The analysis also concluded that even replacing ten-year-old shuttle buses with modern vehicles that used the same fuel source would cut operational emissions by over 60 percent.
PresidiGo Capital Plan: A Technical Analysis of Alternative Transit Fleet Fuels and Transition Strategy
The Presidio of San Francisco is a large park managed by the National Park Service and the Presidio Trust, which itself is a federal agency charged with operating the park without relying on taxpayer support. The area is supported by the PresidiGo transit service, which provides sustainable transportation for users in the area. It owns a fleet of Compressed Natural Gas (CNG) buses and a CNG fuelling station, with a contractor operating and maintaining both.
Recently, the Presidio Trust began reviewing various development proposals that may require the relocation and redevelopment of the bus fueling and storage area. In light of this, the Trust decided to examine the feasibility of transitioning the fleet to alternative fuel technologies. In particular, three scenarios were evaluated: Continuing to use CNG vehicles, using Renewable Natural Gas (RNG) in the future, and converting the fleet to battery-electric buses (BEBs).
The analysis was performed by the Volpe Transportation Center and sought to understand specifically the relative benefits and costs of each scenario, both in terms of monetary cost and in the environmental impact of each method.
The analysis found that replacing the fleet entirely with BEBs would completely eliminate fleet emissions of carbon monoxide (CO), nitrogen oxide (NOx), and volatile organic compounds (VOC).
The use of BEBs would also reduce operations and maintenance costs, such that fully replacing the heavy-duty portion of the fleet with BEBs was projected to save $17,000 per year in fueling costs and $25,000 per year in maintenance costs.
Savings were consistent when the park's continuous-service fleet was examined, with savings of $16,500 per year in fueling costs and of $15,000-$20,000 per year in maintenance costs, despite current technological limitations requiring the use of more vehicles.
Replacing the heavy-duty shuttle fleet with modern CNG buses would reduce CO, NOx, and VOC emissions by 58 percent to 80 percent each, resulting in overall emissions savings of approximately 2,000 kg (2.2 tons) of gases.
By sourcing RNG instead of CNG to fuel the existing fleet, the Trust could realize immediate emissions reductions. Most RNG benefits were achieved during the fuel's production, not its use, which meant that the tail pipe emissions of RNG and CNG were equivalent. Regardless, switching to RNG would reduce CO2-equivalent emissions by 75 percent at minimal or no cost.