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Monatomic Gas Effects on Brayton Cycle Propulsion and Power Systems

Description

Monatomic gases are ideal working mediums for Brayton cycle systems due to their favorable thermodynamic properties. Closed Brayton cycle systems make use of these monatomic gases to increase system performance

Monatomic gases are ideal working mediums for Brayton cycle systems due to their favorable thermodynamic properties. Closed Brayton cycle systems make use of these monatomic gases to increase system performance and thermal efficiency. Open Brayton cycles, on the other hand, operate with primarily diatomic and polyatomic gases from air and combustion products, which have less favorable properties. The focus of this study is to determine if monatomic gases can be utilized in an open Brayton cycle system, in a way that increases the overall performance, but is still cost effective.
Two variations on open cycle Brayton systems were analyzed, consisting of an “airborne” thrust producing propulsion system, and a “ground-based” power generation system. Both of these systems have some mole fraction of He, Ne, or Ar injected into the flow path at the inlet, and some fraction of monatomic gas recuperated and at the nozzle exit to be re-circulated through the system. This creates a working medium of an air-monatomic gas mixture before the combustor, and a combustion products-monatomic gas mixture after combustor. The system’s specific compressor work, specific turbine work, specific net power output, and thermal efficiency were analyzed for each case. The most dominant metric for performance is the thermal efficiency (η_sys), which showed a significant increase as the mole fraction of monatomic gas increased for all three gas types. With a mole fraction of 0.15, there was a 2-2.5% increase in the airborne system, and a 1.75% increase of the ground-based system. This confirms that “spiking” any open Brayton system with monatomic gas will lead to an increase in performance. Additionally, both systems showed an increase in compressor and turbine work for a set temperature difference with He and Ne, which can additionally lead to longer component lifecycles with less frequent maintenance checks.
The cost analysis essentially compares the operating cost of a standard system with the operating cost of the monatomic gas “spiked” system, while keeping the internal mass flow rate and total power output the same. This savings is denoted as a percent of the standard system with %NCS. This metric lumps the cost ratio of the monatomic gas and fuel (MGC/FC) with the fraction of recuperated monatomic gas (RF) into an effective cost ratio that represents the cost per second of monatomic gas injected into the system. Without recuperation, the results showed there is no mole fraction of any monatomic gas type that yields a positive %NCS for a reasonable range of current MGC/FC values. Integrating recuperation machinery in an airborne system is hugely impractical, effectively meaning that the use of monatomic gas in this case is not feasible. For a ground-based system on the other hand, recuperation is much more practical. The ground-based system showed that a RF value of at least 50% for He, 89% for Ne, and 94% for Ar is needed for positive savings. This shows that monatomic gas could theoretically be used cost effectively in a ground-based, power-generating open Brayton system. With an injected monatomic gas mole fraction of 0.15, and full 100% recuperation, there is a net cost savings of about 3.75% in this ground-based system.

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Date Created
  • 2017-05

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Climate Change Effects on Electricity Generation from Hydropower, Wind, Solar and Thermal Power Plants

Description

Climate change is affecting power generation globally. Increase in the ambient air

temperature due to the emission of greenhouse gases, caused mainly by burning of fossil fuels, is the most prominent

Climate change is affecting power generation globally. Increase in the ambient air

temperature due to the emission of greenhouse gases, caused mainly by burning of fossil fuels, is the most prominent reason for this effect. This increase in the temperature along with the changing precipitation levels has led to the melting of the snow packs and increase in the evaporation levels, thus affecting hydropower. The hydropower in the United States might increase by 8%-60% due to Representative Concentration Pathway (RCP) 4.5 and RCP 8.5 scenarios respectively by 2050. Wind power generation is mainly affected by the change in the wind speed and solar power generation is mainly affected by the increase in the ambient air temperature, changes in precipitation and solar radiation. Solar power output reduces by approximately a total of 2.5 billion kilowatt- hour (kWh) by 2050 for an increase in ambient air temperature of 1 degree Celsius. Increase in the ambient air and water temperature mainly affect the thermal power generation. An increase in the temperature as per the RCP 4.5 and RCP 8.5 climate change scenarios could decrease the total thermal power generation in the United States by an average of 26 billion kWh and a possible income loss of around 1.5 billion dollars. This thesis discusses the various effects of climate change on each of these four power plant types.

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Date Created
  • 2020