Making the most of pumped storage

Pumped storage is poised to be a major player in the renewable energy field. Wind generation continues to develop, while renewable portfolio standard mandates loom like clouds over solar panels. It is time to expand green energy resources.

The last time a new pumped storage project was built in the US was over 20 years ago. It’s an old and proven technology but one that has fallen out of favour here. There have been some economic pitfalls, some environmental blunders, and some failures. But today pumped storage is being recognised as a proven and environmentally feasible way to provide the benefits of energy storage, grid stabilisation, and the harnessing of intermittent renewable sources.

Symbiotics, a Utah-based company, is in the preliminary stages of the North Eden pumped storage hydroelectric project, which will provide energy shaping and grid stabilisation for Utah’s coal-reliant energy portfolio.

Why Utah?

The state of Utah has never been synonymous with green energy. Utah’s energy production has always been heavily based on coal mining and coal-fired power plants. In 2006, 90% of Utah’s electricity came from coal. While base load demand has been growing steadily at 2.6% annually, peak power demand has increased 5.4% annually. Although coal is locally available, inexpensive and reliable, it does not provide for daily fluctuations in power demands. Natural gas-fired plants have been added to meet growing peak demand, but this solution adds to the output of carbon dioxide emissions. Additionally, these costly gas plants built to respond to infrequent spikes in demand lie dormant for most of the year.

The benefits of pumped storage hydro power in Utah would be numerous. Utah does not currently use pumped storage but relies on peaking power plants to respond to peak power demand. Pumped storage will be a key feature in the transition away from fossil fuel-fired power plants by providing a method to store intermittent solar and wind power as these resources are developed in the state.

Project location

The North Eden project will be located in Rich County just east of Bear Lake. This is a popular recreation destination shared by southeastern Idaho and northwestern Utah. The upper reservoir will be built on the eastern flank of Black Mountain; the lower will be located in North Eden Canyon. The location of the project is shown in Figure 1.

At this point in the FERC application process, all engineering data is conceptual and preliminary. Detailed engineering will be completed prior to the submission of the final licence application. Figure 2 is a conceptual drawing of project features. This pumped storage project is expected to yield a total of 700MW daily. When the upper reservoir is at full pool the maximum head will be 283m, and the minimum will be 227m. The maximum flow rate from the upper reservoir is estimated at 373m³/sec.

The upper dam, referred to as Black Mountain dam, will be a zoned embankment type with a clay core flanked by shell material sourced from outcrops in the project area. The dam will be approximately 31m high with an emergency spillway running eastward to North Eden Canyon. The spillway will be 9m at its base, with 2:1 side slopes and a longitudinal slope of 5%. The maximum capacity of the spillway will be approximately 75m³/sec. The Black Mountain dam will impound a reservoir with a surface area of 132 acres (53ha) at maximum water surface elevation of 2148m asl. Gross storage capacity will be 10,962 acre-feet (13.5Mm³) and the usable storage capacity will be 10,510 acre-feet (13Mm³). The floor and side slopes of the reservoir will have a multi-layer construction consisting of a 0.6m clay layer overlain with a non-reinforced EPDM geomembrane, which will then be covered by 0.6m of soil. The intake will consist of a glory bowl with a valve to isolate the penstock.

The lower reservoir and dam will be located one mile from the mouth of North Eden Canyon on North Eden Creek. The lower dam will be approximately 52m high, built at a base elevation of 1846m asl in a similar manner to the Black Mountain dam with an impervious clay core surrounded by more pervious outer layers. There will be an emergency spillway for the lower reservoir with a maximum capacity of 10,000ft³/sec (283m³/sec). The spillway will extend to the abutment of the dam in North Eden Canyon. It will be 18m wide at its base, with 2:1 side slopes and a longitudinal slope of 5%.

The lower reservoir will have a surface area of 250 acres (101ha) at maximum water surface elevation of 1891m asl. The gross storage capacity will be 13,761 acre-feet (17Mm³) and the usable capacity will be 11,932 acre-feet (14.7Mm³). Approximately 1000 acre-feet (1.2Mm³) will be dead storage, as the base of the dam will be below the intake/outtake structure. The side slopes and base will be similar to the upper reservoir with a multi-layer composition of clay, EPDM geomembrane and soil. Water discharged from the turbines will enter a stilling basin located approximately 24m below the minimum pool elevation. Placing the intake/outlet structure at this location should alleviate the potential for cavitation during operation.

A 792m long, 9m diameter tunnel with a 32% longitudinal slope will connect the two reservoirs. Transitional penstocks will connect the main tunnel to the pump turbine units. The powerhouse will contain as many as seven variable speed pump-turbines. It will be located on the south side of the lower reservoir and will house the turbine units and a control room.

North Eden Creek will be the primary source of water for the initial fill of the lower reservoir. Water rights will need to be secured, both for the initial fill and annual evaporation maintenance. The advantages of this system are that once the initial fill has occurred, the only water needed will be a small amount to offset annual evaporation from the reservoirs. The precipitation and evaporation balance will result in an annual water loss of approximately 0.2m over the total surface area of both reservoirs.

It is currently proposed that approximately 14km of 230kV transmission line will be constructed to connect the project to an existing 230kV PacifiCorp transmission line located east of the project site. The preliminary plan includes construction of one switchyard/substation near the upper dam site, with the potential need for a second. To minimise possible visual impacts, this substation will be located on the eastern side of the upper reservoir. A second substation may be needed for interconnection.

The North Eden project will be located near a newly proposed transmission line which may be a viable option for interconnection. The benefits of the proximal location to transmission structures are that it will allow the utilities or line owners to stabilise the energy efficiently by adding, reducing or pulling load off the system. The project will help utilise the capacity of large transmission lines more effectively and stabilise Western Electricity Coordinating Council (WECC) infrastructure. The WECC is the energy council that distributes power throughout the western US and British Columbia, Canada.

Project operation

At the initiation of each daily cycle, 10,510 acre-feet (12.93Mm³)of water will be pumped from the lower reservoir to the upper reservoir. Later in the cycle this process will be reversed, with the same amount of water being discharged from the upper reservoir to run through the turbines and generate power. The result will be a total fluctuation of 20m in the lower reservoir and 29m in the upper reservoir. The project is expected to operate in pump mode for 14 hours a day and turbine generation mode eight hours a day, with two hours for reserve in the case of emergencies. The 700MW capacity will be available daily, for up to 10 hours a day. The project is expected to have 75-76% wire to wire efficiency. Table 1 contains the average production in kWh monthly and annually.

Environmental concerns

Symbiotics’ biologists and engineers seek to develop a project that has the least impact possible on the environment. However, the creation of two reservoirs will involve the inundation and subsequent displacement of terrestrial and riparian habitat. Symbiotics has determined the potential impacts and concerns associated with all aspects of the project in its preliminary-application document (PAD) based on existing data. Studies will be required to determine the environmental impacts and mitigation needs of the project. In the next step of the licensing process, Symbiotics will supply a study plan that will be reviewed by the FERC, resource agencies and the public. This will inform future study and mitigation efforts.

The North Eden project will inundate 5km of North Eden Creek and therefore cannot be considered a closed loop system. In addition, three of these five kilometers are considered ‘high quality waters’ and some riverine fish habitat will be permanently lost. However, North Eden Creek currently loses all connectivity to Bear Lake in the summer due to irrigation water rights that use all existing flows. The existing storage right in the system for water rights holders is 400 acre-feet (493,376m³), with an additional 0.11m³/sec diversion rights for the stream. Symbiotics will maintain all downstream irrigation and municipal water rights. The possibility of improving and maintaining the quality of North Eden Creek below the project is being explored as a mitigation option.

The project will require a substantial amount of earthmoving, including the excavation of the reservoirs, the clearing of dam foundations to bedrock, excavation and placement of dam fill materials and excavation for penstocks, spillways and the powerhouse. Roads and road upgrades have been proposed to allow access to project features. All of these activities have the potential to discharge sediment into nearby Bear Lake. In order to receive the Section 401 water quality certification, Symbiotics will develop a soil erosion control plan that will address all concerns associated with project construction.

Fisheries, wildlife and botanical resources

Some of this riverine habitat will be permanently lost due to the project. The Bonneville cutthroat trout (Oncorhynchus clarki utah), a native species that occurs in North Eden Creek, is considered a core population with a conservation management designation. There are some concerns about impacts upon the habitat of this species and mitigation options are being explored.

One proposed option is that Symbiotics will rehabilitate 2km of North Eden Creek between the project and Bear Lake. This would involve fencing off riparian habitat from livestock, restoring stream form and function and reintroducing Bonneville cutthroat trout. Another possibility is the restoration of connectivity to Bear Lake by means of a fish ladder. This would allow natural recruitment of Bonneville cutthroat in North Eden Creek from Bear Lake.

Construction of the project will permanently displace 400 acres (162ha) of shrub-steppe habitat and alter North Eden Canyon’s value as a wildlife corridor on the Bear river plateau. Some recreational hunting takes place in the area for deer, partridge and sage grouse and this could be affected by the project. Before construction commences, a variety of surveys will take place to document wildlife use of the area and delineate habitat for sensitive species. This will aid habitat rehabilitation and mitigation efforts. Some species of concern that could occur in the project area are greater sage-grouse, pygmy rabbit, loggerhead shrike, burrowing owl, willow flycatcher and western toad. Sensitive plant species surveys and noxious weed surveys will inform construction efforts and revegetation efforts following construction.

Visual resources

It is expected that project features will not be visible from Bear Lake, a popular recreation destination. A study has been proposed to document the current character of the local landscape and to investigate and ensure that project features will not be visible from viewpoints within the greater Bear Lake area. In addition, project features will be designed to blend in with their natural surroundings.

The National Register of Historical Places and the state of Utah do not identify any culturally or historically significant sites within the project area. However, all ground disturbing activities will be preceded by a cultural resources survey to prevent displacement of valuable cultural resources.

Project progress

The North Eden project is currently in the licensing stage of project development. Symbiotics was issued a preliminary permit from FERC on 24 December 2008. The pre-application document, which presents the preliminary plan and environmental impacts of the proposed project, was submitted on 22 January 2009, along with a request for the use of the traditional licensing process (TLP).

Symbiotics is currently waiting for FERC to approve their request to use the TLP and will be receiving a response soon. A public scoping meeting is expected to take place in May and, following this, Symbiotics will begin the first year of environmental studies. Submittal of a final licence application could occur as early as December 2010, with construction beginning in 2013.

Progress on this project may be tracked on the FERC website (www.ferc.gov) in the E-library under the North Eden Pumped Storage Hydroelectric Project, Docket No. P-13249.

Rachel Wright is a technical writer at Symbiotics in Portland, Oregon, US. Email: rachel.wright@symbioticsenergy.com

Tables

Table 1

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