Problem 9: (Past exam problem, only part of the exam problem is included)
In a general geothermal power plant, the heat source is geothermal energy from the earth which is in the
form of high temperature steam. This steam at ?200 °C transfers its energy to the power plant working
fluid in a heat exchanger, and the outlet of the heat exchanger is connected to a turbine. The outlet of the
turbine is connected to an air-cooled condenser where the working fluid is condensed at ?50 °C into a
liquid at its saturation condition, which is then pumped back to the heat exchanger. The turbine converts
flow energy to shaft work, which can then be used by an electric generator. If the working fluid at the
inlet and outlet of the turbine are at ?180 °C and ?50 °C, I require your help with an analysis of the turbine
to determine the better working fluid among water and propane. Answer the following:
(a) Draw a schematic of the turbine identifying the various energy flow?
(b) If water enters with a pressure of 0.3 MPa and exits with a pressure of 0.01235 MPa, what is the
enthalpy of the fluid at the inlet and outlet of the turbine?
(c) If propane enters with a pressure of 2.0 MPa and exits with a pressure of 1.7145 MPa, what is the
enthalpy of the fluid at the inlet and outlet of the turbine?
(d) What is the specific power (kW/kg) generated by the turbine in each case?
(e) If both fluids are to produce the same amount of total power ?3 MW, what is the ratio of mass
flowrate for propane to the mass flowrate for water?
