Managing Imbalances

Pumped storage hydropower offers a cost-effective solution

 

The renewable energy segment has grown at a fast pace in the past few years. In India alone, the share of renewable energy sources in the total installed capacity has increased from 10.62 per cent as on March 31, 2011 to 15.92 per cent as on December 31, 2016. With such rapid expansion, the variability in generation, especially in the short-term time frame (owing to cloud movement, wind fronts, calms, etc.), is likely to become a major issue in the coming years. In this respect, pumped storage hydropower (PSH) plants can be used as a flexible generator to mitigate the imbalance in supply and demand.

Since most low-carbon-energy resources cannot flexibly adjust their output to match the fluctuating power demand, there is an increasing need for bulk electricity storage. Major storage technologies include PSH, compressed air energy storage, flywheels, supercapacitors, flow cells and rechargeable batteries such as lithium-ion batteries, and super-conducting magnet energy storage. Among these, PSH is the only mature, cost-effective and widely used technology for commercial, utility-scale electricity storage. With an installed capacity of over 150 GW, these plants comprise around 99 per cent of the world’s electrical energy storage capacity. Hence, this technology can be the backbone of a reliable renewable electricity system.

Currently, over 60 PSH projects are under construction globally, with the majority of these being constructed in Europe, India, China and Japan. This growth is based on the desire to mitigate greenhouse gas emissions and hence balance the growth in intermittent renewable energy generation with a higher number of energy storage projects.

Design

The design of a PSH plant is similar to that of a conventional hydropower plant, with some functional differences. The main difference is that it requires two reservoirs at different levels (upper and lower) to store water, and the stored water is moved to and fro via a pump turbine arrangement. PSH plants utilise off-peak electricity to pump water from the lower reservoir to the one at a higher elevation. Water can be pumped to the higher reservoir when excess or cheaper energy is available. The same water is passed through the hydraulic turbine to generate power during peak demand periods when the price of electricity is high.

Cost-effective option

As per the Energy Technology Systems Analysis Programme of the International Energy Agency and International Renewable Energy Agency [IRENA] Technology Policy Brief on Electricity Storage, April 2012, PSH is one of the cheapest storage options per unit of energy, that is, $2,000-$4,000 per kW, with investment costs largely dependent on the plant site and size. Since PSH is a mature technology, cost reduction is not expected in the future.

In addition, the efficiency of PSH systems is between 70 per cent and 80 per cent. This includes a typical pump and turbine efficiency of 92 per cent, motor and generator efficiencies of 98 per cent and energy losses of 7 per cent in pumping and turbine operation. Moreover, their typical lifetime is over 30 years.

Applications

PSH serves the grid through a wide range of applications.

  • Peak shaving: PSH systems can be used to generate energy for meeting peak demand in a short period of time.
  • Load balancing: Load levelling usually involves the storing of power during periods of light loading (off-peak hours) on the system and delivering it during periods of high demand.
  • Frequency regulation: Hydropower helps maintain the frequency within the given margins by continuous modulation of active power.
  • Backup reserve, spinning reserve: These plants have the ability to enter load into an electrical system from a source that is not online. They can also provide additional power supply that can be made available to the transmission system within a few seconds in case of unexpected load changes in the grid.
  •  Quick-start capability: Hydropower generation can be set up in just a few minutes as compared to the 30 minutes typically taken by other turbines, or the hours needed for steam generation.
  • Black-start capability: These plants have the ability to run at zero loads. When loads increase, additional power can be loaded rapidly.
  •  Voltage support: PSH plants have the ability to control reactive power, thereby ensuring that power flows from generation to load.

Status in India

India started its PSH programme in the 1980s, with the commissioning of the first PSH plant at Nagarjuna Sagar, Andhra Pradesh. The 700 MW plant was commissioned in two stages during 1980-85, with four 100 MW plants being commissioned in the first stage and three 100 MW plants in the second stage. Although envisaged as a PSH plant, it has not been able to function in pump mode due to the unavailability of tail-pool dam.

As per a reassessment study carried out by the Central Electricity Authority (CEA) during the period 1978-87, 63 potential PSH sites were identified, with an aggregate installed capacity of about 96,524 MW, and individual plant capacities varying from 600 MW to 2,800 MW. Of these, seven plants aggregating 2,604 MW were under operation/construction at the time of the study. The project details are given in the table – PSH potential and sites.

Currently, projects aggregating 4,804 MW of capacity are operational and 3,680 MW are under construction.

Since the survey was conducted almost three decades ago, there is a need to reassess the country’s PSH potential. There have been advancements in the technology over the years and the possibility of mines as reservoirs, seawater PSH, etc. are being explored the world over.

Moreover, the assessment needs to be redone, keeping in mind the integration of renewable energy with PSH technology. There is a possibility of finding more technically and economically feasible sites. However, with changes in the environmental norms set by the government, there are chances of cancelling some of the sites identified earlier.

Drawbacks

The main drawback of the PSH system is geographical restrictions. These plants require relatively large water reservoirs, and large elevation variations between the lower and the upper reservoirs to provide sufficient capacity. In addition, they have a longer concept-to-commissioning period and require high upfront capital investment as against other storage options. The construction of conventional PSH systems often involves the damming of a river to create a reservoir, which could have serious environmental impact and has led to the cancellation of a number of projects. However, the potential impact of the systems is site specific and needs to be evaluated on a case-by-case basis.

The way forward

With the renewable energy revolution gaining momentum worldwide, hydropower looks set to become an even more strategic player. As per IRENA’s REmap 2030, hydro capacity could increase up to 60 per cent and the PSH capacity could reach about 325 GW.  Amongst various storage technologies, PSH still has considerable potential for expansion. Huge potential is associated with seawater pumped hydro, which utilises convenient coastal locations.

However, the capital costs for PSH systems are currently as low as they can get. On the other hand, batteries are expected to get significantly cheaper in the coming years. Lithium-ion batteries are currently taking the lead as an efficient energy storage system. Given the scale of lithium-ion production in China, it is likely that the Indian market will soon be swamped with cheap second-life batteries from its neighbour. This reduction in costs, along with easy, location-independent installations for batteries, may hamper the growth of PSH capacity globally.

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