# A Comprehensive Review of Life Cycle Assessments for Domestic Heat Pumps: Environmental Footprint and Future Directions

Mona Aridi, Marie-Lise Pannier, Rima Aridi, Thierry Lemenand

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Mona Aridi, Marie-Lise Pannier, Rima Aridi, Thierry Lemenand. A Comprehensive Review of Life Cycle Assessments for Domestic Heat Pumps: Environmental Footprint and Future Directions. Energy and Buildings, 2025, 336, pp.115605. 10.1016/j.enbuild.2025.115605. hal-05007786

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# A Comprehensive Review of Life Cycle Assessments for Domestic Heat Pumps: Environmental Footprint and Future Directions

Mona Aridi, Marie-Lise Pannier, Rima Aridi, Thierry Lemenand*

Univ Angers, LARIS, SFR MATHSTIC, F-49000 Angers, France62 avenue Notre Dame du Lac, 49000 Angers, France

## ARTICLE INFO

**Keywords:**

Heat pumps

Life Cycle Assessment

Review

Energy-efficient

Environmental impact

## ABSTRACT

With the growing need for economical and energy-efficient space heating solutions in residential buildings, heat pumps have emerged as a promising alternative for domestic heating systems. Heat pumps are a key piece of equipment in the global shift to a safe and sustainable heating technology. This review presents a thorough analysis and synthesis of Life Cycle Assessment (LCA) on domestic heat pumps and commences with the basic ideas behind heat pump technology as well as the need for LCAs in determining the technology's environmental impact. This review systematically addresses several aspects, it (i) examines the existing literature on LCAs conducted for domestic heat pumps, aiming to assess their environmental effect across their life cycle stages; (ii) discusses the key environmental indicators and parameters commonly assessed in LCAs for domestic heat pumps; (iii) states the challenges, limitations, and restrictions in conducting LCAs for these systems. Due to the missing data and inconsistency of relevant parameters such as functional units, refrigerants, electricity mix, and others, this investigation highlights the necessity for standardized methodologies and data quality refinements, which should account for electricity mix variability, refrigerant leakage, and relevant performance metrics. Conducting a comprehensive review of LCA studies over the last two decades offers this study a novelty in identifying inconsistencies in key assessment criteria and proposes a structured approach to standardizing LCA methodologies for heat pumps. This offers recommendations for future research directions to support the development of sustainable heating technologies in residential structures. In conclusion, the LCA conducted on heat pumps reveals valuable insights into their environmental impact across various stages. Moreover, the findings of this LCA contribute to the broader discussion on eco-friendly and energy-efficient residential heating solutions.

## 1. Introduction

Domestic heat pumps have emerged as a pivotal technology in the quest for sustainable and energy-efficient heating solutions. As the global emphasis on reducing carbon emissions intensifies, the adoption of heat pumps in residential settings offers a promising pathway to lower energy consumption and mitigate environmental impacts. These systems operate by transferring heat from external sources – such as air, ground, or water – into homes, providing both heating and cooling with remarkable efficiency.

The urgency to transition towards greener technologies underscores the importance of thoroughly assessing the environmental performance of domestic heat pumps. Life Cycle Assessment (LCA) serves as a comprehensive tool to evaluate the environmental footprint of products and technologies, from raw material extraction to end-of-life disposal. By applying LCA methodologies, stakeholders can gain critical insights

into the sustainability of heat pumps, guiding policy decisions and consumer choices toward more environmentally benign options.

This review aims to consolidate the current understanding of domestic heat pumps through the lens of LCA. By examining the evolution of heat pump technology and LCA methods, and analyzing existing studies on their intersection, this work seeks to highlight the environmental advantages and challenges associated with heat pump deployment. Such an integrative review is essential for fostering advancements in heat pump technology and optimizing its implementation for maximum environmental benefit.

This study offers a novel and comprehensive review of domestic heat pumps through Life Cycle Assessment (LCA), covering the past two decades. The systematic search and evaluation process, leveraging Scopus and Web of Science with a well-defined filtration process, ensures the inclusion of credible and high-quality studies covering all the studies in the domain. It stands out by systematically comparing different heat pump types and key criteria such as refrigerants, COP, energy mixes, and

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: thierry.lemenand@univ-angers.fr (T. Lemenand).</footnote>
Nomenclature

Abbreviations:

<table><tr><td>AC</td><td>Air Conditioning</td></tr><tr><td>ALO</td><td>Agricultural Land Occupation</td></tr><tr><td>ADP</td><td>Abiotic Depletion Potential</td></tr><tr><td>AP</td><td>Acidification Potential</td></tr><tr><td>ASGSHP</td><td>Absorption Solar Ground Source HP</td></tr><tr><td>ASHP</td><td>Air Source Heat Pump</td></tr><tr><td>CB</td><td>Condensing Boiler</td></tr><tr><td>CC</td><td>Climate Change</td></tr><tr><td>CE</td><td>Cancerogenic Effects</td></tr><tr><td>CED</td><td>Cumulative Energy Demand</td></tr><tr><td>CGB</td><td>Condensing Gas Boiler</td></tr><tr><td>CHP</td><td>Cascade Heat Pump</td></tr><tr><td>CML</td><td>Center of Environmental Science</td></tr><tr><td>CNG</td><td>Condensing Natural Gas</td></tr><tr><td>COP</td><td>Coefficient of Performance</td></tr><tr><td>CPT</td><td>Cost Payback Time</td></tr><tr><td>CRR</td><td>CO<sub>2</sub> Reduction Ratio</td></tr><tr><td>EEI</td><td>Energy Efficiency Improvement</td></tr><tr><td>EFW</td><td>Ecotoxicity Freshwater</td></tr><tr><td>EP</td><td>Eutrophication Potential</td></tr><tr><td>EPT</td><td>Energy Payback Time</td></tr><tr><td>EM</td><td>Eutrophication Marine</td></tr><tr><td>Env.</td><td>Environmental</td></tr><tr><td>EQ</td><td>Ecosystem Quality</td></tr><tr><td>EU</td><td>European Countries</td></tr><tr><td>FFD</td><td>Fossil Fuel Depletion</td></tr><tr><td>GAHP</td><td>Gas-driven Absorption Heat Pump</td></tr><tr><td>GB</td><td>Gas Boiler</td></tr><tr><td>GHG</td><td>Greenhouse Gases</td></tr><tr><td>GHP</td><td>Geothermal Heat Pump</td></tr><tr><td>GSHP</td><td>Ground Source Heat Pump</td></tr><tr><td>GWP</td><td>Global Warming Potential</td></tr><tr><td>HH</td><td>Human Health</td></tr><tr><td>HHP</td><td>Hybrid Heat Pump</td></tr><tr><td>HFC</td><td>Hydro Fluorocarbon</td></tr><tr><td>HM</td><td>Heavy Metals</td></tr><tr><td>HP</td><td>Heat Pump</td></tr><tr><td>HT</td><td>Human Toxicity</td></tr><tr><td>HTC</td><td>Human Toxicity Cancer</td></tr><tr><td>HTNC</td><td>Human Toxicity Non-Cancer</td></tr><tr><td>IMPACT</td><td>Integrated Methodology for Pollution and Resource</td></tr></table>

<table><tr><td>ISO</td><td>International Organization for Standardization</td></tr><tr><td>IR</td><td>Ionizing Radiations</td></tr><tr><td>LCA</td><td>Life Cycle Assessment</td></tr><tr><td>LCI</td><td>Life Cycle Inventory</td></tr><tr><td>LCIA</td><td>Life Cycle Impact Assessment</td></tr><tr><td>MAETP</td><td>Marine aquatic ecotoxicity potential</td></tr><tr><td>MRM</td><td>Material Resources: Metals/Minerals</td></tr><tr><td>LU</td><td>Land Use</td></tr><tr><td>NA</td><td>Not Available</td></tr><tr><td>NG</td><td>Natural Gas</td></tr><tr><td>NOx</td><td>Nitrogen Oxides</td></tr><tr><td>ODP</td><td>Ozone Depletion Potential</td></tr><tr><td>PE</td><td>Polyethylene</td></tr><tr><td>PM</td><td>Particulate Matter</td></tr><tr><td>POCP</td><td>Photochemical Ozone Creation Potential</td></tr><tr><td>PV</td><td>Photovoltaic</td></tr><tr><td>PV-GHP</td><td>Photovoltaic Geothermal Heat Pump</td></tr><tr><td>RUMM</td><td>Resources Use Minerals and Metals</td></tr><tr><td>SO<sub>2</sub></td><td>Sulfur dioxide</td></tr><tr><td>SCHP</td><td>Stirling Cycle-based Heat Pump</td></tr><tr><td>SCOP</td><td>Seasonal Coefficient of Performance</td></tr><tr><td>SCOPUS</td><td>Scientific Online Publication System</td></tr><tr><td>SIEC-HP</td><td>Semi-Indirect ceramic Evaporative Cooler HP</td></tr><tr><td>STHP</td><td>Solar Thermal Heat Pump</td></tr><tr><td>TA</td><td>Terrestrial Acidification</td></tr><tr><td>UK</td><td>United Kingdom</td></tr><tr><td>WD</td><td>Water Depletion</td></tr><tr><td>WU</td><td>Water Use</td></tr><tr><td>WSHP</td><td>Water Source Heat Pump</td></tr><tr><td>VGSHP</td><td>Vapor-compression Ground Source HP</td></tr></table>

Units:

<table><tr><td>CO<sub>2</sub>-eq</td><td>Carbon dioxide equivalent</td></tr><tr><td>°C</td><td>Degree Celsius</td></tr><tr><td>GJ</td><td>Gigajoules</td></tr><tr><td>kg</td><td>Kilogram</td></tr><tr><td>kWh</td><td>Kilowatt hour</td></tr><tr><td>m<sup>2</sup></td><td>Squared meter</td></tr><tr><td>MJ</td><td>Megajoules</td></tr><tr><td>Mt</td><td>Megatons</td></tr><tr><td>MWh</td><td>Megawatt hour</td></tr><tr><td>TJ</td><td>Terajoules</td></tr></table>

LCA boundaries, addressing gaps in existing research. The work identifies methodological inconsistencies and proposes actionable recommendations for harmonizing LCA procedures. By consolidating data and highlighting areas for improvement, it provides valuable archival insights for advancing heat pump technology and guiding future research and policy development.

## 2. Literature review

The development and widespread adoption of domestic heat pumps have been shaped by a rich history of technological innovation and environmental awareness. Tracing the trajectory of heat pump technology reveals significant milestones that have enhanced their efficiency, affordability, and applicability in residential contexts. Concurrently, the field of Life Cycle Assessment (LCA) has evolved, offering increasingly sophisticated methodologies to measure and mitigate the environmental impacts of various technologies, including heat pumps. This literature review delves into the advancements in LCA

methods, the historical progression of domestic heat pumps, and the critical importance of integrating these approaches to comprehensively evaluate the sustainability of heat pumps. Through this exploration, the aim is to contextualize the current state of research and identify key areas for future investigation.

### 2.1. Life Cycle assessment

The concept of life cycle assessment (LCA) emerged during the 1960s due to rising concerns about environmental deterioration, particularly regarding restricted resource availability [1]. Since then, an evolution of methods predominantly occurred within university settings contributing to methodological advancement and global cooperation within the scientific realm until the 1990s [2].

Afterward, between 1990 and 2000, a remarkable achievement was recorded in terms of standardization. In this timeframe, LCA gained inclusion in policy papers and legal frameworks. Additionally, numerous prominent life cycle impact assessment techniques, which remain in use
today, stemmed from methodologies developed during this period. These include the CML environmental theme approach, endpoint or damage-focused methods, and the now widely embraced multimedia approach for evaluating potential human and ecological emissions [2,3,4].

Between 2000 and 2010, this era can be defined as a time of elaboration and expansion of applications. As the demand for LCA grew, this period witnessed a notable methodological divergence. The International Organization for Standardization (ISO), while not intending to intricately standardize LCA methods and facing challenges in interpreting certain ISO criteria, led to the emergence of differing approaches. These approaches encompass variations in system boundaries, allocation methods, dynamic LCA, spatially differentiated LCA, risk-based LCA, and economic input-output-based LCA (EIO-LCA), hybrid LCA models, along attributional and consequential LCA [5]. Some of these models may exhibit a strained relationship with the fundamental principles outlined in the ISO standards [2,3,4].

Starting 2010 and during the second decade of the 21st century, the focus of LCA has developed beyond solely environmental impacts to encompass all three dimensions of sustainability –covering aspects related to people, the planet, and prosperity. Furthermore, the scope from primarily examining product-related inquiries is expanded to considering questions at the sector or even economy-wide levels. Additionally, the LCA approach has incorporated factors beyond technological aspects, such as physical constraints (like available resources and land), economic factors, behavioral relations, etc. [6]. This period has witnessed the integration of LCA with other concepts and tools such as Building Information Modelling (BIM) in an attempt to provide real-time environmental impact assessment during the early design stages of buildings and infrastructure projects [1].

Currently and since 2020, the advancements in the LCA methodologies and tools continue to play a crucial role in various sectors and industries as a tool for evaluating the environmental impacts of products, processes, and systems. It is still a fundamental method for assessing the sustainability of products or systems considering their environmental impacts throughout their life cycle, from raw material extraction to end-of-life stage. Moreover, it is subjected to ongoing improvement in terms of methodologies and tools, including the development of databases, software, and modeling techniques, which have enhanced the accuracy, accessibility, and scalability of LCAs. Integration with emerging technologies like artificial intelligence and blockchain is also becoming more prevalent [7]. Interestingly, LCA is increasingly applied in supply chain management, circular economy, energy, and carbon emissions to identify environmental impacts and optimize resource use [8]. It plays an important role in decision-making processes, aiding organizations in evaluating the environmental impact of their choices [9].

## 2.2. Heat pump technology

To ensure clarity throughout this review, we use “heat pump (HP)” when referring to the device itself and “heat pump system” when discussing the complete setup, including its integration with heating and domestic hot water (DHW) applications, as well as its interaction with climate conditions and the energy supply mix.

The performance of a heat pump system depends not only on the efficiency of the heat pump itself but also on several external factors, including the specific application it serves (space heating, DHW production, or a combination of both), local climate conditions (temperature variations, seasonal efficiency), and the energy supply mix (electricity sources and grid emissions intensity). These elements work together dynamically, shaping the system’s efficiency, environmental impact, and overall performance.

The first practical heat pump, developed in the 1870 s by French engineer Charles Tellier, used ammonia to produce industrial ice. Initially used for industrial purposes, heat pumps expanded into

residential heating and cooling in the 1940 s. The invention of the rotary compressor in 1915 by Willis Carrier improved efficiency and reliability [10].

Early to mid-20th century, many efforts were put into developing mechanical refrigeration systems, in which residential heat pumps became commercially available, mainly for air conditioning purposes. Renewed interest in energy efficiency during the 1970 s further promoted heat pump adoption [11]. Late 20th century, the Ground Source Heat Pump (GSHP) and the Air Source Heat Pump (ASHP) gained popularity, where they became more widely adopted, offering a cost-effective and efficient alternative to traditional heating and cooling systems [10].

In the 21st century, the continued advancements in heat pump technology led to increased efficiency, reliability, and adaptability. Since 2010, the integration of renewable energy sources such as solar and geothermal energy into heat pump systems further enhanced their sustainability and reduced their environmental impact. Due to the rapid expansion of urban areas and the corresponding increase in population, there has been a substantial rise in the demand for heat pumps for domestic heating purposes, particularly for both water and space heating [12]. Today, they are widely recognized as efficient and eco-friendly alternatives for both residential and commercial heating and cooling, where ongoing efforts are employed in research and industry to refine heat pump systems [13].

Additionally, variable capacity compressors are a key advancement in heat pump technology, enabling efficient operation under partial loads. Unlike single-speed compressors, they adjust output to match heating or cooling needs, improving energy efficiency, temperature control, and system longevity [14 15]. This technology has been instrumental in enhancing the performance of heat pumps across a wide range of climate conditions, making them more viable for year-round use in regions with significant temperature variations [16].

In addition to GSHP and ASHP, fortunately, many new types of heat pumps have been introduced to the market such as Stirling-based Cycle heat pumps, which operate on a closed-cycle thermodynamic process involving the compression, heating, expansion, and cooling of a working fluid to generate mechanical power. Cascade heat pumps are advanced systems that utilize multiple stages of heat pump technology to achieve higher efficiency and greater temperature differentials than single-stage systems. Hybrid heat pumps combine multiple heat sources, such as an air-source heat pump with a backup heating system like a furnace or boiler, to enhance efficiency and reliability, especially in extreme weather conditions. Absorption heat pumps use a heat source, often natural gas, propane, or solar energy, to generate heat for the system’s operation.

Finally, and most importantly, thermoacoustic heat pumps, an innovative and environmentally friendly technology that utilizes acoustic waves to transfer heat, offering several advantages over conventional systems. Instead of using refrigerants and compressors, they rely on the interaction between high-amplitude sound waves and an inert gas within a sealed resonator to create a temperature gradient, allowing heat to be absorbed from a low-temperature source and rejected at a higher temperature [17]. These systems can achieve high coefficients of performance, especially when utilizing low-grade heat sources like waste heat or solar thermal energy [18]. With minimal moving parts, they offer the potential for extended lifetimes, reduced maintenance, and the ability to be scaled for various applications, from residential to industrial. By eliminating the need for greenhouse gas refrigerants, thermoacoustic heat pumps present a promising and environmentally friendly alternative for efficient heating and cooling solutions [19].

## 2.3. Life Cycle assessment of heat pumps

The refinement of technologies starts with regular ongoing self-assessment to reach the best upgrade. Heat pump technologies offer
<figure><img src="image_3.png" /><figcaption>Fig. 1. Research methodology funnel chart.</figcaption></figure>

environmentally conscious approaches to improve energy efficiency, with the potential to significantly reduce carbon emissions associated with building heating on a large scale [20]. Several studies have focused on either Air Source Heat Pumps (ASHP) or Ground Source Heat Pumps (GSHP) as they encountered a wide distribution globally [21,22,23].

One of the earliest LCA studies applied to a heat pump was published in 2004 by the Environmental Engineering Science Journal. Despite the lack of technical specifications for the LCA study such as the functional unit, the type of refrigerant LCA boundaries, electricity mix, and others this study compared only the heating function of the heat pump against that of a boiler concluding that the heat pumps can be a better alternative in the future [24]. Many studies, compared the performance of domestic heat pumps against each other in an attempt to highlight their drawbacks, seeking an upgrade. Most of those assessments were carried out using the LCA method to quantify the environmental impacts associated with heat pump systems, including energy consumption [25], greenhouse gas emissions [26], resource depletion [27], and potential environmental burdens [28,29].

LCA investigations have various Life Cycle Impact Assessment (LCIA) methodologies such as IMPACT 2002+, CML, Eco-indicator, ReCiPe, and many others, encompassing midpoint or endpoint approaches or even both allowing for a comprehensive evaluation of environmental impacts associated with heat pump systems across different stages of their life cycle. Since 1990, life cycle assessment has been applied in the building industry [30,31,32].

Among the few reviews made on the LCA alone or on heat pumps alone, only Pratiwi addressed the disparities in key aspects of LCA on heat pumps, particularly concerning Geothermal Heat Pumps (GHP), where it was reported that the dissimilarities in the LCIA methods, functional units, and scope of the studies created huge uncertainty and

challenge in the possibility of achieving a fair comparison among the studies reviewed [33].

Another review of the factors affecting the environmental and economic performance of electrically-driven heat pumps concluded that the anticipated project lifespan significantly affects Life Cycle Cost outcomes, often favoring lower initial investment options for shorter durations [34]. Heat Pumps (HPs), especially ground heat exchangers (GHEXs) in GHP systems, frequently exceed their expected operational lifespans of 25–30 years with minimal maintenance [35]. The review deduced that economic analyses typically underestimate HP performance by assuming a conservative 15-year lifespan, despite GHEXs having lifespans of over 30 and 50 years for copper coils and polyethylene pipes, respectively [36]. Consequently, a more advanced approach suggests extending the lifespan to 50 years and accounting for refurbishment costs for less durable components [37].

## 2.4. *Aim of the study*

The aim of this review work is to:

1. Comprehensively assess the environmental performance of domestic heat pumps using the LCA methodology.

2. Appraise the importance of LCA as a crucial method in evaluating the performance of heat pumps,

3. Investigate LCA tools, factors, and metrics in assessing the performance of heat pumps such as the scope of studies (LCA boundaries, functional units, lifetime), Life Cycle Inventory (refrigerants, metrics, database), and Life Cycle Impact Assessment (impact assessment methods, impact categories, midpoint, and endpoint). By
<figure><img src="image_4.png" /><figcaption>Fig. 2. Boxplot for the distribution of years of publication.</figcaption></figure>

synthesizing findings from a range of studies and considering various types of domestic heat pumps, conditions, and metrics,

4. Provide insights into the strengths, limitations, and potential areas for improvement in the environmental sustainability of heat pump systems.

5. Seek to identify gaps in current research and propose recommendations for future studies.

Among the few reviews made on LCA of heat pumps, none of them considered the domestic heat pumps broadly and systematically, the focus was only one type like GHP or one aspect like cost assessment. The novelty of this study lies in:

1. Performing a comprehensive comparison of journal articles published over the last two decades that study the performance of domestic heat pumps using the Life Cycle Assessment method.

2. Carrying out the comparison from different aspects of refrigerant, functional unit, Coefficient of Performance, Seasonal Coefficient of Performance, electricity mix, LCA boundaries, and sensitivity analysis.

3. Addressing multi-gap issues among the considered criteria.

4. Proposing a creative action plan for future work including the development of standardized validated guidelines for LCA methods applied on domestic heat pumps.

# 3. Methodology of LCA studies

## 3.1. Methodology of research: Search strategy and study evaluation

This review aims to evaluate the environmental impacts of domestic heat pumps by presenting a good understanding of their LCA. To do so, an electronic search was conducted across the following global databases Scopus and Web of Science due to their high scientific standards and extensive coverage of papers. Based on the research question, a set of key terms was formulated and defined to initiate the research. Then, an organized, phased search approach was used to filter out the non-relevant articles.

The screening process encountered a set of criteria in which the filtration was achieved, starting with the publication date to focus on articles published between 2003 and 2023 along with the type of publication including only the research articles and reviews and excluding others such as conference proceedings, book sections, reports, ... After keeping the publications in English language only, the inclusion criteria start to focus on the content by reviewing the titles and the abstracts of the articles to assess their relevance to the topic. Articles with non-relevant abstracts or not aligned with the topic were excluded. Finally, after excluding the non-accessible articles and the duplicates among the two databases, a comprehensive full-text reading was carried out to ensure the credibility of the selected articles through their study design, methodology, and relevant outcomes.

The key terms used in both databases are “Domestic heat pumps” OR “Ground-source heat pumps” OR “Water-source heat pumps” OR “Air-source heat pumps” AND “Life Cycle Assessment” OR “Life Cycle Analysis” OR “LCA”, in which 297 articles were addressed in Scopus and 169 in Web of Science. These articles underwent a systematic path of filtering illustrated in Fig. 1.

Out of the English research articles that evaluated the impact of domestic heat pumps on the environment using LCA methods during the

<table><caption>**Table 1**
Distribution of countries in the studied articles (in percentage).</caption><thead><tr><th>Countries</th><th>European countries</th><th>Asian countries</th><th>Others</th></tr></thead><tbody><tr><td>Percentage (%)</td><td>69.5</td><td>22.2</td><td>8.3</td></tr></tbody></table>

last two decades, 36 articles were chosen and retrieved to be discussed in this review.

It is important to mention that the focus of this research is to assess the completeness of academic work related to the LCA of domestic HP. We have intentionally decided to exclude LCA performed by manufacturer of HP through Environmental Product Declaration, although an ever-increasing number of declarations are available. We acknowledge the systematic approach of performing LCA of HP in Environmental Product Declarations, which is framed by several norms such as ISO 14025 [38], or EN 50693 [39] (in the European context). Rather than assessing the quality and completeness of EPDs, this review aims to systematically assess the LCA of domestic heat pumps based on peer-reviewed studies published in research journals. It provides a more detailed analysis of environmental performance across different methodologies, geographic contexts, and operational conditions. Nevertheless, we acknowledge the importance of these standards in shaping industry practices, and future work could explore the alignment between manufacturers and researchers approaches.

### 3.1.1. Data identification

#### 3.1.1.1. Year of publication.
The 36 articles under study were issued between 2010 and 2023, in which 61 % of them were released in the last four between 2020 and 2023. The box plot of the distribution of years of publication, shown in Fig. 2, is negatively skewed due to the existence of the majority of the values to the right side of the axis which resulted in a skewness to the left.

Hence, due to the negative skewness, it can be noticed that considerable attention has been recently drawn to the assessment of domestic heat pumps using the LCA approach.

#### 3.1.1.2. Country of the case studies.
The majority of the articles (69.5 %) were conducted and carried out in European countries along with their case studies, as shown in Table 1, which reveals a widespread adoption of heat pumps. This common acceptance could be attributed to the governmental incentives and policies issued by most of the European governments, especially after the Paris Agreement in 2016 [40].

#### 3.1.1.3. Classifications of heat pumps in the studies.
17 studies, highlighted in green in Fig. 3, aimed to evaluate the environmental impact of heat pumps versus conventional heating systems such as Condensing natural gas (CNG), biomass, and coal boilers. 5 of them compared the conventional systems with GSHP, 4 of them with ASHP, and 8 of them with other heat pump technologies, as illustrated in Table 6

11 articles, highlighted in yellow in Fig. 3, compared a heat pump technology with another one, in which 6 articles evaluated the environmental performance of GSHP versus ASHP, and 5 studies compared miscellaneous types against other types as presented in Table 7

A group of 7 GSHP underwent LCA assessment without any comparisons, and only one study considered an ASHP alone as demonstrated in Table 8 and highlighted in orange in Fig. 3.
<figure><img src="image_5.png" /><figcaption>Fig. 3. Classifications of the articles as per the heating system under study.</figcaption></figure>

<table><caption>**Table 2**
List of criteria to compare the LCA studies.</caption><thead><tr><th>Scope of the studies</th><th>Life Cycle Inventory (LCI)</th><th>Life Cycle Impact Assessment (LCIA)</th><th>Interpretation</th></tr></thead><tbody><tr><td>LCA boundaries</td><td>Refrigerants</td><td>MethodsImpact</td><td>Application of sensitivity</td></tr><tr><td>Functional unit</td><td>Performance metrics</td><td>categories</td><td>analysis</td></tr><tr><td>LCA type</td><td>Electricity</td><td></td><td></td></tr><tr><td>LifetimeBuilding type</td><td>mixDatabase</td><td></td><td></td></tr></tbody></table>

## 3.2. Criteria to compare the LCA of heat pumps

The aim of these 36 research articles is to comprehensively investigate and evaluate the environmental impact of domestic heat pumps through LCA methodologies. Some studies followed a comparative assessment between different types of heat pumps, some compared heat pumps to conventional heating methods, while others chose to study the performance of domestic heat pumps without comparison. Each study explores the ecological footprint of heat pumps across various contexts, including different countries, climates, energy sources, and technological configurations. By examining factors such as energy efficiency, greenhouse gas emissions, resource depletion, and potential environmental trade-offs, these studies seek to provide valuable insights into the sustainability and environmental performance of heat pump systems. Ultimately, this collective body of research aims to inform policymakers, industry stakeholders, and consumers on the optimal deployment and utilization of domestic heat pumps to minimize environmental impact and advance sustainable energy transition efforts.

In order to compare the different articles, a set of criteria, related to the four LCA steps, have been defined. They are listed in Table 2.

# 4. Results

## 4.1. Results based on criteria to perform LCA

### 4.1.1. Scope of the studies

The scope of an LCA study defines the breadth and depth of the analysis, determining the specific aspects of the system that will be assessed. This includes establishing the purpose of the study, identifying the functional unit, and setting the boundaries of the system under investigation. Defining the scope is crucial as it ensures that the study remains focused, relevant, and manageable. It provides a clear

<table><caption>**Table 3**
Functional units used in the studied articles.</caption><thead><tr><th>Functional unit</th><th>kWh or MWh</th><th>MJ or GJ</th><th>Years</th><th>Per m<sup>2</sup></th><th>°C</th><th>NA</th></tr></thead><tbody><tr><td>Percentage (%)</td><td>53</td><td>11</td><td>19</td><td>3</td><td>3</td><td>11</td></tr></tbody></table>

framework for data collection, impact assessment, and interpretation of results. The following sections outline the boundaries, functional units, LCA type, lifetime, and building type for an LCA study.

#### 4.1.1.1. LCA boundaries.
The LCA for 26 studies was carried out using the “cradle-to-grave” approach, in which they tracked the life cycle of the heat pump from the early stage of extraction of the raw materials – namely the “cradle”, passing by the production stage, then transportation, usage, and finally waste disposal stage – namely the “grave”. It should be noted that even if these studies applied a cradle-to-grave approach, some of the life cycle stages may be disregarded: for instance, only two-thirds of them included transportation in their scope. The other 10 studies were performed using the “cradle-to-gate” approach, where the investigation terminates at the second stage after the manufacturing and processing phase. The LCA boundaries of each study are reported in Table 6, Table 7, and Table 8.

It is worth mentioning that some studies stated their framework clearly by stating their scope literally using the “cradle-to-gate” or “cradle-to-grave” key terms. However, several studies didn’t use any of those key terms to clarify their scope. Hence, the classification of their scope was obtained throughout the structure of their study, by tracking the phases and stages being undertaken.

#### 4.1.1.2. Functional unit.
Significant variations were noted in the choice of functional units across the literature. Approximately 53 % of the studies utilized thermal energy measured in Watt-hours (kWh or MWh) as their functional unit, while 11 % opted for thermal energy expressed in Joules (MJ or GJ), and 19 % employed the lifetime (in years) as their functional unit (as shown in Table 3). Interestingly, 11 % of the studies did not specify their functional unit at all. None of the studies mixed different functional units (e.g. energy over a specific lifetime). It is imperative to harmonize the functional units to enable consistent and comparable analysis in LCA studies. A standardized functional unit ensures uniformity and facilitates comparisons among different case studies. This harmonization would streamline benchmarking processes against best practices, thereby identifying areas for improvement and
<table><caption>**Table 4**
Lifetime of the studied heat pumps.</caption><thead><tr><th>Lifetime</th><th>15 years</th><th>20 years</th><th>Various lifespans</th><th>NA</th></tr></thead><tbody><tr><th>Percentage (%)</th><td>13.8</td><td>44.5</td><td>25</td><td>16.7</td></tr></tbody></table>

<table><caption>**Table 5**
Refrigerants of the heat pumps studied in the articles.</caption><thead><tr><th>Refrigerant</th><th>R134a</th><th>R410a</th><th>R22</th><th>R32</th><th>Ammonia</th><th>NA</th></tr></thead><tbody><tr><th>Percentage (%)</th><td>30.6</td><td>19.5</td><td>5.5</td><td>5.5</td><td>2.7</td><td>36.2</td></tr></tbody></table>

bolstering efforts toward achieving sustainability objectives.

Furthermore, in 64 % of the articles, the heat pump was applied to provide heating and cooling to the building. In the 34-remaining percent, the heat pump provided only heating.

#### 4.1.1.3. LCA type.
The LCA approach was rarely mentioned in the articles. Two articles clearly state to frame in an attributional perspective [41 42], and one article applied both attributional and consequential LCA [43].

#### 4.1.1.4. Lifetime.
While some studies (13.8 %) selected a 15-year operational lifetime, a larger proportion (44.5 %) favored a longer duration of 20 years. Additionally, a significant portion (25 %) chose various lifetimes beyond 20 years, and some studies employed a range of lifetimes (15–20 years). It is worth mentioning that among those 25 %, one study considered 80,000 operating hours as a lifespan. The diversity in chosen lifespans underscores the complexity of determining suitable operational lifetimes. In addition, approximately 16.7 % of studies did not specify the operational lifetime of the heat pumps under review. This incomplete reporting hinders comparison and understanding across studies, Table 4.

In essence, the data highlights the need for standardized reporting and transparent documentation of operational lifetimes in LCA studies to facilitate better comparability and reliability of findings.

#### 4.1.1.5. Building type.
Surprisingly, the authors of the articles studied gave very little information about the building in which the heat pump is installed. Yet the type of building determines the system’s energy consumption. Among the articles providing information about the building typology, the building type was a single-family house in nine articles, an apartment in one article, and a multi-dwelling residence in three articles. Two articles considered slightly different building types: a greenhouse and a dormitory respectively. The building sizes ranged between 90 m2 and 220 m2 in single-family houses and between 1 350 m2 and 22 500 m2 for multi-family residences.

Six articles clearly mentioned that the heat pump was installed in a new building, while the heat pump was installed in the old refurbished building in two articles, and in an already refurbished building in one article. Two articles compared the environmental performances of heat pumps in both new, refurbished, and unrefurbished houses.

The lack of consistent reporting on building types, such as new, existing, or retrofitted buildings represented a key challenge in LCA studies of heat pumps as only a few studies provided this information, making it difficult to assess the impact of building type on heat pump performance. This highlights a gap in the literature, underscoring the need for standardized reporting of building characteristics to enable more comprehensive analyses in future studies.

### 4.1.2. Life Cycle Inventory (LCI) of the studies

The Life Cycle Inventory (LCI) phase involves compiling all relevant data for the system under study. This includes detailed information on inputs and outputs throughout the life cycle stages. The following subsections provide specific data related to refrigerants, performance

metrics, electricity mix, and the database used in this LCA study.

#### 4.1.2.1. Refrigerants.
The specific refrigerant utilized in heat pump systems wasn’t disclosed in every study examined. Thirteen out of the total studies, making up 36 % (as indicated in Table 5), omitted this information. Neglecting to consider the impact of refrigerants on environmental burden constitutes a significant disparity. The refrigerants considered in each study are reported in Table 6, Table 7, and Table 8.

In addition to the general performance and energy consumption metrics, refrigerants used in heat pump systems play a crucial role in determining the overall environmental impact. Different refrigerants have varied Global Warming Potential (GWP), with some contributing significantly to greenhouse gas emissions in the event of leaks during the system’s operation [44]. It is essential to consider refrigerant leakage and its effect on the system’s carbon footprint when conducting an LCA. However, many studies either omit this factor (36.2 %) or report inconsistent data (a total of 63.8 %), leading to challenges in comparing the environmental impact of different heat pump technologies. Therefore, future LCA studies should ensure that refrigerant type and leakage rates are consistently included to provide a more comprehensive evaluation of the system’s true environmental impact.

#### 4.1.2.2. Performance metrics.
27 studies, accounting for 75 %, used the Coefficient of performance (COP) comparison ranging between 3.0 and 5.4 and five of them used the Seasonal Coefficient of performance (SCOP) ranging between 2.8 and 3.4 for different types of heat pumps providing a more realistic assessment of annual energy consumption for varying operating conditions throughout the year. In the same manner, in these studies, there was a deficiency in other performance metrics such as energy efficiency Ratio (EER) and energy efficiency Index (EEI), where only two studies considered the EER as an important measure and just one study referred to EEI as a significant value to be discussed. The performance metrics considered in each study are reported in Table 6, Table 7, and Table 8

Performance metrics may be defined through standards such as EN (European Norms), ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), and DIN (German Institute for Standardization); for instance EN 14511–2 [45] in the European context. They play a crucial role in defining performance metrics, testing procedures, and regulatory compliance for heat pumps. However, these standards primarily focus on technical specifications and system efficiency under standardized conditions that may differ from performances observed in real use conditions. Therefore, careful consideration must be taken concerning the source of the performance values quoted in the studies. The articles reviewed did not explicitly specify whether the performance metrics are derived from normative values or from real-use observations. This distinction is important, but due to the lack of explicit information in the sources, we cannot make a definitive statement on the origin of the performance metrics used.

#### 4.1.2.3. Electricity mix.
Heat pumps consume a significant share of electricity during their long-use phase, particularly in regions with high heating demands. It is thus important to know from where the electricity originates, and the impact of the technology production linked with the different mixes.

Surprisingly, not all the studies mentioned the electricity mix or even the source of electricity used for the studied heat pumps, so it was referred to external resources to specify the electricity mix that was mainly used during the time of those studies. Most of the electricity mixes of the European countries were based on nuclear, hydropower, and photovoltaic plants, like Italy, United Kingdom, Switzerland, and Spain. The renewable resources were used partially up to 40 % in the combination of the electricity mix in Italy, Spain, the United Kingdom, and Sweden. However, coal and lignite were mostly used as a main source of electricity in Poland and Greece. The origin of the electricity
<table><caption>**Table 6**
Heat pumps versus conventional systems.</caption><thead><tr><th>Reference</th><th>Type of HP</th><th>Conventional</th><th>Refrigerant</th><th>Electricity mix</th><th>Impact Categories with the highest contribution</th><th>COP / SCOP results</th><th>LCA boundaries/ FU</th><th>SA</th></tr></thead><tbody><tr><td>Famiglietti, Gerevini, et al., 2021</td><td>GSHP (Italy)</td><td>Boilers<br>Combined heat & power (CHP)</td><td>R134a</td><td>1. Electricity is imported from nuclear plants in France.<br>2. Hydropower plants<br>3. Oil fuel plants.<br>4. Photovoltaic plants</td><td>1. IR, HTNC, HTC, EFW<br>2. WU<br>3. AP, EP EM, ET<br>4. RUMM</td><td>SCOP = 4.0</td><td>Cradle-to-grave / 1 kWh</td><td>No</td></tr><tr><td>Russo et al., 2014</td><td>PV-GHP (Italy)</td><td>LPG-HG (hot air generator supplied by liquefied petroleum gas system)</td><td>R410a</td><td>Electricity produced by solar panels & supplied by the national electricity grid according to the Italian electricity mix</td><td>ODP due to the emissions generated within the production of the gas R410a. For ADP, AP, EP ODP, and POCP, the LGPHG plant has the lowest Env. burden. For ADP & GWP, PV-GHP has a less Env. load.<br>The average Env. impact of the CNG heating system operates 44% of its operating time producing 66% additional Env. impact.</td><td>COP varies daily from 3.6 to 4.0 during colder nights</td><td>Cradle-to-grave / NA</td><td>No</td></tr><tr><td>Abusoglu & Sedeeq, 2013</td><td>GSHP (Turkey)</td><td>Coal boiler & CNG</td><td>R134a</td><td>NG (48.6%), coal (28.3%), hydraulic (18.5%), oil (3.4%), renewable (1.1%)</td><td>Reduction in ADP, GWP, IR.</td><td>COP = 2.38</td><td>Cradle-to-grave / 19 °C during heating season</td><td>No</td></tr><tr><td>Zhang et al., 2022</td><td>GSHP (China)</td><td>Coal-based heating</td><td>NA</td><td>Electricity generation in China</td><td>Low ENV impact of GWP, PM, TA, WU, LU.</td><td>NA</td><td>Cradle-to-gate / Annual heat supply/ m²</td><td>Yes</td></tr><tr><td>Pratiwi & Trutnevyte, 2021a</td><td>Geothermal (Geneva)</td><td>Biomass-based</td><td>R1234ze</td><td>Hydropower</td><td>Reduction in EP, AP, GWP, ADPMost Env. impacts for both heating systems occur during operation.</td><td>NA</td><td>Cradle-to-grave / NA</td><td>No</td></tr><tr><td>Naumann et al., 2022</td><td>ASHP (Germany)</td><td>Condensing gas boiler</td><td>R134a</td><td>German electricity marginal mix</td><td>Electricity use, refrigerant leakage (13.8% of the total), & steel in AC devices (3% of the total) are the most vital contributors.</td><td>NA</td><td>Cradle-to-grave / heating over 20 years</td><td>Yes</td></tr><tr><td>Zheng et al., 2016</td><td>ASHP (China)</td><td>Centralized heating</td><td>NA</td><td>Electricity use accounts for 84% of the total GHG emissions</td><td>ADP, ODP, GWP, HH. respiratory effects caused by inorganic substances and air emissions.</td><td>NA</td><td>Cradle-to-grave / 1 year</td><td>No</td></tr><tr><td>Lozano Miralles et al., 2020</td><td>Air-Air heat pump (Spain)</td><td>Biomass system</td><td>R410a</td><td>Wind, solar, and hydropower mix</td><td>GWP, TE, & HNTC out of 18 impact categories. Most of the highest ENV impacts take place in the use phase, due to the electric energy usage.</td><td>NA</td><td>Cradle-to-grave / NA</td><td>Yes</td></tr><tr><td>Marinelli et al., 2020</td><td>DUAL HP (Italy)</td><td>Conventional air & ground source HP systems</td><td>R410aR134a</td><td>Nonrenewable source of production of electricity</td><td>GWP, HH. The highest CO<sub>2-eq</sub> emissions are released during the usage life cycle stage in both heating systems at 18 883 kg of CO<sub>2-eq</sub>, and 104 CO<sub>2-eq</sub> for the furnace & heat pump, respectively.</td><td>SCOP = 3.40 during the first year and 3.35 after 15 years</td><td>Cradle-to-grave / 1 MJ</td><td>Yes</td></tr><tr><td>Addo-Binney et al., 2022</td><td>Cascade HP (CHP) (Canada)</td><td>Typical natural gas furnace</td><td>R410a</td><td>Ontario (Canada)</td><td>HTC, GWP, EP, FFD: HTC was highest among all categories due to the additional infrastructure requirement, GWP, FFD, and EP, which use NG & electricity in the use phase.</td><td>The CHP is designed to operate at lower temperatures while reaching a higher COP than the natural gas furnace system</td><td>Cradle-to-grave / heating per 1 year</td><td>No</td></tr><tr><td>Mahon et al., 2022</td><td>Waste heat-fed (Ireland)</td><td>Biomass-based heating systems & conventional gas boiler</td><td>Ammonia</td><td>Carbon-intensive coal or gas power plants</td><td>HH, CC, EQ. Fossil fuel systems have a higher impact on the Env. They had a higher impact on HH compared to the SCHP, due to the air emissions & toxicity</td><td>NA</td><td>Cradle-to-gate / 1 kWh of heat produced.</td><td>Yes</td></tr><tr><td>Khan et al., 2021</td><td>Stirling-cycle-based (Sweden)</td><td>Oil-based boiler, bio-oil boiler, NG boiler, & biogas boiler</td><td>Not clear</td><td>Waste heat & electricity</td><td></td><td>COP = 2.0 (no discussion or interpretation was drawn).</td><td>Cradle-to-grave / NA</td><td>No</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 6 (continued)**</caption><thead><tr><th>Reference</th><th>Type of HP</th><th>Conventional</th><th>Refrigerant</th><th>Electricity mix</th><th>Impact Categories with the highest contribution</th><th>COP / SCOP results</th><th>LCA boundaries/ FU</th><th>SA</th></tr></thead><tbody><tr><td>Khan et al., 2020</td><td>Stirling (Sweden)</td><td>Gas and oil-fired boilers</td><td>NA</td><td>Renewable electricity</td><td>associated with the fuel combustion.<br>GWP, HH, CC.<br>The GWP after 15 years of use is near – 5000 kg CO<sub>2</sub>.<br>eq.<br>HH, WD, MD, out of 14 IC.</td><td>NA</td><td>Cradle-to-grave / heating per year</td><td>No</td></tr><tr><td>Lin et al., 2021</td><td>Hybrid HP (HHP)(UK)</td><td>Condensing Gas Boiler</td><td>R410aR134a</td><td>33.9 % gas, 24.8 % wind, 13.9 % nuclear, 7.2 % biomass, 6.4 % coal, and 5.7 % solar PV</td><td>HHP potentially saves 30 % of GHG emissions as compared to the CGB.<br>HHP shows 13 % to 48 % emission reduction compared to CGB in TA, POCP, PM, &amp; FFD.</td><td>NA</td><td>Cradle-to-grave / kWh per 20 years</td><td>Yes</td></tr><tr><td>Famiglietti, Toppi, et al., 2021</td><td>Gas-driven absorption GAHP(EU)</td><td>Condensing boiler (CB)</td><td>NA</td><td>Gas combustion</td><td>WU due to the nuclear &amp; hydropower plants CC, ODP, and HT.<br>A reduction of 27 % was found for CO<sub>2-eq</sub> and 25 % for fossil resource consumption.</td><td>SCOP 1.33 for average weather, 1.27 for cold, and 1.40 for warm: 10 % decrease from warm to cold</td><td>Cradle-to-grave / 1 kWh</td><td>Yes</td></tr><tr><td>Rey Martínez et al., 2011</td><td>Split Class HP type (Spain)</td><td>Semi-indirect ceramic evaporative cooler</td><td>R410a</td><td>Electricity consumption is the highest ENV burden in Spain</td><td>IR, CC, ADPThe SIEC-HP is ENV &amp; economically more profitable than the HP.</td><td>COP (Bilbao) = 0.153 T-2.5811COP (Valladolid) = 0.3526 T-7.66</td><td>Cradle-to-grave / not clear</td><td>Yes</td></tr><tr><td>Riva et al., 2021</td><td>Water-Water with PV panel (Italy)</td><td>Natural gas boilers &amp; Air conditioning split units</td><td>R134a</td><td>PV for HP &amp; electricity. A mix of 40 % renewable energy &amp; biofuels &amp; 44 % NG</td><td>The four PV systems have a reduced impact by 57 % in GWP, compared to the conventional heat pump.</td><td>COP = 3.75 best techno-economic performance.</td><td>Cradle-to-grave / 1 kWh</td><td>Yes</td></tr></tbody></table>

mix for each study is reported in Table 6, Table 7, and Table 8 for each study.

Besides the origin of the type of electricity mix, one article considered a dynamic mix, i.e. a mix changing over time because of short-term variations of the electricity demand (daily, weekly, and seasonal variation of consumption) [46]. This allows for more realistic modeling of the variation of environmental impacts over time. In addition, six articles considered a long-term evolution of the technologies providing electricity (e.g. increase in the share of renewable over decades). Two of them are framed in a prospective approach [47,48], while the four remaining apply scenario analysis to test the effect of different electricity mixes.

#### 4.1.2.4. Database.
The analysis of database utilization across the 36 studies on domestic heat pumps, presented in Fig. 4, evaluated through LCA reveals intriguing insights into research practices. Affected by the scope of the study, geographic region, availability of data, and the specific needs of the analysis, the majority of studies, comprising 55.6%, leaned towards employing the “eco-invent” database, showcasing a preference for a widely recognized and established resource. However, the version of eco-invent was mentioned in only 13 of the 20 articles using this database. Additionally, a notable 25 % of studies opted for alternative databases, showcasing the diverse range of sources available for LCA analysis, including the Chinese dataset inventory, the German software tool “Ganzheitliche Bilanzierung” GaBi which means “Holistic Balancing”, the “System for Integrated Environmental Assessment of Products” known as SimaPro database, the “European Reference Life Cycle Database” known as ELCD, and the “New Energy Externalities Development for Sustainability” expressed as NEEDS. This diversity underscores the flexibility in methodological approaches and the recognition of regional or specialized data needs. However, it is concerning that 19.4 % of studies did not specify the database source used, highlighting a gap in transparency and potentially impacting the reproducibility and comparability of results.

It is important to note that the selection of databases in the reviewed studies may be influenced by the publication year, with older studies

potentially relying on databases that were available at the time of publication. More recent studies, especially those after 2020, benefit from the availability of improved and updated databases, providing more accurate and comprehensive data for LCA analyses.

#### 4.1.3. Life Cycle impact assessment (LCIA) of the studies

The LCIA method stands as a pivotal element within LCA, offering valuable insights into the environmental implications of heat pumps throughout their entire lifetime. LCIA can be executed through several methods and techniques contingent upon data availability, study objectives, and scope, such as Eco-indicator 99, recognized for its scientific rigor and practical utility in eco-design [49], featured in 20 % of the articles analyzed in this review. Alternatively, the Center for Environmental Science (CML) method which was created by the University of Leiden in the Netherlands in 2001 [50], was utilized by 14 % of the reviewed papers, distinguishing itself among other approaches.

##### 4.1.3.1. Methods.
The ReCiPe methodology which combines Eco-indicator and CML [50], was notably prevalent in the examined studies, accounting for 36 % of usage. Similarly, IMPACT 2002 + is widely used in academia and industry [51]. Additionally, the Environmental Footprint (EF) package, which encompasses normalization and weighting, was developed by the Directorate General for the Environmental (DG ENV) in 2018 [52]. Each of these methods was employed by only three studies, constituting 8 % of the total.

Various LCIA methods employed differing perspectives throughout the cause-effect chains. Some focused on specific environmental stressors such as greenhouse gases and acidification, utilizing the midpoint perspective. Conversely, others adopted the endpoint approach, evaluating outcomes such as human health or ecosystem quality. Interestingly, certain studies opted to utilize both midpoint and endpoint methodologies. The proposed weighting set from midpoint to endpoint aimed to highlight the relative importance of midpoint indicators and their impact on endpoints.

##### 4.1.3.2. Impact categories.
The impact assessment using several
<table><caption>**Table 7**
Heat pumps versus other heating systems.</caption><thead><tr><th>Reference</th><th>Type of HP</th><th>Compared System</th><th>Refrigerant</th><th>Electricity mix</th><th>Impact Categories with the highest contribution</th><th>COP / SCOP results</th><th>LCA boundaries/FU</th><th>SA</th></tr></thead><tbody><tr><td>Huang & Mauerhofer, 2016</td><td>GSHP (China)</td><td>ASHP</td><td>NA</td><td>Coal-fired power generation, hydroelectric, nuclear, and renewable sources</td><td>GWP, AP, and EP in the production process.</td><td>COP(GSHP) = 3.6<br>COP(ASHP) = 2.0</td><td>Cradle-to-gate / NA</td><td>No</td></tr><tr><td>Li et al., 2020</td><td>ASGSHP (UK)</td><td>VGSHP</td><td>NA</td><td>Not mentioned</td><td>Compared with the VGSHP, the ENV impact of the ASGSHP can be reduced by 42 %, GWP by 39 %, AP by 46 % and dust by 43 %.</td><td>COP of chiller: COP (ASGSHP) = 1.25COP (VGSHP) = 5.4</td><td>Cradle-to-gate / NA</td><td>No</td></tr><tr><td>Aresti et al., 2022</td><td>3 types of GSHP (North Europe)</td><td>ASHP</td><td>Calcium Chloride</td><td>Grid electricity has a lower environmental impact</td><td>ASHP has a higher GWP impact than GSHP.</td><td>GSHP has a higher COP value (better energy efficiency) than ASHP</td><td>Cradle-to-grave / kWh/year</td><td>Yes</td></tr><tr><td>Violante et al., 2022</td><td>GSHP (Italy)</td><td>ASHP</td><td>R32</td><td>Continental European electricity mix</td><td>CC, OD, HT, and WU due to the nuclear hydropower plants.</td><td>SCOP(GSHP) = 3.745 SCOP (ASHP) = 3.3</td><td>Cradle-to-grave / 1 kWh</td><td>No</td></tr><tr><td>Greening & Azapagic, 2012</td><td>GSHP and WSHP (UK)</td><td>ASHP</td><td>R134a</td><td>Coal and NG used in UK electricity</td><td>ASHP has the highest impact on ADP, AP, EP, FAETP, GWP, MAETP, ODP, POCP, and TE. It has the highest impact due to lower efficiencies & higher material requirements.</td><td>COP(GSHP) = 3.9<br>COP(WSHP) = 3.9<br>COP(ASHP) = 2.8</td><td>Cradle-to-grave / 1 kWh</td><td>No</td></tr><tr><td>Sevindik et al., 2021</td><td>GSHP (UK)</td><td>ASHP</td><td>R134a</td><td>40 % NG, 20 % nuclear & wind, 60 % bioenergy, solar, and coal</td><td>CC, TA, PMF, EM.</td><td>COP(GSHP) = 3.4<br>COP(ASHP) = 2.8</td><td>Cradle-to-grave / 1 kWh</td><td>Yes</td></tr><tr><td>Caduff et al., 2014</td><td>Brine-Water HP (Switzerland)</td><td>Air-Water & Water-Water HP</td><td>R134a</td><td>Nuclear & Hydropower</td><td>GWP was recognized as the most crucial impact category. ODP emissions during the production of R134a</td><td>A slight decrease in COP over the size of HP was observed</td><td>Cradle-to-grave / kg/MJ or kW</td><td>Yes</td></tr><tr><td>Famiglietti et al., 2023</td><td>Gas absorption HP (EU)</td><td>Electric HP</td><td>NA</td><td>Coal, biomass, nuclear, & hydropower</td><td>PM by coal, oil, & biomass plants<br>LU by biomassWU by hydropower & nuclear plantsMRM due to the amount of copper used.</td><td>Not clear</td><td>Cradle-to-grave / 1 kWh</td><td>Yes</td></tr><tr><td>Autelitano et al., 2023</td><td>Brine-Water (Italy)</td><td>Air-Air, Water-Water, Air-Water</td><td>R134aR32</td><td>Italian electricity grid</td><td>CC, OD, LU, EP Marine LU due to the incidence of metallic materials (steel & copper) in the production phase.</td><td>COP = 3.5</td><td>Cradle-to-grave / 1 kWh</td><td>Yes</td></tr><tr><td>Nitkiewicz & Sekret, 2014</td><td>Electric Water-Water HP (Poland)</td><td>Absorption Water-Water HP & natural gas boiler</td><td>R134a</td><td>53.5 % from coal & 32.1 % from lignite</td><td>Abs. minerals, CC, AP, ADPElec. CC, R, OD, Eco, AP, EP gas boiler has the lowest damage to human health.</td><td>The higher the COP, the better the efficiency (general comparison only)</td><td>Cradle-to-gate / amount of heat GJ and MJ</td><td>No</td></tr><tr><td>Moore et al., 2017</td><td>Heat Pump Hot Water System (Australia)</td><td>Traditional Hot Water System</td><td>HFC (R410a)</td><td>Grid, photovoltaic solar modules, batteries</td><td>GWP & CC reduction. HP hot water systems: GWP 57 % lower than a conventional electric storage hot water system.</td><td>NA</td><td>Cradle-to-grave / MJ per day</td><td>No</td></tr></tbody></table>

methods and across various multiple categories was recognized and recorded for comparison. It was obtained that most of the impact categories that commonly contribute to the environmental load were the Global Warming Potential (GWP), Ionizing Radiation (IR), Ozone Depletion Potential (ODP), Acidification Potential (ADP), Eutrophication Potential (EP), Water Use (WU), and Land Use (LU), representing the most reported impact categories among the studies. The most contributing impact categories from each study are reported in Table 6, Table 7, and Table 8 for each study.

Beside assessing the environmental impact, eight studies have computed a payback period in order to obtain the time after which the savings become higher than the investment. The saving corresponded to an energy or an environmental saving in five of these studies, while the type of saving was not mentioned in the three other studies.

### 4.1.4. Interpretation

Sensitivity and uncertainty analyses are important and

recommended steps of the interpretation phase of LCA, as they affect the decision reliability unless addressed properly [53 54]. They were carried out in 50 % of the studied articles for significant reasons. Those studies aimed to identify key drivers of environmental impact, assess the robustness and reliability of results, and address uncertainties, providing a clearer understanding of how variations in inputs affect outcomes, and enhancing decision-making. Looking at the sensitivity analysis carried out in those 18 articles helped us realize the most critical parameters influencing environmental impacts, such as reducing electricity generation [55 56 57 58]. Additionally, other important factors were improving heat pump parts such as galvanized steel pipe production [55], updating the compressor [58], or even adjusting parameters of photovoltaic-driven heat pump systems including different PV panel options and installation/operation locations [59].

Another interesting scenario that was proposed by some of the studies was amending the electricity mix [43], especially the energy mix of manufacturers [60], introducing on-site photovoltaic plants [48],
<table><caption>**Table 8**
Heat pumps without comparison.</caption><thead><tr><th>Reference</th><th>Type of HP</th><th>Electricity mix</th><th>Refrigerant</th><th>Impact Categories with the highest contribution</th><th>COP / SCOP results</th><th>LCA boundaries / FU</th><th>SA</th></tr></thead><tbody><tr><td>Koroneos & Nanaki, 2017</td><td>GSHP(Greece)</td><td>Local lignite and oil</td><td>R22</td><td>AP, GWP, EP, OD, CE, HM, WS: 73 % and 14.5 % of the assessment are attributed to the categories of AP and GHG effect respectively. AP dominates by 73.5 %. EP has a minor impact with 9 %.</td><td>COP = 4.0</td><td>Cradle-to-gate / kWh</td><td>No</td></tr><tr><td>Bonamente & Aquino, 2017</td><td>GSHP (Italy)</td><td>Photovoltaic plants (Italy)</td><td>NA</td><td>HH, EQ, IR.</td><td>COP of the storage scenario (2.86) is worse than that of the baseline (3.50). A higher COP was obtained in the improved scenario (3.39).</td><td>Cradle-to-gate / 1 kWh</td><td>No</td></tr><tr><td>Scharrer et al., 2020</td><td>Geothermal (Germany)</td><td>Hydrothermal & Petro-thermal</td><td>NA</td><td>AP, GWP, CED.</td><td>COP = 4.88, it decreases as the storage in temperature increases.</td><td>Cradle-to-gate / kWh</td><td>Yes</td></tr><tr><td>Sevindik & Spataru, 2022</td><td>ASHP(UK)</td><td>Electricity produced from wind energy</td><td>R32</td><td>Reduction in IR, ALO, and GHG by 98 % in CE</td><td>Non-extreme weather conditions resulted in higher COP</td><td>Cradle-to-grave / in years</td><td>No</td></tr><tr><td>Hong et al., 2016</td><td>GSHP(Korea)</td><td>Electricity in Korea</td><td>NA</td><td>GWP, ODP, AP, EP, POCP, RDP.</td><td>COP was 3.83, 4.2, and 4.63 at 15 °C, 25 °C, and 32 °C respectively during heating mode.</td><td>Cradle-to-gate / in years</td><td>Yes</td></tr><tr><td>Zhai et al., 2022</td><td>GSHP(China)</td><td>Renewable energy source</td><td>NA</td><td>GWP, HH, fine PM, WD. Energy consumption & carbon emissions of the GSHP were reduced by 40.5 % and 35.2 %, respectively.</td><td>NA</td><td>Cradle-to-grave / 20 years</td><td>Yes</td></tr><tr><td>Ren et al., 2018</td><td>PE and Steel GSHP(China)</td><td>NA(Southern China)</td><td>R22</td><td>The steel system reduced the CO<sub>2</sub> emissions by 44.9 % in the whole life cycle, so it was energy efficient. More GHG could be reduced by the substitution of diesel oil with RE in the construction stage.</td><td>COP (steel) > COP (PE) in which the increase in summer is 31 % and in winter 62 %</td><td>Cradle-to-gate / 20 years</td><td>No</td></tr><tr><td>Chang et al., 2017</td><td>Geothermal (China)</td><td>Coal-fired electricity</td><td>NA</td><td>AP, GWP. NO<sub>x</sub> emissions are due to the diesel engines of vehicles and construction equipment.</td><td>COP = 5.09</td><td>Cradle-to-gate / NA</td><td>Yes</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Types of databases used for LCA in the studies.</figcaption></figure>

switching to biomethane from natural gas in an attempt to reduce GWP emissions [57] or even doubling the renewable energy share [48]. Reducing costs was one of the suggested scenario propositions as well [55], such as reducing copper usage [58] or even reducing transportation distance [57].

Two studies agreed that boosting the heat pump's Seasonal Performance Factor to 3.9 leads to significant reductions in the heat pump's impacts across all categories, notably a 41 % decrease in GHG emissions compared to Condensed gas boilers [61]. In the same context, a call to increase the Seasonal Performance Factor by 50 % was demanded [48].

Sensitivity analysis interestingly investigated the impact of the LCIA method used on the impact categories' outcomes, in which substantial differences were obtained in toxicity indicators that were attributed to variations in the models used for characterizing toxicity impacts

between EF 3.0 and the ReCiPe Midpoint method [41].

## 4.2. Results analysis of each selected study

### 4.2.1. Comparison of domestic heat pumps and conventional systems

As presented in Fig. 3 in green font, 17 articles compared domestic heat pumps to conventional systems, in which five of those heat pumps were GSHP, four of them were ASHP, and the other eight were a group of miscellaneous types. They are presented in detailed in the following paragraphs as well as in Table 6. The studies by Pratiwi & Trutnevyte (2021a), Khan et al. (2020), and Khan et al. (2021) have been flagged for not presenting key data such as the COP, functional unit, or performing sensitivity analysis. Additionally, Khan et al. (2020) and (2021) did not specify the type of refrigerant used in their study.

#### 4.2.1.1. Results for GSHP versus conventional.
An Italian study assessed the environmental viability of a district heating network providing space heating and domestic hot water in a new building area in Milan using a Life Cycle Assessment [62]. It compared the current and future (2030) environmental profiles of North Milan's district heating network with three individual electric-driven heat pumps. Despite higher CO<sub>2</sub>-eq emissions, district heating could have a nearly equivalent climate change impact by 2030 due to renewable sources integration. Results showed a better Env. profile for the district heating in both scenarios, allowing a reduction of 67 % (present) and 19 % (future).

Another study in Italy compared the environmental impact of a photovoltaic Geothermal Heat Pump (PV-GHP) system for greenhouse heating versus a conventional LPG-HG system [63]. While results did not definitively favor one over the other, PV-GHP is suggested for lower carbon emissions. GHP relying on grid electricity has the highest environmental burden, while LPG-HG is less impactful. Policies should encourage energy-efficient methods like PV-GHP and thermal screens in
addition to supporting energy sources.

Exergoenvironmental analysis was used to evaluate three heating systems in Turkish residential buildings: coal boiler, natural gas boiler, and ground source heat pump [64]. While the ground source heat pump proves thermodynamically efficient, it shows the highest environmental impact. The findings suggest that despite low thermodynamic efficiency, the environmental impact remains significant, urging Turkey to replace coal with natural gas for heating energy generation.

In China, a study assessed the potential of GSHP systems. Optimizing electricity generation and steel production processes is crucial for enhancing its environmental performance [55]. Compared to coal-based heating, GSHP systems can reduce environmental impact by 65%–95% and external costs by 85%, except for metal depletion, which is 1.5 times higher. In Shandong Province, promoting GSHP systems could replace up to 69.4% of district heating, leading to significant reductions in various impacts and external costs. Considering both environmental and economic factors, GSHP systems offer benefits over coal-based heating after around 2.34 years of operation.

The last study assessed the environmental impacts of six geothermal heating and cooling setups in Geneva. Main factors included drilling, district heating construction, and electricity use [65]. Geothermal systems proved effective in cutting carbon emissions in buildings, but mineral resource scarcity remained an issue. Decentralized heat pumps were recommended over traditional systems. Combining geothermal with other sources could further reduce impacts, but careful design is crucial to avoid potential higher impacts compared to natural gas heating.

### 4.2.1.2. Results for ASHP versus conventional.
Among the four articles investigating the impact of ASHP against conventional systems such as condensing gas boilers, biomass systems, centralized heating systems, and DUAL air-ground heat pumps, ASHPs have relatively low GHG emissions. However, they may have higher environmental impacts during manufacturing and disposal; Additionally, their efficiency can be affected by factors such as outdoor temperature, the electricity grid, and system design.

#### 4.2.1.2.1. Condensing gas boiler.
A German study concluded that condensing gas boilers emit fewer pollutants compared to ASHP, which has higher environmental impacts across seven categories [43]. Despite limited greenhouse gas reductions from natural gas-fired systems, ASHPs boast a 70% lower carbon intensity compared to condensing gas boilers. Taking into account the impact of the operational phase, methane leakage during extraction and transportation can still contribute to environmental impacts.

#### 4.2.1.2.2. Centralized heating system.
ASHP and centralized heating systems were compared through a study carried out in China, which concluded that electricity consumption, refrigerant leakage, and steel components in air conditioning (AC) systems are the primary factors contributing to environmental impact [66]. AC systems can result in a 34.5% reduction in CO₂ equivalent emissions compared to hot water radiator heating systems. Air source heat pumps (ASHP) produce lower greenhouse gas emissions compared to centralized heating systems.

#### 4.2.1.2.3. Biomass system.
In Spain, Lozano Miralles compared Air-Air HP with a biomass system to deduce that fuel consumed by the biomass system exceeds that consumed by ASHP [56]. Besides, the inorganic substance air emissions along with the CO₂ emissions caused by the biomass system contribute to climate change damage and human health toxicity (respiratory effects). However, the damage to the two systems occurred during the manufacturing and operation processes. Damages caused by Ecotoxicity are chiefly due to heavy metal emissions in air and water.

#### 4.2.1.2.4. DUAL system.
DUAL Air-ground HP was compared to a conventional air and ground source HP in Italy, and it was denoted that most of the highest environmental impacts occur during the usage phase, primarily due to electric energy usage [60]. In the production

phase, the largest impact stems from the raw materials used in heat pump construction and diesel used in drilling operations. In humid climates, dual-source systems have higher environmental impacts compared to ASHPs when compared to conventional solutions. Despite their higher efficiency, using shorter geothermal probes with GSHPs offers limited environmental advantages.

### 4.2.1.3. Results for miscellaneous types of HP versus conventional.
Eight articles set various types of heat pumps such as the Cascade HP, the Hybrid HP, the Stirling Cycle-based HP, and many others in comparison with several conventional heating systems like the natural gas boiler, the condensing gas boiler, the biomass-based boiler, and many others. Interestingly, all of these comparisons took place in different European countries except for the Cascade HP which was held in Canada.

#### 4.2.1.3.1. Cascade HP versus natural gas furnace.
Relating Cascade HP to a typical natural gas furnace, the highest CO₂-eq emissions occur during the usage phase for both heating systems, amounting to 18 883 kg of CO₂-eq for the furnace and 104 kg of CO₂-eq for the heat pump, respectively [67]. Implementing a cascade heat pump reduces GWP by 85.4% compared to a typical natural gas (NG) furnace. Negative impacts on human health are significantly reduced across all categories, with factors affecting respiratory systems decreasing by nearly tenfold when utilizing the cascade heat pump as an alternative heating system. The cascade heat pump emits approximately 1% of the carbon emissions released by NG furnaces.

#### 4.2.1.3.2. Waste heat-fed HP versus biomass-based and conventional gas boiler.
The waste heat-fed and biomass heating systems were compared in an Irish study, and both had lower environmental impacts than conventional boilers [57]. The biomass-based system outperforms heat pumps. HTC had the highest impact across all categories due to additional infrastructure requirements and energy use. The biomass system reduces GHG emissions by 45% and fossil fuel depletion by 73% compared to boilers. Heat pumps reduce GHG emissions by 42% and fossil fuel depletion by 47%.

#### 4.2.1.3.3. Stirling HP versus multiple types of boilers.
Two Swedish studies were carried out in 2020 and 2021 in an attempt to compare Stirling Cycle-based HP to conventional heating systems and the findings highlighted the significant impact of boiler use on climate change through the CO₂ emission from natural gas combustion [69–68]. Methane emissions occur due to losses in long-distance natural gas pipelines. Additionally, nitrogen oxides from combustion contribute to air pollution and ecosystem damage. Oil-based heating systems are the least environmentally friendly option, with impacts 3 to 4 times greater than Stirling heat pumps. Fossil fuel systems have 3 to 5 times higher negative effects on the environment compared to Stirling heat pumps. Fossil fuel boilers have a higher negative impact on human health than Stirling heat pumps due to air emissions and toxicity [68]. Biomass boilers have a significant impact on environmental quality, with a GWP of around -5000 kg CO₂-eq after 15 years of use. Stirling heat pumps offer an environmental impact reduction of at least 10% to over 40% compared to gas and oil boilers in categories such as climate change, air pollution, and ozone depletion [69].

#### 4.2.1.3.4. Hybrid HP versus condensing gas boiler.
Compared to a Conventional Gas Boiler (CGB), the Hybrid Heat Pump offers potential savings of 30% in greenhouse gas (GHG) emissions in Sweden [61]. It also demonstrates emission reductions ranging from 13% to 48% in categories such as terrestrial acidification (TA), photochemical ozone creation potential (POCP), particulate matter formation (PMF), and fossil fuel depletion (FFD) compared to the CGB. However, the Hybrid Heat Pump emits 3 to 6 times more emissions related to human toxicity (HT), freshwater depletion (WD), and marine eutrophication (MD) than the CGB. In both core scenarios, around 50% of the impact of freshwater depletion and human toxicity is attributed to the production phase. The combustion of natural gas (NG) and electricity production are the primary contributors to the use phase's dominance in all selected impact
categories except for marine eutrophication and human toxicity.

4.2.1.3.5. Gas-driven absorption heat pump versus condensing gas boiler. An interesting comparison between a Gas-driven absorption Heat Pump (GAHP) and a Condensing Boiler (CB) which was carried out in several European countries, concluded that GAHP has a lower environmental footprint compared to the (CB), mainly due to reduced natural gas usage during operation [41]. The use phase has the most significant impact, contributing over 97 % to the overall environmental impact of both energy systems. Heat pumps mitigate climate change impact. Water usage from nuclear and hydropower plants impacts climate change (CC), ozone depletion (OD), and human toxicity (HT). On average, there is a 27 % reduction in CO₂-eq and a 25 % reduction in fossil resource consumption.

4.2.1.3.6. Split gas versus semi-indirect. A Spanish study compared a semi-indirect ceramic evaporative cooler (SIEC) and a heat pipe heat exchanger battery to a Split Class heat pump [58]. The SIEC-HP system offers energy savings, reduced operating costs, lower CO₂ emissions, improved air temperature control, and enhanced indoor air quality compared to conventional heat pumps. Abiotic resources, human health, and ecosystem capacity were the main factors affecting both systems. The heat pump has lower infrastructure requirements but higher electricity consumption, particularly from the compressor.

4.2.1.3.7. Water-water coupled with PV versus natural gas and AC split units. An LCA assessed the environmental benefits of a PV-driven heat pump system for residential buildings in Italy compared to conventional heating and cooling options [59]. The PV-driven heat pumps emitted 71.8 kg/MWh of CO₂ equivalents, while solar absorption systems emitted 150.4 kg/MWh. Sensitivity analysis showed a 57 % lower CO₂ emissions in Trapani compared to Bolzano. An optimal number of PV panels balanced excess electricity production benefits with manufacturing and end-of-life footprints. The four PVs system reduced global warming impact by 57 % compared to conventional heat pumps, and 1 % less than the five PVs solution.

### 4.2.2. Comparison of different HP technologies

Eleven studies investigated the performance of domestic heat pumps in comparison with other heat pumps. There are under a yellow font in Fig. 3 and in Table 7.

4.2.2.1. GSHP versus ASHP. Among the eleven studies that investigated the performance of domestic heat pumps in comparison with other heat pumps, six of them compared GSHP with ASHP, two of them were conducted in China while the other four took place in European countries.

In China, the investigated GSHP systems save about 40 % of energy compared to ASHP [23]. Their main environmental impacts include global warming, acidification, and eutrophication during production, as well as soil temperature changes during operation. This is influenced by China's 2016 electricity mix, which was mostly coal-based but included some hydroelectric, nuclear, and renewable sources.

In the second study, a comparison between absorption solar-ground source heat pumps (ASGSHP) and vapor compression ground source heat pumps (VGSHP) via LCA in China showed ASGSHP's significant reduction in CO₂, CO, SO₂, NOx, and dust emissions compared to VGSHP [70]. This translates to 42 % lower environmental impact, with reductions in GWP by 39 %, AP by 46 %, and dust by 43 %. ASGSHP's impacts are prominent during manufacturing (13 %) and operation (85 %), whereas VGSHP's effects are during operation (98 %). The manufacturing phase of ASGSHP is influenced by parabolic trough collectors (58 %) and absorption heat pumps (29 %), while ground source water pumps contribute 69 % during operation. ASGSHP's effectiveness in providing both summer cooling and winter heating hinges on optimizing heat pump usage and ground heat balance, with future steps focusing on a comprehensive year-round assessment.

A comparative study across seven European countries indicates that

Ground Source Heat Pump (GSHP) systems in residential buildings, even those with nearly Zero Energy Building features, can be more environmentally friendly than Air Source Heat Pump (ASHP) systems. This advantage is influenced by factors such as the thermal properties of the ground, heating and cooling demand, peak energy loads, and the electricity mix. [71]. ASHP systems generally have a higher environmental impact, except in Stockholm, where GSHP systems are less effective by 11 %-18 %. In six out of seven countries (Lefkosia, Seville, Lisbon, Bologna, Belgrade, and Berlin), GSHP systems outperform ASHP by 13 %-43 %. Results are influenced by the electricity mix, with operational processes having the highest impact (42 %-93 %), dependent on renewable energy share.

Comparing GSHP to ASHP in Italy, the second showed higher impacts during operation, with geothermal probes lasting 100 years, they could cover four cycles of GSHP operation, reducing overall impact [72]. The analysis emphasizes the vertical loop's significant impact during manufacturing, mainly due to raw material use. Extending analysis over time and in varied climates could boost GSHP's economic and environmental benefits. Integrating a hybrid geothermal-photovoltaic system could enhance contributions by utilizing electricity from the photovoltaic system to power the geothermal heat pump condenser. However, ASHP shows slightly higher impacts during operation. Overall, GSHP is more energy-efficient and has a lower long-term environmental footprint than traditional air conditioning systems.

Greening & Azapagic [47] compared GSHP, WSHP, ASHP, and gas boilers in the UK, finding that heat pumps generally have higher environmental impacts due to electricity use. ASHP impacts are 82 % higher than gas boilers on average, while GSHP and WSHP are 73 % higher. However, heat pumps show lower impacts in categories like global warming and CO₂ emissions. ASHPs have the highest impacts due to lower efficiencies and material requirements. GSHPs and WSHPs have similar impacts, with WSHPs slightly better. While heat pumps may remain less efficient than gas boilers in regions such as the UK, particularly when considering the efficiency of electricity generation, the potential for improvement with a decarbonized electricity mix could enhance their contribution to climate change targets as the electricity mix appeared to be very influential [53].

[48] showed that among GSHP, ASHP, and gas boilers, the use and manufacturing phases contribute to 74 % and 14 % of environmental impacts on average, as expected. Despite the UK's electricity decarbonization, heat pumps still have higher lifetime impacts than gas boilers in all categories except climate change. Heat pumps have lower carbon intensity than boilers (0.111 and 0.097 kg CO₂-eq for ASHP and GSHP, respectively, compared to 0.241 kg CO₂-eq for boilers). Future scenarios indicate significant reductions in most impact categories.

4.2.2.2. Results for miscellaneous types of HP versus other heat pump technologies. Five different types of heat pumps with different technologies were compared to other heating systems seeking the best performance using the LCA approach:

#### 4.2.2.2.1. Brine-water HP versus air-water and water-water HP.

Through examining brine-water HP, air-water HP, and water-water HP, a Swiss study provides scaling factors linking power output to product properties and life cycle impacts for heat pump and biomass furnace technologies [73]. Key attributes for heat pumps include mass, refrigerant use, and coefficient of performance, while for biomass furnaces, they include mass, electrical input, and efficiency. Findings reveal that mass and refrigerant use increase sub-proportionally to power, with no scaling effect observed for the coefficient of performance and furnace efficiency. Additionally, a sub-proportional growth was noted between power and two environmental impacts (global warming and ozone depletion) during the production phase. These results suggest scaling factors can aid in predicting key characteristics and related environmental impacts in life cycle assessments, particularly for new technologies with limited data.
#### 4.2.2.2.2. Air-source gas-driven HP versus air-source electric-driven HP & condensing gas boiler.
A comparative life cycle analysis was conducted in Europe on three heating technologies: an air-source gas-driven absorption heat pump, an air-source electric-driven heat pump, and a condensing boiler [74]. Sixteen potential impact categories were evaluated, considering each life cycle stage. Data on European electricity decarbonization and green hydrogen production were provided. Results compared environmental profiles using deterministic and stochastic approaches, showing that despite increased renewables, the use phase remained the most significant contributor (91 %). Despite higher emissions in 2020 compared to electric heat pumps (198–200 vs. 170–196 g CO₂-eq/kWhth), the gas absorption heat pump had a lower environmental impact in all scenarios.

#### 4.2.2.2.3. Brine-water HP versus air-air, air-water, and water-water HP.
In an Italian study, the authors developed five empirical correlations for electric heat pump technologies, including various configurations like air-air, brine-water, water-water, and two air-water HPs [42]. They also established correlations for three refrigerant gases such as R32, R134a, and R410a. Validation of these correlations, conducted through deterministic and stochastic approaches, affirmed the reliability of power-appliance mass correlations for evaluating impact categories in Italy. Additionally, power-charge of refrigerant correlations for R32 and R134a were found suitable for carbon footprint analysis. However, improvements in data collection and coverage of additional technologies are recommended, along with better gathering of life cycle inventory data for electric heat pumps.

#### 4.2.2.2.4. Electric water-water HP versus absorption water-water HP.
A comparison of heat sources shows geothermal water and gas boilers, with low-temperature geothermal offering environmental benefits despite fossil fuel usage in Poland [75]. Absorption heat pumps are the most eco-friendly, with the lowest eco-indicator scores. Electrical heat pump impact varies with efficiency and electricity source; in coal-dependent areas like Poland, emissions are high. As efficiency improves and renewables grow, electrical heat pump impact decreases, especially in low-carbon electricity areas like Switzerland. Gas boilers have high eco-indicator levels due to fossil fuel use but emit fewer pollutants. Heat pump COP greatly affects environmental impact, with electrical heat pumps potentially matching or surpassing gas boilers under certain conditions.

#### 4.2.2.2.5. Heat pump hot water system versus traditional hot water systems.
Comparison among five common hot water systems, including electric storage, gas storage, gas instantaneous solar electric thermosiphon, and solar gas instantaneous [76]. Heat pump hot water systems have lower global warming potential (GWP) than electric storage systems due to higher efficiency. Solar heat pump systems also reduce GWP compared to solar electric systems. Combining heat pumps with PV and batteries achieves the lowest GWP. The study recommends replacing electric storage systems with heat pump systems to reduce GWP. Further research includes updated modeling, detailed simulations, and full life cycle assessments. Comparisons between heat pump systems and separate air conditioning units are suggested.

### 4.2.3. Non-comparative LCA of HP

Out of the eight studies, performed between 2016 and 2022, that inspected the performance of domestic heat pumps using the LCA approach without comparing the HP with another system, four of them were conducted in European countries (Greece, Italy, Germany, and the UK), while the other four were conducted in Asian countries (one in Korea and the other three in China). All the studies investigated the GSHP from different perspectives, except for one study that investigated the ASHP in the UK. They are presented on a red font in Fig. 3, and in Table 8.

The Greek study epitomized that acidification and the greenhouse effect accounted for 73 % and 14.5 % of the overall assessment of GSHP in Greece, respectively, with a notable portion of the burden attributed to raw material production and cement [82]. Technologies like GSHP

systems may introduce additional burdens not fully captured by standardized assessments. Installing automatic control systems for GSHP operation could mitigate emissions. The Greek electricity sector, relying on local lignite and imported heavy oil, significantly contributes to air pollution.

Another study evaluates GSHP systems in Italy, analyzing both conventional and alternative modes through different scenarios: a baseline, storage, and improved scenario [83]. Variations in energy efficiency, materials, and construction among these scenarios are revealed through the LCI. While the storage scenario exhibits less environmental performance than the baseline due to lower COPs, the improved scenario, achieved by removing cylindrical heat exchangers, shows similar environmental indicators to the baseline. Furthermore, the fully optimized scenario, with cylindrical heat exchangers removed, a more efficient circulating pump, and GSHP electric consumption covered by a photovoltaic plant, achieves a 60 % decrease in measured midpoint indicators compared to the baseline.

A new energy storage technology, combining a heat pump, thermal storage, and an organic Rankine cycle, was studied for its environmental impact in Germany [77]. Compared to batteries, it proved more eco-friendly when powered by wind energy. Initial findings favor its deployment in northern Germany, but a global analysis is needed for broader application, especially in countries like Indonesia and Turkey with successful geothermal energy use.

The impact of large-scale heat pump adoption in Orkney was examined, aligning with circular economy principles to support the UK's 2050 net zero target [46]. Results show significant reductions in energy demand and GHG emissions with heat pump uptake and energy efficiency measures. Achieving net zero requires commitments across industries, covering manufacturing, energy production, and consumption. The integrated approach allows for flexible scenario analysis, considering both demand and supply sides with a life-cycle perspective. This holistic method is essential for effectively reducing GHG emissions and realizing overall energy, environmental, and financial benefits.

The objective of a study conducted in Korea was to perform a sensitivity analysis of the factors affecting the GSHP system's energy generation and environmental impact [78]. The borehole length emerged as the most significant factor affecting both energy generation and environmental impact. In contrast, U-pipe spacing was found to have the least impact on both energy generation and environmental impact.

In China, GSHP systems mainly impact global warming and human health due to carbon dioxide emissions from electricity generation and polyethylene pipe production [79]. Comparative analysis shows a 40.5 % reduction in energy consumption and a 35.2 % reduction in carbon emissions compared to coal-fired heating and air conditioning systems. GSHPs effectively cut energy use and emissions versus traditional systems. To enhance their environmental performance, the study recommends using renewable energy and steel pipes instead of polyethylene.

The seasonal performance of two Ground Source Heat Pump (GSHP) systems made of polyethylene (PE) and steel were compared using full-scale experiments and LCA in China. Steel systems outperform PE systems, especially in summer [80]. They exhibit lower energy consumption and cost over the life cycle, with reductions of 43.6 % and 24.6 %, respectively, compared to PE systems. Calculations show steel systems have an Energy Payback Time (EPT) of 0.24 years, a Cost Payback Time (CPT) of 1.83 years, and a CO₂ Reduction Ratio (CRR) of 0.45. Additionally, the Coefficient of Performance (COP) for the steel system surpassed that of the PE by 31 % during winter and 62 % during summer, underscoring the superior operational performance of the steel GSHP system compared to the PE system.

Another Chinese study analyzed the life cycle energy consumption and environmental emissions of GHP deployment [81]. Using an LCI approach, the study aimed to provide insights into GHP system design and operation. Results showed a total life-cycle energy consumption of 192 TJ and emissions of 35 Mt SO₂, 45 Mt NOx, and 19,130 Mt CO₂-eq.
GHP systems demonstrated significant energy savings, with 84 % and 83 % reductions in heating and cooling energy compared to conventional systems. Energy and GHG payback times were 0.5 and 0.3 years respectively, with cost-effectiveness achieved in 7.4 years.

# 5. Discussion

## 5.1. Comparison of the performance of heat pump technologies

As an overall interpretation, GSHPs are recognized for their energy efficiency, environmental benefits, and long-term savings, positioning them as a strong choice for residential heating applications. However, the high upfront costs can be a significant barrier. In contrast, advancements in air-to-air heat pump technologies, such as variable capacity compressors and flash injection, have made these systems increasingly desirable in regions like North America, offering a more cost-effective solution without compromising on performance [84].

As the majority of the reviewed studies were applied in European countries with cold climates and weather conditions, the variable capacity air-to-air heat pumps may become an effective choice for residential heating, particularly in North America [85]. Advancements in variable capacity compressors allow these systems to adjust their output based on heating demand, improving energy efficiency and performance in colder climates. These systems are particularly advantageous in areas with fluctuating temperatures, where traditional heat pumps may struggle to maintain efficiency [86]. The ability to operate at partial loads makes them a versatile and cost-effective solution for residential heating needs [87].

It is important to note that in the reviewed studies, the type of air source heat pump (air-to-air or air-to-water) was often not specified, limiting the comparability of results. While there are regional differences in the use of these systems (air-to-air heat pumps being more common in North America and air-to-water systems in Europe), the absence of clear definitions in the reviewed articles prevents a detailed analysis of the specific impact of each system type on performance. In cases where the type of heat pump was mentioned in the studies, this information has been incorporated into the 'Type of HP' column in the corresponding tables.

Several factors contributed to this superiority, including their lower operating costs stemming from heightened efficiency, their versatility in providing both heating and cooling within a single reversible refrigeration cycle, and their remarkable durability and longevity, particularly notable in the prolonged lifespan of the entire system and its underground components compared to traditional heating systems. Furthermore, GSHPs remain unaffected by external weather conditions or seasonal variations, ensuring reliable heating and cooling performance year-round.

Environmentally, GSHPs stand out for their utilization of renewable energy from the ground, aligning with eco-friendly principles by emitting fewer greenhouse gases compared to systems reliant on fossil fuels like gas or oil. This contribution to reducing carbon footprint and enhancing air quality underscores the environmental sustainability of GSHPs.

Regardless of the reasons making GSHP a good choice for residential heating, it still has GWP caused by refrigerants such as R134a [64] and R410a [41 63]. Additional environmental impacts were obtained due to the electricity mix used, which differs between countries, such as the electricity mix in China which depends on coal to almost 60 % in addition to other resources such as nuclear, hydroelectricity, natural gas, wind, and solar power [55]. Like the electricity mix of Italy which contains nuclear, hydropower, and oil fuel plants [41].

ASHP offers advantages when compared to conventional systems, such as lower upfront costs, and year-round heating and cooling capabilities, they are well-suited for moderate climates and require less maintenance. As per consumption, the amount of fuel used by the biomass system, for example, is higher than that used by the ASHP, in

which the CO₂ emissions from the biomass system contribute to climate change and pose risks to human health. However, damage to both systems occurs during their manufacturing and operation stages. [56]. ASHPs have drawbacks including reduced efficiency in cold climates, noise during operation, dependence on electricity, installation, and space requirements, defrost cycles, concerns about refrigerants, and performance variability.

Compared to GSHPs, ASHPs have the highest impact in most of the studies, due to lower efficiencies and material requirements [47 71]. Despite limited greenhouse gas reductions from natural gas-fired systems, ASHPs have lower carbon intensity compared to condensing gas boilers [43]. They produce lower greenhouse gas emissions compared to centralized heating systems [66].

The introduction of a cascade heat pump results in a reduction in GWP compared to a standard natural gas (NG) furnace. There's a substantial decline in negative impacts on human health across all aspects, particularly affecting respiratory systems, with the use of the cascade heat pump as an alternative heating method leading to a nearly tenfold decrease [67]. In the same manner, both waste heat-fed and biomass heating systems exhibit lower environmental impacts when compared to conventional boilers. Among these, the biomass-based system demonstrates superior performance over heat pumps [57]. Moreover, Stirling heat pumps offer environmental impact reduction when compared to gas and oil boilers across various categories such as climate change, air pollution, and ozone depletion [69].

## 5.2. Application of LCA to heat pumps

While the reviewed studies offer valuable insights into the environmental performance of domestic heat pumps through LCA, several limitations are evident across the literature. One notable constraint is the variability in the aspects emphasized by different studies, with some concentrating on energy consumption, heat pump design, or raw materials production. However, few studies delve into critical factors like the impact of electricity mix variability, refrigerant leakage, or performance metrics such as COP, EEI, EER, and other measures. This lack of comprehensive coverage underscores potential gaps in understanding the complete environmental implications of domestic heat pump systems. Future research endeavors should aim to address these limitations by adopting a more holistic approach that considers all relevant factors to furnish a comprehensive assessment of the environmental performance of domestic heat pumps. Consequently, applying appropriate technological advancement to heat pumps can significantly enhance their environmental performance over time.

In reviewing the evaluation of domestic heat pumps using the LCA method, several limitations have emerged that warrant attention and further research. These limitations encompass various aspects of functional units, lifetimes of heat pumps, time horizons considered, availability of data on refrigerant types, diversity in databases used, and variability in metrics and LCIA methods.

Firstly, the issue of non-matching functional units across studies poses a challenge to comparability and consistency in LCA evaluations of different studies. Differences in the aspects of functional units considered hinder direct comparisons between studies and may lead to discrepancies in the assessment of environmental impacts. Similarly, variations in the operational lifetimes or lifespans of heat pumps complicate the interpretation of results and underscore the need for standardized approaches in future research.

Furthermore, the variability in time horizons, encompassing studies focusing solely on current impacts versus those considering future impacts due to mix changes or climate change, adds another layer of complexity to the evaluation process. The incorporation of future impacts necessitates a forward-looking approach that accounts for evolving technological, regulatory, and environmental factors.

Additionally, the presence of missing data on refrigerant types, discrepancies in database types utilized, and variations in metrics such as
<figure><img src="image_7.png" /><figcaption>Fig. 5. Key LCA Steps and Considerations for Evaluating Domestic Heat Pumps.</figcaption></figure>

Energy Efficiency Ratio (EER), Coefficient of Performance (COP), Seasonal Coefficient of Performance (SCOP), and Energy Efficiency Index (EEI) further complicate the synthesis and interpretation of results.

These factors collectively shape the long-term environmental performance of domestic heat pump systems. Technological advancements lead to more efficient and sustainable heat pump designs, reducing both direct and indirect emissions. Changes in energy sources, particularly the shift towards renewable energy, enhance the benefits of using heat pumps by lowering operational emissions. Evolving regulatory frameworks set the direction for future developments and encourage the adoption of best practices, driving continuous improvement in the environmental performance of heat pumps.

To take this research further, it will be interesting to study the content of Environmental Product Declarations from heat pump manufacturers in more detail, in order to explore the extent to which the above-mentioned aspects are taken into account (completeness of the data provided, information on the electricity mix used, source of performance indicators, etc.).

As the field progresses, ongoing research and development, coupled with supportive policies, will be crucial in realizing the full potential of heat pump systems in contributing to a sustainable and low-carbon future. The interplay of these factors underscores the importance of a holistic approach to evaluating and enhancing the performance of domestic heat pumps through the Life Cycle Assessment method.

The development of comprehensive updated guidelines is crucial as it ensures consistency and accuracy in evaluating the environmental performance of domestic heat pumps. By addressing key aspects such as electricity mix variability, refrigerant leakage, and relevant performance metrics, these guidelines can provide a standardized framework. This will improve comparability across studies, enhance the reliability of LCA results, and support the creation of more sustainable and efficient heat pump systems.

In conclusion, addressing these limitations requires collaborative efforts from researchers, policymakers, and industry stakeholders to establish standardized methodologies, enhance data availability, and promote transparency in reporting. By overcoming these challenges, can advance the understanding of the environmental performance of domestic heat pumps and contribute to the development of more sustainable heating solutions for the future.

## 6. Limitations

While this review provides a comprehensive analysis of Life Cycle Assessment (LCA) studies on domestic heat pumps, it is important to

acknowledge certain limitations. First, the review primarily focuses on heat pumps as heating systems in domestic applications, and as such, it does not cover other potential uses of heat pumps (such as in commercial or industrial applications). Additionally, we intentionally chose not to review existing standards such as EN, ASHRAE, or DIN, as they often emphasize technical specifications and do not directly address LCA methodologies. These standards, while essential in defining performance metrics and regulatory requirements, were excluded from the scope of this review.

Furthermore, our study relies heavily on existing LCA literature, which sometimes suffers from inconsistent data, particularly regarding functional units, refrigerants, and energy mix considerations. This lack of standardized data, coupled with regional differences in the electricity mix and refrigerant practices, may affect the generalizability of the findings. We also did not conduct a meta-analysis of primary data or new LCA studies, meaning our review does not offer new LCA results but synthesizes existing findings and their inconsistencies.

Additionally, while we aimed to provide regional context where possible, some studies did not specify the geographical region of their analysis. In such cases, we included regional details only when explicitly provided in the original sources. This limitation is acknowledged as it may influence the interpretation of findings across different geographic contexts.

These limitations highlight the need for further research to develop standardized methodologies and more reliable data to improve the accuracy and comparability of LCAs for domestic heat pumps.

The evaluation of heat pump performance using the Life Cycle Assessment (LCA) approach is influenced by several interrelated factors. As illustrated in Fig. 5, these factors include the scope of the studies, life cycle inventory, LCA methodology, and interpretation techniques. If any of these key elements are missing, as observed in some existing studies, the credibility and robustness of the evaluation may be compromised. Ensuring a comprehensive assessment framework is therefore essential for reliable and comparable LCA results.

## 7. Conclusions

This review has provided valuable insights into the environmental implications of domestic heat pump systems through the lens of LCA. Analysis of the various studies revealed both the strengths and limitations in the current understanding of heat pump performance across various conditions like locations, types of heat pumps, and others and different metrics such as COP and SCOP.

Considering the identified limitations in the reviewed literature, it is
evident that a standardized approach is necessary to ensure comprehensive and consistent evaluations of the environmental performance of domestic heat pump systems. Therefore, it is recommended that future research efforts focus on:

1) The development of standardized guidelines for conducting LCAs of heat pump systems.

2) These guidelines should encompass key aspects such as accounting for electricity mix variability, and refrigerant leakage and incorporating relevant performance metrics.

3) Looking ahead, it becomes increasingly imperative to anticipate and address future environmental challenges associated with heat pump deployment.

4) Factors such as technological advancements, changes in energy sources, and evolving regulatory frameworks may influence the long-term environmental performance of heat pump systems.

By proactively considering these future impacts, decision-making processes and steer can be better informed towards more sustainable pathways.

By adhering to standardized guidelines, researchers can provide more reliable and comparable assessments, thereby facilitating informed decision-making and promoting the adoption of environmentally sustainable heating solutions.

## CRediT authorship contribution statement

**Mona Aridi:** Writing – original draft, Project administration, Methodology, Investigation, Conceptualization. **Marie-Lise Pannier:** Writing – review & editing, Validation, Supervision, Project administration. **Rima Aridi:** Writing – review & editing, Validation, Project administration. **Thierry Lemenand:** Writing – review & editing, Validation, Supervision, Project administration.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

This work was performed within the frame of the doctoral project "Study of the present and future energy and environmental performances of domestic heat pump", granted by the doctoral schoo SIS (Sciences de l'Ingénierie et des Systèmes – Engineering and Systems Sciences), Pays de la Loire Region, France.

## Data availability

No data was used for the research described in the article.

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