Testing An Energy Storage System
There are established test procedures to consistently verify energy storage systems (ESSs)* characteristics, in a technology-neutral way, that are relevant to utility application requirements.
These test procedures provides a set of best practices for characterizing ESSs and measuring and reporting on their performance. They serve as a basis for assessing how individual ESSs will perform with respect to key performance attributes relevant to different applications.
- Rating – The total (or apparent) power expressed in kVA or MVA unit is the product of rms voltage and current following a duty cycle specified by the manufacturer.
- Performance – The performance of an ESS is verified by conducting a Reference Performance Test (RPT). A set of measurements (i.e Round trip energy efficiency, Duty-cycle roundtrip efficiency, Ramp rate test, etc.) are performed to determine baseline system performance that can be subsequently used as a baseline to assess any changes in the condition of the system and rate of performance fade over time and use. RPT test is repeated at regular intervals specified by the manufacturer during cycle testing for same-system comparison purposes. Such intervals can be selected to identify how the testing or operation affects the performance of the system and can be in units of time, number of cycles, or energy throughput.
- Capacity – The ESS is tested for its stored energy capability at selected power in accordance with the manufacturer specification.
- Round-trip Efficiency – Round-trip efficiency indicates the amount of usable energy that can be discharged from a storage system relative to the amount of energy that was put in. This accounts for the energy lost during each charge and discharge cycle due to mechanical, electrochemical or electronic losses. Typical values range from 60% to 95%.
- Response Time – Response time is the amount of time required for a storage system to go from standby mode to full output. This performance criterion is one important indicator of the flexibility of storage as a grid resource relative to alternatives. Most storage systems have a rapid response time, typically less than a minute and are typically very accurate in reference to signals received.
- Ramp Rate – Ramp rate indicates the rate at which storage power can be varied. A ramp rate for batteries can be faster than 100% variation in one to a few seconds.
* DOE, SAND2013-7084