**3. Results and Discussion**

### *3.1. Circular Economy Indicator Calculation*

In Table 3, the elastic modulus and the density of the investigated components, as taken from [30], are presented. Based on these values, the specific stiffness for each component was calculated, as well as the resulting weight, in order for the components to present equal stiffness. As mentioned in Section 2.2, the normalized specific stiffness, i.e., the stiffness ratio of the component under study to the virgin one, has been used in Equation (2) as the relevant CEI. Based on the Table 3 values, the virgin component demonstrates a higher specific stiffness compared to the recycled components, as expected, followed closely by the recycled component comprised of 50% aligned fibers. On the other hand, the recycled component comprised of randomly oriented fibers shows by far the lower quality, resulting in a considerable weight increase compared to the other two components. The poor quality of the randomly oriented recycled components highlights the need for upgrade technologies (mainly alignment) of the recycled fibers in order to be able to compete with the virgin CFRP components in terms of quality.


**Table 3.** Properties of The Investigated Components—Circular economy metric [data adapted from [30].

### *3.2. Environmental and Economic Impact Indicators Calculation*

Based on the obtained weight of each component, the environmental impact and costs were calculated, accounting for the whole lifecycle of the components. The results are presented in Tables 4 and 5, where data have been adapted from relevant works, as described in Section 2.2. The impact relating to the use phase of the components accounts for the different types of fuel that have been considered. The higher values, in terms of environmental impact and costs, are noted in bold.

Based on these results, it becomes clear that the virgin CFRP component presents by far the highest environmental impact and costs with regard to its production and manufacturing. This is owed to the significant energy required to produce PAN fibers as well as the considerable energy requirements of the autoclave manufacturing process. Nevertheless, the impact associated with the production and manufacturing phases contributes only to a small percentage of the overall impact, owing to the use phase impact, which clearly dominates the total lifecycle impact of the component. It is worth noting that nearly 99% of the total impact is owed to the use phase when kerosene fuel is used. A similar situation applies when liquid hydrogen from a conventional or wind source is considered; in this case, over 95% of the total impact is still owed to the use phase. However, when liquid hydrogen from a geothermal source is considered, the use phase environmental impact hardly accounts for 84% of the total impact. This remark highlights that the decarbonization of the aviation sector is expected to shift a considerable amount of the environmental burden to the production and manufacturing phases. On the other hand, the latter remark does not concern the lifecycle costs impact as the costs associated with the use of hydrogen are almost double compared to these of kerosene and over four times larger when hydrogen from renewable sources is used. This is owed to the current high cost of liquid hydrogen and especially the ones produced from renewable sources. Therefore, the use phase cost impact dominates the total lifecycle costs, regardless of the type of fuel utilized. The currently high cost of liquid hydrogen, and especially that deriving from renewable sources, may act as a prohibiting factor for the extensive use of liquid hydrogen, at least for the near future.

**Table 4.** Environmental Impact (LCA) metrics of The Investigated Components.


**Table 5.** Economic Impact (LCC) metrics of The Investigated Components.


When comparing the components under consideration, the lower environmental impact belongs to the recycled component comprised of aligned fibers for which hydrogen from a geothermal source has been used. Although this component is heavier compared to the virgin one, the environmental gains derived from the production phase of the recycled material are sufficient to compensate for the increased GHG emissions of the use phase compared to the virgin one; the latter remark does not apply though to the lifecycle costs. From the above remark, it becomes clear that the environmental impact associated with the production and manufacturing of virgin CFRP components cannot be neglected, and this urges the need to turn to CFRP recycling to avoid the energy-intensive process of PAN fiber production. Moreover, the environmental gains from the implementation of liquid hydrogen from renewable sources are highlighted, although issues concerning liquid hydrogen storage, transportation and infrastructure must also be considered. Yet, for the recycled components to be competitive with the virgin ones, a comparable to virgin quality appears as a mandatory requirement. Moreover, it should be noted that other factors, such as the feasibility of upgrade technologies of the fibers, the efficiency of the recycling processes and the capabilities of remanufacturing methods to produce

recycled components of high quality, as well as the availability of the recycled fibers, must be considered. The worst by far environmental and economic impact concerns the recycled component comprised of randomly oriented fibers. This makes evident that such a component cannot compete with a virgin component, especially when addressed at a high-performance application, and hence, upgrade technologies would be required.
