Pumped Hydro Storage

Introduction

The principles behind pumped hydro storage are fairly simple. When there is an excess of electrical power produced, either by traditional thermal generation systems, or renewable forms, this is used to pump water from a low reservoir to a higher reservoir. When there is demand that cannot be met by the grid, this water is then released to flow through turbines and generate electricity to meet peak demand. A diagram outlining the principles is displayed in Figure 1, from Dinorwig pumped hydro station in North Wales.

Figure 1 - Diagram of Dinorwig pumped hydro station.[1]

Pumped hydro schemes are generally net consumers of electricity, but can effectively be used to stabilize power production and even out gaps between production and demand. It can be especially useful when applied to renewable energy technologies such as wind and photovoltaics, as the energy they produce is highly variable and unpredictable. In some cases, it can even be necessary to reject the supply from these renewables if demand on the grid is insufficient, effectively wasting the produced energy.[2] Some regulatory bodies have actually limited the amount of renewable uptake that is permitted due to their unpredictability, and pumped storage is proposed as a method of increasing renewable uptake while mitigating the problems associated with this variability.[3]

Principles of Operation

The main variable that affects the power output of a pumped hydro station is the size of the upper reservoir, and its vertical height above the lower reservoir.

The equations governing pumped storage are outlined below:

Where:
E is the energy released in Joules
ρ is the density of water
g is acceleration due to gravity
h is the head height
V is the volume of water
ηp is the efficiency of the pumping system and
ηg is the efficiency of the generation system

Factors that affect the efficiency of the pipe work include the smoothness and therefore friction losses, as well as any abrupt changes in direction and cross-section which could cause turbulence. Modern turbines can achieve up to 90% efficiency in generation, however it can be seen from the equations that without 100% efficiency in both pumping and generation, pumped storage will inevitably incur losses. According to the laws of thermodynamics, any energy used to raise the water to a higher level will not be recouped when the same water is used for power generation; however a turnaround efficiency of up to 85% can be achieved from modern designs. [4]

Hydro storage has some advantages, in that heavy rainfall on the upper reservoir could increase the volume of water without needing pumping. Additionally, in tidal schemes, such as La Rance in France, pumping can decrease the lower reservoir or increase the higher reservoir to effectively amplify the effect of the tidal regime, resulting in a net gain of energy after pumping[5]. Conversely, evaporation, or the use of water from the upper reservoir for irrigation or other non-generating purposes, can have a negative effect on the stored capacity.

The main difference between a pumped hydro station and a conventional high or medium-head hydropower station is that water is pumped back up to its starting point in a storage station. The main principles of operation are otherwise similar, and dammed hydropower stations can have pumped storage capability added to them after their original construction[6], and many hydro stations that do not have pumped storage facilities could yet be upgraded.

In general, pumped hydro stations will use existing bodies of water at different heights, if they exist, though many of the projects in operation have required one or both of the reservoirs to be man-made. In some instances, there is no lower reservoir, and the sea is used. The technology is very mature, the first pumped storage stations having been made at the end of the nineteenth century.

The majority of modern pumped hydro stations use a reversible turbine/pump assembly, known as a Francis turbine. This allows electricity to be generated or water to be pumped with the same piece of hardware, by reversing its operation. This has obvious benefits in terms of capital expenditure, by only requiring one piece of hardware to perform both functions.

The Francis turbine consists of a spiral shaped inlet, which channels the incoming high-pressure water through the turbine wheel, or runner. Vanes are generally included to channel the incoming water more effectively, and are often adjustable to allow optimum efficiency with different flow conditions. In large installations, the turbine will generally be designed specifically for the site in question to improve efficiency. As turbines are designed to operate at varying speeds, depending on the head of water, modern turbines, in generation mode, achieve efficiencies in the region of 90%, as shown in Figure 2.

Figure 2 - Variable and Fixed Speed Turbine Efficiencies at Different Head Heights[7]

Pumped storage is the most common form of large-scale energy storage in grid systems, accounting for some 90GW globally, or approximately 3% of instantaneous global production.[8]

Site Constraints

As pumped storage systems require two large bodies of water with a large vertical, and ideally small horizontal separation, the number of naturally occurring sites is small, and these have generally been used already for hydro-electric installations. As previously mentioned, there is scope for existing hydro stations to have pumped storage capacity installed.

For new projects, the environmental and social impacts are similar to any new hydro-electric installation, and require new dams to be built, which flood large areas of land. Alternatively, large man-made lakes need to be constructed. The environmental impacts of flooding large areas of land are well documented, and include the generation of large amounts of methane when land-based vegetation rots underwater. Loss of habitat for natural species, as well as relocation of indigenous peoples and loss of traditional fishing and hunting grounds are also common factors that need to be accounted for. Even where there is an existing watercourse, the change of free-flowing rivers to artificial lakes will cause habitat changes including silting and the rise of new species that displace existing flora and fauna, often far downstream of their actual location. Large artificial reservoirs in tropical countries provide ideal breeding grounds for malarial mosquitoes and bilharzias-carrying snails.[9] Dams also create a barrier to fish migration, which can be mitigated by the use of special fish ladders, however these will always reduce the numbers of fish that migrate successfully. Similarly, fish passage downstream through a turbine is a stressful and often fatal experience. Finally, the vast amounts of steel and concrete used in dams mean they have a significant environmental footprint in their construction phase.

The World Commission on Dams, established in 1998, published a report in November 2000 which outlines a framework for the creation of new hydroelectric projects. This tackled many of these issues and so it is hoped that any new developments would have significantly less impact than they have had in the past.[10]

There are a number of proposals for pumped hydro storage projects that aim to circumvent these issues. Some, for example in Gran Canarias, propose to use already existing reservoirs at different heights and connect them with new water channels.[11] Other proposals put forward the idea of using old mines as a lower reservoir, which could prove cheaper to build, as the reservoir is already excavated. Furthermore, the upper reservoir could be directly on top of the lower one, which would increase efficiency.[12] Other proposals along the same lines involve excavating new caverns for the lower reservoir near existing water bodies, as long as underground rock types are suitable. [13]Finally, more ambitious projects propose building the two reservoirs in the same place, with either the upper or lower reservoirs being positioned inside the other. These would either be in-land in existing or newly made lakes as in the Ringwall-storage-hybrid power plant[14], or in the sea, as suggested by the KEMA Energy Island proposal.[15]

In all of these cases however, the capital cost of a new pumped storage operation is likely to be very high.

Pumped storage and the national grid

Pumped storage is by far the most common form of storage used on a large scale. The only practical limitation to the generation output is the size of the upper reservoir. It has the highest capacity of all forms of storage, as well as the longest discharge time, as outlined in Figure 3, and typical stations are well over 1GW.[16]

Figure 3 - Rated Power and discharge times of various energy storage technologies[17]

Typically, pumped storage is used to balance the load requirements of the grid, by using excess power at off-peak times in order to supply power at times of increased demand. It has generally been used as a means of allowing other fossil-fuel based power stations to continue to run at optimum efficiency, that is, at full load, at all times, as thermal power plants generally are inefficient while starting up, because they need to use a portion of the heat generated in order to get to optimum operating temperature.

Pumped storage is also favourable when compared to peaking thermal power plants, as these often require expensive fuels and hardware that remains idle for long periods. A further benefit of these systems is that they can generate power on demand, needing less than twenty seconds to reach full electrical generation capacity, and can be used as ‘black-start’ systems in the event of grid failure, allowing them to be used to jump-start other power stations and bring the grid back online. Additionally, the speed with which they can change loading means they can aid in frequency regulation on the grid as a whole.

The need to keep the majority of thermal plants, which comprise the bulk of electricity generation, at full output at all times, necessarily means that there are periods where supply outstrips demand. Demand for power varies seasonally, and on a weekly and daily basis, dependent on the amount of light and heat that is naturally available, the working week of offices and industry, and the daily habits of domestic users returning home from work and so on.

This has led to the rise of off-peak tariffs, with industrial and domestic users being encouraged to use off-peak power, in the form of Economy 7 and Economy 10 tariffs in the UK and other demand side management strategies. Indeed in Canada, there was a time in 2006 when energy costs were actually negative.[18]

Electrical storage systems take advantage of this economic situation by using off-peak power to build up reserves, then selling energy back to the grid during the hours of peak demand. Although there are inevitable losses in terms of the total power that is available after storage, the difference in prices between peak and off-peak tariffs make this economically advantageous.

Although the use of energy storage mechanisms does not lead to a reduction in overall power demand, it does mean a reduction in the number and capacity of power stations. The alternative would require enough power stations to produce the power demand at peak times, with the majority sitting idle at off-peak times.

Pumped Storage and a Low Carbon Economy

With the increasing penetration of renewable technologies, particularly solar and wind, there is increased unpredictability of supply. Solar generation will obviously only produce energy during the day, however cloud cover has a significant effect on the amount of energy produced, and production can vary significantly on an hour-to-hour basis. Energy from the wind is even more unpredictable, and depending on local conditions, can be higher during off-peak hours. This unpredictability, coupled with the large peaks and troughs in power output, pose problems for supplying electricity in a traditional grid infrastructure. Some form of supply management and frequency regulation is necessary if these technologies are to be encouraged, and pumped storage is ideal for this. Ludington Pumped Storage Plant in the United States is undertaking a major upgrade with the specific intention of allowing more wind capacity to be installed in its region.[19]

As the uptake of intermittent renewables increases on the path to a low carbon economy, the need to store and manage the energy supply will become ever more important, and the existing infrastructure of pumped hydro storage, as well as new facilities, will be ideally placed to help.

Another interesting idea could be directly combining mechanical wind pumps with pumped storage facilities. These would use wind, directly converted to mechanical energy, to pump water into the storage reservoir. Wind pumps are much simpler technologies than wind generators, and can operate in lower wind speeds.[20] By using the mechanical energy directly for pumping, a more efficient system may be possible than current proposals, which involve converting wind energy to electrical energy in a turbine, then using the electrical energy to pump water in a separate (though very efficient) piece of hardware. The wind pumps ability to work at lower wind speeds may also mean that they could find a use near reservoirs that would not necessarily be viable for standard wind turbines.

Conclusions

Pumped storage is by far the most common means of electrical storage and supply management in the world today. Its widespread use and high availability makes it an ideal means of harnessing the intermittent power of solar and wind renewables, and allows for their widespread adoption. The ability of pumped storage to quickly and effectively manage current frequency on the grid means that it is an effective way of preventing the power spikes and surges that may otherwise occur if renewables were widely adopted in an uncontrolled fashion.

Nevertheless, increasing pumped storage capacity is very site dependant, and the introduction of new capacity is likely to be much more difficult in the form of traditional hydro-electric plants, due to the environmental consequences of creating dams and storage reservoirs. Nevertheless, there is scope for adding pumped storage to existing hydroelectric stations, as well as adding generation and pumping facilities to existing dams that have no generating capacity. Other developments that could add to the capacity include the more novel ideas of using disused mines for underground storage, and creating pumped storage facilities in the middle of pre-existing lakes or the sea, as well as digging storage caverns next to existing ground-level water stores.

Overall, the scope for utilizing existing pumped storage and increasing pumped storage capacity makes it an ideal partner to increasing levels of renewable penetration in the electricity generation mix.

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[1] International Power First Hydro, The Principles of Pumped Storage, Mitsui & Co. Ltd (online) available at: http://www.fhc.co.uk/pumped_storage.htm (accessed 20 Mar 2011)

[2] J.S. Anagnostopoulos and D.E. Papantonis (2008), Simulation and size optimization of a pumped–storage power plant for the recovery of wind-farms rejected energy, Renewable Energy 33, p1686

[3] C. Bueno and J.A. Carta (2004), Wind powered pumped hydro storage systems, a means of increasing the penetration of renewable energy in the Canary Islands, Renewable and Sustainable Energy Reviews 10

[4] Electricity Storage Association (2009), Technologies, (online) available at: http://www.electricitystorage.org/ESA/technologies/ (accessed: 21 Mar 2011)

[5] T. B. Johansson and L. Burnham (1993), Renewable energy: sources for fuels and electricity,  Island Press, p.520

[6] EPDC (1990), Kalayan Pumped Storage (online) available at: http://www.jpower.co.jp/english/international/consultation/detail/se_as_philippines01.pdf (accessed 22 Mar 2011)

[7] European Commission, Status Report on Variable Speed Operation in Small Hydropower, European Commission (online) available at: http://ec.europa.eu/energy/res/sectors/doc/small_hydro/statusreport_vspinshp_colour2.pdf (accessed 22 Mar 2011)

[8] ibid

[9] Survival International (2010), Serious Damage – Tribal Peoples and Large Dams, Survival International (online) available at: http://assets.survivalinternational.org/documents/373/Serious_Damage_final.pdf (accessed 21 Mar 2011)

[10] UNEP, Seven Essential Steps of the Report, (online) available at: http://www.unep.org/dams/documents/Default.asp?DocumentID=664 (accessed 22 Mar 2011)

[11] C. Bueno and J.A. Carta (2004), Wind powered pumped hydro storage systems, a means of increasing the penetration of renewable energy in the Canary Islands, Renewable and Sustainable Energy Reviews 10

[12] I. Venter (2008) Eskom to probe old-mine-based pumped-storage scheme, Mining Weekly (online) available at: http://www.miningweekly.com/article/eskom-to-probe-oldminebased-pumpedstorage-scheme-2008-05-16 (accessed 21 Mar 2011)

[13] EcoFriendly (2008), Water battery: Riverbank Power brings new twist to pumped storage, Eco Friendly Mag (online) available at: http://www.ecofriendlymag.com/sustainable-transporation-and-alternative-fuel/water-battery-riverbank-power-brings-new-twist-to-pumped-storage/ (accessed 21 Mar 2011)

[14] M. Popp (2010), Storage for a Secure Power Supply from Wind and Sun, (online) available at: http://poppware.de/Storage_for_a_secure_Power_Supply_from_Wind_and_Sun.pdf (accessed 21 Mar 2011)

[15] KEMA, Large-scale Energy Storage, (online) available at: http://www.kema.com/services/consulting/utility-future/energy-storage/large-scale-storage.aspx (accessed 21 Mar 2011)

[16] Wikipedia (2011), List of pumped-storage hydroelectric power stations (online) available at: http://en.wikipedia.org/wiki/List_of_pumped-storage_hydroelectric_power_stations (accessed 21 Mar 2011)

[17] R. Miller (2010), Wind Integration Utilizing Pumped Storage (online) available at: http://www.carebs.org/webroot/Richard_Miller.pdf (accessed 21 Mar 2011)

[18] IESO (2006) Monthly Market Report, August 2006, (online) available at: http://www.ieso.ca/imoweb/pubs/marketReports/monthly/2006aug.pdf (accessed 22 Mar 2011)

[19] Consumers Energy (2011), Consumers Energy and Detroit Edison Announce Major Maintenance and Upgrade Project at Ludington Pumped Storage Plant, Press Release (online) available at: http://www.consumersenergy.com/News.aspx?id=4220&year=2011 (accessed 22 Mar 2011)

[20] P. Fraenkel, Wind Pumps, BWEA (online) available at: http://www.bwea.com/ref/pumps.html (accessed 22 Mar 2011)