500kW output over three to eight hours. The competitor AmbriTM offers a liquid metal battery of equivalent life expectancy and built to comparable dimensions to 20- foot shipping containers (20-foot length by 10-foot width by eight- foot height) with 500kW output capability. Both technologies avoid the cycle-to-cycle fade that plagues some electrochemical storage battery technologies.
The vanadium-oxide flow battery can operate with ambient air temperatures between -20-deg C and 50-deg C with 400-volts output and a specific energy density of about 14Wh/Kg. Given that barges have ferried sections of laden freight railway trains across rivers, a tug could be built to carry the filled weight of vanadium-oxide batteries as well as the weight of molten metal batteries that operate at over 300-deg C. Adapting liquid metal battery technology to waterway operation would require innovation in insulation, such as using sections of zirconium-oxide to secure the battery to the vessel structure.
ADVANTAGE OF SCALE
A tug and also a battery powered vessel involve a much larger scale of transportation technology than road going or railway vehicles. The larger scale and comparatively low travel speed allows for viable use of long-life grid-scale battery technologies that would otherwise be impractical for road and even railway transportation. A maritime vessel that sails the inland waterways will outlast road and railway commercial vehicles, enhancing the viability of investing in grid-scale battery technology to store propulsive energy. Commercial waterway vessel operators have competing battery technologies from which to choose for their unique applications.
BATTERIES AND VEHICLE TECHNOLOGY
Battery powered road vehicles expend additional energy carrying the weight of the energy storage
system to climb to higher elevation, requiring light weight batteries with very high specific energy density in excess of 50Wh/kg. Commercial road vehicles such as school buses only operate during AM and PM periods for three to four hours per weekday. Battery powered city transit vehicles can undergo regular partial recharges throughout the service day at terminals and stations. In terms of energy consumed per unit of vehicle weight, road and railway vehicles are less efficient than maritime transportation.
Commercial electric battery propelled waterway vehicles do not have to carry the weight of the energy storage technology up gradients. When in transit to a different elevation at navigation locks, there is scope to partially recharge the batteries. Due to sheer physical size and weight carrying capability, battery powered waterway vehicles can utilize long-life grid-scale batteries with comparatively low specific energy density of 10 to 20Wh/Kg. At the present time, none of the battery technologies applied to road and railway propulsion can offer the extensive useable life expectancy of grid-scale batteries that can be adapted to waterway propulsion.
EVOLVING BATTERY RESEARCH
Research into flow batteries undertaken by the Materials Engineering group at National University of Singapore (NUS) has focused on increasing the energy storage density of redox flow batteries, including higher storage density from vanadium based flow batteries that can apparently hold up to double the energy and at temperatures of up to 80-degrees C. Flow batteries include large tanks of aqueous electrolytic solution of materials such as vanadium oxide and even salt solutions. Pumps flow the aqueous solution through stacks of cells to deliver electric power.
NUS research into Redox flow lithium batteries has focused on offering up to five times the storage
density of vanadium flow batteries which if applied to vessels sailing along inland waterways, should allow for up to five times to sailing distance. A flow battery concept still subject to research and further development at NUS uses two tanks, one tank of titanium oxide with a second tank of lithium-iron- phosphate with potential of holding up to 10 times the storage density of vanadium redox flow batteries. If the research is successful, the technology might find maritime propulsive application.
TUG BARGE ON WATERWAY
Depending on geographic location, barges of 35 feet (10m) beam sail along waterways with a draft of six feet (1.8m) to nine feet (2.7m). The tug pushing an navigating the barge sails in the hydraulic shadow of the vessel ahead of it, allowing the tug to sail an almost identical cross-sectional profile and allowing the weight of grid scale batteries to be spread of a large area of floor surface. A 180-foot tug could carry 12 grid-scale batteries built by Cell-Cube to the same dimensions as a 40-foot shipping container and placed three lengthwise by four widthwise.
Each vanadium-oxide flow battery could deliver 200kW to 500kw for three to eight hours meaning 2400kW (300-Hp) to 6000kW (8,000-HP) with deep-cycle recharge-discharge capability of over 20,000-cycles. Stopping to transit navigation locks provides opportunity to partially recharge the batteries as the tug-barge assembly changes elevation. To help accelerate tug barges and barge trains leaving some navigation locks, it may be possible to install a short distance of trolley cable along the channel bank to allow the tug barge to draw electric power directly from the grid, with batteries blending in as the assembly reaches its cruising speed.
WATERWAY TOUR BOATS
While mega-size ocean going tourist ships attract massive numbers of customers, a
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