Research on the suitability technologies of existing public buildings in hot summer and cold winter zones based on energy consumption and economic coupling optimization

Abstract

In the era of energy conservation and carbon reduction, simulations and analyses were carried out on three typical building types—office buildings, hotel buildings, and commercial buildings—in hot summer and cold winter zones, China, using Ladybug tools based on the Grasshopper platform. The energy consumption benefits and static payback periods for different retrofit technologies were compared. The results show that retrofitting a domestic hot water system is highly suitable for hotel buildings. Retrofitting the lift system is highly suitable for commercial buildings. A comprehensive evaluation of improving the thermal performance of the transparent building envelope, replacing lighting fixture, and improving the efficiency of chiller units shows good suitability for all three building types.

1 Introduction

It has become a global consensus to curb the rapid rise in global temperatures, reduce carbon emissions in human society, peak carbon dioxide emissions, and achieve carbon neutrality as soon as possible. Out of the responsibility of a great nation for the future destiny of all mankind, General Secretary Xi Jinping announced at the 75th United Nations General Assembly on 22 September 2020 that China’s carbon dioxide emissions will peak by 2030 and achieve carbon neutrality by 2060 [1]. According to relevant statistics, the energy consumption of buildings accounts for about 40% of the total energy consumption of society [2]. The proportion is expected to increase steadily for many years to come as industrialization and urbanization continue [3]. And the energy consumption of China’s public buildings is relatively high [4], more than twice the energy intensity of the average urban residential building energy consumption, which is particularly prominent in commercial buildings and hotel buildings [5]. Over the past two decades, the stock of public buildings in China has increased rapidly from about 3.8 billion square meters in 2001 to about 14 billion square meters in 2020 [6], so how to effectively carry out energy-saving and carbon-reducing renovation of existing public buildings has become an important social issue.

Scholars around the world have carried out relevant research and practical work on energy-saving and carbon-reducing retrofitting of existing public buildings. Zhang [7] summarized the appropriate technology and application characteristics of energy-saving transformation of public buildings in hot summer and cold winter zones through the experience of Shanghai’s energy-saving transformation; Song and Chang [8] focused on the Qingdao Shooting Sports Centre project, with the specific technical scheme and energy consumption calculation adopted in green construction and energy saving. Chow et al. [9] used simulation and case studies to explore their energy-saving effects after different retrofit measures, in order to understand the performance of public buildings in hot summer and cold winter zones in the face of climate change. Zhou et al. [10] used a case study approach to investigate effective methods for energy efficiency retrofitting of buildings based on 1 year of testing.

However, the above studies lack economic considerations of appropriate technologies for existing public buildings.

Wan et al. [11] conducted an energy-saving and economic analysis of lighting energy-saving technology, frequency conversion technology, and energy consumption submetering control in a public building. The objective was to derive the most appropriate energy-saving retrofit technology for hot summer and cold winter zones. Astiaso Garcia et al. [12] provided preliminary information about economic costs and energy benefits of some considered interventions in existing public buildings by analysing some feasible interventions in four selected public buildings. Zheng and Lai [13] provided empirical evidence of economic or environmental effectiveness of ESMs (implementing energy-saving measures); this study illustrates a rigorous, pragmatic approach to evaluating retrofit projects in real-world buildings. Wan et al. [2] applied some energy-saving measures in a typical office building based on empirical data from the Beijing area and compared the energy consumption, energy-saving ratio, and lifecycle costs through simulation to determine the best measures.

The aforementioned study did not quantify the suitability of each suitable technology to form a complete system of suitable technologies for energy efficiency improvement in public buildings.

This paper proposes different technologies for the suitability of public buildings in hot summer and cold winter zones, China. Taking a typical office building, a hotel building, and a commercial building in Zhejiang Province as examples, analyses and simulations were carried out using Ladybug tools based on the Grasshopper platform. The energy consumption benefits and static payback periods of each technology were calculated. A comprehensive TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) evaluation model, combining energy consumption and economic optimization, was proposed to evaluate the degree of recommendation of each technology. A complete technical system was thus created.

2 Methods

2.1 Benchmark model construction

The construction method of the benchmark model in study is not fixed; different methods are usually selected according to different research needs. Some scholars [14] directly choose examples as the benchmark model, while others [15] use statistical methods to generate the benchmark model. In order to ensure greater applicability and regional representation, the study employs the use of typical cases of office building, hotel building, and commercial building in Zhejiang Province as the benchmark model.

2.1.1 Office building

The benchmark model of the office building, shown in Fig. 1, consists of a central core with ancillary spaces. The office spaces are arranged symmetrically. The building has a total area of 10118.32 m² and consists of 18 floors with a standard floor height of 3.6 m. To simplify the calculations, the first, top, and standard floors of the office building were selected for modelling and analysis. In terms of hot zone division, the office spaces on the east and west sides and the auxiliary spaces of the core were delineated. Only the evaluation indicators of the standard floors were considered for the calculation.

2.1.2 Hotel building

The benchmark model of the hotel building is shown in Fig. 2, with an interior corridor layout and rooms arranged on either side of the corridor. The building has a total area of 17 941 m² and consists of 15 floors with a standard floor height of 3.6 m. To simplify the calculations, the first, top, and standard floors of the hotel building were selected for modelling and analysis. In terms of hot zone division, the spaces were divided into guest rooms, bathrooms, and corridors. Only the evaluation indicators of the standard floors were considered for the calculation.

2.1.3 Commercial building

The benchmark model of the commercial building, shown in Fig. 3, has an overall layout similar to Chinese character ‘Tian’, with four small atriums distributed in the centre. It consists of two underground floors and five above-ground floors, with a standard floor height of 5.5 m. In the construction of the commercial building model, the thermal performance of different standard floors cannot be treated equally due to the influence of air flow in the atriums. Therefore, a simplified model of the whole building was created. In terms of hot zone division, it was mainly divided into retail space usage areas, auxiliary spaces, and atrium spaces, with evaluation indicators considered for the standard floors.

2.2 Boundary condition setting

2.2.1 Building climate conditions

The weather data of Hangzhou is used as the simulation climate basis for the benchmark model of a typical building in the hot summer and cold winter zones, China, and the surrounding environment of the building used in the simulation is selected to be unobstructed under ideal conditions. Fig. 4 shows the annual temperature and humidity distribution in the region.

2.2.2 Simulation parameter setting

In the energy consumption simulation model, the cooling period was set from 15 June to 15 September, while the heating period was set from 15 December to 20 February of the following year. The schedules for heating and cooling, personnel activities, lighting, and equipment operation were configured in accordance with the Design standard for energy efficiency of public buildings (DB 33/1036–2021). The thermal performance parameters of the buildings are presented in Tables 1–4.

Using Ladybug tools, the annual energy consumption for HVAC (Heating, Ventilation, and Air Conditioning) in the office building was simulated to be 25.7 kWh/m². In contrast, calculations performed using PKPM software under identical conditions gave an annual energy consumption for HVAC of 25.93 kWh/m². This resulted in a relative error of 0.89%, indicating that the accuracy of the calculations is reasonably acceptable.

2.3 Evaluation system construction

2.3.1 Annual energy benefits of buildings per unit area

The total annual energy consumption of a building is a comprehensive evaluation of its energy performance, and optimizing it holds the most direct and practical significance. According to the Design standard for energy efficiency of public buildings (GB50189), the total annual energy consumption of a building includes the annual energy consumption of the heating system, cooling system, lighting system, domestic hot water system, lift system, electrical outlets, and cooking. Among these, the energy consumption of electrical outlets and cooking is primarily related to user behaviour due to their specific usage characteristics and is not considered within the scope of suitability technologies. Therefore, in this study, the calculation formula for the total annual energy consumption of a building is shown in Eq. (1):

where

E—Annual comprehensive energy consumption of the building, kWh.

E_heating—Annual heating energy consumption of the building, kWh.

E_cooling—Annual cooling energy consumption of the building, kWh.

E_lighting—Annual lighting energy consumption of the building, kWh.

E_water—Annual domestic hot water system energy consumption of building, kWh.

E_elevater—Annual lift system energy consumption of building, kWh.

The annual energy consumption of heating systems, cooling systems, and lighting systems is directly related to the building’s performance and is obtained through dynamic simulation using software. However, the annual energy consumption of domestic hot water systems and lift systems is primarily related to equipment performance and building demand and is obtained through formula calculation. The calculation formula for annual domestic hot water system energy consumption is shown in Eq. (2) [16]:

where

ρ—Density of hot water, take 0.983·10³ kg/m³.

i—Effective calculation days, based on 260 working days for office buildings and 360 days for hotel and commercial buildings.

c—Specific heat capacity of water, take 4.2 kJ/(kg °C).

n—Number of people using the building, calculated on the basis of the area occupied per capita.

v—Daily water consumption per capita，L/person，according to the Standard for design of building water supply and drainage (GB 50015–2019), office building take 6 L/person, hotel building take 40 L/person, and commercial buildings take 4 L/person.

t_r—Average temperature of hot water, take 60°C according to GB 50015-2019.

t_l—Average temperature of cold water, take 15°C according to GB 50015-2019.

C_γ—Heat loss coefficient of hot water supply, take 1.10~1.15 according to GB 50015-2019.

ε—Thermal conversion efficiency of water heaters, take 0.7 according to baseline thermal efficiency for centralized hot water supply systems.

The formula to calculate the annual lift system energy consumption [17] is shown in Eq. (3):

where

E_elevater—Annual lift system energy consumption, kWh.

n—Number of lifts, 3 for office buildings, 5 for hotel buildings, and 21 for commercial buildings.

P—Specific energy consumption, mWh/(kg m), according to the Energy performance of lifts, escalators and moving walks—Part 2: Energy calculation and classification for lifts (elevators) (GB/T30559.2-2017), performance class 4 for traction lifts, take 2.43 mWh/(kg m).

t_a—Average annual operating hours of lifts, h; the running time details of office building, hotel building, and commercial building are 1229.84, 1560.30, and 3890 h, respectively.

V—Lift speed，m/s，take 2.50 m/s.

W—Rated capacity of lift, kg, take 1500 kg.

E_standby—Energy consumption in lift standby, W, take 400 W.

t_s—Average annual standby hours for lift, h, office building, hotel building, and commercial building are 7530.16 h, 7199.7 h, and 4870 h, respectively.

The calculation of energy benefits per unit area for different suitability technologies is shown in Eq. (4):

where

C—Annual energy consumption efficiency per unit area of the building, kWh/m².

E_E—Combined value of energy consumption in retrofitted building, kWh.

E_R—Combined value of energy consumption of the benchmark building, kWh.

A—Calculated area of the benchmark building, m².

2.3.2 Static payback period

The static payback period is a measure of the time required to recover the initial cost of a project, calculated based on the project’s annual income without consideration of factors such as the time value of money. The project income is derived from the implementation of suitability technologies, which result in the annual energy consumption per unit area of the optimized building being lower than that of the initial building. This reduction in energy consumption leads to savings in funds associated with reduced electricity consumption. The calculation formula is presented in Eq. (5):

where

P_t—Static payback period, year.

C₀—Initial cost, RMB.

I₀—Annual financial savings after optimization, RMB.

U—Unit cost per retrofit measure, RMB/unit.

Q—Amount of retrofit work per retrofit measure.

E_before—Annual electricity consumption before building renovation, kWh.

E_after—Annual electricity consumption after building renovation, kWh.

P—Electricity price, RMB, in this study electricity price is calculated according to the commercial electricity consumption in Hangzhou City of 0.6964 RMB/kWh.

2.3.3 TOPSIS comprehensive evaluation method

The TOPSIS comprehensive evaluation method, also known as the approximate ideal solution ranking method, derives the closeness of the evaluation units to the optimal solution by determining the positive and negative ideal values of the different evaluation units [18], which can be a good solution to the decision-making problem of multi-attribute objectives, and its calculation formula [18] is as follows:

where

Q_m, |$Q^\ast_m$|—The m-th evaluation solution and its standardized result after forward normalization of each objective function respectively.

|$D^+_m, D^-_m$|—The distance between the m-th evaluation solution and the ideal best and worst solutions, respectively.

|$Q^\ast_{1,n}, Q^\ast_{2,n}$|—The n evaluation solutions for objective functions 1 and 2.

|$Q^\ast_{1,m}, Q^\ast_{2,m}$|—The m-th evaluation solution for objective functions 1 and 2.

|$R^\ast_{m}$|—The closeness coefficient of the m-th evaluation solution.

3 Results

3.1 Results of suitability technologies selecting

The suitability technologies for public buildings mainly involves the renovation of building envelope, lighting systems, HVAC systems, water supply and drainage systems, power systems, and detection and control system [19].

In general, most of the energy consumed in buildings is used for HVAC [20]. It is therefore important to improve the performance of the HVAC system for better operation. Fine and Touchie [21] found that retrofitting HVAC systems alone can reduce the energy required for space heating in residential buildings in Canada by 14%. According to Al-Kodmany [22], most of the existing buildings do not have energy efficient lighting systems, and upgrading the lighting system with energy efficient fixtures will provide significant energy savings of around 70%. Sun et al. [23] found that light emitting diode (LED) lighting is the most feasible and economical lighting retrofit option [24].

Moreover, in hot summer and cold winter zones, China, such as those influenced by geographical latitude and precipitation, summers tend to be humid [25], resulting in a peak in building energy consumption throughout the year, with a significant proportion attributed to air conditioning for cooling [26]. Consequently, enhancing the shading and insulation performance of the building envelope represents a pivotal measure for energy-saving renovations in such regions. In order to identify the most effective means of reducing energy consumption in public buildings, it is essential to conduct research into the suitability of various technologies for use in the building envelope, shading systems, lighting systems, HVAC systems, water supply and drainage systems, as well as power systems. The suitability technologies of different subsystems in public buildings cover a wide range, and in order to make the study more applicable and representative, the most common and efficient technologies are selected for the setup.

The basic suitability technologies are proposed as well as the specific parameter setting values in the simulation software, as shown in Table 5.

3.2 Results of suitability technologies evaluation

The results of the simulations indicate that, under the baseline settings, the annual energy consumption intensity of office buildings is 54.90 kWh/m², with a total annual cumulative electricity consumption of 32820.24 kWh. For hotel buildings, the annual energy consumption intensity is 248.16 kWh/m², with a total annual cumulative electricity consumption of 274324.24 kWh. With regard to commercial buildings, the annual energy consumption intensity is 161.41 kWh/m², with a total annual cumulative electricity consumption of 1 650 583.93 kWh. Following the implementation of suitability technologies, the energy consumption of each building is recalculated through simulation. The discrepancy between the calculated total energy consumption prior to and subsequent to the renovation represents the energy benefits of the suitability technologies. By considering the actual electricity prices during the operation of each building and the initial investment for renovation, the static payback period of the suitability technology can be obtained. The results of the evaluation for each type of building are presented in Tables 6–8.

4 Discussion

4.1 Analysis of office building

As shown in Fig. 5, the bar chart analyses the suitability of different technologies for office buildings. It can be seen that tech-6 and tech-7 significantly outperform the others in terms of annual energy benefits and static payback period, with payback periods within 5 years. Specifically, these technologies involve improving the efficiency of chiller units and retrofitting domestic hot water system.

As shown in Fig. 6, the bar chart presents a comprehensive analysis of the suitability technologies for office buildings in terms of energy benefits and payback period. The indicator central axis scatter plot shows that the evaluation results of all suitability technologies are distributed in three quadrants. The fourth quadrant shows the best evaluation results, with high energy benefits and short payback periods, specifically for the technology involving the improvement of the efficiency of chiller units. The scores in the first and third quadrants are secondary, with either long payback periods with high energy benefits or short payback periods with lower energy benefits. According to the TOPSIS scores, the technologies can be ranked as follows: (1) retrofitting lift system; (2) retrofitting domestic hot water system; (3) replacing lighting fixture; (4) enhancing the thermal insulation of roof; (5) enhancing the thermal insulation of external wall; (6) improving the thermal performance of the transparent building envelope; and (7) adding shading devices to doors and windows.

4.2 Analysis of hotel building

As shown in Fig. 7, the bar chart presents an analysis of the suitability technologies for hotel buildings. It can be seen that tech-6 and tech-7 outperform the other technologies both in terms of energy benefits and static payback period, with payback periods within 5 years. Specifically, these technologies involve improving the efficiency of chiller units and retrofitting domestic hot water system.

As shown in Fig. 8, the bar chart provides a comprehensive analysis of the suitability of technologies for hotel buildings in terms of energy benefits and payback period. The indicator central axis scatter plot shows that the evaluation results of all suitability technologies are distributed in three quadrants. The fourth quadrant shows the best evaluation results, with high energy benefits and short payback periods. In particular, the technologies improving the efficiency of chiller units and retrofitting domestic hot water system rank highest in this quadrant. This is followed by scores in the first and third quadrants, which are characterized by either long payback periods with high energy benefits or short payback periods with lower energy benefits. According to the TOPSIS scores, the technologies can be ranked as follows: (1) retrofitting lift system; (2) enhancing the thermal insulation of roof; (3) replacing lighting fixture; (4) improving the thermal performance of the transparent building envelope; (5) enhancing the thermal insulation of external wall; and (6) adding shading devices to doors and windows.

4.3 Analysis of commercial building

As shown in Fig. 9, the bar chart illustrates the suitability analysis of technologies for commercial buildings. It is clear that tech-1, tech-6, and tech-8 outperform other technologies in terms of both energy benefits and static payback period, with tech-1 and tech-6 achieving payback periods within 5 years. Specifically, these technologies involve improving the thermal performance of the transparent building envelope, improving the efficiency of chiller units, and retrofitting lift system.

As shown in Fig. 10, the bar chart provides a comprehensive analysis of the suitability of technologies for commercial buildings in terms of energy benefits and payback period. The indicator central axis scatter plot shows that the evaluation results of all suitability technologies are distributed in three quadrants. The fourth quadrant shows the best evaluation results, with high energy benefits and short payback periods. In particular, the technologies of improving the efficiency of chiller units and retrofitting lift system rank highest in this quadrant. This is followed by scores in the third quadrant, which are characterized by either long payback periods with high energy benefits or short payback periods with lower energy benefits. According to the TOPSIS scores, the technologies can be ranked as follows: (1) improving the thermal performance of the transparent building envelope; (2) replacing lighting fixture; (3) retrofitting domestic hot water system; (4) adding shading devices to doors and windows. In the second quadrant, the assessment results are the worst, with low energy benefits and long payback periods, particularly for the technology involving enhancing the thermal insulation of roof.

4.4 Summary of technologies

In summary, by conducting comparative analyses of the suitability of different technologies for different building types, and considering the positions of the retrofit results on the indicator central axis scatter plot and the TOPSIS scores, the technologies are classified as highly recommended, moderately recommended, and generally recommended. The summarized results are shown in Table 9. It should be noted that this study is based on simulations and comparative analyses of specific cases, and the conclusions obtained have certain reference value. Different technologies vary in form, degree, and target, all of which can affect the results. In this study, it was found that the total heating load of commercial buildings decreases significantly after the implementation of enhancing the thermal insulation of external walls. However, the total cooling load increases significantly, primarily due to the building’s unique design, which includes large glass windows and expansive atriums with glass roofs. These features cause the building to absorb a substantial amount of solar radiation heat in summer, and the addition of thermal insulation reduces the overall heat dissipation capacity, leading to increased cooling energy consumption. Therefore, in hot summer and cold winter zones, such measures can be generally recommended when the building experiences low solar radiation heat gain. The comprehensive evaluation of improving the efficiency of chiller units is greatly affected by fluctuations in the coefficient of performance (COP) settings and unit pricing, making it a comparatively recommended measure.

5 Conclusion

In this study, typical buildings such as offices, hotels, and commercial buildings in hot summers and cold winters, China, are examined. Simulations were conducted using Ladybug tools to evaluate the suitability of various technologies with a focus on energy benefits and payback periods. TOPSIS theory was employed to couple the evaluation indicators. Based on the evaluation results, the proposed common suitability technologies were classified and rated. These technologies primarily include improving the thermal performance of the transparent building envelope, enhancing the thermal insulation of external wall, enhancing the thermal insulation of roof, adding shading devices to doors and windows, replacing lighting fixtures, improving the efficiency of chiller units, retrofitting domestic hot water system, and retrofitting lift system.

In office buildings, improving the thermal performance of the transparent building envelope, enhancing the thermal insulation of external wall, enhancing the thermal insulation of roof, replacing lighting fixtures, improving the efficiency of chiller units, retrofitting domestic hot water systems, and retrofitting the lift system are moderately recommended technologies. Adding shading devices to doors and windows is a generally recommended technology.

In hotel buildings, retrofitting the domestic hot water system is a highly recommended technology, with annual energy benefits of 11.17 kWh/m² and a static payback periods of 2.57 years. Improving the thermal performance of the transparent building envelope, enhancing the thermal insulation of external wall, enhancing the thermal insulation of roof, replacing lighting fixture, improving the efficiency of chiller units, and retrofitting lift systems are moderately recommended technologies. Adding shading devices to doors and windows is a generally recommended technology.

In commercial buildings, retrofitting the lift system is a highly recommended technology, with annual energy benefits of 37.48 kWh/m² and a static payback periods of 12.59 years. Improving the thermal performance of the transparent building envelope, adding shading devices to doors and windows, replacing lighting fixture, improving the efficiency of chiller units, and retrofitting domestic hot water system are considered moderately recommended technologies. Enhancing the thermal insulation of external wall and enhancing the thermal insulation of roof are considered a generally recommended technology.

Overall, for hotel buildings, priority should be given to retrofitting the domestic hot water system, while for commercial buildings, priority should be given to retrofitting the lift system. The technologies of improving the thermal performance of the transparent building envelope, replacing lighting fixtures, and improving the efficiency of chiller units exhibit good suitability across all three types of buildings. Moreover, significant differences in suitability are observed among the three types of public buildings regarding the technologies of enhancing the thermal insulation of external wall, enhancing the thermal insulation of roof, and adding shading devices to doors and windows.

Author contributions

Li-xia Liang (Conceptualization [equal], Resources [equal], Supervision [equal], Writing—review & editing [equal]), Ze-zhi Jiang (Data curation [equal], Software [equal], Visualization [equal], Writing—original draft [equal], Writing—review & editing [equal]), Jian-kui Wang (Formal analysis [equal], Funding acquisition [equal], Validation [equal]), Lin Lu (Investigation [equal], Methodology [equal], Project administration [equal]).

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article. This work was supported by the Zhejiang Province ‘spearhead’ ‘bell-wether’ research and development project ‘The key technology and equipment development for low-carbon buildings’ (Grant No.: 2023C03153)， Research on the Support System and Evaluation Methods for the Renovation and Renewal of Existing Public Buildings (Grant No.: ZJZX-20230610) and Research on the Implementation Paths for Energy Conservation and Carbon Reduction in Existing Buildings (Grant No.: ZJZX-20240602-03).

References