EXERGY ANALYSIS OF A CAES WITH THERMAL ENERGY STORAGE Giuseppe Grazzini, Adriano Milazzo Dipartimento di Energetica “Sergio Stecco” Università di Firenze, ITALY Abstract A Compressed Air Energy Storage (CAES) with thermal energy recovery is presented. The latter recovers thermal energy in the compression phase and delivers it to the expanding air, eliminating the need for a combustion chamber and therefore avoiding local pollutant emissions. Seeking for maximum system flexibility, reference is made to an artificial storage and compressed air pressure is seen as a design variable. The proposed system has a multistage design with variable configuration, in order to handle the increasing /decreasing pressure in the storage. Hence each compression/expansion stage works in a relatively narrow range of pressure ratio throughout the energy storage and recovery phases. In order to raise its energy recovery efficiency, the proposed system is analyzed in terms of exergy destruction minimization. Some of its design parameters are optimized. Nomenclature c p constant pressure specific heat W & power c v constant volume specific heat β compression ratio D reservoir diameter ε heat exchanger efficiency E stored energy λ (m-1)/m Ex exergy ρ density of reservoir material m polytropic exponent σ allowable stress of reservoir material m & mass flow rate m res mass of reservoir Subscripts n number of stages AC hot storage p pressure AF cold storage R gas constant c compression s reservoir wall thickness e expansion t time h heated T temperature i exit of i-esime stage V storage volume r cooled V mat volume of reservoir material rec recovery W work 0 ambient 1. Introduction Renewable energy sources have often high time variability. This is especially true for wind energy, which however is now the most economically competitive. Therefore energy storage systems, previously used mainly to face demand fluctuations, receive increasing interest. Various energy storage technologies, based on different physical principles, are available (figure 1). Each of them is well suited for a specific power or energy range. Some of these systems, such as flywheels or supercapacitors, have limited capacity. Others, like advanced batteries or fuel cells, have high costs or low availability. Often there is a limit on maximum number of discharge cycles.