Experimental and numerical investigations on heating performance of a series-connected double-layer pipe-embedded wall for ultra-low-temperature heating

To achieve near-zero-energy heating, a series-connected double-layer pipe-embedded wall (S-DPEW) has been proposed as an ultra-low-temperature heating terminal. However, how this concept can be practically implemented, including the appropriate flow configuration and wall design, remains insufficiently studied. Since different configurations and operating conditions need to be compared, a rapid and reliable calculation method is required to support such design-oriented analyses. Therefore, a simplified calculation method for S-DPEW is proposed in this study, and an experimental platform is established for model validation. Based on the validated model, representative flow configurations are compared, and the effects of inlet water temperature, water velocity, insulation thickness distribution, wall width, and outdoor temperature are analyzed. The results show that the simulated intermediate and outlet water temperatures agree with experiments within ±1.0 °C, and the maximum heat flux error is -7.3%. Additionally, parametric results further indicate that inlet water temperature, water velocity, insulation distribution, and wall width mainly affect heating capacity, temperature uniformity and pumping power, outdoor heat loss, and longitudinal water temperature decay, respectively. For representative wall-terminal applications, 60/15–45/30 mm external/internal insulation thickness, 3–4 m wall width, and 0.2 m/s water velocity are recommended under the investigated conditions, which could achieve an effective heating benefit of 55.7–58.3 W/m², an energy utilization rate of 66.9%–73.9%, and a COP of 36.4–46.0. Therefore, this study provides a practical tool and preliminary design reference for S-DPEW-based ultra-low-temperature near-zero-energy heating systems driven by natural energy or waste heat.