Design and fabrication of a separated-type heat recovery module for retrofitting commercial photovoltaic modules: a five-iteration prototyping approach

Conventional photovoltaic-thermal (PVT) collectors are built as monolithic units in which the thermal absorber is permanently laminated to the PV module, a structure that complicates manufacturing, prevents maintenance of individual components, and makes it impossible to upgrade the large stock of PV modules already installed in the field. This study addresses these barriers from a design-and-fabrication perspective by developing a separated-type heat recovery module that can be attached to the rear of commercial PV modules after installation. The core contribution is a manufacturing methodology: the absorber is produced by press forming and NoCoLok brazing, which reduces the workflow from ten to six stages and yields a self-contained 1 m 2 unit that can be combined to match standard PV module sizes. A five-iteration prototyping process was carried out, in which each iteration diagnosed the dominant loss mechanism of the previous one (contact thermal resistance, rear heat loss, and plate flatness) and fed the result back into the design. To demonstrate that the same module attaches to dissimilar PV platforms, it was bonded to a small building-integrated PV module (1.21 m 2 ) and to a standard glass-to-backsheet module (2.79 m 2 ) and tested under ISO 9806 and KS B 8297:2025. The final certified production unit (fifth iteration) achieved an average thermal output of 161.2 ± 17.5 W/m 2 (k = 2); the measured value satisfies the national pass/fail criterion for unglazed PVT (150 W/m 2 ) under the simple-acceptance decision rule applied in the certification scheme, while the 95 % expanded-uncertainty interval overlaps the threshold. Benchmarking against leading commercial collectors shows that the proposed module exhibits one of the highest initial thermal efficiencies among the compared unglazed PVT products. The results establish a reproducible fabrication route with demonstrated retrofit compatibility on two representative PV platforms, providing a practical pathway for converting existing PV installations into hybrid PVT systems.