8.1 Pumped hydro storage: principles and infrastructure

Pumped hydro storage is a game-changer for renewable energy. It's like a giant battery that uses water and gravity to store power. When there's extra electricity, water is pumped uphill. When power is needed, it flows back down, spinning turbines.

This system has two key parts: water storage and power generation. Reservoirs hold water at different heights, while special turbines and generators convert between electricity and water flow. It's a simple yet powerful way to balance the grid.

Pumped Hydro Components

Water Storage and Conveyance

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• Upper reservoir stores water at a higher elevation, acting as the potential energy source for power generation
  • Typically constructed by damming a natural lake or building an artificial reservoir on a hilltop or mountain
  • Must have sufficient capacity to store the water needed for the desired power output and duration
• Lower reservoir receives water discharged from the upper reservoir during power generation and serves as the source for pumping water back up
  • Can be a natural lake, river, or constructed reservoir at a lower elevation than the upper reservoir
  • Must have enough capacity to accommodate the water flow without significant changes in water level
• Penstock is a large pipe or tunnel that conveys water between the upper and lower reservoirs
  • Designed to withstand high water pressure and optimize water flow with minimal friction losses
  • Length and diameter depend on the site topography and desired power output (typically 3-10 meters in diameter)

Power Generation Equipment

• Reversible turbine-generators serve as both pumps and turbines, allowing for bidirectional water flow and energy conversion
  • During pumping mode, they act as pumps, using electricity to move water from the lower to the upper reservoir
  • In generating mode, they function as turbines, converting the potential energy of falling water into electricity
  • Common types include Francis turbines (for medium to high head sites) and Pelton wheels (for high head sites)
• Motor-generators are coupled to the turbines and alternate between motor (pumping) and generator (generating) functions
  • Synchronous machines are typically used, allowing for efficient operation and grid synchronization
  • Rated power output depends on the site characteristics and design, ranging from tens to hundreds of megawatts

Operating Modes

Pumping Mode

• During periods of low electricity demand or surplus renewable energy production, the system operates in pumping mode
  • Electricity from the grid or renewable sources powers the motor-generators, which drive the turbines to pump water from the lower to the upper reservoir
  • Pumping typically occurs during night-time hours when electricity prices are lower or when there is excess wind or solar power available
• Pumping efficiency depends on factors such as the efficiency of the motor-generators, turbines, and the penstock design
  • Modern pumped storage systems can achieve pumping efficiencies of around 80-90%

Generating Mode

• When electricity demand is high or additional grid support is needed, the system switches to generating mode
  • Water is released from the upper reservoir, flowing through the penstock and driving the turbines to generate electricity
  • The motor-generators now function as generators, converting the mechanical energy from the turbines into electrical energy
• Generating efficiency is influenced by the efficiency of the turbines, generators, and the overall system design
  • State-of-the-art pumped storage plants can achieve generating efficiencies of approximately 90-95%
• Round-trip efficiency represents the overall efficiency of the pumped storage system, considering both pumping and generating modes
  • It is calculated as the ratio of energy generated to the energy consumed during the pumping phase
  • Typical round-trip efficiencies range from 70-85%, depending on the system design and technology used

Head Height Considerations

• Head height refers to the elevation difference between the upper and lower reservoirs
  • A higher head height allows for greater potential energy storage and power output for a given volume of water
  • Pumped storage sites with head heights ranging from 200-800 meters are common, with some exceeding 1000 meters
• Higher head heights generally result in more compact system designs, as less water volume is required to achieve the same energy storage capacity
  • This can reduce the size and cost of reservoirs, penstocks, and other civil works
• However, higher head heights also pose challenges in terms of water pressure management and equipment design
  • Specialized turbines, such as Pelton wheels, are used for high head applications to efficiently convert the high-pressure water flow into mechanical energy

Grid Benefits

Frequency Regulation and Grid Stabilization

• Pumped hydro storage provides rapid response to changes in electricity demand and supply, helping to maintain grid frequency stability
  • The system can quickly switch between pumping and generating modes to absorb excess energy or provide additional power as needed
  • Response times are typically in the range of seconds to minutes, allowing for effective frequency regulation
• By balancing supply and demand, pumped storage helps stabilize the grid and reduces the risk of power outages or blackouts
  • It can compensate for the variability of renewable energy sources, such as wind and solar, by storing excess energy during high production periods and releasing it when demand increases

Load Leveling and Peak Shaving

• Pumped storage enables load leveling by storing energy during periods of low demand and releasing it during peak demand hours
  • This helps to flatten the overall electricity demand curve, reducing the need for expensive peaking power plants that only operate during high demand periods
  • By shifting energy consumption from peak to off-peak hours, pumped storage can reduce electricity costs and improve overall grid efficiency
• Peak shaving involves using stored energy to meet the highest demand periods, reducing the strain on the grid and other power generation sources
  • Pumped storage can provide a reliable and dispatchable source of peak power, complementing base-load power plants and renewable energy sources

Improved Capacity Factor and Asset Utilization

• Capacity factor represents the ratio of a power plant's actual energy output over a period of time to its potential output if it were operating at full capacity continuously
  • Pumped storage can improve the capacity factor of the overall electricity system by storing excess energy during low demand periods and releasing it during high demand periods
  • This allows for more efficient use of base-load power plants and renewable energy assets, as their excess output can be stored and used later instead of being curtailed
• By providing energy storage and peak power capacity, pumped hydro can help optimize the utilization of other power generation assets
  • It can reduce the need for spinning reserves (power plants kept running at low output levels to respond to sudden demand changes) and decrease the reliance on less efficient or more polluting peaking power plants

Specialized Systems

Seawater Pumped Storage

• Seawater pumped storage is a variant of conventional pumped hydro that uses seawater instead of freshwater as the working fluid
  • This allows for the development of pumped storage projects in coastal areas where freshwater resources may be limited
  • Seawater is pumped from the ocean into an upper reservoir during periods of low electricity demand and released back into the ocean during high demand periods to generate electricity
• Seawater pumped storage faces additional challenges compared to freshwater systems, such as increased corrosion and marine growth
  • Special materials and coatings are used for equipment and infrastructure to withstand the corrosive nature of seawater
  • Measures to prevent marine organism growth, such as biocides or physical barriers, may be necessary to maintain system efficiency and reliability
• Environmental considerations are crucial in seawater pumped storage projects, as they can impact marine ecosystems and coastal landscapes
  • Careful site selection, design, and operation are necessary to minimize negative impacts on the environment and ensure compliance with regulations
  • Examples of seawater pumped storage projects include the 30 MW Yanbaru project in Okinawa, Japan, and the proposed 300 MW Pacific Ocean Pumped Storage project in Hawaii, USA