Case Study of CO₂ Cascade Air-Source Heat Pump in Public Building Renovation: Simulation, Field Measurement, and Performance Evaluation

Abstract

In cold climates, maintaining indoor comfort in winter requires heating systems to supply consistent and adequate heat at low ambient temperatures, making the proper definition of indoor and outdoor design temperatures critical for equipment selection. In this paper, a flexible parameter-adjustment design approach is proposed, combining on-site testing and simulation to refine heating load calculation, and a CO₂ cascade air-source heat pump (ASHP) renovation project for a cold-climate public building is used as a case study. The optimized approach ensured that the selected ASHP maintained indoor temperature above 20 °C, with the system achieving a heating season COP of 2.89. Even at −22.2 °C, it kept indoor temperature at 22.4 °C, with a COP of 2.70. This study confirms the effectiveness of the approach and offers a practical reference for similar projects.

1. Introduction

In 2023, the heating area in northern China reached 17.3 billion square meters, with annual heating operations generating 500 million tons of CO₂ commissions (29.0 kgCO₂/m²), accounting for 22% of total building operation carbon emissions [1]. Achieving carbon neutrality in building operations remains a challenge. Since 2017, the Central Finance has supported 88 pilot cities in northern China in implementing clean heating programs, and by 2023, the clean heating rate in the northern regions had reached 76% [2], with an increase of 32 percentage points [3]. While the supported pilot cities are mainly located in cold zones, the clean heating rate in severe cold zones still requires substantial improvement. Maintaining comfortable indoor conditions in severe cold zones throughout winter requires heating systems to provide consistent and adequate heat at low ambient temperatures.

Among clean heating technologies, heat pumps are essential for replacing fossil fuels and achieving carbon neutrality in the heating sector [4]. The National Development and Reform Commission has issued the Action Program to Promote High-Quality Development of the Heat Pump Industry, accelerating heat pump adoption in buildings and leading the research and development of advanced technologies [5]. Among the heat pumps installed in China, air-source heat pumps (ASHPs) account for over 90% due to their flexible application [6,7]. However, the decline in ASHP efficiency with decreasing ambient temperature has become a key technical barrier that must be addressed to further promote their adoption in severe cold zones [8]. To improve the acceptability of ASHPs in severe cold zones, several technological advancements have been made, such as cascade heat pump systems, subcooling technology, refrigerant intermediate replenishment technology, deforesting technology, and new refrigerants [8,9,10]. Notably, the CO₂ ASHP has emerged as a promising new technical route for space heating [11], not only due to its excellent heating performance at low ambient temperature and its ability to supply high-temperature hot water [12], but also because it aligns with the policy that China will restrict the production of heat pumps using refrigerants with a GWP over 750 starting in January 2029, in compliance with the Kigali Amendment [13].

As building heating loads and ASHP capacities exhibit opposing trends with fluctuations in outdoor temperature, how to select an appropriate ASHP that balances indoor thermal comfort with ASHP efficiency has become a critical research topic. Wang et al. [14] proposed a balanced temperature (approximately −3 °C to 3 °C) for ASHP selection, where the ASHP is sized to meet 50% to 60% of the building’s design heating load. On days when the outdoor temperature falls below this balanced temperature, the portion of the heating demand exceeding the ASHP’s capacity is supplemented by auxiliary heating equipment. Given the low frequency of extreme cold days, this configuration can largely reduce both investment and operational costs. However, the ASHP investigated in their study has an operation temperature above −12 °C, and this limitation makes the approach unsuitable for severe cold zones where outdoor temperatures can drop to −25 °C to −30 °C. Going further, Wang et al. [15] provided two correction methods for ASHP selection based on a study of 11 cities in cold and severe cold zones. The first method involves lowering the outdoor design temperature specified in the national standard GB 50736—2012 by 0 °C to 3.36 °C, and the second introduces a correction factor, ranging from 1.0 to 1.30, for heating loads calculated in accordance with GB 50736—2012. These correction methods are derived from data from 11 cities and may not be applicable to all northern Chinese cities, indicating the need for a more universal ASHP selection method that is adaptable to cold and severe cold zones.

In the current ASHP-based distributed central heating system project application, most still comply with the GB 50736—2012, in which the outdoor design temperature is derived from daily data with a given unassured rate. As Wang [16] notes, the unassured rate calculated using hourly temperature data is higher than the rate specified in the national standard, which relies on daily temperature data. Using hourly data to determine the outdoor design temperature could therefore yield more accurate heating load calculations for ASHP selection. Wu et al. [17,18] selected ASHPs based on hourly data and found that coupling multiple ASHPs with different nominal capacities increased the annual COP by 6%. Beyond accurately calculating heating loads for ASHP selection, it is also critical to determine the actual heating capacity of ASHPs at low ambient temperatures. Wang et al. [15] and Li et al. [19] proposed evaluation methods for ASHP performance that accounts for the combined effects of temperature and frosting. In addition to the outdoor design temperature, relevant factors related to heating load calculation and indoor comfort include indoor temperature setpoints and building envelope thermal properties, which are also discussed in this research.

To develop a more universal method for ASHP selection that could promote their application in severe cold zones and further improve the clean heating rate in northern China, in this paper, an optimal approach for revising heating load calculation parameters is put forward based on on-site hourly data, and application analysis is conducted using a public building retrofit project located near severe cold zones and equipped with a CO₂ cascade ASHP. The CO₂ cascade ASHP has been applied in several distributed central heating systems across northern China, and field analysis indicates that it operates with high efficiency throughout the heating season [20,21]. By applying the optimal design process, the selected CO₂ cascade ASHP can maintain more comfortable indoor conditions with high efficiency on severe cold days, providing a practical reference for ASHP selection in similar projects.

2. Project Description

The retrofitted public building is located in Chengde City, Hebei Province, China, which is at the conjunction of cold and severe cold zones, with typical heating degree days (HDD) of 3783 and cooling degree days (CDDs) of 20 [22]. The original HVAC system was built in 2009, with a water source heat pump (WSHP) system consisting of fan coil units at the end, providing heating and cooling for a floor area of 8878 square meters. By 2023, after 14 years of operation, the WSHP’s efficiency had decreased significantly due to equipment aging. Thus, an HVAC system renovation was required to provide more comfortable indoor conditions with lower operational costs.

2.1. Scheme Comparison

In order to retrofit the HVAC system, different schemes were compared by evaluating their compliance with current policies, appropriate investment levels, construction feasibility, and operational efficiency. Based on the project realities, three options are proposed for selection.

2.1.1. Water Source Heat Pump

The most straightforward upgrade approach is to replace the original WSHP with a new and more efficient one, requiring no additional construction works. However, the heat source of WSHPs is groundwater, and continued use is restricted by the Hebei Province Groundwater Management Regulations (2018) [23]. Additionally, no alternative high-temperature water sources with adequate flow exist near the project site.

2.1.2. Urban Central Heating System

In northern China, the urban central heating system accounts for 80% [24] and is the most common heating method in urban areas. With the development of Chengde City, there is already a primary heat pipe near the project site. However, the renovation work may be extensive, including laying new connecting heat pipes and changing the existing fan coils to radiators. Even though the cooling load in Chengde is relatively low, air conditioners still need to be installed to provide a comfortable indoor environment for staff in the summer.

2.1.3. CO₂ Cascade Air-Source Heat Pump

The convenience of an ASHP can solve the aforementioned problems. First, it is compatible with the existing fan coil system and requires no retrofitting. Second, the only modification needed is to move the plant room from the basement to an outdoor area adjacent to the building. Third, the ASHP can serve both heating and cooling demand. Given the harsh winter operating conditions, the selected ASHP must be able to provide sufficient heat and maintain high energy efficiency at low ambient temperatures. Moreover, the ASHP units need to be as compact as possible due to the limited site space. The favorable thermodynamic properties of CO₂ [25] give the CO₂ ASHP the advantages of a large heat capacity per unit, high energy efficiency at low ambient temperature, and a compact equipment structure, which is suitable for this renovation project.

2.2. Select Scheme Description

After the evaluation, the CO₂ ASHP heating system was selected, and the renovation was completed in 2023. As shown in Figure 1, the plant room was moved from the basement to a ground-level corner adjacent to the building. The CO₂ ASHP was connected to the existing heat pipe network, with a retrofit length of approximately 135 m. After this connection, the system was ready for commissioning and operation. The compact CO₂ ASHP unit occupied only about 9 square meters, making it highly efficient in limited on-site space. During the renovation, sensors were installed in the heating system (shown in Figure 2) to enable more accurate and flexible online and on-site control. These include one thermo-hygrometer near the CO₂ ASHP to measure operational conditions (T1, H1), several thermo-hygrometers in conditioned rooms on different floors to monitor indoor conditions (T2, H2~T5, H5), two thermometers embedded in the supply and return water pipes (T6, T7), one flow meter to measure the return water flow rate (F1), and two electrical parameter testers to test the input power of the ASHP and the circulating pump (Q1, Q2).

The CO₂ ASHP used in this project is an R134a/CO₂ cascade ASHP consisting of a CO₂ (R744) cycle as a low-temperature cycle and an R134a cycle as a high-temperature cycle (shown in Figure 2). The R134a cycle is also used as a mechanical subcooler for the R744 cycle, thereby reducing the CO₂ temperature at the outlet of the gas-cooler and improving its performance. In the R744 cycle, the compressed high-temperature R744 gas comes out of the compressor and is first sent to the water side heat exchanger, where it heats the supply water for the first time. Then, the wasted heat is transferred to R134a through the plate heat exchanger. After that, the condensed R744 passes the throttle and enters the outdoor evaporator to absorb the heat from the environment. Finally, the heated R744 returns to the compressor, repeating the low-temperature cycle. In the R134a cycle, R134a absorbs heat from the R744 cycle through a plate heat exchanger and becomes a high-temperature gas after the compressor. It then passes through the water side heat exchanger and heats the supply water for the second time. The condensed R134a enters the plate heat exchanger after the throttle and completes the high-temperature cycle.

3. Optimal Design

A highly efficient heating system is one that selects the appropriate capacity of each equipment, especially the CO₂ cascade ASHP in this project, based on the heating load of the building. According to Equation (1), the heat load of civil buildings mainly comprises envelope heat loss and heat loss caused by air infiltration, penetration, and ventilation. When the equipped mechanical ventilation system can maintain the indoor with positive pressure, heat loss from infiltration and penetration can be neglected. For heat loss calculation of each component (see Equations (2)–(6)), the indoor and outdoor temperatures are critical factors affecting the heat load, yet their values specified in current standards are relatively conservative. Additionally, on-site testing of envelope heat transfer coefficients and building airtightness is recommended, especially for renovation projects, as envelope aging over time may impact thermal performance and airtightness. This testing enables more accurate heat loss calculation.

$$Q_B=Q_E+Q_I+Q_V$$

(1)

$$Q_{hr}=\alpha AU\left(t_{n.d}-t_{w.k}\right)$$

(2)

$$Q_{Rr}=\left({1+x}_{cx}\right)\left(1+x_f\right)\left({1+x}_y\right)\left(1+x_g\right)Q_{hr}$$

(3)

$$Q_E=\sum _{r=1}^n{Q}_{Rr}$$

(4)

$$Q_I=0.278\left(V_I+V_P\right){\rho }_w{c}_p\left(t_{n.d}-t_{w.k}\right)$$

(5)

$$Q_V=0.278V_f{\rho }_w{c}_p\left(t_{n.d}-t_{w.k}\right)\left(1-{\eta }_{hr}\right)$$

(6)

where Q_B is the building total heating load, W; Q_E is the heat loss through building envelopes, W; Q_I is the heat loss due to cold air infiltration and penetration, W; Q_V is the heat loss for ventilation, W; Q_hr is the basic heat load of the rth room, W; Q_Rr is total heat load of the rth room, W; α is the correction factor for temperature difference; A is the heat transfer area of envelope, m²; U is the heat transfer coefficient of the main part of the envelope, W/(m²·°C); t_n,d is the indoor design temperature for winter, °C; t_w,k is the outdoor dry bulb temperature for calculate in winter, °C; x_cx is the modification rate for orientation, %; x_f is the modification rate for exterior wind force, %; x_y is the modification rate for ground floor exterior door infiltration, %; x_g is the modification rate for extra high room height, %; V_I is the air flow rate of infiltration, m³/h; V_P is the air flow rate of penetration, m³/h; V_f is the air flow rate of ventilation, m³/h; ρ_w is the density of outdoor air, kg/m³; c_p is the specific heat of outdoor air at constant-pressure, kJ/(kg·°C); η_hr is the heat recover rate, %.

Beyond the requirements of GB 50736—2012, an optimized heating load design process is proposed in Figure 3, which identifies key parameters for refinement to ensure more accurate heating load calculation and proper ASHP capacity selection. Four key parameters require optimization using on-site data:

(1)

Building envelope-related parameters: On-site testing of the heat transfer coefficient and airtightness of building envelopes is essential, especially for renovation projects. Envelope aging over time degrades its thermal performance, and on-site test data is far more reliable than theoretical values.

(2)

Outdoor design temperature: As previously noted, the unassured rate derived from hourly data is higher than the daily-based value specified in current standards [16]. For reference, standards from developed countries such as the ASHRAE Handbook adopt outdoor design temperatures based on hourly data with unassured rates of 0.4% and 1.0% [26]. Thus, local historical hourly data should be used to revise outdoor temperatures for heating load calculation where applicable. The unassured rate can be varied based on building function, with a 1-day (24 h) unassured rate recommended for most scenarios.

(3)

Indoor temperature: Indoor temperature should be optimized based on occupancy preferences. If no specific preference is provided, a setpoint of 20 °C is recommended, as studies confirm it as a widely accepted comfort temperature [27].

Once these parameters are finalized, the building heating load can be calculated and simulated. An ASHP is then selected to match this load, ensuring that its heating capacity at the modified outdoor temperature meets the requirement. Relevant performance data is referenced from product specifications.

3.1. Thermal Performance Testing of Envelope

The heat transfer coefficient of the existing envelope was tested and calculated in accordance with the requirements of the Standard for energy efficiency test of public buildings (JGJ/T 177–2009) [28]. This testing was conducted before designing the heating system to clarify the actual thermal characteristics of the existing building, and the test results of the envelope heat transfer coefficient and airtightness are presented in

Table 1. The heat transfer coefficient of envelope.

Envelope	Exterior Wall	Exterior Roof	Exterior Window
Heat transfer coefficient (W/(m2·°C))	0.483	0.400	2.700 1
Leakage (m3/(m·h))	—	—	2.0

¹ For the heat transfer coefficient of the exterior window, refer to its inspection report.

.

3.2. Indoor and Outdoor Design Temperature Discussion

3.2.1. Indoor Temperature for Design

With the continuous improvement of living standards in China, the minimum indoor heating temperature of 18 °C is no longer sufficient to meet people’s needs. Studies indicate that 20 °C is a more comfortable indoor temperature for individuals in sedentary activities, while 18 °C is the temperature at which most people do not perceive cold [27]. Setting 18 °C as the lower limit in the standards is a comprehensive consideration of energy conservation and initial investment cost reduction, based on the technical conditions available at the time of formulation. Notably, the Regulations on Urban Heat Supply in Heilongjiang Province [29] took the lead in raising the minimum indoor heating temperature to 20 °C in 2021. Improving indoor conditions to a more comfortable temperature may be the future. Additionally, the high efficiency of the CO₂ cascade ASHP at low ambient temperatures and its ability to supply high-temperature hot water can support heating systems in cold climates to achieve warmer indoor conditions [12].

3.2.2. Outdoor Temperature for Design

According to the standard, the outdoor design temperature for heating load calculation in Chengde City is −13.3 °C for the heating system and −15.7 °C for winter air conditioning [30]. These values are determined based on daily temperatures: −13.3 °C is the unassured daily temperature for five days on average for the year, and −15.7 °C is the unassured daily temperature for one day on average for the year. However, the total annual unassured hours differ when calculated using daily and hourly temperatures. Both typical meteorological (TMY) data and the measured data during the 2023–2024 heating season at the project site show that outdoor temperatures below −13.3 °C lasted 321 h and 340 h, respectively, which is significantly longer than the five-day (120 h) threshold (see Figure 4). The same situation occurs at −15.7 °C. To ensure that the selected ASHP provides a higher level of indoor comfort, it is recommended to revise the outdoor design temperature using hourly data.

Given the practical challenge of acquiring 30-year hourly data for projects, TMY data and historical data from the recent 1–2 years are suggested for statistical analysis to improve convenience, pending updates to national standards. TMY data reflects long-term regional climate characteristics, and historical data from the recent 1–2 years can capture the latest climate fluctuations, and their combination balances data accessibility with representativeness. For selecting an appropriate unassured rate, the 0.4% unassured rate (35 h) from ASHREA is referenced as a baseline. However, calculations indicate that the outdoor design temperature yielded from one-year temperature data in cold regions is approximately 3 K higher than that derived from long-term data, which may underestimate low-temperature extremes if using short-term data directly [31]. To address this, a lower 24 h (1-day) unassured rate is adopted, which also aligns with the principle of China’s national standard for winter air conditioning outdoor design temperature of a one-day unassured for the year. For Chengde City, the hourly temperature with a 24 h (1-day) unassured period is −19 °C, considering both TMY data and measured data.

Figure 4. Hours at different outdoor temperatures. Typical meteorological annual data is provided by the Standard for weather data of building energy efficiency JGJ/T 346–2014 [32].

Figure 4. Hours at different outdoor temperatures. Typical meteorological annual data is provided by the Standard for weather data of building energy efficiency JGJ/T 346–2014 [32].

3.3. Heating Load Simulation

The heating load was calculated and simulated using THVAC t20–v8.0, incorporating the previously determined parameters and following the Air Conditioning Design for Civil Buildings [33]. Considering that this existing building is not equipped with a mechanical ventilation system and occupants rarely open windows for fresh air during harsh low-temperature periods, ventilation heat loss was excluded from the calculations. Figure 5 illustrates the heating loads under various outdoor temperatures, with an indoor condition set at 20 °C. As discussed in the previous section, the total heating load used to select the CO₂ cascade ASHP model is 492 kW (−19 °C). The final CO₂ cascade ASHP chosen for the renovation has a nominal heating capacity of 585 kW (−12 °C) and a nominal COP of 2.90 (−12 °C); more detailed parameters are listed in

Table 2. Parameters of selected CO₂ cascade ASHP.

Name	Value	Name	Value
Unit type	Integral unit	Overall dimensions/mm	6155 × 1335 × 2852
Nominal heating capacity (−12 °C)/kW	585	Heating capacity (−19 °C)/kW	515
Nominal input power (−12 °C)/kW	202	Input power (−19 °C)/kW	200
Nominal heating COP	2.90	heating COP (−19 °C)	2.58

. Based on the ASHP’s heating capacity correlation chart, the heating capacity at −19 °C is approximately 515 kW, which satisfies the required heating load.

On the other hand, if the heating load for ASHP selection follows the traditional approach (using the outdoor temperature of −15.7 °C), a smaller-heating-capacity ASHP might be chosen, potentially leading to uncomfortable indoor conditions for approximately 101 h (over 4 days) (see Figure 4).

4. Data Measurement

To evaluate the performance of the CO₂ cascade ASHP heating system, the outdoor temperature and humidity, indoor temperature and humidity, temperature and flowrate of supply and return water, and input power are all measured according to the Standard for Energy Efficiency Test of Public Buildings (JGJ/T 177–2009) [28] and the Evaluation Standard for Application of Renewable Energy in Buildings (GB/T 50801–2013) [34]. All data collection starts after the system has been operating normally for several consecutive hours. The testing device location is shown in Figure 2, and the calculation is shown as Equations (7) and (8).

$$Q_0={\displaystyle \frac{V\rho c\left(t_s-t_r\right)}{3600}}$$

(7)

$$COP={\displaystyle \frac{Q_0}{N}}$$

(8)

where Q_o is the heating capacity of heat pump, kW; V is the flow rate of return water, m³/h; ρ is the average density of supply and return water, kg/m³; c is the average specific heat of supply and return water at constant pressure, kJ/(kg·°C); t_s is the temperature of supply water, °C; t_r is the temperature of return water, °C; COP is the actual coefficient of performance of heat pump; N is the average input power of heat pump, kW.

The uncertainty of on-site heat pump performance coefficient testing is evaluated in accordance with Evaluation and Expression of Uncertainty in Measurement (JJF 1059.1–2012) [35], with equations listed in Equations (9)–(13). The uncertainty of each testing instrument installed in the heating system (shown in Figure 2) is listed in

Table 3. Uncertainty of testing instruments.

Testing Instrument	Uncertainty (k = 2)
Thermo-hygrometer for indoor use	0.2 °C
Thermo-hygrometer for outdoor use	0.2 °C
Thermometer	0.3 °C
Flow meter	2 × 10−3
Electrical parameter tester	5 × 10−3

, and the COP measurement uncertainty is summarized in

Table 4. Expanded uncertainty of heating COP.

Outdoor Temperature/°C	Average COP	Expanded Uncertainty (k = 2)	Relative Expanded Uncertainty
−20	2.83	0.027	0.96%
−15	3.00	0.025	0.84%

. The relative uncertainty of COP is approximately 1% (k = 2), indicating high accuracy and reliability of the test results.

$$u_A\left(\overline{x}\right)={\displaystyle \frac{s\left(x_k\right)}{\sqrt{n}}}=\sqrt{{\displaystyle \frac{{\sum }_{i=1}^n{\left(x_i-\overline{x}\right)}^2}{n\left(n-1\right)}}}$$

(9)

$$u_B\left(x\right)={\displaystyle \frac{a}{k}}$$

(10)

$$u_c\left(y\right)=\sqrt{\sum _{i=1}^N{\left[{\displaystyle \frac{\partial f}{\partial x_i}}\right]}^2{u}^2\left(x_i\right)}$$

(11)

$$y=f\left(x_1, x_2,\dots ,x_N\right)$$

(12)

$$U_C=k{u}_c\left(x\right)$$

(13)

where u_A is the type A evaluation of measurement uncertainty; u_B is the type B evaluation of measurement uncertainty; u_C is the combined standard uncertainty; U_C is the expanded uncertainty; x_i is the measured value for each measurement; $\overline{x}$ is the average of the measured values. n is the number of independent measurements; a is the expanded uncertainty of each instrument according to its calibration certificate; k is the coverage factor; y is the measurement determined by other measurements x₁, x₂,…, x_N by measurement function.

5. Performance Analysis

5.1. Indoor Temperature Measurements

On-site data for the operational performance of the CO₂ cascade ASHP was measured and monitored over a whole heating season. The data revealed that indoor comfort was highly guaranteed, even on the coldest days. During the 2023–2024 heating season, the lowest hourly outdoor temperature was −22.2 °C at 23:00 on 19 December. At the same time, the indoor temperatures for rooms 703 and 1102 were 21.9 °C and 23.0 °C, respectively. During the coldest period, from 19 December to 21, the outdoor temperature varied from −10.1 °C to −22.2 °C, while the indoor temperatures remained within 20.8 °C and 24.1 °C (see Figure 6).

5.2. Heating Coefficient Measurements

With indoor comfort well ensured, the hourly COP of the CO₂ cascade ASHP during the same period was investigated and is shown in Figure 7. The hourly COP from 19 December to 21 was mainly around 2.84. Even at the coldest outdoor temperature of −22.2 °C, the COP remained as high as 2.70. This demonstrates that the CO₂ cascade ASHP operates efficiently at low ambient temperatures.

During extreme cold conditions, frosting and defrosting processes of the ASHP affect its heating performance. Figure 8 presents the minute-by-minute heat supply during the coldest hour. During normal operation, the heat supply remains stable at 510 kw to 525 kW, indicating that the CO₂ ASHP can operate steadily under extreme cold conditions and provide sufficient heat to meet the building’s demand. As shown in Figure 4, the calculated heating load at −22 °C is approximately 520 kW, which matches the supplied heat. The defrosting process started at 23:36 and ended at 23:40, lasting only 4 min. By analyzing the defrost duration during the coldest period, the defrost time proportion is approximately 6%, which is far below the 20% upper limit specified in the standard [36]. Regarding the instantaneous COP during the coldest hour (see Figure 9), the CO₂ ASHP operated efficiently under extreme cold conditions, with the instantaneous COP fluctuating slightly around 2.70 during normal operation. At the defrosting process switchover point, the instantaneous COP varied due to transient system instability. Overall, the CO₂ ASHP maintained an average COP of 2.70 when the outdoor temperature was −22.2 °C, and the defrosting process had minimal impact on its heating performance.

Over the entire heating season, the COP was 2.89. As seen from the distribution of the COP in Figure 10, it showed minimal dependence on outdoor temperature, with only a slight increase in COP when the outdoor temperature rose from −25 °C to 15 °C, which indicates that cold environment temperature has less impact on the energy efficiency of the CO₂ cascade ASHP. The COP values scattered between 2.2 and 3.6 throughout the heating season are influenced by two main factors. First, the building’s actual heating load fluctuates as occupancy rate, solar radiation, outdoor air infiltration, and internal heat gains all vary during actual operation. The system adjusts the supply water temperature accordingly, leading to varying actual COP under different operating conditions even at the same outdoor temperature. Second, as this is an office building, fewer rooms need heating at night. Intermittent operation with frequent startups and shutdowns during this period reduces the COP; this, however, may be optimized by developing suitable control strategies to lower the frequency of startups and shutdowns.

With the unit high efficiency, the ASHP supply heat is compared with the building heating load. As Figure 11 illustrates, the actual heating capacity of the CO₂ cascade ASHP is consistently higher than the heating load. At the lowest outdoor temperature of −22 °C, the design heating load is 528 kW, and the heat supplied is 562 kW, which is 6% higher than the load. Even considering pipe heat loss, the supplied heat still meets the requirement. As previously analyzed, the actual heating load fluctuates, and the supplied heat is also displayed discretely. Figure 11 also indicates that the ASHP heating capacity increases with rising outdoor temperature, which is opposite to the change in building heating load. This phenomenon also indicates that an optimal control strategy is also required to balance heat supply and load, ensuring both energy efficiency and stable indoor comfort.

5.3. Economic and Environment Benefit

The good performance of the CO₂ ASHP validates the effectiveness of the proposed optimal design process, while its high efficiency in supplying sufficient heat in extremely cold climates serves as a necessary condition for applying this method. The optimal process typically increases heating load to reduce uncomfortable periods. If the heat source has low energy efficiency, such as a coal boiler or an electric boiler with an efficiency around 0.9, the increased load will significantly expand equipment size and consume more energy. As shown in

Table 5. Operational cost and carbon emission for different heating sources.

Type	Supplied Heat/kW·h	Efficient	Energy/tce	Cost/Yuan/m2	Carbon Emission/tCO2/a
Central heating	1.10 × 106	-	134.6	32	434.0
CO2 ASHP		2.89	46.5	26	218.8
Electricity boiler		0.97 [37]	138.9	79	652.7

All consumed energy are converted to standard coal equivalent with coefficient of 0.1229 kgce/(kW·h) (for electricity) and 0.03412 kgce/MJ (for thermal energy) [38]. The average electricity fees during heating season is 0.62 yuan/(kW·h) for Chengde. The carbon emission factors are 0.5777 tCO₂/(MW·h) (for electricity) [39] and 0.11 tCO₂/GJ (for thermal energy) [40].

, when the indoor temperature set-point is raised to 20 °C, the operating cost of the optimally selected CO₂ ASHP is only 26 yuan/m². This is 19% lower than the 32 yuan/m² heating fee charged by the central heating system for maintaining 18 °C. Furthermore, as discussed in Section 2, the renovation cost for connecting to the central heating system is significantly higher than that adopting the CO₂ ASHP due to the extensive renovation work required. An electric boiler is introduced as a reference to illustrate that improving indoor comfort and heating assurance rate requires adoption of high-efficiency technologies such as CO₂ ASHPs. Without such efficiency, operating costs and carbon emissions would be three times higher.

Additionally, ensuring sufficient heating under extreme outdoor temperatures is also critical. Ordinary ASHPs, without advanced technologies to extend their operating temperature to −25 °C or lower, will stop working when outdoor temperatures drop below their limit, failing to meet the building heating demand. Overall, with the continuous advancement of low-temperature heat pump technology, the optimized design process is essential in providing thermal comfort with less energy consumed.

6. Conclusions

As ASHP applications expand into severe cold zones under clean heating program initiatives, a more universal ASHP selection method is required. Drawing on recent research, an optimal design process of revising parameters is proposed, with the CO₂ ASHP, a new cold-climate heat pump technology, used as a reference to demonstrate its performance. The conclusions are as follows:

(1)

To enhance indoor comfort reliability and derive more realistic heating loads, a flexible parameter-adjustment design approach combining on-site testing and simulation is proposed. First, testing the heat transfer coefficient and airtightness of building envelopes is strongly recommended. Second, the target indoor condition is defined based on occupant preferences, with a minimum of 20 °C. Third, for heating load simulation, the outdoor temperature should be determined by hourly temperature data rather than daily temperature data, with a recommended unassured rate of one day (24 h). Finally, the building total heating load is simulated using the parameters determined above. The ASHP is then selected based on the optimized heating load and its heating capacity at the refined outdoor temperature. In this paper, CO₂ ASHP is adopted as a case study due to its excellent low-temperature adaptability and high efficiency. Other ASHPs that can also operate stably and efficiently at extremely low temperatures are equally suitable for this approach.

(2)

Using the optimally defined heating load simulation in Chengde, the CO₂ cascade ASHP was selected for a renovated public building and ensured reliable indoor comfort even on the coldest days and operating at high efficiency. During the 2023–2024 heating season, the lowest hourly outdoor temperature reached −22.2 °C, at which the average indoor temperature remained 22.4 °C and the COP of the ASHP was 2.70.

(3)

Based on the above research and analysis, it is recommended to develop a new heating standard based on low-temperature ASHP technology. This standard should provide hourly meteorological parameters or optimized outdoor design parameters for major cities, as well as standardizing appropriate adjustment strategies. All these measures aim at ensuring indoor comfort while improving energy utilization efficiency.