Denis Naberezhnyky and Ben Godfrey discuss the integration of electric buses with the electricity distribution network.
A Thinking City can mean different things to different people. A Thinking City could be described as a city where the use of limited resources and infrastructure is optimised in order to respond to demand in real time, while trying to minimise adverse effects on the provision of other services and use of other infrastructure. By doing so, it is possible to improve the efficiency and effectiveness of existing infrastructure, leading to a more sustainable society that is able to cope with the growing demands placed on our energy and transport infrastructure in cities.
In order to achieve this long-term vision of a Thinking City, the integration or, at the very least, communication between the different elements of city infrastructure is now required.
Imagine a city where all public road transport is electric and is not restricted by battery range as it charges wirelessly directly from the road as it continues its daily operations, producing no local air pollution and drastically reducing national CO2 emissions. In order to make sure that the local electricity distribution network is able to cope with the additional demand on the network, the buses continuously report their progress along the route along with their battery state of charge (SoC). The distribution network is then able to combine this information together with real time data on the quality of the network and prioritises charging of each bus accordingly. At the same time, other buses with excess battery capacity are able to feed power back into the grid in order to provide additional reinforcement if required. This vision of a Thinking City may not be as far from reality as it may first appear.
Demonstration of the possible impacts and opportunities of integrating elements of the transport system with the electricity distribution network is the aim of the Electric Boulevards project, a joint Western Power Distribution (WPD) – TRL, Low Carbon Network Fund (LCNF) project taking place in Milton Keynes, UK, a city some 100km north of London.
An entire bus route going all the way across Milton Keynes between Wolverton and Bletchley is being electrified as part of a demonstration programme. All eight buses on the route will be replaced with electric buses and to ensure that they are able to complete the demanding daily duty cycle without needing to stop for recharging or running out of battery charge, the buses are being equipped with inductive power transfer (IPT) chargers that enable the buses to opportunistically charge during end of route stops through the day.
In order for this concept to be feasible and not to disrupt exiting timetables, charging of the buses must happen over a very short period of time during scheduled end of route stops. This requires a very high level of power transfer of 120kW between the grid and the bus. For comparison, a typical electric car public charging post delivers between 3 and 7kW of power. Use of such high power transfer rates can place a substantial demand on the electricity distribution network and potentially introduce disturbances into it. WPD are investigating the impacts that the use of such buses and chargers can have on their distribution network and together with TRL, are investigating the feasibility of intelligently managing this impact, as well as identifying opportunities for improving the quality of the distribution network through the use of intelligent charging and vehicle to grid (V2G) applications.
The Milton Keynes demonstrator project will be installing two inductive chargers at either end of the number 7 bus route, at Wolverton and Bletchley. In order to fully investigate both the impact of the inductive charging equipment on the distribution network and potential for intelligent management of the chargers based on the condition of the distribution network, (without jeopardising the operation of the demonstrator), the project will install an additional, third charger at another location where the buses stop during the day – Central Milton Keynes (CMK) bus station.
This set up, and in particular the use of the third charger, allows the project to examine the impacts on the distribution network at different times of the day and at different locations. More crucially, it allows varying when and how much power is delivered to the bus from the third charger based on the condition of the network at the time.
Of course, a Thinking City doesn’t have to stop at optimisation of charging of electric vehicles in order to manage the load on the distribution network. Spare capacity of the on-board batteries in the buses could be utilised to feed power back into the network when existing loads become too high, or the power electronics in the charging equipment can be used to provide additional filtering on the network.
The Electric Boulevards project is also investigating the feasibility of such V2G applications. However, during the demonstrator it would be undesirable to discharge the vehicle battery directly and risk insufficient remaining capacity to complete the remainder of the daily duty cycle until a feasibility assessment has been carried out. Therefore, a stationary battery bank is being installed near the CMK charger in order to simulate the on-board battery of the bus by replicating the available capacity and SoC, and providing battery to grid (B2G) capability when necessary. In an actual implementation in a Thinking City, this B2G capability could be provided by the batteries on board the bus as a V2G application, at whichever location it happens to be charging at the time. The additional benefit of having a battery bank near the charger is that the battery can be used to provide the energy and power required to the chargers, instead of placing the entire load directly on the distribution network at peak time. The batteries can then be recharged off peak.
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