Maximum overall efficiency for a solar-driven gas turbine power plant Susana Sánchez-Orgaz 1 , Alejandro Medina 2, * ,† and Antonio Calvo Hernández 2,3 1 Departamento de Física, Ingeniería y Radiología Médica, ETSII de Béjar, Universidad de Salamanca, 37700 Béjar, Salamanca, Spain 2 Departamento de Física Aplicada, Universidad de Salamanca, 37008 Salamanca, Spain 3 IUFFYM, Universidad de Salamanca, 37008 Salamanca, Spain SUMMARY A general model for an irreversible solar-driven Brayton multi-step heat engine is presented. The model incorporates an arbitrary number of turbines (N t ) and compressors (N c ) and the corresponding reheating and intercooling processes; thus, the solar-driven Ericsson cycle is a particular case where N t , N c !1. For the solar collector, we assume linear heat losses, and for the Brayton multi-step cycle, we consider irreversibilities arising from the non-ideal behavior of turbines and compressors, pressure drops in the heat input and heat release, heat leakage through the plant to the surroundings, and non-ideal couplings of the working fluid with the external heat reservoirs. We obtain the collector temperatures at which maximum overall efficiency  max is reached as a function of the thermal plant pressure ratio, and a detailed comparison for several plant configurations is given. This maximum efficiency is obtained in two cases: when only internal irreversibilities are considered and when both internal and external irrever- sibilities (which corresponds to the fully irreversible realistic situation) are simultaneously taken into account. Differences between both situations are stressed in detail. In the fully irreversible realistic case, it is possible to perform an additional optimization with respect to the pressure ratio,  Ã max . In particular, this double optimization leads to a valuable increase in efficiency (between 34% and 65%) for a plant with two turbines and two compressors compared to the simple solar-driven one-turbine one-compressor Brayton engine. Copyright © 2012 John Wiley & Sons, Ltd. KEY WORDS thermodynamic optimization; solar-driven heat engines; multi-step gas turbine; irreversibilities; plant performance Correspondence *A. Medina, Departamento de Física Aplicada, Universidad de Salamanca, 37008 Salamanca, Spain. † E-mail: amd385@usal.es Received 10 February 2012; Revised 28 August 2012; Accepted 30 August 2012 1. INTRODUCTION Because of energy savings and strategies in minimizing environmental impact, solar-driven heat engines are attracting much interest nowadays, and, as a consequence, different heat engine cycle models coupled to a solar collector have been investigated. Thermodynamic studies analyzing different sources of irreversibilities and different optimization criteria have been reported for solar-driven Carnot [1–7], Ericsson [8,9], Stirling [9,10], and Braysson [11–13] cycles. In particular, steam, gas, or combined turbine cycles are realistic examples to generate electricity when the heat source is solar energy. Compared to conventional steam turbines, gas turbines have relatively lower thermal efficiencies but bear the advantage of compact building and low construction costs. Moreover, gas turbines can be operated very dynamically (quick start-up) and at significantly lower pressures. The needed heat input can be supplied at least partially (hybrid systems) by concentrating solar collectors using tower plant or dish/engine technology [14–16]. The turbine exhaust energy could be used in a thermal recuperation process through a bottoming cycle [17,18]. In recent years, several prototypes and experimental facilities of solar-driven gas turbine power plants have been developed [19–22]. They usually work on a hybrid solar/fossil fuel operation, so that a standard combustion chamber can compensate for the intermittent nature of solar irradiance. The future commercial interest of this alternative for electric power generation relies on a reduc- tion of investment and generating costs and on an increase of the plant thermal efficiency [23,24]. Theoretical and computer analyses [25–27] on the effect of the main irreversibility sources over the overall thermal efficiency and the optimal values of some basic thermodynamic parameters are necessary steps in order to design efficient solar-driven thermal plants. INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2967 Copyright © 2012 John Wiley & Sons, Ltd.