Thermal battery

Thermal Battery

Small Residential Thermal Battery example. This is an unencapsulated thermal battery for storing thermal energy formed by burying pipes in an area of earth -- commonly referred to as a GHEX or Geothermal Heat Exchanger. Photo courtesy IGSHPA.
Type	Energy
Working principle	Thermodynamics
Invented	1940's Robert C. Webber American inventor built first heat pump by accident experimenting with deep freezing equipment.
First production	1970's

A Thermal Energy battery is 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.

Thermal batteries come in two basic forms - encapsulated and un-encapsulated Ground Heat Exchanger (GHEX).

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

Thermal Batteries generally fall into 4 categories:

These 4 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.

A ground heat exchanger (GHEX) is an area of the earth that is utilized as an annual cycle thermal battery. These thermal batteries are un-encapsulated areas of the earth into which pipes have been placed in order to transfer thermal energy. Energy is added to the GHEX by running a higher temperature fluid through the pipes and thus raising the temperature of the local earth. Energy can also be taken from the GHEX by running a lower temperature fluid through those same pipes.

GHEX thermal batteries are implemented in two forms. The picture above depicts what is known as a "horizontal" GHEX where trenching is used to place an amount of pipe in a closed loop in the ground. GHEX's are also formed by drilling boreholes into the ground, either vertically or horizontally, and then the pipes are inserted in the form of a closed-loop with a "u-bend" fitting on the far end of the loop. These drilled GHEX thermal batteries are also sometimes called "borehole thermal energy storage systems".