Problem 4.3 - A Geothermal Hybrid Steam Power Plant
A small community of about 500 households have
discovered an underground geothermal brine source that can be used to
boil water at 100°C and would like to use this to generate power.
The following diagram shows the initial design of a low pressure
geothermal plant in which the water is boiled by the geothermal
source to 100°C and subsequently superheated to 200°C by a
wood-fired superheater. Notice that the high pressure of the system
is at 100kPa allowing a convenient
de-aerator
to be placed at the pump outlet.
a) Neatly sketch the complete cycle on the pressure-enthalpy P-h diagram below, indicating clearly all 5 stations on the diagram. Once this is done then use the Steam Tables to determine the following:
b) Assuming that the turbine is adiabatic, determine the power output of the turbine [729kW].
c) Assuming that the feedwater pump is adiabatic, and that the compressed liquid experiences no change in temperature while passing through the pump, determine the power required to drive the pump [0.23kW].
d) Using steam tables, determine the heat transferred to the boiler [6271kW] as well as the heat transferred to the superheater [500kW].
e) Determine the overall thermal efficiency ηth of this power plant [11%]. (Thermal efficiency is defined as the net work done by the system (turbine and feedwater pump) divided by the total heat supplied externally).
f) Discuss the proposed system with respect to its environmental impact and feasibility. Is this a well designed system? What do you consider to be the major advantages and disadvantages of this system? Your discussion should include a comparison of the external fuel used and the turbine power.
Justify all values used and derive all
equations used starting from the basic energy equation for a flow
system.
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