Introduction

Wind energy technology has experienced a rapid growth in recent decades (Larsen 2009). Since the service life of a wind turbine is limited to 20 to 25 years (Gopalraj and Kärki 2020; Jensen and Skelton 2018; Marsh 2017), the issue of wind turbine blade (WTB) disposal is pressing and still unresolved. As indicated by Albers et al. (2009), Europe is expected to generate more than 60.000 tons of WTB waste annually by 2025. Lahuerta et al. (2023) estimate that approximately 7.6 million tons of composite materials from end-of-life (EoL) WTBs will need to be disposed of in Europe by 2050. WTBs are primarily constructed of composite materials consisting of a thermoset matrix and glass fiber reinforcements (Fingersh et al. 2006; Mishnaevsky et al. 2017; Witten and Mathes 2019). However, there is currently no industrially viable technology for recycling glass fiber reinforced polymers (GFRP) (Beauson and Brøndsted 2016; Joustra et al. 2021; Marsh 2017; Schmid et al. 2020; Zhang et al. 2020). In addition, the mechanical properties of glass fibers are degraded during the recycling process, and the cost of recovering glass fibers is significantly higher than the market price of virgin glass fibers (Colledani and Turri 2022). Recycling on its own, especially when it involves low-grade materials, remains strongly connected to a linear economy (Potting et al. 2017). Other treatment strategies besides recycling are needed to avoid unsustainable waste streams such as landfilling or incinerating (Johst et al. 2023b). Landfilling is subject to several taxes, and incineration has a high environmental impact due to carbon emissions (Gopalraj and Kärki 2020).

The repurposing approach is a promising alternative for GFRP WTB waste (Delaney et al. 2021; Johst et al. 2023a; Joustra et al. 2021; Nagle et al. 2022; Ruane et al. 2023). Repurposing aims to use the geometry of a WTB to reuse cut segments for structural or semi-structural applications with a different function (Beauson et al. 2022; Kupfer et al. 2022). The Re-Wind Network (The Re-Wind Network 2023), founded in 2017, is conducting research on the repurposing of fiber-reinforced polymers. The network, consisting of five universities and industry partners, has created a catalog (McDonald et al. 2021) that identifies several potential repurposed applications of WTBs, such as affordable housing (Bank et al. 2018; Gentry et al. 2020), urban furniture (Jensen and Skelton 2018), electrical transmission poles (Alshannaq et al. 2021), or bridges (Ruane et al. 2023; Suhail et al. 2019). The reuse of composite materials from a discarded WTB in civil engineering applications that do not require significant reprocessing is becoming increasingly attractive (Delaney et al. 2021). This is mainly due to weight savings (Beukers and van Hinte 2020), high corrosion resistance compared to metals (Domke and Rübben 1977; Krahwinkel and Gehmert 2011), and resulting low maintenance requirements (Colledani and Turri 2022). In addition, the repurposed structural elements have been found to meet geometric design standards and outperform conventional building materials in terms of flexural stiffness and strength. Moreover, the reduction of environmental impacts throughout the product life cycle has been recognized (Joustra et al. 2021). Because of these advantages, there are already companies (Wings for Living 2023) commercially selling semi-structural components for various applications.

However, it is challenging to identify repurposed applications that cover a large amount of GFRP or other composites and can be mass produced for a specific end market (Beauson and Brøndsted 2016; Hopkinson et al. 2019; Mishnaevsky et al. 2017). Industrial implementation of such solutions requires a reliable source of composite material with continuous dimensions. This appears to be difficult for decommissioned WTBs due to the variety of models and blade geometries (Beauson et al. 2022; Joustra et al. 2021). Furthermore, transporting remanufactured structures remains challenging. Glass fiber content has an impact on processing costs. High glass fiber content requires special cutting equipment. The processing of large-scale GFRP requires health and safety precautions because of occurring micro glass particles (Jensen and Skelton 2018). Another challenge is the evaluation of the residual structural properties of the composites when a certain quality of the composite material is required (Fujita and Masuda 2014; Kucher et al. 2023).

In order to increase the percentage of reused composite materials, repurposed applications for the root sections of WTBs need to be investigated, in particular, due to the continuous structural geometry of the root section (Joustra et al. 2021). It is the part of the blade that is connected to the wind turbine and is shaped as a circular tube (Beauson et al. 2022). It usually consists of thick and massive GFRP walls. For WTBs with a length of more than 60 m, the root section is geometrically characterized by an average diameter of more than 3 m and a length of more than 5 m (Joustra et al. 2021). The aim of this study is to introduce a tiny house concept from the root section structures of a decommissioned WTB that could overcome all the previously mentioned challenges in repurposing. In a proof of concept, it is shown whether a tiny house made from the root section material of the reference model of a 5 MW/61.5 m wind turbine blade (Löffler 2022; Resor 2013) can withstand the mechanical requirements. The concept of the repurposed tiny house was inspired by the architectural style of Frankenwald chalets, a popular tourist accommodation (Löffler 2022).

Furthermore, the environmental impacts and benefits of repurposing the root section of large-scale WTBs as a tiny house are evaluated using the life cycle assessment (LCA) approach. LCA is a widely accepted, holistic, and standardized (ISO 14040 2006; ISO 14044 2006) methodology for assessing the environmental impacts of processes or products. Previous LCA investigations on the repurpose of decommissioned wind turbine components into new structures have shown significant environmental benefits, either compared to conventional waste treatment processes such as landfilling and incineration (Deeney et al. 2021), or compared to conventional manufacturing of new structures from raw materials (Henao et al. 2022). Several possible repurposed structures have been proposed for decommissioned wind turbine components. However, to our knowledge, the utilization of the root section of wind blades for the construction of tiny houses has not been previously investigated from a sustainability perspective.

Material and Methods

5 MW/61.5 m WTB Reference Model as Base Material

The 5 MW/61.5 m wind turbine blade (WTB) reference model serves as the base material within this study to investigate the blade design and determine the structural properties of the repurposed materials. The design of the WTB model was formulated through a collaborative effort between the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories, using insights from an analysis conducted as part of the Dutch Offshore Wind Energy Converter (DOWEC) project (Jonkman et al. 2009; Resor 2013). The choice of this model resulted from the fact that it differs from commercial blades, as it was designed for research purposes with publicly available specifications (Tasistro-Hart et al. 2019). With a length of 61.5 m, it was deliberately designed for a 5-MW wind turbine, a size common to both onshore and offshore wind turbines, and is thus an appropriate representation of today’s wind turbines (Liu and Barlow 2017; Resor 2013).

According to Fig. 1, the reference WTB consists of three different sections: the root, the midspan, and the outboard section (Jonkman et al. 2009). The weight of the blade is 17.7 t in its full scale (Resor 2013). The root section is examined more closely: It has a diameter of 3.4 m and a length of 5.5 m with an average wall thickness of approximately 50 mm. The weight of the root section is approximately 4.7 t, which is about 25% of the total weight of the whole WTB, as a lot of material is needed in the root section area. For the composite material, the weight of each of the main components (1.4 t of epoxy resin, 2.95 t of glass fibers, 0.3 t of foam, 0.05 t of carbon fiber-reinforced prepreg resin) was calculated based on average values reported for the WTB structure and a developed geometrical model based on this data (Resor 2013). The root section of the 5 MW/61.5 m WTB reference model consists of a high weight percentage of GFRP.

Fig. 1
figure 1

Root section of 5 MW/61.5 m WTB reference model

Full size image

Concept of the 5 MW House from Root Section Material

The 5 MW/61.5 m WTB root section allows it to be repurposed as a tiny house with the aim of minimizing cutting effort (i.e., cutting in straight lines) and maximizing the utilization of EoL WTB material. First, an architectural design was developed as a sustainable living unit for vacation with a functional structure (see Fig. 13 in the appendix). The root section is repurposed as the building envelope, and the two shear webs can be used as load-bearing and insulated floor plates. The floor plate is positioned to maintain a distance of 0.5 m between the floor and the inside edge of the building envelope, allowing the user to stand upright and to provide storage space. The proposed concept is shown schematically in Figs. 2 and 3. After dismantling the reference WTB from the wind turbine tower, the concept requires two cutting steps.

Fig. 2
figure 2

Manufacturing concept of the 5 MW house from the root section material of the 5 MW/61.5 m WTB reference model

Full size image
Fig. 3
figure 3

Photorealistic representation of the 5 MW house concept

Full size image

The first cutting step is planned with a water-cooled diamond wire saw (Hechler 2019) to cut out the root section from the rest of the WTB. The second cutting step uses a power saw to cut both shear webs out of the reference WTB. These shear webs are cut to the required dimensions to be used as floor panels for the 5 MW tiny house built from 5 MW/61.5 m WTB root section parts — hereafter referred to as the “5 MW house.”

The cutting processes resulting in following composite structures to be used as repurposed material: cylindrical building envelope and floor plate (Fig. 2). Based on this concept of the 5 MW house structures (Fig. 3), the amount of decommissioned material, the amount of additional input materials, and the energy requirements for cutting and transporting the decommissioned wind turbine components could be quantitatively calculated (cf. “Approach to Assess the Tiny House from an Environmental Point of View” — inventory analysis).

Finite Element Analysis of EoL WTB Segments

The finite element analysis (FEA) is based on the introduced base material (cf. “5 MW/61.5 m WTB Reference Model as Base Material”). In order to prove the concept of the 5 MW house from the decommissioned reference WTB (cf. “Concept of the 5 MW House from Root Section Material”), the FEA was carried out considering the mechanical and thermal requirements for a tiny house. Therefore, the simulation software Ansys (Ansys Workbench 2022 R2, Ansys Inc.; Canonsburg, Pennsylvania, USA) was used together with an AMD Ryzen CPU (AMD Ryzen 7 4700U with Radeon Graphics, 2000 MHz, 8 cores, 9 logical processes). For the developed design concept (see Fig. 2 and Fig. 3), a static mechanical analysis was carried out taking into account snow load, wind load, payload, and self-weight load, as well as a thermal stress analysis for a critical temperature distribution. It was assumed that the front and the back wall of the 5 MW house are not included in the FEA as they are not materials repurposed from WTBs.

Simulation Model

The blade geometry, including blade span \(R\), airfoil profile, chord length, airfoil translation, and airfoil rotation, is based on the definition of the 5 MW/61.5 m wind turbine blade reference model (Resor 2013). A quadrilateral dominant mesh with a maximum element size of 50 mm for the WTB segment’s geometry was used according to the recommendation of Resor (2013). This yields a mesh which consists of 28.763 elements and 28.875 nodes for the WTB segment (Fig. 4a).

Fig. 4
figure 4

Simulation model: a geometry and mesh of repurposed WTB segments, b mounting of the repurposed WTB segments, c projected areas of snow load and payload, d streamlines and surface pressure distribution \({p}_{\mathrm{wind}}\) for the estimation of the wind load, e projected areas of wind load and payload, f temperature distribution for the static thermal analysis and the following thermal stress analysis

Full size image

The material and the layers in the stacks of the 5 MW/61.5 m WTB were used for this conceptual study (Resor 2013) (Tables 1 and 2; Fig. 5). It should be noted that the material properties not provided by Resor (2013) were taken from the Ansys Granta Material Properties Database or were estimated using micromechanical rules of mixture (e.g., Schürmann 2007). Therefore, the material properties of the individual constituents provided by Schürmann (2007) are considered.

Table 1 Mechanical material properties (data from Griffith 2013; Resor 2013; Schürmann 2007)
Full size table
Table 2 Physical, thermal, and thermo-mechanical material properties (data from (Griffith 2013; Resor 2013; Schürmann 2007))
Full size table
Fig. 5
figure 5

Thickness of individual layers of the investigated WTB segment

Full size image

The fiber direction of all orthotropic layers was oriented in the axial direction of the blade. The floor blade is made by two repurposed shear webs. Each shear web consists of a foam core with a thickness of 50 mm and two outer layers of the GF/EP Saertex material with a thickness of 1 mm (Table 2; Resor 2013).

Applied Geometrical Boundary Conditions and Load Cases

The cylindrical building envelope lies on six stands (Fig. 3). The contact areas of the stands were clamped without displacement or rotation (Fig. 4b). A volumetric load representing the self-weight load of the structural components was considered. The connection between the WTB segment and the floor plate was assumed as a fixed constraint. With these assumptions, the following three load cases of the 5 MW house are investigated using FEA:

  • Load case (1): Combination of snow load and payload (see Fig. 4c, isoparametric shell element with isotropic/orthotropic material),

  • Load case (2): Combination of wind load and payload (see Fig. 4e, use both isoparametric solid and shell elements with isotropic/orthotropic material),

  • Load case (3): Thermal stress analysis of the critical temperature distribution (see Fig. 4f, isoparametric solid elements with isotropic/orthotropic material).

According to Genzel and Voigt (2005) and Baar (2021), a snow load with a value of \({p}_{\text{snow}}\) = 1 kN/m2 and a horizontal wind load represented by a wind speed with a value of \({v}_{\text{wind}}\) = 22.5 m/s were considered (Fig. 4c and e). Using a computational fluid dynamics (CFD) software (CFX 2022 R1, Ansys Inc.; Canonsburg, PA, USA), a flow analysis of the airflow around the 5 MW house was performed to determine the distribution of the surface pressure \({p}_{\mathrm{wind}}\) on the windward and leeward side of the 5 MW house (Fig. 4d). For the quantification of the snow load, a smooth pipe was assumed, where the snow slides off from a slope of 45° onwards. A payload on the floor plate of \({p}_{\text{payload}}\) = 1.5 kN/m2 was applied (Ackermann et al. 2014; Baar 2021). An unequal temperature distribution was assumed for the thermal analysis of the repurposed WTB segments. Therefore, a temperature of \(\vartheta =\)−4 °C was considered for the lower part of the cylindrical building envelope and a temperature of \(\vartheta =\) 20 °C was applied to the top of the composite structure (see Fig. 4f). Using these boundary conditions, the resulting temperature distribution was calculated by means of a thermal steady-state analysis. This temperature distribution approximates the situation in central Europe during fall or spring when the ground is frozen and the heat radiation of the sun heats the 5 MW house’s upper parts, which was considered as critical ambient condition.

For each load case, the practically relevant maximum deformations \({u}_{i}\) of the composite structure and the reserve factor of the Puck’s failure criterion \({f}_{\mathrm{R}}\) (Puck et al. 2002) are determined using the resulting stresses within each layer and the parameters of the Puck’s failure criterion (Table 1). A reserve factor of \({f}_{\mathrm{R}}<1\) indicates failure. In case of a reserve factor of \({f}_{\mathrm{R}}\ge 1\), the structure withstands the applied loads. The resulting reserve factor was used for the failure assessment of the 5 MW house for the different loads applied. In addition, the calculated deformations of each load case were evaluated using the Ackermann et al. (2010) deformation limit for structural composites.

Approach to Assess the Tiny House from an Environmental Point of View

To assess the environmental impacts, a comparative LCA was applied to the envisioned 5 MW house and to a commercial tiny house, manufactured from virgin instead of repurposed construction materials. LCA was performed according to ISO standardized procedures (ISO 14040 2006; ISO 14044 2006), and consisted of four stages: (1) Goal and scope definition, (2) Inventory analysis, (3) Impact assessment, (4) Interpretation. The definition of life cycle stages under investigation was based on EU standard EN 15978 for the environmental performance estimation of buildings (2012) and is schematically depicted in Fig. 6.

Fig. 6
figure 6

Value chain of repurposed (a) and commercial (b) tiny houses. The processes included in the system boundaries are indicated with black diagrams and font, while processes excluded from the system boundaries are indicated by gray diagrams and font

Full size image

Goal and Scope

The goal of this study is to compare a commercial tiny house constructed primarily of wood with a house manufactured primarily of a decommissioned WTB root section (the 5 MW house). A cradle-to-grave approach was used. The functional unit was defined as a tiny house, which can provide accommodation for up to two people for 50 years. While there are several environmental impacts associated with the use phase of the structure, such as electricity consumption over the 50 years of lifetime, this stage was excluded, as no difference can be predicted between the two houses in terms of the use phase. Nevertheless, materials with service life of at least 50 years were selected for the inventory of the two houses whenever possible (i.e., when the reference service life was reported) to ensure that the intended lifetime is comparable between the two houses, despite exclusion of the use phase. Repurposing of wind turbine components as tiny houses is an alternative to other EoL processes, such as recycling, repurposing in different applications, incineration, or landfilling, and therefore, an avoided EoL treatment process could be credited to the system boundary of 5 MW houses (e.g., avoided transport of WTB waste to EoL treatment location and landfilling or recycling). However, considering the variety of possible repurpose applications and recycling methods, as well as the possibility to repair and re-use components for wind turbines, selecting a representative EoL treatment process alternative as an avoided process was not possible, and therefore, it was excluded from the system boundaries. Decommissioned components enter the system boundaries without any impact from the materials, manufacture, use, or EoL phase of the original structure (i.e., wind turbines). The impacts of repurposed components’ separation from the wind turbines (e.g., wire sawing) and of their transport to the construction site are credited to the lifecycle of the 5 MW house. The remaining components of the wind turbine are excluded from the system boundaries. While several recycling processes exist to treat waste composite materials from wind turbines (Kalkanis et al. 2019), a prediction of their development by the end of the 50-year life cycle of tiny houses cannot be made accurately by the authors. Therefore, the conventional processes of landfilling and incineration with energy recovery were considered for the structural components of the two houses. The value chain described in this section are also visually presented in Fig. 6. For components assessed based on published environmental product declarations (EPDs), the system boundaries and included processes (e.g., energy, raw materials, waste and byproducts at different stages) are modelled as reported in the corresponding EPD. The system boundaries for the repurposed root section are shown in Online Resource 1 (SI_1, Section 1, Figure S1).

Life Cycle Inventory (LCI)

For the commercial tiny house, LCI data was collected from published EPDs and from the Ecoinvent 3.8 database, modelled using the LCA software SimaPro (version 9.4.0.2). The design of the commercial tiny house was based on the tourist accommodation Frankenwald Chalets (https://frankenwald-chalets.de/), located in Wilhelmsthal, Germany, selected due to their half-elliptical structure, which resembles the structure of the 5 MW house (Figs. 2 and 3), thus making them comparable over multiple criteria (i.e., size, amount of input materials, aesthetics, living space, amenities). The location of the chalets was selected as the construction site of both commercial and 5 MW tiny houses, and all distances for road transport of construction and waste materials (via freight, lorry 3.5–7.5 metric ton, EURO4) were calculated based on this location using Google maps. The size of the commercial house was assumed to be the same as the 5 MW house (5.5 m length, 13 m2 interior floor area). To compare the commercial and 5 MW house, the LCI was limited to materials and processes that are different between the two structures, whereas materials and processes that are the same (e.g., appliances, doors, and windows, and their corresponding manufacture, transportation, application, deconstruction, and EoL treatment) were excluded. Based on this, a bill of materials was created for the two houses (Table 3), and a detailed description of the processes and materials included in the LCI is given in Online Resource 1 (SI_1, Section 1).

Table 3 Bill of materials and corresponding EPDs for the commercial and 5 MW tiny house
Full size table

Impact Assessment

The Life Cycle Impact Assessment (LCIA) was performed using SimaPro for processes modelled based on data from Ecoinvent, whereas for processes and materials modelled based on EPDs, the LCIA results were directly retrieved from the corresponding EPDs. In order to ensure uniformity of the LCIA results, EPDs were exclusively selected that use the EU Standard EN15804 + A2 Standard for LCIA (2019), and the same method (EU Standard EN15804 + A2 version 1.03) was also selected for LCIA in SimaPro. In this method, the environmental impacts are calculated and reported over multiple midpoint impact categories, and the following 19 were included in the assessment: climate change — total (kg CO2 eq); ozone depletion (kg CFC11 eq); ionizing radiation (kBq U-235 eq); photochemical ozone formation (kg NMVOC eq); particulate matter (disease inc.); human toxicity, non-cancer (CTUh); human toxicity, cancer (CTUh); acidification (mol H+ eq); eutrophication, freshwater (kg P eq); eutrophication, marine (kg N eq); eutrophication, terrestrial (mol N eq); ecotoxicity, freshwater (CTUe); land use (Pt); water use (m3 depriv.); resource use, fossils (MJ); resource use, minerals and metals (kg Sb eq); climate change — fossil (kg CO2 eq); climate change — biogenic (kg CO2 eq); and climate change — land use and land use change (LULUC) (kg CO2 eq). All LCIA results presented in the “Results” section stem from the characterization step of the EU Standard EN15804 + A2 method, whereas no normalization or weighing was performed on the LCIA results.

Results

Results of Finite Element Analysis

Deformation and Failure Analysis for a Combination of Snow Load and Payload (Load Case 1)

For the combined snow load and payload, a maximum deformation of 9.8 mm was determined at the roof of the 5 MW house (Fig. 7a). The stresses between the floor plate and the WTB envelope reach the highest values. However, a minimum reserve factor of 5.9 was obtained in this region (Fig. 7b). The CPU runtime for the simulation was 22 s.

Fig. 7
figure 7

a Resulting magnitude of deformations \(|{u}_{i}|\) and b reserve factor of the Puck’s failure criterion \({f}_{\mathrm{R}}\) for a combined snow load and payload

Full size image

Deformation and Failure Analysis for a Combination of Wind Load and Payload (Load Case 2)

For the combined wind load and payload, a maximum deformation of 7.6 mm was determined at the roof of the 5 MW house (Fig. 8a). The stresses between the floor plate and the WTB envelope reach the highest values. However, non-critical loads were identified due to a minimum reserve factor of 5.3 in this region (Fig. 8b). The CPU runtime for the simulation was 265 s.

Fig. 8
figure 8

a Resulting magnitude of deformations \(|{u}_{i}|\) and b reserve factor of the Puck’s failure criterion \({f}_{\mathrm{R}}\) for a combined wind load and payload

Full size image

Deformation and Failure Analysis for an Unequal Temperature Distribution (Load Case 3)

Due to the unequal temperature distribution (Fig. 9a), unequal deformation of the WTB envelope occurs. The highest deformation magnitude was reached at the roof with a value of 9.2 mm (Fig. 9b). For the static thermal analysis, the maximum stresses occurred between the floor plate and the WTB envelope, as in load case 1 and load case 2. However, a minimum reserve factor of 1.9 was determined (Fig. 9c). The CPU runtime for the simulation was 199 s.

Fig. 9
figure 9

a Unequal temperature distribution \(\vartheta\) of repurposed WTB envelope; b resulting magnitude of deformations \(|{u}_{i}|\); c reserve factor of the Puck’s failure criterion \({f}_{\mathrm{R}}\) for an unequal temperature distribution

Full size image

According to Ackermann et al. (2010), the deformation limit \({u}_{d}\) of structural composites with high requirements should not be exceeded, where \({u}_{d}\) is given by Eq. (1).

$${u}_{d}\le \frac{l}{200}$$
(1)

Using the repurposed envelope diameter of 3400 mm as the support width \(l\), Eq. (1) gives a deformation limit of 17 mm. The resulting magnitude of deformation for load case 1 at 9.8 mm, load case 2 at 7.6 mm, and load case 3 at 9.2 mm are all below the limit of deformation. In addition to the magnitude of the observed deformations, the results obtained from the reserve factor of the Puck’s failure criterion \({f}_{\mathrm{R}}\) indicate that the repurposed WTB structures can withstand the investigated loads with reasonable safety margins. This is achieved even in the absence of a front and back wall. Furthermore, it can be assumed that the 5 MW house structure, with the inclusion of both front and back walls become more stable against loads induced by the case of the three considered load cases.

Environmental Assessment of the Tiny House

The results of the comparative LCA for the commercial and 5 MW tiny house are shown in Fig. 10. The environmental impacts were calculated for one house, as per the functional unit definition. For each impact category, the cumulative contribution of materials and processes for each house is expressed as a percentage, compared to the house contributing to the highest (or lowest negative) impact, which is set to 100% (or negative 100%, in the case of negative environmental impacts, i.e., environmental benefits). In Fig. 10a, the impacts of upstream processes are reported, i.e., the production of raw building components (A1–A3), their transportation to the construction site (A4), and the construction of the house, including installation of materials and treatment of packaging waste (A5). In Fig. 10b and c, the full life cycle from cradle-to-grave is assessed for each house, assuming different scenarios for the EoL treatment of the main construction materials (i.e., landfilling in Fig. 10b and incineration with energy recovery in Fig. 10c).

Fig. 10
figure 10

Comparative LCA between the commercial and 5 MW tiny house with different scenarios. a LCA of production and construction phase, including production of materials (A1–A3), transportation to the construction site (A4) and installation (A5). b, c Cradle-to-grave LCA of the two houses assuming that the construction materials are either landfilled (b) or incinerated for energy production (c) at the end of their life cycle

Full size image

When only the production and construction phases are considered (Fig. 10a), the 5 MW house had lower environmental impacts compared to the commercial structure in most impact categories, except for climate change and ozone depletion, with, for example, up to 97% decrease for non-cancer human toxicity and particulate matter potential. In addition to climate change and ozone depletion, when the entire life cycle is considered, assuming landfilling for the EoL treatment (Fig. 10b), the 5 MW house also had higher environmental impacts for marine and terrestrial eutrophication impact categories. Instead, for the EoL scenario of incineration with energy recovery (Fig. 10c), the 5 MW house had lower environmental impacts for most impact categories, except for climate change, marine, and terrestrial eutrophication. Specifically, the total climate change potential of the repurposed house was approximately 2.2 times higher for the 5 MW house when stages A1–A5 are considered, and approximately 3.7 and 1.7 times higher when the full life cycle is considered for the landfill and incineration scenario, respectively. This is partially due to the biogenic climate change potential for wood material, the main construction component for the commercial tiny house, which has a negative environmental impact (i.e., an environmental benefit) for both plywood and timber wood included in the inventory, as a result of carbon dioxide storage from the atmosphere in wood biomass via the process of photosynthesis (Hoxha et al. 2020). Considering the negative biogenic carbon impact, as well as the advantage of biobased materials in terms of resource depletion, compared to non-renewable building components, wood is considered the best building material from a sustainability perspective (Stepien et al. 2022; Yadav and Agarwal 2021). This finding is often confirmed in LCA studies comparing wood with other building materials, with the former resulting in lower environmental impacts when the construction and EoL stages are considered (Abed et al. 2022; Grygierek et al. 2020; Petersen and Solberg 2005; Schenk and Amiri 2022). Besides the negative environmental impacts (i.e., environmental benefits) for biogenic carbon of the commercial tiny house, energy recovery upon incineration also contributed to negative environmental impacts across several impact categories for both houses (Fig. 10c), due to the avoided processes of heat and electricity production modelled in this scenario. Nevertheless, a clear conclusion cannot be drawn regarding which EoL treatment process has the best environmental performance, as the advantages of landfilling over incineration varied per impact category. A comparison of the impacts between the two EoL treatment processes for each house is shown in Online Resource 1 (SI_1, Section 2, Figure S2).

Aside from the cumulative impact of all materials and processes in each impact category, the contribution of different stages per impact category was also calculated for the two houses, and the results are shown in Fig. 11 (for the 5 MW house) and in Online Resource 1 (SI_1, Section 2, Figure S3, for the commercial tiny house). For the 5 MW house under both EoL scenarios, the highest contribution to particulate matter and non-cancer human toxicity was from the production of raw materials and, specifically, the production of the adhesive used to connect the floor to the house envelope, which contributed more than 90% to these impact categories among all materials and processes in stages A1–A5. For most other impact categories in the landfill scenario, the highest contributor was the transport of materials to the construction site (A4; Fig. 11a) and particularly the transport of repurposed wind turbine components, due to the high weight (over 4.7 t) and transport distance (678 km), resulting in over 35% of the marine eutrophication, over 64% of the terrestrial eutrophication, over 59% of total climate change, and over 62% of the ozone depletion potential impact categories along the entire 5 MW house life cycle under the landfill scenario. Instead, the highest contributor to the marine eutrophication impact category for the 5 MW house under the landfilling EoL scenario was the landfill process itself (C4, over 44% of impacts for marine eutrophication along entire life cycle; Fig. 11a). Within the landfill process of the repurposed components, the mixed plastic material used as a proxy for the epoxy resin had the highest contribution to the marine eutrophication impact category (97%), despite only contributing to 30% of the repurposed components’ weight, and the substance responsible for this high impact was mainly the ammonium ions released to water upon landfilling of mixed plastic waste, which contributed to over 88% of the total marine eutrophication impacts within the repurposed material landfilling process. Similarly, for the incineration scenario, the transportation of materials to the construction site (A4) and the incineration process (C3) had the highest contribution to most impact categories examined (Fig. 11b). For example, the transport of repurposed wind turbine components to the construction site contributed to over 26% of total climate change, over 63% of marine eutrophication, and over 68% terrestrial eutrophication potential along the entire life cycle of the 5 MW house under the incineration scenario. The incineration of repurposed components contributed to over 60% of the total climate change potential along the entire life cycle of the 5 MW house, primarily due to the fossil CO2 emissions of mixed plastic incineration, which corresponded to over 90% of the total CO2 eq. emissions of repurposed component incineration, despite the low weight percentage of the epoxy resin compared to the total weight of repurposed components (30%). Based on these findings, and considering the inferior environmental performance of the 5 MW house compared to the commercial house for ozone depletion, marine, and terrestrial eutrophication (Fig. 10b and c), it stands to reason that decreasing the distance between the wind turbine park and the construction site could significantly improve the environmental performance of the 5 MW house. Furthermore, more specialized LCA library processes for composite material components ought to be considered. Mixed plastic waste was used as a broad proxy for the epoxy resin, based on previously published LCA work on composite materials (Hermansson et al. 2022; Stergiou et al. 2022; Vo Dong et al. 2018; Wu et al. 2023). However, given the significant impacts of EoL treatment, a more specific proxy for resins, in combination with more accurate data of composition for different parts of the repurposed components (i.e., cylindrical structure, shear webs), would be needed to improve the understanding of its specific impacts and to contribute to the development of material-specific mitigation strategies for EoL impacts.

Fig. 11
figure 11

Percentage contribution of different LCA stages to the overall impact in each impact category within the full life cycle of the 5 MW tiny house for the two examined EoL scenarios, i.e., landfilling (a) and incineration with energy recovery (b)

Full size image

Considering the significant contribution of the transportation process of construction materials from the manufacturing to the construction site (A4), an additional scenario was assessed for the repurposed tiny house, assuming a different wind park location. Specifically, for this scenario, the wind park Stößen Teuchern Standört Gröbitz was selected, with a distance of 136 km from the construction site. The full life cycle of tiny houses was considered for this scenario, assuming a landfilling process at the EoL stage, and three houses were compared: a 5 MW house with the original location of the wind park (i.e., 678 km), a 5 MW house with the adapted location of the wind park (136 km), and a commercial tiny house. The results of this comparison are shown in Fig. 12. Additional results on the comparative LCA of the 5 MW and commercial tiny house with the adapted location of the wind turbine park, as well as the percentage contribution of different life cycle stages with the adapted location, are shown in Online Resource 1 (SI_1, Section 2, Figures S4 and S5). As can be observed in Fig. 12, while the climate change potential and marine eutrophication were still lower for the commercial tiny house, the modelled decrease in transportation distance for the 5 MW house resulted in a significant decrease of environmental impacts for all impact categories, of up to 51% (for minerals and metals resource depletion). Decreasing the transportation distance resulted in lower impacts of the 5 MW house for ozone depletion and terrestrial eutrophication compared to the commercial house, as opposed to the original distance modelled (Fig. 10b). Overall, these results highlight the potential of the envisioned repurposed application to provide a sustainable solution for the EoL treatment of decommissioned wind turbine components. Finally, these results demonstrate the significant effects of transportation processes, which ought to be carefully considered in process design and sustainability assessments.

Fig. 12
figure 12

LCA of commercial and 5 MW tiny house, with landfill as the EoL treatment process for construction materials, assuming two different scenarios for the source of wind turbine components, corresponding to long distance (678 km) and a short distance (136 km) from the construction site

Full size image

Discussion

Issues that have yet to be addressed, but which warrant consideration, include both compliance with building regulations relevant to tiny houses and the degradation of mechanical properties with composite materials over time. These concerns are addressed in the following sections.

The regulations for tiny houses are governed by various existing laws, the Landesbauordnungen (state building codes) in Germany as building codes, which are largely the same for all German states, and EU regulations, which apply to all structures and regulated or compliant building products. Buildings must be designed, constructed, altered, and maintained in such a way that public safety and order, in particular life, health, and the natural foundations of life, are not endangered. Construction products and types of construction may only be used if they fulfil these principles within a reasonable period and are fit for use (§3(1); Saechische Bauordnung 2016). Buildings must be arranged, constructed, and fit for use in such a way that no hazards or unacceptable nuisances are caused by water, moisture, plant and animal pests, or other chemical, physical, or biological influences (§13(1); Saechische Bauordnung 2016). To ensure this, every building or construction must be officially approved. Tiny houses as detached buildings with up to 400 \({\mathrm{m}}^{2}\) and up to two usage units are defined as building class 1 (§2(3); Saechische Bauordnung 2016) with the lowest general requirements for the construction, such as stability (§12); protection against harmful influences (§13); fire protection (§14); heat, sound, and vibration protection (§15); and traffic safety (§16). The design, material definition, and dimensions of the 5 MW house follow these standards. The foundation, as a steel structure, must be fire-retardant, but there are no requirements for the living floor. The repurposed shear webs as a floor structure with 5 cm of foam must be doubled to achieve the required U-value of 0.35 W/(\({\mathrm{m}}^{2}\mathrm{K}\)) as the heat transfer coefficient (Gebäudeenergiegesetz 2020). In addition, new buildings will have to incorporate a certain amount of renewable energy, for example, through the use of photovoltaic or solar thermal systems. To extend the usability of the 5 MW house, it could even be developed as an Autarc building, following the example of architect Renzo Piano’s Diogene (Vitra International AG 2013) or the Ecocapsule (Ecocapsule 2023), a self-sustaining capsule house.

Another important aspect is that during the service life of WTBs, the properties of the WTBs degrade due to fatigue loads since the WTB rotates (Alshannaq et al. 2021; Joustra et al. 2021). This degradation behavior of the composite material must be considered to ensure the structural integrity of the repurposed composites components. There are few studies in the literature on the residual strength of the composite material of WTBs that were previously in-service and subsequently decommissioned. On the one hand, according to Sayer et al. (2009), Beauson and Brøndsted (2016), and Ruane et al. (2022), no significant damage to the WTB was observed in visual inspection and the composite retained its high strength and stiffness. On the other hand, (Alshannaq et al. 2021) stated that the strength and stiffness of the composite in a WTB degrades in a range of 10–20%. Consideration of degradation was outside the scope of the finite element analysis. However, future studies need to consider the degradation and EoL damage state of the repurposed composite structures.

Building envelopes and load-bearing structures made of GFRP from the 1960s and 1970s (Genzel 2006; Voigt 2007), which are still in use today, prove the predicted durability of these lightweight materials. In particular, the tube house (1970–1994, BRD) by Swiss architect Franz Ulrich Dutler, structural engineer Heinz Isler, CH, can serve as a reference (Genzel and Voigt 2005). Long-term strength is calculated using reduction factors from the short-term strengths determined on the material. In addition to fatigue, ageing due to UV, temperature and assumed manufacturing inaccuracies are also considered. Heinz Isler measured these reductions very narrowly, relying on his own measurements and structural tests (Genzel and Voigt 2021).

Conclusions

Repurposing may become the best option for GFRP from discarded WTBs if recycling is still not economically viable and co-processing is not reasonable due to circular economy principles (Nagle et al. 2022). Therefore, the aim of the current study was to present a conceptual solution for the root section of a blade from the 5 MW/61.5 m wind turbine reference model. The authors believe that the proposed concept could be a high-volume industrial application, especially in light of the housing shortage, using a large amount of reused GFRP in the tiny house market. Firstly, the root section of large-scale WTBs provides reliable composite materials with a continuous geometry. Secondly, the 5 MW house concept requires little reprocessing and transportation of the cut root section components seems to be feasible.

The numerical investigations showed that the 5 MW house can be a suitable application from a mechanical point of view. Deformation and failure analyses were performed through finite element analysis. According to the results obtained from Puck’s failure criterion, it appears that repurposed WTB components (cylindrical building envelope and floor plates) can effectively endure the expected loads with reasonable safety margins. The 5 MW house concept could represent a good practice example within the circular economy approach, as the composite materials maintain highest utility and value. In addition, it was found that environmental benefits can be expected from the use of decommissioned wind turbine components for construction, compared to the use of virgin materials, with a potential for environmental impact decrease of up to 97% over the entire life cycle of tiny houses.

As freestanding buildings with a small base area, tiny houses are subject to very few building codes. In particular, the fire protection requirements, which are currently undefined for GFRP WTBs, are low for this building class and can therefore be satisfied by the 5 MW house concept. Additionally, the thermal insulation requirements, especially those outlined in the Gebäudeenergiegesetz (2020), could also be met conceptually.

In 2015, member nations of the United Nations collectively established a set of 17 global Sustainable Development Goals (SDGs) with the aim of eradicating poverty, safeguarding the environment, and securing prosperity for everyone (UNDP 2023). The tiny house presented in this study as a repurposed application could contribute to several SDGs, e.g., Goal 11 (sustainable cities and communities), Goal 12 (responsible consumption and production), or Goal 13 (climate action).