Joule heating
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Electromagnetism |
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Joule heating (also known as resistive, resistance, or Ohmic heating) is the process by which the passage of an
Joule's first law (also just Joule's law), also known in countries of the former
Joule-heating or resistive-heating is used in multiple devices and industrial process. The part that converts electricity into heat is called a heating element.
Among the many practical uses are:
- An blackbody radiation).
- Electric fuses are used as a safety, breaking the circuit by melting if enough current flows to melt them.
- Electronic cigarettes vaporize propylene glycol and vegetable glycerine by Joule heating.
- Multiple heating devices use Joule heating, such as electric stoves, electric heaters, soldering irons, cartridge heaters.
- Some food processing equipment may make use of Joule heating: running current through food material (which behave as an electrical resistor) causes heat release inside the food.[2] The alternating electrical current coupled with the resistance of the food causes the generation of heat.[3] A higher resistance increases the heat generated. Ohmic heating allows for fast and uniform heating of food products, which maintains quality. Products with particulates heat up faster (compared to conventional heat processing) due to higher resistance.[4]
History
In 1841 and 1842, subsequent experiments showed that the amount of heat generated was proportional to the
Resistive heating was independently studied by
The
Microscopic description
Joule heating is caused by interactions between
A potential difference (voltage) between two points of a conductor creates an electric field that accelerates charge carriers in the direction of the electric field, giving them kinetic energy. When the charged particles collide with the quasi-particles in the conductor (i.e. the canonically quantized, ionic lattice oscillations in the harmonic approximation of a crystal), energy is being transferred from the electrons to the lattice (by the creation of further lattice oscillations). The oscillations of the ions are the origin of the radiation ("thermal energy") that one measures in a typical experiment.
Power loss and noise
Joule heating is referred to as ohmic heating or resistive heating because of its relationship to
Resistors create electrical noise, called Johnson–Nyquist noise. There is an intimate relationship between Johnson–Nyquist noise and Joule heating, explained by the fluctuation-dissipation theorem.
Formulas
Direct current
The most fundamental formula for Joule heating is the generalized power equation:
- is the power (energy per unit time) converted from electrical energy to thermal energy,
- is the current travelling through the resistor or other element,
- is the voltage drop across the element.
The explanation of this formula () is:[6]
Assuming the element behaves as a perfect resistor and that the power is completely converted into heat, the formula can be re-written by substituting Ohm's law, , into the generalized power equation:
Voltage can be increased in DC circuits by connecting batteries or solar panels in series.
Alternating current
When current varies, as it does in AC circuits,
where t is time and P is the instantaneous active power being converted from electrical energy to heat. Far more often, the average power is of more interest than the instantaneous power:
where "avg" denotes average (mean) over one or more cycles, and "rms" denotes root mean square.
These formulas are valid for an ideal resistor, with zero reactance. If the reactance is nonzero, the formulas are modified:
where is phase difference between current and voltage, means
(equal to 1/Z*).For more details in the reactive case, see AC power.
Differential form
Joule heating can also be calculated at a particular location in space. The differential form of the Joule heating equation gives the power per unit volume.
Here, is the current density, and is the electric field. For a material with a conductivity , and therefore
where is the
In the harmonic case, where all field quantities vary with the angular frequency as , complex valued
Electricity transmission
Overhead power lines transfer electrical energy from electricity producers to consumers. Those power lines have a nonzero resistance and therefore are subject to Joule heating, which causes transmission losses.
The split of power between transmission losses (Joule heating in transmission lines) and load (useful energy delivered to the consumer) can be approximated by a voltage divider. In order to minimize transmission losses, the resistance of the lines has to be as small as possible compared to the load (resistance of consumer appliances). Line resistance is minimized by the use of copper conductors, but the resistance and power supply specifications of consumer appliances are fixed.
Usually, a transformer is placed between the lines and consumption. When a high-voltage, low-intensity current in the primary circuit (before the transformer) is converted into a low-voltage, high-intensity current in the secondary circuit (after the transformer), the equivalent resistance of the secondary circuit becomes higher[7] and transmission losses are reduced in proportion.
During the
Applications
Food processing
Joule heating is a flash pasteurization (also called "high-temperature short-time" (HTST)) aseptic process that runs an alternating current of 50–60 Hz through food.[8] Heat is generated through the food's electrical resistance.[8][9][10][11] As the product heats, electrical conductivity increases linearly.[3] A higher electrical current frequency is best as it reduces oxidation and metallic contamination.[8] This heating method is best for foods that contain particulates suspended in a weak salt-containing medium due to their high resistance properties.[4][8]
Heat is generated rapidly and uniformly in the liquid matrix as well as in particulates, producing a higher quality sterile product that is suitable for aseptic processing.[11][12]
Electrical energy is linearly translated to thermal energy as
This method can also inactivate
There are different configurations for continuous ohmic heating systems, but in the most basic process,[11] a power supply or generator is needed to produce electrical current.[10] Electrodes, in direct contact with food, pass electric current through the matrix.[10] The distance between the electrodes can be adjusted to achieve the optimum electrical field strength.[10]
The generator creates the electrical current which flows to the first electrode and passes through the food product placed in the electrode gap.[10] The food product resists the flow of current causing internal heating.[11] The current continues to flow to the second electrode and back to the power source to close the circuit.[10] The insulator caps around the electrodes controls the environment within the system.[10]
The electrical field strength and the residence time are the key process parameters which affect heat generation.[11]
The ideal foods for ohmic heating are viscous with particulates.[11]
- Thick soups
- Sauces
- Stews
- Salsa
- Fruit in a syrup medium
- Milk
- Ice cream mix
- Egg
- Whey
- Heat sensitive liquids
- Soymilk
The efficiency by which electricity is converted to heat depends upon on salt, water, and fat content due to their
Food | Electrical Conductivity (S/m) | Temperature (°C) |
---|---|---|
Apple Juice | 0.239 | 20 |
Beef | 0.42 | 19 |
Beer | 0.143 | 22 |
Carrot | 0.041 | 19 |
Carrot Juice | 1.147 | 22 |
Chicken meat | 0.19 | 20 |
Coffee (black) | 0.182 | 22 |
Coffee (black with sugar) | 0.185 | 22 |
Coffee (with milk) | 0.357 | 22 |
Starch solution (5.5%) | ||
(a) with 0.2% salt | 0.34 | 19 |
(b) with 0.55% salt | 1.3 | 19 |
(c) with 2% salt | 4.3 | 19 |
Benefits of Ohmic heating include: uniform and rapid heating (>1°Cs−1), less cooking time, better
Microbial inactivation in ohmic heating is achieved by both thermal and non-thermal cellular damage from the electrical field.
Decreased processing times in ohmic heating maintains nutritional and sensory properties of foods.[9] Ohmic heating inactivates antinutritional factors like lipoxigenase (LOX), polyphenoloxidase (PPO), and pectinase due to the removal of active metallic groups in enzymes by the electrical field.[13] Similar to other heating methods, ohmic heating causes gelatinization of starches, melting of fats, and protein agglutination.[11] Water-soluble nutrients are maintained in the suspension liquid allowing for no loss of nutritional value if the liquid is consumed.[15]
Ohmic heating is limited by viscosity, electrical conductivity, and fouling deposits.[9][10][11] The density of particles within the suspension liquid can limit the degree of processing. A higher viscosity fluid will provide more resistance to heating, allowing the mixture to heat up quicker than low viscosity products.[11] A food product's electrical conductivity is a function of temperature, frequency, and product composition.[9][10][11] This may be increased by adding ionic compounds, or decreased by adding non-polar constituents.[9] Changes in electrical conductivity limit ohmic heating as it is difficult to model the thermal process when temperature increases in multi-component foods.[9][10]
The potential applications of ohmic heating range from cooking, thawing,
Applications | Advantages | Food Items |
---|---|---|
Sterilisation, heating liquid foods containing large particulates and heat sensitive liquids, aseptic processing | Attractive appearance, firmness properties, pasteurization of milk without protein denaturation | Cauliflower florets, soups, stews, fruit slices in syrups and sauces, ready to cook meals containing particulates, milk, juices, and fruit purees |
Ohmic cooking of solid foods | The cooking time could be reduced significantly. The centre temperature rises much faster than in conventional heating, improving the final sterility of the product, less power consumption and safer product | Hamburger patties, meat patties, minced beef, vegetable pieces, chicken, pork cuts |
Space food and military ration | Food reheating and waste sterilization. Less energy consumption for heating food to serving temperature, products in reusable pouches with long shelf life. Additive free foods with good keeping quality of 3 years. | Stew type foods |
Ohmic thawing | Thawing without increase in moisture content of the product | Shrimp blocks |
Inactivation of spores and enzymes | To improve food safety and enhance shelf life, increased stability and energy efficiency, Reduced time for inactivation of lipoxygenase and polyphenol oxidase, inactivation of enzymes without affecting flavor | Process fish cake, orange juice, juices |
Blanching and extraction | Enhanced moisture loss and increase in juice yield | Potato slices, vegetable purees extraction of sucrose from sugar beets, extraction of soy milk from soy beans |
Materials synthesis, recovery and processing
Flash joule heating (transient high-temperature electrothermal heating) has been used to synthesize allotropes of carbon, including graphene and diamond. Heating various solid carbon feedstocks (carbon black, coal, coffee grounds, etc.) to temperatures of ~3000 K for 10-150 milliseconds produces turbostratic graphene flakes.[17] FJH has also been used to recover rare-earth elements used in modern electronics from industrial wastes.[18][19] Beginning from a fluorinated carbon source, fluorinated activated carbon, fluorinated nanodiamond, concentric carbon (carbon shell around a nanodiamond core), and fluorinated flash graphene can be synthesized.[20][21]
Gallery
-
An incandescent light bulb's filament emitting light
-
thermal imageof a light bulb
-
Electric radiative space heater
-
Electric tabletop hotplate
-
Clothes iron used to remove wrinkles from clothes
-
Soldering iron, used to melt solder in electronic work
-
Hair dryer, produces hot air flow
Heating efficiency
Heat is not to be confused with internal energy or synonymously thermal energy. While intimately connected to heat, they are distinct physical quantities.
As a heating technology, Joule heating has a coefficient of performance of 1.0, meaning that every joule of electrical energy supplied produces one joule of heat. In contrast, a heat pump can have a coefficient of more than 1.0 since it moves additional thermal energy from the environment to the heated item.
The definition of the efficiency of a heating process requires defining the boundaries of the system to be considered. When heating a building, the overall efficiency is different when considering heating effect per unit of electric energy delivered on the customer's side of the meter, compared to the overall efficiency when also considering the losses in the power plant and transmission of power.
Hydraulic equivalent
In the energy balance of groundwater flow a hydraulic equivalent of Joule's law is used:[22]
where:
- = loss of hydraulic energy () due to friction of flow in -direction per unit of time (m/day), comparable to
- = flow velocity in -direction (m/day), comparable to
- = hydraulic conductivity of the soil (m/day), the hydraulic conductivity is inversely proportional to the hydraulic resistance which compares to
See also
- Dielectric heating – Heating using radio waves
- Heating element – Device that converts electricity into heat
- Induction heating – Process of heating an electrically conducting object by electromagnetic induction
- Joule's Second Law– Phenomenon of non-ideal fluids changing temperature while being forced through small spaces
- Molybdenum disilicide – chemical compound
- Nichrome – Family of alloys of mainly nickel and chromium
- Overheating (electricity) – Elevated temperature in an electric circuit
- Resistance wire – wire with high resistivity used to create a resistor
- Electric resistance welding – Welding by passing electric current through work pieces
- Thermal management (electronics) – Regulation of the temperature of electronic circuitry to prevent inefficiency or failure
- Tungsten – Chemical element, symbol W and atomic number 74
References
- ^ a b Джоуля — Ленца закон Archived 2014-12-30 at the Wayback Machine. Большая советская энциклопедия, 3-е изд., гл. ред. А. М. Прохоров. Москва: Советская энциклопедия, 1972. Т. 8 (A. M. Prokhorov; et al., eds. (1972). "Joule–Lenz law". Great Soviet Encyclopedia (in Russian). Vol. 8. Moscow: Soviet Encyclopedia.)
- ^ Ramaswamy, Raghupathy. "Ohmic Heating of Foods". Ohio State University. Archived from the original on 2013-04-08. Retrieved 2013-04-22.
- ^ ISBN 978-0-08-101907-8.
- ^ PMID 25328171.
- ^ a b "This Month Physics History: December 1840: Joule's abstract on converting mechanical power into heat". aps.org. American Physical society. Retrieved 16 September 2016.
- ^ Electric power systems: a conceptual introduction by Alexandra von Meier, p67, Google books link
- ^ "Transformer circuits". Retrieved 26 July 2017.
- ^ OCLC 960758611.
- ^ )
- ^ PMID 25328171.
- ^ ISBN 978-0-08-101907-8.
- ^ )
- ^ ISBN 978-1-4200-7109-2.
- ^ PMID 25328171.
- .
- ^ a b "Kinetics of Microbial Inactivation for Alternative Food Processing Technologies" (PDF). U.S. Food and Drug Administration. May 30, 2018.
- S2CID 210926149.
- ^ "Rare earth elements for smartphones can be extracted from coal waste". New Scientist.
- PMID 35138886.
- ^ Michael, Irving (June 22, 2021). "New method converts carbon into graphene or diamond in a flash". New Atlas. Retrieved 2021-06-22.
- S2CID 235471710.
- ISBN 978-0-7923-3651-8.