Heat pumps. Theory

A heat pump is a device for heat energy transfer from a low-grade heat source to an object to be heated.

The low-grade heat transfer and its “transformation” to the “suitable” temperature range are realized on the principle of refrigerating machine. Phase transitions (the transition from one state to another) of the refrigerant, circulating in a unit loop, are means for the above-mentioned realization.

Vapour-compression heat pumps have gained the most widespread application. Heat pumps, based on an absorption refrigeration cycle, are less widespread.

Both a heat-conductive fluid and an air environment can be the objects to be heated by a heat pump.

Схема переноса тепловой энергииFigure 1.1 – The heat energy transfer diagram.

The concept of heat transfer from the surrounding environment to the object to be heated is illustrated in the figure. The diagram illustrates that the environment heat energy is the main part of energy for facility heating. The electric power is one of the means for low-grade heat energy transformation. The electric power, consumed during the cycle, is also transformed in the heat energy and used for heating, but its part is less by several times than natural heat energy part.

According to the diagram (fig. 1.1) it can be seen that in terms of heat transfer source there are the following types of heat pumps:

  1. Air (the outside air is a heat extraction source).
  2. Surface water (sea, river, lake).
  3. Ground and underground water. Wells drilling is used for this source.
  4. Surface and subsurface ground. Ground probes and also geothermal probes by means of wells drilling.
  5. Artificial low-grade heat. In this case the sewage and waste water, the technological industrial and household processes heat, and the extract air of ventilation systems are used.

Depending on the application of medium and object to be heated, heat pumps are divided into the following main types:

  1. “air-to-air”
  2. “air-to-water”
  3. “water-to-water”
  4. “water-to-air”
  5. “ground-to-water”
  6. “ground-to-air”

Принцип работы теплового насоса. Зимний и летний режимы.

Figure 1.2 – The heat pump operation principle. Winter and summer modes.

Designations on the diagram: a winter mode: ① – the exhaust outside air (given off the heat energy), ② – the air at the outside unit inlet, ③ – the compartment warmed air, ④ – the compartment air at the inside unit inlet. A summer mode: ⑤ – the exhaust (warmed) outside air, ⑥ – the air at the outside unit inlet, ⑦ – the compartment chilled air, ⑧ – the compartment air at the inside unit inlet.

The up-to-date industry presents a wide range of climate control equipment (air conditioners, supply and extract units, chiller-fancoil systems) that enables to heat or to cool compartments, depending on a season, by switching the “conditioning” – “heat pump” modes. It is shown by an example of central conditioner operation scheme (fig. 1.2).

As per a medium application method heat pumps are divided into the following types:

- open-loop;

- closed-loop;

- with direct heat exchange.

In open-type units the water is supplied directly from a natural source into the heat-exchange apparatus (evaporator) of the heat pump to take off the low-grade heat. The open type of heat exchange is mainly applied in geothermal units, where ground waters are used. The water body (river, lake, pond, etc.) are more rarely used as sources. This type is applied if there is enough water amount and upon the condition that such use of ground waters is permitted by law.

In closed-type units for heat take-off the manifolds are installed into sources, in which the intermediate cooling medium is warmed up taking the low-grade heat off the water or the ground. After the intermediate cooling medium has passed through the loop of heat pump evaporator and given off its heat to the refrigerant, it returns into the manifold. Manifolds to be installed into the water body and ground are systems of plastic pipelines. In case of using a well the geothermal probe is a manifold for low-grade heat collection. Antifreeze agents, ethylene glycol and propylene glycol solutions are applied as the intermediate cooling medium.

In units with direct heat exchange the low-grade heat is taken off between a natural source (ground, water body, well) and a pipeline system (the heat pump evaporator) arranged in it. So the refrigerant boiling process in the evaporator occurs due to direct heat removal from a natural source. In such cases the copper is mainly applied as a material for evaporator manufacture. In some cases, depending on the soil or water composition, copper nickel alloys (МНЖ5-1/MNZH5-1 in particular) are used.

When designating the direct heat exchange, abbreviation DX (abbr. from English “direct exchange”) is often used. The use of direct heat exchange ensures a high efficiency and reliability of geothermal heat pumps. Owing to this technology the total length of wells boring is shortened in comparison with open or closed type application.

A series of articles concerning main types and application examples of heat pumps is presented on our website (links are at the beginning of article).

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