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 η _th 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.