ENG 105 Thermodynamics
Due Date: 10th December (Tuesday) by 11:59 PM. Late work will NOT be accepted!
Total Points Achievable: 60 points
Bonus Assignment: A case study on working fluid selection for a geothermal plant
Instructions:
• Answer the provided questions and provide your answers in a report format.
• Fluid property evaluation: You may use the link to the website provided below to obtain the properties of various working fluids. You will have to download a small thermodynamic state calculator to compute the properties of various fluids. The interface of the application is very user friendly.
• Thermo-State Application (Free): https://thermo-state.github.io/
Case Study:
You are tasked with the planning and design of a 50 MW geothermal power plant.
A schematic of the geothermal plant is shown in the figure below. A geothermal powerplant uses the thermal energy of the hot rocks below the earth’s surface to heat the working fluid. Boreholes are drilled several kilometers into the earth’s surface to reach the hot rocks. Next a suitable working fluid is pumped through the boreholes and is heated at high pressure by a heat exchanger (“boiler”) near the hot rocks below the ground level. Finally, the heated working fluid is passed through a power cycle of choice to generate electricity.
The ambient temperature at the ground level, TC is 30 。C. The temperature beneath the surface increases at a rate of 25 。C/km of depth into the lithosphere. You would like the maximum temperature of your cycle, TH, to be at least 200 。C.
a. Determine the depth,H, in km, of drilling needed to reach a TH of 200 。C. (2 pts)
b. Calculate the maximum thermal efficiency that your powerplant can achieve. (3 pts)
You have decided to use a Rankine cycle to design the power cycle for your geothermal plant. Your next task will be to make a decision on the working fluid for the power cycle. Three fluids are being considered namely:
1. Water
2. Ammonia
3. R22
For each fluid, the boiler’s operating pressure should be such that the vapor quality at the turbine exit is at least 88%. Assume the turbine and pump are isentropic, and the working fluid is heated to a temperature of TH at the boiler exit.
c. Determine the operating pressure in the boiler of the cycle for each fluid (10 pts)
d. Determine the state of the fluid at the turbine inlet and draw the T-s diagram of the power cycle for each ofthe working fluids. (5 pts)
e. Calculate the thermal efficiencies provided by each fluid (20 pts)
f. Evaluate the mass of working fluid required for each fluid to generate a power of 50 MW. (5 pts)
g. Compare the densities of the working fluids at the turbine inlet and outlet and comment on how the densities and total volume of the working fluids at these locations may affect your decision in selecting a working fluid. (5 pts)
Report
h. Present your results from the above analysis in a report no longer than 3 pages highlighting relevant information and how you arrive at your selection decision. In addition to efficiencies, consider external factors such as material costs, drilling costs, chemical compatibility, etc. and reference any useful resource that helps you arrive at your decision. (10 pts)