The United States produces about 2700 megawatts (MW) of electricity from geothermal energy, which is comparable to burning 60 million barrels of oil each year. Worldwide about 7000 MW of geothermal electricity are produced. The process is that naturally occurring steam or hot water that is not far below the earth’s surface (especially in places such as Yellowstone National Park and other volcanic and geothermal areas) is brought to the surface and used to heat a working fluid in a binary fluid power generation cycle, such as that shown in Fig 5.2-9. (Geothermal steam and water are not directly injected into a turbine, as the dissolved salts and minerals would precipitate and quickly damage the equipment.) For geothermal water at temperatures less than 200^◦C, isobutane is used as the working fluid. Isobutane is vaporized and superheated to 480 K and 10 MPa in the heat exchanger by the geothermal water, and is then passed through a turbine (which we will assume to be adiabatic and isentropic) connected to an electrical generator. The isobutane next passes through an isobaric condenser that produces a saturated liquid at 320 K. A pressure-enthalpy diagram for isobutane follows.

Fig 5.2-9.

a. At what pressure does the condenser operate? 

b. What are the temperature and pressure of the isobutane leaving the turbine?

c. Determine the work produced by the turbine per kilogram of isobutane circulating, and the flow rate of isobutane necessary to produce 3 MW of electricity. 

d. Draw the cycle of the process on the isobutane pressure-enthalpy diagram. 

e. Obtain the heat or work requirements for the four units of the cycle in the table below.

f. What is the efficiency of the proposed cycle?

 

Transcribed Image Text:

(2) Compressor 1 High-temperature heat exchanger Low-temperature heat exchanger 3 Turbine (4) PWT