By Dr. DF Duvenhage

Seawater-pumped storage is an innovative form of hydroelectric energy storage that harnesses the power of seawater as the lower reservoir in a two-tiered energy storage system. This approach offers a compelling solution for storing and regulating electrical energy. In this article, we explore the working principle, historical context, commercial implementations, and critical criteria for site selection in seawater-pumped storage projects.

Working Principle

Seawater pumped storage operates on a fundamental principle of storing surplus electricity during periods of low demand by using it to pump seawater from a lower reservoir to an upper reservoir. When electricity is needed, the stored water is released, flowing downhill, and driving turbines to generate electricity. This reversible process helps balance the grid and is a crucial asset for grid stability and renewable energy integration.

Major components and sub-systems

These facilities consist of several essential components or sub-systems, each playing a unique role in the process. In this article, we delve into these key elements that make seawater-pumped storage a viable and promising technology.

1. Upper Reservoir

At the heart of a seawater-pumped storage plant lies the upper reservoir, typically situated at an elevated location. This reservoir serves as the storage vessel for seawater, holding it until electricity is needed. The upper reservoir is often constructed with a focus on sealing techniques to prevent leakage and minimize environmental impact.

2. Lower Reservoir

The lower reservoir, as the counterpart to the upper one, is located at a lower elevation. This body of water collects the seawater discharged from the upper reservoir when energy generation is required. The difference in elevation between the two reservoirs is crucial for the plant’s energy storage capacity.

3. Penstock

The penstock is essentially a large pipe or conduit that connects the upper and lower reservoirs. When energy needs to be generated, water from the upper reservoir flows through the penstock to the lower reservoir. The penstock is constructed with durability and water-tightness in mind to prevent leaks and energy loss.

4. Pump-Turbine

The pump-turbine is a versatile machine that serves a dual purpose in seawater-pumped storage. It can act as a pump to lift water from the lower reservoir to the upper reservoir when excess electricity is available. Conversely, it transforms into a turbine to generate electricity by allowing the water to flow back down when power demand surges. These pump-turbines are designed to efficiently convert hydraulic energy into electrical energy and vice versa.

5. Electric Generators and Motors

Connected to the pump-turbines are electric generators and motors. When the pump-turbine operates as a generator, it produces electrical energy that is then fed into the grid. In contrast, when serving as a motor, it consumes electrical energy to pump water back into the upper reservoir. This dynamic interplay helps regulate the grid and optimize energy use.

6. Control and Monitoring System

A control and monitoring system is the brains behind the operation. It orchestrates the switching between pump and turbine modes, adjusts water flow rates, and manages the overall performance of the plant. This system plays a crucial role in maintaining grid stability and ensuring efficient energy storage and retrieval.

7. Seawater Intake and Discharge Structures

These are the entry and exit points for seawater into and out of the plant. The seawater intake structure allows water from the ocean to enter the system, while the discharge structure releases water back into the sea. Proper design and location are essential to minimize environmental impact and ensure efficient seawater use.

Historical Milestones

The Okinawa Yanbaru Seawater Pumped Storage Power Station in Japan holds the distinction of being the world’s first seawater-pumped storage facility. Completed in 1999 at a cost of ¥3.2 billion, it boasted a maximum output of 30 MW. Unfortunately, despite its technological success, this pioneering project was not profitable due to unmet power demand predictions, leading to its dismantlement in July 2016.

Seawater infiltration and dispersion were among the main areas under examination during the testing period of the Okinawa Yanbaru Seawater Pumped Storage Power Station. The following findings were reported:

  • Seawater infiltration and dispersion under normal operations were observed by daily monitoring and inspection. To prevent seawater infiltrating into the surrounding land strata, the entire reservoir surface was lined with rubber sheeting. Assessment of the impervious nature of the sheeting started when reservoir inundation began in August 1998. No water leakage was detected within the upper reservoir for nearly two years, including the test inundation period. Seawater dispersion was regularly checked by measuring the salt content in the surrounding atmosphere, rainwater, and soil. As Okinawa main island is surrounded by the sea, the salt content is usually high. So far, there is no sign of an increase caused by the operation of the upper reservoir. To confirm the effect of seawater infiltration and dispersion, the flora and fauna in the surrounding environment, and the water quality in nearby streams and ponds, were also being monitored.
  • Seawater Corrosion of Power Plant Materials: Stainless steel components, along with fiber-reinforced plastic (FRP-M) piping, showed resilience against corrosion. A careful examination of materials, especially those vulnerable to crevice corrosion, was a vital aspect.
  • Fouling by Marine Creatures: Divers’ inspections of the penstock interior showed no adhesion of marine creatures. The presence of marine life, including various species of fish and crustaceans, indicated a thriving ecosystem around the plant.
  • Operation in Typhoon Conditions: The plant withstood typhoons and maintained normal operations. The resilience of the water-impervious sheets under strong winds was noted, with minimal inflation during the typhoon. Stable generation and pumped storage were maintained with slight fluctuations in output levels.

Overall, the findings from the Okinawa Yambaru seawater pumped storage power plant’s test phase have been encouraging. It demonstrated the feasibility of using seawater for energy storage, even in the face of adverse weather conditions. As the plant continues to operate and undergo further testing, its journey toward commercial viability remains on a promising trajectory.

Commercial Implementations

Currently, the Okinawa Yanbaru facility remains the sole operational seawater-pumped storage power station globally. However, proposals for similar projects have emerged, such as the 300 MW Lanai Pumped Storage Project in Hawaii and potential projects in Ireland. Nevertheless, seawater-based pumped storage projects remain relatively rare when compared to conventional hydroelectric options.

Site Selection Criteria

Choosing an appropriate site is a pivotal aspect of seawater-pumped storage development. Key considerations include:

  1. Topography and Geology: The local topography and geology play an essential role in determining the feasibility and cost-effectiveness of a project.
  2. Surplus Power Availability: The availability of surplus power for the pumping process, primarily during off-peak hours, is a critical factor. Without an excess supply of electricity, the facility’s operation may become unviable.
  3. Feasibility of Seawater Infiltration and Dispersion: Ensuring that seawater remains within the storage system without infiltrating or impacting the surrounding environment is crucial. Linings and monitoring systems may be necessary to prevent infiltration and assess dispersion effects.
  4. Cost and Economic Feasibility: The economic viability of seawater pumped storage depends on factors like electricity costs, construction expenses, and available financing.
  5. Demand for Power: A sufficient demand for electricity within the region is essential to justify the construction of seawater-pumped storage projects.
  6. Technical Challenges: Seawater-pumped storage projects present unique technical challenges, including combating saltwater corrosion and fouling by marine organisms.

Challenges

Using seawater in pumped storage systems (PSS) comes with a set of practical challenges, mainly of a technical nature, due to the potential for seawater leaks from the upper reservoir or penstock, which could lead to significant harm to the environment. It’s crucial to address these issues effectively. In this context, two key technical concerns need attention: firstly, ensuring a secure seal for the reservoir, and secondly, choosing materials that can resist corrosion when building the penstock.

Moreover, it’s important to consider the potential spread of seawater caused by wind across the upper reservoir’s surface. To combat corrosion, it’s equally important to use materials that can withstand it for all the equipment involved in the water movement, including pumps and turbines. When deciding where to place these pump/turbine stations and their intakes along the coast, a thorough assessment of the site’s extreme weather conditions must be part of the planning process.

Overlap with existing industries and technologies

Seawater-pumped storage plants, while cutting-edge and distinct in their purpose, share several fundamental components and systems with existing technologies from various industries. The commonalities between seawater-pumped storage and these established technologies open up exciting opportunities for knowledge transfer and the adoption of best practices. Let’s delve into the overlaps and how they can benefit the development and operation of seawater-pumped storage plants.

1. Desalination Intake Stations

Seawater-pumped storage plants rely on seawater as the working fluid, and desalination intake stations, which draw seawater for desalination processes, have mastered the art of seawater extraction. Techniques used in desalination intake stations can inform the design and location of seawater intake structures for pumped storage, ensuring efficiency and minimal environmental impact.

2. Power Plant Cooling Coastal Discharge Stations

Power plants along coastlines have their cooling water intake and discharge systems. These systems offer insights into the management of seawater flow and temperature control. Seawater-pumped storage plants can adopt cooling system practices to optimize the release of water back into the sea while considering its temperature impact on the marine ecosystem. A benefit of seawater pumped-storage plants is that their discharge does not include any change to the condition of the water itself, meaning that this potential impact is not major. Erosion around the discharge is, however, the main factor that must be considered, since the flow rates can be quite high, and likewise the resulting erosion risk.

3. In-Land Pumped-Storage Hydroelectric Power Plants

Traditional pumped-storage hydroelectric plants, although located inland, share common components with seawater-pumped storage plants. These include penstocks, turbines, generators, and control systems. Lessons learned from the operation and maintenance of inland pumped-storage plants can be readily applied to seawater counterparts. Obviously, this is said with cognizance of the fact that inland systems do not have to contend with adverse water quality since they operate with fresh surface water. It is therefore imperative that the necessary attention is placed on the substantially higher risk of corrosion and wear on these mechanical components operating. Luckily, there are adequate highly experienced engineers from the various heavy industries that need to face marine conditions, who will be able to apply their knowledge towards increasing the wear-resistance of these mechanical components.

4. Mooring Systems in the Oil and Gas Sector

The mooring systems used in the oil and gas industry for offshore platforms and vessels require robust engineering to withstand harsh marine conditions. Seawater-pumped storage plants are often located in the open sea, and insights from mooring systems can enhance the structural integrity and stability of the plant. While seawater-pumped storage plants are invariably located near to coastal areas, their intakes might have to be located further off-shore in order to address coastal erosion risks and impact on marine life. These mooring systems from the oil and gas sector can, therefore, be applied to such scenarios for seawater-pumped storage intakes, in order to capitalize on the existing pool of experience and proven approaches.

5. Environmental Impact Mitigation Techniques

Techniques to minimize environmental impacts, such as those employed in water withdrawal and discharge practices, can be shared across various sectors. Lessons learned in mitigating adverse effects on marine ecosystems can benefit seawater-pumped storage plants.

6. Material Selection and Corrosion Prevention

Materials and coatings used in various marine applications can inform choices for corrosion-resistant components in seawater-pumped storage. Innovations in material selection and anti-corrosion strategies from diverse industries can be integrated to extend the longevity of plant infrastructure. As mentioned earlier, freshwater poses a much lower risk of corrosion and fouling than of seawater. However, the use of seawater in cooling of power stations located near coastal areas, and the types of equipment used in these applications, can be easily applied to seawater-pumped storage plants. Seawater pumps, piping, fittings, and operations and maintenance technicals from such cooling plants can be almost directly applied.

Opportunities for South Africa

Seawater-pumped storage power plants have the potential to provide a reliable and cost-effective solution to South Africa’s energy crisis. The country has been experiencing load shedding due to a shortage of electricity supply, and the increasing large-scale renewable energy generation has created a need for energy storage. The occurrence of inland pumped-storage and hydroelectric power plants in South Africa can provide a foundation for the development of seawater pumped-storage power plants. The metropolitan areas in the Western Cape, Eastern Cape, and KwaZulu Natal provinces are ideal locations for seawater-pumped storage power plants due to their proximity to the coastline.

Figure X shows a Digital Elevation Model Topographical Map of South Africa. This shows the elevation above sea level in meters, at a resolution of 30m2. It is quite clear that the highest elevations are located inland. However, there are various coastal regions where there are sharp elevation changes from 0m above sea level to more than 500m.

Figure X: A Digital Elevation Model Topographical Map of South Africa, https://en-za.topographic-map.com/map-6m7zs/South-Africa/

Considering the greater Cape Town metropolitan area in the Western Cape, shown in Figure Y, a few potential locations can be identified. These points are indicated, along with their elevations in Figure Y. Knowing nothing else, and assuming a similar flow rate to that of the Okinawa Yanbaru Seawater Pumped Storage Power Station of 26m3/s, we can estimate the potential installed capacity of pumped-storage plants located at these points based on the following calculations.

The hydraulic power formula calculates the rate at which water is flowing and the power it carries. It’s calculated as:

P=Q∗ρ∗g∗h*η

Where:

  • P is the hydraulic power (in watts).
  • Q is the flow rate of water (in cubic meters per second).
  • ρ is the density of water (approximately 1000 kg/m³).
  • g is the acceleration due to gravity (approximately 9.81 m/s²).
  • h is the head or height (in meters).
  • η is the efficiency of the plant (as a decimal).

Knowing the elevations above sea level, and assuming a full cycle conversion efficiency of 75% (https://www.eskom.co.za/wp-content/uploads/2022/04/HY-0002-Palmiet-Technical-Brochure-Rev-11.pdf),  we can estimate the potential installed capacity for each location as:

  • 721m: 137,923 MW
  • 585m: 111,907 MW
  • 833m: 159,348 MW
  • 472m: 90,291 MW
  • 586m: 112,098 MW

Figure Y: A Digital Elevation Model Topographical Map of the Cape Town metropolitan area, https://en-za.topographic-map.com/map-6m7zs/South-Africa/

While these installed capacities are obviously much larger than even the installed capacity of the entire South Africa, they neglect a key factor; the area available at that elevation to construct the upper reservoir. A storage area is needed where a dam can be constructed using the terrain, or where engineered water storage can be built in the form of a concrete dam or metal tanks. In order to estimate the volume, and subsequently the footprint of flat surface area, needed for such storage, the formula for gravitational potential energy can be used and is defined as:

GPE=m∗g∗h

Where:

  • GPE is the gravitational potential energy (in joules or watt-seconds).
  • m is the mass of the water (in kilograms).
  • g is the acceleration due to gravity (approximately 9.81 m/s²).
  • h is the height or head (in meters).

Furthermore, the volume can be calculated from the mass of water if the shape of the reservoir is pre-determined. If we assume a reservoir height of 15m, we can determine for various standard-shaped reservoirs, what the footprints need to be for various storage capacities in MWhs.

Clearly, this complicates the identification of locations for seawater-pumped storage. However, this does not mean that there is no potential for its implementation.

Conclusion

By recognizing these overlapping technologies and sharing best practices, seawater-pumped storage can benefit from the collective knowledge of established industries. This cross-pollination of ideas can lead to improved efficiency, enhanced environmental responsibility, and reduced development costs. As we strive for sustainable energy solutions, harnessing the collective wisdom of multiple sectors can play a pivotal role in the success of seawater-pumped storage and other innovative technologies.

The feasibility of such projects depends on several factors, including topography, geology, hydrology, and in the case of inland pumped storage, the availability of freshwater resources. However, some countries with mountainous landscapes, such as Switzerland and South Africa, have been successful in deploying inland pumped storage systems. The availability of seawater allows for larger and more economical schemes without the need to use valuable and limited freshwater resources. However, the capital costs of seawater-pumped storage plants can be high, and the feasibility of seawater infiltration and dispersion at the upper reservoir must be assessed in detail. Overall, the feasibility of seawater-pumped storage power projects depends on a comprehensive assessment of various factors, including topography, geology, hydrology, and the availability of adequate transmission networks.

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