# Solar Hybrid Micro Gas Turbine Based on Turbocharger
## 2 Design, Operation, and Performance Aspects  
### Design and Operation Aspects  
A solar MGT based on a turbocharger (MGT-T/C) is materialized using a turbocharger, a receiver, inlet and outlet ducts, a generator, and a burner in the case of the hybrid configuration. The generator is coupled on the T/C shaft on the compressor side. A high-speed generator, equipped with the relevant electronics for connection to the network is envisaged. Alternatively, a gear box can be used to match (reduce) high turbocharger speed with that of a generator, if the two rotational speeds cannot be directly matched. In the solar-only configuration operation exploits only the available solar energy, while in hybrid configuration, solar energy substitutes (partially or entirely) the fossil fuel energy. Solar energy is captured by the receiver, which heats the air after it exits the compressor. Solar beams are concentrated on the receiver by a heliostat field. The entire set up of MGT-T/C is considered to be placed at the top of the tower, instead of being placed on the ground, to avoid long piping to connect the engine with the receiver. The layout of the described configurations is depicted in Figure 1.  
Figure 1: Solar microturbines based on a turbocharger MGT-T/C configuration. A commercial, widely used turbocharger (SJ180 by Fuyuan turbo) has been selected for setting up the configurations that will be studied. It is employed in ship generator 600 kW diesel engines. Design specifications for the T/C are summarized in Table 1.  
Ducts, and receiver and heating elements (burner) are assumed to add a pressure drop of 2% each at the design point [10; 13]. The shaft/generator is assumed to have efficiency of 98% [24; 25]. Design and manufacturing of combustion chamber for use with T/C has already been demonstrated, achieving a turbine inlet temperature of about 800 \({}^{\circ}\)C [26; 27]. The heliostat field consists of mirrors with 64 m\({}^{2}\) aperture [28] with reflectivity of 97% [28]. The number of mirrors in this study is a design parameter, which should be chosen for minimizing electricity cost. Description of modelling of the heliostat field is included in the solar components modeling tool produced by the authors [20]. For given turbomachinery components, engine performance depends mainly on turbine inlet temperature (TIT). It is well known that the higher the TIT, the better the engine performance. Increasing TIT however, makes necessary the use of more expensive materials. Material properties for the components employed, the turbine in particular, put limits as to how high the TIT can be. In view of these opposing trends, and in order to produce information that can be useful in choosing a design TIT, simulation of the performance of the studied MGT-T/C is carried out for different TIT values. For the design specifications of Table 1, power and thermal efficiency are evaluated and depicted in Figure 2. Power increases with TIT, while thermal efficiency achieves a maximum for TIT of 850 \({}^{\circ}\)C. Here, three cases of turbine inlet temperature are studied: (a) 400 \({}^{\circ}\)C, because it is the nominal inlet turbine temperature of the T/C; (b) 800 \({}^{\circ}\)C, because it is the maximum one experimentally demonstrated [26; 27] (while we are not sure if it is in within the limits of the specific T/C used in this work); and (c) 600 \({}^{\circ}\)C, the intermediate of the two in order to establish a performance trend. The specifications of solar and other components of the MGT-T/C are summarized in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Air mass flow [kg/s] & 1.49 \\ Compressor isentropic efficiency & 0.85 \\ Turbine isentropic efficiency & 0.83 \\ Pressure Ratio & 3.69 \\ Speed [RPM] & 52,000 \\ Turbine Inlet Temperature (TIT) [\({}^{\circ}\)C] & 400 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Turbocharger design specifications. Figure 2: MGT-T/C power and thermal efficiency in function of turbine inlet temperature. TIT—turbine inlet temperature.