*2.4. System Boundary*

To quantify the impacts of the analyzed unit, system boundaries must be determined. The system boundaries adopted in this study are shown in Figure 2.

*Manufacturing*: The first phase across the system boundaries is the manufacturing phase. It includes extraction of raw materials, the production of the parts for the unit, transportation activities, the assembly and packaging of components, and testing analysis at the unit.

*Operation*: The operation phase includes the transport of the HP or boiler to consumers and the processes that follow during the use phase. The processes entail fuel consumption/acquisition, electricity production and usage during boiler operations, the heating process, the hot water cycle, and combustion emissions. Besides water consumption that may occur, the HP also brings about a consumption (due to losses) of helium working fluid.

*Maintenance*: The transport of engineers to and from the site for maintenance of unit is included. The installation or replacement of any component needed during maintenance is excluded, as the contribution may be assumed negligible. If some of the main components are replaced, e.g., the main motor, this assumption is no longer valid. However, the heat pump is engineered to ensure long-life operation of the large critical components.

*Decommissioning*: Lastly, the end of life (EoL) phase is also assessed, considering the heat pump's handling activities after the estimated life span use. In the decommissioning process, disassembly, cleaning, repairing of parts, and final disassembly are considered. In principle, this makes the raw materials available for other use. (In the LCA, this typically results in negative values for environmental impact.)

**Figure 2.** Schematic diagram of the system boundary for life cycle assessment of SE HP.

The system boundary for natural gas-fired boiler and oil boiler are slightly different, consisting of raw material extraction/acquisition, fuel and materials transportation, boiler unit construction, the operation (combustion of fuel), annual maintenance of boiler, and decommissioning of boiler. The production of imported fuel (natural gas) is also considered.

The inventory data sheet used for the development of an LCI network diagram for the Stirling engine-based heat pump (SE HP), natural gas-fired boiler (NGB), and oil boiler (OB) is given in Table 1.


**Table 1.** Inventory data for LCA analysis of Stirling engine-based HP, Natural gas-fired boiler and oil boiler.

\* Based on Ecoinvent v2.0 database (2012).
