Molly Ayala
Thermal batteries are a type of electric battery that stores energy as heat and can be used to store clean energy. They are made of materials that can absorb and store heat well, such as graphite, crushed rock, and bricks. The activation of a thermal battery involves igniting heat pellets (pyrotechnic) in each cell, which then heat the cell components and melt the salt electrolyte. This activates the battery and allows it to supply power. Thermal batteries are known for their high energy density, stable operations, compact size, and applicability in renewable power or industrial settings. They are also used in military applications, such as missiles and bombs, due to their long shelf life, rapid activation, and reliable performance under extreme conditions.
| Characteristics | Values |
|---|---|
| How it works | Stores energy when charged by converting heat into chemical energy and produces electricity when discharged |
| Discovery | Thomas Johann Seebeck (1780–1831) discovered the thermoelectric effect in 1821 |
| Prototype | In 2014, researchers demonstrated a prototype system that uses copper electrodes and ammonia as the electrolyte |
| Performance | Performs without preparation in the most extreme environments and begins providing power almost immediately |
| Composition | Each cell consists of a cathode, an electrolyte, an anode, and a pyrotechnic thermal energy source |
| Stacked cells | Depending on the desired power density and volume, a thermal battery may consist of a single series stack of cells or two or more parallel stacks of series cells |
| Container | The cell stacks are placed in a hermetically sealed stainless steel container |
| Coolants | Deionized water, silicon-based oil, or mineral oil |
| Activation | Thermal batteries are activated when the heat pellets located in each cell are ignited by the heat train and an electrical pulse |
| Materials | Graphite, crushed rock, and bricks |
| Applications | Long shelf life, rapid activation, and reliable performance under extreme conditions; ideal for military and defense applications |
| Advantages | High energy density, stable operations, compact size, and applicability in renewable power or industrial settings |
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What You'll Learn
The thermoelectric effect
The Seebeck effect is the emergence of electromotive force (emf) that develops across two points of an electrically conducting material when there is a temperature difference between them. The emf is called the Seebeck emf (or thermo/thermal/thermoelectric emf). The ratio between the emf and temperature difference is the Seebeck coefficient. A thermocouple measures the difference in potential across a hot and cold end for two dissimilar materials. This potential difference is proportional to the temperature difference between the hot and cold ends. The Seebeck effect was first discovered in 1794 by Italian scientist Alessandro Volta and later rediscovered in 1821 by Russian-born, Baltic German physicist Thomas Johann Seebeck.
The Peltier effect is the presence of heating or cooling at an electrified junction of two different conductors. The effect is named after French physicist Jean Charles Athanase Peltier, who discovered it in 1834. When a current is made to flow through a junction between two conductors, heat may be generated or removed at the junction. The Peltier and Seebeck effects are different manifestations of the same physical process, sometimes referred to as the Peltier-Seebeck effect.
The Thomson effect is an extension of the Peltier-Seebeck model and is credited to Lord Kelvin. It states that the Seebeck coefficient varies with temperature.
In practice, thermoelectric effects are challenging to observe in a single homogeneous conducting material. This is because the overall EMFs from the increasing and decreasing temperature gradients tend to cancel each other out. Thermocouples, which involve two wires of different materials joined in a region of unknown temperature, are used to measure the locally shifted voltage.
Thermoelectric generators are similar to thermocouples but draw current from the generated voltage to extract power from heat differentials. They are optimized to use high-quality thermoelectric materials in a thermopile arrangement to maximize power extraction. While not very efficient, these generators have the advantage of not having any moving parts.
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Cathodes and anodes
The cathode and anode are the two electrodes found in a battery or an electrochemical cell, which facilitate the flow of electric charge. The cathode is the positive electrode, where reduction (the gain of electrons) occurs, while the anode is the negative electrode, where oxidation (loss of electrons) takes place. The anode is an oxidizing metal, such as zinc or lithium, which loses electrons, making it negatively charged. The cathode receives electrons from the anode. Both are submerged in an electrolyte solution, and electricity travels through the conductor from the negative to the positive parts of the battery.
During the charging process in a battery, electrons flow from the cathode to the anode, storing energy that can later be used to power devices. Cathode active materials (CAM) are typically composed of metal oxides. The most common cathode materials used in lithium-ion batteries include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4 or LFP), and lithium nickel manganese cobalt oxide (LiNiMnCoO2 or NMC). Anode active materials (AAM), on the other hand, are generally made from carbon-based materials like graphite, silicon, or a combination of both. Graphite is the most commonly used anode material due to its high electrical conductivity, low cost, and stable structure.
In thermal batteries, the activation of the battery consists of a chain of events. The battery is activated when the heat pellets (pyrotechnic) located in each cell are ignited by the heat train (centre-hole and side heat strips) and the burning is initiated by an electrical pulse to the squib. The burning of heat pellets makes it possible to heat the cell components and melt the salt electrolyte up to the battery operating temperature. When the prescribed current is applied to the squib terminals, the igniter starts and its flame causes the heat train to combust. As a result, the heat elements between cells give off heat. When the electrolyte melts from the heat of the elements, a voltaic reaction occurs and the battery supplies power through the terminals.
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Electrolytes
The movement of these ions creates an electrical potential difference called "voltage". When a battery is connected to an external electric load, the negatively charged electrons flow through the circuit and reach the positive terminal, causing a redox reaction by attracting positively charged ions (cations). This reaction provides power to the connected device.
In rechargeable batteries, electrons and ions can move in either direction through the circuit and electrolyte. When the electrons move from the cathode to the anode, they increase the chemical potential energy, thus charging the battery. When they move in the opposite direction, they convert this chemical potential energy to electricity in the circuit and discharge the battery.
The choice of electrolyte depends on the type of battery. For example, a lead-acid battery typically uses sulfuric acid to create the desired reaction, while zinc-air batteries rely on oxidizing zinc with oxygen. Potassium hydroxide is the electrolyte in standard household alkaline batteries, and lithium salt solutions such as lithium hexafluorophosphate (LiPF6) are commonly used in lithium-ion batteries.
Solid-state batteries use solid electrolytes, such as lithium metal oxides, instead of the liquid or gel polymer electrolytes found in conventional batteries. Solid electrolytes offer several advantages, including improved safety due to their non-flammable nature, a broader range of operating temperatures and voltages, and faster ion transfer, which can reduce charging times. Solid-state batteries also have higher energy densities than traditional lithium-ion batteries due to the use of lithium metal anodes, which have a higher charge capacity.
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Heat management
The passive battery thermal management (BTM) of Li-ion batteries is essential for maintaining peak efficiencies. Active BTM systems, while effective, require additional components and can draw electrical power from the battery. Phase change materials (PCMs) are also used in battery thermal management to address the drawbacks of solid-solid and solid-liquid PCMs, such as low latent heat and leakage issues, respectively. Innovative flexible PCM (FPCM) incorporates flexible polymers to improve performance.
Direct liquid cooling is a critical aspect of thermal management, and the coolant should have specific properties, including electrical insulation, high thermal conductivity, low viscosity, and high thermal capacity. Deionized water, silicon-based oil, and mineral oil are potential coolants for direct liquid cooling. Chen et al. found that direct liquid cooling with mineral oil resulted in the lowest temperature rise but also had the lowest flow rate, potentially impacting cooling performance.
Thermal batteries are designed to focus on storing and releasing heat rather than maintaining a narrow temperature range, as is the case with EV batteries. They are constructed with materials that effectively absorb and store heat, such as graphite, crushed rock, and bricks. Rondo Energy, for instance, develops thermal batteries using conductive bricks that can attain high temperatures for industrial applications.
Additionally, thermal energy storage systems have been employed for thousands of years, and modern thermal batteries offer unique advantages, including long shelf life, rapid activation, and reliable performance under extreme conditions. They are particularly useful for renewable energy sources like wind and solar power, providing a consistent power supply and helping to reduce emissions by utilising waste heat.
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Battery activation
A thermal battery is activated when the heat pellets (pyrotechnic) located in each cell are ignited by the heat train (centre-hole and side heat strips). This burning is initiated by an electrical pulse to the squib. The burning of the heat pellets makes it possible to heat the cell components and melt the salt electrolyte up to the battery operating temperature.
The activation of a thermal battery involves a chain of events. When the prescribed current is applied to the squib terminals, the igniter starts, and its flame causes the heat train to combust. As a result, the heat elements between cells give off heat. When the electrolyte melts from the heat of the elements, a voltaic reaction occurs, and the battery supplies power through the terminals.
The cathode acts as an electron acceptor during discharge, while the anode functions as the electron donor. The electrolyte is a solid-state material that conducts ions between the anode and cathode. The pyrotechnic heat source provides the thermal energy required to melt the electrolyte, activating the battery. This feature allows the battery to transition from dormant to active within milliseconds.
Thermal batteries are made with materials that can absorb and store heat well, such as graphite, crushed rock, and bricks. They are designed to store energy as heat and then release it when needed. This makes them ideal for applications that require a consistent power source, such as renewable energy sources like wind and solar power.
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Frequently asked questions
A thermal electric battery stores energy by converting heat into chemical energy and then produces electricity when discharged.
Thermal batteries are made with materials that can absorb and store heat well, such as graphite, crushed rock, and bricks. They focus on storing and releasing heat rather than maintaining a narrow temperature range to protect electrochemical reactions.
Each cell of a thermal battery consists of a cathode, an electrolyte, an anode, and a pyrotechnic thermal energy source.
Thermal batteries are used in renewable electricity, electric vehicles, and military applications. They are also used in industrial processes and combined heat and power due to their thermal stability and high heat capacity.
