**3. Results**

The following section assesses the environmental impacts associated with Stirling cycle-based heat pump, natural gas-fired boiler, and oil boiler with design capacity of 500 kW and compares them with a Stirling cycle-based heat pump. In the tables and figures, dimensionless values are given, grouping several impact categories, unless clearly indicated with a unit.

## *3.1. Stirling Cycle-Based Heat Pump*

The contributions of the construction, operational use, and decommissioning stages of the Stirling cycle-based HP to the total impact were assessed using SimaPro software. Table 2 shows the impacts associated with the generation of 500 kW of heat using a Stirling cycle-based HP.


**Table 2.** Impact assessment and characterization per impact category for a 500-kW heat output Stirling cycle HP.

\* DALY, disability-adjusted life year; \*\* PDF, potentially disappeared fraction of species.

Figure 3 shows impact assessment for the heat pump on a relative scale. This means that the plotted values are the values in Table 2 divided by the average for the three cases. As can be seen from the graph, the main contributions to the environmental impact are during the construction and decommissioning stages.

**Figure 3.** Relative impact assessment for the heat pump with varying use duration. The different factors are made dimensionless by dividing each value by the average value of the factors.

The analysis shows that almost 80% of the impact stems from the production of raw material for constructing the Stirling cycle HP itself, whereas the operation phase contributes less than 20%. Only a small fraction of the impact is due to the maintenance of the engine.

Figure 4 shows that among the impact categories terrestrial ecotoxicity and respiratory inorganic, the share of the processing is close to 50%. The use of water for the operational phase and maintenance phase and the production of cast iron and copper are the main contributing factors, respectively.

**Figure 4.** Life-cycle assessment of a 500-kW heat output Stirling cycle-based HP.

When decommissioning is included in the assessment, the impact category global warming and respiratory organic effects show negative values, which means a positive effect on the environment. This effect stems from the 90% recycling of the engine's material.

For most categories, the score is positive, which shows that the net effect is damage to the environment. However, in categories such as respiratory organics and global warming, where a score is negative, the benefits are more significant than the burdens. This is because some substances are paired with a negative characterization factor (C.F.). These substances are known to, for example, contribute to global cooling.

For the Stirling cycle-based HP, the primary emission source leading to the impact is the emissions of zinc to air, mainly stemming from copper production. The analysis showed considerable emission of nitrogen oxides and sulfur oxides as well, which contributes to the photochemical ozone formation and acidification. One of the main contributions to the result is water used for cooling in the context of electricity production.

The resource indium also has a significant impact for the Stirling cycle-based HP. Indium appears in lead-zinc mining as a resource input from nature. In the Ecoinvent dataset, it is assumed that this indium is not used, and thus the resource is wasted. However, with rising demand, it would be possible to extract this resource in the process of lead-zinc mining. The indium accounts for about 60% of the total impact. The contributing factor for ozone depletion by a Stirling cycle-based heat pump is the emission of halons resulting from power generation.

A Stirling cycle-based heat pump has an average impact of 0.02 DALY for human health, 2.2 <sup>×</sup> <sup>10</sup><sup>4</sup> PDF·m2·year. for ecosystem quality, <sup>−</sup>4894 kg CO2-eq for global warming and 765,000 MJ for

resource consumption. These values include manufacturing, use for 15 years, and decommissioning at end-of-life, as listed in Table 3.

If impacts of the Stirling cycle-based H.P are analyzed over the years, the result shows that one year (including manufacturing phase) of the daily operation of 500-kW heat output Stirling cycle HP emits 8114 kg CO2-eq with 836,067 MJ energy needed for the extraction/manufacturing of materials. Daily operation of this H.P for eight years (including manufacturing phase) emits 9610 kg CO2-eq requiring 865,853 MJ energy. Finally, after 15 years of operation including manufacturing and the decommissioning phase, 767,212 MJ energy is needed with overall negative emissions of −4894 kg CO2-eq.

**Table 3.** The environmental footprint of the 500-kW heat output Stirling cycle HP for construction, use and end-of-life decommissioning.


\* DALY, disability-adjusted life year; \*\* PDF, potentially disappeared fraction of species.

The ECO INDICATOR 99 method was used to analyze further the damage on human health, ecosystem quality, and climate change, as shown in Figure 5. The Pt unit (a dimensionless value) measures the impact of these damages. A value of 1 Pt refers to one-thousandth of the yearly environmental impact of one average European inhabitant.

**Figure 5.** Damage assessment and characterization for Stirling cycle-based HP.

The figure shows that the major impact the Stirling cycle-based HP is on human health. From the analysis of the results, it seems clear that the most critical material in terms of environmental impact is copper (used in the electromotor of the Stirling cycle). The reason is that copper production, although typically 41% recycled copper is used, contributes to the emission of direct atmospheric arsenic emission.

Moreover, the environmental impact for one year of operation is almost the same as for eight years of operation. This shows that the main impact is associated with the production/extraction of raw material for the equipment. It makes clear that, over the 15 years of operation, the additional impact on human health, ecosystem, and climate change is not significant.

#### *3.2. Natural Gas-Fired Boiler (NGB)*

A natural gas-fired boiler shows a more significant environmental impact compared to a Stirling cycle HP. For all the options that supply heat by burning natural gas (or oil, as discussed below), the emissions of mercury to air are the crucial values. Table 4 shows the life cycle impacts associated with natural gas boiler for 1, 8, and 15 years of operation. A significant emission source is the emission of bromochlorodifluoromethane. This emission results from the typically long-distance transportation of natural gas in pipelines as it is used for fire suppression within natural gas pipelines infrastructure. The chromium (VI) emissions from iron production process contribute to the human toxicity and cancer effects.

The CO2 emissions from burning natural gas have the main impact on climate change during boiler use. Some further climate change effects stem from methane emissions that mainly occur due to losses during the transport of natural gas (imported from Denmark via North Sea lines) in long-distance pipelines (methane being the main component of natural gas). The use of a natural gas boiler also results in considerable emissions of particulate matter from the combustion process. Finally, the emission of nitrogen oxides during the combustion process at the heat pump results in photochemical ozone formation, acidification, and terrestrial and marine eutrophication.


**Table 4.** Impact assessment and characterization for the 500-kW heat output natural gas-fired boiler (NGB).

\* DALY, disability-adjusted life year; \*\* PDF, potentially disappeared fraction of species.

## *3.3. Oil Boiler (OB)*

The analysis of the life cycle footprint of an oil boiler shows a similar split (Table 5) of the total impact as for natural gas. In addition, here, the emissions of the burning process, especially CO2, contribute most to the impact category climate change. A prominent difference is that the emissions from the oil burning process also contribute most in the impact categories photochemical ozone formation, terrestrial eutrophication, and marine eutrophication. Electricity (needed during the equipment construction phase) contributes very little in most categories, being also, per MJ of heat produced during the use phase, smaller than for a natural gas boiler. The oil boiler has higher impacts on acidification compared to natural gas, a large extent the result of sulfur dioxide emissions. These emissions result primarily from the oil production (refining) process. For the oil boiler, emissions of copper and zinc to air both contribute to the environmental impact, stemming mainly from the burning process.

For heat from an oil boiler, the emission of bromotrifluoromethane (with a high ozone-depleting potential) from oil production is an important input. The emission stems from leakage, losses at filling, and false alarms.


**Table 5.** Impact assessment and characterization for oil boiler (OB).

The comparison of damage assessment and characterization of Stirling cycle-based HP, oil boiler (OB), and natural gas-fired boiler (NGB) during their life span of 15 years is given in Table 6.

**Table 6.** Impact assessment and characterization for construction, 15 years of use, and end-of-life decommissioning of a Stirling cycle-based HP (SE HP), an oil boiler (OB), and a natural gas-fired boiler (NGB) for 500-kW heat output.


Similar to Figure 3, a comparison of relative impacts (normalized around the average value) for the three technologies is given in Figure 6.

**Figure 6.** Comparing the relative impact assessment of the technologies. The different factors are made dimensionless by dividing each value by the average value of the factors.

#### **4. Discussion**

The Stirling cycle-based heat pump technology causes lower to non-significant environmental impacts compared to a natural gas-fired boiler or an oil-fired boiler. The toxicity originates to the largest part from chromium (VI) emissions into the water in all considered technologies. The unit process is responsible for emissions with the main impact on human toxicity, cancer effects, and global warming.

Figure 7 shows the relative distribution of impacts associated with the use of SE HP, NGB, and OB. Among all categories, human impact is the largest contributing category by these heating technologies following climate change (i.e., global warming potential).

The unit processes with the most significant direct emissions are the processes in which fuels are burned. The largest impacts then come from the oil boiler, followed by the natural gas boiler. The emissions with the highest influence in this category are, besides CO2, sulfur dioxide and particulate emissions. For the Stirling cycle-based HP construction, nickel and lead manufacturing are the main contributors, besides copper, which is used in the electromotor.

For the impact on climate change, a substantial reduction is possible by replacing a natural gas or oil boiler with a high temperature heat pump. A reduction of up to 15% of the original impact is possible for the options that do not use natural gas. For the oil boiler, a reduction by almost one third is possible. For particulate matter, the oil-fired boiler gives a much higher environmental burden, comparatively. Many impact categories show similar results since the same emissions (see, e.g., emission of nitrogen oxides to air) is responsible for various environmental problems.

**Figure 7.** Comparison of damage assessment and characterization Stirling cycle-based HP (SE HP), oil boiler (OB), and natural gas-fired boiler (NGB) on a relative scale.

Figures 8–10 give a comparison of environmental impact for nine damage categories for construction + 1 year of operation (Figure 8), construction + 8 years of operation (Figure 9) and construction + 15 years of operation followed by decommissioning (Figure 10).

**Figure 8.** Comparison of damage assessment and characterization for a Stirling cycle-based HP (SE HP), an oil boiler (OB), and a natural gas boiler (NGB) on a relative scale for one year of operation (excluding decommissioning).

**Figure 9.** Comparison of damage assessment and characterization Stirling cycle-based HP (SE HP), oil boiler (OB), and natural gas boiler (NGB) on a relative scale for eight years of operation (excluding decommissioning).

**Figure 10.** Comparison of damage assessment and characterization for a Stirling cycle-based HP (SE HP), an oil boiler (OB), and a natural gas-fired boiler (NGB) on a relative scale for 15 years of operation (including decommissioning).

The already mentioned paper by Stamford et al. [11] gives a similar study done for a much smaller 1-kW Stirling engine HP and compares it with a gas-fired boiler. They concluded that the S.E. micro-CHP system offers an environmental and economic advantage over the oil boiler by 30%, similar to what is found here.

#### **5. Conclusions**

The study evaluated the environmental sustainability of a Stirling cycle-based HP using the LCA approach. The analysis conducted above shows that the manufacturing phase has the most impact during the life span (15 years) of a Stirling cycle-based HP in terms of environmental impacts. The results show that, for the Stirling cycle HP to produce 4 GWh heat output (including manufacturing phase, operation phase, and decommissioning phase), the global warming potential at the end of its life span is −5000 kg CO2 equivalent and acidification potential 202 kg SO2 equivalent.

This study also compared the environmental impacts of a Stirling cycle-based heat pump with that of an oil boiler and a natural gas-fired boiler for 500-kW heating. The major impacts of the oil boiler and the natural gas-fired boiler are during the use phase of the engine.

For future work, the comparison should be conducted concerning the economic sustainability of the Stirling cycle-based HP and its comparison with a natural gas-fired boiler, oil boiler, and, if possible, an electric heater boiler. This would be beneficial in providing a still broader picture of how Stirling cycle-based HP technology can replace NGB, OB, and electric boilers in terms of lower environmental and economic impacts.

**Author Contributions:** Conceptualization, R.Z., U.K. and T.-M.T.; methodology, R.Z. and U.K.; software, U.K., and R.Z.; validation R.Z. and T.-M.T.; formal analysis, U.K. and R.Z.; investigation, U.K., R.Z. and T.-M.T.; resources, U.K., R.Z. and T.-M.T.; data curation, U.K., R.Z. and T.-M.T.; writing—original draft preparation, U.K.; writing—review and editing, R.Z. and T.-M.T.; visualization, T.-M.T.; supervision, R.Z. and T.-M.T.; project administration, R.Z. and T.-M.T.; funding acquisition, R.Z. and T.-M.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** The European Commission within the Horizon2020 Fast track to innovation project "HIGHLIFT" grant agreement number 831062 funded this work.

**Acknowledgments:** The authors acknowledge the collaborators Olvondo Technology, AstraZeneca, and Åbo Akademi University for all the necessary support provided to carry out this research work.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Disclosures:** One of the authors, T.-M. Tveit, is working in a company developing and marketing the heat pump being studied. All authors are participating in an EU-financed project developing the technology.
