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Glossary of Thermal Terms

 

Ferrotec's Thermoelectric Technical Reference Guide is a comprehensive technical explanation of thermoelectrics and thermoelectric technology.

13.0 Glossary of Thermoelectric and Thermal Terms

AMBIENT TEMPERATURE: Temperature of the air or environment surrounding a thermoelectric cooling system; sometimes called room temperature.

ASPECT RATIO: The numerical ratio of the length (height) to cross-sectional area of a thermoelectric element. An element’s L/A aspect ratio is inversely proportional to its optimum current.

BISMUTH-ANTIMONY: A thermoelectric semiconductor material that exhibits optimum performance characteristics at relatively low temperatures.

BISMUTH TELLURIDE: A thermoelectric semiconductor material that exhibits optimum performance in a "room temperature" range. An alloy of bismuth telluride most often is used for thermoelectric cooling applications.

BTU: British Thermal Unit: The amount of thermal energy required to raise one pound of water by one degree Celsius at a standard temperature of 15°C.

CALORIMETER: A scientific apparatus used to measure the evolution or absorption of heat. Thermoelectric modules, when used in a calorimeter, may exhibit much higher sensitivity than conventional thermopiles.

CASCADE MODULE (MULTI-STAGE MODULE): A thermoelectric module configuration whereby one module is stacked on top of another so as to be thermally in series. This arrangement makes it possible to reach lower temperatures than can be achieved with a single-stage module.

CFM: Cubic Feet per Minute: The volgenerallyumetric flow rate of a gas, typically air, expressed in the English system of units. For thermoelectric applications, this refers to the amount of air passing through the fins of a forced convection heat sink.

CLOSED-LOOP TEMPERATURE CONTROLLER: A temperature controlling device having some type of temperature sensor (thermocouple, thermistor, RTD, etc.) that will transmit or "feed back" temperature data to the controller. Based on the returned information, the controller will automatically adjust its output to maintain the desired temperature.

COEFFICIENT OF PERFORMANCE (COP): A measure of the efficiency of a thermoelectric module, device or system. Mathematically, COP is the total heat transferred through the thermoelectric device divided by the electric input power. COP sometimes is stated as COPR (Coefficient of Performance as a Refrigerator) or as COPH (Coefficient of Performance as a Heater).

COLD SIDE OF A THERMOELECTRIC MODULE: The side of a module that normally is placed in contact with the object being cooled. When the positive and negative module leads are connected to the respective positive and negative terminals of a DC power source, heat will be absorbed by the module's cold side. Typically, the leads of a TE module are attached to the hot side.

CONDUCTION(THERMAL): The transfer of heat within a material caused by a temperature difference through the material. The actual material may be either a solid, liquid or gas (or a combination) where heat will flow by means of direct contact from a high temperature region to a lower temperature region.

CONVECTION (THERMAL): The transfer of heat by means of air (gas) movement over a surface. Convection actually is a combined heat transfer process that involves elements of conduction, mixing action, and energy storage.

COUPLE: A pair of thermoelectric elements consisting of one N-type and one P-type connected electrically in series and thermally in parallel. Because the input voltage to a single couple is quite low, a number of couples normally are joined together to form a "module." 

DEGREES KELVIN: Absolute temperature scale where absolute zero (0K) represents the point where all molecular kinetic energy of a mass is zero. When calculating the temperature dependent properties of semiconductor materials, temperature values must be expressed in degrees Kelvin. On the Celsius scale, 0°C equals 273.15K; in respect to quantity, one Kelvin degree equals one Celsius degree. Note that the (°) symbol normally is not used when denoting degrees Kelvin.

DELTA-T: The temperature difference between the cold and hot sides of a thermoelectric module. Delta T may also be expressed as "DT" or "DT." 

DENSITY: The mass of a material per unit volume; often expressed as pounds per cubic foot or grams per cubic centimeter. 

DICE: A general term for blocks of the thermoelectric semiconductor material or "elements" prepared for use in a thermoelectric module. 

DIE: An individual block of thermoelectric semiconductor material used in the fabrication of a module. A die may also be called an "element," "leg," or "thermoelement." 

EFFICIENCY: For thermoelectric coolers, mathematical efficiency is the heat pumped by a module divided by the electrical input power; for thermoelectric generators, efficiency is the electrical output power from the module divided by the heat input. To convert to percent, multiply by 100. See definition of Coefficient of Performance.

ELEMENT: An individual block of thermoelectric semiconductor material. See definition of DIE.

EMISSIVITY: The ratio of the energy emitted by a given object to the energy emitted by a black-body at the same temperature. Emissivity is dependent upon an object’s material and surface finish.

ENERGY: Energy is the physical quantity which, in the context of thermoelectrics, generally is used to express a unit of heat or electricity. Energy may be stated in British Thermal Units (BTU) or watt-hours. It is important to note the difference between energy and power. Power is the rate at which energy is being used, and power may be stated in BTU/hour or watts. The relationship between power and energy is Power = Energy / Time.

FIGURE-OF-MERIT (Z): A measure of the overall performance of a thermoelectric device or material. Material having the highest figure-of-merit also has the highest thermoelectric performance.

FORCED CONVECTION HEAT SINK: A heat sink that incorporates a fan or blower to actively move air over the heat sink’s fins. Greatly improved cooling performance may be realized with a forced convection system when compared to a natural convection heat sink.

HEAT LEAK: The amount of energy gained or lost by an object being thermoelectrically controlled due to heat transfer to or from external media. Heat transfer may occur due to conduction, convection, and/or radiation.

HEAT LOAD: The quantity of heat presented to a thermoelectric device that must be absorbed by the device’s cold side. The term heat load, when used by itself, tends to be somewhat ambiguous and it is preferable to be more specific. Terms such as active heat load, passive heat load or total heat load are more descriptive and less uncertain as to meaning.

HEAT OF FUSION: More correctly called Latent Heat of Vaporization. The amount of heat energy required to change a given mass of a substance from a liquid to a gas without changing the temperature of the substance. To change water into stream, for example, requires a heat input of about 971 BTU/pound or 540 calories/gram.

HEAT PUMP: A general term describing a thermoelectric cooling device, often being used as a synonym for a thermoelectric module. In somewhat less common usage, the term heat pump has been applied to a thermoelectric device operating in the heating mode.

HEAT PUMPING CAPACITY: The amount of heat that a thermoelectric device is capable of pumping at a given set of operating parameters. Frequently, this term will be used interchangeably with the expression maximum heat pumping capacity. The two terms are not strictly synonymous, however, because maximum heat pumping capacity specifically defines the maximum amount of heat that a module will pump at the maximum rated input current and at a zero temperature differential.

HEAT SINK: A body that is in contact with a hotter object and that expedites the removal of heat from the object. Heat sinks typically are intermediate stages in the heat removal process whereby heat flows into a heat sink and then is transferred to an external medium. Common heat sinks include natural (free) convection, forced convection and fluid cooled.

HEAT TRANSFER COEFFICIENT: A numerical value that describes the degree of coupling that exists between an object and a cooling or heating fluid. The heat transfer coefficient actually is an extremely complex value that encompasses many physical factors.

HEIGHT TOLERANCE (MODULE): The maximum variation in height or thickness of a thermoelectric module referenced to its nominal specified dimension. Most Ferrotec modules are available in two tolerance ranges of +/-0.03mm (+/-0.001") and +/-0.3mm (+/-0.01"). When more than one module will be installed between a given pair of mounting surfaces, the maximum height variation of all modules should not exceed 0.06mm (0.002").

HOT SIDE OF A THERMOELECTRIC MODULE: The face of a thermoelectric module that usually is placed in contact with the heat sink. When the positive and negative module leads are connected to the respective positive and negative terminals of a DC power source, heat will be rejected by the module’s hot side. Normally, the wire leads are attached to the hot side ceramic substrate.

INTERSTAGE TEMPERATURE: The temperature between specific stages or levels of a multi-stage or cascade module.

JOULE HEATING: Heat produced by the passage of an electrical current through a conductor or material due to the internal resistance.

KINEMATIC VISCOSITY: The ratio of a fluid’s viscosity to its density; typically units are centimeters squared per second and feet squared per second.

LATENT HEAT: Thermal energy required to cause a change of state of a substance such as changing water into ice or water into steam.

LEAD TELLURIDE: A thermoelectric semiconductor that exhibits its optimum performance within a temperature range of 250-450°C. Lead telluride is used most often for thermoelectric power generation applications.

LIQUID COOLING: A heat sink method involving the use of water or other fluids to carry away unwanted heat. When comparing alternative heat-sinking methods, liquid cooled heat sinks normally provide the highest thermal performance per unit volume.

MASS FLOW RATE: The weight of a fluid flowing per unit of time past a given cross-sectional area. Typical units include pounds per hour-square foot and grams per second-square centimeter.

MAXIMUM TEMPERATURE DIFFERENTIAL (MAXIMUM DT): The largest difference that can be obtained between the hot and cold faces of a thermoelectric module when heat applied to the cold face is effectively zero. DTmax or Dmax is one of the significant thermoelectric module/device specifications.

MAXIMUM HEAT PUMPING CAPACITY (MAXIMUM Qc): The maximum quantity of heat that can be absorbed at the cold face of a thermoelectric module when the temperature differential between the cold and hot module faces is zero and when the module is being operated at its rated optimum current. Qmax is one of the significant thermoelectric module/device specifications.

MODULE: A thermoelectric cooling component or device fabricated with multiple thermoelectric couples that are connected thermally in parallel and electrically in series.

MULTI-STAGE MODULE (CASCADE MODULE): A thermoelectric module configuration whereby one module is mechanically stacked on top of another so as to be thermally in series. This arrangement makes it possible to reach lower temperatures than can be achieved with a single-stage module.

NATURAL CONVECTION HEAT SINK: A heat sink from which heat is transferred to the surrounding air by means of natural air currents within the environment. No external fan, blower or other appliance is used to facilitate air movement around the heat sink.

N-TYPE MATERIAL: Semiconductor material that is doped so as to have an excess of electrons.

OPTIMUM CURRENT: The specific level of electrical current that will produce the greatest heat absorption by the cold side of a thermoelectric module. At the optimum current, a thermoelectric module will be capable of pumping the maximum quantity of heat; maximum temperature differential (DTmax) typically occurs at a somewhat lower current level.

PELTIER EFFECT: The phenomenon whereby the passage of an electrical current through a junction consisting of two dissimilar metals results in a cooling effect; when the direction of current flow is reversed heating will occur.

PHASE CHANGE: The change of a substance from a liquid to solid, liquid to gas, etc. A phase change occurs, for example, when water freezes and turns into ice. See Heat of Fusion and Heat of Vaporization.

POWER SUPPLY: Any source of DC electrical power that may be used to operate a thermoelectric device.

P-TYPE MATERIAL: Semiconductor material that is doped so as to have a deficiency of electrons.

RADIATION (THERMAL): The transfer of heat energy by electromagnetic waves as a result of a temperature difference between two bodies. In thermoelectric cooling applications, radiation losses are quite small and usually have to be considered only for multi-stage coolers operating near a DTmax condition.

RESISTIVITY (ELECTRICAL): Resistivity is a bulk or inherent property of a material that is unrelated to the physical dimensions of the material. Electrical resistance, on the other hand, is an absolute value dependent upon the cross-sectional area (A) and Length (L) of the material. The relationship between Resistivity (r) and Resistance (R) is: r = (A/L) (R)

SEEBECK EFFECT: The phenomenon whereby an electrical current will flow in a closed circuit made up of two dissimilar metals when the junctions of the metals are maintained at two different temperatures. A common thermocouple used for temperature measurement utilizes this principle.

SI: An abbreviation for System International, the international standard metric system of units.

SILICON-GERMANIUM: A high temperature thermoelectric semiconductor material that exhibits its optimum performance within a temperature range of 500-1000°C. Silicon-Germanium material most often is used for special thermoelectric power generation applications that utilize a radioisotope/nuclear heat source

SINGLE-STAGE MODULE: The most common type of thermoelectric cooling module using a single layer of thermoelectric couples connected electrically in series and thermally in parallel. Single-stage modules will produce a maximum temperature differential of approximately 70°C under a no-load condition.

SPECIFIC GRAVITY: The ratio of the mass of any material to the mass of an equal volume of water at a temperature of 4°C.

SPECIFIC HEAT: The amount of thermal energy required to raise the temperature of a given substance by one degree compared to the energy required to raise the temperature of an equal mass of water by one degree. The specific heat of water is 1.000.

SUBSTRATE: A plate or sheet of thermally conductive and electrically insulated material on which a thermoelectric module is fabricated. A typical module has two individual substrates each having a metalized pattern to conduct electric current. Thermoelectric elements are sandwiched between the two substrates to form a completed module. Most substrates used in thermoelectric coolers are made of alumina ceramic although berylia ceramic and other materials may be used in special circumstances.

THERMAL COEFFICIENT OF EXPANSION: A measure of the dimensional change of a material due to a change in temperature. Common measurement units include centimeter per centimeter per degree Celsius and inch per inch per degree Fahrenheit.

THERMAL CONDUCTANCE: The amount of heat a given object will transmit per unit of temperature. Thermal conductance is independent of the physical dimensions, i.e., cross-sectional area and length of the object. Typical units include watts per degree Celsius and BTU per hour per degree Fahrenheit.

THERMAL CONDUCTIVITY: The amount of heat a material will transmit per unit of temperature based on the material’s cross-sectional area and thickness.

THERMAL GREASE: A grease-like material used to enhance heat transfer between two surfaces by filling in the microscopic voids caused by surface roughness. Most thermal greases, also known as Transistor Heat Sink Compound or Thermal Joint Compound, are made from silicone grease loaded with zinc oxide. Non-silicone based compounds are also available which in most cases are superior but more expensive than silicone-based alternatives.

THERMAL RESISTANCE (HEAT SINK): A measure of a heat sink’s performance based on the temperature rise per unit of applied heat. The best heat sinks have the lowest thermal resistance.

THERMOELECTRIC DEVICE: A general and broad name for any thermoelectric apparatus. The term Thermoelectric Device has recently been modified to exclude thermoelectric modules in favor of thermoelectric assemblies.

THERMOELECTRIC GENERATOR: A device that directly converts energy into electrical energy based on the Seebeck Effect. Bismuth telluride-based thermoelectric generators have very low efficiencies (generally not exceeding two or three percent) but may provide useful electrical power in certain applications.

THERMOELECTRIC HEAT PUMP: Another name for a thermoelectric module or thermoelectric cooler. The term Heat Pump has been used by some specifically to denote the use of a thermoelectric module in the heating mode, but this usage is uncommon.

THERMOELEMENT: Another name for a thermoelectric element or die.

THERMOPILE: When a thermoelectric module is used in a calorimeter application it is frequently called a thermopile. Some have used the word thermopile as a synonym for thermoelectric module regardless of application, but such use is unusual.

THOMSON EFFECT: The phenomena whereby a reversible evolution or absorption of heat occurs at opposite ends of a conductor having a thermal gradient when an electrical current passes through the conductor.

VISCOSITY: A fluid property related to the interaction between fluid molecules that determines the fluids resistance to sheering forces and flow.

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