Definitions

Electric_heating

Electric heating

Electric heating is any process in which electrical energy is converted to heat. Common applications include heating of buildings, cooking, and industrial processes.

An electric heater is an electrical appliance that converts electrical energy into heat. The heating element inside every electric heater is simply an electrical resistor, and works on the principle of Joule heating: an electric current through a resistor converts electrical energy into heat energy.

Alternatively, a heat pump uses an electric motor to drive a refrigeration cycle, drawing heat from a source such as ground water or outside air and directing it into the space to be warmed. Such systems can deliver two or three units of heating energy for every unit of purchased energy.

Space heating

Although they all use the same physical principle to generate heat, electric heaters differ in the way they deliver that heat to the environment. Several types are described in the sections below.

Radiative heaters or "space heaters"

Radiative heaters contain a heating element that reaches a high temperature. The element is usually packaged inside a glass envelope resembling a light bulb and with a reflector to direct the energy output away from the body of the heater. The element emits infrared radiation that travels through air or space until it hits an absorbing surface, where it is partially converted to heat and partially reflected. This heat directly warms people and objects in the room, rather than warming the air. This style of heater is particularly useful in areas which unheated air flows through. They are also ideal for basements and garages where spot heating is desired. More generally, they are an excellent choice for task-specific heating.

They operate silently. Radiant heaters present the greatest potential danger to ignite nearby furnishings due to the focused intensity of their output and lack of overheat protection.

Convection heaters

In a convection heater, the heating element heats the air next to it by conduction. Hot air is less dense than cool air, so it rises due to buoyancy, allowing more cool air to flow in to take its place. This sets up a constant current of hot air that leaves the appliance through vent holes and heats up the surrounding space. They are ideally suited for heating a closed space. They operate silently and have a lower risk of ignition hazard in the event that they make unintended contact with furnishings compared to radiant electric heaters. This is a good choice for long periods of time or if left unattended. They are very safe heaters and there is a very low chance of getting burned.

In the United Kingdom, these appliances are sometimes called electric fires, because they were originally used to replace open fires.

Fan heaters or "forced convection heaters"

A fan heater is a variety of convection heater that includes an electric fan to speed up the airflow. This reduces the thermal resistance between the heating element and the surroundings, allowing heat to be transferred more quickly.

They operate with considerable noise caused by the fan. They have a moderate risk of ignition hazard in the event that they make unintended contact with furnishings. This type of heater is a good choice for quick heating of enclosed spaces.

Storage heating

A storage heating system takes advantage of cheaper electricity prices, sold during low demand periods such as overnight. In the United Kingdom, this is branded as Economy 7. The storage heater stores heat in clay bricks, then releases it during the day when required.

Domestic electrical underfloor heating

These systems are called radiant heating systems, regardless of whether they include a heat exchanger (also called a radiator) or are electrically powered.

When a home radiant heat system is turned on, current flows through a conductive heating material. For high-voltage radiant heat systems, line voltage (110 V or 230 V) current flows through the heating cable. For low-voltage systems, the line voltage is converted to low voltage (8 to 30 V) in the control unit (which contains a step-down transformer) and this low voltage is then applied to the heating element.

The heated material then heats the flooring until it reaches the right temperature set by the floor thermostat. The flooring then heats the adjacent air, which circulates, heating other objects in the room (tables, chairs, people) by convection. As it rises, the heated air will heat the room and all its contents up to the ceiling. This form of heating gives the most consistent room temperature from floor to ceiling compared to any other heating system.

Heat pumps

A heat pump uses an electrically-driven compressor to operate a refrigeration cycle that extracts heat energy from the outdoor air or from ground water, and upgrades its temperature to a level high enough to use for space heating. The working fluid boils at a low temperature, absorbing heat in an outdoor heat exchanger, then the resulting vapor is compressed and condenses to liquid form in a condensor inside the building. Heat from the condensor is absorbed by the air in the building (and sometimes also used for domestic hot water). In the summer months the cycle can be reversed to provide air conditioning. Heat pumps may obtain low-grade heat from the outdoor air in mild climates; in areas with average winter temperatures well below freezing, ground-source heat pumps tap geothermal energy from groundwater.

Water heating

Immersion heater

Water heating by electricity is usually done by an immersion heater. This consists of a metal tube containing an insulated electric resistance heater. Domestic immersion heaters (usually rated at 3 kilowatts in the UK) run on the normal domestic electricity supply. Industrial immersion heaters (such as those used in electric steam boilers) may be rated at 100 kilowatts, or more, and run on a three-phase supply.

Electrode heater

With an electrode heater, there is no wire-wound resistance and the liquid itself acts as the resistance. This has potential hazards so the regulations governing electrode heaters are strict .

Environmental and efficiency aspects

The efficiency of any system depends on the definition of the boundaries of the system. For an electrical energy customer the efficiency of electric space heating can be 100% because all purchased energy is converted to building heat. However, if the power plant supplying electricity is included, the overall efficiency drops. For example, a fossil-fuelled power plant may only deliver 4 units of electrical energy for every 10 units of fuel energy released. Even with a 100% efficient electric heater, the amount of fuel needed for a given amount of heat is more than if the fuel was burned in a furnace or boiler at the building being heated. If the same fuel could be used for space heating by a consumer, it would be more efficient overall to burn the fuel at the end user's building. Not all fuels are suitable for building heating; for example, emissions controls required for coal combustion are too expensive for household-scale furnaces.

In Sweden the use of direct electric heating has been restricted since the 1980s for this reason, and there are plans to phase it out entirely - see Oil phase-out in Sweden - while Denmark has banned the installation of electric space heating in new buildings for similar reasons. In the case of new buildings, low-energy building techniques can be used which can virtually eliminate the need for heating, such as those built to the Passive House standard.

In order to provide heat more efficiently, an electrically driven heat pump can raise the indoor temperature by extracting heat from (e.g. cooling) the ground, the outside air, or waste streams such as exhaust air in order to use it as a heat source. This can cut the electricity consumption to as little as 20% of that used by resistive heating and thus reduce the environmental impact.

Electrical space heating can still be economic where electricity supplies are low-cost. Where the primary source of electrical energy is hydroelectric, nuclear, wind, or other carbon-free source, it may not be practical to exploit that resource directly in heating applications but grid electricity can be conveniently used. Electric space heating is useful in places where air-handling is difficult, such as in laboratories.

Economic aspects

The operation of electric resistance heaters to heat an area for a long period of time is generally considered to be costly. However intermittent or partial day use can be more cost efficient than whole building heating since there savings due to superior zonal control.

Example: A lunch room in an office setting has limited hours of operation. During low use periods a "monitor" level of heat (50 °F/10 °C) is provided by the central heating system. Peak use times between the hours of 11:00–14:00 are heated to "comfort levels" (70 °F/21 °C). Significant savings can be realized in overall energy consumption since infrared radiation losses through thermal conductivity are not as large with a smaller temperature gradient both between this space and unheated outside air as well as between the refrigerator and the (now cooler) lunch room.

Industrial electric heating

See also Induction furnace, Electric arc furnace

Electric heating is widely used in industry.

Advantages of electric heating methods over other forms include precision control of temperature and distribution of heat energy, combustion not used to develop heat, and the ability to attain temperatures not readily achievable with chemical combustion. Electric heat can be accurately applied at the precise point needed in a process, at high concentration of power per unit area or volume. Electric heating apparatus can be built in any required size and can be located anywhere within a plant. Electric heating processes are generally clean, quiet, and do not emit much byproduct heat to the surroundings. Electrical heating equipment has a high speed of response, lending it to rapid-cycling mass-production equipment.

The limitations and disadvantages of electric heating in industry include the higher cost of electrical energy compared to direct use of fuel, and the capital cost of both the electric heating apparatus itself and the infrastructure required to deliver large quantities of electrical energy to the point of use. This may be somewhat offset by efficiency gains in using less energy overall to achieve the same result.

Design of an industrial heating system starts with assessment of the temperature required, the amount of heat required, and the feasible modes of transferring heat energy. In addition to conduction, convection and radiation, eletrical heating methods can use electric and magnetic fields to heat material.

Methods of electric heating include resistance heating, electric arc heating, induction heating, and dielectric heating. In some processes (for example, arc welding), electric current is directly applied to the workpiece. In other processes, heat is produced within the workpiece by induction or dielectric losses. As well, heat can be produced then transferred to the work by conduction, convection or radiation.

Industrial heating processes can be broadly categorized as low-temperature (to about 400 C (730 F)), medium temperature (between 400 C and 1150 C (730-2100 F)), and high temperature (beyond 1150 C (2100 F)).

Low temperature processes include:

Medium temperature processes include:

  • melting plastics and some non-metals for casting or reshaping,
  • annealing,
  • stress-relieving, and
  • heat-treating.

High-temperature processes include:

See also

References

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