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.

Contents 1 Space Heating 2 Water Heating 3 Environmental and Efficiency Aspects

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 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 has 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 in which unheated air flows through. Radiative heaters are also ideal for basements and garages where spot heating is desired. More generally, they are an excellent choice for task-specific heating. Radiative heaters operate siliently. One disadvantage is that they do present the greatest potential danger of igniting nearby furnishings, due to the focused intensity of their output and the 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 cycle sets up a constant current of hot air that leaves the appliance through vent holes and heats up the surrounding space. Convection heaters are ideally suited for heating closed spaces. They operate silently, and compared to radiative heaters, they have a lower risk of ignition hazard in the event that they make unintended contact with furnishings. Convection heaters are a good choice for situations in which the heater will be used for long periods of time or left unattended. They are very safe heaters, and there is a very low chance of getting burned.

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. The use of the fan reduces the thermal resistance between the heating element and the surroundings, allowing heat to be transferred more quickly.

These heaters operate with considerable noise, which is 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 lower electricity prices for electricity sold during low-demand periods, such as overnight. The storage heater stores the 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. 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, which consists of a metal tube containing an insulated electric resistance heater. Domestic immersion heaters (usually rated at 3 kW 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 kW or more, and run on a three-phase supply.

Electrode Heater

With an electrode heater, there is no wire-wound resistance; the liquid itself acts as the resistance. This system 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-fueled 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 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 because of the lack of efficiency at the power plant, and there are plans to phase it out entirely, while Denmark has banned the installation of electric space heating in new buildings. In the case of new buildings, low-energy building techniques can be used that can virtually eliminate the need for heating.

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 method can cut the electricity consumption down to as little as 20% and can reduce the environmental impact.

Electrical space heating can still be economical where electricity supplies are low-cost. Where the primary source of electrical energy is hydroelectric, nuclear, wind, or another 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.