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.
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.
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.
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.
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 × 104
·year. for ecosystem quality, −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.
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.
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.
3.3. 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.
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
Similar to Figure 3
, a comparison of relative impacts (normalized around the average value) for the three technologies is given in Figure 6