Interest in pumped storage (PS) is among the forefront of major infrastructure development possibilities in the mature US hydropower market. Both large and mid-size PS plants are in the early development pipeline, and many would call for large tunnelling works.

The drivers behind the resurgence in PS have two key factors – a bigger need for energy storage in the power market; and, a greater requirement for electricity transmission grids to have quick, capable and big resources to draw upon to stay stable. The matter of stability is a rising issue in the booming era of intermittent renewables, such as wind and solar power; as their inputs vary, the grid wobbles, fractionally, but enough to demand that attention is given to ways and means of solving the new problem.

Among the spin-off beneficiaries of the handling naturally fluctuating wind and solar generation is hydropower – especially pumped storage systems, which are large, fastresponse power plants, and call for major civil engineering works. Research led by Argonne National Laboratory has investigated the potential for more PS in the US, and discusses the tools and market conditions needed to help support more development, and draw in funding.

The US has dozens of potential PS projects in the pipeline. They are spread across the nation and applications for development are lodged with the Federal Energy Regulatory Commission (FERC). See the box panel for a list of many envisioned projects with FERC licences and preliminary permits, such as Eagle Mountain, Haiwee Ridge and Bison Peak.

Presently, though, and despite their benefits to the grid, project applications have to shuffle through a long federal permitting and licensing process. The government is looking for ways to shorten the time to two years, and so far is pursuing this goal for one type of PS system – "closed loop", which are effectively enclosed, sealed off water systems.

Demand drives PS resurgence
The concept of pumped storage is not new. It involves an interplay of large flows of water between two reservoirs at different elevations. While hydropower schemes need only one reservoir, or river, and water flows down through a power plant to spin turbines and generators, in pumped storage the extra action is to send flows back uphill. To do so, the second, lower reservoir is needed.

Traditionally, pumped storage was used for "time shifting" the value of water resources: water would be released to flow downhill and generate at times of greatest demand and so achieve best prices, like in conventional hydropower; but, then, at times of lower demand – and hence cheaper electricity – power would be drawn from the grid to pump water back uphill to repeat the cycle.

For the same unit flow of water, therefore, the profit and value in classic pumped storage is derived primarily from the price difference, or margin, between electricity sold and used.

The up and down cyclic flows were generally predictable, often over the course of a day, and also longer. But the extremely fast-response nature of the technology led to shifts in what the plants may be usefully called upon to do in both generation and pumping modes. As such, some of the operational regimes have been changing, which can become more of an economic challenge, and therefore balancing act, for plant owners.

But there’s no getting away from the speedy muscle that both hydropower and pumped storage plants can offer grids. They can step in when others’ generation drop away, or – for the latter – absorb some of the extra power generated by switching to pump mode; the pumped water is stored uphill as potential energy.

For example, a recent scoping study by Northwest Hydroelectric Association (NWHA) found PS to be important for ‘wind integration and restoration of operational flexibility’ in the federal power system of the Columbia River basin. To date, however, the region has only one PS plant, John W Keys III, 314MW, held by the US Bureau of Reclamation, although more facilities are proposed.

Consequently, in the era of carbon concerns – which sees governments calling for, and electricity markets building, more renewable plants such as wind and solar – there is growing interest in developing new PS projects and keeping tight hold of existing plants. The US has about 40 PS plants with combined capacity of approximately 22GW, according to the Argonne report.

Last year, utility Pacific Gas & Electric (PG&E) celebrated the 30th anniversary of its 1.2GW Helms PS plant, in California; the scheme’s reservoirs and powerhouse are linked by more than 4 miles (6.4km) of 28ft (8.5m) diameter tunnels.

In 2013, FERC relicensed the 365MW Yard Creek PS plant to keep providing its key services to New Jersey; the plant is co-owned by Jersey Central Power & Light and PSEG Fossil, LLC, and the reservoirs are linked by a short tunnel.

The value of PS assets is further shown by the reconstruction of the Taum Sauk scheme, in Missouri, over 2005-10 after the dam of the upper reservoir failed; FERC relicensed the 442MW scheme, owned by Ameren Missouri, last year. Tunnels and shafts are used to convey flows between the upper reservoir and powerhouse.

Federal R&D funding is promoting greater diversity in generation technologies and energy storage systems, such as large-scale batteries. For fast-response systems, gas-fired plants are another solution, and they can be built near load centres of grids without much trouble – a key advantage over pumped storage. However, they too have their challenges in wear and tear as grid demands lead to more changes in operating regimes.

Simply more plants are needed to provide energy storage and grid support. PS is a proven and established technology, but efforts are underway to add to its offering in terms of operational range, flexibility, and water use, including:

  • increasing the operational performance range by using more sophisticated "variable speed" pumpturbines and generators in some powerhouses;
  • increasing flexibility in the potential locations for PS plants through fresh ways to employ different water bodies, and possibly construction of lower reservoirs – including underground construction concepts or use of old mines (surface or below ground); and,
  • minimising how PS systems might interrupt local environments during operations as water is taken and released; the concept to solve the challenge is "closed-loop", which would separate the PS system from natural flowing river systems and open bodies of water after initially filling reservoirs and conduits, and allowing only for occasional top ups.

Constructing underground lower reservoirs was envisaged for some schemes a decade or so ago but none from that phase were built, although the large-scale tunnelling concept has not been ruled out for the future.

Some lower reservoir concepts are focused on using disused mines, but the concept can run into problems, such as for Green Energy Storage’s planned 150MW Weed Heights closed loop scheme, in Nevada. A disused open copper pit mine would form the lower reservoir, and be linked by a tunnel to the upper water body. However, FERC recently rescinded the preliminary permit for the scheme – barely a few months after it was granted – over concerns about heavy metals.

Common to many existing and new concepts, though, will be tunnels and other underground structures in varying degree – see the box panels for more details on project examples, such as Eagle Mountain, White Pine, New Summit and Iowa Hill. Despite the various development challenges, including technological and market barriers, Argonne says in its recent report that the recognised benefits of PS is pushing plans for about 50 projects totalling more than 40GW of new capacity.

Loop the loops
In August 2014, FERC announced a pilot study to shorten the licensing process for two niches in hydropower, one of which is closed-loop PS, which FERC says ‘must not be continuously connected to a naturally flowing water feature.’ The need for the pilot study comes out of the Hydropower Regulatory Efficiency Act of 2013, which explicitly requires a two-year option to be examined.

According to FERC data, interest in closed-loop PS schemes surged into early permitting and licensing applications over 2008-11. More recently, and though overall the PS application numbers after 2011 have fallen and are more in line with the trend of low numbers over many previous years, applications for pre-permits for open-loop schemes are again in the majority. With the challenges recognised for permitting and licensing, the government is looking for ways to accelerate the process, and the proposed two-year timeframe could see another bounce for closed-loop schemes.

Whether closed-loop or open-loop, however, many PS projects feature tunnel works to greater or lesser degree – headrace and tailrace tunnels, access tunnels and shafts, and powerhouse caverns, such as at Bison Peak project in California, which recently gained a preliminary permit from FERC.

The need for energy storage and grid stability is not going away, and will only increase as the generation mix becomes more diverse. The prospect arises of much potential underground construction work in the pipeline of PS projects coming along.