Browsing by Author "Bandhauer, Todd, advisor"

Now showing 1 - 4 of 4

Open Access
Evaluating thermal efficiency and economic impacts in supplying energy demands for direct air capture
(Colorado State University. Libraries, 2024) Siegel, Madeleine Hope, author; Bandhauer, Todd, advisor; Quinn, Jason, committee member; Herber, Daniel, committee member
Direct Air Capture (DAC) technologies that remove CO2 directly from the atmosphere are needed to meet international goals of limiting atmospheric temperature increase before 2100. Operating costs, including the cost of energy inputs, currently limit the rapid deployment of DAC systems. An abundance of untapped and abandoned geothermal resources provides an opportunity to utilize this thermal energy beneath the Earth's surface to reduce the financial and energy costs of DAC. In this study, thermodynamic models of applicable renewable energy scenarios for fulfilling heating and electrical requirements of DAC were analyzed using ASPEN Plus. Individual components were optimized within the geothermal-DAC coupled systems to quantify specific costs of implementation. The results were integrated into a techno-economic analysis (TEA) to provide a holistic perspective to optimize DAC coupled renewable energy systems. The levelized cost of energy (LCOE) for DAC was reduced from $175/t-CO2 removed to as low as $66/t-CO2 removed, guiding large-scale deployment of DAC, and supporting decision-making in the future.
Open Access
Experimental investigation of an advanced organic Rankine vapor compression chiller
(Colorado State University. Libraries, 2022) Grauberger, Alex Michael, author; Bandhauer, Todd, advisor; Quinn, Jason, committee member; Windom, Bret, committee member; Sharvelle, Sybil, committee member
Thermally driven chilling technologies convert heat into cooling. These systems can support increasing cooling demands using waste heat in a variety of applications. Commercial thermally driven chilling technologies suffer from several implementation challenges, including high capital costs, limited equipment lifecycles, rigid working principles, and large physical formats, and thus are not implemented widely. Organic Rankine vapor compression cooling systems are a pre-commercial technology which can address the limitations of commercial alternatives. Organic Rankine vapor compression cooling systems couple an organic Rankine power generation cycle to a standard vapor compression chilling cycle. These systems can use benign, pressurized refrigerants as working fluids which allows for reduced heat exchanger costs over commercial thermally driven alternatives without environmentally impactful fugitive emissions. Refrigerants are released from cooling technologies during charging, leaking connections, and/or improper/unregulated disposal. Furthermore, the coupling of the two individual cycles allows the use of high-speed compression and expansion machinery as well as multiple methods of heat recuperation. High-speed fluid machinery and heat recuperation strategies reduce the format and cost of the technology while simultaneously improving the longevity and operational flexibility. Current organic Rankine vapor compression efforts are limited from an absence of experimental validation. This study aims to fill this research gap through investigating a prototype organic Rankine vapor compression system enhanced with a high-speed, centrifugal turbo-compressor, sub cycle and cross cycle heat recuperation, compact heat exchanger technologies, and benign, next-generation refrigerants at an industry-relevant scale of 300 kW. A thermodynamic model was created and a system heat-to-cooling coefficient of performance (COP) of 0.65 was simulated with 91°C liquid waste heat, 30°C condenser coolant, and 7°C chilled water delivery where a 5°C inlet to outlet temperature difference was specified for each stream. A full-scale prototype was fabricated and tested following standards for performance rating of commercial water chilling technologies to validate the performance simulation. Experimental testing of the prototype yielded a thermal COP of 0.56 and a cooling duty of 264 kW under its baseline operating conditions. The baseline test conditions were identical to the simulated conditions except the temperature difference across the condensers, which was 1.7°C greater due to a 25.6% lower condenser coolant flowrate. The lower condenser coolant flowrate, a vapor compression condenser refrigerant outlet vapor mass quality of 6.2% instead of the modeled 1°C of subcooling, and elevated system pressure losses limited the efficiency and cooling duty of the prototype over the simulated values. A scenario analysis on the test data was complete to show the prototype could surpass the simulated performance prediction with a COP of 0.66 at 300 kW of cooling if the operational limitations associated with prototype were corrected. This performance is competitive with commercial single-effect absorption systems and is possible because the turbomachinery efficiencies were high. The isentropic efficiency values for the turbine and compressor were 76.7% and 84.8% respectively at the baseline conditions during experimentation and the two devices had a 100% power transmission efficiency within experimental error. Following the assessment of baseline performance, operational characteristics of the technology were quantified at off-design boundary conditions and normalized to those of the baseline to identify performance trends. It was shown that prototype thermal performance generally improved with increasing waste heat supply temperature, increasing chilled water delivery temperature, decreasing condenser coolant temperature, and decreasing chilling duty. These trends are consistent with performance simulations in literature. However, performance improvements at off-design operation were often challenged by variations in turbine and compressor efficiency as well as the efficacy of heat recuperation strategies. Such changes to component performance characteristics at varying boundary conditions have not been previously quantified in practice and, thus, have historically been neglected in analytical investigations of organic Rankine vapor compression systems. Understanding the off-design component performance characteristics allows for the creation of validated organic Rankine vapor compression performance models. Such models will be critical to understanding the true energy savings potential of organic Rankine vapor compression systems as they are continuously investigated.
Open Access
Heat transfer enhancement in two-phase microchannel heat exchangers for high heat flux electronics
(Colorado State University. Libraries, 2020) Hoke, Jensen, author; Bandhauer, Todd, advisor; Windom, Bret, committee member; Venayagamoorthy, Karan, committee member
Laser diodes are semiconductor devices that emit high intensity light with a small spectral bandwidth when a forward voltage is applied. Laser diodes have a high electrical to light conversion efficiency which can be greater than 50%. These robust, high efficiency laser sources are used in medical and manufacturing fields and, if their power can be increased, show promise in inertial confinement fusion and defense applications. Individual diode emitters are arrayed into bars with a footprint of 1 mm by 10 mm to increase their light output power. These bars are further combined into arrays with the light emitting edges stacked close together. As the spacing in these arrays are reduced to increase brightness, thermal management becomes the limiting factor for each bar. State of the art diode arrays can have heat fluxes exceeding 1 kW cm-2. Effective thermal management strategies are key because the diode's output wavelength, bandwidth, efficiency and lifetime are temperature dependent. Commercially available high powered laser diode arrays are traditionally cooled using a single-phase fluid passing through conduction coupled copper-tungsten channels. These heat exchangers have high thermal resistances which require the coolant to be significantly subcooled before entering the device. High working fluid flow rates are required to reduce thermal gradients in the diode bars and working fluid conditioning is required to reduce corrosion in the cooling plates. Many of these issues can be addressed by cooling the diodes with a two-phase working fluid in a corrosion resistant, silicon microchannel heat exchanger. The high heat transfer coefficients associated with flow boiling, as well as the high surface area to volume ratios in microchannel arrays allow the working fluid temperature to be much closer to that of the diode which reduces the cooling load on a system level. Additionally, as heat is added to a two-phase fluid, there is virtually no change in temperature. Therefore, the working fluid flow rate can be much lower than a comparable single-phase heat exchanger, which reduces pump work. However, using a two-phase working fluid presents its own unique set of challenges. This work presents a novel approach to increasing the effective critical heat flux and reducing thermal resistance in an array of 125 high aspect ratio silicon microchannels (40 µm × 200 µm) subjected to heat fluxes up to 1.27 kW cm-2. R134a is used as the two-phase working fluid and outlet vapor qualities up to 80.7% are reported. The silicon heat exchangers are manufactured using a DRIE MEMS process that allows fine control over feature sizes. The performance of traditional plain walls is compared to a novel sawtooth structuring pattern that increases available heat transfer area by 41% and provides bubble nucleation sites. A 17% decrease in thermal resistance is reported for one of the area enhancement schemes and critical heat flux is increased in both area enhanced parts. A thermal FEA model is used to determine heat transfer coefficients and local heat fluxes within the test section. This model is used to investigate alternate patterning schemes. An adjustment to the Bertsch two-phase heat transfer coefficient is also suggested for smaller microchannels geometries and higher heat fluxes. Examination of the model results show that performance increase observed in the area enhanced test sections is driven by an increase in bubble nucleation sites. The additional area available for heat transfer has little effect because reduction of heat flux at the fluid wall interface reduces two-phase heat transfer coefficients. This effect is driven by the relative importance of nucleate boiling in these small channels.
Open Access
Transient experimentation and modeling of a multi-microchannel evaporator
(Colorado State University. Libraries, 2020) Richey, Joshua Johnhenry, author; Bandhauer, Todd, advisor; Young, Peter, committee member; Simske, Steve, committee member
Laser diodes are semiconductor devices that convert electrical energy into light. Often, diodes are arrayed closely together to produce high optical output. Commercially available diode arrays show electro-optical efficiencies of ~50%, resulting in high heat fluxes from these compact devices. Active thermal management is warranted to prevent decreases in performance or damage to the device. Two-phase cooling in microchannels has shown great promise in steady-state studies, dissipating heat fluxes over 1.1 kW cm-2. The very high latent heat and buoyancy-driven effects associated with two-phase cooling produce large heat transfer coefficients, minimizing undesirable temperature gradients across the diode. Previous research has primarily focused on steady-state operation. Although promising steady-state results were documented, little is known on the effects of transient heat loads on microchannel flow boiling. Laser diodes can rapidly change their optical output, inducing extreme transient heat loads. Furthermore, cold start up, where the diode is stepped up directly to maximum optical output, produce transient heat loads which are especially concerning. These transient challenges need to be fully understood to usefully implement two-phase cooling of laser diode arrays. The current study investigates the effects of transient heat loads on a multi-microchannel evaporator. A silicon multi-microchannel heat exchanger with small hydraulic diameters (52 µm) and a surrogate laser diode heater, developed by Lawrence Livermore National Laboratory, is integrated into a two-phase pumped loop to perform transient experiments with pulsed and ramped heat loads. When exposed to pulsed loads, infrared temperature measurements and flow visualization showed extreme superheat temperatures (~50°C) before the onset of boiling. After the onset of boiling, unexpected flow instabilities were seen, followed by a delay in steady-state two-phase boiling that could not be explained by thermal mass of the test section. Transients in the flow conditions were also documented, and ramping heat loads showed promise in mitigating the peak temperature and flow instabilities. Furthermore, a transient thermal suite (ATTMO) developed by P C Krause and Associates (PCKA), is utilized to model the microchannel evaporator. The thermal suite is augmented to model the dynamics seen under a pulsed heat load. A reduced-order, non-computationally demanding method using a logistic function to describe the transient heat transfer coefficient is implemented into ATTMO. The transient modeling results showed a good correlation (average error of (±2.04°C) with the experimental data collected. A direct relationship between onset of boiling temperature and growth rate is shown. The results from this study show potentially dangerous peak temperatures for laser diodes. Mitigation strategies should be investigated and implemented to avoid the extreme superheat temperatures. The non-computationally demanding model developed in this research can be used in future studies to rapidly investigate the effects of heat loads and different operational parameters.