Access to this full-text is provided by MDPI.
Content available from Applied Sciences
This content is subject to copyright.
Citation: Rabczak, S.; Mateichyk, V.;
Smieszek, M.; Nowak, K.;
Kolomiiets, S. Evaluating the Energy
Efficiency of Combining Heat Pumps
and Photovoltaic Panels in
Eco-Friendly Housing. Appl. Sci. 2024,
14, 5575. https://doi.org/10.3390/
app14135575
Academic Editors: María Isabel
Rodríguez Rojas and Montserrat
Zamorano
Received: 31 May 2024
Revised: 21 June 2024
Accepted: 24 June 2024
Published: 26 June 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
applied
sciences
Article
Evaluating the Energy Efficiency of Combining Heat Pumps and
Photovoltaic Panels in Eco-Friendly Housing
Sławomir Rabczak 1, Vasyl Mateichyk 2,3, * , Miroslaw Smieszek 2, Krzysztof Nowak 1
and Serhii Kolomiiets 3
1Faculty of Civil Engineering, Environment and Architecture, Rzeszow University of Technology,
35-959 Rzeszow, Poland; rabczak@prz.edu.pl (S.R.); krzynow@prz.edu.pl (K.N.)
2Department of Technical Systems Engineering, Rzeszow University of Technology, al. Powstancow
Warszawy 10, 35-959 Rzeszow, Poland; msmieszk@prz.edu.pl
3Department of Ecology and Environmental Protection Technologies, National Transport University,
01010 Kyiv, Ukraine; s.kolomiiets@ntu.edu.ua
*Correspondence: vmate@prz.edu.pl; Tel.: +38-050-078-92-60
Abstract: This article aims to analyze the energy efficiency of combining heat pumps with photovoltaic
(PV) panels in energy-efficient homes. The research methodology involved a detailed energy balance
analysis, assessment of the impact of mechanical ventilation, location, heat loss, and the choice and
operation of heat sources, with a particular focus on heat pumps in synergy with PV installations.
The results demonstrate that integrating heat pumps with PV panels can significantly reduce the
demand for external energy sources and lower the operating costs of buildings, while contributing to
their energy self-sufficiency. This study highlights that such a combination of technologies is key to
promoting sustainable development and achieving energy efficiency goals in the residential sector.
The results of this analysis expand knowledge about the effectiveness of such systems and provide
practical recommendations for designers and engineers interested in implementing renewable energy
technologies in modern energy-efficient buildings, taking into account the impact of these solutions
on reducing CO2emissions as well.
Keywords: heat pump; photovoltaic panel; energy efficiency; CO2emissions
1. Introduction
In the face of global challenges related to climate change, such as global warming and
increased carbon dioxide concentration in the atmosphere, sustainable management of
energy resources has become an indispensable priority in designing modern residential
homes [1–6]. Growing ecological awareness and pressures to reduce carbon footprints ne-
cessitate rethinking the use of available technologies and shaping living spaces sustainably.
Studies and analyses conducted in various parts of the world confirm that integrating mod-
ern technological solutions with appropriate design can significantly contribute to reducing
emissions of harmful substances and improving the energy efficiency of buildings [7–11].
Modern technologies, such as heat pumps and photovoltaic (PV) installations, play
a significant role in achieving ecological goals, such as reducing CO
2
emissions and de-
creasing energy consumption from non-renewable sources [
10
,
12
–
17
]. Heat pumps, by
utilizing thermal energy from the environment (air, water, or earth), can efficiently heat
and cool buildings while consuming significantly less electrical energy than traditional
systems [
10
,
14
,
18
]. Meanwhile, photovoltaic installations, by converting solar radiation
into electrical energy, are becoming an increasingly popular solution, enabling the pro-
duction of green energy directly at the point of consumption [
15
,
16
]. Combining these
technologies not only reduces the use of traditional energy sources but also lowers the
operational costs of buildings, which is particularly important from a long-term economic
perspective [19,20].
Appl. Sci. 2024,14, 5575. https://doi.org/10.3390/app14135575 https://www.mdpi.com/journal/applsci
Appl. Sci. 2024,14, 5575 2 of 14
Moreover, the goal set out in the article is consistent with the 2030 Agenda for Sus-
tainable Development, which in paragraph 31 “calls for the broadest possible international
cooperation aimed at accelerating the reduction of global greenhouse gas emissions and
addressing the issue of adaptation to the adverse effects of climate change” [21].
2. Review of the Literature
Integrated systems, combining heat pumps with photovoltaic installations, are becom-
ing a key element of sustainable development strategies in modern ecological construction.
The introduction of such systems not only allows for the energy self-sufficiency of buildings
but also significantly reduces their impact on the natural environment [
7
,
9
,
22
]. For instance,
studies [
7
] have demonstrated that in appropriately designed buildings utilizing both heat
pumps and photovoltaic panels, it is possible to achieve up to an 80% reduction in carbon
dioxide emissions compared to using traditional heating methods. Such integrated ap-
proaches to energy management in residential buildings are becoming a model to follow in
designing future architectural projects, promoting technologies that are both economically
viable and environmentally friendly.
The integration of heat pumps and photovoltaic panels into building systems rep-
resents a forward-looking direction that is already gaining importance today due to its
potential benefits for both users and the planet. By utilizing renewable energy sources and
maximizing operational efficiency, it is possible to significantly reduce maintenance costs
and substantially limit the negative impact of human activities on climate change.
Research conducted in various climatic regions, such as the northern United States and
Canada, shows that the combination of PV systems with heat pumps can be economically
viable, offering returns on investment of up to 2.7%, while also increasing the energy self-
sufficiency of residential buildings [
13
,
23
]. In study [
13
], it was noted that integrating these
systems into cooler climates not only improves energy efficiency but also provides econom-
ically beneficial solutions that can significantly exceed initial investment expectations.
Analyses [
19
] have highlighted the possibilities of optimizing energy consumption
through the smart control of thermal systems combined with PV installations. A developed
heuristic model for planning the operation of heat pump systems integrated with PV
installations allows for a significant increase in the efficiency of using energy produced by
solar panels, thereby reducing dependence on the power grid and operational costs [
19
].
The results of these studies indicate that properly programmed systems can automatically
adjust their operation to current energy demand and weather conditions, thereby increasing
the level of energy self-sufficiency.
Additionally, research [
20
] has shown that hybridization of various energy-generation
technologies, including heat pumps and PV panels, can lead to even greater optimization
and energy efficiency. Technical-economic assessments of various hybrid schemes for solar
and gas power plants have demonstrated that such an approach can significantly reduce
CO
2
emissions and other associated operational costs, while simultaneously increasing the
reliability of the energy system.
These results, illustrating the synergistic combination of modern technologies, em-
phasize the importance of integrating heat pump systems with photovoltaics as a key
component in designing sustainable, energy-independent, and economically viable residen-
tial homes. Focusing on these aspects could lead to a revolution in residential construction,
where every home is capable of independently managing its energy needs, thereby mini-
mizing its impact on the natural environment.
In Eastern Europe, where renewable energy is gaining importance in the context of
energy independence and the implementation of the Paris Agreement objectives [
19
,
24
],
systems combining heat pumps and PV installations also demonstrate significant potential
for increasing energy efficiency. The role of renewable energy sources in shaping the energy
policies of the region is becoming increasingly important, and investments in green energy
bring tangible benefits, both ecological and economic. Studies conducted in Krakow show
how managing the operating time of heat pumps can significantly increase the energy
Appl. Sci. 2024,14, 5575 3 of 14
self-sufficiency of homes, achieving up to 18% in monthly savings [
14
,
25
]. This analysis
proves that intelligent energy-management systems can effectively adjust the operation of
devices to the current needs of users and weather conditions, resulting in reduced demand
for energy from external sources.
Across the European Union, with political support and subsidies, PV installations
and heat pump systems are increasingly used in new construction projects, supporting
the achievement of ambitious goals to reduce greenhouse gas emissions by 55% by 2030,
compared to 1990 levels [
26
,
27
]. This reflects a broader trend towards increasing energy
efficiency and sustainable development. The dynamic development of PV technology in
Europe, supported by favorable regulatory and financial frameworks, favors the faster
adaptation of these technologies in residential and commercial projects [26]. Additionally,
the increased availability and economic attractiveness of heat pumps, as well as advance-
ments in photovoltaic technologies, open new possibilities for designers and engineers who
are looking for effective ways to integrate these systems to maximize energy self-sufficiency
while minimizing the negative impact on the environment.
Simultaneously, the engagement of European institutions in promoting and supporting
energy-sustainable solutions indicates a growing awareness of the importance of green
energy in achieving not only climate goals but also in improving the quality of life of
residents. Through appropriate policies and investments, it becomes possible to shape a
future where sustainable construction and energy management become the standard, not
the exception.
According to 2023 data [
28
], 2.64 million heat pumps were sold in Europe. This
increase in sales indicates a growing market interest in solutions that enhance the energy
efficiency of buildings. Additionally, there is a steady decrease in the prices of photovoltaic
panels, which results from both technological progress and increased production scale.
These changes are supported by the development of new technologies that allow for the
production of higher efficiency panels at lower material costs [29].
This trend is crucial because lower investment costs can contribute to faster devel-
opment and broader implementation of these technologies in residential homes. This
paradigm shift aims not only to increase energy efficiency but also to improve the energy
self-sufficiency of residential buildings, which is significant in the context of increasing
energy price volatility. The reduction in initial costs allows for faster investment payback,
making such installations more accessible to a wider range of consumers.
Moreover, the increased efficiency of these systems contributes to lower energy bills for
end-users, which further motivates their adoption. In response to these trends, the energy
policies of many European countries are increasingly focusing on supporting renewable
energy initiatives, which include not only subsidies and tax incentives but also educational
programs aimed at raising awareness about the benefits of using modern heating and
energy production systems [29].
These market and regulatory changes demonstrate how dynamically the renewable
energy sector is developing, adapting to new economic and environmental challenges. This
represents an important step towards building a sustainable energy future on the European
continent. These initiatives, by increasing the accessibility of eco-friendly technologies,
have the potential to significantly contribute to achieving global climate goals, reducing
dependency on fossil fuels, and minimizing the adverse effects of climate change.
The aim of this paper is to examine the energy efficiency of combining heat pumps
and photovoltaic panels in environmentally friendly buildings. These systems, increasingly
used in modern construction, offer the possibility of significantly reducing dependency
on conventional energy sources and reducing carbon footprints. A variant analysis of
the energy balance of a heat pump operating in a monoenergetic system with an electric
heater and in a bivalent system with a gas boiler will be conducted. Both cases will be
considered as systems powered by electricity from the power grid, or optionally powered
by a photovoltaic installation.
Appl. Sci. 2024,14, 5575 4 of 14
This analysis will include a detailed examination of the energy efficiency of both
systems under various climatic and operational conditions, with particular attention paid
to their ability to integrate with local power networks, as well as the potential benefits of
energy autonomy. The results of this analysis aim not only to increase knowledge about
the efficiency of such systems but also to provide practical guidance for designers and
engineers interested in implementing renewable technologies in modern energy-efficient
construction. Particular emphasis will be placed on identifying best practices in energy
consumption management, which can contribute to the optimization of operational costs
and increase the durability of systems.
Additionally, this work aims to highlight the impact of these solutions on reducing
CO
2
emissions, analyzing data from actual installations and simulation modeling. By
comparing emissions associated with traditional heating methods with those achievable
using a combination of heat pumps and photovoltaic panels, this paper will contribute to the
discussion on the best strategies for achieving goals related to green energy transformation.
Considering the detailed discussion presented in the article, the structure of the work
is as follows: Section 3provides a comprehensive description of the experimental setup and
methodologies used to assess the synergy between heat pumps and photovoltaic panels,
focusing on a single-family building located in a temperate climate zone in Eastern Europe.
Section 4analyzes the outcomes of the extensive analysis of the building’s energy demand
over the year, including monthly and annual heat demand values, variations in the heat
pump system’s coefficient of performance relative to external temperatures, the economic
implications of operating the system, and an assessment of CO
2
emissions from various
heating system configurations. Potential reductions achievable through the integration
of PV panels are also discussed. Section 5synthesizes findings from the empirical data,
offering conclusions about the practical and environmental benefits of integrating heat
pumps with photovoltaic panels in residential buildings, and discusses implications for
energy policy, emphasizing the need for ongoing investments in renewable technologies
and the integration of more efficient heating solutions to effectively combat climate change.
3. Materials and Methods
The analysis focused on a single-family building located in a climate zone with a
temperate climate in Eastern Europe. Characteristic of this zone are cold winters and warm
summers, which directly affect the energy demand for heating and cooling. The building is
designed for four occupants, equipped with a domestic hot water system (DWS) featuring
a storage tank with a capacity of 150 L. Water in the tank is heated from 10
◦
C to 60
◦
C. The
total hourly heat demand for DWS purposes, considering water heating in the tank over
8 nighttime hours
, is 1.1 kW. Additionally, the central heating system (CHS) is designed to
operate in external temperatures down to
−
20
◦
C, with an hourly heat demand of 8 kW.
The total heat demand of the building is 10.2 kW.
The mechanical ventilation system (MVS) in the analyzed building, equipped with a
rotary heat exchanger with a capacity of 300 m
3
/h, is crucial for maintaining high indoor
air quality and energy efficiency. The heat recovery efficiency in the analyzed system is
73%, which corresponds to an energy demand for heating ventilation air at the level of
1.1 kW. Energy requirements for cooling during the summer period were not analyzed.
Figure 1illustrates the schematic of the analyzed baseline system. It is assumed that
the air source heat pump (AHP) will operate with an electric heater in a monoenergetic
system to ensure continuous heat supply for the building. The electric heater activates
when the coefficient of performance (COP) for heating drops below 2.8. This value was
selected as the minimum due to the economic efficiency of the AHP operation, given
current electricity prices.
Appl. Sci. 2024,14, 5575 5 of 14
Appl. Sci. 2024, 14, 5575 5 of 14
selected as the minimum due to the economic efficiency of the AHP operation, given cur-
rent electricity prices.
Figure 1. Diagram of the cooperative system involving an air source heat pump (AHP) with an
electric heater in a monoenergetic system. The components are labeled as follows: 1—building in-
stallations; 2—air source heat pump (AHP); 3—electrical grid; 4—electric heater (EH); 5—electric
power for the heat pump (NAPH); 6—electric power for the electric heater (EHM). This layout il-
lustrates the flow of energy and the integration of components necessary to maintain efficient heat-
ing within the building.
To perform the calculations, it was necessary to determine the parameters of the air
source heat pump (AHP). For this purpose, data from the existing market model HPA-O
10 Premium by Stiebel Eltron was used. Table 1 shows the technical data of the heat pump.
Table 1. Technical data of the heat pump.
Energy efficiency class of the heat pump W35 A+++
Heating power at A7/W35 (EN 14511 [30]) 7.84 kW
Heating power at A2/W35 (EN 14511 [30]) 8.33 kW
Heating power at A-7/W35 (EN 14511 [30]) 9.54 kW
Coefficient of Performance at A7/W35 (EN 14511 [30]) 5.09
Coefficient of Performance at A2/W35 (EN 14511 [30]) 4.14
Coefficient of Performance at A-7/W35 (EN 14511 [30]) 3.26
SCOP 35 °C (EN 14825 [31]) 4.70
Sound power level (EN 12102 [32]) 55 dB (A)
Limit of use of lower source min./max. −20/40 °C
Limit of use on the heating side max. 65 °C
Refrigeran
t
R410A
The Coefficient of Performance (COP) at the working point A2/W35 is 3.26, whereas
at A7/W35 it is 4.14. Based on this information, interpolation of the COP values depending
on the external temperature was conducted using a linear equation:
𝐶𝑂𝑃
𝐴
∙𝑇
𝐵 (1)
where:
A—constant of the equation, A = 0.176,
B—constant of the equation, B = 2.908,
T
z
—value of the external temperature, °C.
The bivalent temperature, at which the additional electric heater in the heat pump is
activated, is −0.6 °C. Below this temperature, the COP value drops below 2.8. In such a
situation, the AHP begins to operate in a parallel system with the electric heater until a
COP of 2.0 is reached, below which operation is limited only to the electric heater.
Figure 1. Diagram of the cooperative system involving an air source heat pump (AHP) with an electric
heater in a monoenergetic system. The components are labeled as follows: 1—building installations;
2—air source heat pump (AHP); 3—electrical grid; 4—electric heater (EH); 5—electric power for the
heat pump (NAPH); 6—electric power for the electric heater (EHM). This layout illustrates the flow of
energy and the integration of components necessary to maintain efficient heating within the building.
To perform the calculations, it was necessary to determine the parameters of the air
source heat pump (AHP). For this purpose, data from the existing market model HPA-O 10
Premium by Stiebel Eltron was used. Table 1shows the technical data of the heat pump.
Table 1. Technical data of the heat pump.
Energy efficiency class of the heat pump W35 A+++
Heating power at A7/W35 (EN 14511 [30]) 7.84 kW
Heating power at A2/W35 (EN 14511 [30]) 8.33 kW
Heating power at A-7/W35 (EN 14511 [30]) 9.54 kW
Coefficient of Performance at A7/W35 (EN 14511 [30]) 5.09
Coefficient of Performance at A2/W35 (EN 14511 [30]) 4.14
Coefficient of Performance at A-7/W35 (EN 14511 [30]) 3.26
SCOP 35 ◦C (EN 14825 [31]) 4.70
Sound power level (EN 12102 [32]) 55 dB (A)
Limit of use of lower source min./max. −20/40 ◦C
Limit of use on the heating side max. 65 ◦C
Refrigerant R410A
The Coefficient of Performance (COP) at the working point A2/W35 is 3.26, whereas
at A7/W35 it is 4.14. Based on this information, interpolation of the COP values depending
on the external temperature was conducted using a linear equation:
COP =A·TZ+B(1)
where:
A—constant of the equation, A= 0.176,
B—constant of the equation, B= 2.908,
Tz—value of the external temperature, ◦C.
The bivalent temperature, at which the additional electric heater in the heat pump is
activated, is
−
0.6
◦
C. Below this temperature, the COP value drops below 2.8. In such a
situation, the AHP begins to operate in a parallel system with the electric heater until a
COP of 2.0 is reached, below which operation is limited only to the electric heater.
Appl. Sci. 2024,14, 5575 6 of 14
4. Results
The calculation results have been divided into three groups for better visualization
of the problem. The annual heat demand for the various systems in the building was
determined based on statistical data on the occurrence of temperature values at different
hours throughout the year. The data were obtained from a meteorological station located in
Poland in Jasionka, near the main city of the Podkarpackie region, Rzeszów. Such data are
available on a government website [
33
]. It is assumed that the CHS system will not operate
during the summer period due to high external air temperatures. It should be noted that
similar calculations can be made by entering data into a spreadsheet created by the authors
for any location in the world.
4.1. Heat Demand
Table 2presents the monthly heat demand values for various systems in the build-
ing. As the data show, the highest heat demand occurs in the winter months (January,
February, December), which is typical for a continental temperate climate characterized by
cold winters.
Table 2. Monthly heat demand values for the building.
Month QCHS QDWS QMVS Qtotal
kWh/month kWh/month kWh/month kWh/month
1 3656 271 503 4430
2 2644 245 364 3253
3 2834 271 390 3495
4 1708 263 235 2206
5 0 271 149 420
6 0 263 75 338
7 0 271 62 333
8 0 271 59 330
9 0 263 111 373
10 1953 271 269 2493
11 2588 263 356 3206
12 3159 271 435 3865
Qannual, kWh/a 18,541 3194 3009 24,743
Qannual, GJ/a 66.8 11.5 10.8 89.1
The total annual heat demand for the analyzed building, Q
total
, is 24,743 kWh. The
seasonal coefficient of performance for heating, SCOP, in this case, is 4.19.
4.2. Coefficient of Performance (COP)
Figure 2shows the variation of the COP (Coefficient of Performance) with external
temperature over the analyzed statistical year. The highest values are achieved during the
summer months, when the external temperature reaches its peak for the year.
Appl. Sci. 2024,14, 5575 7 of 14
Appl. Sci. 2024, 14, 5575 7 of 14
Figure 2. Change in the heat pump’s COP (Coefficient of Performance) value depending on the ex-
ternal temperature.
4.3. Cost Analysis
Based on the data provided, the costs associated with the heat demand for the build-
ing were calculated using an average price of 0.25 Euro per kWh of electricity. Table 3 lists
the monthly operating costs of the heat pump related to the electricity demand for the
heat pump compressor (NAHP) and the costs associated with activating the electric heater
(EHM) during periods when the heat pump achieves low COP values.
Table 3. Monthly electricity demand for the NAHP compressor and the EHM electric heater, and
the percentage of electricity demand covered by the EHP electric heater.
Month NAHP EHM EHM + NAHP EHP
kWh/m kWh/m Euro/m %
1 633 3163 949 83.3
2 1030 34 266 3.2
3 1082 19 275 1.8
4 570 0 142 0.0
5 414 0 104 0.0
6 222 0 55 0.0
7 186 0 47 0.0
8 174 0 44 0.0
9 303 0 76 0.0
10 754 0 189 0.0
11 959 76 259 7.4
12 1001 1169 542 53.9
Total 7328
(26.4) *
4462
(16.1) * 1947 149.6
* GJ/a.
EHP represents the percentage of the electric heater’s demand in the total electricity
demand. For the entire year, the power of the electric heater accounts for 12.5% of the total
electricity demand for the discussed system. The annual costs associated with providing
heat for the building for the aforementioned purposes amount to 2947 Euro/a. This is a
relatively high cost due to the low external air temperatures from October to January, and
the high price of electricity. Therefore, it was decided that an alternative bivalent system
Figure 2. Change in the heat pump’s COP (Coefficient of Performance) value depending on the
external temperature.
4.3. Cost Analysis
Based on the data provided, the costs associated with the heat demand for the building
were calculated using an average price of 0.25 Euro per kWh of electricity. Table 3lists
the monthly operating costs of the heat pump related to the electricity demand for the
heat pump compressor (NAHP) and the costs associated with activating the electric heater
(EHM) during periods when the heat pump achieves low COP values.
Table 3. Monthly electricity demand for the NAHP compressor and the EHM electric heater, and the
percentage of electricity demand covered by the EHP electric heater.
Month NAHP EHM EHM + NAHP EHP
kWh/m kWh/m Euro/m %
1 633 3163 949 83.3
2 1030 34 266 3.2
3 1082 19 275 1.8
4 570 0 142 0.0
5 414 0 104 0.0
6 222 0 55 0.0
7 186 0 47 0.0
8 174 0 44 0.0
9 303 0 76 0.0
10 754 0 189 0.0
11 959 76 259 7.4
12 1001 1169 542 53.9
Total 7328
(26.4) *
4462
(16.1) * 1947 149.6
* GJ/a.
EHP represents the percentage of the electric heater’s demand in the total electricity
demand. For the entire year, the power of the electric heater accounts for 12.5% of the total
electricity demand for the discussed system. The annual costs associated with providing
heat for the building for the aforementioned purposes amount to 2947 Euro/a. This is a
relatively high cost due to the low external air temperatures from October to January, and
Appl. Sci. 2024,14, 5575 8 of 14
the high price of electricity. Therefore, it was decided that an alternative bivalent system
where the AHP works in conjunction with a gas boiler (GB) would be analyzed. Figure 3
shows the diagram of the aforementioned system.
Appl. Sci. 2024, 14, 5575 8 of 14
where the AHP works in conjunction with a gas boiler (GB) would be analyzed. Figure 3
shows the diagram of the aforementioned system.
Figure 3. Diagram of the cooperative system between the heat pump (AHP) and the gas boiler (GB)
in a bivalent system, where: 1—building installations; 2—heat pump (AHP); 3—electrical grid; 4—
gas boiler (GB); 5—electric power for the heat pump (NAPH).
The price of 1 kWh of gas is assumed to be 0.11 Euro in Poland, but in Ukraine for
example, only 0.065 Euro. This seems to be a more favorable solution compared to the
monoenergetic variant operating in a parallel system due to the lower price per kWh of
gas compared to electricity. Table 4 presents the calculation results, where the heat de-
mand obtained from the AHP and the gas boiler GB is determined. The AHP operates up
to a COP of 2.8; below a COP of 2.8, the system provides heat solely from the GB due to
the lower cost of gas compared to the price of electricity needed for the NAHP compres-
sor.
Table 4. Summary of monthly heat demand supplied by the gas fuel (GB), electricity demand for
the heat pump compressor (NAHP), the cost of electricity for the heat pump (NAHPM), and the
cost of gas fuel (GBM).
Month Qtotal NAHP GB NAHPM GBM NAHPM + GBM
kWh/m kWh/m kWh/m Euro/m Euro/m Euro/m
1 4430 633 3163 158 348 506
2 3253 1030 34 257 4 261
3 3495 1082 19 271 0 271
4 2206 570 0 142 0 142
5 420 414 0 104 0 104
6 338 222 0 55 0 55
7 333 186 0 47 0 47
8 330 174 0 44 0 44
9 373 303 0 76 0 76
10 2493 754 0 189 0 189
11 3206 959 76 240 8 248
12 3865 1001 1169 250 129 379
Total 24,743
(89.1) *
7328
(26.4) *
4462
(16.1)* 1832 489 2 321
* GJ/a.
The total annual cost associated with the electricity for AHPM and the gas fuel for
GBM amounts to 2321 Euro/a, which is 21.3% lower than the monovalent heat pump sys-
tem (AHP) with an electric heater (EH). This indicates that the cooperation between AHP
and GB is more cost-effective. Further in the analysis, the CO2 emissions resulting from
the operation of the analyzed heat pump installation variants were determined.
Figure 3. Diagram of the cooperative system between the heat pump (AHP) and the gas boiler (GB) in
a bivalent system, where: 1—building installations; 2—heat pump (AHP); 3—electrical grid; 4—gas
boiler (GB); 5—electric power for the heat pump (NAPH).
The price of 1 kWh of gas is assumed to be 0.11 Euro in Poland, but in Ukraine for
example, only 0.065 Euro. This seems to be a more favorable solution compared to the
monoenergetic variant operating in a parallel system due to the lower price per kWh of gas
compared to electricity. Table 4presents the calculation results, where the heat demand
obtained from the AHP and the gas boiler GB is determined. The AHP operates up to a
COP of 2.8; below a COP of 2.8, the system provides heat solely from the GB due to the
lower cost of gas compared to the price of electricity needed for the NAHP compressor.
Table 4. Summary of monthly heat demand supplied by the gas fuel (GB), electricity demand for the
heat pump compressor (NAHP), the cost of electricity for the heat pump (NAHPM), and the cost of
gas fuel (GBM).
Month Qtotal NAHP GB NAHPM GBM NAHPM + GBM
kWh/m kWh/m kWh/m Euro/m Euro/m Euro/m
1 4430 633 3163 158 348 506
2 3253 1030 34 257 4 261
3 3495 1082 19 271 0 271
4 2206 570 0 142 0 142
5 420 414 0 104 0 104
6 338 222 0 55 0 55
7 333 186 0 47 0 47
8 330 174 0 44 0 44
9 373 303 0 76 0 76
10 2493 754 0 189 0 189
11 3206 959 76 240 8 248
12 3865 1001 1169 250 129 379
Total 24,743
(89.1) *
7328
(26.4) *
4462
(16.1) * 1832 489 2 321
* GJ/a.
The total annual cost associated with the electricity for AHPM and the gas fuel for
GBM amounts to 2321 Euro/a, which is 21.3% lower than the monovalent heat pump
system (AHP) with an electric heater (EH). This indicates that the cooperation between
Appl. Sci. 2024,14, 5575 9 of 14
AHP and GB is more cost-effective. Further in the analysis, the CO
2
emissions resulting
from the operation of the analyzed heat pump installation variants were determined.
4.4. CO2Emissions
Unit CO
2
emissions from gas combustion are set at 55.5 kgCO
2
/GJ, while for the
combustion of lignite coal, they are 110.0 kgCO
2
/GJ, reflecting the production of electricity
in power plants fueled by these sources [
34
]. The calculations were performed in accordance
with the guidelines provided by the International Panel of Climate Change for statistical
data for 2021 [
35
]. Two variants for electricity supply are considered: one produced in a
gas-fired power plant, and the other in a coal-fired power plant, labeled respectively as
“gas” and “coal” on Figure 4for the heat pump systems with a gas boiler (AHP + EH), and
a heat pump cooperating with a gas boiler (AHP + GB).
Appl. Sci. 2024, 14, 5575 9 of 14
4.4. CO
2
Emissions
Unit CO
2
emissions from gas combustion are set at 55.5 kgCO
2
/GJ, while for the com-
bustion of lignite coal, they are 110.0 kgCO
2
/GJ, reflecting the production of electricity in
power plants fueled by these sources [34]. The calculations were performed in accordance
with the guidelines provided by the International Panel of Climate Change for statistical
data for 2021 [35]. Two variants for electricity supply are considered: one produced in a
gas-fired power plant, and the other in a coal-fired power plant, labeled respectively as
“gas” and “coal” on Figure 4 for the heat pump systems with a gas boiler (AHP + EH),
and a heat pump cooperating with a gas boiler (AHP + GB).
For the AHP + EH system, all the electricity comes from the power plant, for which
an energy input factor of 3.0 is assumed [34]. For the AHP + GB system, the energy input
factor for electricity from the power plant is also 3.0, but for gas-generated energy locally
produced in the gas boiler, an input factor of 1.3 is assumed [34]. The emission of gas is
the product of energy consumption, the energy input factor, and the unit CO
2
emission.
Figure 4. The annual CO
2
emissions for each installation variant.
Assuming electricity production solely from a coal-fired power plant, the AHP + GB
installation shows about 32.9% lower CO
2
emissions compared to the installation with an
electric heater (EH). When covering the electricity demand from a natural gas-fired power
plant, the difference is not as significant, showing a 21.5% disadvantage for the installation
with the electric heater. As can be seen, the installation with GB generates lower CO
2
emis-
sions in every case.
4.5. System with PV Panels
The main issue concerning CO
2
emissions is the demand for electrical energy, which
reaches 11,790 kWh annually for the installation with an electric heater (AHP + EH), while
for the installation with a gas boiler (AHP + GB), this value is only 62.2% of the above
value, i.e., 7328 kWh. Assuming that the demand for electrical energy would be covered
by an additional photovoltaic (PV) installation designated solely for the purposes of the
heat pump (excluding the demand for electrical energy for building-lighting and other
purposes such as induction stoves, refrigerators, washing machines, dryers, etc.), the size
of the PV installation can be roughly determined, assuming 1 kWp for every 1300 kWh of
demand. Thus, the size of the PV installations for the respective systems can be estimated
at about 15.5 kWp for the AHP + EH system, and about 9.5 kWp for the AHP + GB system.
Figures 5 and 6 show the schematics of the analyzed systems.
Figure 4. The annual CO2emissions for each installation variant.
For the AHP + EH system, all the electricity comes from the power plant, for which
an energy input factor of 3.0 is assumed [
34
]. For the AHP + GB system, the energy input
factor for electricity from the power plant is also 3.0, but for gas-generated energy locally
produced in the gas boiler, an input factor of 1.3 is assumed [
34
]. The emission of gas is the
product of energy consumption, the energy input factor, and the unit CO2emission.
Assuming electricity production solely from a coal-fired power plant, the AHP + GB
installation shows about 32.9% lower CO
2
emissions compared to the installation with an
electric heater (EH). When covering the electricity demand from a natural gas-fired power
plant, the difference is not as significant, showing a 21.5% disadvantage for the installation
with the electric heater. As can be seen, the installation with GB generates lower CO
2
emissions in every case.
4.5. System with PV Panels
The main issue concerning CO
2
emissions is the demand for electrical energy, which
reaches 11,790 kWh annually for the installation with an electric heater (AHP + EH), while
for the installation with a gas boiler (AHP + GB), this value is only 62.2% of the above
value, i.e., 7328 kWh. Assuming that the demand for electrical energy would be covered
by an additional photovoltaic (PV) installation designated solely for the purposes of the
heat pump (excluding the demand for electrical energy for building-lighting and other
purposes such as induction stoves, refrigerators, washing machines, dryers, etc.), the size
of the PV installation can be roughly determined, assuming 1 kWp for every 1300 kWh of
demand. Thus, the size of the PV installations for the respective systems can be estimated
at about 15.5 kWp for the AHP + EH system, and about 9.5 kWp for the AHP + GB system.
Figures 5and 6show the schematics of the analyzed systems.
Appl. Sci. 2024,14, 5575 10 of 14
Appl. Sci. 2024, 14, 5575 10 of 14
Figure 5. Diagram of the heat pump system (AHP) with an electric heater (EH) cooperating with a
photovoltaic installation (PV), where: 1—building installations; 2—heat pump (AHP); 3—electrical
grid; 4—electric heater (EH); 5—inverter; 6—photovoltaic panels (PV); 7—electric power for the heat
pump (NAHP); 8—electric power for the electric heater (EHM).
Figure 6. Diagram of the cooperative system between the heat pump (AHP) and the gas boiler (GB)
in a bivalent system, where: 1—building installations; 2—heat pump (AHP); 3—electrical grid; 4—
gas boiler (GB); 5—inverter; 6—photovoltaic panels (PV); 7—electric power for the heat pump
(NAHP).
The estimated installation costs for these variants are 14,800 Euros for the AHP + EH
system, and 9500 Euros for the AHP + GB system. The installation of a PV system would
eliminate costs associated with electricity charges, which amount to 2947 Euros per year
for the AHP + EH system, and 1832 Euros per year for the AHP + GB system. This means
that the investment in PV panels would pay for itself after about 7.0 years for the variant
with the electric heater. A similar payback period, estimated at about 6 years, is expected
for the installation equipped with the GB gas boiler, as calculated from the cash flows
(Table 5). This indicates that, in both cases, after approximately 6–7 years, the operating
costs of the system will reduce to 0 Euros for the AHP + EH system, which represents a
100% reduction in annual bills, and to 489 Euros for the AHP + GB system, which repre-
sents a 79% reduction in annual bills (remaining costs are associated with the gas fuel
needed for the gas boiler).
Figure 5. Diagram of the heat pump system (AHP) with an electric heater (EH) cooperating with a
photovoltaic installation (PV), where: 1—building installations; 2—heat pump (AHP); 3—electrical
grid; 4—electric heater (EH); 5—inverter; 6—photovoltaic panels (PV); 7—electric power for the heat
pump (NAHP); 8—electric power for the electric heater (EHM).
Appl. Sci. 2024, 14, 5575 10 of 14
Figure 5. Diagram of the heat pump system (AHP) with an electric heater (EH) cooperating with a
photovoltaic installation (PV), where: 1—building installations; 2—heat pump (AHP); 3—electrical
grid; 4—electric heater (EH); 5—inverter; 6—photovoltaic panels (PV); 7—electric power for the heat
pump (NAHP); 8—electric power for the electric heater (EHM).
Figure 6. Diagram of the cooperative system between the heat pump (AHP) and the gas boiler (GB)
in a bivalent system, where: 1—building installations; 2—heat pump (AHP); 3—electrical grid; 4—
gas boiler (GB); 5—inverter; 6—photovoltaic panels (PV); 7—electric power for the heat pump
(NAHP).
The estimated installation costs for these variants are 14,800 Euros for the AHP + EH
system, and 9500 Euros for the AHP + GB system. The installation of a PV system would
eliminate costs associated with electricity charges, which amount to 2947 Euros per year
for the AHP + EH system, and 1832 Euros per year for the AHP + GB system. This means
that the investment in PV panels would pay for itself after about 7.0 years for the variant
with the electric heater. A similar payback period, estimated at about 6 years, is expected
for the installation equipped with the GB gas boiler, as calculated from the cash flows
(Table 5). This indicates that, in both cases, after approximately 6–7 years, the operating
costs of the system will reduce to 0 Euros for the AHP + EH system, which represents a
100% reduction in annual bills, and to 489 Euros for the AHP + GB system, which repre-
sents a 79% reduction in annual bills (remaining costs are associated with the gas fuel
needed for the gas boiler).
Figure 6. Diagram of the cooperative system between the heat pump (AHP) and the gas boiler (GB) in
a bivalent system, where: 1—building installations; 2—heat pump (AHP); 3—electrical grid; 4—gas
boiler (GB); 5—inverter; 6—photovoltaic panels (PV); 7—electric power for the heat pump (NAHP).
The estimated installation costs for these variants are 14,800 Euros for the AHP + EH
system, and 9500 Euros for the AHP + GB system. The installation of a PV system would
eliminate costs associated with electricity charges, which amount to 2947 Euros per year for
the AHP + EH system, and 1832 Euros per year for the AHP + GB system. This means that
the investment in PV panels would pay for itself after about 7.0 years for the variant with
the electric heater. A similar payback period, estimated at about 6 years, is expected for the
installation equipped with the GB gas boiler, as calculated from the cash flows (Table 5).
This indicates that, in both cases, after approximately 6–7 years, the operating costs of
the system will reduce to 0 Euros for the AHP + EH system, which represents a 100%
reduction in annual bills, and to 489 Euros for the AHP + GB system, which represents a
79% reduction in annual bills (remaining costs are associated with the gas fuel needed for
the gas boiler).
After an average operational period of about 15 years for this type of installation, it
is estimated that the final savings amount to €23,516 for the AHP + EH installation, and
€20,668 for the AHP + GB gas boiler installation.
Appl. Sci. 2024,14, 5575 11 of 14
Table 5. Comparison of cash flows in a system with PV panels.
AHP + EH AHP + GB
Year PV Installation
Cost
Yearly Cost
without PV Cash Flow PV Installation
Cost
Yearly Cost
with PV Cash Flow
Euro Euro/a Euro/a Euro Euro/a Euro/a
1 14,800 2947 −17,747 9500 2321 −11,821
2 0 2947 −14,800 0 2321 −9500
3 0 2947 −11,853 0 2321 −7179
4 0 2947 −8905 0 2321 −4859
5 0 2947 −5958 0 2321 −2538
6 0 2947 −3010 0 2321 −218
7 0 2947 −63 0 2321 2103
8 0 2947 2884 0 2321 4424
9 0 2947 5832 0 2321 6744
10 0 2947 8779 0 2321 9065
11 0 2947 11,727 0 2321 11,385
12 0 2947 14,674 0 2321 13,706
5. Conclusions
The analysis conducted reveals a challenging decision regarding the application of a
heat pump system operating in either a monovalent configuration with an electric heater
or in a bivalent configuration with a gas boiler. On the one hand, considering the financial
aspect, the solution with a gas boiler seems to be more favorable, being cheaper to operate.
However, with the use of PV panels, the investment return in a dedicated PV installation
is achieved in 6–7 years, but there still remains an annual fee for gas fuel, which is not
the case with an electric heater installation. The electric heater solution also offers savings
throughout the assumed 15-year lifespan of the installation, amounting to nearly €3000
more in favor of the EH installation. This is a significant aspect of the issue.
On the other hand, an equally important ecological aspect is the release of CO
2
emissions into the atmosphere, which is more favorable with the gas boiler installation,
averaging a reduction of between 21.5 and 29.6% when there is no additional PV installation.
With the implementation of PV panels, there is a reduction in emissions by approximately
88.3% for the AHP + GB installation powered by a coal-fired power plant, and by 79.1%
for a natural gas-fired power plant. In the case of the AHP + EH installation, the use of PV
panels results in a 100% reduction in CO
2
emissions, bringing them down to zero annually.
Taking into account that the total lifespan of the installation is approximately 15 years,
we can estimate the total CO
2
emissions that will occur in the analyzed cases with the use of
PV installations. Since the monovalent AHP + EH installation relies entirely on electricity,
achieving a 100% reduction in CO
2
emissions allows for the elimination of 756 kg CO
2
from the environment over the 15-year period when powered by electricity produced in
a coal-fired power plant. Similarly, for a gas-fired power plant, the avoidable emissions
amount to 381 kg of CO2.
This solution fully supports the goals of the 2030 Agenda, which is aimed at reducing
greenhouse gas emissions into the atmosphere.
In general, the main conclusions from the analysis presented above can be
condensed into:
1. Financial aspects:
•The gas boiler solution is cheaper to operate;
•PV panel investment return in 6–7 years;
•Electric heater (EH) installation has no annual gas fuel fee;
Appl. Sci. 2024,14, 5575 12 of 14
•EH installation saves nearly €3000 more over a 15-year lifespan.
2. Ecological aspects:
•
Gas boiler installation reduces CO
2
emissions by 21.5–29.6% without PV panels;
•
PV panels reduce CO
2
emissions by 88.3% (coal-fired power) and 79.1% (natural
gas-fired power) for AHP + EH.
Comparing these results with other studies, it is evident that the integration of heat
pumps and photovoltaic systems is a widely supported approach for achieving energy
efficiency and sustainability in residential buildings. For instance, studies in the northern
climates of the U.S. and Canada demonstrated that combining PV systems with heat pumps
can offer returns on investment, while also increasing energy self-sufficiency [
13
]. Similarly,
other research [
23
] conducted highlights the optimal sizing of solar-assisted heat pump
systems for residential buildings, emphasizing the significant reduction in CO
2
emissions
and operational costs, further supporting our findings on the benefits of hybrid heat pump
and PV systems.
Author Contributions: Conceptualization, S.R., V.M. and K.N.; methodology, S.R., M.S. and K.N.; soft-
ware, S.R. and K.N.; validation, S.R., S.K. and K.N.; formal analysis, S.R. and V.M.; investigation, S.R.,
M.S. and K.N.; resources, S.R., V.M. and K.N.; data curation, M.S., S.K. and K.N.;
writing—original
draft preparation, S.R., V.M. and K.N.; writing—review and editing, M.S., S.K. and K.N.; visualization,
S.R., S.K. and K.N.; supervision, S.R. and V.M.; project administration, S.R. and M.S. All authors have
read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The original contributions presented in the study are included in the
article, further inquiries can be directed to the corresponding author.
Conflicts of Interest: The authors declare no conflicts of interest.
Nomenclature
A constant of the equation (-)
AHP air source heat pump
B constant of the equation (-)
COP coefficient of performance (-)
MVS mechanical ventilation system
DWS domestic hot water system
CHS central heating system
EH electric heater
EHM electric power for the electric heater (kWh/month)
EHP percentage of electricity demand covered by the electric heater (%)
GB gas boiler
GBM costs of gas fuel (Euro/month)
NAHP electric power for the air heat pump (kWh/month)
NAHPM costs of electricity for the heat pump (Euro/month)
PV photovoltaic system
Qannual annual costs for heat demand (kWh/a)
QDHS heat demand for district heating system (kWh/month)
QDWS heat demand for district water system (kWh/month)
QDVS heat demand for ventilation system (kWh/month)
Qtotal total annual heat demand (kWh/month)
SCOP seasonal coefficient of performance for heating (-)
Tzvalue of the external temperature (◦C)
Appl. Sci. 2024,14, 5575 13 of 14
References
1.
Jåstad, E.O.; Bolkesjø, T.F.; Trømborg, E.; Rørstad, P.K. The Role of Woody Biomass for Reduction of Fossil GHG Emissions in the
Future North European Energy Sector. Appl. Energy 2020,274, 115360. [CrossRef]
2.
Alobaid, F.; Busch, J.-P.; Stroh, A.; Ströhle, J.; Epple, B. Experimental Measurements for Torrefied Biomass Co-Combustion in a
1 MWth Pulverized Coal-Fired Furnace. J. Energy Inst. 2020,93, 833–846. [CrossRef]
3.
Garcia-Fernandez, B.; Omar, O. Sustainable Performance in Public Buildings Supported by Daylighting Technology. Sol. Energy
2023,264, 112068. [CrossRef]
4.
El Sayary, S.; Omar, O. Designing a BIM Energy-Consumption Template to Calculate and Achieve a Net-Zero-Energy House. Sol.
Energy 2021,216, 315–320. [CrossRef]
5.
Rabelo, S.N.; Paulino, T.F.; Machado, L.; Duarte, W.M. Economic Analysis and Design Optimization of a Direct Expansion Solar
Assisted Heat Pump. Sol. Energy 2019,188, 164–174. [CrossRef]
6. Rabczak, S.; Nowak, K. Evaluating the Efficiency of Surface-Based Air Heating Systems. Energies 2024,17, 1252. [CrossRef]
7.
Péan, T.; Costa-Castelló, R.; Fuentes, E.; Salom, J. Experimental Testing of Variable Speed Heat Pump Control Strategies for
Enhancing Energy Flexibility in Buildings. IEEE Access 2019,7, 37071–37087. [CrossRef]
8.
Peña Asensio, A.; Arnaltes Gómez, S.; Rodriguez-Amenedo, J.L. Black-Start Capability of PV Power Plants through a Grid-
Forming Control Based on Reactive Power Synchronization. Int. J. Electr. Power Energy Syst. 2023,146, 108730. [CrossRef]
9.
Long, J.; Xia, K.; Zhong, H.; Lu, H.; Yongga, A. Study on Energy-Saving Operation of a Combined Heating System of Solar Hot
Water and Air Source Heat Pump. Energy Convers. Manag. 2021,229, 113624. [CrossRef]
10.
Dolgun, G.K.; Güler, O.V.; Georgiev, A.G.; Keçeba¸s, A. Experimental Investigation of a Concentrating Bifacial Photo-
voltaic/Thermal Heat Pump System with a Triangular Trough. Energies 2023,16, 649. [CrossRef]
11.
Kut, P.; Pietrucha-Urbanik, K. Most Searched Topics in the Scientific Literature on Failures in Photovoltaic Installations. Energies
2022,15, 8108. [CrossRef]
12.
Licharz, H.; Rösmann, P.; Krommweh, M.S.; Mostafa, E.; Büscher, W. Energy Efficiency of a Heat Pump System: Case Study in
Two Pig Houses. Energies 2020,13, 662. [CrossRef]
13.
Pearce, J.M.; Sommerfeldt, N. Economics of Grid-Tied Solar Photovoltaic Systems Coupled to Heat Pumps: The Case of Northern
Climates of the U.S. and Canada. Energies 2021,14, 834. [CrossRef]
14.
Pater, S. Increasing Energy Self-Consumption in Residential Photovoltaic Systems with Heat Pumps in Poland. Energies 2023,
16, 4003. [CrossRef]
15.
Ghasemipour, S.; Sameti, M.; Sharma, M.K. Annual Comparative Performance of Direct Expansion Solar-Assisted and Air-Source
Heat Pumps for Residential Water Heating. Int. J. Thermofluids 2024,22, 100651. [CrossRef]
16.
Sameti, M.; Syron, E. 100% Renewable Industrial Decarbonization: Optimal Integration of Solar Heat and Photovoltaics. Smart
Energy 2022,8, 100087. [CrossRef]
17.
Verma, V.; Meena, C.S.; Thangavel, S.; Kumar, A.; Choudhary, T.; Dwivedi, G. Ground and Solar Assisted Heat Pump Systems for
Space Heating and Cooling Applications in the Northern Region of India—A Study on Energy and CO
2
Saving Potential. Sustain.
Energy Technol. Assess. 2023,59, 103405. [CrossRef]
18.
Reffat, R.M.; Ezzat, R. Impacts of Design Configurations and Movements of PV Attached to Building Facades on Increasing
Generated Renewable Energy. Sol. Energy 2023,252, 50–71. [CrossRef]
19.
Kemmler, T.; Thomas, B. Design of Heat-Pump Systems for Single- and Multi-Family Houses Using a Heuristic Scheduling for
the Optimization of PV Self-Consumption. Energies 2020,13, 1118. [CrossRef]
20.
Rashid, K.; Safdarnejad, S.M.; Ellingwood, K.; Powell, K.M. Techno-Economic Evaluation of Different Hybridization Schemes for
a Solar Thermal/Gas Power Plant. Energy 2019,181, 91–106. [CrossRef]
21. THE 17 GOALS | Sustainable Development. Available online: https://sdgs.un.org/goals (accessed on 7 June 2024).
22.
Wang, Y.; He, X.; Liu, Q.; Razmjooy, S. Economic and Technical Analysis of an HRES (Hybrid Renewable Energy System)
Comprising Wind, PV, and Fuel Cells Using an Improved Subtraction-Average-Based Optimizer. Heliyon 2024,10, e32712.
[CrossRef]
23.
Franco, A.; Fantozzi, F. Optimal Sizing of Solar-Assisted Heat Pump Systems for Residential Buildings. Buildings 2020,10, 175.
[CrossRef]
24.
Ciuman, P.; Kaczmarczyk, J.; Jastrz˛ebska, M. Simulation Analysis of Heat Pumps Application for the Purposes of the Silesian
Botanical Garden Facilities in Poland. Energies 2023,16, 340. [CrossRef]
25.
Kurz, D.; Nowak, A. Analysis of the Impact of the Level of Self-Consumption of Electricity from a Prosumer Photovoltaic
Installation on Its Profitability under Different Energy Billing Scenarios in Poland. Energies 2023,16, 946. [CrossRef]
26.
Gr˛ebosz-Krawczyk, M.; Zakrzewska-Bielawska, A.; Glinka, B.; Gli ´nska-Newe´s, A. Why Do Consumers Choose Photovoltaic
Panels? Identification of the Factors Influencing Consumers’ Choice Behavior Regarding Photovoltaic Panel Installations. Energies
2021,14, 2674. [CrossRef]
27.
Busu, M.; Nedelcu, A.C. Analyzing the Renewable Energy and CO2 Emission Levels Nexus at an EU Level: A Panel Data
Regression Approach. Processes 2021,9, 130. [CrossRef]
28.
European Heat Pump Association. Market Data-EHPA 2023. Available online: https://www.ehpa.org/market-data/ (accessed
on 24 April 2024).
Appl. Sci. 2024,14, 5575 14 of 14
29.
Ziabina, Y.; Kwilinski, A.; Lyulyov, O.; Pimonenko, T.; Us, Y. Convergence of Energy Policies between the EU and Ukraine under
the Green Deal Policy. Energies 2023,16, 998. [CrossRef]
30. Available online: https://sklep.pkn.pl/pn-en-14511-1-2012e.html (accessed on 24 April 2024).
31. Available online: https://sklep.pkn.pl/pn-en-14825-2014-02e.html (accessed on 24 April 2024).
32. Available online: https://sklep.pkn.pl/pn-en-12102-1-2022- 12e.html (accessed on 24 April 2024).
33.
Data for Energy Calculations of Buildings. Available online: https://www.gov.pl/web/archiwum-inwestycje-rozwoj/dane-do-
obliczen-energetycznych-budynkow (accessed on 24 April 2024).
34.
Available online: https://www.kobize.pl/pl/article/monitorowanie-raportowanie-weryfikacja-emisji/id/318/tabele-wo-i-we
(accessed on 24 April 2024).
35. Available online: https://www.ipcc-nggip.iges.or.jp/public/2006gl/ (accessed on 24 April 2024).
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual
author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to
people or property resulting from any ideas, methods, instructions or products referred to in the content.
Available via license: CC BY 4.0
Content may be subject to copyright.
