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Generating Power Using Geothermal Resources 177

and multistage turbines. Lower temperature geothermal resources can be used to generate power
using binary plants that employ a working fluid (usually an organic compound such as isopentane
or propane, or an ammonia–water solution) that has a boiling temperature significantly below that
of water. In a binary plant the geothermal water flows through a heat exchanger, transferring its heat
to the working fluid, and is then reinjected into the reservoir. Binary plants are becoming the fastest
growing part of the geothermal energy market. They have the advantage of requiring lower tempera-
ture resources, they emit no gases into the atmosphere, and can be built in modular form.

Problems
9.1 That is a dry steam resource and why is it favored over other types of geothermal
resources for power generation?
9.2 Assume that steam entering a turbine is at 235°C, has a flow rate of 5 kg/s, it leaves the
turbine at 35°C, and the efficiency of the turbine is 84%. What is the power output of
the turbine, assuming an ideal process?
9.3 At what pressure will the fluid in Problem 9.2 flash to steam?
9.4 Assume that the pressure at the top of a reservoir 3000m underground is 35 MPa, and
3
it contains a fluid at 235°C. Assume the fluid density is 820 kg/m and that it is flowing
in the well at 3m/s. Assume also that the bottom pressure is 25 MPa, the well diameter
is 0.3m, that the surface roughness of the pipe is 4.57 e-05 meters and the absolute
viscosity is 0.00107 kg/(m-s). At what depth in the well will the fluid flash?
9.5 How many kg/s of cooling water is necessary to cool 60°C water that is exiting a tur-
bine to 25°C if the flow rate of the hot water is 5 kg/s?
9.6 What factors determine the sustainability of a geothermal resource? What operational
strategies can be employed to enhance the sustainability?
9.7 What is a Rankine cycle? Why is it important for the use of geothermal resources for
power generation?
9.8 What is the approximate minimum temperature for a useful geothermal power genera-
tion system that employs binary technology? What determines this temperature?

reFerences
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