Thermochemical Energy Storage | Principle, Types, and Requirements

Introduction

Thermochemical energy storage is highly efficient for saving energy and reducing greenhouse gas emissions. Compared to other types of energy storage, like sensible heat (storing heat by changing temperature) and latent heat (storing heat through phase changes), thermochemical storage can store the most heat without losing any energy over time.

Materials commonly used in thermochemical storage systems include silica gel/water, magnesium sulfate/water, lithium bromide/water, lithium chloride/water, and NaOH/water. These materials are effective at storing a lot of heat.

Using materials with a porous structure containing reactive substances can improve the processes of heat and mass transfer. Additionally, chemical heat pumps, which operate on chemical reactions, can store even more heat at high temperatures compared to conventional heat pumps.

Combining thermochemical energy storage with long-term seasonal thermal energy storage (TES) techniques can further improve the system’s thermal performance while maintaining energy efficiency and environmental benefits.

Principle of a Thermochemical Energy Storage

1. Charging the Storage Unit: Heat is added to a reaction that absorbs heat (endothermic reaction), creating separate products that are stored separately.
2. Releasing the Energy: When energy is needed, the stored products are combined, releasing the stored heat.

This process can run continuously with the right reactor setup. It uses two reactors and storage tanks:

• Reactor 1 (Charging): Heat is added, causing solid component A to turn into solid component B and releasing water vapor.
• Storage: Component B is stored until needed.
• Reactor 2 (Discharging): When energy is needed, component B is combined with water, releasing the stored energy. The reaction turns component B back into component A.
• Cycle Repeats: Component A is sent back to Reactor 1 to start the cycle again.

This system allows for efficient storage and release of energy as needed.

Types of Thermochemical Energy Storage

There are three types of thermal energy storage systems: sensible heat, latent heat, and thermochemical.

1. Sensible Heat Storage: Uses materials like water or rock to store and release heat by changing their temperature. It’s the most practical option for reducing energy use and CO2 emissions and is commonly used in homes.
2. Latent Heat Storage: Uses materials that store energy without changing temperature but by changing their state (solid to liquid or vice versa). These “phase change materials” are used in solar energy and building materials to absorb and store excess heat.
3. Thermochemical Storage: Stores energy through chemical reactions.

These systems can store excess heat for hours, days, or even months, depending on the technology used.

Requirements of Thermochemical Energy Storage

Materials used for thermochemical energy storage (TCES) must be affordable, non-toxic, and able to undergo reversible reactions. These reactions must remain stable through many cycles and occur within a specific temperature range, up to 200°C. This allows the system to store heat from solar energy and industrial waste sources. Applications within this temperature range include district heating, the food industry, and paper production. Suitable materials for these reactions include salt hydrates and boric acid (H3BO3).

These solid materials are used in a three-phase suspension reactor, which has several benefits. The liquid suspension helps improve heat and mass flow. The current experimental setup includes a stirred reactor using thermal oil as the suspension medium. Solid reactants are mixed into the oil, heated, and converted through a gas-solid reaction. The reaction produces water vapor, which is collected after condensing. The solid product stays in the suspension until it is needed again.

Advantages of Thermochemical Energy Storage

1. High Energy Density: TCES systems offer greater energy storage density than sensible and latent heat systems, allowing for more compact storage solutions.
2. No Thermal Losses: Energy is stored through chemical reactions, preventing thermal losses over time and ensuring efficient long-term storage.
3. Versatile Temperature Range: TCES operates efficiently across a wide range of temperatures, making it suitable for various applications, including industrial and residential uses.
4. Enhanced Energy Efficiency: Direct conversion of heat to chemical energy and vice versa improves overall system efficiency and reduces energy consumption.
5. Environmental Benefits: By enabling efficient use of renewable energy and waste heat, TCES helps reduce greenhouse gas emissions and supports sustainability.

Disadvantages of Thermochemical Energy Storage

1. Complexity: TCES systems are complex to design and implement, requiring advanced materials and precise control mechanisms.
2. High Initial Costs: The materials and technology needed for TCES are often expensive, leading to high upfront investment costs.
3. Material Degradation: Some reactive materials used in TCES can degrade over time, reducing efficiency and lifespan.
4. Safety Concerns: Handling and storing reactive chemicals can pose safety risks, requiring stringent safety measures and protocols.

Frequently Asked Questions (FAQs)

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