Pumped-storage key to energy storage

Accommodating supply variations

By Morand Fachot

With rising electricity consumption and costs, and the need to balance increasing levels of intermittent RE (renewable energy) generation from wind and solar systems, EES (electrical energy storage) solutions are being pursued. These use and store energy efficiently and help improve grid stability and flexibility. Long-established pumped-storage hydropower currently represents the largest and most flexible EES solution and is experiencing significant growth.

Upper and lower basin of Limberg II pumped storage plant, Austria (Photo: Voith) Upper and lower basin of Limberg II pumped storage plant, Austria (Photo: Voith)

IEC support for EES systems

The IEC is a strong supporter of EES. The IEC MSB (Market Strategy Board) recently published two White Papers, the first on EES, the second analysing the role of large-capacity EES systems, integrating large-capacity RE sources. Both White Papers stress the crucial importance of EES in future installations.

IEC TC (Technical Committee) 120: Electrical Energy Storage (EES) Systems has been created to prepare International Standards for such systems.

A number of new energy storage technologies under development contribute to compensate for the variations in availability and grid fluctuation of some renewable energy resources. Of them all, pumped-storage hydropower is the most cost effective and technically viable. It has also been around the longest – its first reported use was in Italy and Switzerland in the 1890s. The 1930s saw the development of reversible hydroelectric turbines that could operate both as turbine-generators and (in reverse) as electric motor driven pumps (see article on hydropower ine-tech, January 2012).

The work of experts within IEC TC 4: Hydraulic turbines, also supports the construction, operation and maintenance of pump-turbine storage sites.

Interest in pumped-storage is increasing, particularly in those regions and countries with the most variable renewable resources and where new installations of traditional hydropower are harder to achieve. The vast majority of installations are currently found in Europe, Asia (Japan in particular) and the US

Principle of pump-turbine arrangement

The technique provides the most widely used form of bulk-energy storage in the short term. Pumped-storage hydropower currently accounts for more than 99% of installed storage capacity for electrical energy worldwide: around 127 GW (gigawatts), according to the EPRI (Electric Power Research Institute – the research arm of America’s power utilities) and Germany's Fraunhofer Institute.

A combination of water and gravity is used to capture off-peak power and release it at times of high demand. Pumped-hydro facilities typically take advantage of natural topography, using two reservoirs built at different heights.

The “head” refers to the vertical height of the fall of the water stream (or river) from the upper reservoir. Higher heads provide a greater pressure and therefore greater hydropower potential. Because reservoirs for pumped storage can be built at sites fed by small streams, which also make up for water lost through evaporation, topographical restrictions for selecting the sites to construct large capacity plants are far less significant than with conventional hydropower.

Off-peak electricity is used to pump water from the lower to the upper reservoir, turning electrical energy into gravitational potential energy. When power is needed, water is released back down to the lower reservoir, spinning a turbine and generating electricity along the way.   

Plants that do not use pumped storage are referred to as conventional hydroelectric plants. These have significant storage capacity and may be able to play a role similar to pumped-storage plants in the electric grid by deferring output until needed. Whereas the primary goal of pumped storage is short term power and intermittent storage, large hydro reservoirs are primarily intended for long term storage and base power, but they can also be used on short notice for levelling the variability of renewable energy

Established benefits of pumped storage

Historically, hydroelectric installations are long established leaders in EES because of their excellent response in three main areas:

  • They are highly cost-effective, reducing electricity costs by using electricity produced at off-peak times when the price is lower. Systems demonstrate low maintenance costs and typically achieve one of the highest cycles per lifetime at some of the lowest costs. High-performance hydropower equipment can frequently run without interruption for extended periods of time and hydroelectric plants have a life-time of at least 50 years, making hydropower a profitable long term investment.
  • EES systems offer support for users needing to improve power supply reliability as well as to compensate for variations of availability and fluctuations in intermittent RE sources such as wind and solar.
  • EES maintains and improves power quality, frequency and voltage by providing spinning reserves that can come online quickly and are needed to maintain system frequency stability during emergency operating conditions and unforeseen load swings. While thermal plants are less able to respond to sudden changes in demand, pumped-storage plants, like all hydroelectric plants, can respond to load changes within seconds, helping instantaneously maintain the balance between generation and net load so as to avoid brownouts, blackouts and overloads.

Pursuing increased capacity, efficiency and flexibility

The market for energy storage is dynamic, reflecting the current state of development in the industry as well as fluctuations within it. Demand is driven by several key trends, including the proliferation of intermittent RE sources and the resultant need for spinning reserves, the onset of the smart grid concept and a shift to plug-in hybrid and electric vehicles. Although utilities are building capacity to meet so-called needle peaks in electricity usage, these only occur for a small number of hours each year. It is therefore expensive and inefficient to size capacity to these peaks, and energy storage can play a large role in providing peaking generation.

Hydroelectric technology storage facilities have always provided the highest capacity in terms of electricity-generating forms of energy storage. While continuing to perform this critical task, facilities are also making a valuable contribution to power grid efficiency and flexibility.

Market applications

The hydro industry maintains its dynamism through the exploration of improved and optimized technologies, development of innovative business models for long and short duration applications and cost-effective technology and project development. Long-established hydro facilities with large storage reservoirs, including pumped-hydro storage, continue to perform as global technology leaders in both developed and emerging markets.

Experts involved in IEC TC 4 are seeing increased demand for clean energy pumped-storage installations because of the known benefits of pump-turbines. In addition, associated production and monitoring equipment provides increased revenues for producers and more affordable pricing for end-users.

Pumped-storage facilities worldwide still expanding

Most of the global installed pumped-storage generating capacity is to be found in Asia – which currently holds over 60 GW of cumulative installed capacity – in Europe and in the USA.

The October 2012 TC 4 Plenary Meeting took place in Japan, which is a prime global user of pumped-storage facilities. The country's complex geological features and an abundance of rainfall have created a number of small river systems that provide opportunities for the development of hydropower. As hydropower resources have been developed, Japan has tapped into pumped-storage as a way of increasing the supply of peaking power.

Recently commissioned units are in operation at the 1 200 MW Omarugawa pumped-storage project, where the head is 646 m. These currently offer the world's highest head adjustable speed, although a higher adjustable unit (where the head exceeds 700 m) is now being constructed at the Kazunogawa pumped-storage facility and will be commissioned in 2014.

China is also developing pumped-storage facilities. It is to start building a 3 600 MW hydroelectric pumped-storage facility in Hebei Province, according to HydroWorld.com. The first phase of the project, offering a capacity of 1 800 MW, is expected to take seven years to complete. The plant is intended to help retain some of the province’s high wind turbine output, some 5% of which was reportedly lost last year. In the same province a 1 024 MW pumped-storage plant was completed in 2008.

In September 2012 an official from India's West Bengal state said the government was planning a 1 000 MW pumped-storage plant in the state's Purulia District. The official indicated that the USD 470 million project would help diversify the region's energy portfolio which currently places a reliance of "90% on power generated from thermal power stations".

In 2009 the European Union had 38,3 GW net pumped-storage capacity (36,8% of world capacity) out of a total of 140 GW of hydropower, which represented 5% of total EU net electrical capacity.

The 1 060 MW Goldisthal pumped-storage plant on the Schwarza River, which started commercial operation in October 2004, is the biggest hydroelectric project in Germany and the most modern in Europe. The Goldisthal project is unique because two of the four vertical Francis pump-turbine units feature variable-speed (asynchronous) motor-generators. This benefits its operator, Vattenfall Europe, by providing power regulation during pumping operation, improved efficiency at partial load conditions, and – for grid stabilization purposes – high dynamic control of the power delivered. There are other projects in Slovenia, Austria, and Switzerland.

In 2010 the United States had 40 pumped-storage plants which accounted for about 16% of renewable capacity and 2% of the country's energy capacity, supplying 21,5 GW of pumped-storage generating capacity (20,6% of world capacity for this category).

Some 40 pumped-storage projects providing an additional 31 GW could be developed in the US in the future to balance variable generation from wind and solar sources. The largest hydroelectric pumped-storage plant in the world, with 2 100 MW peak power, is the Bath County plant located in Virginia, USA.

IEC TC 4 expertise central to pumped-storage expansion

TC 4 hydro experts are in demand for their technical expertise in the construction, operations and maintenance of pump-turbine storage sites. Without their work and involvement, it would be impossible to develop such facilities, although they are vital to ensuring the future of the world's energy supply.

TC 4 has frequently scheduled Plenary Meeting locations that also include technical visits to turbine-pump storage installations. Recent trips have been to the Goldisthal pumped-storage plant in Germany; the Kazunogawa pumped-storage power station in Japan was visited in October 2012. The technical contributions of TC 4 experts enable the developing energy mix to be optimized, including proper integration with new sustainable energy sources, and increasing demands for improved power grid efficiency and flexibility to be met.

The development of many new pumped-storage projects and the modernization, operation and maintenance of the numerous existing installations mean that the agenda of TC 4 hydro experts is likely to remain full for the long term .

Upper and lower basin of Limberg II pumped storage plant, Austria (Photo: Voith) Upper and lower basin of Limberg II pumped storage plant, Austria (Photo: Voith)
Reversible pump-turbine in the Goldisthal pumped-storage station (Photo: Hydroprojekt Ingenieur GmbH Reversible pump-turbine in the Goldisthal pumped-storage station (Photo: Hydroprojekt Ingenieur GmbH
Shiobara 900 MW pumped-storage plant (Courtesy of Tokyo Electric Power Co.) Shiobara 900 MW pumped-storage plant (Courtesy of Tokyo Electric Power Co.)