# Solar Hybrid Micro Gas Turbine Based on Turbocharger
## 4 Discussion  
As shown above, for a given TIT, there is an optimum number of mirrors, for which LEC is minimized. The optimum LEC and the corresponding produced energy and number of mirrors in function of TIT for solar-only and hybrid configurations are summarized in Figures 9 and 10, respectively. The performance in the studied range (400-800 \({}^{\circ}\)C) is depicted with the solid line, while the dotted line depicts the performance of the studied configurations if they could operate with higher turbine inlet temperature. For the solar-only configuration, it is observed that the produced power increases with the size of heliostat field. This happens because of the fact that the given thermal power is only from the Sun, and thus bigger heliostat fields result in higher amount of given thermal power. Furthermore, increase of the maximum TIT is favorable because it results in an increased efficiency. Increase of the TIT results in energy increase and LEC decrease, but with declining rates (Figure 9) caused by the declining rate of thermal efficiency augmentation (Figure 2). The comparison of solar-only and hybrid configurations shows that the solar-only configuration is more favorable because it demonstrates lower LEC due to the relative low fuel thermal efficiency of the hybrid configurations. However, this configuration has the disadvantage that the produced power is dictated by the available solar power, while in the hybrid configuration, produced power is available at any time. This disadvantage could be overcome by using energy storage configurations (e.g., batteries). Hybrid configuration could be set to operate as solar-only and set to hybrid/fuel operation only when is mandatory. Figure 10: Optimum LEC value and the corresponding produced energy and number of mirrors for each TIT (hybrid configuration). Figure 9: Optimum LEC value and the corresponding produced energy and number of mirrors for each TIT (solar-only configuration). As mentioned above, fuel cost value can vary, and thus the optimum LEC obtained for the hybrid configurations may alter. In order to examine the effect of fuel cost on the performance of hybrid configuration, the optimum LEC was obtained for three fuel cost values: the two extreme ones for the period of 2012-2017 mentioned earlier and an intermediate one, the one of Table 3. The values obtained together with the one for the solar-only configuration are shown in Figure 11. Fuel cost has a strong impact on the economic performance of the plant. While solar-only LEC is altogether lower than that for hybrid, for \"medium\" and \"high\" fuel cost, the trend reverses for \"low\" fuel cost. Furthermore, it was found that for higher cost, the optimum LEC is obtained with a higher number of mirrors, as can be seen in Figure 12.  
The results presented here show that such a configuration may be promising, as economic results for solar hybrid MGT of 100 kW show that the LECs vary in the range of 250-700 $/MWh (according to the chosen set up) [10], while the LEC of a typical diesel generator is about 500 $/MWh [10]. We saw that in the solar-only configuration the heliostat field size has stronger effect on produced energy than on the LEC. In other words, a bigger heliostat field will result in increased produced energy with a relative low penalty on LEC. Deviations of produced power and LEC from the values for minimum LEC, as the result of augmenting the heliostat field size, are shown in Figure 13. A significant  
Figure 11: Hybrid configuration optimum LEC value for three fuel cost values. Figure 12: Corresponding number of mirrors for optimum LEC value for three fuel cost values (hybrid configuration). increase of energy production can be achieved with a relatively small increase of LEC (e.g., a 9.5% increase in energy is achieved with 1.8% increase of LEC).