Heat storage, also known as thermal energy storage (TES), involves capturing and retaining thermal energy for later use. This technology functions much like a thermal battery, allowing surplus heat or cold to be saved for hours, days, or even months. It provides a flexible way to manage energy supply and demand, reducing reliance on conventional energy sources and improving overall energy efficiency.
Understanding Heat Storage Mechanisms
Heat can be stored through several fundamental mechanisms. Sensible heat storage is the most straightforward method, involving the increase or decrease of a material’s temperature without changing its physical state. The amount of energy stored depends on the material’s mass, its specific heat capacity, and the temperature change it undergoes, following the equation Q=mcΔT, where Q is the stored heat, m is the mass, c is the specific heat capacity, and ΔT is the temperature change. This approach is widely used due to its simplicity and cost-effectiveness, although it has a lower energy storage density compared to other methods.
Latent heat storage involves storing heat during a material’s phase change, such as melting or freezing, at a nearly constant temperature. This “hidden” heat, known as latent heat, is absorbed or released as the material transitions from one state to another, like ice melting into water at 0°C. Phase Change Materials (PCMs) are designed for this purpose, offering a much higher energy storage density than sensible heat storage, meaning more energy can be stored in a smaller volume.
Thermochemical heat storage involves storing energy through reversible chemical reactions. During charging, an endothermic (heat-absorbing) reaction occurs, converting reactants into products that can be stored separately. When energy is needed, these products are recombined in an exothermic (heat-releasing) reaction. This method boasts high energy density and the potential for long-term, loss-free storage, as the energy is held in chemical bonds rather than as heat directly, making it suitable for seasonal storage applications.
Materials Used in Heat Storage
A variety of materials are employed for heat storage. For sensible heat storage, common materials include water, rocks, and concrete. Water is effective due to its high specific heat capacity and availability, capable of storing up to 250 MJ/m³ for a 60°C temperature change. For higher temperatures, molten salts, such as a mixture of 60% sodium nitrate and 40% potassium nitrate, are used, offering higher storage capacities and operating up to 600°C. Solid materials like alumina, silicon carbide, graphite, cast iron, and refractory bricks are also used for high-temperature sensible heat storage in industrial applications, reaching up to 1500°C.
Phase Change Materials (PCMs) are used for latent heat storage, absorbing and releasing energy during their phase transitions. These materials are categorized as organic, inorganic, and eutectic compounds. Organic PCMs include paraffin waxes and fatty acids, known for chemical stability and congruent melting, but may have lower latent heat values. Inorganic PCMs, such as salt hydrates, offer high latent heat values and are non-flammable and cost-effective, but may have issues such as supercooling, phase segregation, and corrosiveness. Examples include sodium sulfate decahydrate for solar energy systems and dodecanoic acid (lauric acid) for domestic hot water applications.
Thermochemical heat storage utilizes materials that undergo reversible chemical reactions. Examples include salt hydrates and boric acid (H3BO3). These materials must be affordable, non-toxic, and capable of stable, reversible reactions within a defined temperature range, often up to 200°C for applications like district heating or food processing. Research also focuses on composite materials that combine salts with porous structures to enhance heat and mass transfer properties.
Practical Applications
Heat storage technologies are used in diverse settings, from individual homes to large-scale industrial complexes and entire energy grids. In residential applications, heat storage is used in solar water heaters, where thermal energy from solar collectors is stored in insulated water tanks for later use. Passive solar building designs also incorporate heat storage by using materials like concrete or masonry to absorb solar heat during the day and release it slowly at night, to regulate indoor temperatures and reduce heating demands. Some homes use large underground water tanks to store summer heat from solar panels for winter space heating.
Industrial uses of heat storage improve energy efficiency and recover waste heat. Industries like manufacturing, chemical production, and food processing generate waste heat, which can be captured and stored to preheat processes or provide general heating, reducing fossil fuel consumption. For instance, regenerative accumulators in glass or steel industries store energy from furnace fumes in refractory solids, reaching 1300°C, to preheat combustion air for subsequent cycles, improving thermal efficiency. Thermal energy storage solutions are being developed to provide high-temperature heat (over 1000°C) for heavy industries such as steel, cement, and iron, utilizing materials like graphite or refractory bricks heated by renewable electricity.
On a large scale, heat storage is used in modern energy systems, particularly in Concentrated Solar Power (CSP) plants and district heating networks. CSP plants use molten salts to store solar heat at high temperatures, allowing electricity generation after sunset or during cloudy periods. For example, the Solana Generating Station in the U.S. can store six hours of generating capacity in molten salt, and the Cerro Dominador plant has 17.5 hours of storage. District heating systems employ large underground hot water tanks or pit thermal energy storage (PTES) to store heat centrally for multiple buildings, enabling seasonal storage. These large-scale systems can store heat for months, providing flexibility and supporting the transition to fully renewable energy supplies.
The Role of Heat Storage in a Sustainable Future
Heat storage is important for a sustainable energy future by addressing challenges with renewable energy and enhancing overall energy efficiency. Renewable sources like solar and wind power are intermittent. Heat storage systems help overcome this by capturing surplus energy generated during peak production times and making it available when demand is high or when renewable sources are not producing, ensuring a more stable and reliable energy supply. This capability reduces the need for fossil fuel-based backup power plants and helps integrate a higher percentage of renewable energy into the grid.
Beyond renewable energy integration, heat storage improves energy efficiency by reducing waste and optimizing consumption. Many industrial processes and power generation facilities produce waste heat that can be recovered and reused, lowering energy consumption and greenhouse gas emissions. In buildings, heat storage allows for shifting energy demand away from peak hours, reducing strain on the electrical grid and potentially lowering energy costs for consumers. This “peak shaving” ability improves the overall efficiency of energy systems by minimizing energy losses and maximizing the utility of generated heat.
Heat storage technologies are becoming cost-effective and are a viable alternative to battery storage for certain applications, especially for long-duration thermal energy management. As research continues to improve storage materials and system designs, heat storage is expected to become a key component of global clean energy infrastructure. By providing flexible energy management and enabling greater renewable energy penetration, heat storage supports efforts to decarbonize heating and cooling sectors and move towards a more resilient and environmentally responsible energy landscape.