Browsing by Subject "Monte Carlo Simulation"
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Item Open Access Assessing Climate Change under Uncertainty: A Monte Carlo Approach(2008-04-25T07:47:25Z) Park, JeeyoungClimate change has emerged as one of the most multifaceted manifestations of global change of our time. However, there is less confidence about exactly how the climate will change in the future, and lesser confidence still about the adjustments it will induce to natural and human system. Thus, policy formulation for climate change poses a great challenge because of a problem of decision-making under uncertainty. To facilitate climate policy decision, quantification of the uncertainty in climate outcomes under possible policies is needed. This paper presents an approach to assess climate change under uncertainty using Monte Carlo simulations. Here, I find that in the absence of climate policy, the 95% bound on temperature change in 2100 is 5.79°C. The stringent climate policies with aggressive emissions reductions over time lower significantly the temperature change, compared to no policy case.Item Open Access Benefits of Energy Efficiency Standards: The Case of Residential Refrigerators(2021-04-29) Jain, Abhishek SanjayAn integral part of the modern American household, refrigerators are ubiquitous. Characterized by their constant load profile, refrigerators account for a substantial part of residential energy demand. Improving energy efficiency standards for refrigerators can offer a sizeable demand reduction and therefore be a pivotal part of climate change mitigation strategy. This study quantifies the benefits of implementing energy efficiency standards for residential refrigerators by considering three policy scenarios. An in-house Monte-Carlo model is developed to replicate important considerations in the rulemaking process through which the Department of Energy (DOE) sets the conservation standards. Our results indicate that the existing standards for refrigerators can realize savings equal to the electricity consumption of about 400,000 average U.S. homes by 2025. While these savings are critical, the benefits of this conservation standard are inequitable. Our study provides insights into how the benefits are realized across all the consumers in the country and provides recommendations for improving the rulemaking process.Item Open Access Energy, Economics, and Politics: An Analysis of Decisions to Pursue Large Hydropower Projects in Bhutan and Nepal(2017-11-15) Ridel, RobertLarge hydropower projects are on the rise. Once subject to universal condemnation by environmentalists and largely abandoned by international donors, these projects have been reborn as a critical component of “sustainable development” in the age of climate change. This resurgence of interest in large hydropower projects raises questions for policymakers about whether these projects provide a net benefit to their constituents. While rigorous economic analyses could provide guidance to the policymakers grappling with these questions, the complexity of this endeavor and the politics that almost always enmesh large infrastructure projects mean that decisions to pursue these projects are likely influenced by a different calculus—one that may vary across countries based on their unique history and political dynamics. Nepal and Bhutan, two developing countries that hold tremendous hydropower potential, provide an illustration of how these decisions are being carried out and what is driving them. This paper examines the political discourse surrounding hydropower projects in each country and how the projects fit within their development narrative. It also includes a cost-benefit analysis of two large projects that were recently pushed forward, Punatsangchhu I (or “Puna I”) in Bhutan and Arun III in Nepal. This paper concludes that the overwhelming economic benefit of the hydropower generated by these projects has driven and will continue to drive development in both Bhutan and Nepal, with the potential to bring substantial net benefits to the region. As a result, people who are concerned about the social and environmental costs of projects may be more successful in arguing for stronger safeguards for projects rather than a complete halt in their development. The economic value of clean energy for an area that has not been fully electrified is simply too large for policymakers to ignore. This paper also considers the role of political and international dynamics in development choices and the shadow price of capital, and how these factors could explain why Bhutan has made more progress in hydropower development than Nepal.Item Open Access Equilibrium and Non-equilibrium Monte Carlo Simulations of Microphases and Cluster Crystals(2012) Zhang, KaiSoft matter systems exhibiting spatially modulated patterns on a mesoscale are characterized by many long-lived metastable phases for which relaxation to equilibrium is difficult and a satisfactory thermodynamic description is missing. Current dynamical theories suffer as well, because they mostly rely on an understanding of the underlying equilibrium behavior. This thesis relates the study of two canonical examples of modulated systems: microphase and cluster crystal formers. Microphases are the counterpart to gas-liquid phase separation in systems with competing short-range attractive and long-range repulsive interactions. Periodic lamellae, cylinders, clusters, etc., are thus observed in a wide variety of physical and chemical systems, such as multiblock copolymers, oil-water surfactant mixtures, charged colloidal suspensions, and magnetic materials. Cluster crystals in which each lattice site is occupied by multiple particles are formed in systems with steep soft-core repulsive interactions. Dendrimers have been proposed as a potential experimental realization. In order to access and understand the equilibrium properties of modulated systems, we here develop novel Monte Carlo simulation methods. A thermodynamic integration scheme allows us to calculate the free energy of specific modulated phases, while a [N]pT ensemble simulation approach, in which both particle number and lattice spacing fluctuate, allows us to explore their phase space more efficiently. With these two methods, we solve the equilibrium phase behavior of five schematic modulated-phase-forming spin and particle models, including the axial next-nearest-neighbor Ising (ANNNI) model, the Ising-Coulomb (IC) model, the square-well linear (SWL) model, the generalized exponential model of index 4 (GEM-4) and the penetrable sphere model (PSM). Interesting new physics ensues. In the ANNNI layered regime, simple phases are not found to play a particularly significant role in the devil's flowers and interfacial roughening plays at most a small role. With the help of generalized order parameters, the paramagnetic-modulated critical transition of the ANNNI model is also studied. We confirm the XY universality of the paramagnetic-modulated transition and its isotropic nature. With our development of novel free energy minimization schemes, the determination of a first phase diagram of a particle-based microphase former SWL is possible. We identify the low temperature GEM-4 phase diagram to be hybrid between the Gaussian core model (GCM) and the PSM. The system additionally exhibits S-shaped doubly reentrant phase sequences as well as critical isostructural transitions between face-centered cubic (FCC) cluster solids of different integer occupancy. The fluid-solid coexistence in the PSM phase diagram presents a crossover behavior around T~0.1, below which the system approaches the hard sphere limit. Studying this regime allows us to correct and reconcile prior DFT and cell theory work around this transition.
Item Open Access LIFE CYCLE ASSESSMENT OF GREENHOUSE GAS EMISSIONS FROM NAM THEUN 2 HYDROELECTRIC PROJECT IN CENTRAL LAOS(2011-04-25) Zhou, JiaoniConventional energy generation techniques such as coal and oil power plants release large amounts of greenhouse gases (GHGs) due to fossil fuel combustion while renewable energy sources, particularly, hydroelectric generation, are considered as climate-benign since they do not emit fossil carbon to produce energy. However, dams and their associated reservoirs are not entirely GHG-neutral and their emissions need further investigation. In particular, reservoirs contribute to a major source of methane emission owning to the energy and material input in the construction and the decommissioning phase as well as the anaerobic decomposition of flooded biomass in the operation phase. This master’s project evaluates the greenhouse gas (GHG) emissions associated with Nam Theun 2 hydroelectric power plant in central Laos. A life cycle perspective is coupled with two Monte Carlo Simulations and time-specific global warming potential (GWP) for methane, all to predict the statistically most likely 100-yr GWP of Nam Theun 2. The stochastic models indicate GHG emissions of 2.5±0.5 gCO2 eq/kWh GWP in the construction phase, 75±5 gCO2 eq/kWh GWP in the operation phase and 0.60±0.05 gCO2 eq/kWh GWP in the decommissioning phase. The operation phase emission estimation is larger than previously believed (around 10 gCO2 eq/kWh for hydropower) due to the accounting for biomass decomposition. However, the hydropower plant is still significantly lower than the lifetime GWP of a typical coal plant (800-1000 gCO2 eq/kWh). A literature review on the topic of reservoir GHG emissions is conducted, an introduction of the Nam Theun 2 project is presented and GHG mitigation recommendations are provided for various stages of a dam’s life.Item Open Access Quantifying the Economic Risk of Wildfires and Power Lines in San Diego County(2014-04-25) Johnson, JesseSan Diego Gas & Electric Company has proposed retrofits to seven of its transmission lines to reduce the lines’ potential for igniting fires and to increase their ability to withstand damage from wildfires. Since the company’s ratepayers will ultimately pay for the cost of these retrofits through electricity rates, the benefit of the projects in terms of wildfire risk reduction is a matter of public policy interest. This study estimates the range of potential monetary losses that the company could incur due to wildfires and compares those losses to the costs of the transmission line retrofits as a means of evaluating their risk reduction benefit. The study uses a Monte Carlo simulation to estimate the losses for the company from wildfires in a given year. The model outputs the number of ignitions from the transmission lines, the acreage of the resulting wildfires, the property damage caused by those fires, the length of transmission line damaged by wildfires, and the costs of repairing those lines. The model is parameterized using empirical observations of transmission lines ignitions, wildfire sizes, and property values for San Diego County. Results suggest that although the expected value of losses is not large enough to justify the investment in the retrofits, the high risk of losses (driven by rare but extremely damaging events) may justify the investment. The transmission lines in closest proximity to populated areas are the best candidates for retrofits. The study provides a possible framework for regulators and electric utilities to discuss the public benefit of safety-related infrastructure investments as part of the regulatory process.Item Open Access Validation of the dosimetry for a Lay-down Total Skin Irradiation techniques by Monte Carlo Simulation(2019) Li, RuiqiTotal skin irradiation (TSI) with electron beam has been very effective for patient with Mycosis fungoides. We recently developed and implemented a technique of laying down position for patients who are too frail for the standard standing position. In this study, we validated these measurements with Monte Carlo (MC) simulation which can provide more information on dose distributions and guidance on further optimization of the technique. The laydown technique consists of 6 equi-spaced beam directions relative to the patient cranial-caudal axis, similar to the standup technique. For the AP/PA directions (vertex fields), patient is placed directly under the gantry at 195cm source-to-skin distance (SSD) and 3 overlapping fields with gantry angles 60˚ apart are used. For the four oblique directions, patient is repositioned on the floor parallel to the gantry rotation axis at SSD of 305 cm with gantry at 300˚. A customized 0.25 mm Cu filter was placed in the linac interface mount to further broaden the beam. Each treatment fraction consists of 10 fields and 3 of them are unique. The Monte Carlo simulation was performed within the EGSnrc environment, using the phase space file provided by the linac vendor. The following quantities were studied and compared with the measurements: for each field/direction at the treatment SSDs, the percent depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output on the flat water phantom; the composite dose distribution on a cylindrical phantom of 30 cm diameter. Cu filter increases the beam FWHM by 44% but also reduces the output by 60%. The central regions within ±10% of the prescription dose were 170×70 cm2 for vertex fields and 140×80 cm2 for oblique fields. Profiles and output factors for both vertex fields and oblique fields agreed within 3% between MC and measurements. Vertex fields has dmax at (0.55: MC; 0.67: measurement)cm and R80 at (1.15; 1.40)cm, oblique field has dmax at (1.05; 0.86)cm and R80 at (1.55; 1.40)cm. When all fields are combined on the cylindrical phantom, the dmax shifted toward surface region. The composite dose distribution has the surface dose at (99.0; 95.2) %, dmax at (0.15; 0.15)cm, and R80 at (0.55; 0.75)cm. The maximum X-ray contamination at the central axis was (2.2; 2.1)%, and reduced to 0.2% at 40 cm off the central axis. Cylindrical phantom of 20 cm and 40 cm diameters for patient size simulation shows the surface dose of 93% and 103%, compared to 30 cm diameter. The Monte Carlo results in general agree well with the measurement, which provides secondary support in our commissioning procedure. In addition to those measurable quantities, the Monte Carlo simulation can provide further information such as the full dose distribution of the patient phantom, and the ability to investigate and optimize techniques such as different filter design, SSD and field size variations.
Item Open Access Validation of the Stand-up Technique for Total Skin Irradiation by Monte Carlo Simulation(2019) Tseng, Wen-ChihPurpose/Objective(s): The standard total skin irradiation (TSI) procedure for patients with Mycosis fungoides at our clinic is the Stanford technique where dual electron beams are directed toward patient standing at an extended source to skin distance (SSD) of 300 cm. Patients rotate along the cranial-caudal axis in 6 directions to get full coverage to skin surface. The purposes of this study are to validate the commissioning dosimetric data using Monte Carlo (MC) systems, and to investigate the effect of scattering filter on the standard stand-up technique with a single beam.
Materials/Methods: The first MC system is the EGSnrc environment with BEAMnrc and DOSXYZnrc packages, which has been the standard MC simulation system used in the radiation therapy field. In this study, extended SSD with electron beam was tested, which is not a common use of EGSnrc. The second system is the VirtuaLinac, a recently developed cloud-based application from Varian for research purpose based on GEANT4 platform. For both MC systems, the same phase space files which have been previously validated were used. At each direction, a dual-field electron beam with jaws opening of 36 × 36 cm2 and gantry angle at ±19° degrees from horizontal direction was used. The following quantities were studied and compared with the measurements during commissioning: for each field/direction at the treatment SSD, the percentage depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output on a flat solid water phantom; the composite dose distribution on a cylindrical phantom of 30 cm diameter. For the investigation part, the materials (Cu, Fe, Au, Zn, Ag) were chosen because of their stability and availability. The thickness ranges from 0.05 mm to 0.55 mm, depending on characteristics of materials. The extended source to skin distance (SSD) from 250 cm to 350 cm were studied. For each material, we vary the thickness and SSD, to evaluate following quantities: percent depth dose (PDD), profiles and output at dmax, and compared them with the standard dual beams at treatment SSD.
Results: For the dual-field electron beam from one direction, the average(maximum) difference in profiles between EGSnrc/VirtuaLinac and measurement were -5.5% (-8.7%) and 0.9% (2.0%). Both dmax (1.1 cm) and R50 (2.1 cm) in PDD of both MC systems agreed well with the measurements within 1 mm. The X-ray contamination at 15 cm depth was 0.5%/0.6% for EGSnrc/ VirtuaLinac, compared with the measured value of 0.8%. The output was -2.4%/-3.2% difference for EGSnrc/VirtuaLinac when compared with measurement. When radiation from all six directions are combined on a cylindrical phantom, the ratio of output at the surface from 6 directions to a single direction, defined as B-factor, is 3.1 from both MC systems and the measurement. The dmax also shifted toward the surface at 0.15 cm. The X-ray contamination of all fields was 1.2 % and 1.3% for EGSnrc and VirtuaLinac, compared with 2% in the measurements. For the investigation part, no material shows acceptable profile flatness (±10% within the central 160 cm) at 250 cm SSD. At 300 cm SSD, Au (0.1 mm), Ag (0.25 mm), and Cu (0.45 mm) are acceptable. Zn (0.45 mm) requires 325 cm SSD to meet the requirement. For these 4 configurations, the dmax is 0.87-0.99 cm, similar to dual beam (0.97 cm); R50 is 1.85-1.91 cm, compared with dual beam of 2.06 cm; the output ranges from 0.025-0.029, lower than the dual beam (0.080). The composite fields for 4 configurations, the dmax is 0.1 cm, compared with dual beam (0.16 cm). The surface dose is 97%, similar to dual beam (96%). B-factor is 3.3-3.4, compared with dual beam (3.1). The maximum x-ray contamination is 3%, slightly higher than dual beam (2%).
Conclusions: The results from both Monte Carlo systems in general agree well with the measurement for the validation part. Furthermore, MC results suggest the stand-up TSI technique can be implemented using a single beam if the customized filter is used. In addition to those measurable quantities, the Monte Carlo simulation can provide further information such as the full dose distribution of the patient phantom, thus become the foundation for investigations for future technique optimizations.