Real Results and Discussion
The amount of power the power plant needed to produce was based on the number of households in New Paltz, 6,132 and the average power usage per household in the United States of 1.24 kW. Based on these estimates New Paltz uses 7,639.45 kW of power. The plant also needed to be rated for twice the required load so that some power could be sold to other cities. This made the maximum possible power requirement 15,278.90 kW.
For the purposes of this study, the mass flow rates, maximum temperatures and isentropic efficiencies for all pumps and turbines were kept constant across all designs at 16 kg/s, 900℃ and 50%.
The two heat sources considered for this project were coal and natural gas. Anthracite was chosen as the coal source to be analyzed due to its high energy density. The heating value for anthracite is 30 MJ/kg and the heating value for natural gas was based on the national average of 38 MJ/m3.[3] Cost and delivery of the fuel source was outside the scope of this project and was not considered.
Using the Hudson River as the prime location for the power plant, the pipe dimensions can be calculated based on the collected data on the river’s mass flow rate and debit. The pipe will only require a small percentage of the river’s mass flow rate-- a value of 10 kg/s was used for this design, as well as its speed of flow at 2.5m/s. The pipe to be used is steel-- the pipe must then not exceed a stress of 420 MPa and be able to withstand 22 MPa of pressure. After calculating for the diameter, the pipe must be 2.257m. This diameter is easily found in the market with thicknesses ranging from 0.05m to 0.25m. The calculations done in order to solve for these variables can be found in Appendix E: Hand Calculations, Pipe Dimensions.
The real results under these constraints for each system studied are analyzed below.
For the purposes of this study, the mass flow rates, maximum temperatures and isentropic efficiencies for all pumps and turbines were kept constant across all designs at 16 kg/s, 900℃ and 50%.
The two heat sources considered for this project were coal and natural gas. Anthracite was chosen as the coal source to be analyzed due to its high energy density. The heating value for anthracite is 30 MJ/kg and the heating value for natural gas was based on the national average of 38 MJ/m3.[3] Cost and delivery of the fuel source was outside the scope of this project and was not considered.
Using the Hudson River as the prime location for the power plant, the pipe dimensions can be calculated based on the collected data on the river’s mass flow rate and debit. The pipe will only require a small percentage of the river’s mass flow rate-- a value of 10 kg/s was used for this design, as well as its speed of flow at 2.5m/s. The pipe to be used is steel-- the pipe must then not exceed a stress of 420 MPa and be able to withstand 22 MPa of pressure. After calculating for the diameter, the pipe must be 2.257m. This diameter is easily found in the market with thicknesses ranging from 0.05m to 0.25m. The calculations done in order to solve for these variables can be found in Appendix E: Hand Calculations, Pipe Dimensions.
The real results under these constraints for each system studied are analyzed below.
Simple Rankine Cycle
The simple Rankine cycle had an efficiency of 20.72% but the power output fell short of the desired amount by approximately 2 MW. While this system could sufficiently power the town it would fall short of the stated goal for excess power for sale. It also has the second highest fuel source requirement, which should be taken into consideration when determining the net cost for this system.[4] Even though it does not meet the design requirement it does end up being the cheapest system to build because of its simplicity and low amount of required parts.
The parametric study with regards to temperature above shows that the increase in minimum temperature is inversely proportional to the efficiency and work output of the system.
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Modified High/Low Pressure Turbine
The High/Low Pressure Turbine system is the only system that met the power output under the design constraints. The one major drawback is that it has the highest fuel source requirement, which is to be expected since the steam is heated twice per cycle. This system also requires a second turbine which will increase the cost which should be taken into consideration when choosing the final design
The parametric study with regards to temperature for this system shows that cycle efficiency decreases dramatically with the decrease increase in minimum temperature in a non-linear fashion.
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Modified Closed Feedwater Heater
The Modified Closed Feedwater Heater system had the worst power output, falling approximately 7 MW short of the required power output, and efficiency of all the systems, but has the lowest fuel source requirement. A greater temperature range is needed for this system to be effective and the system is unfortunately limited by the projects constraints. In addition to these drawbacks this system also requires additional piping for the heat exchanger and two pumps which add to the cost of the system.[5]
The parametric study with respect to minimum temperature shows a decrease in efficiency, which can be attributed to the decrease in the usefulness of the heat exchanger in this system. If the difference in temperature is small the benefit of the heat exchange is minimal.
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Modified Open Feedwater Heater
The Modified Open Feedwater Heater system falls approximately 4 MW short of the required power output and suffers from the same issues as the closed feed water system in that the increase of the minimum temperature greatly decreases the benefit of the heat transfer. It is slightly better than the closed feed system because it does not suffer the losses associated with the intricate heat piping system needed for a heat exchanger. The system does however have the second highest efficiency of the systems studied. But the additional pump needed from the system increases the cost of the system and effectively reduces the efficiency.
The parametric study with respect to the increase minimum temperature shows a drop in efficiency and power. Which is to be expected because a higher minimum temperature decreases the benefit of the mixing chamber.
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