Lifecycle CO₂ Reduction by Implementing Double Window Casement Systems in Residential Units in Korea

Abstract

This study investigated lifecycle CO₂ (LCCO₂) emission reductions through application of double-window casement systems (DWCS) in residential units in Korea, compared with conventional single windows (SWs). The DWCS is a double window system that is energy-efficient, reducing energy consumption during operation. However, this system incorporates increased embodied CO₂ emissions. We evaluated LCCO₂ reductions associated with use of the DWCS by calculating CO₂ emissions during space conditioning as well as the embodied CO₂ emissions of the DWCS. The results showed that use of DWCS in a residential unit during the cooling season had 26.2 and 27.4 t CO₂ fewer emissions than SWs in the natural ventilation and minimum ventilation modes, respectively. Although implementation of DWCS is expected to substantially reduce LCCO₂ emissions, the large embodied CO₂ emissions of the aluminum frame reduce the benefits of the DWCS.

1. Introduction

1.1. Background

Recently, climate change has become a difficult challenge for society and governments. There is worldwide agreement on the necessity for mitigation of global warming resulting from increased greenhouse gas (GHG) emissions. GHG emissions are mainly due to anthropogenic sources such as economic activity and consumption of fossil fuels for energy. Under these circumstances, building energy consumption plays an important role. According to the International Energy Agency, residential and commercial buildings are responsible for about 35.9% of energy use globally [1]. Developed countries such as the UK or USA have higher energy consumption in the building sector, making up about 40% of the total national energy consumption [2]. Consequently, many nations have released action plans to mitigate climate change, containing detailed strategies for achieving substantial CO₂ reductions. The Korean government has participated in this movement and has announced a plan to reduce CO₂ emissions by 30% by 2020. In Korea, building energy consumption accounts for about 30% of the total energy consumption and residential buildings consume 53% of this amount [3]. In addition, residential building construction accounts for more than 40% of new construction. Thus, improvements in the energy efficiency of residential building in Korea are necessary to meet CO₂ reduction targets. In many cases, it may be desirable to reduce energy demand by enhancing the performance of passive systems (or the building fabric) before applying more efficient facility or renewable energy systems [4].

Double-layered envelope systems such as double-skin façades are effective in reducing the energy consumption of office buildings and residential buildings [5,6,7,8,9,10,11,12]. The energy-efficient properties of a double-layered envelope system can be maximized by applying natural cooling strategies [13,14,15,16,17,18]. In addition, a double-layered envelope system can be implemented as a compact window type, the double window system. Double window systems also have energy-saving benefits such as increased thermal resistance, simplicity of solar control and natural ventilation ability [19,20,21,22,23,24,25,26,27,28,29,30]. Therefore, double window systems could contribute to the reduction of national CO₂ emissions in Korea.

1.2. Purpose

The main objective of this study was to evaluate reductions in LCCO₂ emissions with the implementation of double-window casement systems (DWCS) in residential buildings in Korea. Conventional double-window systems (DWs) without shading devices are already in use in residential buildings in Korea for heat load reduction. However, because conventional DWs lack built-in shading devices, they permit undesirable solar transmission during the cooling season, increasing the cooling load. In this study, we considered a DWCS, which is a box-type double-skin façade designed to reduce the heating and cooling loads of residential buildings. We evaluated its benefits in terms of lifecycle CO₂ (LCCO₂) reductions assuming that the DWCS was implemented in a residential building in Korea.

2. Methods

To evaluate the potential benefits of the DWCS, we compared the DWCS to conventional single windows (SWs). We first defined the geometry of the DWCS, then evaluated the resulting reductions in heating and cooling loads, and finally performed a complete LCCO₂ assessment. We focused on the net LCCO₂ change from application of the DWCS due to reduced operating energy and increased embodied energy. Components of the residential building other than the window system were considered to be identical and were not included in the assessment.

An LCCO₂ assessment for a building includes quantitative assessment of the total CO₂ emissions generated during the entire lifespan of the building. The life cycle of a building is divided into the construction/transportation stage, the operation and maintenance stage, and the demolition and disposal stage. To perform an LCCO₂ assessment for a residential building, the life cycle of the building should be divided into these stages and an overall CO₂ assessment method should be determined after developing a CO₂ assessment approach for each stage [31]. The present assessment included the critical factor of embodied energy, which represents resource consumption during production and transportation. Embodied energy includes all of the primary energy used by a product or process, including fuel and electricity [32]. Estimates of embodied energy for each window system were converted into embodied CO₂ emissions.

To evaluate LCCO₂ reduction during the operation stage, heating and cooling loads were calculated using the energy simulation model ESP-r (Energy System Performance-research) [33]. ESP-r is an open source energy simulation tool in the fields of various built environments: thermal, visual, electrical and etc. This program has been extensively validated by many case studies [34]. Assumed energy consumption for plug load and domestic hot water were added to the operational energy consumption for the LCCO₂ assessment. In addition, embodied CO₂ emissions for SWs and the DWCS were calculated using CO₂ emission factors.

3. Window Systems

In contrast to conventional windows such as SWs or DWs used for residential buildings in Korea, DWCS have a shading device between the external and internal windows. Conventional DWs have very little or no cavity space between the external and internal windows, preventing installation of a shading device in the gap. However, the new DWCS window system has a greater cavity depth of 20 cm for shading device installation. In addition, the internal window is designed to tilt and is designed to enhance air-tightness and regulate the natural ventilation rate. A schematic of the DWCS is depicted in Figure 1.

Figure 2 illustrates the operational strategy of the DWCS. When outdoor temperatures are very cold, the external and internal windows are closed for heat loss reduction and all shading is removed to maximize solar radiation. During intermediate seasons, window operation varies with the outdoor air temperature. Under these conditions, the external window is always fully open and the internal window controls the incoming air flow rates. If the outdoor temperature is warm, requiring rapid ventilation of excess heat from the indoor space, the internal window can be fully opened for maximum natural ventilation. Shading is provided during intermediate seasons. When the outdoor air temperature is too hot for natural ventilation, the external window is fully opened to prevent overheating in the cavity between the external and internal windows. The internal window is closed to exclude the hot outdoor air and shading is provided. Mechanical ventilation is provided when natural ventilation is not available. Figure 3 and Figure 4 show the geometries of the SW and DWCS. Window systems consist of aluminum, glass, thermal breaks, and sealant.

Figure 1. Conceptual geometry of the DWCS.

Figure 1. Conceptual geometry of the DWCS.

Figure 2. Operation of the DWCS.

Figure 2. Operation of the DWCS.

Figure 3. Composition of a SW.

Figure 3. Composition of a SW.

Figure 4. Composition of a DWCS.

Figure 4. Composition of a DWCS.

4. CO₂ Emissions during Space Heating and Cooling

4.1. Target Building

A residential building adopting DWCS was selected for evaluation of space heating and cooling loads. The target building is located in Incheon, Korea. The target building and the configuration of the selected residential unit are depicted in Figure 5 and Figure 6. The building and most of the windows are installed on the main façade and facing southeast. The climate in this region is hot and humid during summer and cold and dry during winter. Such a climate pattern results in drastic variations in heating and cooling energy consumption with seasonal change.

Figure 5. Target building.

Figure 5. Target building.

Figure 6. Selected residential unit and window geometry.

Figure 6. Selected residential unit and window geometry.

Table 1 lists the materials used for windows in the target residential unit, in the case of SW and DWCS. The total volumes of the materials used for manufacturing were calculated using a CAD program. Sectional areas and lengths of the window components were obtained and multiplied to calculate the frame volume.

Table 1. Materials in windows per residential unit.

Item	SW	DWCS
Frame (cm3)	177,643	271,156
Glass (m2)	45.6	91.2
Polyamide (m3)	0.09	0.33

Table 1. Materials in windows per residential unit.

Item	SW	DWCS
Frame (cm3)	177,643	271,156
Glass (m2)	45.6	91.2
Polyamide (m3)	0.09	0.33

4.2. Simulation Inputs

To estimate the heating and cooling loads, an energy simulation was performed. To compare SWs and the DWCS, two different simulation models were established. Simulation model images developed by the dynamic simulation model tool ESP-r are depicted in Figure 7. Simulation cases for the comparative study are presented in Table 2. Cases 1 and 3 assumed SWs with low thermal resistance and an internal shading device. Cases 2 and 4 assumed implementation of the DWCS for improved thermal performance. The optical and thermal performances of the window systems are shown in Table 3 and Table 4. The U-factor of the external wall was set to 0.3 W/m²·K.

Figure 7. Residential unit modeling by ESP-r of (a) Model 1 (SWs) and (b) Model 2 (DWCS).

Figure 7. Residential unit modeling by ESP-r of (a) Model 1 (SWs) and (b) Model 2 (DWCS).

Table 2. Simulation cases.

Case	Window	Ventilation Mode
Case 1	SWs	Minimum ventilation (0.7 ACH)
Case 2	DWCS
Case 3	SWs	Natural ventilation
Case 4	DWCS

Table 2. Simulation cases.

Case	Window	Ventilation Mode
Case 1	SWs	Minimum ventilation (0.7 ACH)
Case 2	DWCS
Case 3	SWs	Natural ventilation
Case 4	DWCS

Table 3. Properties of the SWs.

Item	External Window
Modeling Module	CFC (Complex Fenestration Construction)
Outer glazing	Solar transmission: 0.771
	Reflection (front): 0.070
	Reflection (back): 0.070
Inner glazing	Solar transmission: 0.771
	Reflection (front): 0.070
	Reflection (back): 0.070
Shading device	Depth: 50 mm Angle: 45°
Overall U-factor (including frame)	3.0 W/m2·K

Table 3. Properties of the SWs.

Item	External Window
Modeling Module	CFC (Complex Fenestration Construction)
Outer glazing	Solar transmission: 0.771
	Reflection (front): 0.070
	Reflection (back): 0.070
Inner glazing	Solar transmission: 0.771
	Reflection (front): 0.070
	Reflection (back): 0.070
Shading device	Depth: 50 mm Angle: 45°
Overall U-factor (including frame)	3.0 W/m2·K

Table 4. Properties of the DWCS.

Item	External Window	Internal Window
Module	CFC (Complex fenestration construction)	TMC (Transparent multi-layer construction)
Outer glazing	Solar transmission: 0.771	Solar transmission: 0.514 Absorption (front): 0.182 Absorption (back): 0.138
	Reflection (front): 0.07
	Reflection (back): 0.07
Inner glazing	Solar transmission: 0.771
	Reflection (front): 0.07
	Reflection (back): 0.07
Shading device	Depth: 50 mm Angle: 45°	-
Overall U-factor (including frame)	3.0 W/m2·K	2.0 W/m2·K

Table 4. Properties of the DWCS.

Item	External Window	Internal Window
Module	CFC (Complex fenestration construction)	TMC (Transparent multi-layer construction)
Outer glazing	Solar transmission: 0.771	Solar transmission: 0.514 Absorption (front): 0.182 Absorption (back): 0.138
	Reflection (front): 0.07
	Reflection (back): 0.07
Inner glazing	Solar transmission: 0.771
	Reflection (front): 0.07
	Reflection (back): 0.07
Shading device	Depth: 50 mm Angle: 45°	-
Overall U-factor (including frame)	3.0 W/m2·K	2.0 W/m2·K

To appropriately operate each window system, separate control methods were adopted for the heating and cooling seasons. Control strategies during the cooling period are shown in Table 5. Two ventilation strategies were established, cross-ventilation and single-sided ventilation. When the outdoor air was a comfortable temperature, cross-ventilation was used. However, if the indoor temperature approached the cooling or heating set-point temperature, the doors were closed and the ventilation mode was changed to single-sided. This mode reduces outdoor air intrusion to the indoor space and maintains a moderate indoor temperature. For the space cooling device, a package air conditioning system (PAC system) was selected and the basic heating and cooling module (ideal heating/cooling calculation module) was used to represent the air cooling system in ESP-r. The cooling set-point temperature was 26 °C. During the heating season, minimum ventilation of 0.7 ACH (Air Change per Hour) was applied by a mechanical ventilator to the indoor space and an infiltration rate of 0.36 ACH was assumed for the cavity space of the DWCS. Korean residential buildings typically have a traditional radiant floor heating system, known as ondol. Thus, heat flux was provided to the floor fabric to simulate this floor heating model. A total of 25 kW of boiler energy was applied and the maximum heat flux to the each room was proportional to the floor area. The indoor heating set-point temperature was 20 °C. These heating and cooling methods are depicted in Figure 8.

The air-flow network (AFN) of the energy simulation used the standard orifice model to evaluate natural ventilation effects as expressed by Equation (1). The simulation used a fixed discharge coefficient (C_d = 0.65). The operable window areas in the partially and fully open conditions are shown in Table 6. The operable areas of the SW and internal window of the DWCS were controlled by the outdoor air temperature:

$$Q=C_d\bullet A\bullet {(2\bullet \frac{\left|\mathrm{\Delta }P\right|}{\mathrm{\rho }})}^{\frac{1}{2}}$$

(1)

Q: Mass flow (kg/s); C_d: Discharge coefficient; ΔP: Pressure difference (Pa); ρ: Fluid density (kg/m³); A: Area of opening (m²).

Climate data for Incheon in 2010 were used for the simulation. ESP-r provides a module that converts the climate data format from that of the Korea Meteorological Administration (KMA). Thus, global horizontal solar radiation data furnished by KMA was used in the simulation and diffuse radiation was calculated by the climate modification module of ESP-r, which uses the Muneer model.

Table 5. Window control strategies during the cooling season.

Category	Control Strategies
Cooling equipment	Set point temperature: 26 °C
	Operating period: 00:00–24:00
	Cooling device module for the simulation: Ideal cooling
Ventilation strategy	Cross ventilation conditions: - Window operation reflects the outdoor temperature (To) - Indoor doors are fully open - Window operation 1 (SW and internal window of DWCS): 23 °C > To ≥ 20 °C: partially open, 25 °C > To ≥ 23 °C: fully open, To < 20 °C or ≥25 °C: closed - Window operation 2 (external window of DWCS) To ≥ 20°C: fully open To < 20 °C: closed Single-sided ventilation conditions: - Operates the indoor doors based on the indoor temperature (Ti) - Ti > 25.5 °C or < 20.5 °C: close indoor doors

Table 5. Window control strategies during the cooling season.

Category	Control Strategies
Cooling equipment	Set point temperature: 26 °C
	Operating period: 00:00–24:00
	Cooling device module for the simulation: Ideal cooling
Ventilation strategy	Cross ventilation conditions: - Window operation reflects the outdoor temperature (To) - Indoor doors are fully open - Window operation 1 (SW and internal window of DWCS): 23 °C > To ≥ 20 °C: partially open, 25 °C > To ≥ 23 °C: fully open, To < 20 °C or ≥25 °C: closed - Window operation 2 (external window of DWCS) To ≥ 20°C: fully open To < 20 °C: closed Single-sided ventilation conditions: - Operates the indoor doors based on the indoor temperature (Ti) - Ti > 25.5 °C or < 20.5 °C: close indoor doors

Table 6. Operable window area for natural ventilation.

Item	Living Room		R1	R2	R3	R4
	South	North	South	South	South	North
Floor area (m2)	60		24	12	12	11
Fully opened (m2)	1.3	2.4	1.3	1.3	1.3	1.3
Partially opened (m2)	0.3	0.55	0.3	0.3	0.3	0.3

Table 6. Operable window area for natural ventilation.

Item	Living Room		R1	R2	R3	R4
	South	North	South	South	South	North
Floor area (m2)	60		24	12	12	11
Fully opened (m2)	1.3	2.4	1.3	1.3	1.3	1.3
Partially opened (m2)	0.3	0.55	0.3	0.3	0.3	0.3

Figure 8. Heating and cooling methods in the energy simulation.

Figure 8. Heating and cooling methods in the energy simulation.

4.3. Results

The calculated annual space heating and cooling energies are shown in Table 7 and Figure 9. The heating load was reduced by about 23% when the DWCS was used, regardless of the ventilation conditions. On the other hand, the cooling load was reduced by 46% and 61% in the minimum ventilation and natural ventilation modes, respectively, with implementation of the DWCS. The average natural ventilation rates of Cases 3 and 4 during cooling period are 20.2 air changes per hour (ACH) and 12.9 ACH respectively. Thus, energy savings are increased by natural ventilation during the cooling season. The average ventilation rate of DWCS is lower than SW. The heating load made up the majority of the overall space conditioning load. Based on this annual total heating and cooling load analysis, the DWCS reduced energy consumption under all ventilation conditions.

Table 7. Calculated heating and cooling loads.

Case	Window Type	Ventilation Mode	Heating Load (kWh)	Cooling Load (kWh)
Case 1	SW	Minimum	12,540	4,183
Case 2	DWCS	Minimum	9,662	2,277
Case 3	SW	Natural	12,540	2,830
Case 4	DWCS	Natural	9,673	1,098

Table 7. Calculated heating and cooling loads.

Case	Window Type	Ventilation Mode	Heating Load (kWh)	Cooling Load (kWh)
Case 1	SW	Minimum	12,540	4,183
Case 2	DWCS	Minimum	9,662	2,277
Case 3	SW	Natural	12,540	2,830
Case 4	DWCS	Natural	9,673	1,098

Figure 9. Space conditioning load reduction: (a) minimum ventilation conditions and (b) natural ventilation conditions.

Figure 9. Space conditioning load reduction: (a) minimum ventilation conditions and (b) natural ventilation conditions.

5. Lifecycle CO₂ Emissions Assessment

5.1. Lifecycle CO₂ Assessment Overview

LCCO₂ assessment includes the entire lifespan of a building. In this study, we considered the following three stages: (1) the construction and transportation stage; (2) the operation and maintenance stage; and (3) the demolition and disposal stage. CO₂ emissions during the operation stage were calculated based on the energy simulation, which determined the heating and cooling loads. These results were used as input data for CO₂ emissions during the operation stage. CO₂ emissions during the construction stage can be calculated by inter-industry analysis. LCCO₂ inventories in Korea generally furnish data in the form of CO₂ emissions per unit cost. However, recent studies have developed a limited LCCO₂ inventory consisting of mass- or volume-based data. In this study, embodied CO₂ emissions during the construction process were calculated using the results of evaluated heating and cooling load in advance. CO₂ emissions during transportation, maintenance, and the demolition and disposal stage are difficult to calculate; we roughly assumed that these stages required about 30% of the embodied CO₂ emissions of the window system [31,35].

5.2. CO₂ Emissions during Operation

The end-use energy consumption of the residential unit is calculated by using the evaluated heating and cooling loads. Subsequently, CO₂ emissions during the operation stage were calculated. Heating and cooling loads were converted to end-use energy using the efficiency of the space conditioning equipment. The residential unit was assumed to use a gas boiler as the heating device and a PAC system as the cooling device. Natural gas was assumed to be the heat source for the boiler and electricity was assumed to be the energy source for the PAC system. End-use energy consumption was obtained as follows:

$$\text{Natural gas consumption }({\mathrm{m}}^3)= \frac{\text{Heating load }(\text{kWh})/\text{Boiler efficiency}}{\text{Lower calorific value of natural gas}}$$

(2)

$$\text{Electricity consumption for cooling }\left(\text{kWh}\right)= \frac{\text{Cooling load }\left(\text{kWh}\right)}{\text{Coefficient of performance}}$$

(3)

Key input values for calculating the end-use energy consumption are presented in Table 8.

Table 8. Values for end-use energy calculation.

Factor	Input Value
Boiler efficiency	85%
Coefficient of performance of the PAC system	2.7
Low calorific value of natural gas	11.1 kWh/N·m3

Table 8. Values for end-use energy calculation.

Factor	Input Value
Boiler efficiency	85%
Coefficient of performance of the PAC system	2.7
Low calorific value of natural gas	11.1 kWh/N·m3

In addition to the heating and cooling energy, energy consumption for heating domestic hot water and electricity for plug load were estimated. The base energy consumption was calculated as follows:

$$\text{Heat energy for hot water}\left(\text{MJ}\right)=14,504$$

(4)

$$\text{Plug load }\left(\text{MJ}\right)=98.7\bullet A+5965$$

(5)

where A is the floor area of the residential unit (m²).

The Korean government uses this equation to evaluate the performance of low-energy housing [36]. These equations express the typical energy consumption of the baseline model, conventional residential buildings. The total floor area of the residential unit of the target building was 119 m².

The CO₂ emissions factor is multiplied with the end-use energy consumption to obtain the CO₂ emissions during operation stage. CO₂ emissions factors for each energy source, natural gas and electricity, are shown in Table 9 [37].

Table 9. CO₂ emissions factors by energy source.

Energy Source	Emissions Rate
Natural gas	0.00259 t CO2/N·m3
Electricity	0.469 t CO2/MWh

Table 9. CO₂ emissions factors by energy source.

Energy Source	Emissions Rate
Natural gas	0.00259 t CO2/N·m3
Electricity	0.469 t CO2/MWh

The calculated annual end-use energy consumption for each case for the residential unit is shown in Table 10. In addition, estimated yearly CO₂ emissions are depicted in Figure 10 and Figure 11. Annual CO₂ emissions from heating and cooling energy were reduced by 26.9% under minimum ventilation conditions and 27.7% under natural ventilation condition. Reductions in the heating load contributed much more than reductions in the cooling load to annual CO₂ emissions reductions due to implementation of the DWCS. When the hot water and plug load were considered, annual operational CO₂ emissions were reduced by 15% under both minimum ventilation conditions and natural ventilation conditions.

Table 10. Annual end-use energy consumption.

Case	Window Type	Ventilation Mode	Natural Gas (m3)	Electricity (kWh)
Case 1	SW	Minimum	1756	6218
Case 2	DWCS	Minimum	1451	5513
Case 3	SW	Natural	1756	5715
Case 4	DWCS	Natural	1451	5076

Table 10. Annual end-use energy consumption.

Case	Window Type	Ventilation Mode	Natural Gas (m3)	Electricity (kWh)
Case 1	SW	Minimum	1756	6218
Case 2	DWCS	Minimum	1451	5513
Case 3	SW	Natural	1756	5715
Case 4	DWCS	Natural	1451	5076

Figure 10. Reduction in annual CO₂ emissions for space conditioning.

Figure 10. Reduction in annual CO₂ emissions for space conditioning.

Figure 11. Reduction in annual CO₂ emissions for overall operation.

Figure 11. Reduction in annual CO₂ emissions for overall operation.

5.3. Evaluation of Embodied CO₂ Emissions

In this study, the embodied CO₂ emissions of the aluminum frame, glass, and thermal break were included, as these make up nearly the entire volume of the window systems. Sealants and other components were considered accessory materials.

Embodied CO₂ emissions were estimated considering the amounts of the materials required to manufacture the SW and DWCS, as follows:

$${\text{Embodied CO}}_2\text{ emissions}=\text{Volume}\times \text{Density}\times {\text{CO}}_2\text{ emissions factor}$$

(6)

Embodied CO₂ emissions per material unit area or weight are shown in Table 11 [38]. We assumed that all of the materials were manufactured in Korea. Table 12 shows the density of the aluminum used for frame manufacturing.

Table 11. Embodied CO₂ emissions factor.

Material	Unit	CO2 Emissions (domestic material) (kg CO2/unit)
Glass	m2	26.3690
Metal window frame	kg	17.3816
Polyamide (thermal break)	kg	751.5498

Table 11. Embodied CO₂ emissions factor.

Material	Unit	CO2 Emissions (domestic material) (kg CO2/unit)
Glass	m2	26.3690
Metal window frame	kg	17.3816
Polyamide (thermal break)	kg	751.5498

Table 12. Material properties.

Material	Density
Aluminum	2.7 g/cm3
Polyamide	1.1 g/cm3

Table 12. Material properties.

Material	Density
Aluminum	2.7 g/cm3
Polyamide	1.1 g/cm3

As noted above, CO₂ emissions during the maintenance and disposal stage and other stages were added to the embodied CO₂ emissions [31,35]. Detailed percentages are as follows relative to CO₂ emissions during the material production process: accessory materials (2%), transportation and installation (5.5%), maintenance (10%), and demolition and disposal (12.5%).

The amounts of the embodied CO₂ emissions for the residential windows are shown in Table 13. The aluminum frame accounted for the majority of the embodied CO₂ emissions. The total embodied CO₂ emissions of the SW and DWCS were 12.4 and 21.1 t CO₂, respectively, relatively large values. For SWs, the embodied CO₂ emissions were about 1.64–1.70 times the annual CO₂ emissions from operations, while the DWCS embodied CO₂ emissions were 3.27–3.38 times larger than those of annual operations. Based on these results, use of an aluminum frame in the DWCS may not be ideal to achieve LCCO₂ reductions.

Table 13. Total embodied CO₂ emissions.

CO2 Emissions (domestic material) t CO2	SW	DWCS
Frame	8.3	13.6
Glass	1.2	2.4
Polyamide	0.06	0.24
Other	2.87	4.80
Total	12.4	21.1

Table 13. Total embodied CO₂ emissions.

CO2 Emissions (domestic material) t CO2	SW	DWCS
Frame	8.3	13.6
Glass	1.2	2.4
Polyamide	0.06	0.24
Other	2.87	4.80
Total	12.4	21.1

5.4. Assessment Results

To calculate the LCCO₂, we assumed that the life spans of residential buildings and window systems are 40 and 20 year, respectively. Lots of Korean local governments set the life span of residential building to 40 year [39]. Korean building code suggests 10 year and 25 year as the life spans of aluminum window systems for partial and full renovation, respectively [40]. In this evaluation life span of the window systems is assumed to 20 year, half of the building life span.

The assessment results are shown in Table 14 and Table 15. In minimum ventilation mode, implementation of the DWCS reduced CO₂ emissions by 27.4 t CO₂ over the lifecycle of the residential unit. In natural ventilation mode, about 26.2 t CO₂ emissions were reduced.

Table 14. Calculated LCCO₂ emissions.

Window	Ventilation Mode	LCCO2 Emissions (t CO2)
		Operating	Embodied
SW	Minimum ventilation	298.8	24.8
DWCS		254.0	42.2
SW	Natural	289.2	24.8
DWCS	Ventilation	245.6	42.2

Table 14. Calculated LCCO₂ emissions.

Window	Ventilation Mode	LCCO2 Emissions (t CO2)
		Operating	Embodied
SW	Minimum ventilation	298.8	24.8
DWCS		254.0	42.2
SW	Natural	289.2	24.8
DWCS	Ventilation	245.6	42.2

Table 15. Calculated LCCO₂ reduction.

Window	Ventilation Mode	LCCO2 Emissions (t CO2)	Total Reduction (t CO2)
SW	Minimum ventilation	323.6	-
DWCS		296.2	27.4
SW	Natural ventilation	314.0	-
DWCS		287.8	26.2

Table 15. Calculated LCCO₂ reduction.

Window	Ventilation Mode	LCCO2 Emissions (t CO2)	Total Reduction (t CO2)
SW	Minimum ventilation	323.6	-
DWCS		296.2	27.4
SW	Natural ventilation	314.0	-
DWCS		287.8	26.2

6. Conclusions

In this study, we calculated the reduction in LCCO₂ emissions due to implementation of DWCS instead of SWs in a residential building in Korea. The results can be summarized as follows:

(1)

A total of 26.2–27.4 t CO₂ was reduced by implementation of the DWCS rather than SWs in the residential unit in Korea.

(2)

Most of the reduction in LCCO₂ emissions resulted from reduced heating energy consumption. Reductions in cooling energy were comparatively small for the residential unit.

(3)

The aluminum frame incorporated very high embodied CO₂ emissions and reduced the LCCO₂ benefits of the DWCS system.

Based on these results, implementation of the DWCS can reduce LCCO₂ emissions. However, the CO₂ emissions benefits are substantially reduced by the high embodied CO₂ emissions of the larger aluminum frame of the DWCS. Optimization of the frame material and geometry are suggested to further reduce LCCO₂ emissions.