3.2. Calculation Method
Based on the different materials of curtain wall panels, they can be roughly categorized into glass curtain walls, metal curtain walls, stone curtain walls, and artificial board curtain walls. Due to the significant differences in materials and construction methods among different types of curtain walls and the possibility of multiple types of curtain walls being used in the same project, the carbon emissions are calculated separately for each type of curtain wall in this paper. Then, the total carbon emissions of the entire curtain wall project are obtained by adding up the emissions from each type as follows:
In the equation, represents the total carbon emissions of curtain walls over the lifespan of a single building or buildings in kgCO2e; represents the carbon emissions per unit area of the j-th type of curtain wall in kgCO2e; represents the area of the j-th type of the curtain wall in m2; represents the number of replacements of the j-th type of curtain wall over the entire lifespan.
According to the delineation of different stages within the boundary of the building of the curtain wall, the carbon emissions per unit area of different types of curtain wall can be represented as follows:
In the equation, represents the carbon emissions per unit area of the curtain wall during the material acquisition stage, represents the carbon emissions per unit area of the curtain wall during the processing and production stage, represents the carbon emissions per unit area of curtain wall during the transportation stage, represents the carbon emissions per unit area of the curtain wall during the installation and construction stage, represents the carbon emissions per unit area of curtain wall during the use and maintenance stage, and represents the carbon emissions per unit area of the curtain wall during the dismantling stage. For building curtain walls, the carbon emissions at different stages can be categorized into three parts: materials, energy consumption, and combustion of fossil fuels, where the combustion of fossil fuels refers to the generation of greenhouse gases.
Considering the differing statistical practices across various stages of engineering, no uniform calculation formula has been established. Therefore, each stage is computed separately. The carbon emissions during the material acquisition stage of the unit area curtain wall can be represented as follows:
In the equation, denotes the consumption per unit area of a specific material i for a given curtain wall j, in kg/m2 or m2/m2, while represents the carbon emission factor of material i, in kgCO2e/kg or kgCO2e/m2. Building curtain wall materials encompass profiles, glass, stone, aluminum panels, fireproof insulation materials, sealing materials, fasteners, embedded parts, hardware materials, packaging materials, etc. Typically, the manufacturers provide the carbon emission factors of materials.
The processing and production stage of curtain walls mainly includes the processing of profiles, assembly of unit modules, assembly of opening fans, assembly of sun shading louvers, glass gluing, and storage packaging processes. During the processing and production process, materials such as packaging, cleaning, and auxiliary materials are involved, as well as energy consumption from equipment operation and on-site transportation. The carbon emission intensity of curtain wall processing and production per unit area can be expressed as follows:
In the equation,
represents the consumption per unit area of a specific energy
i for curtain wall
j, in (kW·h)/m
2 or L/m
2 or kg/m
2, where energy types may include electricity, petroleum, coal, etc.;
represents the carbon emission factor of energy
i, measured in kgCO
2e/(kW·h), kgCO
2e/L, or kgCO
2e/kg;
represents the carbon emissions per unit area generated by the combustion of fossil energy
i for curtain wall
j, which can be calculated using the following formula:
In the equation, represents the consumption per unit area of a specific type of fossil energy i for curtain wall j, in L/m2 or kg/m2; represents the average lower heating value of fossil energy type i, in GJ/L or GJ/kg; represents the carbon content per unit heat value of fossil energy type i, in kgC/GJ; represents the carbon oxidation rate of fossil energy type i, in %.
The transportation stage of building curtain walls primarily involves two parts: transporting the acquired materials to the processing workshop and then transporting them to the construction site after processing. Transportation within the workshop or site, as well as the external transportation of waste during the dismantling phase, are not included. Unlike other stages, carbon emissions during transportation are typically calculated based on different transportation methods and distances. The carbon emissions of curtain wall transportation per unit area can be expressed as follows:
In the equation, denotes the quantity per unit area of the material i for transportation mode k of the curtain wall j, in kg/m2; represents the transportation distance of the material i for transportation mode k of the curtain wall j, in km; signifies the carbon emission factor of the transportation mode k, in kgCO2e/(kg·km).
The calculation of carbon emissions during the installation and construction phase of unit-area curtain wall construction relies on Equations (4) and (5), encompassing measures such as the on-site storage of components, on-site transportation, auxiliary installation of scaffolding, installation processes, and curtain wall cleaning.
Similarly, the calculation of carbon emissions during the use and maintenance phase of the unit-area curtain wall is also based on Equations (4) and (5). The stage involves activities such as replacing aged materials, repairing faulty components, daily cleaning of curtain walls, and energy consumption of control systems like electric sunshades. When the photovoltaic curtain wall is applied, in Equation (4) should include the electricity generation resulting from the photovoltaic, with a negative value indicating carbon reduction due to photovoltaic capacity.
The calculation of carbon emissions during the dismantling phase of a unit-area curtain wall is likewise based on Equations (4) and (5), primarily including the material and energy consumption from dismantling and auxiliary measures during the dismantling process, as well as carbon emissions from waste transportation.
3.3. Case Study
A specific aluminum-glass curtain wall is considered, comprising both framed and photovoltaic curtain walls. The building is designed to endure for 50 years. The area of the framed glass curtain wall is 8000 m
2, with a designated service life of 25 years, while the photovoltaic curtain wall spans 1000 m
2, similarly engineered for a 25-year period. The design drawing for the curtain wall section is shown in
Figure 2.
The material quantities and carbon emission factors during the material acquisition phase of the curtain wall are outlined in
Table 2. When calculating the consumption of materials, material loss should be considered and converted based on the service life of the materials and the projects. Utilizing Equation (3), the carbon emissions
during the material acquisition phase of the curtain wall were calculated to be 310.9 kgCO
2e/m
2.
The material and energy consumption, as well as the carbon emission factors during the processing and production stage of the curtain wall, are delineated in
Table 3. These factors primarily encompass the electricity consumption of processing machinery and packaging materials. Based on Equations (4) and (5), the carbon emissions
during this stage were calculated to be 1.13 kgCO
2e/m
2.
Table 4 presents the materials, energy consumption, and carbon emission factors for the transportation phase of the curtain wall. Usually, aluminum alloy profiles and glass materials are sourced from manufacturing companies, resulting in longer transportation distances. Steel components can be purchased from the market, allowing for sourcing from nearby locations. Semi-finished products typically come from processing plants near the construction site, thus minimizing transportation distances.
Considering the carbon emissions during the return journey of the transportation vehicle, the transportation distance needs to be calculated twice. Consequently, employing Equation (6) yielded the carbon emissions for the transportation phase as 12.38 kgCO2e/m2.
The materials, energy consumption, and carbon emission factors during the installation and construction stage of the curtain wall are detailed in
Table 5. The carbon emissions primarily encompass the energy consumption during the construction process and the utilization of non-recyclable auxiliary materials. The average low-heat
of diesel, essential for calculation purposes, stands at 42.652 × 10
−3 GJ/kg. Correspondingly, the carbon content per unit calorific
is estimated to be 20.2 kgC/GJ, with a carbon oxidation rate of 99%. Consequently, the carbon emissions
of the curtain wall installation and construction stage were derived through the application of Equations (4) and (5), resulting in 4.37 kgCO
2e/m
2.
Table 6 presents the annual material and energy consumption, along with carbon emission factors during the usage phase of the curtain wall. With a service life of 25 years, the carbon emissions
resulting from the consumption of materials or energy during this stage amounted to 17.02 kgCO
2e/m
2, calculated using Equations (4) and (5).
The material and energy consumption, as well as the carbon emission factors during the dismantling phase of the curtain wall, are presented in
Table 7. The activity level data and carbon emission factors during the transportation process are shown in
Table 8. Based on Equations (4)–(6), the carbon emissions
during the dismantling phase of the curtain wall were calculated to be 5.43 kgCO
2e/m
2.
Based on the cumulative analysis, the carbon emissions per unit area of the framed glass curtain wall for this building project amounted to 351.23 kgCO
2e/m
2. The photovoltaic curtain wall of the project generates approximately 100 kWh/m
2 of electricity annually. The design lifespan realistic with photovoltaic products is 25 years. Utilizing the electricity carbon emission factor from
Table 6, the carbon reduction during its usage phase was calculated to be −100 × 25 × 0.9419 = −2354.75 kgCO
2e/m
2. Apart from the usage phase of electricity generation, other phases’ configurations were similar to the framed glass curtain wall, with negligible differences. Referring to the carbon emission calculation results of the framed glass curtain wall, the cumulative carbon emissions per unit area of the photovoltaic curtain wall in this building project were −2354.75 + 351.23 = −2003.52 kgCO
2e/m
2.
The total carbon emissions
of the building curtain wall over its lifecycle amounted to 1612.640 tCO
2e, calculated according to Equation (1). The carbon emissions data for different stages are illustrated in
Figure 3. Insights from the case study reveal the following: (1) Excluding photovoltaic components, the material acquisition stage contributes nearly 90% of carbon emissions, serving as the primary source of emissions for glass curtain walls. Consequently, mitigating emissions should primarily focus on material reduction; (2) The scientific application of photovoltaic curtain walls can significantly reduce the carbon emissions associated with building curtain walls.