Multi-angle thinking about electric heating?

1 The prospect of electric heating is promising.

1. 1 The reform of heating system is imperative and will involve many aspects, but its core problem is to turn welfare heating into commercialization and monetization of heat. This major change will make the level of heating cost an important factor in choosing heating mode. (refer to the Chinese website of architecture)

1.2 What are the costs of various heating methods? According to the notice issued by Beijing Municipal Price Bureau 200 1, the heating price per square meter of central heating residential buildings in each heating season is: coal-fired boiler direct supply 16.5 yuan, coal-fired boiler intermittent supply 19 yuan, urban heating network 24 yuan, and oil, gas or electric boiler heating 30 yuan. As coal-fired boilers have been eliminated, among the above heating prices, only 24 yuan /m2 of urban heating network and 30 yuan /m2 of oil, gas or electric boilers can be used as reference and comparison standards.

1.3 What is the cost of electric heating? There is a unified evaluation standard, that is, the limit value of building heat consumption index stipulated in the national industry standard "Design Standard for Energy Efficiency of Civil Buildings" (JGJ26-95). For example, the heat consumption index of ordinary residential buildings in Beijing area does not exceed 20.6W/ m2, and the total heat consumption during the whole heating period does not exceed 6 1.8kWh day, 24 hours a day. Even if household electric heating is adopted, the electricity charge of 6 1.8 /kWh is 27. 19 yuan /m2, which is slightly higher than that of 24 yuan /m2 of urban central heating, but lower than that of 30 yuan /m2 of central gas-fired boiler room. But also has the advantages of saving operating costs, such as: intermittent heating mode suitable for use requirements can be implemented, and the average room temperature during the whole heating period can be appropriately reduced on the premise of ensuring thermal comfort during use. So even if the simplest household electric heating is adopted, the heating cost is acceptable.

1.4 Beijing is actively planning to put forward the third step of building energy efficiency. Based on the comparison of energy consumption in the past 80 years, energy saving has been achieved by 30% in the first step, 50% in the second step and 65% in the third step. In other words, heating energy consumption can be reduced to one-third of the 80-year level. If implemented, the heat consumption index of ordinary residential buildings will not exceed 18W/ m2. According to the above method, the electricity charge for direct heating can be reduced to 24 yuan/m2.

1.5 Recently, the Beijing Municipal Economic Commission, the Planning Commission, the Municipal Administration Commission and the Price Bureau jointly issued the document (2002) 100 to implement preferential prices for electric heating during off-peak hours. During the heating period, from 23: 00 every day to 7: 00 the next day, regardless of the nature of electricity consumption and the heating object, it is charged at 0.20 yuan/kWh, and the cost of electric heating is lower.

1.6 In fact, many far-sighted developers have clearly seen the market value of energy-efficient buildings. The index of building heat consumption should be regarded as an important index of building quality and housing price. Recently, you can see an advertisement in the media-"Farewell to the era of heating and air conditioning". Of course, it is not really "farewell to the era of heating and air conditioning", but another heating and air conditioning method is adopted, and this less used method can only adapt to low-energy buildings with little energy consumption in winter and summer. The real copy point of this advertised building should actually be low energy consumption, rather than its heating and air conditioning methods with certain hidden dangers. When the energy consumption has been reduced to a lower level, simpler heating and air conditioning methods can be adopted, such as using electricity as the energy source for heating and air conditioning.

1.7 electric heating is more attractive than traditional centralized heat source or gas-fired household independent heat source in terms of accurate energy consumption measurement, simple charge, simplified construction procedures and property management, flexible heating for residents, and convenient room temperature regulation.

2. Improve the energy efficiency of electric heating.

In recent years, electricity supply has been sufficient in many parts of the country. Starting from environmental protection and balancing the seasonal surplus of electricity, relevant departments strongly encourage the use of electric heating. However, China's power production mainly depends on thermal power generation, and the average thermoelectric conversion efficiency of thermal power generation is about 33%, plus the transmission and distribution efficiency is about 90%. Direct heating with electric heaters, electric heaters and electric water heaters is an inefficient way of energy utilization. From coal to electricity to electricity to heat, the comprehensive energy efficiency is only about 30%, far lower than that of coal-fired, oil-fired or gas-fired boilers, and even lower than that of cogeneration heating. Generally speaking, it has not reduced but increased the pollution factors to the atmospheric environment, which is only the transfer of polluted land. Therefore, the development of electric heating should maximize the efficiency of energy utilization. The main ways are: electric heat pump heating, which can improve the efficiency of electricity-heat conversion, regenerative heating and radiation heating, which can make full use of off-peak electricity consumption.

3 About electric heat pump heating

The maximum electrothermal conversion efficiency of direct electric heating can only be 1: 1, while electric heat pump heating can make the conversion efficiency (heating coefficient) reach 1: 2.5 or 1: 3, that is to say, the energy consumption of 1kW can get 2.5 or 3 kW of heat energy. Electric heat pump mainly includes air heat pump, water loop heat pump and ground source heat pump. Ground source heat pump can be divided into surface water, groundwater and ground source heat pump.

Air heat pump air conditioning, that is, the so-called "cold and warm" air conditioning different from single cooling type. Air heat pump type cold and hot water unit, which is a kind of equipment that can both cool and heat, is the so-called "household central air conditioning". Air heat pump is used for heating, and the heating capacity is limited at low temperature in winter. The capacity of the unit or auxiliary heat source should be determined according to the heating capacity under the most unfavorable conditions in winter. In addition, the noise of the outdoor unit and its impact on the environment should be properly solved.

Due to the heat storage function of the earth, the temperature of groundwater and soil at a certain depth can be stably maintained at a level slightly higher than the local annual average temperature all the year round. For example, if the annual average temperature in Beijing is 1 1.4℃, the temperature of groundwater and soil with a depth of 6m can be maintained at around 13- 14℃ all the year round. Therefore, in the area where the annual average temperature is 10-20℃, the ground source heat pump can be used as a cold and heat source. Through electric heat pump, heat can be absorbed from groundwater or soil in winter, and condensation heat that needs to be removed during refrigeration can be absorbed from groundwater or soil in summer. Ground source (ground pipe) heat pump is only suitable for villa residential areas with very low building volume ratio. The feasibility of groundwater source heat pump mainly depends on whether it has reliable conditions for extracting and recharging groundwater resources and whether it meets the local hydrogeological management regulations.

The investment cost of electric heat pump heating is relatively high, but it also has the function of cooling air conditioning in summer and should not be directly compared with the system with single heating function. Compared with the investment cost of other heating methods, the additional cost of heat pump function should be compared with the additional cost of single refrigeration function.

4 About electric heat storage heating

In addition to the electric heat pump, if electric heating is used for direct heating, it is best to use the heat storage method, that is, to convert electric energy into heat energy during the low period of the power grid, and to store the heat energy while heating, so as to prepare for the transformation from stored heat to heating during the peak period of the power grid. The main way of electric heat storage heating is centralized electric boiler with heat storage device. Electric thermal storage heating makes use of the abundant power in the trough period of the power grid, and can obtain preferential electricity price when measuring with time-sharing watt-hour meter, which greatly reduces the heating cost. Water is the simplest heat storage medium, so it needs a large volume of water to store heat with water. A house with a building area of 10000 square meters needs about 80 cubic meters of heat storage water. If the phase change heat storage medium is used, the volume can be reduced by about 65,438+0/3.

This kind of electric heater which can store heat can be used for household electric heating. The heat storage electric heater is actually solid heat storage, similar to an electric iron. The heat capacity (specific heat) of water is greater than that of ordinary substances, but the heating temperature limit of water under normal pressure is 100℃. Although the heat capacity (specific heat) of other substances such as Fe2O 3 is only 1/4.3 of that of water, its heating temperature can be as high as 800℃, and its density is relatively high (about 4 times that of water), which is the same volume as water. When the time-sharing watt-hour meter can be used, the use of electric heaters that can store heat can also greatly reduce the heating cost.

5 About radiant heating of ceiling with electric heating film

When electric heat pump or electric energy storage cannot be used for heating due to construction investment and other reasons, it is better to use more energy-saving radiation heating method for direct electric heating.

The human body generates heat due to metabolism, so it needs to dissipate heat through evaporation, convection and radiation to maintain physiological heat balance. Radiation heating reduces the radiation heat dissipation of human body, and the physiological heat balance needs to reduce the room temperature to increase the convective heat accordingly. Therefore, radiation heating at lower room temperature can achieve the same thermal comfort as convection heating at higher room temperature, that is, the so-called equivalent thermal comfort, which is why radiation heating is more energy-saving than convection heating. For residential buildings, the energy saving range of radiant heating is about 10% compared with convection heating.

Electrothermal film radiation is a way of electrothermal radiation heating. Radiant heating with electrothermal film ceiling has been widely used in residential buildings for some time because of its low investment cost and simple technology.

The ceiling radiant heating system with electrothermal film consists of electrothermal film, switch box, thermostat and junction box. The infrared ray radiated by the electric energy through the film heating element is used as the radiation heating source.

The electrothermal film consists of two basement membranes, an upper bus and a lower bus, and a plurality of conductive strips connected in parallel or in series. The function of the basement membrane is to protect the bus bars and conductive bars, insulate and dissipate heat, so it requires good insulation, thermal conductivity and certain mechanical strength. The bus bar is used to connect the power supply and the conductive bar, and it is required to transmit the required current, and it is well connected with the power supply and the conductive bar. There are two basic forms of conductive strips: parallel connection or series connection. Using conductive ink or metal strip to convert electric energy into heat energy requires high conversion efficiency, uniform and stable resistance, and the surface temperature is not higher than the allowable working temperature of the basement membrane.

In order to reduce the heat generated by the electric heating film from being transmitted to other rooms, an incombustible insulating layer should be laid on one side of the electric heating film. Generally, centrifugal glass wool plate with a thickness of not less than 50mm is adopted. On the other side, a decorative layer with small thermal resistance is installed, and gypsum board is generally used.

The objective evaluation of this heating method should be "available", but "comfort is not high" The so-called "availability" means that the heating cost is acceptable for energy-saving houses. The so-called "low comfort" is mainly due to the fact that the heating density of electric heating film products at home and abroad is generally 130- 160 W/m2, which makes the surface temperature of radiator above 40℃, and some even as high as 45℃, far exceeding the limit of the national standard "Code for Design of Heating, Ventilation and Air Conditioning" on the surface temperature of radiator. According to the above specifications, the room height is 2.5-3 m, the panel temperature is 28-30℃, the room height is 3-4 m, and the panel temperature is 33-36℃.

Radiant heating is not "warming fire", but reducing the radiation heat dissipation of human body under the condition of reasonable ambient radiation temperature. Under normal circumstances, the human body does not need to get radiant heat. The surface temperature of radiator is too high, which makes people feel uncomfortable when baking. It is obviously inappropriate for people to stay for a long time. Roof electrothermal film heating is mostly used in short-term inhabited or uninhabited places abroad, but it is not the main means of heating houses. The simple introduction and application of residential heating is a low-standard application of this technology. Experiments show that it is possible to reduce the heat flux density of radiator and improve the temperature gradient of heating room. If we can't develop and provide products with low heat flux density, the application of electric heating film in residence will be limited and denied.

6. About ground radiant heating with electric heating cable

Ground cable radiant heating is another way of electric heating radiant heating. The measured data prove that the thermal comfort of floor radiation heating is obviously better than that of ceiling radiation. In the past, electric heating cables were mainly used for pipeline anti-freezing protection, outdoor ground ice melting in winter and raising the ground temperature of outdoor venues (such as football fields). The investment cost of floor radiant heating is much higher than that of electrothermal film ceiling radiant heating, which limits the application of this heating method in residential buildings. Due to the multi-channel introduction of foreign products and price competition, the investment cost has been reduced to the level of radiant heating with electric heating film ceiling, which has become a new hot spot after direct heating with electric heating film.

The system consists of electric heating cable, switch box, ground thermostat, indoor thermostat and junction box. Similar to the well-known low-temperature hot water floor radiant heating, in order to reduce the heat generated by the electric heating cable from being transmitted to other rooms, an insulating layer should be laid at the lower part of the electric heating cable, concrete should be filled around the electric heating cable, and a ground layer should be laid at the upper part.

Electric heating cable is a kind of cable which can generate heat after being electrified. It consists of solid single or multiple resistance wire heaters, insulation layer, grounding conductor, metal shielding layer, waterproof sheath and cold and hot conductor joints. There are alloy wires and carbon fiber wires as heating elements. Because the heating stability and strength of alloy wire are better than that of carbon fiber wire, alloy wire electric heating cable should be the first choice for floor radiant heating.

Electric heating cables can be divided into single-loop electric heating cables and double-loop electric heating cables. Single-loop electric heating cable has two terminals, which need to be connected by wires to form a loop. The structural difference between double-loop electric heating cable and single-core cable is that a cold conductor loop is added inside the cable, and a loop is formed inside the cable itself. In this process, a sheath is added, which can be connected to the conductor at one end.

Compared with single-core cable, double-core cable is more convenient and flexible to install, especially suitable for pipeline antifreeze and outdoor applications. In addition, it also plays a certain role in weakening the electromagnetic field (the electromagnetic radiation of single-core cable is only 0.3- 1% of the safe electromagnetic radiation).

The power range of the electric heating cable is 6-30w/m. Like low-temperature hot water radiation heating and electric heating film radiation heating, the arrangement of the electric heating cable for floor radiation heating should be determined according to the heating load of the room, the uniform temperature field and the limit value of the average surface temperature of the radiator in the Code for Design of Heating, Ventilation and Air Conditioning. Generally, the power used should not be greater than 65,438+08W/m. 。

According to the national standard "Code for Design of Heating, Ventilation and Air Conditioning", according to the length of personnel's stay, the requirement for long-term stay is 24-26℃, which should not exceed 28℃, and higher temperature can be used for short-term stay or non-stay. The ground radiant heating with electric heating cable can ensure that the above requirements can be met, so as to obtain a reasonable vertical temperature field distribution and maintain good indoor temperature stability.

7 on the safety of direct electric heating

No matter which direct heating method is adopted, safety is a crucial issue.

Both ceiling radiant heating with electrothermal film and floor radiant heating with electrothermal wire should have the highest temperature limit that heating elements can reach and effective protective measures, especially effective control and protection under unfavorable conditions of shielding or covering. Due to the one-sided pursuit of low cost and low capacity configuration, it is not only difficult to reach the heating room temperature, but also makes the heating element overheat after long-term continuous operation. Lack of high-performance temperature control configuration can not guarantee the safety of operation.

It is controversial whether conductive ink or metal strip should be used as the conductive strip in the electrothermal film. There is also a debate about whether the electric heating cable should adopt single loop or double loop. Except for some foreign materials from different channels, national or local technical standards are urgently needed to standardize them, and the standardization work lags behind the application.

On the basis of engineering practice, Beijing has actively carried out the standardization of electric heating. At present, we should adhere to a prudent policy to effectively prevent accidents.

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