Heat pumps

A heat pump is a device for transferring heat energy from a source to a consumer. Unlike spontaneous heat transfer, which always occurs from a hot body to a cold one, a heat pump transfers heat in the opposite direction. The heat pump requires an external power source to operate. The most common heat pump design consists of a compressor, thermal expansion valve, evaporator and condenser. The coolant circulating inside these components is called a refrigerant.

Refrigerators and air conditioners are known examples of heat pumps. Heat pumps can be used for both heating and cooling. When a heat pump is used for heating, it implements the same type of thermodynamic cycle as a refrigerator, but in the opposite direction, releasing heat in the heated room and taking heat from the colder ambient air.

The International Energy Agency predicts heat pumps will provide 10% of heating energy needs in OECD countries by 2020 and 30% by 2050

The basis of the heat pumping equipment park operated today in the world is formed by vapor compression heat pumps, but absorption, electrochemical and thermoelectric ones are also used.

When using conventional heating with an energy source with which mechanical work can be obtained, the amount of heat entering the heating system is equal to this work.

If this work is used to drive a heat pump, then the heat received by the heated body will be greater than the work being done. Let the temperature of the water in the heating system be equal, and the temperature of the environment surrounding the heated room is equal, and. Then the amount of heat received by the heating system. Thus, the less the temperature of the heating system differs from the ambient temperature, the greater the benefit of the heat pump compared to the direct conversion of work into heat.

The value is called the transformation ratio of the heat pump. The transformation ratio of the heat pump, or heat pump heat supply system (TST) “Ktr” is the ratio of the useful heat removed to the heat supply system to the consumer to the energy spent on the operation of the heat pump heat supply system, and is numerically equal to the amount of useful heat received at the temperatures Tout and Tin , per unit of energy spent on the drive of the VT or TST. The actual transformation ratio differs from the ideal one described by formula (1 1) by the value of the coefficient taking into account the degree of thermodynamic perfection of the GTST and irreversible energy losses during the cycle. The dependences of the real and ideal transformation ratios (Ktr) of the heat pump heat supply system on the temperature of the heat source of low potential Tin and the temperature potential of the heat removed to the heating system Tout are given. When plotting the dependencies, the degree of thermodynamic perfection of TST h was taken equal to 0.55, and the temperature head (the difference between the temperatures of the freon and the coolant) in the condenser and in the evaporator of the heat pumps was equal to 7 ° C. These values ​​of the degree of thermodynamic perfection h and the temperature difference between the freon and the heat carriers of the heating and heat collection system seem to be close to reality from the point of view of taking into account the real parameters of the heat exchange equipment (condenser and evaporator) of heat pumps, as well as the associated costs of electrical energy to drive circulation pumps, automation systems , shut-off and control valves.

In general, the degree of thermodynamic perfection of heat pump heat supply systems h depends on many parameters, such as: compressor power, quality of production of heat pump components and irreversible energy losses, which, in turn, include:

heat energy losses in connecting pipelines;

losses to overcome friction in the compressor;

losses associated with imperfect thermal processes in the evaporator and condenser, as well as imperfect thermophysical characteristics of freons;

mechanical and electrical losses in motors, etc.

Table 1-1 shows the “average” values ​​of the degree of thermodynamic perfection for some types of compressors used in modern heat pump heating systems.

Table 1-1. The efficiency of some types of compressors used in modern heat pump heating systems

Power, kWt Compressor type Efficiency
(degree of thermodynamic perfection), fraction of units.
300−3000 Open centrifugal 0,55-0,75
50-500 Open piston 0,5-0,65
20-50 Semi-hermetic 0,45-0,55
2-25 Sealed, with R-22 0,35-0,5
0,5-3,0 Sealed, with R-12 0,2-0,35
<0,5 Sealed <0,25

Like a chiller, a heat pump consumes energy to implement a thermodynamic cycle (compressor drive). The conversion factor of a heat pump – the ratio of heat output to power consumption – depends on the temperature levels in the evaporator and condenser. The temperature level of heat supply from heat pumps can currently vary from 35 ° C to 55 ° C, which allows using almost any heating system. Energy saving reaches 70%. The industry of technically developed countries produces a wide range of vapor compression heat pumps with thermal power from 5 to 1000 kW.

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