# Solar Hybrid Micro Gas Turbine Based on Turbocharger
## 1 Introduction  
The first solar thermal power plant (STPP) built mainly uses troughs and oil as heat transfer fluid (HTF), materializing a Rankine cycle [1]. While direct steam generation from troughs as well as the use of other materials as HTF is still under investigation today [2; 3; 4], steam generation using a tower receiver is already in commercial state either with the use of molten salts [5] or directly in the receiver [6]. Solar thermal power in the Brayton cycle is used to preheat the air before it enters the combustion chamber, in order to lower the needed amount of fuel. A heliostat field harnesses the Sun beams and reflects them to a receiver, which is placed at the top of a tower. The compressed air is fed into the receiver to be heated, before its entrance into the combustion chamber. Several European Union (EU) funded projects [7; 8; 9] and research works study this option [10; 11; 12; 13]. This technology is in experimental stage [7; 8; 9].  
The main difficulties encountered in the concept of this hybrid Brayton cycle are related to the combustion chamber and the extraction of compressed air fed into the receiver. Higher thermal stresses occur in the combustion chamber as a result of the higher inlet air temperature. Furthermore, combustion stability problems may occur as a result of the wide fuel flow variation and high inlet air temperature. Such difficulties have been studied by several groups [7; 8; 9]. The in-line arrangement of modern gas turbine components makes redesign of the engine obligatory in order to permit the extraction of air to be fed to the receiver [8; 9; 14].  
Finally, in economic terms, installations with high power output are favored [15], but the use of small gas turbines or microturbines (MGTs) in small scale plants (kW to MW scale) is recognized to offer advantages in order to achieve lower investment cost [16].  
The main problem with STPP today is that investment cost is high, resulting to high levelized electricity cost (LEC) that makes them far from competitive with other renewable technologies. The factthat mass production would be necessary in order to reduce the capital cost of CSP plants has been recognized and addressed in recent publications [17]. The idea behind the present work is to investigate if STPPs can be materialized by using low-cost components, taking advantage of existing mature technologies in other fields. The possibility of using a turbocharger (T/C) in the solar hybrid MGT scheme and its performance is investigated. Turbochargers for reciprocating engines are mass-produced, their cost being thus comparatively lower than other turbo-components, because of economies of scale. Furthermore, the use of a turbocharger has some advantages such as easy air extraction to feed the receiver.  
Both solar-only and hybrid schemes are studied, while their performance is assessed for three maximum cycle temperatures. For every scheme, an optimum rotational speed schedule is determined and used. The effect of heliostat field size on performance (produced energy and electricity cost) is investigated, in order to identify the one that leads to the lower LEC.  
The tool used to carry out the engine performance studies is PROOSIS [18] utilizing TURBO [19] and SOLAR [20] libraries. It is noted that the performance of compressor and turbine components is represented by suitable performance maps, interrelating speed, mass flow rate pressure ratio and efficiency, as is usually done for this kind of modelling. The maps for the studies presented in this paper have been produced using the software TurboGENIUS [21], which is based on existing widely accepted methods [22; 23].