Power Reservoirs

Critical role of pumped storage plants in load balancing

The lack of large-scale commercial energy storage solutions poses significant challenges for system operators as they integrate increasing levels of renewable energy generation into the grid. The installed renewable capacity in India is projected to increase to 175 GW by 2022 from the current installed base of 75.76 GW, taking its share in the total installed capacity from 22 per cent in January 2019 to nearly 33 per cent in 2022. In such a scenario, pumped storage plants (PSPs) can provide an ideal solution for energy storage and load balancing. Pumped storage is a mature technology that offers the benefits of simple design, ease of scalability and operation, and quick ramping.

Current status

Currently, India has nine PSPs with an installed capacity of 4,786 MW. Of these, six PSPs aggregating 3,306 MW are working in both generation and pumping mode. These are the 400 MW Kadamparai (Tamil Nadu), the 150 MW Bhira (Maharashtra), the 900 MW Srisailam LBPH (Andhra Pradesh), the 250 MW Ghatghar (Maharashtra), 900 MW Purulia (West Bengal), and the 706 MW Nagarjuna Sagar (Telangana) PSPs.

The remaining PSPs aggregating 1,480 MW are working only in generation mode owing to the unavailability of lower reservoir/tail pond and vibration issues. These include the 1,200 Sardar Sarovar (Gujarat), the 240 MW Kadana Stages I and II (Gujarat), and the 40 MW Panchet Hill (West Bengal) PSP. The Kadana PSP is facing vibration problems and an original equipment manufacturer has been appointed to resolve these issues. Meanwhile, the Panchet Hill and Sardar Sarovar projects do not have a tail pond. While the tail pond for the Sardar Sarovar PSP is under construction, the project developer (Damodar Valley Corporation) is facing land acquisition issues in the construction of a tail pond for the Panchet Hill PSP.

Potential

The Central Electricity Authority (CEA) has estimated the overall potential of PSPs in the country at 96,524 MW across 63 projects. About 39,684 MW, or 41.2 per cent of the total potential, lies in the western region. This is because of the steep gradient of the rivers originating in the Western Ghats. The southern region comes next with 18.4 per cent of the total potential, followed by the north-eastern region (17.5 per cent), northern region (13.5 per cent) and eastern region (9.4 per cent). State-wise, Maharashtra has the highest PSP potential of 27,094 MW followed by Madhya Pradesh (11,150 MW) and Mizoram (10,450 MW). Only 5 per cent of this potential has been harnessed so far while another 1 per cent is under development.

Role in load balancing

PSPs can provide energy balancing, stability, storage capacity and ancillary grid services such as network frequency control and reserves. This is due to the ability of PSPs to quickly respond to large load changes. PSPs can act as load in pump mode by raising water to the upper reservoir during times of surplus power and run in generating mode during power-deficit situations. PSPs are used by utilities across the world to balance load in a power system as they enable large generating sources to operate at peak efficiencies.

Some other benefits associated with PSPs include large energy storage and long discharge time, high ramp rate, voltage support, grid stability, and black-start facility. PSPs can be simultaneously used as spinning and non-spinning reserves. Like conventional hydro generators, the PSP systems running in generating mode can respond to frequency deviations within a few seconds, thereby contributing to overall grid balancing and stability. In both turbine and pump modes, excitation in generators and motors can be adjusted to contribute to reactive power load and stabilise voltage. Further, reactive power stability can be achieved by operating the PSP in synchronous condenser mode.

The role of hydropower projects including PSPs in load balancing assumed greater significance in July 2018, when the Central Electricity Regulatory Commission (CERC) approved the pilot project on hydro as a fast response ancillary service (FRAS). While the pilot is recommended for central sector hydropower stations, the scope of the project will be increased in future. The FRAS from hydro stations can help handle the sharp fluctuations and intermittency associated with renewable generation.

Currently, a total of 20 hydroelectric plants aggregating 8,604 MW are a part of the FRAS. They are operated by central sector utilities such as NHPC Limited, SJVN Limited, THDC India Limited, NTPC Limited, the Bhakra Beas Management Board and North Eastern Electric Power Corporation Limited. Under FRAS operation, all constraints declared by the hydro stations will be honoured. The total energy delivered during the day will be maintained as “declared capacity” by the hydro station. The schedules of the beneficiaries will not be disturbed in the despatch of FRAS. The regional power committees will issue weekly the FRAS accounts along with reserves regulation ancillary service accounts. The incentive will be paid from the deviation settlement mechanism (DSM) pool on a mileage basis at the rate of 10 paise per kWh both for “regulation up” and “regulation down”.

Issues and challenges

Environmental concerns are one of the biggest roadblocks to the development of hydropower projects including PSPs. Also, PSPs require very specific geographical features that limit unit sizing. These projects entail a high capital cost and a long gestation period. In the open- loop-type PSP, a dam is constructed along the main stem river, which alters the ecology of the river system. As a result, project developers find it difficult to secure environmental clearances. Therefore, they should choose project sites where construction would have minimum environmental impact.

Also, close- loop-type PSPs can be developed wherever feasible since they have minimal environmental impact on river systems as the project is separated from the river. PSPs should be located in areas with an existing upper/lower reservoir. This would be more cost and time effective. One option is to set up PSPs near abandoned mines. This will also help project developers to overcome land acquisition challenges to some extent. There are about 82 abandoned mines in the country as per the Indian Bureau of Mines. A fresh survey needs to be conducted to explore the possibility of utilising abandoned mines and seawater to set up PSPs.

The justification of the business case of PSPs is also an issue. As PSPs use more electrical power in pumping mode, the economic viability of the peak energy produced from these plants is a problem. PSPs utilise grid power during off-peak hours when the frequency is high and tariffs are low, and supply power during the peak demand period, or whenever required. It is therefore necessary to have adequate regulatory frameworks that incentivise peaking power generation through higher tariffs. Also, given their role in grid balancing, PSPs may be considered as a grid element instead of a power generation source and their development can be supported by the Power System Development Fund or the National Clean Energy Fund.

Outlook

There is a large pumped storage potential in the country waiting to be tapped. Currently, three PSPs aggregating 1,205 MW are under construction – the 1,000 MW Tehri PSP in Uttarakhand by THDC India Limited, the 125 MW Kundah Pump Storage Phase-I (PSP) by Tamil Nadu Generation and Distribution Corporation (TANGEDCO), and the 80 MW Koyna Left Bank by Maharashtra State Power Generation Company Limited. While the Tehri PSP is slated for commissioning in 2020-21, the Koyna and Kundah PSPs are expected to be commissioned by 2022-23.

The detailed project report for the 1,000 MW Turga PSP in West Bengal has been approved by the CEA and the project has also received environmental and forest clearances. Construction of the project is expected to begin soon. Further, three PSP projects aggregating 2,920 MW are under survey and investigation. These are the 600 MW Upper Indravati and the 320 MW Upper Kolab PSP in Odisha, and the 2,000 MW Sharavathy PSP in Karnataka. The commissioning of these PSPs in the coming years is expected to provide much-needed relief to system operators in balancing the grid.

With inputs from presentations by Surajit Chakraborti, Resident Director, West Bengal State Electricity Distribution Company Limited, and Alok Kumar, Deputy General Manager, NRLDC, Power System Operation Corporation Limited

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