In addition to the promise of emissions-free transit, the challenges of operating a bus fleet on an island led Martha’s Vineyard Transit Authority (VTA) to begin converting to an electric bus system in 2018. Since the VTA also needed to be able to operate when the main grid goes down, the authority opted to implement a microgrid, which uses on-site solar power and battery energy storage for a resilient independent energy source that also significantly reduces carbon emissions. Pioneering a decarbonizing model for transit, the combined system supports a more reliable and environmentally responsible public transportation network for the 20,600 year-round residents on Martha’s Vineyard Island who live in six towns connected by the bus service. The all-electric bus fleet will eliminate 36,000 tons of carbon dioxide over ten years of driving 1.4 million miles annually.
The single-user microgrid designed by Arup, PXiSE and Vermont Energy Investment Corporation is an energy distribution, storage, and generation network that can be disconnected from the main grid during power outages to then tap into its own stored electricity and solar-generated power. This system also allows the agency to reduce its demand on the grid during peak hours and enables it to charge vehicles overnight through its energy storage infrastructure without interrupting service.
Located at VTA’s existing depot in the town of Edgartown, the microgrid combines a new 700 kW DC solar PV array, battery energy storage and a diesel generator as back-up, as well as 16 vehicle charging stations, with 20 more to be added in the coming years. The fleet is charged largely through solar energy.
The network will also include induction charging stations that recharge the buses on route, allowing them to stay in service for their 200 to 300 miles-a-day circuit without detouring to recharge. Planned for operation in spring 2022, the stations are wireless charging plates embedded in the ground at bus stops. The vehicles will drive over the charging stations as passengers board and exit the vehicle, enabling buses to be partially charged numerous times throughout the day at each stop. With on-site battery storage systems to provide energy cost reduction, and emissions free backup power, these charging stations easily blend in with Martha’s Vineyard’s natural landscape.
Accessible by boat and air only, the 88-square-mile island depends on imported fossil fuels for electricity and to power vehicles. This not only translates into higher fuel and energy costs, but it also contributes to greenhouse gas emissions and leaves the island vulnerable to power outages during major weather events. As an electrified fleet combined with a single-user microgrid, the VTA’s new transit system brings a cleaner, more resilient and economic model to Martha’s Vineyard.
36,000 tons of carbon to be eliminated over 10 years
700 kWDC solar PV array
32electric buses serving 6 towns
The natural evolution to a microgrid
Once the VTA began transitioning to electric buses, the agency had to address the issue of how to reliably, sustainably, and cost-effectively keep the buses charged. Options included deploying diesel or natural gas generators for when the grid is not available or when electricity is at peak cost. For the VTA, using diesel generators to regularly charge bus batteries would defeat their cost-savings and sustainability objectives. This led to their decision to install a large onsite PV system at the depot to generate their own energy. Solar provides 100% clean renewable energy for the VTA at a lower price than if purchased from the utility, which also would have higher carbon content.
Solar is free, abundant, and zero carbon, however, it must be stored in a way that can be tapped into when needed. The VTA chose to pair battery storage with its solar energy system to manage utility costs and allow them to operate when the utility is down, thereby creating a microgrid.
Optimizing electric fleet operations
Along with operating cost savings and reduced carbon emissions, in comparison to diesel buses, the VTA’s electric fleet is also quieter inside and out, has no adverse impact on local air quality, and requires less maintenance. Unlike a diesel vehicle that can be quickly fueled—often conveniently drawing from large stores onsite—a battery electric bus, requires a longer time to recharge.
This is where the VTA’s microgrid plays a key role in supporting its fleet operations and costs. It uses complex algorithms and forecasts to balance and optimize energy costs, carbon emissions and resilience. The microgrid also calculates each bus’s energy consumption for the next day and plans charging for the fleet accordingly. Similarly, in the event that utility power is lost, the microgrid starts its onsite generators to charge the fleet overnight, when solar is not available, thereby ensuring that all buses are at full capacity as scheduled the next morning. Additionally, the VTA’s microgrid incorporates maintenance needs and maintains safe electrical infrastructure operations—all without user intervention.
Translating microgrid operations for transit
Moving to a microgrid required the VTA to fundamentally rethink how they fuel their fleet. For example, once a battery electric bus is on the road, its operation is much like a diesel bus, however when it returns to the depot, facility operations and recharging depart considerably from diesel procedures, upending many conventional transportation processes. Therefore, Arup worked in close partnership with the agency to provide microgrid engineering to design a system that supports the VTA’s transportation requirements for reliable, regular transit while leveraging the microgrid’s smart technology for a new level of operational efficiency.
The Arup Journal 2022 Issue 2
Learn more about this project in the Arup Journal. Applying a gender lens to sustainable transport system design, mass timber construction in the USA, and mitigating the impact of climate change on coastal areas – discover more in the Arup Journal.Download