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Research Experiences

My two main research projects were (1) designing a high-temperature concentrated solar thermal collector for hydrogen generation, and (2) optimizing operational efficiency of a hydrogen fuel cell for vehicle applications. These projects directly relate to my GCS topic of making solar energy economical. Converting methane to hydrogen for electricity generation aims to make energy generation more sustainable. By optimizing both hydrogen production and hydrogen utilization, we can create a cheap, efficiency system for electricity generation from solar. The following summarizes my work each semester.

 

Table of Contents

  • Solar Thermal
    • ME 493
    • Pratt Fellows Summer Research
    • ME 394
    • ME 391
    • EGR 393
  • Fuel Cell
    • ME 491
    • ME 392
    • ME 490

 

ME 493: Engineering Research Fellows Projects

Supervisor: Dr. Nico Hotz

Lab: Thermodynamics and Sustainable Energy Laboratory

Dates: August 2018 – December 2018

Hours: 8 hours/week · 15 weeks = 120 hours

Continuation of Pratt Fellows Summer Research (Summer 2018). I continued my 2 main projects, (1) developing a high-temperature solar selective absorption coating, and (2) modeling the solar collector. For the coating, this semester I focused on characterization in an actual thermal collector. While the optical properties were measured with a spectrophotometer, the performance of the coating must be measured in an actual collector. This was done in our vacuum insulated cylindrical collector, with a thermocouple attached to the bottom of the substrate. Under 40x concentration, the coating was unfortunately unable to reach high temperatures. This is likely due to thermal stresses impacting the integrity of the coating. Further research must be conducted to troubleshoot why this multilayer coating fails.

On the modeling project, this semester I created 3D model of the collector in COMSOL Multiphysics, building on the collector design from previous semester. The dry methane reforming process was used to avoid complications with phase change modeling. The finite element model included the coupled effects of heat transfer, laminar flow, and chemical reaction. Instead of modeling the catalyst as a porous media (which would have exceeded computational capacity since the active areas are so small) the catalyst was treated as a bulk volume and the catalytic properties were tuned against experimental values obtained in the lab. The results of the finite element model were promising and indicated a concentration ratio of 20x was sufficient to convert 100% of inlet methane at a flowrate of 100 SCCM. I presented this work at the 2018 COMSOL Conference and won a Best Paper Award.

 

Pratt Fellows Summer Research

Supervisor: Dr. Nico Hotz

Lab: Thermodynamics and Sustainable Energy Laboratory

Dates: May 2018 – August 2018

Hours: 40 hours/week · 12 weeks = 480 hours

Part of Pratt Fellows and continuation of ME 394 (Spring 2018). My summer research focused on 2 main projects: fabricating the high-temperature solar selective absorption coating, and finite element modeling of the solar collector. For the coating, over the summer I focused on fabrication techniques beyond the TiSi absorber considered in the previous semester. Specifically, I included an antireflective coating applied on top of the TiSi involving alternating layers of TiO2 (deposited with RF sputtering) and SiO2 (deposited with chemical vapor deposition). This prevented losses due to reflection and improved optical performance. I also considered different substrates such as copper and aluminum to create a physically applicable sample. The final results were measured using a spectrophotometer to determine absorptivity, emissivity, and reflectivity at various wavelengths. These results indicated a selectivity of 24 with a cutoff between high absorptivity and low emissivity at 1400 nm.

The second project was finite element modeling of the solar collector with COMSOL Multiphysics. I first focused on 2D modeling of phase change to model steam methane reforming. However, phase change of fluid flow was difficult to model. The large density change across a sharp interface created instabilities in the finite element model. The solution was to use higher order polynomials in each element, but this functionality was not available in the current license I had. Therefore I shifted focus to dry methane reforming, which does not involve phase change as the methane is reacted with carbon dioxide instead of steam. Coupling heat transfer, laminar flow, and chemical reaction became the focus of the rest of my summer, which continued into the fall semester.

 

ME 394: Engineering Research Fellows Projects

Supervisor: Dr. Nico Hotz

Lab: Thermodynamics and Sustainable Energy Laboratory

Dates: January 2018 – May 2018

Hours: 8 hours/week · 15 weeks = 120 hours

Continuation of ME 391 (Fall 2017). This semester, I started fabrication of a high-temperature solar selective absorption coating based on NREL’s #6A patented design. The coating is based on an alloy of Ti and Si in the form of TiSi-a or TiSi2 as the main absorber, with SiO2 and TiO2 acting as reflectors to retain light and heat. I spent time at SMIF (Shared Materials Instrumentation Facility) using the sputtering machines to form thin films of Ti and Si, then used the rapid thermal annealing system to anneal the thin films and produce a TiSi alloy. This semester, I focused on getting the multilayer coating correct and therefore used glass as the substrate. Other substrates were considered over the summer.

 

ME 391: Undergraduate Projects in Mechanical Engineering

Supervisor: Dr. Nico Hotz

Lab: Thermodynamics and Sustainable Energy Laboratory

Dates: August 2017 – December 2017

Hours: 8 hours/week · 15 weeks = 120 hours

Continuing the project started in EGR 393 (Spring 2017). The goal for this semester is to refine the mathematical model developed last semester to include other important parameters – specifically including the heat input required for the reaction of catalytic steam reforming and how the reaction rate influences heat transfer.

I will also be applying the results from the mathematical model to build an actual concentrated solar thermal collector. The focus of manufacturing will be the high-temperature solar absorption coating. The current coating used in the lab degrades under excessive temperatures, and therefore a new coating must be manufactured that can withstand high temperatures. Many such coatings are available in literature with simulated results. The goal of this portion of the project is to manufacture a high-temperature coating using the equipment at SMIF and use it to validate the developed mathematical model.

 

EGR 393: Research Projects in Engineering

Supervisor: Dr. Nico Hotz

Lab: Thermodynamics and Sustainable Energy Laboratory

Dates: January 2017 – May 2017

Hours: 8 hours/week · 15 weeks = 120 hours

As part of my Independent Study for Spring 2017, I worked with Dr. Hotz to develop a mathematical model of a solar thermal collector. One of the lab’s projects is developing a non-concentrating, residential solar thermal collector to generate the heat required to reform biofuels and produce hydrogen for use in fuel cells that generate electricity. Because hydrogen is more energy dense than biofuels, this is a more favorable energy storage mechanism. In past projects they have been able to generate temperatures up to 270ºC, which is enough to reform methanol at low flow rates.

My project involved expanding this project to include concentrated solar power. Although methanol can be reformed at low temperatures, other biofuels require higher temperatures, along the order of 700-900ºC. Concentrated solar power is one way to generate these high temperatures. I developed a mathematical model of the system using MATLAB, using heat transfer principles to determine the temperatures of each part of the system. My code determined the outlet temperature of methane as a function of concentration ratio and flow rate, offering important design insight for the optimal characteristics to generate hydrogen from methane.

 

ME 491: Undergraduate Projects in Mechanical Engineering

Supervisor: Dr. Josiah Knight and Dr. Nico Hotz

Dates: May 2018 – August 2018

Hours: 8 hours/week · 15 weeks = 120 hours

Continuation of ME 392 (Spring 2018). Once we fully characterized the fuel cell and understood the parameters we needed to optimize, we conducted a full-scale optimization of the system. We optimized cathode and anode side gas pressures, gas and membrane temperature/humidity, load point, fan speed, short circuiting, and anode purging to maximize efficiency of our fuel cell. Because this is a multidimensional problem, we used a gradient descent optimization to find the global extremum. This was made less complicated due to the steady state operation of the fuel cell. Vehicle loads are typically dynamic, with brief periods of high loads interspersed between a low baseline load. To minimize the side of our fuel cell, we used a supercapacitor bank for load leveling, allowing the fuel cell to operate at a constant power value. For the Shell Eco-Marathon competition, we used active supercapacitor charging with a DC/DC converter to step up the fuel cell voltage. This novel idea led us to win 1st place at the competition and also take home a Technical Innovation Award.

However, the DC/DC converter had about 5% losses, which is detrimental to overall efficiency. Therefore, following the competition, we switched to passive supercapacitor charging with carefully selected supercapacitors to match the motor voltage while still having enough capacity to store the required amount of energy through the race. We implemented this new control strategy for our Guinness World Record attempts over the summer. This was successful, as on July 21, 2018, we officially broke the record for fuel efficiency at 14,573 MPGe. We are currently publishing our work on fuel cell optimization.

ME 392: Undergraduate Projects in Mechanical Engineering

Supervisor: Dr. Josiah Knight and Dr. Nico Hotz

Dates: January 2018 – May 2018

Hours: 8 hours/week · 15 weeks = 120 hours

Continuation of ME 490 (Fall 2017). Following some initial characterization and preliminary optimization last semester, we’ve developed a more sophisticated control system for running the fuel cell and measuring power outputs without having to manually read displays. This has allowed extensive data collection and thus more fine-tuned experimentation. We have been working on developing a standard baseline for fuel cell operation at its most ideal conditions. We are now starting to put constraints based on vehicle operation during the race and attempting to optimize efficiency for the duration of the race. One variable we are optimizing is purge frequency. As the fuel cell runs, its power output and efficiency decrease and the old hydrogen must be purged from the fuel cell. The frequency and duration of this purge can be optimized to bring efficiency back up while wasting the minimum amount of hydrogen. Another variable we are testing is humidity. Theoretically, humidifying the hydrogen increases efficiency by decreasing ohmic losses in the membrane. We are attempting to implement a passive bubbler to humidify the hydrogen, which should increase efficiency. Other variables we are interested in are temperature and pressure.

The goal is to implement the optimized fuel cell into a prototype vehicle for the Shell Eco-Marathon competition in April. The fuel cell will provide the necessary propulsion power for completing the track in time. We are competing for the world record of the most efficient vehicle, and hopefully optimizing the fuel cell efficiency can help us break the record.

 

ME 490: Transportation Energy (Duke Electric Vehicles Hydrogen Fuel Cell)

Supervisor: Dr. Josiah Knight and Dr. Nico Hotz

Dates: August 2017 – December 2017

Hours: 8 hours/week · 15 weeks = 120 hours

In the 2017-2018 academic year, Duke Electric Vehicles is planning on expanding our organization by also building a hydrogen fuel cell vehicle. This is an extension of the work I’m doing in the Thermodynamics and Sustainable Energy Laboratory, as the ultimate goal is producing hydrogen for use in fuel cells, whether they are residential-based or located in vehicles. This is related to my GCS focus as it attempts to solve the problem of storing solar energy. Generated hydrogen from the solar thermal collector can be compressed and stored on-board the vehicle, and then be used for energy to drive the car.

My work focuses on the requirements of a hydrogen fuel cell and how to incorporate a fuel cell into a moving vehicle. There are many specific challenges required in developing a fuel cell vehicle, one of which is the power requirement of the vehicle during acceleration and while climbing hills. The fuel cell is unable to produce enough power during these high-intensity portions, so the idea is to store excess energy produced by the fuel cell in super-capacitors and discharge this power as needed. Mechanically, one challenge is finding the best way to transfer hydrogen from a compressed tank through a pressure regulator, flowmeter, and flashback arrestor to the fuel cell at an inlet pressure of 0.5 bar. Another challenge is prevent excessive motion of the fuel cell when traveling over bumps. The goal is to characterize the fuel cell to maximize efficiency while running, and incorporate other efficiency improvements such as temperature regulators and humidifiers.