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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power plants based on heat pumps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Alternative energy sources Introduction Heat supply in Russia, with its long and rather severe winters, requires very high fuel costs, which are almost 2 times higher than the costs of electricity supply. The main disadvantages of traditional sources of heat supply are low energy efficiency (especially in small boiler houses), economic and environmental efficiency (traditional heat supply is one of the main sources of pollution in large cities). In addition, high transport tariffs for the delivery of energy carriers exacerbate the negative factors inherent in traditional heat supply. It is impossible not to take into account such a serious thermodynamic drawback as the low exergy efficiency of using the chemical energy of fuel for heat supply systems, which in heating systems is 6-10%. Extremely high costs for heat networks, which are probably the most unreliable element in district heating systems. The specific accident rate for pipelines with a diameter of 1400 mm is one accident per year per 1 km of length, and for pipes of a smaller diameter - about six accidents. Considering that the total length of heating networks in Russia is 650 thousand km, and 300 thousand km need to be completely replaced, it becomes obvious that the construction and maintenance of heating networks in working order require costs commensurate with the cost of thermal power plants or district boiler houses. All of the listed negative factors of traditional heat supply urgently require the intensive use of non-traditional methods. One of these methods is the beneficial use of dissipated low-temperature (5-30°C) natural heat or industrial waste heat for heat supply using heat pumps. Heat pumps, due to the fact that they are spared from most of the listed disadvantages of district heating, have found wide application abroad, if in 1980 there were about 3 million heat pump installations in the USA, 0,5 million in Japan, 0,15 in Western Europe, 1993 million, then in 12 the total number of operating heat pump installations (HPU) in developed countries exceeded 1 million, and the annual output is more than 2020 million. Mass production of heat pumps has been established in almost all developed countries. According to the forecast of the World Energy Committee, by 75 in the advanced countries the share of heating and hot water supply with the help of heat pumps will be XNUMX%. Basic designations, indices and abbreviations Quantity notation
Indexes
Abbreviations
Principle of operation of the heat pump The principle of operation of a heat pump follows from the works of Carnot and the description of the Carnot cycle, published in his dissertation in 1824. A practical heat pump system was proposed by William Thomson (Lord Kelvin) in 1852. heating purposes. In justifying his proposal, even then, Thomson pointed out that the limited energy resources would not allow continuous burning of fuel in furnaces for heating and that his heat multiplier would consume less fuel than conventional furnaces. Thomson's proposed heat pump (HP) used air as the working fluid. The ambient air was sucked into the cylinder, expanded as it cooled, and then passed through a heat exchanger, where it was heated by the outside air. After being compressed to atmospheric pressure, the air from the cylinder enters the heated room, being heated to a temperature above ambient. In fact, a similar machine was implemented in Switzerland. Thomson stated that his HP is capable of producing the required heat using only 3% of the energy used for heating. Heat pump installations were further developed only in the 20s and 30s of the 20th century, when the first installation designed for heating and hot water supply using the heat of the surrounding air was created in England. After that, work began in the USA, leading to the creation of several demonstration plants. The first large heat pump plant in Europe was put into operation in Zurich in 1938-1939. It used the heat of river water, a rotary compressor and a refrigerant. It provided heating of the town hall with water at a temperature of 60 C at a power of 175 kW. There was a heat storage system with an electric heater to cover the peak load. During the summer months, the installation worked for cooling. In the period from 1939 to 1945, 9 more such installations were created in order to reduce coal consumption in the country. Some of them have been successfully operating for more than 30 years. So, in 1824, Carnot first used the thermodynamic cycle to describe the process, and this cycle remains the fundamental basis for comparing with it and evaluating the efficiency of HP. A heat pump can be thought of as a reversed heat engine. The heat engine receives heat (Fig. 1.1.1) from a high-temperature source and dumps it at a low temperature, giving useful work. A heat pump requires work to generate heat at low temperatures and deliver it at higher temperatures.
It can be shown that if both of these machines are reversible (i.e., thermodynamic processes do not contain heat or work losses), then there is a finite limit to the efficiency of each of them, and in both cases this is the ratio Qн/W. If this were not so, then it would be possible to build a perpetual motion machine simply by connecting one machine to another. Only in the case of a heat engine, this ratio is written in the form W/Qn and is called thermal efficiency, while for a heat pump it remains in the form Qn/W and is called the heat conversion coefficient (Kt). If we assume that heat is isothermally supplied at a temperature TL and isothermally removed at a temperature TH, and compression and expansion are performed at constant entropy (Fig. 1.1.2), work is supplied from an external engine, then the conversion coefficient for the Carnot cycle will look like: Кт = TL /( TN - TL ) + 1 = TN / ( TN - TL )
Thus, no heat pump can have a better performance, and all practical cycles only realize the desire to get as close as possible to this limit. Classification of heat pumps At present, a large number of heat pump installations have been created and are being operated, differing in thermal schemes, working fluids and equipment used. According to the designation of various classes of installations, in the literary sources known to us, there is no single established opinion, there are various designations and terms. In this regard, the classification of installations is of great importance, which makes it possible to consider their properties in accordance with one or another group. All types of heat pump installations can be classified according to a number of similar features. Each of them reflects only one characteristic feature of the installation, therefore, in the definition of a heat pump installation, there may be two or more features. The classification of heat pump installations should be carried out primarily according to their cycles of operation. There are several main types of heat pumps:
All heat pumps, according to the principle of interaction of working bodies, can be combined into two main groups: 1) open cycle, in which the working body is taken and released into the external environment; 2) a closed cycle, in which the working fluid moves along a closed circuit, interacting with the source and consumer of heat only through heat exchange in surface-type apparatuses. There are one- and two-stage and cascade HPIs, as well as HPIs with serial connection of heated and cooled heat carriers with their countercurrent movement. By appointment: stationary and mobile, for the accumulation of thermal energy and its transport and disposal of waste heat. By performance: large, medium, small. By temperature regime: high-temperature, medium-temperature and low-temperature. According to the mode of operation: stationary, non-stationary, continuous or cyclic, non-stationary with a thermal energy accumulator. By type of refrigerant: air, ammonia, freon, on mixtures of refrigerants. By type of energy consumed: driven by an electric motor or gas turbine or gas turbine, operating on secondary energy resources, etc.
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