Thermal energy storage (TES) is a technology that allows the storage and release of heat or cold at a later time. TES can be used to balance the supply and demand of energy, especially from renewable sources such as solar and wind, which are intermittent and variable. TES can also improve the energy efficiency of buildings, industries, and power plants, by reducing the peak load and enhancing the performance of heating and cooling systems. TES can help reduce greenhouse gas emissions, lower energy costs, and increase the reliability and resilience of energy systems.
TES has many applications in different sectors and regions, such as:
- Heating and cooling of buildings, using seasonal TES or diurnal TES
- Power generation, using concentrated solar power (CSP) or combined heat and power (CHP)
- Industrial processes, using waste heat recovery or process heat integration
- Transportation, using cold storage for food or ice storage for air conditioning
- Agriculture, using greenhouse heating or crop drying
In this blog post, we will explore the different types of TES, their advantages and disadvantages, and some examples of materials and technologies used for each type. We will also discuss the potential and challenges of TES for the future of energy systems.
Types of thermal energy storage
TES can be classified into three categories, based on the way heat is stored and released: sensible heat, latent heat, and thermo-chemical heat storage.
- Sensible heat storage is the most common and simple type of TES, where heat is stored by raising or lowering the temperature of a liquid or solid medium, such as water, molten salts, metals, or rocks. The amount of heat stored depends on the mass, specific heat, and temperature difference of the medium. Sensible heat storage has a low storage capacity, but a high efficiency, low cost, and high safety.
- Latent heat storage is a more advanced type of TES, where heat is stored by changing the phase of a material, such as melting, freezing, vaporizing, or condensing. The material is called a phase change material (PCM). The amount of heat stored depends on the mass, latent heat, and phase transition temperature of the PCM. Latent heat storage has a high storage capacity, but a low efficiency, high cost, and low safety.
- Thermo-chemical heat storage is the most novel and complex type of TES, where heat is stored by breaking or forming chemical bonds in a reversible reaction, such as hydration, dehydration, oxidation, or reduction. The material is called a thermo-chemical material (TCM). The amount of heat stored depends on the mass, enthalpy, and equilibrium constant of the reaction. Thermo-chemical heat storage has a very high storage capacity, but a very low efficiency, very high cost, and very low safety.
In the following sections, we will discuss each type of TES in more detail, and provide some examples of materials and technologies used for each type.
!Sensible heat storage)
Sensible heat storage
Sensible heat storage is the most widely used type of TES, as it is simple, reliable, and economical. The principle of sensible heat storage is to store heat by increasing or decreasing the temperature of a liquid or solid medium, without changing its phase. The heat can be released by reversing the process, i.e., decreasing or increasing the temperature of the medium.
The most common and widely used option for sensible heat storage is water tanks, which can store hot or cold water for heating or cooling purposes. Water tanks can be classified into two types: stratified and mixed. Stratified water tanks have a layer of hot water on top of a layer of cold water, separated by a thermocline. Mixed water tanks have a uniform temperature throughout the tank, achieved by stirring or pumping the water. Stratified water tanks have a higher storage capacity and efficiency than mixed water tanks, but they require more careful design and operation to maintain the stratification.
Some alternative options for sensible heat storage are molten salts, metals, or underground thermal energy storage (UTES). Molten salts are mixtures of salts that melt at high temperatures, such as sodium nitrate and potassium nitrate. They can store heat at temperatures up to 600°C, and are used for CSP plants. Metals are materials that have high thermal conductivity and specific heat, such as aluminum, copper, or steel. They can store heat at temperatures up to 1000°C, and are used for industrial processes. UTES is a method of storing heat in the ground, using boreholes, aquifers, or caverns. It can store heat at temperatures up to 90°C, and is used for seasonal heating and cooling of buildings.
!Latent heat storage)
Latent heat storage
Latent heat storage is a more advanced type of TES, as it can store more heat in a smaller volume and at a constant temperature. The principle of latent heat storage is to store heat by changing the phase of a material, such as melting, freezing, vaporizing, or condensing. The material is called a phase change material (PCM). The heat can be released by reversing the process, i.e., changing the phase of the material back to its original state.
The main challenges and opportunities of using PCM for TES are:
- Finding suitable PCM that have high latent heat, low cost, high stability, and low environmental impact
- Enhancing the thermal conductivity and heat transfer of PCM, which are usually low and slow
- Integrating PCM with other components and systems, such as heat exchangers, pipes, pumps, or containers
Some examples of PCM types and applications are:
- Ice is a PCM that freezes and melts at 0°C, and has a latent heat of 334 kJ/kg. It can be used for cold storage or air conditioning, by using ice tanks, ice slurry, or ice-on-coil systems.
- Paraffin is a PCM that melts and solidifies at various temperatures, depending on the carbon chain length, and has a latent heat of 200-250 kJ/kg. It can be used for heating or cooling, by using paraffin capsules, panels, or tubes.
- Salt hydrates are PCM that dehydrate and hydrate at various temperatures, depending on the salt type and composition, and have a latent heat of 250-500 kJ/kg. They can be used for heating or cooling, by using salt hydrate composites, pellets, or bricks.
!Thermo-chemical heat storage)
Thermo-chemical heat storage
Thermo-chemical heat storage is the most novel and complex type of TES, as it can store very large amounts of heat for very long periods of time, without any heat losses. The principle of thermo-chemical heat storage is to store heat by breaking or forming chemical bonds in a reversible reaction, such as hydration, dehydration, oxidation, or reduction. The material is called a thermo-chemical material (TCM). The heat can be released by reversing the reaction, i.e., forming or breaking the chemical bonds.
The main advantages and limitations of using thermo-chemical storage for TES are:
- Very high storage capacity and density, as the heat is stored in the form of chemical energy, which is much higher than thermal energy
- Very low heat losses, as the heat is stored in the form of chemical products, which are stable and inert
- Very high storage temperature, as the heat is released by exothermic reactions, which can reach up to 1000°C
- Very low efficiency and power, as the heat is released by slow and complex reactions, which require catalysts and reactors
- Very high cost and risk, as the materials and technologies are expensive and hazardous
Some examples of thermo-chemical storage systems and materials are:
- Metal hydrides are TCM that absorb and release hydrogen at various temperatures and pressures, depending on the metal type and composition, and have an enthalpy of 20-200 kJ/mol. They can be used for heating or cooling, by using metal hydride beds, reactors, or tanks.
- Sorption is a process that involves the adsorption or absorption of a gas or a liquid by a solid or a liquid, such as water by zeolites or salts. It can store heat at various temperatures and pressures, depending on the sorbent and sorbate type and concentration, and have an enthalpy of 50-500 kJ/kg. It can be used for heating or cooling, by using sorption chillers, heat pumps, or modules.
- Methanol synthesis is a reaction that converts carbon dioxide and hydrogen into methanol and water, and vice versa. It can store heat at temperatures of 200-300°C and pressures of 50-100 bar, and have an enthalpy of 90 kJ/mol. It can be used for power generation, by using methanol synthesis reactors, turbines, or fuel cells.
Conclusion
In this blog post, we have learned about the different types of TES, their advantages and disadvantages, and some examples of materials and technologies used for each type. We have also discussed the potential and challenges of TES for the future of energy systems.
TES is a promising technology that can help achieve a sustainable, clean-energy future, by enabling the integration of renewable energy sources, improving the energy efficiency of buildings, industries, and power plants, and reducing the greenhouse gas emissions, energy costs, and reliability and resilience issues of energy systems.
However, TES also faces many technical, economic, and social barriers, such as the lack of standardization, regulation, and awareness, the high initial investment and operational costs, and the safety and environmental risks. Therefore, more research and development, demonstration and deployment, and policy and market support are needed to overcome these challenges and promote the adoption and diffusion of TES.