To achieve the goals of carbon peaking and carbon neutrality (the carbon peak and neutrality goals), China needs to complete the transition from fossil energy to zero-carbon energy. In this regard, cities are the “main battlefields” to reach the carbon peak and neutrality goals since most energy is consumed by buildings, transportation, and industry in cities and surroundings. How the urban energy supply system develops is crucial to achieving the carbon peak and neutrality goals. The urban energy supply system mainly meets the following demands:

  • Electricity demand, including electricity consumption of urban buildings, industry, transportation, and other municipal facilities;

  • Heating demand, including heating for the building and industrial sectors;

  • Natural gas demand, including natural gas for fuel, power, raw material, or supplementary electricity purposes.

The urban energy supply system consists of an electricity transmission and distribution system, a gas supply system, and a heat supply system. The three systems have been independent of each other for a long time. However, under the vision of carbon neutrality, the three systems will undergo radical changes in their structures and need to work together and support each other. The urban energy supply system must be studied and planned as a whole system to finish the overall task of low-carbon energy supply.

The urban energy system involves the transition from primary energy to end-use energy, the transmission and distribution from the source of supply to the demand side, and the reception and conversion at terminals. The possible carbon emissions will be generated mainly in the energy conversion process of the urban energy supply system. Natural gas becomes the main source of direct carbon emissions in the final consumption of urban energy. Avoiding the use of natural gas as much as possible is one of the important tasks to realize carbon neutrality.

In the field of natural gas application related to buildings, it is entirely possible to replace gas with electricity and heat. Cooking has seen a gradual trend toward the comprehensive development of electric cooking. There is no doubt that the way forward for producing domestic hot water (DHW) will be the replacement of gas with electricity, and the gas for both urban heating and industrial steam can also be substituted with electricity and heat. Natural gas, as the fuel for automobiles and other means of transport, will also be replaced with electricity.

Therefore, natural gas will be substituted in all of the above-mentioned areas. However, in specific places including factories and power plants, it is necessary to reserve a small amount of natural gas as a chemical raw material and to fuel thermal power plants for seasonal peak shaving and safe standby of power grids. Dedicated gas pipelines can be provided in these factories and power plants for them to use gas in a centralized manner. The gas pipe networks mainly oriented toward building demands in cities will be likely to be canceled gradually. Electricity and heat supplies will be at the core of the urban energy supply system. How to realize the zero-carbon transition of power generation and heat sources becomes critical to the comprehensive realization of the carbon neutrality goal for the urban energy supply system.

6.1 Power Transition for New Power System

In cities, with the gradual replacement of fossil energy, electric energy will be more widely used. Nevertheless, thermal power generation with high carbon emissions is still the major means of power generation in China. Therefore, the zero-carbon transition of power is the foundation for establishing a new power system and also the key to the realization of carbon neutrality for the urban power supply system.

The coal-fired power plants will be gradually replaced by wind power, solar PV power, hydropower, and other renewable energy as well as nuclear power. The obvious characteristic that distinguishes renewable energy generation from thermal power is that the generating capacity of the former depends on the natural climatic conditions, leading to a prominent contradiction between supply and demand. Cities, as the main consumers, guarantee the effective consumption of renewable energy generation through the dynamic change of their demands, which is core to realizing carbon neutrality.

The dynamic balance between electricity supply and demand can be divided into short-term balance and long-term balance. The short-term balance mainly refers to the supply–demand issue of intraday electricity. Some short-term problems of balancing electricity supply and demand can be solved effectively by multiple methods of energy storage. Moreover, a flexible building energy terminal that is formed by the connection of a building and an intelligent charging pile system with electric vehicles will largely relieve the short-period contradiction between supply and demand for the urban electric power supply system.

However, it is very difficult for the above-mentioned energy storage and regulation measures to relieve the long-term contradiction between electricity supply and demand. In China, on the whole, wind power is abundant in spring and winter, power generated by solar PV is plentiful in summer and scarce in winter, and hydropower resources are rich in summer and autumn. Nuclear power is suitable for carrying the base load. The supply–demand relationship between renewable energy generation and electrical power loads exhibits an obvious seasonal difference. The seasonal imbalance between electricity supply and demand can be addressed by the following four means: energy storage; a further increase in the installed capacity of zero-carbon power generation; the retention of some thermal power plants and the installation of Carbon Capture and Storage (CCS) systems; and the change in electricity demands.

Energy storage methods such as pumped storage and chemical battery energy storage require heavy investment and are uneconomical for long-term power regulation. One of the seasonal regulation methods is to develop regulated hydropower with a storage capacity to store the hydropower resources in the wet season for power generation in the dry season. This method has been adopted in Northern European countries to effectively solve the difficulty in seasonal peak shaving of electricity. However, in China, hydropower resources account for a relatively small proportion and can only solve less than a quarter of the seasonal regulation problems.

Increasing the installed capacity of wind power, solar PV power, etc. can make up for the deficiency in power generation during the period of peak power load in winter and summer. But excessive generation may also lead to more curtailment of wind/solar PV power in other seasons.

Some thermal power plants can be retained for peak load of electricity in winter and summer and use coal, natural gas, biomass, etc. as fuels. The carbon dioxide emitted after the burning of the fuels can be captured by CCS. Negative carbon emissions are achieved by using CCS to recover CO2 after power generation with biomass fuels. Therefore, as long as biomass fuels in thermal power reach a certain proportion, zero and even negative carbon emissions can be realized. Analyses indicate that both the investment and operation costs of the CCS system are lower than the costs of hydrogen generation and storage.

In addition, electricity consumption can be decreased in winter and summer and increased in spring and autumn. To reduce the peak power load in winter, heating methods with higher energy efficiency should be adopted as much as possible. The electricity consumption of heating heat pumps can be reduced effectively by utilizing the waste heat from thermal power plants, nuclear power plants, and other industries.

6.2 Challenges Facing Low-Carbon Transition of Heating

How to realize zero-carbon heating is another key issue in realizing the carbon neutrality goal for the urban energy supply system. At present, the great majority of heat sources consume fossil fuels. Only about 10% of them use electricity and industrial waste heat for heating. It is necessary to think hard about such questions as how to achieve the carbon peak and neutrality goals in the heating field, how to obtain zero-carbon heat sources, and how to follow the path of carbon neutrality.

Whether coal is to be retained is the first focus of concern for the industry. At present, there are still lots of coal-fired boilers. Under the carbon peak and neutrality goals, coal-fired boilers with low energy conversion efficiency and high carbon intensity are the first to be shut down and replaced. If coal-fired thermal power plants can effectively recover all waste heat emitted by them, their energy consumption and carbon emissions will decrease significantly compared to coal-fired boilers. Most authorities suggest that thermal power plants will still play an irreplaceable role in the power system, which means that thermal power plants will remain in place for a long time.

Although the state and local governments have vigorously promoted natural gas heating in recent years, the current scale is still relatively small due to two main reasons. One is the high cost of gas; the other is the insufficient security for the gas supply. Therefore, natural gas is not the future direction of development.

The constraint imposed by carbon neutrality on the future use of fossil energy makes electrified heating, including direct electric heating and electric heat pumps, highly valued. Direct electric heating is to convert electric energy directly into heat energy and requires a small investment, thus being beneficial to the consumption of curtailed wind and solar PV power. Electric heat pumps extract low-grade heat from the soil, air, rivers, lakes, and seas, with high energy efficiency.

However, due to the low density of the heat offered by low-temperature heat sources in the natural environment, it is difficult to meet the demands of urban buildings with a high floor area ratio. Therefore, this heating method is more applicable to rural areas with a low building density and southern cities where the density of heating loads is relatively low.

In 2019, the Building Energy Research Center (BERC), Tsinghua University proposed a “Clean Heating 2025” mode for northern urban heating (NUH). The main characteristics of the mode are as follows: First, heat sources are dominated by the abundant thermal power plants and other industrial waste heat in China; second, the low return water temperature is adopted during transmission in the heating network; third, heat and power coordination; fourth, natural gas-fired boilers are used as the heat source for peak shaving.

This clean low-carbon heating mode can reduce the carbon emissions from heating by 80% without a significant increase in the heating cost compared to heating with coal-fired and gas-fired boilers. This mode has been demonstrated in some Chinese cities, with a prominent effect of energy saving and emission reduction. However, this mode still cannot be treated as the final mode for the comprehensive realization of carbon neutrality in China because carbon dioxides are still emitted from the use of natural gas for peak shaving.

There are two paths to zero-carbon heating abroad, i.e. the centralized heating route in Northern European countries and the electric heat pump heating route adopted in the United States and other countries. Both paths make full use of the low-grade heat energy and utilize the relatively centralized low-grade heat energy whenever possible. Northern Europe witnesses the large-scale utilization of the waste heat emitted by biomass and garbage power plants, the industrial waste heat, the ocean thermal energy, and other thermal energy warmer than air, all of which, whether used directly or after being heated by heat pumps, need to be transmitted to buildings through heating networks. Since cities of a certain size in Northern European countries have set up pipe networks for heating, the heating networks can be fully utilized to transmit the above-mentioned centralized low-grade heat energy to consumers, thus realizing zero-carbon heating at a low cost. This is also the fifth-generation heating mode advocated in Northern Europe and other regions. For countries like the United States without heating network facilities, air source and ground source heat pumps and even direct electric heating are used generally, but considering the investment and operation costs, they are less economical than the low-grade heat transmitted in a centralized manner.

In the field of industrial heating, fossil energy is still dominant among heat sources. Steam generation with electric boilers has such problems as low energy efficiency and high cost. Industrial heating requires higher temperature parameters, but steam generation with air source and ground source electric heat pumps has a low Coefficient Of Performance (COP) and a high cost, thus it is very hard to adapt to the requirements of industrial heating.

In summary, from the perspective of heating, heat pumps can avoid the direct use of fossil energy but are subject to the low temperature and low density of heat available from low-temperature heat sources. Therefore, getting high-density low-temperature heat sources has become the key to the development of low-carbon heating.