Tunnelling often features in hydropower projects, and possibly moreso with pumped storage schemes. The advantage that pumped storage has over other hydro systems is it re-uses what amounts to the same water, or at least the water from the same bodies, which then become known as the upper and lower reservoirs.
The water is then used for fast-response generation either to meet peak demands or provide some support, or ancillary, services to the transmission grid, such as improved ‘black start’ capability, load balancing, reserve power and voltage support, and so on. Further arguments for pumped storage include being able to support intermittent, or erratic output from other renewables, like wind power, by both storing energy and local grid balancing.
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However, the challenge that pumped storage, and most hydro projects, have faced is that, unlike thermal plants they cannot be sited anywhere: their locations depend on topography and hydrology. That is a disadvantage for potential growth and market size as the grid needs, and values, generation capacity as near as possible to load centres.
Concepts to recirculate water in, generally, closed-loop or diversion systems offer a fresh opportunity for pumped storage projects, depending on certain conditions: either the lower reservoir or the entire system has to be underground, and so natural surface features or absolute elevations of the entire project become irrelevant. These would give rise to significant tunnelling opportunities. The schemes always need an open stretch of water for the upper reservoir.
No such project has yet been built. In the 1990s, there was a pumped storage project to be built at Mt Hope, New Jersey, that would have had an underground powerhouse and also a network of tunnels to form the lower reservoir. It was to have had 2GW of installed capacity and be a one-off scheme, but the project did not come to pass. The design eventually changed to have only half the capacity and then a multi-stage layout of different elevation steps was envisaged.
Now, two different concepts are being explored for pumped storage projects that can be sited in non-conventional areas. One type, the ‘Aquabank’ system, is being developed for potential projects in the US and Canada by Riverbank Power Corporation, a Canadian limited liability company. It has a superficial similarity to what was conceived before but the 1GW projects are being developed to a standardised design.
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By GlobalDataThe other type offers potentially even more flexibility for locating a pumped storage facility because it needs neither an upper pond nor lower reservoir of caverns, and it does not require a water body to constantly tap or with which to pass the flow back and forth. Instead, it would use extremely deep, water-filled pairs of sealed shafts, one of which contains a large concrete weight.
The dynamic generation system is being developed with a standardized design approach and as it is modular it is scalable. The system is called the ‘Gravity Power Module’ and is being developed by California-based Gravity Power.
Riverbank Power – cavern concept
Riverbank Power is an independent power producer backed by private financial muscle seeking opportunities in a resurgent era for renewables. The company is pulling together knowledge and expertise, both inhouse and among a team of industry suppliers, to pursue a vision of constructing large pumped storage projects close to grid load centres.
While it does not hold intellectual property, the company plans to realise the business opportunity using its ‘Aquabank’ concept, a standardised design concept for buried, generally closed-loop, large pumped-storage schemes. For tunnellers, the concept of the 1GW installed capacity projects would offer major design and excavation work.
In terms of scale, the concept design calls for long access tunnels leading to the underground complex, which would be constructed approximately 1800ft (550m) – 2,000ft (610m) below mean sea level. Drilland- blast excavation would be employed mostly, it is anticipated. The powerhouse and associated caverns would be the deepest of the tunnel works to suit the hydraulic design of the system.
The primary features of the Aquabank schemes, however, are the six immense galleries that would receive the discharge flow from the powerhouse cavern and function together as the lower reservoir. The galleries will have combined capacity to store the discharge flows from six hours of generation in the powerhouse.
The galleries are to be connected at their bases by a tunnel for water conveyance. A further series of tubes across the tops of the caverns will to provide ventilation and be connected to a single, independent shaft to the surface.
Each gallery is to be about 90ft (27.4m) wide by 150ft (45.7m) high. In total, the combined length of the galleries is to be approximately 14 000ft (4270m), which would make each, on average, approximately 2330ft (712m) long. To be excavated in suitable geology, the galleries are to free-stand in unlined rock.
The powerhouse is to hold four reversible pump-turbines and the estimated annual energy production from a standard plant is 2190GWh. Water will be conveyed down four x 13ft (4m) diameter vertical penstocks, each almost 2200ft (670m) in length and lined with concrete and steel, to the powerhouse.
Riverbank Powe estimates the cost of developing a project and bringing it online to be USD 1.5-2bn with an estimated construction period of four-to-five years.
The company has been investigating more than a dozen potential sites across North America in the last few years for its Aquabank plants, and the most advanced in planning is located at Wiscasset, Maine. Another early runner at Sparta, New Jersey, was dropped last year.
More recently, applications have been submitted for projects at Tomkins Cove, New York and at Granite Falls, Minnesota.
Wiscasset project
Development work on the Wiscasset project began in earnest two years ago and in early 2009 the Federal Energy Regulatory Commission (FERC) issued the scheme with a preliminary permit.
Geology at the site has been described as ‘excellent’ from the one borehole drilled so far, by Boart Longyear. Packer tests showed little seepage into the rock mass which, at the depths of about 1,700ft (518m) – 2,200ft (670m), is primarily granitic gneiss. The general geology of the area is dominated by the Cape Elizabeth Formation, which is schist.
Initial design work envisages a 3.7 mile (5.9km) long permanent access ramp, constructed with a D-shaped section that is 38ft (11.6m) wide and 26ft (7.9m) high. The ramp tunnel would have a maximum Slope of 10 per cent. This has been chosen to help reduce the gallery length. Additionally, separate shafts would be constructed that would not only serve ventilation purposes for the galleries but also allow separate access to, and egress from, the powerhouse complex.
The powerhouse is expected to be more than 145ft high, possibly up to 160ft (48.8m). Although the length and width were not stated, comparison to the other project plans could see a length of about 400ft (122m). The dimensions of powerhouses in the Aquabank system are adjustable to suit the geology and constraints of particular sites, and the layouts in relation to the galleries can also be altered.
Kuo-Bao Tong, project manager of the Wiscasset scheme, said, “The volume typically remains the same, just the geometry and/or orientation may vary based on the drilling programme and the determined insitu stresses.”
Tong is also helping to oversee early development progress on the other candidate sites.
The upper ‘reservoir’ for the Wiscasset scheme is to be the tidal Black River, and therefore while the pumped storage scheme would return the water it would not, in principle, be a strict recirculation from that flowing, open body. The scheme is classed, therefore, not as closed-loop but diversion pumped storage.
Given the involvement of such a large, flowing open body of coastal water, and also fisheries, it is understandable that environmental aspects have been a particularly important part of the development work for the project. These are critical to resolve for the scheme to be approved, and so efforts are underway to ensure there are minimum impacts on the quality and temperature of the water from when it is extracted and through the conveyance, storage and return steps.
Riverbank is developing the project through a subsidiary, Riverbank Wiscasset Energy Center, registered in New York.
Sparta Project
Riverbank has four main criteria for initial site selection: prospective geology, hydrology, distance to the grid, and support of local communities. Each needs to be cleared before any test drilling begins, including geology to both minimize structural support to the excavation and permanent reinforcement required but also to minimise any seepage.
The process was followed for the Aquabank project intended for Sparta, which was one of Riverbank Power’s top candidate sites. It was to have employed a flooded quarry, Limecrest Quarry, as an to upper reservoir, and be operational by about 2015.
However, when initial drilling began in early 2009 the data were not good news. The geology was too poor to support the proposed major tunnelled structures and the scheme was cancelled just over a year ago.
The Sparta project was dropped barely two months after the Federal Energy Regulatory Commission (FERC) had issued a preliminary permit for the development work, and approved the option of a priority application.
Tomkins Cove project
In January 2009, Riverbank Power submitted an application for a preliminary permit to develop the Tomkins Cove project, in New York state. FERC issued a preliminary permit just over a year. Riverbank Power worked through its whollyowned, Delaware-registered, subsidiary Riverbank USA Holdings Corporation.
The upper reservoir for the project is to be the flooded Tilcon Tomkins Cove quarry, near Stony Point, in Rockland County. Water will be conveyed through openings along a 350ft (106.7m) long intake structure to flow under gravity through 4 x 13ft (4m) diameter penstocks.
The powerhouse is to be 415ft (126.5m) long by 160ft (48.8m) high by 220ft (67.1m) wide, including the machine hall and transformer gallery. The generator floor is to be about 1600ft (488m) below sea level.
Six storage galleries are to be excavated at a depth of approximately 1800ft (550m) below mean seal level.
No borehole has been drilled yet. It is estimated the rock at the depth of the excavated structures is granitic gneiss.
Granite Falls project
The preliminary permit application was submitted in January 2010 for the Granite Falls project, which is sited close to the town of Granite Falls and near the east bank of Minnesota River, in Chippewa County.
Water will be conveyed from the upper reservoir down a 20ft (6.1m) diameter, approximately 1800ft (550m) long steellined penstock and then through double bifurcations to the four generating units in the powerhouse cavern.
The early design estimate is for a 380ft (115.8m) long by 83ft (25.3m) wide by 165ft (50.3m) high cavern.
The rock type at depth is believed to be granitic gneiss but so far no borehole has been drilled.
While the lower reservoir is to be the standard tunnel and cavern arrangement, the upper reservoir is to be constructed with enclosed earth embankments.
Riverbank Power – development team
The primary investor behind Riverbank Power is Blackrock.
Riverbank Power has agreements with various specialist firms through the supply chain for hydro development and underground works, including geological consultant Continental Placer, and mining design and project managers Groupe RSW and SNC-Lavalin. In addition, there is support from mining contractor Redpath.
The company said it would consider other strategic partners including with leading engineering firms and contractors.
Tunnel layout and location of Riverbank Power’s proposed pumped storage project at Wiscasset, Maine, which would call for major cavern excavation Wiscasset Bridge – Looking north accross the bridge of US Route 1, traversing the river into Wiscasset, Maine Location map of three projects sites being investigated by Riverbank for large-scale underground pumped storage projects The pristine Reid State Park, some 20 miles to the south of Wiscasset
