Electric heating 3508315 225688747 2008-07-14T22:24:47Z Mild Bill Hiccup 5202324 copyedit {{For|the album by Roger Taylor|Electric Fire}} '''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 [[electricity|electrical]] [[appliance|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. 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. ==Design variations== 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=== {{main|Convector heater}} (Only a stub) In a convection heater, the heating element heats the air next to it by [[Heat conduction|conduction]]. Hot air is less [[density|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"=== {{main|Fan heater}} A fan heater is a variety of convection heater that includes an [[fan (mechanical)|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; however, they should not be left unattended. ===Storage heating=== {{main|Storage heater}} 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=== {{main|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. ===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 boiler]]s) 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 <ref>http://www.tlc-direct.co.uk/Book/7.11.2.htm</ref>. ==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 electical 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#Household_Furnace|furnace]] or [[Boiler#Hydronic_boilers|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 {{Fact|date=March 2008}}. 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.<ref name="AECB1">[http://www.aecb.net/PDFs/green%20electricity%20illusion.pdf The Green Electricity Illusion], ''[[Association for Environment Conscious Building|AECB]]'', published 2005-11-11, accessed 2007-05-26</ref> In the case of new buildings, [[low-energy building|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 use electricity to provide heat efficiently, a [[heat pump]] driven by electricity can boost the temperature of the heat in the ground, in the outside air, or in waste streams such as exhaust air in order to use it as a heat source. This can cut the electricity consumption down to as little as 20% and can reduce the environmental impact. Electrical space heating can still be economic where electicity 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. ==Mathematical analysis== According to [[Joule's Law]], the heat power produced by a resistor is: :<math>P=IV</math> where :''P'' is the power in [[watt]]s :''I'' is the current in [[ampere]]s, and :''V'' is the potential difference in [[volt]]s, and according to [[Ohm's Law]] ''I'' and ''V'' are related as follows: :<math>V=IR</math> where :''R'' is the resistance of the heating element, in [[Ohm (unit)|ohms]]. We can combine these two formulae to obtain the heat output from the heating element in terms of either current or voltage: <math>P=I^2R=\frac {V^2} R</math> For heaters powered by AC mains, ''I'' and ''V'' are the [[root mean square]] (RMS) values of current and voltage. ==Industrial electric heating== See also [[Induction furnace]], [[Electric arc furnace]] ==See also== {{Portal|Energy}} *[[Thermal efficiency]] *[[Central heating]] *[[Energy conservation]] *[[HVAC]] *[[Renewable energy]] *[[Underfloor heating]] ==References== {{reflist}} [[Category:HVAC]] [[Category:Heating methods]] [[ja:電気暖房]] [[nl:Elektrische verwarming]] [[sv:Elvärme]] [[yi:עלעקטריק הייצונג סיסטעם]]