Pumped storage hydro plants are a type of energy storage system that utilizes the potential energy of water to store and generate electricity. This method stores energy in the form of gravitational potential energy of water, pumped from a lower elevation reservoir to a higher elevation. They are typically built in hilly or mountainous regions where there are two reservoirs located at different elevations that are namely Upper Reservoir and Lower Reservoirs.
Advantages of Pumped Storage Hydro Plants:
Pumped storage hydro plants are characterized by their high energy storage capacity, longer lifespan, and relatively low operational costs.
- To meet variable renewable energy balance
Capable of responding quickly within seconds to minutes to the need for additional generation thus it is used as a spinning reserve.
- To cope up peak peak-hour demand
Generating while releasing water from the upper reservoir and hence supplying expensive energy
All this is done by storing energy during off-peak hours only. During this period pumping water to the upper reservoir by consuming inexpensive energy. This process is known as Peak shaving or Load leveling.
Need of Pumped Storage Hydro Plants:
The coal or nuclear-based thermal power generation cannot steeply rise or fall matching the load demand. The thermal power generators cannot normally be shut off at night and brought online in the morning to match system loads.
Thermal power stations such as coal-fired plants and nuclear power plants continue operating at peak efficiency and run as base-load power plants.
If the thermal power stations are to share base and peak loads, then the power system will be subjected to frequency excursions. Moreover, when the thermal plants have to run at low loads for prolonged periods they need fuel oil support thereby increasing the cost of generation, which is not desirable and not recommended.
In the case of renewable energy sources like Wind and Solar power stations, it is essential to note that generation from Wind power varies from hour to hour and from season to season, often increasing after evening hours. Similarly, generation from solar power is available only during day time, but also varies hourly and by season. The power generated from these sources at a certain hour on a future date of a year is uncertain. As the share of renewable energy increases, it is naturally hard to balance energy generation and consumption at all times.
The most reliable option for energy storage is the development of a pumped storage scheme, which utilizes the surplus power available during the Off-peak period to pump up the water for storage and meets the On-peak demand by utilizing the stored water during peak demand. PSH can be made available at short notice.
PSH also provides a solution in the form of stabilizing the operation of the grid by controlling electrical network frequency.
The Integrated Renewable Energy Project is made possible with the help of PSH.
Integrated Solar, Wind, and PSH Plants:
Integrated solar, wind, and pumped hydropower project operation is advantageous. The closed-loop system will benefit from being able to use the electricity generated by the nearby solar or/and Wind project to pump water uphill during the day before dispatching load and flexible energy with PSH at periods of peak demand such as in the evening.
Major Components of PSH:
1. Upper Dam: Used to store water in the Upper reservoir.
2. Lower Dam: Used to store water in the Lower reservoir.
3. Headrace Tunnel: Water Flows from the upper reservoir are diverted to a headrace tunnel. It is proposed to provide concrete lining for the entire length of the tunnel.
4. Surge Tank/Surge shaft: Connects between dam and Powerhouse. It provides better regulation of the flow of water.
5. Penstock:
A large pipe (the “penstock”) delivers water from the reservoir to the turbine. It connects between the surge tank and the turbine.
6. Pressure shaft:
The pressure shaft is the steeply sloped or vertical portion of the penstock between the head race and the turbine. For long-term stability and O&M aspects, the pressure shaft will be provided with a steel liner to withstand the internal water pressure and suitably backfilled with concrete.
6. Shaft type Powerhouse: To house the reversible pump-turbine units required for power generation and pumping.
7. Tailrace Tunnel: Water after power generation, is let back into the lower reservoir through a tailrace tunnel
Operation of Pumped Storage Plants:
A pumped-storage plant generates electricity by allowing water to fall through a turbine generator. Once the pumped-storage plant generates electricity, it pumps that water from its lower reservoir back to the upper reservoir — unlike conventional hydropower projects where the water flows downriver.
- Renewable Energy such as wind or solar used to pump water from a lower reservoir to an upper reservoir during times of low demand.
- When demand increases, or wind/solar production drops, water runs downhill from the upper reservoir.
- Water runs through a turbine, creating electricity.
- A more stable, less variable supply results from adding electricity from the turbine to the original renewable power.
The above operation is made possible with the use of reversible PTG Units.
Reversible Pump-Turbine-Generator (PTG) Units:
The reversible PTG unit shall be made up of a single-stage Francis pump-turbine and a synchronous generator/motor.
The PTG unit is a vertical-axis reversible-type hydroelectric unit composed generator-motor and a pump-turbine allowing two rotational directions of the rotating part.
In one direction, by virtue of the energy supplied by the electrical network, the electrical machine operates as a synchronous motor and drives the hydraulic machine in pumping mode allowing it to pump the water from the lower reservoir to the upper reservoir. The power requirement for the pumping mode will be drawn through the same switchyard and the transmission lines by back-feeding the generator transformers.
In the opposite direction, the hydraulic machine operates as a turbine with water being supplied from the upper reservoir and drives the electrical machine as a synchronous generator enabling it to generate electric power.
PTG Unit Types:
- Two types of Reversible type pump turbine machines are available viz. Constant speed is where the machines operate at a fixed speed and Variable speed machines.
- Constant/fixed speed units are cost efficient and the variable speed units offer better utility of the power.
