2.1

Scenario setting based on progressive energy efficiency targets

Table 1 begins with a review of existing mandatory and voluntary building codes in the HSCW region, along with an examination of international low-energy building standards. Based on the national low-carbon development strategy and relevant standards, the study proposes three scenarios for retrofitting existing residential envelopes based on progressive energy efficiency targets. These scenarios cover the retrofitting wants with different energy saving objectives.

Table 1.

Existing mandatory and voluntary building codes in the HSCW zone.

Note: U: heat transfer coefficient(W/(m2·K)); N50: Indoor and outdoor pressure difference of 50 Pa

Scenario 1: Retrofitting aims for a 50% energy-saving target based on the current mandatory standard in the HSCW zone, JGJ 134-2010. The building type targeted was prevalent in central Chinese cities from the 1980s to the early 1990s, with basic thermal insulation measures in its envelope. Scenario 2: Retrofitting targets a 65% energy saving based on China’s 2015 Green Retrofitting Standard. These standard mandates a 35% improvement in building envelope thermal performance. Guangxi’s updated Residential Building Energy Efficiency Design Standard in 2022 aligns with this, prompting an increase in retrofit targets. Scenario 3: Retrofitting aims for a 75% energy saving target according to China’s Ultra Low Energy Building (ULEB) standards. The 2019 national standard, “Technical Standard for Near Zero Energy Buildings” (GB/T51350-2019), emphasizes renewable energy use and efficient systems, with specific envelope performance requirements.

2.2

Case study

This retrofit project is in an old neighborhood in Guilin City, chosen for its typical characteristics like high energy consumption. The building, constructed in 1990, is a multi-storey residential building with six floors. Each floor has six units with standard layouts. It relies on self-contained air conditioning and manual window opening for ventilation. Energy consumption data from sixth-floor flats were monitored in 2022 to calibrate the simulation model.

2.3

Simulation

A simulation model of the building was created using DesignBuilder software based on original CAD drawings and field data. Each floor was divided into six flats with standard layouts. We excluded decorative facade structures, internal architectural details, and outdoor features as they minimally affect indoor thermal conditions. The analysis focused on retrofitting the building envelope and HVAC system. Components of the envelope were derived from field data and CAD drawings. Thermal properties of external walls, roof, and windows were calculated using software. Table 2 show the details of the area parameters of the façade, roof and windows as well as the calculated thermal performance parameters for the case building and the “reference building”.

Table 2.

Thermal performance parameters of the envelope of the case building and the reference building (W/(m2·K).

To assess modeling accuracy, ASHRAE Standard 14-2014 is widely accepted, with the mean square error (MBE) and the coefficient of variation of the root mean square error (CVRMSE) being the two commonly used metrics assessed⁸, with equations (1) and (2). The deviation of the simulated temperatures from the monitored data was checked by the mean bias error (MBE) and root mean square error (RMSE), which are given in ASHRAE Standard 14 as -10% ≤ MBE_hourly ≤ 10% and CV(RMSE)_hourly ≤ 30% for hourly data, and -5% ≤ MBE_monthly ≤ 5% and CV(RMSE)_monthly ≤ 5%. 5% and CV(RMSE)_monthly≤15%.

2.4

Key performance assessment

2.4.1

Environmental indicators (carbon emissions).

This study evaluates environmental impact by considering both greenhouse gas emissions saved by the retrofit program and the embodied carbon of materials used in passive technologies. To quantify these benefits, total CO₂-equivalent emissions saved are assessed, allowing for easier comparison and management of greenhouse gas emissions. Specific and operational emissions are calculated using carbon emission factors from building standards and literature, particularly following the “China Building Carbon Emission Calculation Standard” (GB/T 51366-2019)⁹. Major building materials and energy sources have specific carbon emission coefficients, as shown in Table 3. Total carbon emissions before and after retrofit (TCE), implied energy of materials (CE_mcp), emissions during the operational phase (CE_op), and carbon emission reduction potential (CERP) are calculated according to equations (3)-(6) .

Table 3.

CE factors for major building materials and energy sources.

Category	Source	CE factor	Unit
Material	Concrete and steel	Concrete	554	kg CO2/m3
External wall material	Clay Brick	336	kg CO2/m3
Cement	221	kg CO2/m3
Paint	1.50	kg CO2/t
Glazed window	Single glazed windows	202	kg CO2/m2
Double glazed windows	245	kg CO2/m2
Triple glazed windows	308	kg CO2/m2
XPS	5220	kg CO2/t
Interior flooring and walls	Lightweight aggregate concrete	146	kg CO2/m3
Ceramic tile	950	kg CO2/t
Energy	Energy emission factor	Electricity	0.94	kgCO2/kWh

where TCE is the total carbon emissions, kgCO₂eq/m². CE_mcp and CE_op denote the implied energy of the material and the emissions during the operational phase, respectively.

where n is the number of material types and M_i is the number of materials in category i; F_i is the carbon emission factor of materials in category i in Table 3 in kgCO₂/per unit.

where EO_i is the annual electricity and coal consumption during the operation phase in kWh/a. EF_i is the carbon emission factor of the energy source in kgCO₂/per unit as shown in Table 3.

where CERP is the Carbon Emission Reduction Potential, which includes carbon emissions during the operation phase and implied carbon emissions from materials; CE_B is the CE of the baseline building in kgCO₂eq/m²; and CE_D is the CE of the retrofitted building in kg CO₂eq/m².

2.4.2

Economic indicators (net present value).

Net Present Value (NPV)¹⁰ is a financial metric used to assess investment profitability by comparing cash flows with initial costs over time. A positive NPV indicates profitability, while a negative NPV suggests potential losses. It’s commonly used to evaluate investment opportunities and their associated risks. NPV considers costs and anticipated returns, while the Simple Payback Period (SPP) measures the time to recoup initial costs. A shorter SPP implies quicker returns. NPV and SPP are calculated using provided formulas in equations (7) and (8) .

where B is the incremental benefit from retrofitting and C is the investment cost of retrofitting. d is the social discount rate and t is the remaining service life of the building.

where C_i represents the total original investment and B_i represents the total energy-saving benefit.

The main factors considered for economic efficiency include retrofit measure cost, energy savings, measure life, discount rate, and energy price. Costs were determined through consultation with local contractors and MOHURD’s Energy Retrofit Guide. For example, the cost of XPS insulation is RMB 50/m², double glazing is RMB 120/m² and air tightness improvement is RMB 30-100/m². The building life is 50 years and the electricity price is RMB 0.68/kWh with a discount rate of 3%.

3.1

Simulation model validation

To verify the model’s accuracy, we collected the total monthly electricity bills for the 6-storey flat during field research. We then compared the monthly electricity consumption with the simulation results. The results of the comparative analysis are shown in Table 4. After calculation, we can conclude that the monthly energy consumption difference MBE value is 0.25%, which is less than the allowed minimum error of ±5%, and the CVRMSE value is 3.79%, which is also less than the defined baseline value of 15%, which are within the acceptable range of the ASHRAE’s Guidelines. Therefore, the above results indicate that the simulation results of the model are in good agreement with the measured results and the model is reliable.

Table 4.

Comparison of actual electricity consumption and simulation results.

3.2

Retrofit strategies based on progressive energy efficiency goals

For the 50% energy-saving target scenario, retrofit options were chosen based on HSCW district standards and relevant literature (Table 5). Measures included exterior wall and roof insulation, window improvement, and airtightness enhancement. Combining various options led to 270 schemes, with 136 meeting the target. These schemes achieved energy savings ranging from 44.56% to 55.06%, with heating energy consumption between 3471.57 and 3878.94 kWh and cooling energy consumption between 3070.56 and 4410.58 kWh. The most efficient scenarios featured 15mm XPS insulated external walls, 60mm XPS insulated roofs, double clear 6mm/6mm air external windows, and 0.6 ac/h airtightness.

Table 5.

Retrofit measures under the 50 % energy savings target scenario.

To achieve the 65% energy-saving target, we analyzed the envelope’s thermal performance using Guangxi’s Energy Saving Design Standard and the Green Building Standard. Several retrofit options were proposed, including exterior wall and roof insulation, window improvements, and airtightness enhancements (Table 6). These options were evaluated to upgrade thermal insulation and meet the target. Under Scenario 2, measures were labeled W4 to W13 for walls, R11 to R13 for roofs, G4 and G5 for windows, and A4 to A6 for airtightness. Of 180 scenarios, 70 achieved the 65% target, with energy savings from 57.18% to 70.44%. The most efficient schemes featured 65 mm XPS insulated walls, 75 mm XPS insulated roofs, double LoE (e2=.1) Tint 3 mm/6 mm Arg-type windows, and 0.1 ac/h airtightness.

Table 6.

Retrofit measures under the 65% energy savings target scenario.

For the 75% energy-saving target, the scenario is retrofitted to Ultra Low Energy Building (ULEB) standards, employing both passive and active techniques (Table 7). Passive strategies include exterior wall and roof insulation, window enhancements, and airtightness improvements. Active measures involve enhancing the heating and cooling system’s COP. Of 210 scenarios, 119 met the 75% energy-saving target. Energy savings ranged from 74.06% to 76.13%. The most efficient retrofit measures included thick XPS insulation for walls and roofs, double LoE (e2=0.1) Clear 3 mm/13 mm Arg windows, and achieving a 0.05 ac/h airtightness rating. Additionally, active measures included retrofitting heating and cooling systems for improved COP. Overall, these retrofits achieved significant energy savings.

Table 7.

Retrofit measures under the 75 % energy savings target scenario.