On the possibility of using an industrial steam turbine as an air expander in a Compressed Air Energy Storage plant

Small/medium size CAES systems (1–10 MW) could be efficiently and successfully employed for off-grid and self-consumption applications and for the delivery of ancillary services on the lower grid levels. A critical issue affecting the feasibility of such systems is related to the availability of efficient and affordable air expanders. Taking into consideration that typical CAES applications are characterized by inlet pressure levels in the range of 40–60 bar, an attractive opportunity to reduce development efforts and investment costs is to resort to the consolidated steam turbine engineering practice. In the present paper, the possibility of using existing building blocks developed to assemble industrial steam turbines to arrange air expanders for CAES applications is explored. A general model for off-design calculation capable of simulating, for a given turbine geometry, steam and air operations as well, has been developed and applied to a case study. On the basis of available information about an existing industrial steam turbine, a possible arrangement for an air expander has been set up and investigated. Results have evidenced a performance loss in terms of both power output and efficiency with respect to steam operations. Nevertheless, the air expander behavior in a wide range of operation can be considered satisfactory. Therefore, the use of industrial steam turbines technology might be considered interesting for the applications under consideration.