OPTIMIZED CONTROL OF BATTERY BANKS IN RENEWABLE ENERGY SYSTEMS TO MITIGATE DIESEL-GENERATOR EMISSIONS IN THE AMAZON REGION OF PERU

June 2002

Traditionally, Peru has relied on hydroelectric systems to supply the majority of its electricity. At present, these systems account for 82% of the energy generated by the national grid system. By contrast, isolated rural communities in Peru use diesel generators as remote-area power-supply (RAPS) systems to meet their electrification needs. Whilst such generators can provide power in areas remote from the grid, they can be expensive and can produce significant quantities of carbon dioxide and related pollutants. In order to provide a more cost-effective, lower-polluting option, a program to develop and demonstrate the viability of large-scale, solar-diesel-generator-battery RAPS systems in the Amazon Region of Peru was commenced. Electric Applications Inc. staff were employed to develop advanced operating strategies that both minimize pollution from diesel-generator operation and maximize battery life, and evaluate battery performance under this duty. The schedule was based on the partial-state-of-charge (PSoC) principle, and involves operating the batteries below a full state-of-charge (SoC) for extended periods, i.e., 28-days. Two 24-V battery banks comprising thick, flat-plate gelled-electrolyte technology, termed 24V1 and 24V2, were operated under the preliminary strategy. Each bank has completed 4 months of service, i.e., four, 28-day, ‘master cycles’. Bank 24V1 commenced cycling immediately after conditioning, whereas 24V2 was first subjected to a procedure developed specifically to age the batteries. Using a method developed for estimating SoC based on open-circuit voltage (OCV) measurements, it is found that controlling charge-discharge by ‘Ah counting’, results in a 3% and 13% decrease in the SoC at the end of the master cycle for the new (24V1) and the aged (24V2) batteries, respectively. Cycling also shows that the maximum voltage of both banks attained during the daily charge period increases significantly during each master cycle. This can give rise to charging inefficiency. Given this behavior, and the observed limitation of the Ah counting method, it is recommended that the number of PSoC cycles per master cycle be reduced to 14.It is further recommended that the SoC, as determined by Ah counting, should be adjusted on a regular basis, using the OCV-SoC relationship. The diesel generator should commence charging if the SoC decreases below 40% (as determined by Ah counting). If this occurs within 13 PSoC cycles, the generator should cease operation when the average cell voltage in the battery string reaches 2.45 V. If the charging is activated at the end of the 14th PSoC cycle, a battery-conditioning step should be applied.

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