Building-integrated photovoltaic roof tiles
An LCA analysis of material and energy emissions NEPD-2709-1409-NO
AAR4817 Emissions as Design Drivers: Theory Spring Semester 2022
MSc in Sustainable Architecture Norwegian University of Science and Technology
Abstract
The increasing demand for zero-energy and zero-emission buildings could be fulfilled with low environmental impact building products and renewable energies. BIPV roof tiles have the potential to produce green energy to counterbalance embodied emissions without compromising the architectural aesthetics. The report investigates the impact and energy consumption of the Skarpnes concrete roof tile, to find out how BIPV can contribute to a sustainable future in energy and construction sectors. The study evaluates the performance of the roof tile in three LCA scenarios, showing that the product has the lowest emissions and EPBT compared to ceramic and zinc solutions.
Keywords
life cycle assessment (LCA), roof tile, environmental impact, BIPV, embodied energy, environmental product declaration (EPD)
Table of contents
Abstract
Table of contents
List of abbreviations
List of figures
Lists of tables
Introduction Methods
Goal and scope
Life cycle inventory
Results
Discussion
Conclusions
References
Appendix: Renduzer and Excel data
List of abbreviations
Concepts
BAPV
BIPV CLT
Building-attached photovoltaics
Building-integrated photovoltaics
Cross-laminated timber
Energy payback period
Environmental product declaration
Greenhouse gas
GWP GWP/kWh
LCA
TNRPE TRPE Study cases S
Global warming potential
Global warming potential per energy produced
Life cycle assessment
Total non-renewable primary energy
Total renewable primary energy Scenario
Scenario 1. Material: Products Scenario 2. Material: Building parts/components Scenario 3. Energy: BIPV/BAPV
Product
Product: Skarpnes Falt concrete roof tile
Product: Braas Turmalin ceramic roof tile
Product: Rheinzinc double-standing seam titanium zinc slates
Building part/component
Building part: Glulam sloped roof
Building part: Glulam sloped roof with concrete roof tiles
Building part: Glulam sloped roof with ceramic roof tiles
Building part: Glulam sloped roof with titanium zinc roof slates
Building part: CLT sloped roof
Building part: CLT sloped roof with concrete roof tiles
Building part: CLT sloped roof with ceramic roof tiles
Building part: CLT sloped roof with titanium zinc roof slates
Photovoltaic
PV1
Building-integrated photovoltaic concrete roof tile
Building-integrated photovoltaic ceramic roof tile
Building-attached photovoltaic titanium zinc roof slate
List of figures
Figure 1
Figure 2
Figure
Figure
Figure
Figure 6
Concrete roof tile (Source: Skarpnes AS).
Scenarios analyzed in the report.
GWP and composition of Scenario 1 study cases.
Mass and GWP by-product of Scenario 2 study cases (BP1).
Mass and GWP by-product of Scenario 2 study cases (BP2).
Comparison of FutureBuilt Zero and NS 3720 calculation methods for BP1 (50 years service life assumption).
Figure 7
Figure 8
GWP and EPBT of Scenario 3 study cases.
Comparison of FutureBuilt Zero and NS 3720 calculation for BP1-P1 and BP2-P1.
List of tables
Table 1
Table 2
Table 3
Table 4
Table 5 Table 6
Technical data and EPD information of roof tiles.
Layer composition (from top to bottom) and Reduzer data of the sloped roofs.
Technical data of BIPV and BAPV roof tiles.
LCA results for roof tiles in Scenario 1.
LCA results for sloped roofs in Scenario 2.
LCA results for BIPV and BAPV roof tiles in Scenario 3.
ROOF TILE
Introduction
In the current context of climate change and global warming, companies, public institutions, and particular clients are demanding zero-energy and zero-emission buildings1 (1). While society awareness of sustainability and clean energies facilitates the achievement of the 17 Sustainable Development Goals of the United Nations, the growing interest in building products with lower greenhouse gas (GHG) emissions is still a challenge for the energy and construction sectors, in terms of affordability, availability, and efficiency of the existing technologies (2).
According to the European Parliament (3), the building stock is responsible for approximately 36% of all CO2 emissions and 40% of the energy use in the European Union. Specifically, sloped roofs have a considerable impact on global energy performance, being directly exposed to weather conditions (4) and “absorbing the highest thermal load of a building structure” (5, p. 74). The energy loses through the roof depends on the climate, orientation, insulation —thickness and thermal conductivity—, and the Solar Reflectance Index of the finishing layer which, in the case of residential constructions, is usually covered with roof tiles.
Regarding environmental performance, pitched roofs account for around 25% of the total emissions of the building (6). The existing literature that analyses GHG emissions of roof construction systems using Life Cycle Assessment (LCA) techniques —reviewed by Carretero-Ayuso and García-Sanz-Calcedo (7)—, shows that the environmental impact of sloped roofs is determined by the embodied energy, CO2 emissions, waste, transportation distances, and service life of the materials used. Other socioeconomic parameters, like the difficulty of execution, labor, and material costs, can influence the choice of roof typology. Within this framework, this report investigates the concrete flat roof tile manufactured by Skarpnes AS, using the information available in the Environmental Product Declaration (EPD) NEPD-2709-1409-NO (8) In particular, the present study is focused on the Skarpnes Flat ‘solcelletakstein’ (9), a building-integrated photovoltaic (BIPV) roof tile developed with funding from the European Union’s Horizon 2020 project.
The selection of this product arises from the previously explained demand for zero-emission buildings. BIPV roof tiles have the potential to produce renewable energy to counterbalance the embodied emissions of the materials without compromising the architectural aesthetics (Figure 1), thus contributing to the holistic LCA of the environmental impact of buildings.
1 Moschetti et al. state that the zero-energy building concept is being displaced by the zero-emission building, which provides a holistic approach to use renewable sources of energy —solar electric photovoltaic (PV), biomass or geothermal heat pump, etc.— to compensate embodied emissions in building materials during the entire life cycle.
Methods
Goal and scope
This report proposes the following research question: “How the BIPV concrete roof tile from Skarpnes AS can, in comparison with other roof solutions and PV technologies, contribute to a sustainable future both in construction and energy sectors?”. Therefore, the main goal of this work is to analyze the GHG emissions and embodied energy of Skarpnes roof tile in contrast with other BIPV roof tiles in three different scenarios (S)(Figure 2):
S1. Material: Products. Comparison of three main environmental impact and energy indicators — global warming potential (GWP), total non-renewable primary energy (TNRPE), and total renewable primary energy (TRPE)— of Skarpnes Flat concrete roof tile (Product 1; P1), Braas Turmalin ceramic roof tile (Product 2; P2) and Rheinzink double-standing seam titanium zinc slates (Product 3; P3).
This scenario only analyzes the embodied energy and emissions of the roof tile, regardless of the photovoltaic system, and assumes two system boundaries: from cradle-to-gate —product emissions
A1-A3— and cradle-to-grave —indicative GWP using FutureBuilt Zero and NS3730 standards—.
S2. Material: Building parts/components. Comparison in Reduzer of the main environmental impact indicator (GWP) of two standard sloped roof construction systems, considering a system boundary from cradle-to-grave, the same transportation distance (500 km), different years of service life —50 (German EPD) and 60 (Norwegian EPD) years— and different calculation methods — FutureBuilt Zero and NS 3720—.
S3. Energy: BIPV/BAPV. Comparison of three main environmental impact and energy indicators — global warming potential per energy produced (GWP/kWh) and energy payback period (EPBT)— of the S1 products, including building-integrated photovoltaics (BIPV) for P1 and P2, and buildingattached photovoltaics (BAPV) for P3. In this case, the calculation method developed by Linjord et al. (10) is applied, presuming a system boundary from cradle-to-gate (A1, A2, and A3 stages).
Ceramic ZincConcrete
Material Energy
Products Components
BIPV/BAPV
Figure 2. Scenarios analyzed in the report.
BIPV CONCRETE ROOF TILELife cycle inventory
The environmental impact and energy indicators correspond to the following products (P), building parts/ components (BP), and photovoltaics (PV). The declared unit for the products is kg, whereas the component amount implemented in Reduzer is 1 m2 of roof area.
Scenario 1 (S1). Material: Products
Table 1 defines the technical data and EPD information of the products analyzed in Scenario 1: Skarpnes Flat concrete roof tile (P1)(8), Braas Turmalin ceramic roof tile (P2)(11), and Rheinzink double-standing seam titanium zinc slates (P3)(12)
Table 1. Technical data and EPD information of roof tiles. (8)(11)(12)
P 1 Technical data Weight Dimensions Density Area density Composition
5.4 kg 33 x 42 x 2 cm 1948 kg/m3 38.96 kg/m2
Cement (19.01%), Aggregate (80.9%), Chemicals (0.09%)
EPD information Reference Declared unit Service life Location System boundaries
NEPD-2709-1409-NO kg 60 years
Grimstad, Norway A1, A2, A3, A4
P 2 Technical data Weight Dimensions Density Area density Composition
4.4 kg 27 x 47 x 2 cm 1733.65 kg/m3 34.67 kg/m2 Clay (80.9%), Water (19.01%), Engobe/Glaze (0.09%)
EPD information Reference Declared unit Service life Location System boundaries
EPD-BRA-20170030-ICD1-DE kg
50 years
Oberursel, DE A1, A2, A3, A4, A5, C2, C4, D2
P 3 Technical data Weight Dimensions Density Area density Composition
5.6 kg/m2 53 x 200 x 0.07 cm 7200 kg/m3 5.6 kg/m2 Zinc (99.74%), Cooper (0.15%), Titanium (0.1%), Aluminum (0.015%)
Scenario 2 (S2). Material: Building parts/components
EPD information Reference Declared unit Service life Location System boundaries
EPD-RHE-20180073-IBA1-EN kg
50 years
Datteln, DE A1-A3, C2, C4, D
Table 2 expresses the layer composition and data entered in Reduzer for the building parts studied in Scenario 2: sloped roof with glued laminated timber structure (BP1) and with cross-laminated timber structure (BP2)2
The criteria to choose the products for each layer corresponds to the two materials with the lowest indicative GWP available in the Reduzer catalog as of March 7, 2022. The calculation methods are Future Built Zero and 2 Scenario 2 studies ten alternatives of the two building parts, as can be consulted in Reduzer: BP1-P1 Glulam roof with concrete roof tiles; BP1-P2-50 years Glulam roof with ceramic roof tiles; BP1-P2-60 years Glulam roof with ceramic roof tiles; BP1-P3-50 years Glulam roof with titanium zinc roof slates; BP1-P3-60 years Glulam roof with titanium zinc roof slates; BP2-P1 CLT roof with concrete roof tiles; BP2-P2-50 years CLT roof with ceramic roof tiles; BP2-P2-60 years CLT roof with ceramic roof tiles; BP2-P3-50 years CLT roof with titanium zinc roof slates; and BP2-P3-60 years CLT roof with titanium zinc roof slates.
NS 3720, assuming several LCA conditions: same transportation distance (500 km), same estimated service life (60 years), and different estimated service life for P2 and P3 products (50 years).
Regarding building parts, the roof construction systems selected for the analysis correspond to a standard Norwegian sloped roof approved by SINTEF (13). It is also important to mention that the layers composed of timber —like glulam beams and battens—, are calculated in percentages, considering 360 mm of separation between battens (24% of wood in 1 m2) and 600 mm between glulam beams (17% in 1 m2).
Table 2. Layer composition (from top to bottom) and Reduzer data of the sloped roofs. (13)
B P 1
Layer Roof tiles
Battens Roof board Insulation
Glulam Vapor barrier Battens Ceiling board
TOTAL
mm 20 60 20 200 0.2 30 12.5
Quantity 1 m2 0.009 m3 1 m2 1 m2 0.03 m3 1 m2 1 m2
Product
P1 Skarpnes (8)
P2 Braas (11)
P3 Rheinzink (12)
Royal-impregnated timber (14)
Hunton Undertak (15)
Hunton Trefiberinsolasjon (16)
Standard limtrebjelke (17)
Baca Dampsperre (18)
Royal-impregnated timber (14)
Norgips Standard (STD) (19)
BP1-P1 BP1-P2 BP1-P3
B P 2
Layer Roof tiles
Battens Roof board Insulation CLT
Vapor barrier Battens Ceiling board
TOTAL
mm 20 60 20 200 100 0.2 30 12.5
Quantity 1 m2
0.009 m3 1 m2 1 m2 0.1 m3 1 m2 1 m2
Product
P1 Skarpnes (8)
P2 Braas (11)
P3 Rheinzink (12)
Royal-impregnated timber (14)
Forestia Standard board (20)
ISOVER Kretsull (21)
Cross-laminated timber (22) Gram Dampsperre (23)
Royal-impregnated timber (14) Gyproc Robust (24) BP2-P1 BP2-P2 BP2-P3
Scenario 3 (S3). Energy: BIPV/BAPV
Transport distance (km)
Service life (yrs)
Mass A1-
(kg/m
BIPV CONCRETE ROOF TILETransport distance (km)
Service life (yrs)
Wastage (%)
Mass A1-
(kg/m
Table 3 describes the technical data of photovoltaics technologies evaluated in Scenario 3: BIPV concrete roof tile (PV1), BIPV ceramic roof tile (PV2), and BAPV titanium zinc slates (PV3). Since Reduzer does not estimate the GWP per energy produced, the holistic calculation method employed by Linjord et al. (10) is considered as the main reference for this scenario. The academic publication investigates the current stateof-the-art of BIPV roof tiles, establishing the average values of CO2 emissions and energy indicators for solar cells produced using the modified Siemens and Elkem processes in Norwegian solar conditions, with an annual average energy production of 117 kWh/m2 per BIPV roof tiles and 168.5 kWh/m2 per BAPV.
P V 1
Technical data Type Dimensions Peak power
Primary energy demand from production
Results
Table 3. Technical data of BIPV and BAPV roof tiles. (9)(10)(25)
BIPV 40 x 28 cm 13 W p 813.9 kWh/m2
P V 2
Technical data Type Dimensions
Peak power
Primary energy demand from production
BIPV 42 x 22 cm 13 W p 826.45 kWh/m2
P V 3
Technical data Type Dimensions Peak power
Primary energy demand from production
BAPV 100 x 50 cm 68 Wp 1356.7 kWh/m2
As stated in the methodology, this report focuses the life cycle impact assessment on the embodied energy and GHG emissions indicators in three different scenarios:
Scenario 1 (S1). Material: Products
Table 4 and Figure 3 summarize the LCA results of the products (P1, P2, and P3) analyzed in Scenario 1, measuring the environmental impact in indicative global warming potential (GWP; kgCO2eq/kg), total nonrenewable primary energy (TNRPE; MJ), and total renewable primary energy (TRPE; MJ) from cradle-to-gate (A1-A3) and cradle-to-grave. The indicative GWP is obtained from Reduzer, while the resource use values are calculated directly from the EPDs.
Table 4. LCA results for roof tiles in Scenario 1.
P 1 Indicative GWP (kgCO2eq/kg)
FutureBuilt Zero NS 3720
Product emissions A1-A3 TNRPE A1-A3 (MJ) TRPE A1-A3 (MJ) Total primary energy consumption
0.24 0.24 0.14 0.52 0.26 0.78
P 2 Indicative GWP (kgCO2eq/kg) FutureBuilt Zero NS 3720
Product emissions A1-A3 TNRPE A1-A3 (MJ) TRPE A1-A3 (MJ)
Total primary energy consumption
0.57 0.84 0.29 4.53 0.35 4.88
P 3 Indicative GWP (kgCO2eq/kg)
FutureBuilt Zero NS 3720
Product emissions A1-A3 TNRPE A1-A3 (MJ) TRPE A1-A3 (MJ) Total primary energy consumption
4.4 6.6 3.1 30.3 13.1 43.4
The impact assessment shows that the concrete roof tile (P1) has the lowest GWP of the three cases studied, both for cradle-to-gate —product emission A1-A3— and cradle-to-grave —indicative GWP—. In particular, the ceramic roof tile (P2) has a similar environmental impact to P1 if is compared to P3, which emits 27 times more CO2 than P1 using the Norwegian standard (NS 3720) and 18 times more using FutureBuilt Zero. It is also important to mention that P1 has the same GHG emission —9 kgCO2eq/kg— despite the calculation method implemented, whereas the indicative GWP results of P2 and P3 are 50% higher according to NS3720. These values are caused by the replacements executed before the end of life, assumed in the Norwegian standard.
kgCO2e/kg
composition of Scenario
kgCO2e/kg
Regarding embodied energy, the concrete roof tile (P1) also uses fewer resources than P2 and P3 —around 6 and 56 times less MJ in total, respectively—, due to the diverse energy consumption during the extraction and manufacture process of concrete, ceramic, and zinc. Concrete embodied energy coefficient is 0.94 MJ/kg, while ceramic is 2.5 MJ/kg and zinc is 51 MJ/kg, one of the highest coefficients of all construction products (26)
Another significant embodied energy indicator is the use of non-renewable primary energy sources, which are 2 (P1), 13 (P2), and 2.3 (P3) times higher than renewable sources.
Scenario 2 (S2). Material: Building parts/components
Table 5 reviews the LCA results of the building parts (BP1 and BP2) studied in Scenario 2, measuring the environmental impact —total and by-product— in global warming potential (GWP; kgCO2eq/m2). The calculations, carried out in Reduzer, assume an estimated service life of 60 years and transportation distance of 500 km for every product, except C2 and C3. Ceramic roof tiles (P2) and titanium zinc slates (P3) are studied both in the Norwegian (60 years) and German standard (50 years), where the EPDs and original products are manufactured. The results also show the disparity between the calculation methods used in Norway: FutureBuilt Zero and NS3720.
Table 5. LCA results for sloped roofs in Scenario 2.
B P 1
Layer Roof tiles
Battens Roof board Insulation Glulam Vapor barrier Ceiling board TOTAL
Product
P1 Skarpnes (8)
P2 Braas (11)
P3 Rheinzink (12)
Royal-impregnated timber (14)
Hunton Undertak (15)
Hunton Trefiberinsolasjon (16)
Standard limtrebjelke (17)
Dampsperre (18)
Norgips Standard (STD) (19)
BIPV CONCRETE ROOF TILEP 2
Layer Roof tiles
Battens Roof board Insulation Glulam Vapor barrier Ceiling board
Product
P1 Skarpnes (8)
Braas (11)
Rheinzink (12)
Royal-impregnated timber (14)
Forestia Standard board (20)
ISOVER Kretsull (21)
Cross-laminated timber (22) Gram Dampsperre (23)
Robust (24)
(kg/m
years'
The impact assessment for Scenario 2 (Figures 4 and 5) indicates that roof tiles have a significant role in roof construction systems, both regarding mass —from cradle-to-gate— and GHG emissions. P1 represents 48.1% of the total mass of the glulam sloped roof (BP1) and 33.9% in the CLT sloped roof (BP2); P2 accounts for 45.5% of the mass in BP1 and 31.5% in BP2; and P3 stands for 12.5% in BP1 and 7.3% in BP2. Therefore, P1 and P2 emissions —9 and 14-20 kgCO2eq/m2, respectively— are balanced with their mass contribution, but P3 emissions —18-25 kgCO2eq/m2, depending on the estimated service life— are considerably high in comparison with its mass contribution.
In addition to the roof tiles, the structure also plays a decisive role in the environmental impact of roof construction systems. Following the criteria of selecting products in Reduzer with the lowest indicative GWP possible, BP1 uses glued laminated timber beams, whereas BP2 employs cross-laminated timber panels. Although glulam and CLT have a similar indicative GWP (0.14 versus 0.17 kgCO2eq/m2 in FutureBuit Zero), CLT accounts for a higher impact because of its mass contribution, representing around 40-50% of the total mass of BP2, in comparison with approximately 20-30% of the global mass in the case of the glulam beams. The rest of the products —insulation, roof and ceiling board, battens, and vapor barrier— present emissions around 1-3 kgCO2eq/m2, almost negligible in comparison with the environmental impact of roof tiles and timber structure.
BP2-P1 CLT sloped roof with concrete
tiles
BP2-P2 CLT sloped roof with ceramic roof tiles
BP2-P3 CLT sloped roof with titanium zinc roof slates
Figure 5. Mass and GWP by-product of Scenario 2 study cases (BP2).
life of 50 years.
Regarding the different service life considered, the graphics obtained from Reduzer (Figure 6) shows the substantial environmental impact of replacements before the Norwegian service lifetime (60 years). When a service life of 50 years is assumed —according to German EPDs of P2 and P3— BP1-P2, BP1-P3, BP2-P2, and BP2-P3 emissions are about 125% higher than BP1 and BP2, which presume a service life of 60 years for every product.
Consequently, and bearing in mind the global CO2 emissions of the components, the glulam sloped roof with concrete roof tiles (BP1-P1), calculated with the FutureBuilt Zero method, has the lowest environmental impact (18 kgCO2eq/m2) of the ten alternatives analyzed in Reduzer, while the CLT sloped roof with 50-years-servicelife titanium zinc roof slates, calculated with the NS 3720 standard, produces the highest GHG emissions (75 kgCO2eq/m2).
BIPV CONCRETE ROOF TILEBP1-P1 Glulam sloped roof with
BP1-P2 Glulam sloped roof with ceramic roof
BP1-P3 Glulam sloped roof with titanium zinc roof slates
Scenario 3 (S3). Energy: BIPV/BAPV
Table 6 reviews the LCA results of the BIPV and BAPV technologies (PV1, PV2, and PV3) explored in Scenario 3, measuring the global warming potential per energy produced (GWP/kWh; gCO2eq/kWh) and energy payback period (EPBT; years). The table indicates two values for GWP/kWh, assuming the average results from Norwegian BIPV/BAPV manufacturers (10)—average GWP/kWh— and the specific results for the roof tiles studied in this paper —specific GWP/kWh—. The EPBT is also calculated considering the average GWP/kWh.
Table 6. LCA results for BIPV and BAPV roof tiles in Scenario 3.
P V 1
Embodied GWP (gCO2eq/kWh)
Average GWP/kWh
Specific GWP/kWh
EPBT
Primary energy demand from production
56.1 58.8 5.94 813.9 kWh/m2
P V 2 Embodied GWP (gCO2eq/kWh)
Average GWP/kWh Specific GWP/kWh
EPBT
Primary energy demand from production
57.65 119 6.03 826.45 kWh/m2
P V 3
Embodied GWP (gCO2eq/kWh)
Average GWP/kWh Specific GWP/kWh
EPBT
Primary energy demand from production
59.3 102 6.9 1356.7 kWh/m2
The impact assessment for Scenario 3 (Figure 7) shows that the BIPV concrete roof tile (PV1) has the lowest GWP per energy produced —56.1-58.8 gCO2e/kWh—, both considering the average and specific results, and assuming an annual energy production of 117.3 kWh/m2 for BIPV and 168.5 kWh/m2 for BAPV. Thus, the values only account for the Norwegian solar conditions.
However, PV2 and PV3 have significant variations in the two embodied GWP results: the specific GWP/kWh represents the double CO2 emissions than the average GWP/kWh for the standard Norwegian BIPV ceramic roof tile and BAPV titanium zinc slates. Given that P2 and P3 roof tiles are manufactured in Germany, the service life, GHG emissions, wastage, and transportation distance may differ substantially from Norway. Supposing the uncertainty of these values, it is considered that the results offered in the academic publication of Linjord et al. (10) are more accurate.
Regarding the energy payback period (EPBT), PV1 demands the lowest primary energy during the production process. Hence, the time required to produce the same energy used to manufacture the photovoltaic and the roof tile is also the lowest: 5.94 years. PV2 results are very similar to PV1, while PV3 has the highest EPBT (6.9 years) “due to the aluminum stand that is used to attach the panel to the roof” (10)
PV1: Energy GWP 58.8 gCO2e/kWh
Figure 7. GWP and EPBT of Scenario 3 study cases.
Primary energy demand from production 813.9 kg/m2 Energy payback period 5.94 years
PV2: Energy GWP 119 gCO2e/kWh
Primary energy demand from production 826.45 kg/m2 Energy payback period 6.03 years
PV3: Energy GWP 102 gCO2e/kWh
Primary energy demand from production 1356.7 kg/m2 Energy payback period 6.9 years
BIPV CONCRETE ROOF TILEROOF TILE
Discussion
The overall results of this report show that the BIPV concrete roof tile from Skarpnes presents the best environmental performance —concerning GHG emissions and embodied energy in comparison with the ceramic roof tile from Braas and the titanium zinc slate from Rheinzink— in the three scenarios analyzed applying LCA techniques. In Scenario 1, P1 has the lowest global warming potential (0.24 kgCO2eq/kg), product emissions (0.14 kgCO2eq/kg) and total primary energy consumption (0.78 kgCO2eq/kg). In Scenario 2, BP1-P1 (glulam sloped roof) and BP2-P1 (CLT sloped roof) have the lowest global warming potential (18 and 26 kgCO2eq/ m2, respectively), but the highest mass of all building parts studied (81 and 115 kg/m2). Finally, in Scenario 3, PV1 has the lowest global warming potential per energy produced (56.1 gCO2e/kWh), primary energy demand for production (813.9 kWh/m2), and energy payback period (5.94 years).
Consequently, the research question —“how the BIPV concrete roof tile from Skarpnes AS can, in comparison with other roof solutions and PV technologies, contribute to a sustainable future both in construction and energy sectors?”—, stated in the goal and scope section, can be answered positively, at least in the matter of GHG emissions and embodied energy, and bearing in mind the limitations and assumptions made in this report.
To obtain a grounded response to the key of the research question —the contribution to a ‘sustainable future3’ to the construction and energy sectors—, as well as to review the meaning, scalability, and accuracy of the results offered in the report, this discussion section will evaluate several technical indicators to confirm if the concrete roof tile is a sustainable building product: circularity, service life, wastage, PV efficiency, CO2 emissions, affordability, and availability (7)
Circularity and estimated service life are significant in respect of GHG emissions. Scenario 3 simulates the same service life for the PV and the surface material in the BIPV and BAPV roof tiles because there are no EPDs available for these types of building products. Thus, this assumption doesn’t show the actual situation: while roof tiles have an estimated service life of 60 or 50 years —depending on the EPDs (Norwegian or German, respectively) used—, several studies indicate that BIPV and BAPV systems have a service life of between 30 and 50 years (28). Consequently, the PV cells will need a replacement before the roof tile, which reduces the global service life of the BIPV roof tile and increases the CO2 emissions. However, it is also important to contemplate that the BIPV roof tile presents an energy payback period of 5.9 years, so this type of building product has a substantial potential to counterbalance embodied GWP, being a suitable solution for zero-emissions buildings. Furthermore, BIPV systems can be installed by any craftsmen in every roof orientation and are less affected by malfunction and shadows because PVs are connected in parallel (9).
3 The term ‘sustainable future’ or ‘sustainable development’ represents a scenario that “meets the needs of the present without compromising the ability of future generations to meet their own needs (16, p. 39)”.
Regarding wastage, Reduzer considers a standard 10% waste for roof tiles. However, since the roof layout needs to be designed and approved by the grid company and solar system installers —or ordinary carpenters and electricians in the case of dwelling projects—, the actual waste may be lower. In connection with photovoltaics efficiency, the values given in Scenario 3 respond to the Norwegian solar conditions (117.3 kWh/m2). If the analysis assumes the Spanish solar conditions of about 173 kWh/m2 of annual solar irradiation, the energy payback period will be 1.2 years (10). Although Norway does not present optimal conditions for solar energy production, it does have the lowest European GHG emission intensity per electricity generated, whereas Spain is the 17th country in this ranking (29). Thus, if the exchange of electricity from renewable sources across European countries is promoted —especially after the breakdown of economic and energy relations with Russia—, the Nordic countries could benefit from Mediterranean climate conditions for energy production, and Southern countries could reduce their CO2 energy emissions, in order to successfully achieve the Sustainable Development Goals by 2030.
The accuracy of the CO2 emissions results from Scenario 2, which compares sloped roof construction systems, can also be discussed. As the component comparison carried out in Reduzer (Figure 8) only evaluates the environmental impact of 1 m2 of roof area, the values obtained do not offer a global view of the sloped roof performance within the building. If a standard two-story building of 100 m2 with a 25º gable roof is studied in
BP1-P1 Glulam sloped roof with concrete roof tiles
BP2-P1 CLT sloped roof with concrete roof tiles
Figure 8. Comparison of FutureBuilt Zero and NS 3720 calculation for BP1-P1 and BP2-P1.
Climate change:
kgCO2e
CONCRETE ROOF(Wastage)
(Used)
(Used)
(Used)
ROOF TILE
Reduzer’s project section, BP1-P1 has a GWP of 2013 kgCO2eq/m2 in FutureBuilt Zero and 3088 kgCO2eq/m2 in NS 3720, with a total mass of 8927 kg/m2; BP2-P1 has a GWP of 2924 kgCO2eq/m2 in FutureBuilt Zero and 5271 kgCO2eq/m2 in NS 3720, with a total mass of 12718 kg/m2. In this global scenario, the GHG emissions in both building parts are quite similar, despite the substantial difference in the mass.
In terms of affordability and availability, all scenarios adopt a 500 km transportation distance with the purpose of comparing the products in the same framework. Nevertheless, this assumption does not reflect the European supply chain and energy market instability, affected by the pandemic, political conflicts, and electricity and fuel prices. In the current uncertain context, locally produced materials but with high CO2 emissions (e.g., concrete) may be selected by constructors, instead of low-impact products (e.g., cross-laminated timber) that are usually imported from Austria, Germany, Sweden or Latvia, or Lithuania (30). In that regard, the BIPV roof tile from Skarpnes presents several advantages: the product is manufactured in the south of Norway and customers can apply for ENOVA support to finance a percentage of their investment (9)
Conclusions
Considering the previous discussion about the sustainability indicators —implemented in the LCA analysis of Carretero-Ayuso and García-Sanz-Calcedo (7)—, it can be concluded that the BIPV roof tile from Skarpnes does contribute to future environmental sustainability. The results from the three scenarios demonstrate that the concrete roof tile is the product with the lowest CO2 emissions and embodied energy in comparison with other commonly used materials (ceramic and zinc), but it is also the BIPV system that more quickly counterbalance the energy consumed during the fabrication process. This property is fundamental in the climate change and global warming context, in which more sustainable buildings with zero emissions and zero energy consumption are being demanded. In fact, the European Union directive on the energy performance of buildings (3) established that, by 2050, the entire building stock must be transformed to nearly zero-energy buildings. To fulfill this goal and pursue a more ambitious sustainable future with a zero-emission building stock, the BIPV concrete roof tile can play an essential role.
In respect of the research impact of this report, the methodology implemented can be a suitable tool for future studies concerning GHG emissions and embodied energy using LCA techniques. While the reviewed literature mainly focuses on the environmental impact of building products, this work proposes a holistic approach to the life cycle impact assessment. The product is analyzed in three different scenarios, investigating the emissions in the material (product and building part scale) and the whole system (BIPV), with the purpose of balancing the embodied material emissions with the renewable energy produced by the photovoltaics.
Therefore, the main goal of this research, in terms of knowledge transfer, is to establish an evaluation method to scientifically verify the choice of a specific building product for a zero-emissions building project.
References
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Appendix: Reduzer and Excel data
The obtained results were calculated with Reduzer —for cradle-to-grave system boundary— and the information available in the EPDs —for cradle-to-gate system boundary and energy analysis—.
In Reduzer, the project named ‘Theory: Irene González Fernández’ contains the components created, as well as the tests carried out in the software, in three versions:
Final Delivery. This version includes the ten variations of components BP1 and BP2, used in Scenario
2. Each component includes the same code as in the report, which can also be consulted in the list of abbreviations at the beginning.
Discussion. BP1-P1 Glulam roof with concrete roof tiles. The version refers to the test mentioned in the CO2 emissions discussion, which analyses a two-story building of 100 m2 with a 25º gable roof to compare the global warming potential in a 1 m2 of roof area with the GWP of the entire glulam sloped roof.
Discussion. BP2-P1 CLT roof with concrete roof tiles. The version refers to the test mentioned in the CO2 emissions discussion, which analyses a two-story building of 100 m2 with a 25º gable roof to compare the global warming potential in a 1 m2 of roof area with the GWP of the entire CLT sloped roof.
On Teams, the Excel file uploaded in the ‘Final reports’ folder, contains the information from the EPDs and the manual calculations developed in Scenario 3.
BIPV CONCRETE ROOF TILE