Numerical and experimental analysis of heat transfer modes in various coil tube configurations of solar cavity receivers

• A novel two-zone solar cavity receiver with helical + spiral coil configuration is proposed. • Experimental and CFD analysis conducted under varying solar energy inputs (7–14 kW). • Novel receiver achieved 69% efficiency, surpassing basic helical model (∼55%) at 7 kW. • Enhanced convective (21%) and radiative (16%) heat transfer modes in novel design. • At higher flux (14 kW), spiral coils caused a pressure drop, reducing thermal efficiency. An experimental and Computational Fluid Dynamics (CFD) analysis was conducted to evaluate the modes of heat transfer (HT) and their effects on energy efficiency for different solar energy inputs on cylindrical cavity receiver models with two distinct coil tube configurations. The cavity absorber surface absorbs the high-intensity heat flux from the dish reflector, which is then conducted to the cylindrically coiled tube surfaces of the receiver. The basic model features a helical-coil tube configuration that serves as a single-zone heat exchanger throughout its length. In contrast, the novel model introduces a two-zone heat exchanger design. The second half employs two spiral coils in a parallel, aiming to enhance convective and radiative HT. In the first phase of this research, both receiver models were tested with a 12 m² dish reflector. The HT and energy efficiency results for the two models were compared under a steady-state average solar energy input of 7 kW, based on collected experimental data. The novel receiver model achieved a maximum energy efficiency of 68.98%, compared to 54.74% for the basic model, with enhancements of 21% and 16% for convective and radiative HT modes, respectively. However, the conductive mode of HT in the novel model was 13% lower than that of the basic single-zone heat exchanger. In the second phase, CFD analysis compared the HT and thermal performance of both receiver models for average solar energy inputs of 7 kW and 14 kW to evaluate performance under higher operating conditions. The CFD results for the 14 kW input indicated that the basic receiver model performed better than the novel model in terms of HT enhancements and energy efficiencies, achieving a 70.6% efficiency.