American Journal of Applied Science and Technology
61
https://theusajournals.com/index.php/ajast
VOLUME
Vol.05 Issue02 2025
PAGE NO.
61-65
10.37547/ajast/Volume05Issue02-15
Heat pumps and their applications
M.R. Nazarov
Bukhara State Pedagogical Institute, Uzbekistan
S. Salimov
Bukhara State Pedagogical Institute, Uzbekistan
Received:
24 December 2024;
Accepted:
26 January 2025;
Published:
28 February 2025
Abstract:
Recently, there has been a significant interest in heat pumps (HP) both in our country and abroad. The
areas of their application have also expanded. This is primarily due to rising energy prices and environmental issues.
Therefore, research on increasing the energy efficiency of heat pumps remains a relevant topic.
This paper examines the design and working principle of heat pumps. Additionally, it analyzes the different types
of heat pumps and ways to improve their energy efficiency.
Keywords:
Heat pumps (HP), World Energy Committee (WEC), higher-temperature div.
Introduction:
According to forecasts by the World
Energy Committee (WEC), by 2020, 75% of heating
supply (both municipal and industrial) in developed
countries will be provided by heat pumps. Heat pumps
have been successfully used in households and
industries in Europe and the United States for more
than 25 years [1,2].
A heat pump is essentially a refrigeration machine that
transfers heat from a lower-temperature div to a
higher-temperature div, thereby increasing its
temperature. Heat pumps serve as alternative energy
sources, allowing the production of affordable heat
without harming the environment. Their unique
feature is the ability to convert low-potential heat from
the environment
—
such as the ground, water, or air
—
into high-potential thermal energy for consumers. In
many countries, this eco-friendly technology has only
recently become widely adopted.
Just like in refrigeration machines (RMs), heat pumps
transfer heat from a low-temperature medium to a
higher-temperature medium.
It is also worth noting that heat pump technology has a
long history, dating back to the 1950s. This technology
has been well-developed by foreign specialists and is
widely applied in construction projects across Europe,
the U.S., and Japan. The majority of these projects
involve comprehensive energy supply systems that
include ventilation, heating, hot water supply, and heat
recovery. Undoubtedly, such projects are much more
efficient than conventional heating methods. However,
they are also actively supported and subsidized by the
government.
Uzbekistan’s Potential for Heat Pumps
Uzbekistan has great potential for utilizing solar energy.
One of the key measures for its utilization is converting
it into low-potential thermal energy, particularly for
hot water supply and heating needs. However, due to
technical and economic limitations, autonomous solar
heating systems have not yet gained widespread use in
the country. The main reason is the inability to receive
stable heat from solar systems throughout the year.
The high dependence on fossil fuels in our energy
supply is becoming a problem due to limited reserves
of oil and gas [8].
Many scientific and popular science studies have been
dedicated to heat pump design and operation
principles [3,4,5,12].
A heat pump is essentially a refrigeration machine that
transfers heat from a lower-temperature medium to a
higher-temperature
medium,
increasing
the
temperature of the latter. Heat pumps serve as
alternative energy sources, allowing the production of
affordable heat without harming the environment.
The working principle of a heat pump is based on the
fact that any object with a temperature above absolute
zero contains thermal energy. The amount of energy
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
stored is directly proportional to the mass and specific
heat capacity of the substance.
For example, seas, oceans, the atmosphere, and
underground water have enormous thermal energy
reserves due to their large mass. Therefore, a portion
of this vast energy resource can be extracted using heat
pumps for heating buildings and other thermal
processes without negatively impacting the global
environmental balance.
Compression of Refrigerant by the Compressor
At the next stage, the refrigerant in its gaseous state
enters the compressor. Here, the compressor
compresses the refrigerant (Freon), which causes a
sharp increase in pressure and, consequently, an
increase in temperature. A similar process occurs in a
household
refrigerator
compressor.
The
only
significant
difference
between
a
refrigerator
compressor and a heat pump compressor is that the
heat pump compressor has significantly higher capacity
[9,10].
Heat Transfer to the Heating System (Condensation)
After compression in the compressor, the high-
temperature refrigerant enters the condenser. In this
case, the condenser is also a heat exchanger, where
during condensation, heat is transferred from the
refrigerant to the heating system’s working medium
(e.g., water in an underfloor heating system or radiator
heating system) (see Fig. 1).
In the condenser, the refrigerant transitions from a gas
phase back to a liquid phase. This process is
accompanied by the release of heat, which is used for
the home heating system and domestic hot water
supply (DHW).
Reduction of Refrigerant Pressure (Expansion)
Now, the liquid refrigerant needs to be prepared to
repeat the working cycle. To achieve this, the
refrigerant passes through a narrow opening in the
thermostatic expansion valve (TXV).
After being pushed through the narrow expansion
valve opening, the refrigerant expands, causing its
temperature and pressure to drop. This process is
similar to spraying an aerosol from a can. After
spraying, the can becomes cold for a short time. This
occurs because of the sudden pressure drop as the
aerosol is released, leading to a corresponding drop in
temperature.
At this stage, the refrigerant is again under conditions
where it can boil and evaporate, which is necessary for
absorbing heat from the heat carrier.
Everyday Heat Pump Applications
In everyday life, people interact with heat pumps
without realizing it
—
for example, in a household
refrigerator.
A refrigerator extracts thermal energy from inside the
cooling chamber (from food and air) and transfers it to
the surrounding air through a hot panel, usually located
on the back of the refrigerator. Essentially, it heats the
room in which it is placed.
Figure 1. Structure and Operating Principle of a Heat Pump
All thermal machines (such as internal combustion
engines, refrigerators, steam engines, etc.) operate
cyclically. The term "cycle" ("cyclic process") refers to
the continuous change in the state of a system (working
fluid), which eventually returns to its initial state, from
which these changes began.
Graphically, a cyclic process (cycle) is represented as a
closed loop. In thermodynamics, cycles consist of a
strictly defined sequence of some of the simplest
thermodynamic processes (isoprocesses), through
which the working fluid returns to its original state [10].
In 1824, engineer S. Carnot first used the
thermodynamic cycle to describe and analyze the
operation of an ideal heat engine. Essentially, the
Carnot cycle efficiency defines the theoretical limit for
the maximum efficiency of a thermal machine within a
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
given temperature range.
This cycle remains a fundamental benchmark for
comparing and evaluating heat pump efficiency, as a
heat pump can be considered a reversed heat engine.
In direct cycles (also known as power cycles or engine
cycles), useful work is produced. In reverse cycles (also
called refrigeration cycles), energy must be supplied for
the process to occur.
The Second Law of Thermodynamics dictates the
direction of spontaneous thermodynamic processes,
stating that heat cannot spontaneously transfer from a
colder div to a warmer div.
"A Heat Pump is a Refrigerator in Reverse"
In articles popularizing heat pumps, one often
encounters the phrase:
"A heat pump is just a refrigerator working in reverse."
It is important to understand that both refrigerators
and heat pumps operate on the same thermodynamic
cycle
—
the reversed cycle.
Figure 2. Thermodynamic Diagram of a Heat Pump (1) and a Heat Engine (2)
In the first case, the goal is to create a lower
temperature inside the refrigerator compartment. By
expending additional energy, the heat from inside the
refrigerator is removed into the surrounding
environment.
In the second case, the goal is to create a higher
temperature inside a building. By expending additional
energy, heat from the surrounding environment is
transferred into the building, effectively cooling the
environment.
A heat engine (Figure 2) absorbs heat (QH) from a high-
temperature source and releases it (QL) at a lower
temperature (TL) while performing useful work (A).
A heat pump, on the other hand, requires an input of
work (A) to extract heat (QL) at a low temperature (TL)
and transfer it to a higher temperature (TH).
It can be shown that if both of these machines are
reversible (i.e., the thermodynamic processes do not
involve heat or work losses), there exists a fundamental
efficiency limit for each system.
In both cases, this efficiency limit is defined by the ratio
QH/A [10].
Basic Working Principle of the Most Common Vapor-
Compression Heat Pump System (Figure 3):
1.
In the external heat exchanger (evaporator),
thermal energy from the outdoor environment or
another available heat source is transferred to the
working fluid of the heat pump (typically a refrigerant
such as Freon), which circulates through the internal
circuit.
2.
The refrigerant absorbs heat, evaporates, and
moves toward the compressor. The compressor
compresses the refrigerant, causing its temperature to
rise significantly.
Figure 3. Schematic Diagram of a Heat Pump
3.
The compressed refrigerant then passes
through the internal heat exchanger (condenser),
where it condenses and releases heat into the
consumer system. This can be used for:
American Journal of Applied Science and Technology
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American Journal of Applied Science and Technology (ISSN: 2771-2745)
o
Direct air heating
o
Heating of a liquid heat carrier for the
building's heating system or industrial processes
o
Hot water production for consumers
4.
The refrigerant then moves through the
expansion valve, which reduces the pressure and
lowers the temperature of the refrigerant.
As the high-pressure refrigerant flows through a
capillary or expansion valve into the evaporator, a
sudden pressure drop occurs, leading to evaporation.
This absorbs heat from the evaporator walls, which in
turn draws heat from the ground or water loop,
constantly cooling the evaporator.
The compressor then draws in the refrigerant from the
evaporator, compresses it, causing a sharp
temperature increase, and pushes it into the
condenser.
Inside the condenser, the compressed and heated
refrigerant releases its stored heat (approximately 85-
125°C) to the heating circuit and returns to a liquid
state. This cycle continuously repeats.
Understanding the Working Principle of a Heat Pump
To grasp how a heat pump functions, it is essential to
understand key thermodynamic concepts, including:
•
Low-potential heat
•
Carnot cycle
•
Heat energy transfer
•
Refrigerant (Freon)
•
Compressor operation, etc.
Heat Pump Efficiency and the "Efficiency Paradox"
When discussing the energy efficiency of a heat pump,
it is important to note that, like a refrigeration machine,
it has extremely high efficiency. This can sometimes
appear counterintuitive or even paradoxical.
The COP (Coefficient of Performance) of a heat pump is
often greater than 1, sometimes reaching values of up
to 5. This means that the device produces more heat
energy than the electrical energy it consumes, which is
a key advantage of heat pump technology.
Although this may seem unusual, it is not a violation of
energy conservation laws. The phenomenon is easily
explained using the well-known Carnot efficiency
formula for maximum theoretical efficiency of a heat
engine [10,11].
Heat Pump Performance Coefficient Formula
In technical thermodynamics, the efficiency of heat
pumps is measured using a special parameter called the
Heating Efficiency Coefficient (ε_отоп), which is
calculated using the following formula:
1
2
1
/
1
1
T
T
A
Q
отоп
−
=
(1)
Thus, when using a heat pump, the heated space
receives more energy than it would through direct
heating.
As stated above, according to formula (1), when the
temperature difference between the environment and
the heated space is small, the latter receives
significantly more heat than is released during fuel
combustion. This may seem paradoxical. However, in
reality, there is no paradox in heat pumps and dynamic
heating, which becomes clear when considering the
concept of internal energy quality, associated with the
chaotic thermal motion of molecules.
CONCLUSION
Based on the research conducted on heat pumps, the
following conclusions can be drawn:
1.
The structure and working principle of heat
pumps were studied from a thermodynamic
perspective, and their advantages over other thermal
machines were demonstrated.
2.
The use of heat pumps in air conditioning,
space heating, and other applications significantly
reduces electricity consumption (as well as fuel and
other energy resources), making it energy-efficient and
economically beneficial.
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