCM-VW has a slightly higher electrical efficiency of 7.79%, its PCM stays predominantly in the solid state due to huge heat loss under cold weather, while the critical role of PCM phase transition is proved by comparison with two additional systems without PCM: daytime melting and nighttime solidification effectively reduce peak PV temperatures and improve indoor thermal comfort. In Lhasa, where consistently favorable irradiation enables PCM melting in both systems, the thermal efficiency of OPVPCM-VW (23.10%) remains lower than that of IPVPCM-VW (26.16%). During the non-heating season, IPVPCM-VW may increase potential cooling demand in Hefei; however, for Lhasa's climate, it provides superior heat preservation during cold nights while maintaining comparable noon cooling performance to OPVPCM-VW. These findings provide guidance for the structural design of double-layer PV ventilated windows tailored to regional applicability.