Electric Vehicle Applications of Flow Batteries

Published on 8th June 2012

Synopsis

Flow batteries have a unique capability; rapid and safe recharge of electric vehicles (EV’s) by electrolyte exchange. This is seen as the most efficient, practical route for bringing renewable energy (RE) to transport; ‘well to wheel’ efficiency is more than four times better than Hydrogen and because the ‘electric fuel’ is recycled continually, operating costs can equal conventional diesel fuelled vehicles. 24 hour vehicle availability can overcome the energy and power limitations of flow batteries.

New proprietary refueling technology described in this paper, will initially be used in the urban environment where zero emission vehicles are essential and later, with improvements in ‘electric fuel’ energy and power density, to the wider 50m global vehicle fleet where the market demand for automotive electric drive systems is now expected to exceed $25Bn by 2020.

Low carbon, zero emission urban transport – competing with diesel

To compete successfully with the flexibility and low cost of established diesel and LPG vehicles, electric city vehicles – taxis, buses and delivery vans need to have a highly flexible operating capability with minimum downtime. Many city vehicles operate 18 hours per day or more and simply cannot be serviced by conventional batteries due to excessive down time required for recharging. There are also severe limitations on the availability of fast charging points in cities due to lack of parking space.

Currently flow battery technology achieves an electrolyte energy density, around 25Wh/kg similar to industrial lead acid batteries and therefore cannot match the >100Km range between charges achieved by advanced batteries such as Lithium Ion. Flow battery technology can largely offset this disadvantage by rapid refueling and considerably lower operating costs.

Boost charging of conventional batteries can close the operational range shortfall but costs increase because of shortened battery life, peak time electricity use and unproductive downtime all contributing to higher operating costs per kilometre. Despite this there are many examples of successful city operations – notably the ATAC Rome fleet of 52 electric minibusses and the EA Technology Wavedriver postal vehicles with on-board fast charging.

Note that average city centre bus loop route in the UK is less than 12km. Flow battery powered vehicles can be safely refueled in five or six minutes – replacing two to three hours of operating time and coming close to diesel costs. Note that they will also be recharged electrically at off peak rates.

Flow battery refueling – how it works

Shown here is an artist’s impression of the proprietary flow battery refueling system being developed by RE-fuel Technology Ltd. The system will simultaneously pump in fully charged electrolyte from the base station while extracting discharged electrolyte from a vehicle. Features are:

• Vanadium Redox Flow Battery (VRFB) technology with enhanced electrolyte energy density. System will operate with any fully liquid flow battery system including air cathode types.
• Twin Catholyte and Anolyte electrolyte tanks with internal moving membrane separate charged from discharged electrolytes. Tanks are sealed to atmosphere and always full, so that venting is eliminated and tank volume is minimized on board the vehicle and in base station.
• A four way (two in, two out) dry-break coupling incorporates selective porting valve which ensures that appropriate tank ports are aligned before fuel pumping commences
• Safe non flammable ‘electric fuel’ allows high flow rates and energy transfer at 600kW equivalent. Customers may be billed on a per kWh basis
• Electrolyte is continually re-energised in the refueling station by wind, solar or grid energy according to availability. Off peak baseload power can be utilized and undesirable grid loads during peak times can be avoided. Vehicle may also be recharged by mains power.

Electric minibus feasibility study – Vanadium Redox Flow Battery (VRFB) power

A hypothetical feasibility study using currently available VRFB technology has been carried out on the Ecobus design. The Ecobus is a lead acid battery powered minibus which is operating in the Italian cities of Rome, Bologna and Milan and recently by EATechnology in Liverpool UK. Working within the existing battery envelope and based on the RE-fuel system with 30Wh/L electrolyte ED, a city based minibus could cover a distance of 30km between refueling points giving 2 to 3 hours of city centre operation. The refueling process could be carried out in 5-10 minutes fitting into a conventional diesel city bus schedule – the bus could operate for 24 hours a day if required. Other potential applications include Taxis, delivery vans and 24 hour fork lift trucks.

In conclusion, although domestic and urban recharging is now being rapidly deployed for small commuter vehicles there is limited availability of sites and lack of operating flexibility for urban transport operations. High EV operating costs mainly driven by battery depreciation are currently being overcome by state subsidies which are not sustainable.

Flow battery systems offer a unique refueling capability which can overcome both the limited autonomy and the high cost of advanced batteries making EV’s cost competitive with diesel fuel for city transport they also offer a smooth interface with zero carbon renewable power and maximum use of off peak baseload grid energy. Increased electrolyte ED and specialized lightweight vehicles will increase range between refueling stops and could open much wider global markets in the future. To make this happen a co-operative effort in the flow battery business is needed now to include partners among the forward thinking city transport authorities.