Pumped Storage: A battery with a promising potential

Even though most people are familiar with the basics of conventional hydro, pumped storage is still a quite unfamiliar concept which provides solutions to some of the vital problems faced in the general power systems. One of the important usages of pumped storage comes in to play in matching constantly-changing supply from power producers with constantly-changing demand of power consumers by acting as a facility that can supply extra power when demand increases by taking in extra power when supply stays high while demand drops. Other than that, it is also considered as the largest capacity form of grid energy storage available, providing a large scale energy storage option for intermittent power sources. These points would be discussed extensively later in the article as it is important to first identify what pumped storage is exactly.
  
What is pumped hydroelectric storage?

A pumped hydroelectric storage facility typically consists of pumps/generators connecting an upper elevation reservoir and a lower elevation reservoir (As shown in Figure 1).

Figure 1: Basic overview of a pumped hydroelectric storage facility (Extracted from http://www.bbc.co.uk/bitesize/standard/physics/energy_matters/generation_of_electricity/revision/3/)

This method stores energy in the form of potential energy of water, pumped from the lower elevation reservoir to the higher elevation reservoir. The pumps utilize relatively low-cost electricity from the grid during off-peak hours to move water from the lower reservoir to the upper reservoir to store energy. During periods of high electricity demand (peak-hours), the stored water is released through turbines to produce electric power.

Pumped storage as a load balancing tool:

This specific function could be efficiently portrayed through the Sri Lankan power system, where the daily electricity demand fluctuates significantly and the late evening peak demand (around 2000 MW) is more than double the off-peak demand (around 800 MW). This situation leads to the requirement of developing generation facilities to serve the peak demand specifically.  (Sri Lankan daily load curve (in general) is shown in Figure 2)

Figure 2: Sri Lankan Electricity demand load curve (Extracted from
http://www.nalakagunawardene.com/tag/power-load-curve/)

Currently this high peak demand is satisfied through diesel/heavy fuel fired thermal plants (if the hydro generation capacity is not enough) which leads to a higher generation cost per unit whereas during off peak, plants with lower generation costs (e.g. coal fired power plants) are part loaded due to low demand making them inefficient (under-utilized).

This imbalance points us to the function of a pumped storage as a load balancing tool where during off peak, the pumps could utilize low cost energy by full loading the coal fired power plants and during the peak, the stored water could be used to satisfy the high demand presenting an economically advantageous situation than the normal conditions.

Even though pumped storage might be a novel concept in the Sri Lankan context, it has been utilized effectively in many countries over the world for this specific function.

Pumped storage as a battery for renewable energy:

It is known fact that one of the main drawbacks of electricity generation using renewable energy sources like wind, solar etc. is the high amount of intermittency present and the lack of suitable energy storage system which compensates to this intermittency.

Pumped storage is considered as a possible solution to this issue as it provides an energy storage opportunity, as potential energy in water in larger capacities. Already pumped storage systems are built on research basis which use wind turbines or solar power to drive water pumps directly, thus providing a more efficient system to smooth out the variability of energy captured from the wind or sun. (Example: Ringwall- storage-hybrid power plant in Germany)

Prospects for future:

Even though there are many new research concepts and novel ideas popping out regularly regarding  pumped storage technology, few stands out among those as really bright prospects for the future. One of them is utilizing sea-water for a pumped storage plant which Japan has already pioneered. The Okinawa seawater PHS station, which has commenced operation in 1999, is the world’s first seawater pumped storage facility.

Also researchers had proposed the possibility of utilizing an underground cavern as the lower reservoir for a pumped storage project. Recent examples include the proposed Summit project in Norton, Ohio, the proposed Maysville project in Kentucky (underground limestone mine), and the Mount Hope project in New Jersey, which have used a former iron mine as the lower reservoir.

Where Sri Lanka stands regarding this technology:

Sri Lanka as a country primarily based on hydro resources to generate power, should possess the suitable terrains with significant elevation difference between the two reservoirs and significant amount of water resource. Studies have already been conducted in this regard and several possible sites have been identified for a pumped storage facility. One of the main candidate sites is “Kiriketi” which is based on Kiriketi Oya- North of Samanalawewa Reservoir. More details about these studies could be found in the original research paper “PLANNING OF PUMPED STORAGE POWER PLANTS IN SRI LANKA” by MTAP Wickramarathna published in SLEMA Journal,Vol 14,No.2, September 2011.
(http://www.slema.org.lk/news_events/SLEMA-Journal/SLEMA-Journal-Sept-2011.pdf)

It could be concluded that, in the Sri Lankan context, a pumped storage facility could be a valuable addition to the power system as it provides a platform to operate the system more economically by utilizing the low cost coal power plants coming up and an opportunity to integrate more renewable energy sources to the system.

References:

http://thinkprogress.org/climate/2013/08/27/2524501/hydro-pumped-storage-climate-change/
http://www.renewableenergyworld.com/rea/blog/post/2013/11/pumped-storage-in-the-spotlight
http://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

Article By: Asith Kaushalya and Chathuranga Fernando

Posted in: Power System