Evaluation of lead-acid valve-regulated (AGM) cells for frequency regulation duty in electrical power grids
After having been installed for frequency regulation duty in the Chino/USA plant (14MW/40MWh) from 1988 until 1997 and in the Berlin-Steglitz/Germany (17MW/14MWh) plant from 1986 until 1994, Lead Acid batteries have faded from the limelight in this application due to technical issues and a changing business environment.
The massive installation of wind and solar power plants and the ensuing need of transient energy management and storage, has however led in recent years to consider batteries as the most appropriate vehicle to achieve this. Whereas Lithium-Ion batteries have captured most of the attention of investors and the press, VRLA/AGM based batteries with more than xx00MW power capability have been installed and are operated successfully.
For the qualification of batteries for grid-connected energy regulation and storage duty, two test methodologies, the IEC 61427-2 and the PNNL 22010/Sandia 3078, can be used.
Test results obtained with 2V1000Ah VRLA/AGM cells, using both test profiles, will be reported. The data show the need of fine-tuning of depth-of-discharge per service day in frequency regulation duty and the round-trip coulombic efficiency η achieved under test conditions.
When operating in such a grid stabilization duty, at any instance of time the battery must be able to deliver and accept the contractually agreed power and energy levels so not to incur in financial penalties.
For both methodologies appropriate battery SoC stabilization routines have been therefore defined and tested. In these test, the cells where equipped with conventional and carbon-modified negative active mass and two levels of electrolyte concentration.
The simulated frequency regulation duty according to IEC 61427-2 clause 6.2 and PNNL clause 5.3.2 was carried out at a SoC of about 50% (C10) for uninterrupted periods of up to 4 months without the need of significant time-outs for maintenance.
As the cells were equipped with reference electrodes, the polarization behavior over elapsed frequency regulation duty time of the positive and negative active mass was recorded. The data show that only minimal overvoltage changes (Δ <0.020Vpc over several months) occur and that the positive active mass is performance limiting. This has important bearing on future battery development strategies.
Note: This work is supported by an ALABC/CBI contract 1618 STFRLL
The author is involved in Lead Acid Battery research, development and production since 1969 and is, since 2007, an independent consultant. He is also active in IEC Battery standardization activities and current Chairman of IEC TC21 Secondary Cells and batteries.