# Biaxial-Type Concentrated Solar Tracking System with a Fresnel Lens for Solar-Thermal Applications
## 3 Results and Discussions  
Figure 3 shows the temperature profile of the heating head with solar tracking. In Figure 3a, with light focused on the bottom of the heating head, the temperature began to rise rapidly because the sunlight irradiation covered the largest area of the heating head. In this condition, the heat loss of the heating head is very small and it will cause a larger initial slope of temperature profile. As time goes on, the focal point began to shift away from the heating head and the slope of the curve became less steep. Soon after a re-tracking of the trace of the sun by the solar tracker, the temperature rose quickly again. Then the slope of the temperature curve became stable because of the balance of heat loss of the heating head and heating source from the sunlight and the final stable temperature can reach 718 \({}^{\circ}\)C. Comparing the temperature curves for both the upper and lower surfaces of the heating head, it can be seen that the higher temperatures are on the top of the heating head. In addition, since the largest areas of the top surface of the heating head are heated by the sunlight, due to the different heat dissipation rates at other regions of the heating head, there was a temperature difference between the top and the bottom surfaces. In Figure 3b, with light focused on the top of the heating head, the temperature rises slowly because there is less heating area for the sunlight. The heat loss area of the heating head is very large so that the stable temperature only reaches 523 \({}^{\circ}\)C. This is because the operating principle for the Stirling engines depends on the temperature difference between hot and cold ends of the engine to produce work. Therefore, since the goal of the current system is to obtain a maximum temperature difference, having focal point projects covering the whole heating head surface is a better method than being located exactly on top of the heating head surface. Therefore, thermal insulation cotton wrapped around the engine body to prevent heat dissipation is a good method. Moreover, the slope of the temperature curve also depends on the material of the heating head. Since thermal conductivity of the material affects the speed of temperature change, when a higher thermal conductivity material is used, temperature will increase quickly with sunlight heating. However, when the focal point is shifted, heat dissipates sooner and the temperature also drops faster. In summary, the control accuracy of the focal point position, the tracking time interval, the material selection of the heating head and the design of the heating head all affect the efficiency of the temperature rise. Figure 3: Heating temperature diagram with different focus location of the heating head. (**a**) Sunlight cover the heating head (**b**) Sunlight focused on top of the heating head. Although the theoretical efficiency of solar heating power with Stirling engine is higher than high-concentrating photovoltaic (HCPV), it may be limited by the accuracy requirement for the focus control and needs a frequently tracking operation to make sure the focus of solar power can be gathered on the engine heating head. Therefore, the accuracy of the solar tracking system is very important for solar heating application. In our experiment, because the thermal conductivity of copper is superior to steel, a copper heating head was applied to enhance the heating system. As shown in Figure 4, the temperature profile of the copper heating head shows the same trend as the solar irradiation measured directly from the sunlight by pyrheliometer. The heating head reaches its maximum temperature over 1000 \({}^{\circ}\)C. In the temperature profile as shown in Figure 4, the small temperature fluctuation is due to the shifting interval from the tracking system.