Abstract:Prefabricated building structures are of great significance for energy conservation and emission reduction in the field of construction in mainland China, improving the engineering quality and labor productivity level of buildings, and realizing the sustainable development of building construction. In order to explore the energy saving and emission reduction effect of prefabricated building structures in their physicalization stage, and to provide an analytical basis for their whole life cycle, on the basis of analyzing the calculation and classification of carbon emissions, the construction activities are divided into several main carbon emission sources, and then the use and consumption of each emission source are counted. Then through the construction process of the prefabricated structure, the corresponding carbon emission system boundary is established, the carbon emission source is divided into direct carbon emissions and indirect carbon emissions, and each item is classified and calculated according to the carbon emission intensity of commonly used primary energy and secondary energy and commonly used building materials, and finally the complete carbon emission calculation idea of the building stage is proposed.
Keywords: Prefabricated building structure Materialization stage Direct carbon emissions Indirect carbon emissions Calculation method
0 Introduction
In 2009, the mainland has promised the international community carbon emission reduction targets: by 2020, the carbon dioxide emissions per unit of GDP on the mainland will be reduced by 40% to 45% compared with 2005, and will be included in the medium- and long-term planning of national economic and social development as a binding indicator, and corresponding domestic statistics, monitoring and assessment methods will be formulated.
According to statistics, the amount of carbon dioxide emitted by the construction sector accounts for more than 35% of the total emissions every year. Therefore, the construction industry is facing the need to meet the environmental friendly requirements of low-carbon environmental protection while meeting the appropriate rapid growth of building volume and the overall improvement of building quality.
Prefabricated prefabricated structure (referred to as PC structure) is a concrete structure made of prefabricated components as the main components, assembled and connected. It uses modern science and technology to carry out a comprehensive and systematic transformation of the traditional construction industry, and realizes the sustainable development of building construction by optimizing resource allocation, reducing resource consumption, and improving the engineering quality, functional quality, environmental quality and construction labor productivity level of construction.
The materialization stage refers to the construction stage of the building structure, including the production, transportation, installation and construction process of prefabricated components.
1 Carbon emissions calculations
Carbon emission calculation methods for the materialization stage of buildings can be divided into two categories:
The first is to statistically calculate the carbon emission data of each construction process, such as the carbon emissions of the temporary support system, the carbon emissions of the steel bar lashing, the carbon emissions of concrete pouring, etc., and finally the carbon emission data of each construction process is accumulated. The feasibility of this approach is low due to the complexity of the construction process in the physicochemical stage of the prefabricated structure, and the lack of research results on carbon emissions for each construction process, which lacks the support of basic data.
The second is to divide the construction activities into only a few major carbon emission sources, and then count the use and consumption of each source. Because there are already many research results on carbon emissions caused by carbon emission sources such as fossil fuels and electricity, building material production, human life, transportation, etc., these research results provide favorable data support for this method. Therefore, method 2 can be used to calculate and analyze the carbon emissions in the materialization stage of the prefabricated structure.
The sources of carbon emissions for any product include both direct and indirect carbon emissions. Direct carbon emissions refer to the carbon dioxide emitted directly into the atmosphere by consuming fossil fuels, electricity, and man-hours in the processing of finished products. Indirect carbon emissions refer to the carbon emissions implied by the constituent components of the finished product. For example, product M and product N produce product K through processing. Then the direct carbon emissions of product K are the carbon emissions generated by processing activities, and the indirect carbon emissions are the carbon emissions implied by product M and product N itself. If you continue to track the carbon emissions of product M and product N, you will find that they themselves include both direct and indirect carbon emissions.
The production of each product in society will require the input of many other products, and the carbon emissions of the whole society are precisely a network system, and the products are contained in each other. But if a small proportion of associations is discarded, in general, the carbon emission system of the whole society can be simplified into a hierarchical system. For example, fossil fuels, electricity, and working hours can be used as the first level, cement, yellow sand, gravel, formwork, steel bars and other raw materials can be used as the second level, and higher products such as buildings can be used as the third level. Under such a hierarchical system, the carbon emissions of each known product can be easily calculated as long as the carbon emissions of each known product are counted.
2 Carbon emission system boundaries
The process system for the materialization stage of prefabricated building structures is shown in Figure 1. The entire materialization stage is divided into three stages: the production stage of prefabricated components (formwork + reinforcement + embedded parts + other + ready-mixed concrete (cement + sand + gravel + other) → the → storage of concrete precast components), the transportation stage (loading and transportation), the installation and construction stage (temporary support system + construction machinery→ on-site hoisting + on-site part → construction products).
The characteristics of integrated prefabricated building structures first divide the carbon emission sources of buildings into two categories. The first is direct carbon emissions (construction personnel + component transportation + construction machinery), that is, carbon emissions caused by fossil fuels, electric energy, other energy sources, and personnel inputs in the process of building construction. For example, co2 emissions caused by the use of electricity by the construction machinery burning diesel fuel, gasoline and other primary energy sources directly into the atmosphere. The second is indirect carbon emissions (prefabricated components + temporary support systems + other scattered materials + water), that is, the carbon emissions of products and raw materials such as water, building materials, office supplies and other products consumed during the construction of buildings, see Figure 2.
3 Carbon intensity of primary energy sources
When calculating carbon emissions, it comes down to tracking the consumption of basic energy sources. Basic energy sources include fossil fuels and electricity. The carbon emissions of a certain energy source can be derived by multiplying the consumption of that energy source by the carbon intensity of that energy source.
3.1 Carbon emission intensity of fossil fuels
According to the IPCC National Greenhouse Gas Inventory Guidelines Volume II I Energy, the carbon intensity of the fuel is available. The carbon emission intensity of commonly used fossil fuels is shown in Table 1.
3.2 Carbon emission intensity of electricity
The carbon intensity of electricity is weighted and averaged based on the amount of electricity generated by various types of electricity. The carbon intensity of mainland electricity in 2009 is shown in Table 2.
4 Direct carbon emissions
4.1 Carbon emissions of construction personnel
In order to reflect the advantages of the prefabricated structure in terms of labor savings, the CO and emissions generated by personnel are also included in the carbon emission source system. The carbon emission intensity per unit of working hours multiplied by the number of working hours is the carbon emissions of the construction workers.
The number of man-hours per unit refers to the number of man-hours required during the installation and construction phase of the prefabricated components. According to the analysis, in 2002, the average annual carbon emissions of Chinese were 1.37426t/person", which was converted to 3.77kg/person per day. Although each working hour is 8h, the life of the worker is basically around the site, so the carbon emission intensity value per unit working hour is conservatively taken as 3.77kg/working hour.
4.2 Carbon emissions from the transportation of prefabricated components
The consumption of CO2 available fuels from the various types of tools produced by prefabricated components during the transport phase is multiplied by the carbon emission intensity of the corresponding fuel type (Table 1) to derive the carbon emissions at the transport stage.
4.3 Carbon emissions of construction machinery
The sources of CO during the installation and construction of prefabricated components of construction machinery include electricity and fossil fuels consumed during the use of machinery, maintenance consumption, and amortization costs. The electricity and fossil fuels sections can be derived by simply having statistics and then multiplying them by the corresponding carbon intensity. Because the cost of construction machinery is often very high, its manufacturing process will also consume a lot of raw materials and energy, emitting a amount of CO2 that cannot be ignored, so it is necessary to consider the maintenance and amortization of machinery caused by considerable carbon emissions. These two parts can be calculated by translating the financial losses from maintenance and amortization into local electricity consumption, which is then calculated by the carbon intensity of the electricity.
5 Indirect carbon emissions
Indirect carbon emission sources can be considered to be the consumption of raw materials and intermediate products during the construction of buildings, and these raw materials and intermediate products will also directly and indirectly emit CO2 to the environment during their production process, so in order to examine the carbon emissions of buildings in the physical and chemical stage from the perspective of the whole life cycle, it is necessary to incorporate the hidden carbon emissions of these raw materials and intermediate products themselves. The co and emission intensities of the main building materials are shown in Table 3.
5.1 Carbon emissions of prefabricated components
Prefabricated components require the consumption of ready-mixed concrete, steel bars, embedded parts and other auxiliary materials. Both their manufacturing and production require CO2 emissions into the environment. According to the definitions of direct and indirect carbon emissions in section I of this paper, carbon emissions from precast components include carbon emissions from the production activities of precast components and the implied carbon emissions of mixed concrete, steel reinforcement, embedded parts and other auxiliary materials. Among them, the carbon emission intensity values of ready-mixed concrete with common labels are shown in Table 4; The carbon emission intensity of steel bars, embedded parts and other auxiliary materials is shown in Table 3; Carbon emissions from prefabricated component production activities are fossil fuels, electricity, and man-hours consumed in the activity multiplied by their respective carbon intensities.
5.2 Carbon emissions of supporting materials
According to the survey, most of the supporting materials are leased. For simplicity, the rental fee is converted into local electricity consumption at the same price, and then the carbon footprint is calculated.
5.3 Carbon emissions from water use
Water itself does not produce CO2, but the process of producing and transporting water will produce carbon emissions due to the consumption of energy and other materials, according to the energy consumption working fluid energy and other value reference table, the unit energy consumption of new water energy consumption is 7.535MJ/t, the standard coal coefficient is 0.2574kgce/t, which is converted into a carbon emission intensity of 0.6324kg/t. The water consumption is the production water during the construction phase of the prefabricated components. Because carbon emissions from domestic water are already counted as carbon emissions by people, they are no longer included in the total water consumption. Carbon emissions from water consumption during the production phase of prefabricated components are already included in the carbon emissions of prefabricated components and are therefore not included in the scope of water use.
5.4 Carbon emissions from other sporadic materials
Due to the large number of sporadic materials, the respective dosages are very small. For the sake of simplicity, the cost of sporadic materials is converted into local electricity consumption on the basis of equality, multiplied by the electricity carbon emission intensity, as the carbon emissions of sporadic materials.
6 Conclusion
Carbon emissions from buildings are mainly concentrated in the use phase, but have the highest carbon intensity during the construction phase, so it is necessary to study how to reduce the carbon intensity of the construction phase. Based on the structural characteristics and construction process of the integrated prefabricated building, this paper first determines the system boundary of carbon emissions, that is, the calculation range of carbon emissions. The carbon emission sources in the materialization stage of prefabricated building structures are then divided into direct carbon emissions and indirect carbon emissions. By accumulating the carbon emissions of these carbon emission sources, the total carbon emissions of the materialization stage of the prefabricated building structure can be calculated: C structure = C ancient connection + C indirect = (C personnel + C cloud transmission + C machinery) + (C component + C support + C water + C zero mile).
In order to obtain the carbon emissions of each carbon source, it is necessary to obtain each carbon emission. Discharge consumption and carbon intensity. On-site statistics on fossil fuels, electricity, man-hours, as well as prefabricated components, supports, and water, the consumption of each carbon source can be obtained. The work of counting carbon source consumption is also conducive to on-site construction management. The monthly carbon source statistics of the construction unit are conducive to the self-inspection of the construction unit's own construction quality, progress and efficiency, and it is convenient to make timely adjustments so that the project can be carried out in an orderly and smooth manner. Consumption requires actual statistics on site, and the carbon intensity of carbon sources requires more in-depth research by the relevant industry.
Through the calculation ideas proposed in this paper, according to the carbon emission intensity value of existing products, the carbon emissions of prefabricated building structures in the materialization stage can be easily obtained, which is conducive to comparing the advantages of prefabricated building structures in energy conservation and emission reduction compared with cast-in-place structures. At the same time, it is also conducive to analyzing the distribution of carbon emission sources of prefabricated building structures, identifying the main sources of carbon emissions and proposing targeted carbon reduction measures. With the continuous deepening of the research on the carbon emission intensity value of related social products, the carbon emission calculation results of prefabricated prefabricated building structures will become more and more accurate, which can be used as an assessment index for relevant government departments to measure the green construction level of construction enterprises.