Case study:

Opportunity Charging Wireless for buses in Madrid (Spain)

Abstract Within the last 15 years, many cities and authorities have decided to test and implement electric buses into their public transit networks. Besides using conductive plugs to charge the buses overnight at the depot, they also can be wirelessly charged with inductive energy transfer technology. When implementing wireless charging pads into the streets and considering charging time in operation planning, a bus with a small battery (e.g. 60 kWh) can complete a whole day of driving. Study case in Madrid has proven that this technology is applicable for everyday service. Within Europe, 17 cities are currently or going to use wireless charged electric buses.

Modern municipalities must manage local air pollution, which is caused by individual automobiles and heavy road traffic. One attempt is to electrify the local bus fleet. Around the world, pilot projects are running and testing different alternatives for electric bus systems, and none are perfect equals in terms of vehicle components used and operational integration. So far, there is no technical standard that has proven to be the best solution to fit in all cases. All-electric buses have a common attribute: their engines run on electricity. The drive train and other electric components of buses receive their power from a battery or overhead catenary lines. If a combustion engine retrofitted the onboard power supply chain to connect to an electrical system, it becomes a hybrid bus. One option to recharge the built-in batteries of electric buses is to use wireless inductive charging technology.

Recharging Strategies
Electric buses can be recharged in three different operational scenarios which presenting in this study: Overnight Charging, Opportunity Charging and In-Motion Charging.

Overnight Charging
A typical day operation of a bus consists of phases in which it runs on the lines and on which it is at the depot. For example, the bus leaves at 4 a.m. and returns at 8 p.m. The time at the depot is using for maintenance, cleaning, and preparation for the upcoming tour. An electric bus can use this period to recharge the batteries. The time to fully recharge depends on the battery capacity and the power output of the transformer. Overnight Charging is the strategy in which the time in the depot is the only chance to recharge the bus for the following operating day. It means that these vehicles need large batteries. For example, the 2018 EVOBUS eCitaro is a 12 m long Battery Electric Vehicle (BEV) that has lithium-ion batteries with a capacity of 243 kWh that lasts in a worst-case summer scenario for 150 km.

Opportunity Charging
Opportunity charging describes operational cases in which buses are not only recharged at the depot but also designated charging stations throughout the network. Buses do not have to drive back to the depot to recharge. Time and energy can potentially be saved. As a result, smaller batteries are needed onboard the buses. Accurate planning is necessary to ensure that no vehicle runs out of energy. Backup solutions for different scenarios must ensure stable operation in case of incidents, construction works, or detours. Nowadays, the minimum turning time at terminal stations is optimized to compensate for route delays. If delays occur, recharging time adds more waiting time as an additional constraint and modifications to bus tours may be necessary. To save time, recharging stations can equip with inductive charging pads. As soon as the bus occupies the station, it is detected, and the process of recharging initiated.

In-Motion Charging
In-Motion Charging enables electric buses to recharge while moving. This has the advantage that no extra time for recharging must consider during an operating day. When setting up smart, On-Line Electric Vehicles (OLEV) that have access to In-Motion Charging can run perpetually. As with opportunity charging, the flexibility restricts due to the fixed lines that must be called at regularly to make sure that the state of charge (SOC) is always high enough to reach the next power lane. Testing sites have shown that as few as 17 % of the network length equipped with shaped magnetic field in resonance (SMFIR) technology is enough for sufficient power transfer It is highly probable that setting up a redundant network of lanes for a reliable bus network will require more infrastructure. While this strategy is only used in combination with overhead catenary power supply, for the time being, there have been testing projects around the globe to equip roads with copper coils to wirelessly charge the batteries on the go.

The Technology of IPT Charge Bus

Wireless charging stations along the route top the state-of-charge (SOC) throughout a day of operation. At night the battery is fully recharged for the next day. IPT has also proven usability in many cases in Europe. An advantage of IPT is that the modular integration of components allows the installation of 100 kW modules up to 300 kW.

The positioning of the elements of the charging station is also variable. A typical setup is showing in the picture below. Charge Pad (1) and Track Supply (2) are installed directly underneath the road surface.

Monitoring and cooling units are placed in a case next to the bus stop. Depending on the setup, the gap between the roadside and vehicle side can modify. In Madrid, the air gap measures 130 mm and leads to a 10 % loss of power while transferring.

Madrid, the capital of Spain, has a population of 3.2 million inhabitants. The responsible authority for public bus transit is Empresa Municipal de Transportes de Madrid, S. A. (EMT). In 2016, bus line 76 was equipped with wireless charging equipment. Its average route length is 7 km, connecting the edge of the City Center with an outskirt. The map shows the geographic location of the route relative to the City Center. The charging stations are located at both termini of the way. The horizontal gap between the bottom and the top coils varies between 130 mm and 150 mm and is the result of different vehicle configurations (load, tires, etc.) and uneven surfaces. The vertical tolerance for vehicle positioning is 100 mm and must ensure by the driver.

The power of each charging station is 100 kW. and the lithium battery of the bus has a capacity of 124 kWh, which equals 154 Ah at ~ 650 V. They are configured to operate at a state of charge between 23 % and 100 % (Diaz, 2015). Effectively, 95.48 kWh using in operation. Considering the efficiency of 93 % of IPT, it takes theoretically 62 minutes for a full recharge. The minimum headway on the line is 10 minutes (MOOVIT, n.d.). During operation, an average of 6.95 minutes of recharging time at each terminus (Diaz, 2015) resulted in a minimum SOC of 55 Ah (~ 36 %) as shown in the figure below. This value can be interpreted as stable enough in case a charging period has to be skipped, to reach the depot and to compensate for the ageing of the battery.

Wireless charging is a cutting-edge technology that many transit authorities consider worth adopting. Inductive pads for opportunity charged electric buses – the specific configurations of batteries and charging power differ. More projects are coming up in the future within Europe. The main issues are the reduction of flexible operation and the integration of high-voltage equipment within the public space.

Mr Bittler from IPT expects that more manufacturers, especially those entering the market, will soon produce buses using induced power supply technology such as the “Vero” from the company TAM Europe. Ongoing standardization and reduction of costs, especially for high-power charging stations, will increase the economic efficiency and therefore the adoption of wireless charging. He states that market leaders in bus manufacturing fail to integrate new technologies in their existing products. IPT received positive feedback from the IPT networks in Turin, Utrecht, Milton Keynes, London, and Madrid. As new players join the market, and battery technologies continue to develop, popular wireless charging use may come to success.

Author: Sawilla, Swenja; Schütt and Oskar
Source: Department of Transport System Management University of Applied Sciences Karlsruhe


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