|The Solo EV. Click to enlarge.
UK-based Optare PLC has introduced the Solo EV, a full battery-electric bus. The Solo EV is a progression of the established Solo family of buses. The Solo EV is available in lengths of 8.1m, 8.8m and 9.5m and widths of either 2.3m or 2.5m.
Replacing the usual diesel engine is an all-new electric drive, featuring an Enova Systems P120 AC induction motor rated at 120 kW and powered by two banks of Valence Lithium Ion Phosphate batteries. The two packs work in parallel and provide 307V with a total capacity of 80 kWh. The batteries are housed in two steel crates set either side of the centrally mounted motor for excellent weight distribution.
|The Solo EV power pack. Click to enlarge.
Behind the motor are the cooling radiator, electric power-assisted steering pump, motor controller unit and charger unit. On the right, above one battery crate, is the electric air compressor.
The motor controller contains the power inverter to drive the motor, two 8kW AC inverters for the power steering and air compressor drives, and a 24V DC-DC converter to charge the standard vehicle batteries. This is contained in one fully sealed and water-cooled unit.
The cooling system is non-pressurized and cools the motor, motor controller and battery charger. A single electric fan draws air from the inside of the engine bay and blows it out of the underside. The water pump is controlled so that it is used during normal driving and also during overnight charging. The pneumatic system is supplied from a rotary vane compressor which couples to a standard Solo air-dryer allowing the normal service items to be carried over.
Effective heating for the driver and passengers is achieved using an Eberspacher Hydronic M12 auxiliary heater, housed in the area normally occupied by the diesel fuel tank and connected into the standard Solo heating circuit. The heater, header tank and pump are easily accessed for servicing. The installation ensures that the front circuit is prioritized for rapid screen demisting but also has sufficient capacity to maintain the cabin at a comfortable temperature of 21 °C (70 °F).
Vehicle top speed is limited to 90 km/h (56 mph) although this can be further restricted for in service requirements.
The vehicle controls are the same as for a diesel powered Solo apart from amendments to the driver display to accommodate a battery ‘fuel’ gauge and warning lights. The familiar driving experience is retained with overrun braking set to mimic the driving characteristics of the conventional vehicle.
The vehicle has regenerative braking with the drive motor being used to slow the vehicle and recover energy to the batteries. This increases vehicle range and reduces brake component wear. This system is supplemented by the Solo’s foundation brakes.
No retarder is required as the motor is used as a generator under braking conditions to achieve retardation and provide power to the batteries. Lifting off the throttle automatically signals the motor control system to produce a small amount of regeneration to mimic normal diesel engine braking. First pressure on the brake pedal starts to increase the level of regeneration up to its maximum, just prior to the service brakes coming into play. This recovers up to 60 kW of energy back into the batteries during deceleration and gives the same retardation as the electromagnetic retarder fitted to the diesel Solo.
An on-board charger can be plugged into a standard 3 phase outlet at the depot; a full charge can be achieved in less than 8 hours. Optional boost charging can be undertaken through the day either at the depot or using a suitable power outlet. The charger is programmed to match the specific charge routine of the batteries and also equalizes the batteries at the end of the procedure. This latter phase gives improved battery life and charge utilization.
The battery charger is water cooled to improve thermal management and communicates with the motor controller unit so that the water pump and 24V DC-DC converter are switched on during charging. The unit shuts all systems down at the end of the cycle. The charging process is fully automated and the vehicle can be left connected without risk of overcharge.
Optare is investigating range extension, achieved by adding a third set of batteries under the floor or on the roof. Due to the design of the Solo this can be accommodated within the axle weight limitations, and could improve the range by nearly 150%. When introduced, this could be made available as an optional upgrade to existing vehicles.
Roy Stanley, the founder and currently chairman of the Tanfield Group Plc, parent of Smith Electric Vehicles, is a Non-Executive Director of the Optare Group. As of 15 July 2008, Stanley held 28.4% of Optare Group shares.
(A hat-tip to John!)