10.2 Energy Storage Technologies for Ocean Energy Systems

Ocean energy systems face unique challenges in grid integration due to their intermittent nature. Energy storage technologies play a crucial role in smoothing out power output and ensuring reliable electricity supply. From mechanical to electrochemical solutions, various storage options can be tailored to meet specific needs.

This section explores key energy storage technologies for ocean energy systems. We'll cover pumped hydro, flywheels, compressed air, batteries, hydrogen, and supercapacitors. Understanding these options helps optimize ocean energy integration and maximize its potential in the renewable energy mix.

Mechanical Energy Storage

Pumped Hydro Storage

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• Stores energy by pumping water from a lower reservoir to an upper reservoir during periods of excess electricity
• When electricity is needed, water is released from the upper reservoir to generate power through a turbine
• Largest capacity form of grid energy storage and provides long-duration storage (hours to days)
• Requires specific geographical features (large elevation differences and water availability)
• Established technology with high round-trip efficiency (70-85%)

Flywheel Energy Storage

• Stores kinetic energy in a spinning rotor or flywheel
• Electricity is used to accelerate the flywheel, and energy is extracted by using the flywheel's rotational energy to drive a generator
• Provides short-duration, high-power storage with fast response times
• Suitable for applications requiring frequent charge/discharge cycles and high power output
• High round-trip efficiency (80-95%) and long cycle life, but relatively low energy density

Compressed Air Energy Storage

• Stores energy by compressing air in underground caverns or above-ground tanks during periods of excess electricity
• Compressed air is released to drive a turbine and generate electricity when needed
• Can provide long-duration storage (hours to days) and large-scale capacity
• Requires suitable geological formations (salt caverns, aquifers, or depleted gas fields) for underground storage
• Above-ground storage tanks have lower capacity and are more expensive
• Round-trip efficiency varies (40-70%) depending on the system design and heat recovery

Electrochemical Energy Storage

Battery Energy Storage Systems

• Store electrical energy through reversible chemical reactions in electrochemical cells
• Various battery technologies available, including lithium-ion, lead-acid, and flow batteries (vanadium redox, zinc-bromine)
• Provide a wide range of power and energy capacities, from kilowatts to megawatts and kilowatt-hours to megawatt-hours
• Suitable for both short-duration (minutes to hours) and long-duration (hours to days) storage applications
• Modular and scalable, allowing for flexible deployment and expansion
• Round-trip efficiency varies by technology (60-95%)

Hydrogen Storage

• Stores energy by producing hydrogen through water electrolysis using excess electricity
• Hydrogen can be stored as a compressed gas, cryogenic liquid, or in solid-state materials (metal hydrides)
• Stored hydrogen can be used in fuel cells to generate electricity or burned directly in gas turbines
• Provides long-duration, seasonal storage and can be transported for use in various applications (power generation, transportation, industrial processes)
• Requires infrastructure for hydrogen production, storage, and distribution
• Round-trip efficiency is relatively low (30-50%) due to conversion losses in electrolysis and fuel cells

Supercapacitors

• Store electrical energy in an electric field between two electrodes separated by an electrolyte
• Provide high power density and rapid charge/discharge capabilities, but lower energy density compared to batteries
• Suitable for applications requiring short-duration storage (seconds to minutes) and high power output
• Long cycle life (millions of cycles) and high round-trip efficiency (85-98%)
• Can complement batteries in hybrid energy storage systems to handle high power demands and extend battery life

Thermal and System Considerations

Thermal Energy Storage

• Stores thermal energy in materials with high heat capacity or through phase change (latent heat storage)
• Common materials include water, molten salts, and phase change materials (PCMs) like paraffin wax or salt hydrates
• Can be integrated with concentrating solar power (CSP) plants to store excess heat and generate electricity during off-sun hours
• Provides medium to long-duration storage (hours to days) and can improve the dispatchability of CSP plants
• Round-trip efficiency varies (50-90%) depending on the storage material and system design

Energy Storage Sizing

• Determining the appropriate size (power and energy capacity) of an energy storage system based on the application requirements and system constraints
• Factors to consider include the expected power demand, duration of storage needed, available space, and economic feasibility
• Oversizing the storage system leads to higher costs and underutilization, while undersizing may result in insufficient capacity to meet the desired performance
• Optimization techniques and simulation tools can be used to determine the optimal storage sizing for a given application

Round-trip Efficiency

• Measure of the amount of energy that can be retrieved from a storage system relative to the amount of energy put into it
• Accounts for energy losses during the charging, storage, and discharging processes
• Varies significantly among different storage technologies (30-98%)
• Higher round-trip efficiency reduces the overall cost of stored energy and improves the economic viability of the storage system
• Factors affecting round-trip efficiency include the inherent efficiency of the storage technology, system design, and operating conditions (temperature, pressure, charge/discharge rates)