Pumped storage peaks in the US

13 February 2012

Bison Peak has been described as one of the most exciting and promising pumped storage projects on the horizon. Upon a target completion date of 2019 it could become the sixth highest head pumped storage project in the world. Project consultant Matthew Shapiro discusses the unique challenges and solutions that need to be tackled during the development of this US scheme.

The Tehachapi area of southern California is no stranger to being on the cutting edge of new energy projects. It is one of several gusty regions in which the wind power industry got its start nearly thirty years ago. It is today the site of the largest wind project in the US. It may also now play host to one of the most innovative pumped storage projects in the world.

Several years ago, the longtime owners of a mountainous parcel in the Tehachapi Mountains recognized that they had a topographic anomaly that could be valuable: a relatively flat mountain top surrounded by a number of ravines up to 914m below. They knew a significant vertical drop was one of the basic necessities for a pumped storage project, and that theirs was world class.

The Burbank-based Lucchese family, who have owned the land for three decades, had been nascent wind developers themselves and were familiar with the world of renewable energy. They were also aware of what large-scale energy storage might offer to intermittent wind and solar resources, particularly in the California power market.

The Bison Peak owners engaged Matthew Shapiro of Gridflex Energy and soon afterwards the project began to take form. The Bison Peak pumped storage project took concrete steps toward development in the form of a preliminary permit application submitted to the Federal Energy Regulatory Commission (FERC). The original application was withdrawn and resubmitted in May 2011 in order to accommodate additional lower reservoir options and an expanded study boundary. The preliminary permit is expected to be granted early in 2012.

While the project has a number of alternative configurations that will determine its ultimate sizing, it was filed as a 1000MW project based on its highest-head options - 853-933m - and a 500MW project using lower head options. If the highest head option stands, the project would have the sixth highest head in the world, and by far the highest of any existing or proposed pumped storage project in the US. Even the lower head options are world class, with maximums ranging from 658-762m.

The site

The project is located in the Tehachapi Mountains, approximately 161km north of Los Angeles. The mountains run northeast-southwest and overlook the Antelope Valley to the east. The name of the project comes from the designation on a marker placed on the top of Covington Mountain. Its 2408m summit would be the site of an approximately 0.2km2 upper reservoir formed with a ring embankment approximately 2.1km in length and rising 15-21m above grade. The upper reservoir would have an estimated capacity of 5500 acre-feet (6.8Mm3). Several lower reservoir options were identified within 3.2km of the upper site. To meet the specifications for the project, the lower reservoir options had to match the storage capacity of the upper site with a dam size that would remain within acceptable parameters.

The highest head reservoir option lies to the south of the forebay site on Covington Mountain. With a dam height of 94.5m and crest of 366m - most likely utilising an RCC gravity and/or arch design - the South option would feature a maximum head of 927m. While technically challenging, such a head would allow the project an estimated 16,500MWh of energy storage potential.

A second option, named the Tejon alternative, involves creation of the lower reservoir to the west of the forebay site, at the upper end of Tejon Canyon. This alternative would also afford an extremely high head: 872m at its maximum. The tentative dam dimensions for this option are 457m in crest length by 79m high.

The remaining lower reservoir siting options lay to the east and northeast of the upper reservoir site, in Horsethief and Sawmill Canyons. The Horsethief alternative involves a head range of 640-762m, while the Sawmill alternative offers a head range of 536-658m – the lowest of the alternatives considered. Dam heights for these are estimated at 95m, with crest lengths from 305-366m.

The conduit lengths for Bison Peak options range from 2.9km to 4km, with length/head ratios ranging from 4.2 to 4.5. Headrace diameters would range from 3.9m to 5.8m, depending on lower reservoir selected and ultimate project size.

On-site transformers would step up project output to 345kV, with a new line, 19-22.5km in length, running to either of two 500kV substations currently under construction by Southern California Edison as part of the Tehachapi Renewable Transmission Project. Interconnection options to Los Angeles Department of Water and Power and to Pacific Gas & Electric are being considered as well.

Storage time versus power

The South and Tejon options, representing the highest head options, would allow for an estimated 16,500MWh of storage. This is more than adequate for a daily peaking cycle of six to eight hours of full power generation at 1000MW, or even a wider, intermediate-level cycle if desired.

The Horsethief and Sawmill options offer an estimated 10,500MWh and 12,500MWh of storage, respectively. Although these were filed as 500MW options, they could still offer sufficient storage for up to 1000MW on a daily peaking cycle. The final sizing will be based on the balance between the storage available, economies of scale, and market priorities.

Modelling carried out by Gridflex Energy suggests that a longer duration of storage is necessary for the purpose of firming wind resources over a semi-weekly period. This function is a distinct possibility in the Tehachapi region. A more conventional pumped storage model calls for only enough storage time necessary to meet peaking demands and for participation in the ancillary services market.

Challenges and solutions

Head & technology

The highest head single-stage pump turbines currently installed serve a head of approximately 700m. Several of the Bison Peak options would exceed this level. While there are pumped storage projects with significantly higher heads in operation, all of these require multi-stage pump turbines.

In order to avoid the greater complexity and lower flexibility of multi-stage equipment, Bison Peak looked to pump-turbine vendors for an innovative solution. To date, two designs have been offered as potentially meeting the needs of the highest head options: andritz Hydro’s ternary design with Pelton runners, and alstom’s double-stage regulated design. Both types have been installed in only one location each to date – the ternary design at Austria’s Kopswerk II project (818m), and the double-stage regulated design at South Korea’s Yang Yang plant (798m).

The ternary design involves a separate pump and turbine sharing a common motor-generator. This design, in addition to a hydraulic short-circuit feature, allows for changing modes in less than 20 seconds. The Kops II plant in Austria, using this equipment, is said to go through 60 mode changes per day. This ability would create exceptional value in the ancillary services market. The ternary system is able to offer the same kind of operating range that variable speed pump-turbines afford.

Alstom’s double-stage design features two adjustable distributors, one for each runner. This allows for adjustable-speed operation.

Either option would be more expensive than conventional designs, but they would likely pay for themselves. As filed with FERC, the 1000MW options specified 4 x 250MW units, and the 500MW options 4 x 125MW units.

Seismic challenges

California is no stranger to the construction of infrastructure projects near fault lines, as they are ubiquitous in much of the state. Design standards and engineering solutions alike have had to meet the seismic conditions. The Bison Peak pumped storage project would be located near a fault that is relatively inactive but has the potential for a significant seismic event. For the project, this has implications for dam design as well as location of the lower dam and reservoir.

In fact, the South option would require that the tailrace actually traverses the fault zone. In recognition of the significant shear potential of the fault, innovative approaches are being considered. These include the design of a tailrace with shear fuses that would allow it to break at preferred points in a major shear event, with a vault constructed around the highest risk section. Pre-designed access would facilitate repair and replacement.

The South option will require careful analysis of the prospects for tunneling through the fault and the conditions for dam siting. Whether this option can and will be pursued will depend on a variety of factors to be determined during feasibility studies.

The market

The residents of Kern County, California are accustomed to large energy projects. The county is already home to more than 700MW of wind power capacity, and one wind developer alone is constructing 1500MW of new wind capacity in the area. They are not alone: in addition to other wind projects, some of the world’s largest photovoltaic solar projects are also being constructed in the Tehachapi area as well. The county has set a goal to see 10,000MW of renewable resources developed by 2015. Looking beyond the local region, the state has the highest renewable energy portfolio standard in the nation. By the year 2020, California utilities must derive 33% of their energy sales from renewable sources. This is significant to the value proposition for the Bison Peak pumped storage project because renewable energy integration and enhancement is today’s driver for pumped storage development.

There are two challenges that Bison Peak can help to address. First, wind energy is variable in nature. In California, wind’s contribution to meeting load with certainty at any particular time - particularly at a time when demand is high - garners a low rating. Second, wind generally peaks during the evening and early morning hours. Through a significant amount of flexible and fast-responding storage capacity, Bison Peak is intended to serve as an ideal partner to the natural patterns of a clean energy resource.

California’s grid operator anticipates the need for several thousand megawatts of regulation capacity in order to maintain the crucial balance between load and generation in the post-2020 period. Recent studies suggest that 3000-4000MW of fast-acting energy storage (which includes pumped storage) may be required by 2020 to help integrate renewable resources. However, the regulation market is not the only value driver for the Bison Peak project. New rules will require California utilities to shutter 22,000MW of once through cooling generators—mostly gas-fired—that today help support grid balance. In addition, some California utilities are divesting their shares of out-of-state coal generation in order to meet greenhouse gas reduction goals. This loss of capacity, coupled with a moderate increase in load (after demand-side measures are taken into account), means that there is a market for firm capacity in California’s future. Bison Peak’s ability to provide firm peaking-to-intermediate capacity will be another driver of project economics. Finally, California represents one of the few substantial arbitrage opportunities—at least during the summer, when the difference in value between off-peak and peak energy is significant.

In addition to the physical needs of the market, California offers institutional support for projects like Bison Peak in the form of two laws. First, the state’s renewable energy portfolio law allows utility investment in bulk energy storage for renewables to offset penalties that a utility might face for falling behind on the mandatory renewable energy acquisition goals. Second, the state has a law on the books requiring the study of possible mandatory targets for energy storage acquisition by the state’s utilities. The details of regulations relating to this law are now being negotiated.

Licensing and permitting

Pumped storage projects – even closed-loop projects like Bison Peak – generally fall under FERC jurisdiction as hydroelectric projects. Thus, FERC licensing drives the ultimate timeline for project development. Within that timeline will be local and state-level permitting requirements. In California, the State Water Resources Control Board (SWRCB) handles the water quality certification that FERC requires of pumped storage projects. The SWRCB, in turn, requires an Environmental Impact Report (EIR) before issuing such a certification. An EIR will also be required by Kern County for a conditional use permit.

The FERC clock officially begins when Bison Peak receives a preliminary permit, which is expected in the first part of 2012. Theoretically, a FERC licence could be obtained within three years. Several recent high-profile pumped storage projects that have been under development for a decade or more appear to make this target overly ambitious, but there are two mitigating factors that apply to those projects. First, they began development in a different era – i.e., at a time when the market for bulk energy storage had not yet reached the maturity it is approaching today (driven, again, by the magnitude of renewable resource development). Second, some of those projects have had controversial elements or multi-jurisdictional complications, eg, a long transmission line through national forest land or groundwater or land use issues.

Among the strengths of the Bison Peak project are its relatively low risk for environmental sensitivity and land use conflicts. And as a closed-loop project, no natural waterways are affected. These factors will contribute to a smoother licensing process. Bison Peak also believes that through careful coordination of the federal, state, and county-level processes, redundancy can be minimised and the process streamlined. Furthermore, if a Pre-Application Document (PAD) is filed relatively quickly and can be followed up by a draft licence application without significant delay, the proponents believe that the project could be licensed within three to four years. (If the Hydropower Improvement Act currently pending in Congress passes, it will improve the chances that low impact projects like Bison Peak can be licensed within such a time frame). Allowing four years for construction, the target online date would be 2018-19.


While awaiting the formal start of the FERC timeline, the project has been moving through some key early considerations. Among early goals was confirmation that fill water could be obtained from local water agencies. Following consultation with those agencies, several options have been established for the initial fill requirement. Hydrologic assessment will determine the extent to which groundwater under the project site may be sufficient to provide for evaporation losses. Scoping and pricing of feasibility-level environmental and engineering studies has been completed. The all-important business case analysis has been undergoing refinement from the beginning, and market outreach has been initiated. The project was one of those submitted to the Department of Energy last spring for matching grant funding. While SMUD’s Iowa Hill pumped storage project was the only one selected for funding, the Bison Peak project application was commended in the DOE review for its closed-loop design, the fact that the project would sit in large part on land owned by the project owner, the use of innovative technology, and a sophisticated understanding of the value of storage in the California renewable energy market.

The pace of continued progress will be driven by the continued availability of project financing. The Bison Peak owners are exploring new partnerships on an ongoing basis. The variety of visitors hosted at the site—from engineers to wind energy developers, construction firms to public officials—exemplifies the diversity of interest in what such a project can do.

Looking forward

A pumped storage project is a major undertaking, particularly for an independent developer. Whether a project is undertaken by a utility or an independent, it needs to have an exceptional combination of features, location, and business case in order to make it to construction. The Bison Peak pumped storage project seems to have that combination:

• Very high head for efficient use of water and civil works.

• Excellent length/head ratio.

• No impact on natural waterways.

• Location entirely on private land.

• Proximity to transmission and the most intensive region of wind and solar development in the nation.

• An energy market that recognises the value of storage.

The first priority of the project owners is to sustain forward progress through the crucial early stages, where geotechnical considerations and project constructability are the key questions to be addressed. Looking beyond, the Bison Peak project has to be counted as one of the most exciting and promising pumped storage projects on the horizon.

Matthew Shapiro is the CEO of Gridflex Energy, LLC and a consultant to the Bison Peak pumped storage project. More information about the project can be found at http://bisonpeakenergy.com.

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