Carbon emission reduction in public buildings of extreme cold regions: A study on enclosure structure and HVAC system parameter optimization

The implementation of strategies aimed at curtailing energy consumption and carbon emissions in buildings is of paramount importance. Priority should be accorded to the reduction of embodied carbon emissions from construction materials and operational carbon emissions during the usage phase. A public building situated in an extremely cold region is chosen as the subject of this study. An initial investigation is conducted into the impact of various enclosure structure materials on embodied carbon. The impact of different heating, ventilation and air conditioning (HVAC) systems on the total carbon emissions of public buildings are studied as well. The findings indicate that the amalgamation of W3 + R + G4 + H4 culminates in the least total carbon emissions. Upon establishing the foundational scheme for the public building, an optimization (nondominated sorting genetic algorithm II [NSGA II] and Criteria Importance Through Intercrieria Correlation [CRITIC]) of the enclosure structure parameters is initiated to examine the optimal parameters of the public building's enclosure structure. The findings reveal that a decrease in the heat transfer coefficient could trigger an increase in total carbon emissions. This is attributed to the fact that the carbon emissions embodied in the production process of these materials could potentially outweigh the reduction observed in operational carbon emissions. Further adjustments were also made to the parameters of the HVAC system in the buildings. The findings indicate that within the context of public buildings, given the presence of optimal parameters and an HVAC system that utilizes solar energy for both heating and cooling in the American Society of Heating, Refrigerating and Air‐Conditioning Engineers, Inc. 6 A climatic zone, the efficiency of the cooling system can exert a significant influence on the operations carbon emissions reduction.


| INTRODUCTION
The global operations of buildings significantly contribute to energy consumption and emissions, accounting for 30% of global final energy consumption and 26% of global energy-related emissions. 1In 2022, the energy demand of buildings saw a substantial increase, escalating to 135 EJ.The surge in energy demand has led to a corresponding rise in CO 2 emissions from building operations, reaching around 10 GtCO 2 . 2 In the United States, the combined end-use energy consumption by the residential and commercial sectors accounted for about 29% of total end-use energy consumption in 2022. 3Given these figures, it is crucial to implement measures aimed at reducing energy consumption and carbon emissions from buildings.Such measures could potentially lead to significant improvements in global energy efficiency and contribute to the mitigation of climate change.
The data indicate that 55.51% of the total carbon emissions are attributed to the production phase of building materials, while the operational phase of buildings accounts for 42.52%.The construction phase contributes a mere 1.97%. 4Embodied carbon is the sum of CO 2 emissions from various manufacturing and construction processes.Reducing embodied carbon from construction materials is essential to effectively addressing climate change.Therefore, reducing the embodied carbon emissions from building materials and the operational carbon emissions during the use phase should be prioritized.
By augmenting the quality of the materials utilized in the building's protective structure and amplifying the thickness of the insulation layer, a substantial reduction in the building's heat transfer coefficient can be achieved. 5This not only mitigates the energy required to sustain a comfortable internal temperature but also curtails the carbon emissions generated during operation.7][8] Consequently, it is of paramount importance to implement multiobjective optimization techniques to ascertain the optimal enclosure structure parameters that will concurrently minimize the building's operational energy consumption and total carbon emissions.
Benincá et al. used multiobjective optimization and nondominated sorting genetic algorithm II (NSGA-II) to model the thermal behavior of large residential social housing multifamily buildings, with the goal of determining the optimal positioning of solar energy while minimizing cooling and heating requirements. 9Similarly, Liu et al. designed an efficient and intelligent hybrid approach to optimize design parameters and address the mismatch among green buildings' multiple objectives. 10Wu et al. developed a comprehending optimization method based on NSGA-II to explore the multiobjective optimization of nearly zero energy buildings to reduce carbon emissions, increase energy efficiency, and optimize thermal comfort in four typical climate regions in China. 11Merlet et al. suggested a way to help building stock or real estate managers improve building efficiency, specifically targeting the retrofits of walls and windows of the buildings. 12osamo et al. designed a system to study the impact of building factors on energy use and determine the best design, which combined building information modeling (BIM), machine learning, and NSGA II. 13 In comparison to other multiobjective optimization algorithms, NSGA II exhibits notable attributes such as expeditious nondominated sorting, intensive crowd comparison operations, robust global search capabilities, and high computational efficiency.These features render NSGA II well-suited for addressing complex problems characterized by multiple conflicting objectives.][16][17] The Criteria Importance Through Intercrieria Correlation (CRITIC) method provides a systematic way to handle multiobjective optimization to find the optimal solution by accounting for conflicting relationships among objectives and estimating their relative importance. 18,19he heating, ventilation and air-conditioning system is responsible for approximately half of the energy consumed.In 2023, the total energy consumption in the U.S. was significantly influenced by the residential and commercial sectors, contributing about 19.7% and 17.2% respectively, amounting to a combined contribution of 36.9%. 20As of 2022, the thermal processes in architectural structures are anticipated to yield 2.4Gt of direct carbon emissions, coupled with 1.7Gt of indirect carbon emissions.The carbon emissions from global refrigeration processes account for over 7% of the total global emissions, with a projection indicating a twofold increase in global refrigeration energy demand by 2050. 21he incorporation of technologies such as solar photovoltaic systems and geothermal heat pumps has emerged as a significant strategy for mitigating operational carbon emissions in buildings.Saffari et al.'s study showed that retrofitting deep building structures with heat pumps under low environmental temperatures can lead to a significant decrease in primary energy by over 70% and a 68% reduction in carbon emissions. 22The potential for neglect and degradation of heritage buildings can be prevented by aligning with existing energy efficiency and decarbonization requirements, and preserving their historical value through the use of renewable energy derived from air-to-water heat pumps, as evaluated by Pochwała et al. 23 An innovative approach to enhance the performance of direct expansion solar-assisted heat pump systems was proposed by Kutlu et al., which effectively harnessed solar energy to reduce energy consumption in buildings. 24A photovoltaic technology integrated with the front glass wall was developed by Marei et al., contributing to energy conservation within buildings.It was found that the photovoltaic-integrated front wall supplies approximately 394 kWh/m 2 of electrical energy to the building daily. 25A method to estimate the influence of the level of building photovoltaic integration on the photovoltaic energy balance was introduced by Lillo-Bravo et al., which incorporates a natural ventilation or forced cooling system. 26owever, it is noteworthy that, in comparison to traditional cooling and heating methods, heat pump systems, which fulfill the thermal load demand of buildings, consume electrical energy. 27Decarbonization from energy efficiency in China will be mainly affected by the end uses of heating and appliances. 28,29onsequently, it is imperative to optimize the parameters of the HVAC system with renewable energy, using operational carbon emissions as objective functions, to determine the optimal HVAC system parameters that minimize operational carbon emissions.
In summary, this study is centered on public buildings situated in regions characterized by severe cold climates.The initial phase involves the construction of a building model utilizing DesignBuilder.The primary objective is to identify a combination of enclosure structure and HVAC systems that minimizes total carbon emissions, thereby establishing a solid foundation for subsequent investigations.In the ensuing stage, an optimization of the enclosure structure parameters is undertaken, employing total carbon emissions and operational energy consumption as the objective functions.This approach facilitates the determination of the optimal enclosure structure parameters that concurrently minimize both total carbon emissions and operational energy consumption.In the final phase, an optimization of the HVAC system parameters with solar energy supply for cooling and heating is executed, with operational carbon emissions as the objective function.This strategy enables the identification parameters of the HVAC system with a solar energy supply that optimally reduces operational carbon emissions, thereby contributing to the overall efficiency and sustainability of the public building operations in severe cold regions.

| Building simulation
A simulation model of a public building has been constructed using DesignBuilder.The building, located in Changchun City (an American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) 6 A climate zone in DesignBuilder), boasts a total area of 3231.3 m 2 and a form factor of 0.27.The average window-to-wall ratio is 0.25, with the east façade at 0.16, the west at 0.18, the south at 0.36, and the north at 0.22.The simulation model of the building can be seen in Figure 1.
Figure 2 delineates the schematic representation of the energy supply system.The left segment of the figure embodies the conventional energy supply paradigm, whereas the right segment illustrates the energy supply system that is the subject of the present research.

| Extent of carbon emissions estimation
As previously mentioned, the embodied carbon of building materials and the carbon emissions during the operational phase of a building account for over 90% of the total carbon emissions over the building's entire The simulation model of the building.4][35] Therefore, this study primarily includes embodied carbon emissions of the building's enclosure structure, heating carbon emissions, and cooling carbon emissions when analyzing the total carbon emissions and potential carbon reduction of public buildings.
where C T signifies the total carbon emissions from the building (kg CO 2 e), C Embodied represents the embodied carbon emissions of the building (kg CO 2 e), the embodied carbon data can be obtained from the Design Builder simulation, and where E denotes the energy consumption of the building (kWh), coefficient of performance (COP) represents the coefficient of performance of the energy supply system, and CE signifies the carbon emission factor of the energy supply system.CE heating signifies the carbon emission factor of heating energy supply.CE cooling signifies the carbon emission factor of cooling energy supply.E O signifies the operational energy consumption of the building (kWh).The subscripts "heating" and "cooling" respectively indicate heating and cooling processes.
According to the statistical data from the China Urban Greenhouse Gas Working Group, the carbon emission factor of the Jilin Province power grid is 0.889 kg CO 2 e/kWh, 36 and the carbon emission factor of natural gas is 1.481 kg CO 2 /kgce. 37

| Optimization function for enclosure structure parameters
During the optimization of enclosure structure parameters, the NSGAⅡ 38 method is selected.The heat transfer coefficients of the external wall, roof, and external window are taken as independent variables, and C T and E O of buildings are taken as objective functions, as shown in Equation (4).
A weighting method is introduced to find the optimal solution.The Criteria Importance Through Intercrieria Correlation (CRITIC) 39 method is used to determine the weights between the objectives.The CRITIC method can calculate the intensity of comparison and conflict between the objectives, comprehensively measure the objective weights and consider the impact of the objectives on the function, as shown in Equation (5).
Where w 1 and w 2 represent the objective weights, indicating the degree of influence of C T and E O on the objective function.

| Optimization function for HVAC system parameters
The optimization scheme for the HVAC system is shown on the right side of Figure 2. A heat pump with power The schematic representation of the energy supply system.grid and solar energy supply is selected for cooling and heating for the public building.
The COP of the Heating system, the COP of the Cooling system, and the proportion of renewable energy supply are taken as independent variables.The operational carbon emissions of the building is taken as objective functions, as shown in Equation (6).
When using a heat pump with power grid and solar energy supply for cooling and heating, the C Operation of the building is as shown in Equation (7).
Herein, P r denotes the proportion of solar energy supply; EF e and EF s respectively represent the carbon emission factors of grid electricity and solar power generation.
According to the statistical data from the China Urban Greenhouse Gas Working Group, the carbon emission factor of solar power generation is 0.085 kg CO 2 e/kWh. 36| RESULTS AND DISCUSSIONS

| The impact of different enclosure structure materials on embodied carbon emissions
The simulation model of the public building has initial heat transfer coefficients for its enclosure structures as follows: exterior wall 0.45 W/(m 2 •K), roof 0.43 W/(m 2 •K), and exterior window 2.2 W/(m 2 •K).
7][8] Ensuring that the heat transfer coefficients of the exterior wall, roof, and exterior window remain constant, the study first investigates the impact of different enclosure structure materials on the building's embodied carbon.Five schemes are chosen for the exterior wall, roof, and exterior window, as shown in Tables 1-3.
As shown in Figure 3, the amount of embodied CO 2 emitted by the exterior wall structure schemes W-W4 is as follows: W3 < W1 < W2 < W4 < W. The calculation results in this article show that the brickwork outer layer of the wall accounts for 59.57%-75.28% of the total embodied carbon of the exterior wall, making it the layer with the most embodied carbon in ASHRAE 6 A climate zone.
To withstand the freeze-thaw cycles in frigid environments and endure the harsh climatic conditions prevalent in these regions, exterior wall structures in cold areas typically exhibit substantial mass.Notably, the brickwork outer layer of the exterior wall constitutes a significant proportion of the overall mass.The quality of the brickwork outer layer materials investigated in this study accounts for 42.60%-71.77% of the total wall mass.It presents the primary contributor to embodied carbon emissions.
In contrast, the mass of insulation materials constitutes a mere 0.2%-0.91% of the total wall mass.Its embodied carbon emissions represent only 1.47%-29.99% of the total embodied carbon for the entire wall.
Therefore, it is suggested to choose low-carbon bricks or other materials for the outer layer of the wall to reduce the embodied carbon of the exterior wall.It also highlights the importance of material innovation and material manufacturing process in total carbon reduction of life cycle buildings.
When the outer layer material of the wall is the same, the Concrete Block material has less embodied CO 2 than Thermalite, and the insulation layer has the greatest T A B L E 1 Five schemes for exterior wall enclosure materials.impact on embodied CO 2 .This is consistent with the conclusions of previous studies that embodied energy penalty outweighs operational energy savings when the insulation is over-thickness. 8s shown in Figure 4, the amount of embodied CO 2 emitted by the roof structure schemes is as follows: R < R3 < R2 < R1 < R4.In the entire roof structure, the concrete material layer accounts for 29.26%-94.58% of the total embodied carbon of the roof structure, making it the layer with the greatest impact on the total embodied CO 2 of the roof structure in ASHRAE 6 A climate zone.

Layers
Indeed, the mass of concrete materials constitutes 67.31%-95.29% of the total mass in the roof structure.
Concrete presents the predominant contributor to embodied carbon emissions.This is followed by the roof panel layer, which accounts for a range of 0.42%-62.4% of the total mass in the roof structure, which accounts for 2.76%-84.78% of the total embodied carbon of the roof structure.The insulation material layer accounts for 6.16%-58.36% of the total embodied carbon of the roof structure, and the asphalt material layer accounts for 2.66%-10.82%.
As shown in Figure 5, the amount of embodied CO 2 emitted by the exterior window structure schemes is as follows: G < G1 < G4 < G2 = G3.However, although windows with fewer layers of glass and without sun shading produce less embodied carbon, in severe cold The embodied carbon of different exterior wall enclosure materials.
The embodied carbon of different roof enclosure materials.
TENG and YIN | 2681 climates, a combination of three layers of glass with sun shading is generally adopted. 30

| The impact of different HVAC systems on total carbon emissions
In the quest to identify a combination of enclosure structure and HVAC systems that minimizes total carbon emissions, and steered by empirical engineering instances, a selection of five distinct HVAC schemes has been made.The COP and energy efficiency of HVAC systems significantly impact their effectiveness in energy conversion and utilization.Consequently, these factors directly influence the carbon emissions generated during the operational phase of building heating and cooling.These schemes are instrumental in dissecting the ramifications of various HVAC systems on the total carbon emissions of buildings.
As delineated in Table 4, these five viable HVAC systems amalgamate with the scheme of external walls, roofs, and windows during the embodied carbon calculation to predict the total carbon emissions.
As depicted in Figure 6, the results suggest that the total carbon emissions of the building oscillate within the range of 243.11-394.72 t CO 2 e/a.An exhaustive analysis unveils that the amalgamation of W3 + R + G4 + H4 culminates in the least total carbon emissions.Despite the fact that an augmented Coefficient of Performance (COP) of the HVAC system results in a diminished carbon emission factor of the heat and cold sources, thereby curtailing the operational carbon emissions, it is noteworthy that the carbon emissions from natural gas are inferior to those from the power grid.Consequently, even when the heating fuel is natural and the COP of H4 is less than that of H3, the carbon emissions of H4 persistently remain lower.
In light of these empirical findings, it is judicious to employ a heat pump with a superior COP in the HVAC system of a building.Concurrently, efforts should be made to augment the integration of renewable energy sources to cater to the energy needs of the HVAC system.This strategy will not only reduce the carbon emission factor of the heat and cold sources but also contribute significantly to the reduction of the public building's carbon emissions.
The impact of different heating, ventilation and air conditioning (HVAC) systems on total carbon emissions.

| Optimization of enclosure structure parameters
Building upon the foundation of the W3 + R + G4 + H4 scheme, a multiobjective optimization of the enclosure structure parameters is embarked upon to scrutinize the optimal parameters of the public building's enclosure structure.The spectrum of the enclosure structure design parameters is established in alignment with the "Near Zero Energy Building Technical Standard of China," 40 encompassing the heat transfer coefficient for the external wall, the heat transfer coefficient for the roof, and the heat transfer coefficient for the external window.Harnessing this spectrum, and employing NSGAⅡ and CRITIC, in conjunction with formulas ( 4) and ( 5), a population size of 200 and an iteration count of 200 are instituted to analyze the total carbon emissions and operational energy consumption of public buildings.
The data presented in Figure 7 indicate that as the heat transfer coefficient decreases, the operational energy consumption of public buildings will experience a reduction.However, the total carbon emissions of the building will increase with the decrease of the heat transfer coefficient.
The rationale behind this is that the carbon emissions embodied in the production process of different materials could potentially supersede the decrement observed in operational carbon emissions. 16,25,30y leveraging the CRITIC method, the weights of the enclosure structure parameters for the total carbon emissions and operational energy consumption of the building can be ascertained, as delineated in Table 5.
Through the process of optimization, the optimal parameters for the enclosure structure are determined as  witnesses an increase of 7.46%.The operational energy consumption undergoes a decrease of 39.86%, and the total carbon emissions observe an increase of 3.11%.

| Optimization of HVAC system parameters
Drawing upon the optimal parameters of the enclosure structure, a further refinement of the parameters of the HVAC system with power grid and solar energy supply for cooling and heating in public buildings is undertaken.In adherence to the "Energy Saving Design Standards for Public Buildings of China," 41 6) and ( 7) are employed to optimize HVAC system parameters, thereby facilitating a comprehensive analysis of carbon emissions of public buildings.The optimal results pertaining to the HVAC system with solar energy supply for cooling and heating design parameters are depicted in Figure 8.
Figure 8 posits that when COP cooling = 4, COP heating is approximating 6, and P r is approximating 1, the C Operation are reduced.when COP heating = 2, COP cooling is approximating 6, and P r is approximating 1, the rate of reduction in C Operation is most pronounced.
The findings of Figure 8 indicate that when the proportion of solar power supply is close to 1, enhancing COP cooling has a more substantial carbon reduction effect than enhancing COP heating .
As a consequence of the diminished heat transfer coefficient, there is a decrease in the dissipation of heat from the building's interior, thereby reducing the building's thermal demand during the winter season.Conversely, there is an escalation in the energy consumption for cooling the building.Consequently, the COP for cooling exerts a more significant influence.A superior COP cooling value signifies a more efficient system, implying its capability to deliver the necessary cooling utilizing less energy.
Hence, the efficiency of the cooling system in an HVAC system with a solar energy supply can exert a substantial influence on the further operational carbon emissions reduction of public buildings in extreme cold regions.
Consequently, the influence of the parameters of the HVAC system with solar energy supply for cooling and heating in public buildings on reducing carbon emissions from building operations is ranked as follows: COP cooling -P r > COP heating -P r > COP cooling -COP heating .

| CONCLUSIONS
The empirical evidence suggests that 55.51% of the total carbon emissions are ascribed to the production phase of building materials, while the operational phase of buildings is responsible for 42.52%.Consequently, it is of utmost importance to prioritize the reduction of embodied carbon emissions from building materials and operational carbon emissions during the use phase.A simulation model of a public building in severe cold regions (an ASHRAE 6 A climate zone in DesignBuilder) has been meticulously constructed utilizing Design-Builder.The building has been selected as the subject of this investigation.The deductions are as follows: 1.A comprehensive analysis discloses that the combination of W3 + R + G4 + H4 results in the minimum total carbon emissions.Despite the fact that an increased COP of the HVAC system leads to a reduced carbon emission factor of the heat and cold sources, thereby diminishing the operational carbon emissions, it is noteworthy that the carbon emissions from natural gas are inferior to those from the power grid.As a result, even when the heating fuel is natural and the COP of H4 is less than that of H3, the carbon emissions of H4 consistently remain lower.It has been empirically demonstrated that the selection of the heating source for a building's HVAC system significantly influences both the operational emissions and the overall carbon emissions.2. As the heat transfer coefficient decreases, the operational energy consumption of public buildings will witness a reduction.However, the total carbon emissions of the building will escalate with the decrease of the heat transfer coefficient.This suggests that a reduction in the heat transfer coefficient could potentially lead to an increase in the total carbon emissions.The reasoning behind this is that the carbon emissions embodied in the production process of different materials could potentially outweigh the reduction observed in operational carbon emissions.
The manufacturing process of building materials (embodied carbon) has a large impact on building carbon emissions.3. The optimal parameters of the public building's enclosure structure possess the potential to reduce the required energy for heating buildings located in regions characterized by extreme cold.As a consequence of the diminished heat transfer coefficient, there is a decrease in the dissipation of heat from the building's interior, thereby reducing the building's thermal demand during the winter season.Conversely, there is an escalation in the energy consumption for cooling the building.Consequently, the COP for cooling exerts a more significant influence.A superior COP cooling value signifies a more efficient system, implying its capability to deliver the necessary cooling utilizing less energy.Hence, for public buildings with optimal parameters and HVAC system with solar energy supply for cooling and heating in ASHRAE 6 A climate zone, the efficiency of the cooling system can exert a significant influence on the operations carbon emissions reduction.
The findings presented in this study lay a theoretical foundation for the targeted mitigation of carbon emissions in public infrastructure in ASHRAE 6 A climate zone.Furthermore, they offer pivotal educational strategies and theoretical guidance for the cultivation of innovative talents in the field of carbon reduction technology.

F I G U R E 5
The embodied carbon of different window enclosure materials.TA B L E 4 Five schemes for HVAC systems.

H
with the initial parameters of the W3 + R + G4 scheme, the heating energy consumption experiences a reduction of 89.45%, while the cooling energy consumptionF I G U R E 7The trend of total carbon emissions and operational energy consumption under different heat transfer coefficients of the enclosure structure: (A) Wall; (B) Roof; (C) Window.

T A B L E 5 F
The weights of total carbon emissions and operational energy consumption.I G U R E 8 The trend of C operation .
T A B L E 2 Five schemes for roof enclosure materials.Five schemes for windows enclosure materials.