The principle of operation of a heat pump. Scheme, description

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The principle of operation of a heat pump. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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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 major heat pump plant in Europe was commissioned 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 with 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.

Principle of operation of the heat pump
Fig.1.1.1. Thermodynamic diagram of a heat pump and a heat engine: 1 - heat pump; 2 - heat engine.

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 temperature TL and isothermally removed at 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:

Kt \u1d TL / ( TN - TL ) + XNUMX \uXNUMXd TN / ( TN - TL )

Principle of operation of the heat pump
Ris.1.1.2

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.

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