Investigation of a two stage Rankine cycle for electric power plants

A two stage Rankine cycle for power generation is presented in this paper. It is made of a water steam Rankine cycle and an Organic Rankine bottoming Cycle. By using an organic working fluid with higher density than water, it is possible to reduce the installation size and to use an air-cooled condenser. In order to search suitable working fluids for our application, nine potential candidates from four different organic fluid families and ammonia are tested. The performances of the two stage Rankine cycle operating with those different working fluids are evaluated. The influences of design external temperatures and the steam turbine outlet pressure on the system are analyzed. Optimal points are found at different cold source temperatures and steam turbine outlet pressures for each fluid. System efficiency can also be enhanced by introducing a regenerator for some of the selected working fluids.

► A two stage air cooled Rankine cycle with bottoming ORC is presented.
► Its efficiency is computed for pure fluids of four different families and for ammonia.
► There is an optimal exhaust steam pressure varying with ambient temperature.
► For dry organic fluids, regeneration allows to reduce the steam turbine size.