Annual performance prediction of a hybrid brayton thermosolar plant

We present a thermodynamic model for the prediction of the performance records of a solar hybrid gas turbine power plant, with the aim to deliver to the grid an steady power output. The overall thermal efficiency depends on the efficiencies of the involved subsystems and the required heat exchangers in a straightforward analytical way. Numerical values for input parameters are taken from a central tower heliostat field recently developed near Seville, Spain. Real curves for irradiance and external temperature are taken. The values of several variables in yearly terms are predicted and the fuel consumption assuming natural gas is estimated, as well as greenhouse emissions. Moreover, we present an analysis of the losses coming from each plant subsystem. This kind of analysis can contribute to improve the design of these facilities in order to get better performance. INTRODUCTION During the last years several experimental projects have tried to develop hybrid solar gas turbine technologies in which concentrated solar power coming from a central receiver plant is used to heat pressurized air that performs a Brayton cycle [1]. This technology is suitable for regions with favorable solar irradiance conditions, usually linked to water shortage. These power plants can be combined with other cycles in order to improve their overall efficiency. The term hybrid refers to the fact that during low solar radiation periods a combustion chamber ensures an stable power release to the electricity grid and avoids the use of storage systems. Apart from R+D projects, prototypes, and experimental installations, several research works have been published in the last times. Some of them make use of commercial simulation environments or in-house developed software which allows a detailed description of all plant components and specific calculations on the solar subsystem [2]. But, in this way, it is not easy to extract direct physical information about the main losses sources in the plant and to plan global strategies for the optimization of the plant design and operation as a whole. This is why we follow next modus operandi instead of previous one. On the other hand, there are several theoretical works starting from the Brayton ideal cycle that thereafter include refinements in order to recover realistic output records [3; 4; 5]. Usually, in these works the model for the concentrated solar subsystem, although including the main heat transfer losses, is simple. This favors to obtain closed analytical expressions for thermal efficiencies and power output in terms of a reduced number of parameters, with clear physical meaning each, and then check the model predictions for particular design point conditions, with fixed values of solar irradiance and ambient temperature. Also it allows to guide optimization strategies and to suggest where an investment in enhancing the losses gives more results. Thereby, this paper helps to improve the design of these facilities, getting information about the main irreversibility sources and the bottlenecks of the overall plant efficiency. THERMODYNAMIC PLANT MODEL We consider a central tower hybrid solar installation as depicted in Fig.1. A single step recuperative closed Brayton cycle is hybridized in order to release to the grid an stable power output, independent of solar irradiance conditions. The model is simple and uses a reduced number of parameters with a clear physcial meaning each. Thus, analytical expressions can be reached. Our hybrid plant consists of three main susbsystems: a solar collector, a combustion chamber, and a heat engine. The whole system receives two energy inputs. On the one hand, a heat input, GAa, comes from the sun, where G is the direct solar irradiance and Aa, the aperture area of the solar field. The direct solar irradiance is not constant over time, but it depends on the sun position during the day, the meteorological conditions and the seasonal fluctuations. The solar useful heat flux, |Q̇HS|, is transferred to the working fluid through a solar receiver, considered as non-ideal heat exchanger with effectiveness εHS. So the effective solar heat flux received in the heat engine is |Q̇HS| = εHS|Q̇HS|. The efficiency of the solar collector, ηs, is then defined as the ratio between the heat useful flux it provides, |Q̇HS|, and solar energy input: ηS = |Q̇HS| GAa = |Q̇HS| εHSGAa . On the other hand, the energy input at the combustion chamber is ṁ f QLHV , being ṁ f the fuel mass flow rate and QLHV , its corresponding lower heating value. The fuel mass rate will be also considered as time dependent because it should compensate solar irradiance fluctuations. Combustion chamber releases a heat rate |Q̇HC| to its heat exchanger, whose effectiveness is 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics