electrochemistry Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

, a thermogalvanic cell is a kind of

galvanic cell A galvanic cell or voltaic cell, named after the scientists Luigi Galvani and Alessandro Volta, respectively, is an electrochemical cell in which an electric current is generated from spontaneous Oxidation-Reduction reactions. A common apparatus ...

in which

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...

is employed to provide

electrical power Electric power is the rate at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt, one joule per second. Standard prefixes apply to watts as with other SI units: thousands, millions and billions of ...

directly. These cells are

electrochemical cells An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...

in which the two

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...

s are deliberately maintained at different temperatures. This temperature difference generates a

potential difference Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to mo ...

between the electrodes. The electrodes can be of identical composition and the

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

solution homogeneous. This is usually the case in these cells.Chum, HL; Osteryoung, RA (1981). “Review of thermally regenerative electrochemical systems. Volume 2”. Solar Energy Research Institute pp. 115–148. This is in contrast to galvanic cells in which electrodes and/or solutions of different composition provide the electromotive potential. As long as there is a difference in temperature between the electrodes a

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

will flow through the circuit. A thermogalvanic cell can be seen as analogous to a

concentration cell In battery technology, a concentration cell is a limited form of a galvanic cell that has two equivalent half-cells of the same composition differing only in concentrations. One can calculate the potential developed by such a cell using the Nern ...

but instead of running on differences in the concentration/pressure of the reactants they make use of differences in the "concentrations" of thermal energy.Quickenden, TI; Mua, Y (1995). “A review of power generation in aqueous thermogalvanic cells”. J Electrochem Soc 142 (11): 3985–94. The principal application of thermogalvanic cells is the production of electricity from low-temperature heat sources (

waste heat Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility ( ...

and solar heat). Their energetic efficiency is low, in the range of 0.1% to 1% for conversion of heat into electricity.

History

The use of heat to empower galvanic cells was first studied around 1880. However it was not until the decade of 1950 that more serious research was undertaken in this field.Agar, JN (1963). “Thermogalvanic cells”. Advances in electrochemistry and electrochemical engineering (Ed. Delahay, P, and Tobias, CW) Interscience, New York; vol. 3 pp. 31–121.

Working mechanism

Thermogalvanic cells are a kind of

heat engine In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state ...

. Ultimately the driving force behind them is the transport of

entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

from the high temperature source to the low temperature sink. Therefore, these cells work thanks to a thermal gradient established between different parts of the cell. Because the rate and

enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant p ...

of chemical reactions depend directly on the temperature, different temperatures at the electrodes imply different chemical equilibrium constants. This translates into unequal chemical equilibrium conditions on the hot side and on the cold side. The thermocell tries to approach an homogeneous equilibrium and, in doing so, produces a flow of chemical species and electrons. The electrons flow through the path of least resistance (the outer circuit) making it possible to extract power from the cell.

Types

Different thermogalvanic cells have been constructed attending to their uses and properties. Usually they are classified according to the electrolyte employed in each specific type of cell.

Aqueous electrolytes

In these cells the electrolyte between the electrodes is a water solution of some salt or hydrophylic compound. An essential property of these compounds is that they must be able to undergo

redox reaction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

s in order to shuttle electrons from one electrode to the other during the cell operation.

Non-aqueous electrolytes

The electrolyte is a solution of some other solvent different from water. Solvents like

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a lig ...

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscible wi ...

, dimethyl sulphoxide and

dimethyl formamide Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the major ...

have been successfully employed in thermogalvanic cells running on copper sulfate.

Molten salts

In this type of thermocell the electrolyte is some kind of salt with a relatively low melting point. Their use solves two problems. On one hand the temperature range of the cell is much larger. This is an advantage as these cells produce more power the larger the difference between the hot and cold sides. On the other hand, the liquid salt directly provides the anions and cations necessary for sustainment of a current through the cell. Therefore, no additional current-carrying compounds are necessary as the melted salt is the electrolyte itself.Kuzminskii, YV; Zasukha, VA; Kuzminskaya, GY (1994). “Thermoelectric effects in electrochemical systems. Nonconventional thermogalvanic cells”. J Power Sources 52: 231–42. Typical hot source temperatures are between 600–900 K, but can get as high as 1730 K. Cold sink temperatures are in the 400–500 K range.

Solid electrolytes

Thermocells in which the electrolyte connecting the electrodes is an ionic material have been considered and constructed too. The temperature range is also elevated as compared to liquid electrolytes. Studied systems fall in the 400–900 K. Some solid ionic materials that have been employed to construct thermogalvanic cells are AgI, PbCl₂ and PbBr₂.

Uses

Given the advantages provided by the working mechanism of thermogalvanic cells, their main application is electricity production under conditions where there is an excess of heat available. In particular thermogalvanic cells are being used to produce electricity in the following areas.

Solar energy

The heat collected from this process generates steam, which can be used in a conventional steam turbine system to make electricity. In contrast to the low-temperature solar thermal systems that are used for air or water heating in domestic or commercial buildings, these solar thermal electricity plants operate at high temperatures, requiring both concentrated sunlight and a large collection area, making the Moroccan desert an ideal location. This is an alternative approach to the more widely used “photovoltaic” technology for producing electricity from sunlight. In a photovoltaic system, the sunlight is absorbed in the photovoltaic device (commonly called a solar cell) and energy is passed to electrons in the material, converting the solar energy directly into electricity. Sometimes, solar thermal electricity and photovoltaics are portrayed as competing technologies and, while this may be true when deciding on the way forward for a specific site, in general they are complementary, using solar energy as extensively as possible.

Thermal generators

Waste heat sources

Thermogalvanic cells can be used to extract a useful quantity of energy from waste heat sources even when the temperature gradient is less than 100C (sometimes only a few tens of degrees). This is often the case in many industrial areas.

References

{{Galvanic cells Electrochemistry