Study on the operation strategies and carbon emission of heating systems in the context of building energy conservation

Coal‐fired thermal power must be flexible to enable the grid absorption of inconsistent photovoltaic (PV) and wind power. Combined heat and power (CHP) coal‐fired plants are the primary source for district heating systems. This paper uses a 330 MW subcritical CHP unit as an example to carry out the study. With the promotion of building energy efficiency, when the thermal index is reduced to below 20 W/m2, the low‐load operation of CHP can meet the wind power and PV feed‐in demand and guarantee residential heating without the need for flexibility modification. Meanwhile, more renewable energy generation can reduce carbon emissions from the power supply, further contributing to reducing carbon emissions from buildings. The impacts of different envelope parameters and supplementary heat sources on building carbon emissions are also studied. The conclusion shows that the degree of their impact on carbon emissions ranks as ESMs (energy supply modes) > Factor D (infiltration N50) > Factor A (external wall heat transfer coefficient) > Factor C (window heat transfer coefficient) > Factor B (roof heat transfer coefficient). When the building's heating energy consumption gradually decreases, the distributed heat pump unit can replace the coal‐fired boiler to supply the peak heat load demand. In the future, China's district heating systems can be gradually changed from the current CHP and coal‐fired boilers to CHP and distributed heat pumps.

International Energy Agency (IEA) Net Zero by the 2050 Scenario. 3 Although renewable power sources are ecofriendly and produce no greenhouse gas emissions during operation, the electric grid suffers a significant challenge due to the high penetration of these uncertainty sources. The curtailment for fluctuating renewable power generation can be frequently observed in China, the United States, and other countries. 4 Therefore, different kinds of technologies have been developed to enhance the overall flexibility of the power system to reduce the curtailment of variable renewable electricity.
Coal still accounted for 27% of the global primary energy supply in 2021 5 and has played an essential role in meeting many countries' energy security and energy access goals. Combined heat and power (CHP) coal-fired power plants apply heat resources that would otherwise be wasted to supply heat for residential heating sectors or industrial use, resulting in increased energy efficiency. 6 The district heating (DH) system is integrated with the CHP using a heat distribution network of pipes. 7 Conventional CHP plants operate in a heat-controlled mode, which narrows the power regulation range of the CHP plants. 8 Therefore, the heat-power decoupling technologies, which produce power and heat independently, require further development to increase flexibility in nations where CHP and DH play a pivotal role in space heating, such as China, Danish, and Germany. 9 The flexibility of CHP can be mainly enhanced from the following aspects: (i) reducing the minimum operation load, 10 (ii) increasing the load change rates, 11 and (iii) more frequent start-up and shut-down control strategies. 12 Richter et al. 13 integrated a Ruth's storage with the coal-fired power plants to increase flexibility by achieving fast reaction times of the net power when charging or discharging the storage vessel. Zhang et al. 14 evaluated the potential benefits of electric boilers and pumped hydro to reduce the curtailment of wind electricity in wets Inner Mongolia. Low-pressure turbine renovation with a bladeless shaft or renovation with zero-power output can also increase the flexibility of coal-fired power plants. 15 However, fewer scholars investigate operation strategies for CHP units in the context of building energy conservation and heating load reduction. Advancements in energy savings in buildings will substantially limit residential heating load intensity. 16 For instance, insulation and double-glazed windows reduced heating demand by 34.4%. 17 The heating demand would scale down when retrofitting existing buildings into nearly zero energy buildings (nZEB) and passive houses. 18 As promising means to reduce carbon emission and energy consumption, zero/low-energy buildings have been attracting increasing attention. 19 Sun et al. 20 proposed transparent insulation as a component material of windows to reduce heat loss and effectively use solar heating, which can decrease building energy consumption by 35.8%. Lami et al. 21 recycled the wasted air in windows ventilation to optimize the energy saving. The results showed that an energy saving of 83.1% was achieved when the available flow rate of wasted air was shared with three double-glazed windows. Short et al. 22 reduced carbon emissions by modifying the ventilation system and improved the energy efficiency building by utilizing natural ventilation. Kim et al. 23 employed a variety of renewable energy sources to supply energy to an eco-energy town where the heating load demand was 67% lower than the average heating load demand in Korea. Truong et al. 24 achieved 135.7 MWh annual savings in the building's thermal demand by improving building airtightness, ventilation heating recovery rate, and wall insulation.
In DH systems where CHP is the primary heating source, the use of renewable energy to replace traditional coal-fired boilers for supplemental heating can effectively reduce carbon emissions. 25 Gustafsson et al. 26 investigated how the global CO 2 emissions are influenced by the different energy systems. It is concluded that the calculated primary energy number is lower for heat pump systems, but the global CO 2 emissions are lowest when DH uses mostly biofuels and is combined with solar PV systems. Bode et al. 27 studied the control strategy of heat pumps to further reduce building energy consumption. The Danish government promoted the use of heat pumps for heating and progressively reduced the use of natural gas. 28 Poulsen et al. 29 designed a ground source heat pump system incorporating a foundation pile heat exchanger in the Vejle Rosborg stadium in Denmark, which can improve thermal comfort through efficient passive cooling. Heat pumps can be selected as an effective supplemental heating technology.
Therefore, this paper investigates flexible operation strategies for CHP units in the context of building heating load reduction. The impacts of different envelope parameters and supplementary heat sources on building carbon emissions are also studied. The results of this study provide detailed guidance for selecting the optimal CHP operating measures when considering the decreasing demand of the space heating sector. This paper investigates how to consider the low carbon development of building energy efficiency and power systems without completely abandoning fossil energy generation. The results are also applicable to other climatic zones with DH systems.

| A CHP unit
In our study, a 330 MW subcritical coal-fired power plant was selected. The inlet parameters at rated conditions are 16.7 MPa, 538°C, and 538°C for the live steam pressure, live steam temperature, and reheat steam temperature, respectively. The four turbines, namely, HP (highpressure), IP (intermediate-pressure), LP (low-pressure), and boiler feedwater pump turbines, are used to produce power. The heat regenerative system contains three HP heaters, four LP heaters, a deaerator, and other heaters.
This 330 MW subcritical coal-fired power plant produces a rated pumping capacity of 500 t/h and a heating capacity of 324.7 MW. The design heating index is 55 W/m 2 . The coal-fired power plant needs to coheat with coal-fired boilers to meet the thermal demand of a 10,000,000 m 2 residential area.
According to the flexibility transformation requirements of China's coal-fired thermal power plant, thermal power plants must operate below 50% load in winter to effectively guarantee wind power and PV access to the grid. 30 The maximum heating capacity of coal-fired generating units is about 194 MW when the electricity generation capacity drops to 50%.

| A typical residential building
A typical 8-story residential building designed in the Changchun area (ASHARE/C climate zone 6A) is used as an example to study the effect of different supplementary heat sources on the building operation carbon emissions (CO 2,operation ) in winter. Also, the effect of changing the building envelope parameters on CO 2,operation is analyzed. The building area is 3863.31 m 2 and the simulation model constructed in Designbuilder software is shown in Figure 2. The basic envelope parameters of the studied building are shown in Table 1, which are obtained from field investigations.

| Thermal energy demand of the DH networks
The thermal energy demand of the DH networks can be calculated by Equations (1)-(7) as follows 31 : where t w represents the outdoor ambient temperature (°C). t′ w (°C) is the outdoor computational temperature, defined according to the unguaranteed-days method, and the number of unguaranteed days in the Chinese heating standard is 5 days (5 days ). Besides, 5 in Equation (1) indicates that the heating period starts when the mean outdoor temperature is lower than 5°C and ends when it is higher than 5°C. R n is a nondimensional value, taking values from 0 to 1, and b is the coefficient of R n . R n and b can be derived from Equations (2) and (3) as where N and N zh are one specific day and the total days of the heating period, respectively. Five in Equation (3) represents 5°C. μ is the correction factor, which can be obtained from Equation (4). t pj indicates the outdoor mean temperature during the heating period (°C) The geometry of the heating load duration curve can be expressed as follows: where Q n is the specific heating load when the outdoor temperature equals to t w , Q′ n indicates the design heating load which can be calculated by Equation (7), β 0 is a coefficient obtained from Equation (6), t n is the inside air calculating temperature for heating (generally set at 18°C), F refers to the building area (m 2 ), and q f stands for the thermal index (W/m 2 ). Besides, 5 in Equation (6) represents 5°C.

| Contributions of CHP in DH in China in the foreseeable future
A comparison of the power generation structures of China, the United States, and OECD-Europe is shown in Figure 3 and Table 2. 32 US electricity generation in 2020 (4.252 × 10 6 GWh) is 99% of that in 2005 (4.295 × 10 6 GWh), remaining almost unchanged. To reduce carbon emissions from the electricity supply, the United States reduced the share of coal-fired generation from 50% in 2005 to 20% in 2020. Meanwhile, the percentage of natural gas, wind power, and PV power generation in the United States has increased by about 2.1, 19.1, and 222.7 times, respectively. The proportions of nuclear, hydroelectric, and other energy generation remain nearly constant.
The amount of electricity generation in OECD-Europe in 2020 (3.536 × 10 6 GWh) is 99% of that in 2005 (3.571 × 10 6 GWh). OECD-Europe reduces carbon emissions from the electricity supply by replacing coal-fired generation with wind, photovoltaic, and other energy sources (e.g., biomass, tidal, waste, etc.). Coal-fired generation accounted for 30% of electricity generation in 2005, reducing it to 13% in 2020. Nuclear, natural gas, and hydroelectric generation remain virtually unchanged.
China's electricity generation capacity in 2020 (7.798 × 10 6 GWh) is 3.1 times higher than in 2005 (2.5 × 10 6 GWh). Since China's natural gas dependence is 43%, it cannot replace coal for power generation with natural gas on a large scale down. Therefore, the way for China to reduce carbon emissions from the power supply is to significantly increase the share of wind power, PV, and hydropower. China's coal-fired power generation percentage is reduced from 80% in 2005 to 64% in 2020.
This clearly shows that fossil energy generation methods fueled by coal and natural gas are unlikely to be eliminated soon. CHP units have higher thermal efficiency and fuel utilization than pure condensing units. Therefore, urban buildings in China will still use centralized DH with coal-fired CHP units as the primary heat source in winter.

| Flexible operation strategies for CHP units
The design parameters of the central regional heating are as follows: the heating period is about 166 days, and t′ w , t pj , and t n are −24°C, −8.5°C, and 18°C, respectively.
The heating load duration diagram of the DH system can be calculated according to Equations (1)-(7), as shown in Figure 4. The peak heating load demand is 1980 GJ/h, and the maximum heating capacity of the coal-fired power plant is 1168.92 GJ/h (324.7 MW). The gray area in Figure 4 represents the heating load satisfied by the coal-fired boilers.

| 2425
Before the flexibility transformation, when the electrical load of the coal-fired power plant is reduced to 50%, the heating capacity drops to 698.4 GJ/h (194 MW), which is seriously insufficient for heating demand. Therefore, one of the main tasks of flexibility transformation is to ensure that the heating capacity of the coal-fired power plant is not reduced by heat-power decoupling. This task is typically accomplished by electric boilers, 12 thermal heat storage tanks, 33 and lowpressure turbine renovation. 15 However, the building heating index shows a decreasing trend as nZEB spreads. As shown in Figure 5, when the building heating index is reduced to 20 W/m 2 , the heating capacity of the CHP generator at 50% electrical load can still meet the residential heating demand. There are similarities between this parameter and the results of previous studies. The German passive house stipulated that the heating index should be less than 10 W/m 2 . 34 The results of Feng et al. 35 showed that the heating index of a residential nZEB in a cold region of China was 14.6 W/m 2 .
In this case, the 330 MW CHP unit only needs to reduce the power generation to 50% to meet the requirements of China's policy, 30 which not only contributes to the integration of variable renewables (such as wind power and PV) but also satisfies energy requirements of the residential heating sector. It also avoids large-scale renovation of CHP units. Moreover, the coal-fired heating boilers in the DH system can be eliminated and CHP can be directly used for centralized heating, thus effectively minimizing heating carbon emissions. 36

| Impact of different supplementary heat sources on building carbon emissions
As can be seen from Figure 5, supplementary heat sources are still needed to supply DH together with CHP until the building is completely converted to nZEB.
The supplementary heat source design scheme is shown in Table 3. The CO 2,operation are composed of CO 2,heating (heating carbon emissions) and CO 2,electricity (electricity consumption carbon emissions). Under ESM-1 (energy supply modes), the value of CO 2,heating (399.23 g CO 2 /kWh 37 ) is taken from the standard coal consumption for heating of CEC, and that of CO 2,electricity (565 g CO 2 /kWh 38 ) is taken from the Annual Development Report of China Electricity Industry 2021. In ESM-2, ESM-3, and ESM-4 modes, the CO 2,electricity is 565, 385.55, 39 and 230.7 g CO 2 /kWh, 40 respectively. Moreover, the COP of the heat pump with ASHARE/C climate zone 6A is set to 2, and then the CO 2,heating is 0.5 of CO 2,electricity . The CO 2,electricity values of the United States and Europe can be regarded as the target values of China's future CO 2,electricity . With the continuous optimization of the power supply structure, the CO 2,electricity of China are showing a decreasing trend annually.
The parameters in Table 1 are varied in the range of ±20% and ±40%, as shown in Table 4. The effects of the heat transfer coefficient of the exterior wall, heat transfer coefficient of roof, heat transfer coefficient of exterior windows, and airtightness on CO 2,operation are studied.
As shown in Figure 6, during the decrease of Factor A (External wall heat transfer coefficient) from level 5 (0.7 W/m 2 k) to level 1 (0.3 W/m 2 k), the CO 2,operation F I G U R E 5 Heating load duration diagram at different heating indexes.
T A B L E 3 Supplementary heat source design scheme. Energy supply methods CO 2,heating (g CO 2 /kWh) CO 2,electricity (g CO 2 /kWh) shows a descending trend. Under ESM-1, CO 2,operation drops from 204.25 to 169.78 t/a, and the rate of change for CO 2,operation per level is 8.62. However, the influence of different ESMs on CO 2,operation appears to be more significant. Under level 1, as the energy supply methods change from ESM-1 to ESM-4, CO 2,operation decreases from 169.78 to 59.71 t/a, and the rate of change of CO 2,operation per ESM is 36.69.
Level 3 is the basic envelope parameter of the building, and level 1 is lower than the limits of the thermal envelope parameters proposed in the Chinese standard (energyefficient design standards for residential buildings in severe cold and cold regions JGJ . Once an existing building is retrofitted to level 1, 60%-75% energy savings are required to meet the energy requirements specified in the Chinese nZEB standard. However, upgrading the thermal performance of the envelope is currently difficult and can significantly increase the cost of building retrofits. The optimization of energy supply methods can reduce building energy consumption and carbon emissions more significantly. With the reduction of Factor A, the building heating energy consumption gradually decreases, and the distributed heat pump unit can replace the coal-fired boiler to supply the peak heat load demand (from ESM-1 to ESM-2), thus reducing CO 2,operation . As heating energy consumption decreases, CHP can operate at lower loads (as shown in Figure 4), allowing the grid to accept more variable renewable energy generation. At lower CO 2,electricity , the heating carbon emissions of the heat pump unit can be further reduced (from ESM-2 to ESM-4).
Similar conclusions can be drawn in Figures 7-9. As shown in Figure 7, during the decrease of Factor B (roof heat transfer coefficient) from level 5 (0.56 W/m 2 k) to level 1 (0.24 W/m 2 k), the CO 2,operation shows a descending trend. Under ESM-1, CO 2,operation drops from 190.49 to 184.23 t/a, and the rate of change for CO 2,operation per level is 1.57. However, the influence of different ESMs on CO 2,operation appears to be more significant. Under level 1, as the energy supply methods change from ESM-1 to ESM-4, CO 2,operation decreases from 184.23 to 63.92 t/a, and the rate of change of CO 2,operation per ESM is 40.1. Figure 8 exhibits that during the decrease of Factor C (window heat transfer coefficient) from level 5 (3.08 W/ m 2 k) to level 1 (1.32 W/m 2 k), the CO 2,operation shows a descending trend. Under ESM-1, CO 2,operation drops from 203.34 to 171.7 t/a, and the rate of change for CO 2,operation per level is 7.91. However, the influence of different ESMs on CO 2,operation appears to be more significant. Under level 1, as the energy supply methods change from ESM-1 to ESM-4, CO 2,operation decreases from 171.7 to 60.37 t/a, and the rate of change of CO 2,operation per ESM is 37.11.
As Factor D (infiltration N 50 ) decreases from level 5 (5.04 ac/h) to level 1 (2.16 ac/h), the CO 2,operation shows a descending trend, as illustrated in Figure 9. Under ESM-1, CO 2,operation drops from 206.73 to 166.37 t/a, and the It can be concluded that with the reduction of Factor A (external wall heat transfer coefficient), Factor B (roof heat transfer coefficient), Factor C (window heat transfer coefficient), and Factor D (infiltration N 50 ), the CO 2,operation tends to decline. But ESM has a more significant effect on CO 2,operation . The degree of their impact on CO 2,operation ranks as ESM > Factor D > Factor A > Factor C > Factor B.

| CONCLUSIONS
In previous studies, CHP units will need to be operated at reduced loads and undergo a series of flexibility modifications to enable the grid to absorb more unstable renewable generation. However, the building heating load demand will gradually decrease as the building energy retrofit progresses. In this paper, a 330 MW subcritical CHP unit is used as an example to study its flexible operation strategy in the context of building energy retrofitting. To compare the impact of different supplementary heat sources on building carbon emissions, this paper conducts a study with an 8-story residential building as an example. The impacts of different envelope parameters and supplementary heat sources on building carbon emissions are also studied. The following conclusions are obtained.
(1) When the residential heating index is reduced to below 20 W/m 2 , the CHP unit can meet the future residential heating demand by directly operating at a reduced load without complicated flexibility retrofits. (2) The energy demand can be reduced by changing the parameters of the envelope. Externally, the building's carbon emissions are further reduced by reducing the carbon emissions from the power supply of the electrical system. Under different ESMs, the CO 2,electricity are 565 g CO 2 /kWh (from the Annual Development Report of China Electricity Industry 2021), 385.55 g CO 2 /kWh (from the US Energy Information Agency) and 230.7 g CO 2 /kWh (from the European Energy Agency), respectively. The degree of their impact on CO 2,operation ranks as ESM > Factor D (infiltration N 50 ) > Factor A (external wall heat transfer coefficient) > Factor C (window heat transfer coefficient) > Factor B (roof heat transfer coefficient).
(3) China's DH systems can be gradually changed from the current CHP and coal-fired boilers to CHP and distributed heat pumps. Coal-fired power generation will continue to account for a large share in the future.