Narada Power Sources demonstrates successful PSoC lead acid battery operation in an ALABC project

Narada recently concluded a three-year ALABC project (1315 STD1) in which an advanced lead-acid battery was to be adapted for power-assist and power-recovery duty in battery/diesel hybrid Rubber Tired Gantry (RTG) port cranes, and the results have produced a positive outlook for lead batteries in demanding new energy storage applications.

Background

RTG cranes are ubiquitous in container ports, and a vast number of these mobile units are not connected to the power grid and are powered by a diesel engine.  This diesel engine based operation is however not very energy efficient, consuming up to 2 liters of fuel per single container movement and emitting copious smoke and atmospheric pollutants during its operation. 

Narada, in collaboration with a Chinese container port operator, has identified an opportunity for the lead-acid battery to halve the fuel consumption and increase diesel engine combustion efficiency. This was to be achieved by recovering the otherwise wasted energy produced by regenerative braking during the lowering of the container and using this energy to power-assist a downsized diesel engine during container raising.

The technical challenges in this project resided in being able to accept and deliver power in 15-second bursts with an VRLA/AGM battery for over 14,000 cycles, or approximately 1 month of 24/7 simulated container transfer operation without the battery reaching any operational voltage limits. A total of over 168,000 container transfers was set as a lifetime goal.

This repetitive power-assist/power-storage operation requires that the battery be operated in a partial state-of-charge (PSoC), an operation mode that requires an adaptation of its internal design and ultimate operation parameters. Based on technical solutions implemented in Start-Stop SLI batteries (also operating in PSoC), the modification of the negative active mass with carbons was the first avenue of performance optimization explored. In this experimental phase, multiple allotropes of carbon ranging from carbon black, graphite, activated carbon, milled carbon fibers and multilayer graphene were incorporated in the negative active mass so as to enhance to voltage stability under PSoC operation. 

Results

The electrical tests of the ensuing batteries were complemented by the quest of understanding how a carbon addition was modifying the active mass structure and possibly influencing battery operation. This research task yielded some new data on carbon-negative active mass interaction during manufacturing steps. The research team was however surprised when the analysis of the cycling data showed that the impact of any carbon on the hybrid RTG-crane battery performance was minor compared to the effects of the concentration of the battery electrolyte and the actual state-of-charge at which the power-assist and power recovery operation was carried out.

The experimental effort was thus redirected to focus on these two factors. The power-assist and power recovery operation algorithm was adapted in such a way that the most important signal of performance of the battery, its voltage on discharge, would steer the level of power-assist energy, provided during the container hoisting phase, to the diesel engine circuit. This would also yield additional fuel saving opportunities.

The improvement was immediately forthcoming with the number of successful power-assist cycles achieved, going from a meager 700 to over 26.000! The hybrid RTG crane battery was thus operated at a state-of-charge of about 20% of rated and an associated electrolyte density much below 1.20g.ml-1. 

The investigation was then rounded off by fine-tuning these operating conditions and by confirming the reproducibility of the found performances. 

The ultimate service life of the Narada VRLA/AGM-based RTG crane battery and in pulsed power service was also confirmed in the laboratory, yielding a possible life of more than 230,000 power-assist cycles equivalent to more than 1,200 equivalent C10-rated capacity throughputs. The latter number is considered a major breakthrough demonstrating that well-designed VRLA/AGM cells are serious contenders for demanding energy storage duties.

The discussions with the Chinese port operator have resulted in the decision to equip one of their cranes with such a battery, first to enhance workstation-to-workstation battery-powered crane mobility, and to be then followed up by a full-fledged battery supported power-assist and power-recovery operation.