# Design Problem Applied Thermal Engineering - Take Home Exam. (Due by sfh41483

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Design Problem: Applied Thermal Engineering – Take Home Exam. (Due June 17, 2008)

1. Several geothermal power plants are in operation and more are being built since the heat
source of a geothermal plant is the hot geothermal water which is “free energy.” An 8-MW
geothermal power plant is being considered at a location where geothermal water at 160°C is
available. Geothermal water is to serve as the heat source for a closed Rankine power cycle with
refrigerant-134a as the working fluid. Specify suitable temperatures and pressures for the cycle,
and determine the thermal efficiency of the cycle. Justify your selections.

2. A 10-MW geothermal power plant is considered at a site where geothermal water at 230°C is
available. Geothermal water is to be flashed into a chamber to a lower pressure where part of the
water evaporates. The liquid is returned to the ground while the vapor is used to drive the steam
turbine. The pressures at the turbine inlet and the turbine exit are to remain above 200 kPa and 8
kPa, respectively. High-pressure flash chambers yield a small amount of steam with high
availability whereas lower pressure flash chambers yield considerably more steam but at a lower
availability. By trying several pressures, determine the optimum pressure of the flash chamber
to maximize the power production per unit mass of geothermal water withdrawn. Also
determine the thermal efficiency for each case assuming 10 percent of the power produced is
used to drive the pumps and other auxiliary equipment.

Turbine

Flash
chamber

230°C
Geothermal
water

Figure 1. A Schematic for Design Problem 2.
2

TAKE HOME EXAM. (Due June 17, 2008)

1. Write an essay on air-, water-, and soil-based heat pumps. Discuss the advantages and the
disadvantages of each system. For each system, identify the conditions under which that system
is preferable over the other two. In what situations would you not recommend a heat pump
heating system?

2. Write a computer program to determine the effects of pressure ratio, maximum cycle
temperature, regenerator effectiveness, and compressor and turbine efficiencies on the net work
output per unit mass and on the thermal efficiency of a regenerative Brayton cycle. Assume the
working fluid is air which is at 100 kPa and 300 K at the compressor inlet. Also assume constant
specific heats for air at room temperature. Determine the net work output and the thermal
efficiency for all combinations of the following parameters:
Pressure ratio:   5, 8, 14
Maximum cycle temperature:       1000, 1400, 1600 K
Regenerator effectiveness:    70, 80 percent

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