## 3 Results  
Figure 7 depicts the variation of the cavity surface temperature along the cavity tube for the three investigated cavity receivers using thermal oil and a dish diameter of 1.9 m (see Table 1). Results show that the surface temperature of the hemispherical cavity receiver is the highest compared to the other cavity receivers. This is because of the higher heat flux intensity on the hemispherical cavity receiver (see Figure 8 as generated with SolTrace when \(I_{sun}\) is set to 1000 W/m\({}^{2}\)). Note that the surface temperature data for all of the investigated cavity receivers in Figure 7 compares well with the cavity heat flux data in Figure 8. The presented results in Figure 7 can be compared with the reported results by Refs. [17; 18] for rectangular and cylindrical cavity receiver as solar dish absorber, respectively. In this research, variation of cavity surface temperature was presented and compared for three shapes of cavity receivers including hemispherical, cubical, and cylindrical cavities as a new achievement. The heat flux (Figure 8) in the hemispherical cavity receiver is highest in the central element of the cavity tube where most of the solar rays intersect with the receiver. Note that, for the cylindrical cavity receiver, there is a sudden increase in heat flux at the 15 th element. This is due to high amounts of concentrated solar irradiance at the top surface of the cylindrical cavity receiver where the 15 th to 21 st  
Figure 6: Deviation between experimental and numerical thermal efficiency results for the hemispherical cavity receiver. Figure 7: Variation of the cavity surface temperature along the cavity tube length with pure thermal oil for weather conditions of 20 October 2016, Tehran, Iran. elements are located. This sudden increase is also observed in the 53 rd to 64 th elements of the cubical cavity receiver. These elements, located at the top wall of the cubical cavity, absorb higher amounts of solar heat flux compared to the cavity elements located at the side walls of the cubical cavity receiver. Smaller heat flux spikes are also observed on the side wall elements. This is due to the tracking error of \(1^{\circ}\) for the investigated solar dish concentrator. The intensity of the solar heat flux is, therefore, higher at two cavity side walls of the cubical cavity receiver compared to the other two side walls. Consequently, this causes a sudden increase or decrease in heat flux from the 1 st to the 52 nd element. Similar results were presented by Refs. [17; 18] for the rectangular and cylindrical cavity receivers. In the current study, a comparison of heat flux distribution was presented for three investigated cavity receiver including hemispherical, cubical, and cylindrical cavities as a new achievement. A similar pattern is observed for the cavity surface temperatures in Figure 7. The net heat transfer rate along the length of the three investigated cavity receivers is displayed in Figure 9. Note that the net heat transfer rate per element is the highest for the hemispherical cavity receiver. This is because of the high heat flux rate on the hemispherical cavity receiver, as discussed in the previous paragraph. Figure 9 depicts variation of the net heat transfer rate along the cavity receivers using weather conditions of 20 th October 2016, Tehran, Iran. As observed from Figure 10, the maximum solar power  
Figure 8: Variation of heat flux rate along the cavity tube in the cavity receivers with pure thermal oil, when the solar beam irradiance is 1000 W/m\({}^{2}\) (for \(\eta_{\mathit{eff}}=100\%\)). Figure 9: Variation of the net heat transfer rate along the cavity receivers with pure thermal oil using weather conditions of 20 October 2016, Tehran, Iran. is at the 7 th element in the hemispherical cavity receiver. Furthermore, variation of available solar power along the cavity tubes for different investigated cavity receives is presented in Figure 10 based on the calculated results by the SolTrace software.