*2.2. LCA Methodology*

To evaluate the sustainability of heat pump and compare that to other, more conventional but potentially more polluting heaters, reliable scientific tools that consider the entire lifetime of a product are needed. LCA is an evaluation tool that assesses and quantifies environmental impacts associated with product/process throughout the life cycle of a product known as "cradle to grave" analysis. The manufacturing and use of resources (i.e., materials and energy), as well as emission to the environment (land, air, and water), are calculated for each process. The significance of impact on environmental categories such as climate change, human and ecotoxicity can then be assessed for several so-called impact categories.

For this study, the LCA methodology was applied using the commercial software SimaPro 9.0. *SimaPro* is a software tool that collects, analyzes, and monitors the sustainability performance of any product or process. Two types of data are used to model the functional systems: foreground and background data [17]. The foreground data, which include data about technology, efficiency, and installed capacity, were taken from industrial technical data sheets and literature. Background data, which include information about raw material manufacturing and fuel use for transportation,

construction, and decommissioning of functional unit, were obtained from the Ecoinvent (v.3.5) database, which is embodied in the SimaPro software. SimaPro contains many LCI (Life cycle impact) databases, besides the well-known Ecoinvent v3 database, e.g., theAgri-footprint database and the ELCD database. The Ecoinvent database is a data source for studies and assessments based on ISO 14040 and 14044. The Ecoinvent LCI data are utilized to conduct the life cycle assessment, water footprint assessment, life cycle management, carbon footprint assessment, environmental performance monitoring, product design, and eco-design or Environmental Product Declarations (EPD) [17]. The ISO standardization makes LCA scientifically well-supported while databases for it are continuously expanding. Limitations of LCA are the need for reference or comparison data in the databases used, which sometimes need to be added by the user, or, for research purposes, lack of data for a new process or product. For the current study, database as well as real-life heating system data were sufficiently available.

The heating systems studied are: (a) Stirling cycle-based heat pump (SE HP); (b) oil boiler (OB); and (c) natural gas-fired boiler (NGB). These three systems were evaluated for locations in Sweden with identical climatic conditions. The choice of the location was according to the location of the heat pump under study, which is in Gothenburg, Sweden. The results of the analysis might be different depending on the chosen location.

The LCA methodology is comprised of four main stages of analysis: (i) defining goal and scope; (ii) data collection for life cycle inventory (LCI); (iii) identifying the environmental impact of all the inputs and outputs (LCIA); and (iv) interpreting the results. Several assessment methods have developed over time to classify and characterize the environmental performance of a system: Eco-indicator 99, EDIP 2003, CML 2001, IMPACT 2002+, ReCiPe Endpoint, CML 2 baseline 2000, BEES, TRACI 2, EDIP 2, etc. [18]. The methods used for the current analysis are IMPACT 2002+ and Eco-indicator 99. IMPACT 2002+ v Q2.2 [19] combines the midpoint/damage-oriented approach to impact categories such as human toxicity, carcinogenic effects, non-carcinogenic effects, respiratory effects (due to inorganics), etc. The Eco-indicator 99 [20] method specifies the environmental impact in numbers or scores. This score is scaled in such a way that each point signifies the annual environmental load of an average [European] citizen. The impact categories to be investigated for this study are *respiratory e*ff*ects*, *climate change*, *ozone layer depletion*, and *acidification* because they have been found to be the significant ones in this kind of studies.

For each phase during the manufacturing, operation, and decommissioning, inventory lists, including raw materials and fuel acquisition/manufacturing and air/water emissions, were computed and categorized into the impact categories. The data for impact categories were from information provided by Pre Consultants [21]. Through characterization, the environmental impacts were determined for each category. Lastly, it was investigated which practice has the major impact on the environment during manufacturing and decommissioning, respectively, as these phases are independent of the duration of use of a product between these two phases.
