Power-to-Gas and zero-emission buses in Uppsala : challenges and Opportunities from a techno-economic perspective

Abstract: This thesis has explored the use of power-to-gas technology as a means to produce hydrogen for fuel cell electric buses. The municipal council of Uppsala are looking at options to electrify the local city and regional bus fleet but have been faced with power capacity constraints on the local electrical grid. For an extended period of the day, the bus depots may only utilize 1.5 MW of power, which has proven a hurdle as battery electric buses were considered to be introduced. This thesis explored the techno-economic feasibility of using hydrogen to electrify buses, as the power consumption of fuel production can be shifted to hours where the grid is less constrained. Additionally, the coproducts of the electrolytic hydrogen production were considered as a potential cost reduction or revenue. In order to evaluate the economic feasibility, a model was created. Different scenarios of fuel cell electric bus fleets were described, and their assumed refueling strategy was derived from historic refueling behavior. Existing electricity demand for both depots were described, and the city bus depot, which only just started operating, had its electricity load profile modeled for a year from historical data in conjunction with insights from a previous thesis study. The model subsequently evaluated two different aspects, first the potential of power-to-gas without power constraints, and secondly the potential of a system faced with the power constraints. From the simulations, the model behavior was described, and the equipment sizes were determined based on meeting a set of criteria. The equipment sizes, energy consumption, hydrogen, and oxygen production and so forth, was then used to estimate the levelized cost of hydrogen. Oxygen was considered to be used as high value medical oxygen or as low value oxygen for local processes, such as for aeration in the local wastewater treatment plant. The heat developed from cooling the electrolyzer was considered for the site district heating demand as well as low value return heat. As a result, the levelized cost of hydrogen was shown to converge towards a cost of 35 – 43 SEK/kg H2, as the scale of the scenarios were increased. In the constrained scenarios, there were only two fleet sizes which could operate with the given constraints and criteria. The levelized cost of hydrogen of the largest possible constrained scenario was then used to compare the total cost of ownership for a fuel cell electric bus, a HVO diesel bus, and a compressed biogas bus. It showed that fuel cell electric buses with the existing capital cost, and the projected cost of fuel, is close to competitive with compressed biogas buses on price. The thesis also highlighted the industry price targets for fuel cell electric buses and determined the necessary fuel price to reach price parity with the existing buses. There is a plethora of aspects which could have been explored in better detail, and there are questions raised which could be explored in future studies. The model may include more dynamics, such as a stochastic behavior of fuel demand, a more rigid control method, an increased amount of renewable energy and so forth. Future studies could explore means to reduce the electrical load of the depots to improve the conditions for fleet electrification.