Low-Carbon Heating Mode Dominated by Waste Heat Utilization

Abstract

In China, the current power supply and heating sources are mainly dominated by thermal power generation and coal combustion, leading to higher carbon emissions. In the future, the main task is to promote the large-scale and high-proportion transition to new energy sources in the power system and provide zero-carbon heat sources for building heating and non-process industrial production to achieve carbon neutrality.

7.1 Analysis of Future Power and Heating Source Structure

7.1.1 Power and Heating Systems Under Energy Transition

In China, the current power supply and heating sources are mainly dominated by thermal power generation and coal combustion, leading to higher carbon emissions. In the future, the main task is to promote the large-scale and high-proportion transition to new energy sources in the power system and provide zero-carbon heat sources for building heating and non-process industrial production to achieve carbon neutrality. With the construction of a new type of power system primarily based on renewable energy and the gradual popularization of electrified heating, urban energy will be mainly concentrated in the fields of electricity and heating, making carbon emissions from power and heat generation a key factor in achieving urban carbon neutrality. Therefore, it is necessary to further understand the development of power sources in the future power system and the interplay between electricity and heating.

This chapter considers two scenarios for achieving zero-carbon heating by 2050: one is based on centralized heating with waste heat utilization, and the other is based on comprehensive electrification for heating. A simplified model is established to reflect the changes and balance of electricity supply and demand on a daily basis. Based on this, the capacities and operation of various power generation technologies are determined, along with the optimal heating combinations. The model is applied to different regions in China, and specific calculations are made for each region.

7.1.2 Renewable Energy Resources and Power Output Characteristics

In China, the onshore wind energy resource at a height of 100 m has a potential development capacity of 86.9 billion kilowatts, mainly concentrated in the “Three North” regions (Northwest, North China, Northeast). The offshore wind resource, at water depths of 5–50 m and a height of 70 m, has a potential capacity of approximately 500 million kilowatts.

The maximum technical development potential for hydropower in China is about 500–600 million kilowatts, of which 355 million kilowatts have been developed so far (conventional hydropower only, excluding pumped storage). With the completion and operation of the Jinsha River Wudongde, Baihetan, Yalong River Lianghekou, and Dadu River Shuangjiangkou hydropower stations, in addition to the existing Yarlung Zangbo River and Nu River hydropower bases, the development of major hydropower bases in China has been basically completed. By 2050, the installed capacity of conventional hydropower in China will reach 510 million kilowatts.

The power output characteristics of various renewable energy sources lead to temporal and spatial mismatches in power supply and demand. Wind and solar energy exhibit significant seasonal variations, with around 60% of wind power concentrated in spring and winter, and around 60% of solar power concentrated in summer and autumn. Additionally, at the regional level, wind and solar power output characteristics differ, with the northwest, north, and northeast regions having better wind resources, while the northwest and north regions have better solar resources. Centralized photovoltaic and distributed photovoltaic show distinct seasonal differences in output; centralized photovoltaic exhibits a 25% difference in average output between winter and summer, while distributed photovoltaic shows a 50–60% difference.

Furthermore, hydropower exhibits a "summer surplus and winter shortage" seasonal characteristic, and its generation capacity varies according to seasonal changes. Each year is divided into periods of abundant water, normal water, and water scarcity. Taking Sichuan province as an example, the abundant water period is from June to October, the normal water period is in May and November, and the water scarcity period is from December to April of the following year. During the abundant water period in summer, hydropower generation can be high due to the abundant water supply, while during the water scarcity period in winter, hydropower generation may decrease or even stop. Hydropower stations with annual regulation or above can store excess water during the abundant water period for use during the water scarcity period. According to statistics for water and hydropower stations in different river basins operated by the State Grid Corporation, the proportion of hydropower stations with annual regulation or above in the Yangtze River basin and Yellow River basin is 43.6% and 21.7%, respectively.

Nuclear power is an important source in the future zero-carbon power system. As of April 2020, China had a total of 18 nuclear power plants (including those under construction) with a total installed capacity of 45.9 million kilowatts. Among them, the number of nuclear power units in northern regions has been increasing, with 17 units in commercial operation or under construction, and a total installed capacity of 16.71 million kilowatts. According to relevant nuclear power plans, China's coastal areas can accommodate nuclear power resources ranging from 178 to 206 million kilowatts.

7.1.3 Regional Power System Model and Power Balance Results for Future New Power System

• (1) Regional Power System Model

  To analyze the impact of future power systems on heating in northern regions, the country is divided into 21 power grid regions. The 17 provinces and regions in the north are treated as independent areas, while the southern region is divided into four areas: Southwest Grid, Southern Grid, East China Grid, and Central China Grid (excluding Henan). The regions are numbered as follows: 1. Xinjiang, 2. Qinghai, 3. Ningxia, 4. Gansu, 5. Shaanxi, 6. Southwest, 7. Southern, 8. Central China (excluding Henan), 9. East China, 10. Henan, 11. Shaanxi, 12. Shandong, 13. Southern Hebei, 14. Western Inner Mongolia, 15. Northern Hebei, 16. Beijing, 17. Tianjin, 18. Eastern Inner Mongolia, 19. Heilongjiang, 20. Jilin, 21. Liaoning. The model includes 14 types of power generation installations: 1. Coal-fired power, 2. Coal-fired power with Carbon Capture, Utilization, and Storage (CCUS), 3. Gas-fired power, 4. Gas-fired power with CCUS, 5. Biomass power, 6. Biomass power with CCUS, 7. Onshore wind power, 8. Offshore wind power, 9. Centralized photovoltaic, 10. Distributed photovoltaic, 11. Nuclear power, 12. Run-of-river hydropower, 13. Daily regulated hydropower, 14. Seasonal regulated hydropower. The model also considers the transmission and distribution limitations between regions and presents the daily power supply and demand balance for each region.

  In this subsection, the model does not consider heating demand. The optimization objective is to minimize the total cost of the power supply system, including investment costs for power sources, transmission grids, and energy storage, as well as operational costs. The online capacity of thermal power generation (coal-fired, gas-fired, and biomass) and nuclear power cannot exceed the installed capacity. Biomass power generation must meet the constraints of available biomass resources in each region, while nuclear power must meet the constraint of 7,000–8,000 operating hours per year. The carbon emission limitation requires net emissions from coal-fired and gas-fired power plants to be equal to the CO₂ captured by CCS from biomass power plants. In addition, nuclear power and conventional hydropower are not involved in the optimization calculations. Nuclear power is assumed to have a development capacity of 178–206 million kilowatts along the coastal areas, and conventional hydropower is assumed to have a development scale of 500 million kilowatts.

• (2) Future Power and Heat Demand

  In China, the heat demand includes high-density heat demand for winter heating in northern urban areas and heat below 150 °C for non-process manufacturing, totaling 13 billion GJ, equivalent to 3.6 trillion kWh of heat. The future choice between centralized heating with low-grade waste heat utilization and comprehensive electrification will significantly impact the quantity and characteristics of future power load requirements. This will be further analyzed in the following simulations. For the future scenario with a total electricity consumption of 14 trillion kWh in 2050, including electricity for industrial steam production above 150 °C and electricity for rural heating in the north and decentralized heating in the south, the electricity demand for heating in northern urban areas (5.45 billion GJ) and non-process industries (7.6 billion GJ) for heat preparation and transportation are not included. The electricity load exhibits distinct winter and summer peaks, with a daily peak total load of 1.92 billion kilowatts.

• (3) Power Balance Results for Future New Power System

  According to the analysis method of the regional power system model, the electricity load and renewable power generation are optimized on a daily basis. Under the scenario of 14 trillion kWh total electricity consumption, the total installed capacity required for the optimized power system is 730 million kilowatts, of which 610 million kilowatts are from wind and solar power, accounting for 83.7% of the total capacity. The proportion of wind and solar power in total electricity generation is 71.8%, with a wind and solar curtailment rate of 5.7%. The daily maximum electricity demand for thermal power is 480 million kWh nationwide, considering the variation in output throughout the day, the reserved peaking thermal power capacity should be between 550 to 600 million kilowatts, accounting for 7.5% to 8.2% of the total installed capacity, generating 0.8 trillion kWh of electricity, representing 5.7% of the total annual electricity generation. By implementing Carbon Capture and Storage (CCS) for emissions from coal-fired and gas-fired power plants and achieving a biomass fuel proportion above 25%, the amount of CO₂ captured from biomass power plants can exceed the CO₂ emissions from coal and gas power plants, achieving overall zero emissions.

  The key challenge for the power system is to address the seasonal and daily fluctuations in power supply and demand. With the increasing proportion of fluctuating renewable energy generation, such as wind and solar power, in the future, the hourly and seasonal fluctuations in the system will increase. This requires strategies to manage intra-day peaks using pumped storage and electrochemical storage, and to adjust seasonal fluctuations using hydro and thermal power. Figure 7.1 shows the daily variation in electricity demand for various power sources nationwide, indicating significant curtailment of wind and solar power during late winter and spring, and limited curtailment during autumn. Thermal power plants serve as peaking power sources to balance the seasonal mismatch between renewable energy supply and power demand, primarily compensating for the electricity deficit in winter and summer. Seasonal adjustments account for 25% of the total hydroelectric power generation, increasing the average power output in winter by 60 million kilowatts compared to the full runoff method.

  Fig. 7.1

  

  Daily variation of power generation demand for various power sources nationwide

7.1.4 Future Integrated Planning and Analysis of New Power and Heating Systems

• (1) Future Integrated Optimization Model of New Power and Heating Systems.

  To meet the heat source requirements for centralized heating, low-grade waste heat and discarded heat from nuclear power plants, peaking thermal power plants, industrial processes, data centers, and large substations can be recovered and integrated into the required heating parameters through heat pumps. However, to address the temporal mismatch between heat source emissions and heat demand, large-scale seasonal energy storage is required. Although the total amount of waste heat resources and discarded wind and solar heat exceeds the total heat demand, there are spatial, geographical, and temporal matching issues. Therefore, this study also establishes a regional heating balance model to examine whether each region has sufficient waste heat resources and determine the required seasonal heat storage capacity for each region.

• (2) Waste Heat-Based Heating Scheme.

  Based on the model calculations, the future retained peaking thermal power plants will produce 5.08 billion GJ of waste heat annually, and nuclear power plants will emit 7.2 billion GJ of low-grade waste heat annually. In addition to waste heat from power plants, there are also 4.8 billion GJ of industrial waste heat and 4.2 billion GJ of heat from data centers, substations, and sewage that can be utilized. Furthermore, up to 2.26 billion GJ of discarded wind and solar energy can be converted into heat. The total heat source resources amount to 23.5 billion GJ, which is sufficient to meet the 13 billion GJ demand for urban heating in northern areas and heat below 150 °C for industrial processes.

Based on the data from previous chapters, the future waste heat resource and heat demand for each region are compiled. After considering the potential conversion of discarded wind and solar energy into heat, only Beijing, Tianjin, Heilongjiang, and Jilin have heat demand exceeding waste heat resource in these regions. Ningxia and Henan have a relatively balanced waste heat resource and heat demand. Other regions have abundant heat source resources.

Large-scale seasonal heat storage facilities can efficiently recover waste heat and discarded wind and solar energy throughout the year, balancing the temporal mismatch between heat supply and heating demand, significantly improving the flexibility and reliability of the heating system. Based on the 21 energy-consuming regions, the daily variation of waste heat production and heat demand for building heating and industrial production in each region is determined. As building heating requires more heat in winter and far exceeds the heat demand, while heat supply in spring, summer, and autumn greatly exceeds heat demand, a large-scale seasonal heat storage system, in the form of large heat storage tanks, can store excess heat from spring to autumn for use in winter. The storage medium for heat storage is water, which has low cost, with a storage cost of 100–150 yuan/m³. In regions with abundant heat sources, it is essential to choose the lowest-cost heat storage and minimize the long-term storage time to reduce heat losses, thus reducing the investment in seasonal heat storage. For regions with a scarcity of heat sources, larger heat storage capacity is needed based on demand matching. Additionally, waste heat can be extracted using electric heat pumps or seasonal discarded wind and solar energy can be converted into heat using electric boilers. This heat can be stored through seasonal heat storage to alleviate the contradiction of inadequate waste heat resources in some regions.

According to the optimized model results, a large-scale seasonal heat storage capacity of 1.14 billion GJ is required nationwide, assuming a heat storage temperature of 90/20 °C. This requires the construction of large heat storage tanks with a total capacity of 4.32 billion m³, necessitating an investment of 5.5 trillion yuan. Most regions in the southern area do not require seasonal heat storage. Based on the daily variation of real-time heat storage throughout the year, the stored heat is almost depleted by late winter, and then waste heat and discarded wind and solar energy are stored. Some provinces release a portion of the stored heat in summer to avoid consuming peak electricity during that season, and then they release peak heat for load balancing during the severe cold period in winter.

7.2 Dominant Waste Heat Utilization as the Footing for Urban Heating

To figure out how to achieve carbon neutrality in heating, the heating and power systems must be put together for analysis. The comprehensive use of air-source heat pumps, ground-source heat pumps, and other electric heat pumps will lead to a significant increase in the electrical power load in winter, an exacerbation of the shortfall in zero-carbon electricity in winter, increased investments in power generation, transmission, consumption, and other links, and a substantial rise in the heating cost.

In fact, there will be abundant thermal power plants and nuclear power plants for peak shaving, and other waste heat resources in China. China holds immense potential to recycle these resources, which can be used as the main heat sources for NUH and industrial heating. In terms of the efficiency and economy of waste heat recycling, thermal power, and nuclear power have outstanding advantages. Waste heat from the cold end of a steam turbine can be recycled effectively by such means as extracting steam from the steam turbine, raising the backpressure of exhaust steam, and local provision of additional heat pumps.

According to the analysis of the supply–demand balance in the power system, as the seasonal peak shaving power supplies in the future power supply structure under carbon neutrality, thermal power plants will need an installed capacity of about 500 million kW. The corresponding waste heat emissions will be close to 5 billion GJ. The waste heat from these thermal power plants may serve as important zero-carbon heat sources in the future.

Another main source of waste heat emissions is coastal nuclear power. Although the future installed capacity of nuclear power will reach only 200 million kW, the annual waste heat emissions will be more than 7 billion GJ. This waste heat, if emitted directly, will cause thermal pollution to the surrounding environment. This waste heat from nuclear power plants can be utilized to make up for the shortage of zero-carbon heat sources in coastal areas and alleviate pollution to the coastal ecology. All of the above-mentioned waste heat from thermal and nuclear power plants, if utilized, can offer approximately 12 billion GJ of heat per year and thus can serve as the main heat source to provide zero-carbon low-grade heat for NUH and industrial heating in China.

There is also plenty of waste heat available in the industrial field. In the future, the steel, non-ferrous, and building material industries will be scaled down but still retained in a sufficient proportion, while the chemical industry, as the primary provider of various materials, will develop enormously. A lot of waste heat will be emitted from the production processes in these industries. In addition, some emerging industries will also discharge a large amount of waste heat. For example, data centers that have grown fast in recent years will emit nearly 2 billion GJ of waste heat in the future. The considerable amounts of heat emitted from waste incineration in each city can also be recycled.

The above-mentioned industrial and other waste heat, if recovered, will provide at least about 7 billion GJ of heat per year. Compared with the waste heat from power plants, this waste heat is more scattered and more expensive to recycle, but its temperature is between 30 °C to 50 °C and is easier to recycle than geothermal and ambient air sources.

In the future, the industrial heat demands in China can be classified by grade. The demand for high-temperature heat above 150 °C is mostly concentrated in the process industries, at about 5 billion GJ per year; the demand for low-temperature heat below 150 °C is mainly from the non-process industries, at about 7.6 billion GJ per year.

In the future, the floor space of NUH in China will be close to 22 billion m². With the gradual promotion of building energy efficiency, the building heating demands will reach about 5.4 billion GJ in total.

In conclusion, the heat required for industrial heating (below 150 °C) and NUH will be 13 billion GJ. In terms of the balance between supply and demand, the amount of waste heat resources will be greater than the heating demand. Hence, the waste heat can serve as the main heat source to address the needs of industrial heating (below 150 °C) and NUH.

Moreover, when wind and solar PV power grow up and become the main power sources according to the planning for zero-carbon electricity, there will be certain wind and solar curtailments in spring and autumn. Based on the current patterns of seasonal changes in electricity consumption, it is reckoned that the optimal range of wind and solar curtailments will be 5–8%.

The future wind and solar PV power in China will total 9 PWh, and even the 5% wind and solar curtailments will reach 0.45 PWh. How to deal with so much curtailed wind and solar PV power at a low cost is also an issue that must be taken seriously.

If this portion of electricity is directly converted into heat, the regulation and storage of the heat through seasonal heat storage to supply heat for building heating and industrial production may be the best solution in terms of comprehensive economy. The curtailed wind and solar PV power may provide up to 2 billion GJ of heat and may also play an important role in the future zero-carbon heat source system.

7.3 Construction of the Low-Carbon Heating Mode Dominated by Waste Heat Utilization

The urban low-carbon heating mode means the utilization of the waste heat generated in various production processes to meet the needs of urban and industrial heating and thus improve energy efficiency and reduce carbon emissions. However, to realize this mode, it is necessary to overcome the three mismatches below:

First, a temporal mismatch between the waste heat resource and the heating load. There are obvious differences in terms of season, daytime, and nighttime because the waste heat emissions from such sources as power plants and factories vary with production conditions and urban heating is primarily affected by climate.

Second, a spatial mismatch between the waste heat source and the heating load. To avoid environmental pollution, large thermal power plants, nuclear power plants, and other sources of high-temperature waste heat are usually far away from urban centers, but the traditional transfer methods have limitations on distance. How to transfer the distant waste heat to cities is a technological challenge.

Third, a temperature mismatch between the waste heat resource and the heating demand. Most of the waste heat resources are discharged at a lower temperature, in the 20–50 °C range, but the heating networks require a larger temperature difference between the supply and return water to ensure the transfer capacity; besides, the end consumers have different temperature requirements, for example, buildings require a temperature of about 40–60 °C, and industrial consumers require higher temperatures; and factors including volume utilization rate and safety also need to be considered for the seasonal heat storage system. The temperature difference between systems needs to be coordinated.

To sum up, the key to the construction of the urban low-carbon heating mode dominated by waste heat utilization lies in solving the three mismatches, and on this basis, multiple energy forms can be complemented by each other and integrated together.

7.3.1 Storing Heat to Solve the Temporal Mismatch Between Waste Heat and Heating

We can use the thermal, nuclear, and industrial waste heat and the spring wind and solar PV power to supply heat. However, these resources are unstable and change with the generating capacity and production output, and the heating demand also changes with the season and air temperature. For instance, thermal power plants discharge more waste heat during peak shaving in winter and summer, while nuclear power plants and some industries emit waste heat throughout the year. However, heating is required only in winter and needs to be regulated depending on air temperature. To solve the temporal mismatch, we need to accomplish seasonal heat storage. Furthermore, the abandoned wind and solar PV power in spring and autumn can also be converted into heat for storage, which also needs seasonal heat storage.

One seasonal heat storage method is to build large water storage tanks with heat-insulating top covers. Stored water is layered naturally, with the high-temperature hot water at the top and the low-temperature cold water at the bottom. When there is excess heat, the bottom cold water is extracted, heated, and then stored at the top. When heating is required, the top hot water is extracted for heat release and then sent back to the bottom. This enables the storage and utilization of heat. This method has been used for solar heating in Northern European countries.

There may be concern about whether seasonal heat storage is cost-effective, whether lots of heat is lost during long-time heat storage, whether a heavy investment in heat storage equipment leads to an increase in cost, etc. If the calculation is done based on a water temperature of 90/20 °C, the investment in heat regenerators will be only RMB 3 per kWh, but the investment in chemical cells will reach up to RMB 1,000 per kWh, indicating a difference of more than 300 times between the two. Even if the method of increasing heat by 6 times via heat pumps after electricity storage in cells is considered, the cost difference between the two will be more than 50 times. Hence, heat storage is more economical. Furthermore, the cost of heat storage will drop with the growth of scale, because the per unit surface area gets smaller and the investment per unit volume gets lower when the heat regenerator grows in size. With regard to heat loss during long-time heat storage, the Fourier number reflecting the unsteady-state heat transfer of the heat regenerator is directly proportional to time and inversely proportional to the square of the scale. When the scale is increased by a factor of 10, the time will be magnified by a factor of 100, so a half-year will not be a very long period of heat storage. Therefore, a heat regenerator that is big enough in volume will have a relatively small loss of heat even in seasonal heat storage. In addition, an increase in the difference in heat storage temperature may also lead to an increase in the density and efficiency of heat storage and is favorable to heat transfer and the recovery of industrial waste heat. Therefore, we should use seasonal water storage tanks and lower the return water temperature to below 20 °C when building a new zero-carbon heating system.

The investment and operation costs of the heat regenerator are about RMB 50–90 per GJ, and the waste heat in the non-heating period will be wasted if there is no seasonal heat storage. The waste heat is zero-cost, and it is cost-effective if the cost of heat storage is lower than that of the conventional heat source. The cost of RMB 50–90 per GJ is equivalent to that of a natural gas-fired boiler, so heat storage is economically acceptable. Besides, the replacement of natural gas-fired boilers with heat storage can also help reduce carbon emissions. From a safety point of view, natural gas may be insufficient in winter, but heat storage facilities may release heat at any time and thus be more reliable. Seasonal heat storage facilities, if any, can not only address the seasonal imbalance between supply and demand but also eliminate the contradiction between supply and demand resulting from the intraday waste heat fluctuations.

7.3.2 Developing the Large-Temperature-Difference and Long-Distance Transfer Technology to Solve the Spatial Mismatch in Waste Heat Utilization

Most coal-fired thermal power plants and boilers in northern cities are old and will be phased out. So, they need to be replaced by waste heat as the main heat source in cities. However, waste heat is often far from cities and needs to be transmitted through long-distance heating networks. There is a concern that this will cause such problems as high investment, high energy consumption, and losses. Long-distance heat transfer is economically feasible due to the following reasons: First, waste heat recycling projects are generally large in scale and require large-diameter transfer pipelines, whose cost per unit of heat is lower than that of small-diameter pipelines. Second, the large-temperature-difference heat transfer technology, namely reducing return water temperature to increase the temperature difference between supply and return water, is adopted to improve the transfer capacity and reduce the cost. The current water supply temperature is generally under 130 °C, and the return water temperature may be lowered to below 30 °C and even below 20 °C through the absorption heat exchange process. In this way, the temperature difference between supply and return water may be increased from 60 K to above 100 K, causing an increase in the transfer capacity by nearly 70% and thus a significant reduction in the long-distance transfer cost. Third, in terms of heat sources, the cost of waste heat recycling is lower than that of natural gas-fired boilers, coal-fired boilers, and even the extraction of steam from thermal power plants. Since the return water temperature is low, the waste heat can be recovered through direct heat exchange. As shown in Fig. 7.1, in a heating period of 4 months, the economical heating distance for the large-temperature-difference heating network exceeds 200 km compared with natural gas-fired boilers. The large-temperature-difference heating network is also economically feasible compared to other conventional methods within a distance of 80 km. If there are seasonal heat storage facilities on the heat user side, the pipelines can be operated based on the time for waste heat generation rather than the time of demand. In this way, the annual operation time and economy of pipelines can be improved (Fig. 7.2).

Fig. 7.2

Relationship between long-distance heating cost and transfer distance

Large-temperature-difference and long-distance heating projects have been widely applied in many cities, and the Gujiao-Taiyuan Large-Temperature-Difference and Long-Distance Heating Project is a typical example. In this project, the waste heat from the Gujiao Power Plant is used to heat the 76 million m² of buildings in Taiyuan City 40 km away and two long-distance pipelines with a diameter of 1.4 m are built, including a 15 km mountain tunnel. The investment in this project is RMB 6.7 billion, but the heat investment per square meter of buildings is equivalent to about RMB 90 only, the heat loss due to radiation and the energy consumption along the way are very low, and the comprehensive heating cost (transfer to urban areas) is less than RMB 40 per GJ, which is lower than that of coal-fired boilers. After the start of this project, similar projects have been implemented one after another in Yinchuan, Shijiazhuang, and other cities to utilize the waste heat from power plants for the realization of clean and low-carbon heating.

Fig. 7.3

Schematic diagram of combined heat and water technology

Since nuclear power plants are mostly distributed in coastal areas, the combined heat and water technology can be adopted to greatly reduce the cost of long-distance transfer of waste heat. This technology consists of three main parts, i.e. combined heat and water generation, combined heat and water transportation, and heat and water separation, as shown in Fig. 7.3. For combined heat and water generation, the extraction steam is utilized for the desalination of seawater, and then hot fresh water is generated and transmitted to cities. For combined heat and water transportation, fresh water and waste heat can be transmitted simultaneously with only one pipe. For heat and water separation, waste heat is extracted from the fresh water and released to the heating network through absorption heat exchange at the city end, and then the fresh water at a normal temperature is fed into the urban water supply system. This technology can not only solve the problem of water shortage in coastal cities but also reduce transmission costs. The economical transfer distance may reach 400 km compared with that for natural gas heating. According to analyses, the waste heat from nuclear power plants in northern coastal areas can be fully utilized to supply 5 billion m² of buildings with zero-carbon heat and to generate 4 billion tons of fresh water per year. This technology has been successfully operated and verified in the Haiyang Nuclear Power Plant and is ready for popularization and application.

7.3.3 Adopting the Temperature Change Technique to Solve the Temperature Mismatch in the Links of Waste Heat Utilization

To utilize waste heat for building and industrial heating, a unified heating network system needs to be established to connect multiple heat sources and sinks. To coordinate the demands of all aspects, the water supply temperature may be set at 90–95 °C, and the return water temperature may be set at 20–25 °C. However, the temperature of waste heat varies, and the temperature requirements for heating purposes are also different. Therefore, the temperatures of water in the heating network need to be adjusted through some equipment when being accessed or used. The equipment mainly consists of heat exchangers and heat pumps. They can be used for raising or lowering the grade of hot water, generating high-pressure steam, etc. In this way, the temperature mismatches encountered in heat transfer can be solved.

Typical heat transfer and grade improvement may fall into the following four scenarios:

• (1) The average temperature of waste heat sources is higher than that of supply and return water in the heating network, and the temperature difference for heat transfer is sufficient;

  The waste heat from thermal and nuclear power is mainly from the exhaust steam of steam turbines and generally at a temperature of 30–50 °C and may be heated by the extraction steam to a temperature higher than that of water supply in the heating network. To this end, the waste heat from exhaust steam is recycled efficiently through multistep heating of the circulating water in the heating network. When the circulating water in the heating network is heated to a temperature higher than the above-mentioned range, the first solution is to raise the backpressure of the steam turbine to heat the water in the heating network. The water in the heating network may be directly heated to above 120 °C by the further use of the extraction steam at 0.3–1.0 MPa. This steam sacrifices a certain generating capacity for high-temperature heat to heat the heating network and is equivalent to an electric heat pump with a COP of 4–6. In addition, the extraction steam can also be used for driving the absorption heat pump to recover the waste heat from the exhaust steam. The circulating water is heated first to a relatively high temperature (for example, 90 °C) and then heated by the extraction steam to 120 °C, thus further improving the efficiency of waste heat utilization in the power plant. Also, a steam ejector may be used to recover the waste heat from the exhaust steam. Despite its functions similar to those of the absorption heat pump and its relatively low investment, the steam ejector is subject to great limitations by variable working conditions. For the utilization of waste heat from multiple steam turbines, the backpressure of the steam turbines may be raised in sequence to heat the return water in the heating network through series connections, then the absorption heat pump may be adopted selectively, and finally, the steam is extracted for heating. This multistep heating method may reduce energy consumption by 50% compared to the extraction steam for heating from traditional steam turbines and is equivalent to heating by a heat pump with a COP of 7–10.

  In the utilization of industrial waste heat, including waste heat from slag washing water in steel mills, the waste heat temperature may also be higher than the average temperature of supply and return water in the heating network. In this case, the temperature transducer based on the second-class absorption heat pump may be used to recover the waste heat and heat the circulating water in the heating network to a temperature higher than that of the waste heat; when necessary, the peak shaving heat source or electrically-driven heat pump may be combined to further heat the circulating water to a temperature required for water supply in the heating network.

• (2) The average temperature of waste heat sources is lower than that of supply and return water in the heating network, or the temperature difference for the heat transfer with the heating network is insufficient;

  A lot of low-temperature waste heat is generated in industrial production and may be raised to higher temperatures by heat pumps. The temperature and amount of industrial waste heat vary with industries; therefore the suitable methods of waste heat collection need to be selected on a case-by-case basis. If there is both industrial waste heat and the waste heat from a power plant in a heating system, we may first utilize the former for low-temperature heating and then utilize the latter for high-temperature heating, thus achieving the most optimized energy efficiency and economy.

• (3) The average temperature required on the demand side is lower than that of supply and return water in the heating network, and the temperature difference for heat transfer is sufficient;

  The required temperature of heat for building heating in northern China is low, but the temperature of hot water in the urban heating network is very high. We can use the absorption heat exchanger to reduce the temperature of return water in the heating network, use the temperature difference between the water supply of the primary network and that of the secondary network to drive the absorption heat pump, and lower the temperature of return water of the primary network to below 20 °C without consuming extra energy. This not only improves the transfer capacity of the heating network but also creates conditions for the recovery of waste heat. In the future, the decentralized temperature reduction method for returning water should be popularized, such as installing absorption heat exchangers in buildings.

• (4) The average temperature required on the demand side is higher than that of supply and return water in the heating network.

  High-temperature steam is required for industrial heating, but most of the current heat sources are coal and natural gas with high carbon emissions. To achieve zero carbon, we may consider the following methods:

  • Steam generation with electric boilers: This method is characterized by low efficiency, high electricity consumption, and high cost.

  • Extraction of low-temperature waste heat with electric heat pumps: In this method, air, geothermal energy, and other low-temperature energy in nature are used to drive electric heat pumps, which increase the water temperature to generate steam. However, this method has heavy investment and a low COP, and the natural heat sources at low temperatures have low grade and density, making them difficult to meet the industrial requirements for high grade and large capacity.

  • Steam generation and transmission to industrial consumers by thermal and nuclear power plants: In this method, clean energy from thermal and nuclear power plants can be utilized to generate high-temperature steam, which is transmitted to industrial consumers through steam pipelines. Nevertheless, it is difficult to transmit steam over long distances through steam pipelines due to large heat radiation and pressure loss.

Hence, we propose a new method of industrial heating: The waste heat from thermal and nuclear power plants and the industrial waste heat are collected, transferred to industrial consumers over long distances through circulating hot water, and subject to temperature change at the user end to make steam. The specific operation is as follows:

• Heat water to above 90 °C at the collection locations of the waste heat from thermal and nuclear power plants and the industrial waste heat, and then transmit the water through circulating long-distance pipe networks to industrial consumers;

• Generate steam of different grades by temperature rise and flashing in stages or by compression after flashing at industrial consumers depending on demand;

• Return the remaining low-temperature circulating water to the collection locations of the waste heat from thermal and nuclear power plants and the industrial waste heat for reheating.

This new heating method has the following advantages:

• Zero-carbon emissions can be achieved;

• The energy efficiency and heating economy of the system can be improved;

• Heating parameters can be adjusted flexibly based on the requirements of different consumers;

• The investment cost can be reduced by sharing some facilities with urban heating.

For the non-process industry in northern China, this new heating method is more advantageous.

The grade and pressure requirements vary in industrial heating, so we can also adopt different heating methods based on the practical situation:

• Low-grade heating demand can be met by waste heat and heat pumps locally, thus saving the cost of long-distance transfer;

• High-temperature and high-pressure steam can be transmitted directly by thermal and nuclear power plants but is subject to distance limitations;

• New industrial projects with high-pressure steam requirements may be built near nuclear power plants or use biomass boilers, electric boilers, and other clean energy;

• The chemical industry demanding a lot of stable and high-parameter steam may consider the direct supply of steam from modular nuclear reactors.

These methods are conducive to realizing zero carbon emissions and carbon emission reduction in industrial heating.

7.4 Mode and Planning of Low-Carbon Heating Dominated by Waste Heat

Based on the foregoing analyses, we establish a low-carbon heating mode dominated by waste heat. This mode has five characteristics, namely waste heat utilization, long-distance heat supply, low-temperature return water, heat and power coordination, and heat storage for peak shaving

• (1) Recovery of waste heat. Make full use of various waste heat.

• (2) Low-temperature return water. Reduce the temperature of return water in the heating network to efficiently utilize waste heat and improve the transfer capacity of the heating network.

• (3) Long-distance heat supply. Use the large-temperature-difference and long-distance transfer technology and the combined heat and water technology to supply cities with heat at a low cost.

• (4) Heat and power coordination. Utilize heat storage to realize the mutual coordination and support between the heating network and the power grid.

• (5) Heat storage for peak shaving. Seasonal heat storage replaces fossil-fuel boilers for peak shaving and serves as a safe standby.

In the future, the development of the urban heating system in China will be dominated by waste heat utilization. However, waste heat utilization is not only affected by the total amount of resources but also limited by factors including spatial distribution and the difficulty in collecting waste heat. Therefore, urban heating should be planned based on specific cases.

Table 7.1 shows the heat supply of future heat sources. The waste heat from thermal and nuclear power, which has the advantages of high efficiency, low cost, and easy implementation, shall receive priority in recycling. Industrial waste heat including waste heat from the process industry, data center, and power transformer shall be recycled. Wind and solar PV power curtailed in non-heating seasons can be converted into heat. The consumption of electric energy is required in centralized heating, which mainly involves the electric energy required for making steam for non-process industries, the electric energy required for the collection and transmission of industrial waste heat, and the electric energy required for a few decentralized heating methods such as electric boilers. For civil heating, air source, ground source, and other decentralized heating methods may be used in regions that cannot be covered.

Table 7.1 Composition of heat source for future urban heating in China

China’s coastal areas are dotted with many nuclear power plants and large thermal power plants. The eastern coastal areas and the surrounding areas are developed areas and also the gathering places of industries and populations in China, with concentrated demands for industrial and domestic heating. Therefore, for one thing, based on the industrial layout planning, petrochemical and other consumers with high heat consumption and a great demand for high-parameter steam are built near nuclear power plants. The waste heat from nuclear power plants is converted directly into steam for transmission to these industrial consumers of high-pressure steam. For another, the long-distance heat supply technology is adopted to transfer the waste heat through hot water transmission to a wider range of areas to meet the non-process industrial heating (above 150 °C) and urban heating demands. In combination with the characteristic that power plants are distributed along the coast, the combined heat and water technology may be utilized to increase the economical transfer distance of the waste heat from these power plants to over 300 km. Based on the coastal nuclear power layout and planning, this economical transfer distance may be utilized to radiate the waste heat from nuclear power plants to cities and large industrial heat consumers in coastal areas and even inland areas. Thus the heating demand for industrial and civil purposes in eastern China will be met. Moreover, the 10 billion tons of desalinated seawater supply per year will effectively relieve the water shortage in eastern China. In areas south of Shandong, heating is mainly to meet the industrial demands for steam; while in areas north of Shandong (included), heating is to meet both domestic and industrial demands. The eastern coastal areas and the areas further inland have the densest population and industries in China, and the majority of industrial and civil heating can be supplied by the waste heat from nuclear power plants.

Heating in the central and western regions of China relies more on the waste heat emitted from thermal power plants and the process industry. Thermal power plants are uniformly distributed in the central and western regions. The waste heat from thermal power plants can serve as the main heat source, which can be utilized together with the waste heat from other process industries of a certain scale and transferred through the long-distance hot water networks to meet the main demands of central and western provinces and cities demand low-carbon heating. In Jilin and Heilongjiang provinces in northeast China, the capacity of thermal power plants at present is relatively small, and new thermal power plants with a certain capacity may be built in the future to utilize the abundant biomass resources in this region as the fuel to achieve zero-carbon electricity generation and heating; also, the construction of nuclear power plants with a proper capacity may be considered in this region for mutual support with the utilization of the waste heat from thermal power, thus achieving zero-carbon heating for urban areas in this region (Fig. 7.4).

Fig. 7.4

Time sequence of development in the urban heating plan

In terms of the time sequence of construction, as shown in Fig. 7.4, although thermal power plants are gradually transformed into peak shaving power sources for power grids, they will remain one of the main power sources for the next 10 to 20 years and discharge a lot of waste heat, which can be recovered at a low cost and with little difficulty. Thus, by 2035, the top priority of low-carbon heating will be the deep recovery of the waste heat from thermal power plants to replace the traditional coal-fired boilers and medium- and small-sized thermal power plants. In addition to the utilization of waste heat from exhaust steam, natural gas-fired power plants have more need to recover the flue gas heat with greater potential. Independent natural gas-fired boilers will be used to recover the flue gas heat in the short term and will be gradually shut down in the long term. The qualified gas-fired boilers (near the heating networks) will be interconnected with the heating networks for peak shaving for the heating networks and will be phased out with the development of seasonal heat storage. For future heating, the increasing heat demand will be met mainly through the recovery of the low-grade waste heat of existing heat sources, and new heat sources will be built as few as possible. The projects for comprehensive recovery of waste heat from nuclear power are combined with the time sequence of construction of seasonal heat storage. The 14th Five-Year Plan period will witness the implementation of industrial heating and projects that combine large-scale heating with seasonal heat storage. The heating mode based on the waste heat from nuclear power will be popularized and applied comprehensively from the start of the 15th Five-Year Plan. Non-process industrial heat sources and heating networks are constructed to replace the existing small coal-fired power plants, mainly in Shandong, Jiangsu, Zhejiang, Fujian, and other provinces where non-process industries are concentrated. They have been constructed in succession since the start of the 14th Five-Year Plan. In the link of the heating network, comprehensive reconstruction of heating networks is carried out to reduce the return water temperature. The return water temperature will be lowered to below 20 °C through the installation of absorption heat exchangers or some electric heat pumps at terminals.

Coal-fired and gas-fired boilers as the main heat sources at present will be fully shut down by 2035. From 2035 onward, the power system will have the coal-fired power plants shut down gradually, or have their operation time reduced significantly, and the proportion of heating by the waste heat from thermal power plants will also be reduced gradually. Then, large seasonal heat storage can be built on a large scale to store the low-grade waste heat from non-heating seasons for a supplement. The utilization of the waste heat from nuclear power will be promoted comprehensively with the popularization of seasonal heat storage and the continual improvement of the combined heat and water technology. The overall reduction of the supply and return water temperatures in the urban heating networks will enable the utilization of industrial waste heat to become more economical and to develop on a large scale. Meanwhile, renewable energy generation will gradually become dominant. A significant amount of curtailed wind and solar PV power will be converted into heat for urban heating, and coupled with heat storage, the utilization of curtailed wind and solar PV power will contribute over 10% of the heat after 2035.

The heating system is expected to achieve carbon neutrality in all respects by 2050. At that time, low-density buildings that are difficult for heating networks to cover will adopt air-source and ground-source heat pumps and other decentralized heating methods for heating; apart from this, the heat sources for NUH and industrial heating (below 150 °C) in China will be mainly the waste heat from thermal power, nuclear power, and other industries, and the heat converted from the electricity consumed for recycling the waste heat and from the curtailed wind and solar PV power.