US Energy Storage Update

US energy storage plants

Tehachapi is nominally a wind energy-related project attached to the 4.5 GW.
When it opened, in September 2014, it was credited with being the largest battery storage project in the North of the US, with 604,832 Li-ion cells housed in 10,872 modules.

This blog focuses on the Borrego Springs, Tehachapiand and Notrees energy storage projects and examines the long-term impact that they will have on the future project pipeline in the US.

According to the Energy Storage Association’s US Energy Storage Monitor, 60.3 MW of storage was deployed in the third quarter of 2015, a twofold year-on-year increase.

Much of this comes from the early adoption of electrical energy storage.

The US has pioneered the use of batteries for grid-scale storage applications.

California, utilities started commissioning significant projects as far back as 2012. And increasingly other states in te US, are emerging as key markets for grid-scale energy storage, with utilities there initiating projects, too.

Especially California has a long experience of grid-scale electrical energy storage.

US: What to expect in future?

This experience is critical going forward because one of the big challenges facing electrical storage deployment is not just how to follow best practice now, but also what to expect in future.

We felt it would be important to review a number of pioneering US projects and assess how their performance has lived up to expectations. The three projects chosen for analysis in this report all belong to investor-owned US utilities and have significant operational experience. They are:

  1. The Borrego Springs microgrid project owned by SDG&E in California
  2. The Notrees wind storage demonstration project owned by Duke Energy in Texas
  3. The Tehachapi wind energy storage project owned by SCE in California

Borrego Springs microgrid

The successful microgrid Borrego Springs is a leading microgrid project designed to show how utility storage assets could help to stabilize output from third-party-owned distributed generation assets, predominantly solar, in a remote community.

  • The Borrego Springs community is served by a single sub-transmission line and SDG&E chose to install a microgrid there to improve the reliability of electricity supply while avoiding the need for additional transmission capacity.
  • Batteries
    • Lithium nickel cobalt aluminum, 1.6 MW, 4.7MWh
    • Battery supplier: Saft (sub-station storage) and S&C Electric (community storage)
    • Power electronics: Parker Hannifin (sub-station storage) and Kokam (community storage)
    • Applications: load following, renewables capacity firming, transmission congestion relief and distribution upgrade deferral
  • The microgrid features a mix of technologies, including two 1.8 MW Caterpillar diesel generators, about 700 kW of rooftop PV and 125 home area network systems.
  • The storage portion of the microgrid is also heterogeneous, initially comprising a 0.5 MW/1.5 MWh battery system for peak load reduction at the local substation, plus three 25 kW/50 kWh community and six 4 kW/8 kWh residential battery systems.
  • This initial configuration was subsequently enhanced, with the substation storage capacity rising to 1.5 MW/4.5 MWh.
  • Operational highlights
    • The microgrid initially served 1,060 customers in the community, and in September 2013 managed to maintain a power supply to users during the hottest hours of the day following an outage caused by flash floods.
    • In 2015, SDG&E received a $5 million California Energy Commission grant to extend the infrastructure across the entire Borrego Springs metered customer base of 2,800.
      The expansion is due for completion in mid-2016 and involves the integration of a nearby PV plant, the 26-MW Borrego Solar project owned by NRG.
    • In May 2015, the community was due to suer a 10-hour outage while the utility replaced poles carrying the distribution line.
      In the event, Borrego Springs was switched over to the power supply from the NRG solar plant, which provided more than half the energy needed for the entire community. The rest came from the microgrid’s batteries and gensets.

Notrees: adapting battery chemistry to applications

Duke Energy developed Notrees alongside a 153 MW wind power project in partnership with the Energy Reliability Council of Texas (ERCOT) and the DoE. The project is notable for being one of the biggest battery installations in the country.

  • Batteries
    • Advanced lead-acid/ Lithium ion (Li-ion), 36 MW, 24 MWh
    • Battery supplier: Xtreme Power (phase 1) and Samsung SDI (phase 2)
    • Power electronics: Younicos
    • Applications: ramp control, electric energy time shift, renewables capacity firming, frequency response and voltage control
  • Operational highlights
    • When Notrees was completed, in December 2012, its main intended purpose was to provide renewable integration services such as frequency response, ramp control, voltage support and energy time shifting for the wind farm next to it.
    • It was expected that the advanced lead-acid batteries supplied by vendor Xtreme Power would be appropriate for these use cases.
    • However, according to Younicos, lead-acid proved a poor fit for a growing trend towards the provision of grid services, such as frequency regulation, at Notrees.
    • This prompted Duke to announce an upgrade in battery technology in June 2015 .
      • A first phase of the repowering project, involved replacing 18 MW of the 36 MW of lead-acid batteries with Li-ion.
      • This is now being run in parallel with the remaining lead-acid-based storage.
      • A second phase, due for completion before the end of 2016, will see the entire facility converted to Li-ion.
      • Operation of both lead-acid and Li-ion technologies is managed automatically by a control system installed by Younicos, which takes signals from ERCOT and from the wind farm.
  • Duke Energy has commissioned a significant upgrade:
    The experience underscores the need to consider carefully which applications you will be developing before you choose a particular battery chemistry for your project.

Tehachapi: dealing with integration issues

Tehachapi is nominally a wind energy-related project attached to the 4.5 GW Tehachapi Wind Resource Area, although in actual fact the plant was conceived as a two-year test bed for a wide range of potential grid applications.

When it opened, in September 2014, it was credited with being the largest battery storage project in the North of the US, with 604,832 Li-ion cells housed in 10,872 modules.

  • Batteries
    • Lithium ion, 8 MW, 32 MWh
    • Battery supplier: LGChem
    • Power electronics: ABB
    • Applications: electric supply capacity, renewable capacity firming, transmission congestion relief, distribution upgrade deferral and voltage support
  • Operational highlights
    SCE has encountered a number of challenges in the development of Tehachapi:

    • It was created in response to a DoE demonstration project request and the design parameters were consequently constrained by the DoE’s requirements.
      The utility was originally only able to find one vendor, A123 Systems, which could meet the project specification for grid-scale Li-ion technology at the right price.
      Before implementation, however, A123 Systems went into receivership as a result of manufacturing problems, forcing SCE to source alternative vendors.
    • By the time of its second request for proposals, SCE was able to source 5 vendors that could meet its technical requirements “within the cost envelope”.
    • The company chose LGChem to provide the batteries and overall energy storage system.
    • For the power conversion and system integration work it was necessary to bring in ABB.
    • SCE opted to create a ‘mini system’ based on two racks of cells only, but with all the integration components in place.
    • To get this to work properly took several months and 11 versions of the control software.
    • Even after commissioning, the system has been subject to problems. Issues with the design of the system led to the catastrophic failure of one of its four transformers, which required all of them to be replaced over a period of around 5 months.
    • SCE is currently considering extending Tehachapi’s original two-year lifespan to 10 years and using the facility for commercial applications. Which applications these might be is still unclear.
    • As part of its demonstration remit, the project is being used to evaluate 13 applications and eight separate use cases, testing multiple applications simultaneously at the facility.
    • In the meantime, SCE is using its experience from Tehachapi to inform the development of other storage projects, including a 2.4 MW/3.9 MWh plant that is due to be awarded during 2016.
    • Most of the problems in Tehachapi’s development have been nothing to do with storage. The transformer problem was due to mistakes with design and the integration problems had to do with it being the first of a kind


Problems with the US Notrees and Tehachapi projects illustrate that getting grid-scale energy storage right is a complex affair. This issue is likely to wane with growing storage portfolios, but underscores the need for experienced project partners.

  • It is notable that Notrees and Tehachapi also both suffered from the loss of their first-choice battery vendors.
    This highlights the widely held wisdom of sticking to diversified, mainstream suppliers with strong balance sheets and track records.
  • None of the utilities listed in this report has published a return-on-investment calculation for its project.
  • All are pressing ahead with further storage plans.



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