Engineering:Thermal Battery

A thermal energy battery is a physical structure used for the purpose of storing and releasing thermal energy—see also thermal energy storage. Such a thermal battery (a.k.a. TBat) allows energy available at one time to be temporarily stored and then released at another time. The basic principles involved in a thermal battery occur at the atomic level of matter, with energy being added to or taken from either a solid mass or a liquid volume which causes the substance's temperature to change. Some thermal batteries also involve causing a substance to transition thermally through a phase transition which causes even more energy to be stored and released due to the delta enthalpy of fusion or delta enthalpy of vaporization.

History of thermal batteries

Thermal batteries are very common, and include such familiar items as a hot water bottle. Early examples of thermal batteries include stone and mud cook stoves, rocks placed in fires, and kilns. While stoves and kilns are ovens, they are also thermal storage systems that depend on heat being retained for an extended period of time.

Types of thermal batteries

Thermal batteries generally fall into 3 categories:

• Encapsulated thermal batteries
• Phase change thermal batteries
• Other thermal batteries

These 3 types of batteries are each unique in their form and application, although fundamentally all are for the storage and retrieval of thermal energy. They also differ in method and density of heat storage. A description of each type of thermal battery follows.

Phase change thermal battery

Phase change materials used for thermal storage are capable of storing and releasing significant thermal capacity at the temperature that they change phase. These materials are chosen based on specific applications because there is a wide range of temperatures that may be useful in different applications and a wide range of materials that change phase at different temperatures. These materials include salts and waxes that are specifically engineered for the applications they serve. In addition to manufactured materials, water is a phase change material. The latent heat of water is 334 joules/gram. The phase change of water occurs at 0°C (32°F).

Some applications use the thermal capacity of water or ice as cold storage; others use it as heat storage. It can serve either application; ice can be melted to store heat then refrozen to warm an environment which is below freezing (putting liquid water at 0°C in such an environment warms it much more than the same mass of ice at the same temperature, because the latent heat of freezing is extracted from it, which is why the phase change is relevant), or water can be frozen to "store cold" then melted to make an environment above freezing colder (and again, a given mass of ice at 0°C will provide more cooling than the same mass of water at the same temperature).

The advantage of using a phase change in this way is that a given mass of material can absorb a large quantity of energy without its temperature changing. Hence a thermal battery that uses a phase change can be made lighter, or more energy can be put into it without raising the internal temperature unacceptably.

Encapsulated thermal battery

An encapsulated thermal battery is physically similar to a phase change thermal battery in that it is a confined amount of physical material which is thermally heated or cooled to store or extract energy. However, in a non-phase change encapsulated thermal battery the temperature of the substance is changed without inducing a phase change. Since a phase change is not needed many more materials are available for use in an encapsulated thermal battery.

One of the key properties of an encapsulated thermal battery is its volumetric heat capacity (VHC), also termed volume-specific heat capacity. Typical substances used for these thermal batteries include water, concrete, and wet sand.

An example of an encapsulated thermal battery is a residential water heater with a storage tank. This thermal battery is usually slowly charged over a period of about 30–60 minutes for rapid use when needed (e.g., 10–15 minutes). Many utilities, understanding the "thermal battery" nature of water heaters, have begun using them to absorb excess renewable energy power when available for later use by the homeowner. According to the above cited article, "net savings to the electricity system as a whole could be $200 per year per heater – some of which may be passed on to its owner".

Other thermal batteries

There are some other items that have historically been termed "thermal batteries". In this group is the molten salt battery which is a device for generating electricity. Other examples include the heat packs that skiers use for keeping hands and feet warm (see hand warmer). These are a chemical battery which when activated (with air in this case) will produce heat. Other related chemical thermal batteries exist for producing cold (see instant cold pack) generally used for sport injuries.

The one common principle of these other thermal batteries is that the reaction involved is generally not reversible. Thus, these batteries are not used for storing and retrieving heat energy.

See also

• Thermal energy storage
• Seasonal thermal energy storage
• Ground-coupled heat exchanger
• Geothermal heat pump
• International Ground Source Heat Pump Association
• Steam accumulator