Technical pathways of dual carbon control in China’s buildings sector

Reducing carbon emissions in the buildings sector is of great significance to the realization of China’s carbon peak and neutrality goals. By analyzing factors influencing buildings carbon emissions at the operational stage, this paper applies the China Building Carbon Emission Model (CBCEM) to make medium and long-term forecasts of China’s building operation carbon emissions, discussing the goals and realization paths of China’s dual carbon goals in the buildings sector. The results show that building operation carbon emissions, according to the current development model in the buildings sector, will peak in 2038–2040 with a peak carbon emission of about 3.15 billion tons of CO2; however, by 2060, carbon emissions will still be 2.72 billion tons of CO2, falling short of China’s dual carbon goals. The carbon saving effects of three scenarios, namely clean grid priority, building photovoltaic priority and energy efficiency enhancement priority, were measured and shown to be significant in all three scenarios, but the building photovoltaic priority and energy efficiency enhancement priority scenarios were superior in comparison.


Introduction
In recent years, the global warming trend has accelerated sharply, and according to the IPCC report [1], the global average temperature rose by about 0.87°C between 2006 and 2015 compared to the pre-industrial global average temperature, and the main cause of global warming is the emission of greenhouse gases such as carbon dioxide [2]. Therefore, in order to mitigate global warming, countries have successively proposed carbon peaking and carbon neutrality targets, and as of June 12, 2020, 125 countries and regions have set carbon neutrality (or similar concepts) targets through enacting regulations, executive orders, policy declarations, and submitting Intended Nationally Determined Contributions (INDCs) [3]. In September 2020, China's president, Xi Jinping, stated that China's carbon dioxide emissions strive to peak by 2030 and work towards carbon neutrality by 2060 [4]. As a large carbon producer, China's commitment to carbon neutrality not only demonstrates its role as a leading country, but also its determination to tackle climate change [5].
Energy consumption in the buildings sector is an important contributor to greenhouse gas emissions [6]. Among them, carbon dioxide emissions related to building operations can be divided into direct and indirect emissions [7]. Direct emissions are carbon emissions from the direct combustion of fossil energy during building operation, mainly from central heating boilers in northern towns and cities, heating, cooking and domestic hot water in residential buildings; indirect emissions mainly refer to carbon emissions related to electricity used in buildings and heat from co-generation in central heating in northern towns and cities. According to IEA data, the carbon emissions generated from the building operational stage in China totaled about 2.1 billion tons of CO 2 in 2019, accounting for about 20% of the total national carbon emissions. China is still in the development stage compared to developed countries, and as urbanization and economic development continue, the total amount and proportion of carbon emissions from building operations in China will increase significantly [8], making peak carbon and carbon neutrality more difficult to achieve. Many researchers have explored the role of building energy efficiency measures in addressing climate change mitigation [9]. These studies cover topics such as the use of renewable energy in buildings [10], heating strategies [11], total building energy consumption [12], and the impact of various interventions on the medium and long-term development of building operating energy consumption [13]. According to the method proposed by IPCC, Coondoo et al. [14] looked at the Granger causality test between carbon dioxide emissions and per capita income, and hypothesized different income-environment connections in different nations. Using historical data from 2004 to 2012, Antanasijevic et al. [15] calculated the intensity of greenhouse gases in Europe. In addition, Ahmad et al. [16] developed an empirical model to analyze the relationship between carbon emissions, energy consumption and economic growth in India. For building carbon emissions, Zhou et al. [17] analyzed different building energy conservation and emission reduction policies, and proposed that China's building sector can achieve a peak in 2030 through policy implementation. Tan et al. [12] established a bottom-up model to analyze and predict the impact of low-carbon building policies and energy structure transitions on the medium and long-term development of carbon emissions in the building sector. Their study showed that in the coordinated emission reduction scenario considering energy structure transition, carbon dioxide emissions can peak before 2030. From the perspective of China's total building carbon emission control, Yang et al. [18] established a China building carbon emission model with a bottom-up approach to predict the future carbon emissions of China's building sector and their findings revealed that in order to effectively manage building sector carbon emissions, building size, building energy efficiency, and energy mix must all be addressed at the same time. In short, in order to achieve China's carbon peak and carbon neutrality goals, it is critical to combine carbon emissions in different scenarios in the buildings sector and to carry out specific carbon reduction work in two areas: buildings themselvesincluding energy efficiency in new buildings and the clean-up of the electricity gridand the application of photovoltaics in buildings.
In order to realize the dual goals of carbon peak and carbon neutrality in China, it is critical to establish clear carbon reduction targets and explicit implementation steps in the buildings sector. To this end, this paper examines the macro variables of carbon emissions from building activities based on China's core national conditions, drawing on the experience of industrialized countries. This research uses the China building carbon emission model (CBCEM) to forecast carbon emissions from building operations in China during the medium and long term. It also analyzes the goals and methods to attaining China's dual carbon goals in the buildings sector, with the goal of providing a fundamental foundation and technical reference for realizing China's strategic goal of carbon neutrality in the buildings sector.

Carbon emission calculation methods and control paths
Building energy consumption, broadly defined, encompasses the entire energy consumption process, from building material manufacture to building construction to building operation and use, with operation energy consumption accounting for the majority of total building energy consumption [8].. Therefore, the most effective strategy to minimize a building's overall carbon emissions is to calculate and determine the total energy consumption during the operational stage of the building and develop an acceptable energy saving and carbon reduction pathway. The following is the typical process for calculating carbon emissions in the building operation phase: using a typical building as an example, the baseline scenario energy consumption in the building operation phase is first calculated by taking into account all of the factors that influence building energy consumption. Next, the carbon emissions per unit of energy consumption for each scenario are calculated year by year, i.e., the carbon emission factor, by setting up multiple possible carbon reduction scenarios and evaluating the predicted reduction benefits. Then, the carbon emission in the year-over-year building operating stage under several paths can be calculated by calculating the energy consumption in the baseline scenario and the carbon emission factors for each scenario.

Methods for calculating energy consumption in building operation
Factors such as population size, floor space, urbanization rate, building energy efficiency, and changes in energy structure all influence overall carbon emissions in the operational stage of a building [19]. The analysis of building operation carbon emissions is mainly based on macro characteristics such as population size, floor space, and building operation energy intensity.

Population growth and distribution forecasts
The intensity of energy use in different climatic zones varies greatly due to China's vast size and vast climatic differences between regions, and it is particularly important to measure the energy consumption of buildings in different climatic zones and accurately determine the population distribution in different climatic zones. The 2019 Statistical Yearbook of the National Bureau of Statistics and Cui Zhenwu's team from the Population and Development Research Center at Renmin University of China [20] have made a medium and long-term development forecast for the total population in each climate zone, taking into account the population share of each province, autonomous region and municipality directly under the Central Government, as well as urban population share and population migration, among other factors. As shown in Fig. 1, the results of the study show that China's total population will reach its peak in 2030, and then decline year by year for the next 30 years, with the rate of decline increasing year by year, and the total population of China will reach 1.193 billion in 2060. In addition, population distribution varies greatly between climatic zones, with hot summer and cold winter regions and colder regions having a larger proportion of the population requiring heating in winter, which implies the population is still growing until 2030 in terms of population development and distribution projections, posing a challenge for the buildings sector to achieve a carbon peak by 2030.

Forecast of floor area
The floor area is mainly associated with the total population, the area per capita and the distribution of the population [21]. For rural dwellings, the per capita area is about 39.7m 2 . Considering the difference between the current rural dwelling pattern and that of towns, which are mostly single dwellings, and the gradual change of the dwelling pattern to that of towns, the per capita dwelling area is projected to remain unchanged; for urban dwelling buildings, the per capita dwelling area is over 70m 2 in the United States, 55m 2 in Denmark, Norway and Canada; in France, Germany and Japan, the residential area per capita is 40m 2 . This research proposes that the urban residential floor space per capita in 2060 will be 50m 2 and will increase linearly from 2015 onwards, based on the experience of the abovementioned developed countries and taking into account China's national conditions [17]. In addition to China's national circumstances, the current state of public buildings in China, as well as the development tendency, are examined. With the advancement of digital technology, home offices and cloud offices, video conferencing and other digital techniques are more commonly employed in the workplace, limiting the growth of public structures such as office buildings and hotel buildings in terms of per capita area. Based on the existing state of public buildings, the above development tendency, and the resource conditions in China, it is estimated that the area per capita of public buildings will expand by 42% (compared to the current level as of 2021), reaching 21.4m 2 /person. The floor space of different types of buildings is calculated using the analysis and present status of China's housing patterns and housing area, as well as the medium and long-term population distribution in different climatic zones, as shown in Fig. 2.

Energy consumption per unit of floor area
To begin, we obtained data for the energy intensity per unit area of urban residential, rural residential, and public buildings in different climate zones based on energy consumption data from 2001 to 2015, which included the average household energy intensity of residential buildings, the average area of urban and rural households, and the energy intensity per unit area of public buildings. Second, we forecasted the medium and longterm building energy consumption intensity per unit area by studying changes in China's building energy consumption from 2001 to 2015, as well as the results of Fig. 1 The forecast of population development and distribution in 2011-2060 [10] earlier predictions of China's primary energy consumption. China's building sector, in particular, consumes 20% to 25% of the country's main energy. According to the findings of the reference study [22], the share of building energy consumption in overall energy consumption should be kept below 25% to ensure sustainable growth. Consequently, the maximum contribution of the building sector to total energy consumption is set at 25%.

Methods for calculating building operational carbon emissions
Building operational carbon emissions are influenced by a number of factors, and for this reason, the following assumptions are made in this paper: 1) the total building area used for calculation is only related to the number of people and the floor area per capita; 2) the energy efficiency retrofit of existing buildings meets the energy efficiency design standards for new buildings; 3) the energy intensity of the same type of buildings in the same climate zone is consistent with that of a typical building; 4) zero-energy and zero-carbon buildings have the feasibility of large-scale promotion; and 5) within one climate area, building energy use of the same building type is the same. Based on the above assumptions, the CBCEM was adopted to predict China's building operation carbon emissions in the medium and long term, which was calculated as shown in eq. (1) [18]: m e e;i;k;m Á A e;i;k þ e n;i;k;m Á A n;i;k þ e r;i;k;m Á A r;i;k Where: e n,i,k,m is the energy consumption intensity of class m for new buildings of class k in climate zone i, kgce/(m 2 ·a); e e,i,k,m is the energy consumption intensity of class m for retrofit buildings of class k in climate zone i, kgce/(m 2 ·a); A n,i,k is the annual new, existing and retrofit floor area of buildings of class k in climate zone i, m 2 ; E m is the carbon emission of class m energy factor.
After determining the total building energy consumption using factors such as population, floor area, and energy consumption per unit area of the building, it is necessary to establish the carbon emission factors under various paths by constructing reasonable carbon emission reduction paths, and finally obtain the building carbon emissions under various emission reduction paths, according to the calculation method of Eq. (1). Figure 3 depicts building carbon emissions in various climate zones under the baseline scenario.

Design of Carbon emission control pathways
Carbon reduction solutions in the operational stage of a building can be classified into two primary categories based on a study of the entire life cycle of energy consumption: enhancing the cleanliness of building energy usage and lowering building energy consumption. Three viable carbon reduction strategies are proposed in this paper: the priority of clean grid dependence, the priority of building photovoltaic application, and the priority of improving energy efficiency and regulation.

Priority of clean grid dependence
Cleaner power grids rely entirely on building energy electrification and significant grid reduction to achieve carbon neutrality in the buildings sector. By following a green and low-carbon development path, we will gradually transform from a fossil-based energy grid to a clean energy grid, moving away from fossil energy dependence.
The key to achieving clean energy is to implement "two alternatives", namely, a "clean alternative" in energy development, which involves replacing fossil energy with clean energy sources such as solar, wind, and water, and a "electric alternative" in energy consumption, which involves replacing coal and oil with electricity and increasing the proportion of electric energy in final energy consumption.

Priority for building photovoltaic applications
We can reduce our dependency on high-carbonemitting energy sources and improve the cleanliness of the energy supply side of buildings by aggressively developing building photovoltaics. Structures presently absorb over half of total energy consumption, according to data, and the United States has set a goal of decreasing energy consumption by 50% in new buildings and gradually rehabilitating 15 million existing buildings to reduce energy consumption by 30%. One of the key measures is the promotion of gridconnected rooftop photovoltaic systems that combine photovoltaics with buildings. Solar modules are integrated with building materials, with unique materials and processes utilized to turn photovoltaic modules into roofs, facades, windows, and other shapes that can be used directly as building materials, lowering the cost of power generation.

Priority for upgrading energy-saving regulations
China's building energy efficiency standards have been systematically upgraded three times, having completed a three-step strategy of 30%, 50% and 65% energy savings compared to the 1980s building standards; buildings in cold and extremely cold regions have taken the lead in implementing an energy efficiency rate of 75, and new building energy efficiency is almost on par with that of developed countries. There has been work done to promote near-zero energy buildings, which have the highest worldwide degree of energy efficiency, as well as research on zero-carbon structures. Buildings will be steadily upgraded to ultra-low, near-zero, and zero energy efficiency levels in the future as increasingly stringent mandated energy efficiency criteria for new construction and retrofits are developed. Sub-scenarios of paths and quantified relevant indicators are developed to better illustrate the three paths described in this paper, as shown in Table 1.

Prediction of energy consumption in the operation stage of buildings
The energy consumption of building operations under the base model from 2011 to 2060 can be calculated using population, floor area, and energy consumption per unit area of building operation data from previous years. Meanwhile, the trend of energy consumption changes under the base model, priority of clean grid reliance, priority of building photovoltaic application, and priority of energy-saving strong regulation upgrading scenarios can all be obtained by designing three carbon reduction pathways, as shown in Fig. 4.
In Fig. 4, the baseline nearly aligns with the grid cleaning line. The primary cause reason of this phenomenon is that, although the form of energy consumption has changed, the total amount of energy consumption hasremained constant. For example, the initial energy consumption method is coal, and the current energy consumption method is electricity, but the total energy consumption has not changed. In addition, the base-case scenario shows that building operating energy consumption will continue to grow at a relatively fast rate until 2030, and then the rate of growth will decline, peaking at around 1671 million tons of standard coal in 2049. Subsequently, building operating energy consumption will decline slowly to a level as low as 1648 million tons of standard coal in 2060, a reduction of only 1.38% compared to the peak energy consumption.  Among the three carbon reduction priority pathways, building operational energy consumption under the priority photovoltaic application in buildings and the energy efficiency enhancement pathway are similar to the base model energy consumption trend until 2030. However, the growth rate of energy consumption under the two paths above has decreased dramatically after 2030, resulting in a large reduction in overall energy consumption. Their peak energy consumptions are 82.60% and 83.08% of the peak energy consumption of the basic mode respectively. The peak time of building operation energy consumption is advanced to 2041. In addition, the energy-saving effect under the path of building photovoltaic application priority and energy-saving mandatory improvement is obvious. Until 2060, the building operation energy consumption of the above two paths is only 87.39% and 82.44% of its peak value, which is a significant improvement over the basic mode. The energy consumption of building operation over the years under the priority path of a clean power grid is basically consistent with that of the basic mode, owing to the fact that the main function of improving the cleanliness of power grid is to reduce the carbon emission per unit energy consumption, which has no effect on the value of building operation energy consumption.

Prediction of carbon emission in the operation stage of buildings
With the energy structure under different paths completely taken into consideration, the overall building energy consumption under the three paths may be determined. As shown in Fig. 5, the carbon emission factors for each path are analyzed to determine the final results of building carbon emissions under the baseline scenario, cleaning the grid priority, building energy efficiency standard improvement priority, and building renewable energy priority scenarios. Figure 5 shows that, while carbon emissions have fallen in the priority of clean grid reliance scenario, energy consumption has not decreased. It is also worth noticing that whilethe power consumption increases, the power grid emission factor decreases. Additionally, though the carbon emission in the building sector decreases, the building sector has not made relevant changes. As a result, this scenario will raise the electrical grid's transformation pressure. In the scenarios of priority for building photovoltaic applications and priority for upgrading energy-saving regulations, due to the photovoltaic power generation of the building itself and the improvement of the performance of the building envelope, both the energy consumption and the demand of the building itself for the power grid are reduced, which will consequently lesson the transformation pressure of the power grid. In the baseline scenario, with the implementation of the 2016 version of energy efficiency standards for new buildings, the mandatory energy efficiency standards are upgraded once every 10 years at the current upgrade pace of around 25%. As can be seen in Fig. 2, the peak of carbon emissions from building operations under the baseline scenario occurs near 2039, with a peak of 3.15 billion tons of CO 2 , which will not meet the target of reaching the carbon peak by 2030. Moreover, total building carbon emissions is projected to increase at a faster rate in 2030, with the baseline scenario using 2020 building carbon emissions as a baseline, increasing by 32.73% by 2030. In addition, it is estimated that the total building carbon emissions vary significantly between climate zones with hot-summer-andcold-winter regions accounting for a larger share. It should be noted that cold regions and hot-summer-andcold-winter regions are more likely to have a higher In this paper, we have designed three priority pathways for carbon emission reduction technologies and identified the trends of carbon emission changes in building operation under different pathways, which can be used as a guide for future carbon emission pathways in China. However, in the context of China's current technology and national conditions, there are still issues that could thwart the implementation of the scheme if only one abatement pathway technology approach is considered. Thus, the feasibility of implementing the three schemes should also be assessed in the context of China's current situation.

The impact of land resources on clean energy development
Under the background of the carbon neutrality goal, the electric quantity after accelerated electrification of the terminal energy consumption department is predicted to reach about 15 trillion kWh by various research institutions. If all photovoltaic power supply is adopted, it is necessary to use an area of 100,000 km 2 of China's territory to lay photovoltaic power to meet the electric quantity demand of the whole society. China's total land resources are plentiful, but its per capita land resources are insufficient. In other words, the land types are diverse, yet the proportion of land resources is imbalanced. In fact, all kinds of land resources are damaged to varying degrees. In addition, the current situation of land resource shortage in China has become the main problem restricting photovoltaic development due to the contradiction between photovoltaic agriculture and arable land red line, unclear land jurisdiction, imperfect supervision system, and high cost of photovoltaic construction land.

Feasibility of buildings photovoltaic development
At present, China's urban and township housing construction area is rapidly increasing, providing sufficient conditions for the development of building photovoltaic resources. Photovoltaic power generation can be built on no less than 50% of the total roof area of Party and government buildings in China; photovoltaic power generation can be put on no less than 40% of the total roof area of schools, hospitals, village committees, and other public facilities. The percentage of solar power generation placed on the total roof area of industrial and commercial buildings must be at least 30%.The percentage of photovoltaic power generation installed in the total roof area of rural residents shall not be less than 20% [23]. Assuming that urban buildings have an average of 10 storeys and rural residential buildings have an average of 3 storeys, 50% of the urban roof area can be used for photovoltaic installation. It is roughly estimated that 450GW of photovoltaic can be installed in cities and 350GW in rural areas [23].

The intensity of implementation of China's mandatory building energy efficiency regulations
In terms of the development and implementation of the energy efficiency regulations, higher building energy efficiency standards and earlier implementation of the regulations could significantly reduce the cumulative carbon emissions of buildings and contribute to the rapid achievement of the zero-carbon goal. However, the implementation of the policy also needs to take into account China's economic situation, the growth rate of building energy efficiency technologies and the actual development of relevant industries in relation to the economic development level. Premature and overly stringent implementation of strong regulations may instead raise the cost of building construction and lead to increased carbon emissions. Near-zero energy buildings are 10%-15% more energy efficient than ultra-low energy buildings, and considering the current state of technological development, it is technically feasible to implement the policy after 2030; zero energy buildings are the same as near-zero energy buildings in terms of energy efficiency, and in the context of vigorous development of renewable energy, it is also technically feasible to partially implement the zeroenergy standard after 2035. Given the current economic development of different parts of China, it is proposed that Beijing, Tianjin, Hebei, Yangtze River Delta, Shandong and Henan should take the lead in implementing the standard.
Overall, it is more feasible to implement a comprehensive three-line parallel development path that takes into account China's actual economic and technological conditions, maximizes existing resources, and fully considers the development constraints of each carbon reduction technology path.

Future prospects
In order to create sufficient prerequisites for the implementation of the above-mentioned three carbon reduction pathways, this paper offers the following suggestions and prospects in terms of the design of carbon reduction pathways, government policy support and the promotion of technological development.

Developing a clear roadmap for carbon peak and neutrality
At present, developed countries are stepping up the development of their own carbon neutral roadmaps in the buildings sector, and some countries have set clear targets for buildings to move towards lower energy consumption and lower carbon emissions. Given of the bulk of buildings in China, the uneven regional economic development and the differences in climatic characteristics, there are many challenges as to how to clarify the path of carbon reduction in the building sector through separate requirements for urban and rural buildings, residential and public buildings, new buildings and renovation of existing buildings. There are also challenges as to how to achieve the overall goal of carbon neutrality in China by stages and regions, and to provide guidance to the construction industry, the building energy efficiency industry and industries related to renewable energy building applications in China. This is an urgent need for China's carbon neutral work. It is proposed that special research projects should be set up to support the indepth study of China's building energy efficiency development technology roadmap, the formulation of medium and long-term planning for buildings energy efficiency, and the service of national urbanization and energy saving and emission reduction work.

Building a system of policies and standards for carbon neutrality
Taking into account the experience of industrialized nations, the evaluation method based on building energy consumption and carbon emissions has steadily become the most important evaluation method in the standard. A unified, easy-to-use energy consumption and carbon emission compliance determination tool is the most indispensable control tool and management platform in future energy efficiency standards, which should be used by the government to assess and monitor the actual amount of carbon reduction in buildings. In order to achieve assessable quantitative targets for carbon reduction, it is recommended that unified building energy consumption and carbon emission evaluation tools for government authorities should be expedited as soon as possible and their use be promoted.

Increasing investment in research and development of key zero-carbon technologies
We should improve the level of scientific and technological innovation and accelerate the research and development of key technologies for carbon-neutral buildings. We should focus on a number of key technology products for building energy efficiency, green buildings, lowcarbon buildings and communities, with a focus on making breakthroughs in such areas as zero-carbon/energy-consuming building integration technology, intelligent operation and maintenance of public buildings, building adaptation, multi-energy complementarity and synergy between source, network, storage and use, DC power supply and energy storage for buildings (light, storage and direct flexibility), digital cities, and carbon emission big data. It is imperative that we focus our efforts on the industrialization of miniaturized equipment for heating and air conditioning, as well as hightemperature heat pumps and high-performance building components. Furthermore, we should conduct research on building electrification projects and promote efficient electrification application technologies and equipment in the fields of building heating, domestic hot water, cooking and other energy uses. We should utilize all types of waste heat resources and biomass energy sources. As a final step, we need to further develop zero-carbon building energy systems and make technological gains in the field of low-carbon and zero-carbon construction.