Multi-scenario research on the low-carbon transition in energy-deficient cities-view from the power supply side

Cities are the places where power supply demand is highly concentrated. How to ensure power supply security while realizing low-carbon transition is the key challenge faced by the construction of new power systems. In large cities with a shortage of local power generation resources, this problem is particularly prominent. The study takes Guangzhou city as a case study area, which has the typical characteristics of “serious energy shortage and extremely high-power load density” in China. This study explores the differences in power supply capacity, structure, carbon emissions, and power supply costs under the six different scenarios set up from the power supply side for future power supply structures. The optimal power supply modes have been matched for cities with different development characteristics and needs. The change in power supply structure and the realization path under the optimal power supply modes are discussed. The results show that the power supply capacity of Guangzhou is constantly improving under different scenarios, with the highest self-sufficiency rates of power production capacity and power production quantity reaching 80% and 50%, respectively, by 2030. Despite this, Guangzhou still does not have the basic conditions for building a new power supply system through the development of local high-proportion renewable energy only due to the limitation of resource endowment. The combination with increasing the ratio of renewable energy in outsourcing power is an inevitable choice for low-carbon transition for the city. This study provides a new idea on how to build a new power supply system for an energy shortage city. It can be used as a reference for safe power supply and low-carbon transition for the power supply sectors of other cities.


Introduction
China is undergoing a major low-carbon transition to ensure the low-carbon transition of the energy system and achieve the target of "carbon peaking by 2030 and carbon neutrality by 2060" (hereinafter referred to as dual carbon targets).The power industry is the main force of transformation.Power sources will play a crucial role and have a significant impact on the overall evolution process of the new power supply system.Therefore, extensive and in-depth theoretical research is needed.
Domestic and foreign scholars have carried out a heated discussion on whether the power side should transition to a high proportion of renewable energy or to a combination of low-carbon technologies.Some studies suggest that using a very high proportion of renewable energy (mainly wind and solar) can achieve deep emissions reductions in the power system.For example, studies have mentioned that using 100% wind-water-solar energy in all energy sectors in 139 countries would prevent global warming of 1.5°C and millions of deaths from air pollution each year by 2050, thereby reducing social energy costs.These roadmaps, although far more radical than those required by the Paris Agreement, are still technically and economically feasible [1][2][3][4].Some other studies support a deep transition of the power sector through the use of a combination of low-carbon technologies.These studies do not completely deny the transition path to a high proportion of renewable energy.Instead, they analyze the shortcomings of this kind of path and suggest that relying on a wider range of low-carbon technologies would be the best approach to the deep power transition [5].
BP World Energy Resources Out-look [6] released by BP PLC in 2023 pointed out that security was placed in a more prominent position in the adjustment of the global energy supply pattern.Ensuring the safe and reliable operation of the new power supply system is the most critical test and a major issue that the new energy system must face with caution [7].It is also an effective breakthrough point to solve the difficult problem of " energy trilemma".Domestic and international researchers have also carried out some studies on power supply security from the perspectives of power supply safety, power grid security, power demand guarantee, power technology support, mechanism design, etc. [8][9][10][11][12][13][14][15].However, some shortcomings also exist in these studies.For example, in most cases, these studies emphasized more on optimizing or improving the security and flexibility of the power supply operation through different technologies and methods.However, they lack adequate consideration of the endowments of various types of power generation resources and the connection with governmental planning.Its practical application value was also insufficiently explored.
Guangzhou, the capital of Guangdong Province in southern China (see Figure 1 for its geographical location), is one of the largest energy-consuming mega-cities in China and a typical energy input city as well.Fossil energy, such as coal, oil, and natural gas, relies on domestic supplies from other provinces or importation.The local renewable energy endowment is poor.The development potential of renewable energy sources other than solar energy is currently close to saturation.The local power production capacity is relatively limited.In 2022, the power consumption of Guangzhou was 115.5 billion kilowatts, with a self-sufficiency rate of 46.55% in power supply capacity and a selfsufficiency rate of only 31.63% in power supply quantity.It has the typical characteristics of severe energy deficiency and extremely high-power load density.
The study takes Guangzhou city as a case study area, which has the typical characteristics of "serious energy shortage and extremely high-power load density" in China.This study explores how cities with scarce resources can realize a low-carbon transition and build up their new power supply systems while ensuring the security of their power supply.The aim is to seek a breakthrough to build a new type of power system.added: -3% system in social fields of Guangzhou"

Urban and rural households
Average growth rate of permanent population: 2.4% Change of per capita electricity consumption: 23% "The 14th Five-Year Plan for Guangzhou's National economic and social Development and the outline of the vision goals for 2035" and "The 14th Five-Year Plan for population development and construction of public service system in social fields of Guangzhou" 2.2.Scenarios prediction of power supply and demand

Setting up scenario
Based on the different combinations of variations of the installed equipment structure, adoption of carbon reduction measures at the terminals, adoption of the technologies for energy efficiency improvement, and the phased development of power supply from the power supply side, six different power supply scenarios were set up.The detailed scenario descriptions are given in Table 2.
Table 2. Scenarios and description.

Business as Usual (BAU)
All kinds of power generation installations are constructed in accordance with the policy documents and development plans issued in Guangzhou, and all power generation technologies remain at the current level.

Renewable energy promotion (A1)
Some ratio of new installed capacity of renewable energy is added on the basis of BAU scenario.All power generation technologies remain at the current level, and no further carbon capture and storage (CCS) measures or ultra-low emission measure are introduced.

Technological progress (A2)
The generation hours and equipment energy efficiency are further improved, with the introduction of the measures of CCS and ultra-low emission on the basis of the BAU scenario.

Phased development of power supply security and low-carbon emissions (A4)
On the basis of the power supply development characteristics of Guangzhou in the post-pandemic period and under the constraint of the target of carbon peaking, the main goal of the "14th Five-Year Plan" is to ensure power supply security.The goal of the "15th Five-Year Plan" is to achieve carbon peaking and carbon emission reduction.Most of the planned installation of thermal power units will be completed in the "14th Five-Year Plan", and the planned installation of renewable energy is postponed to the "15th Five-Year Plan".CCS and ultra-low emission measures will be introduced.

Self-sufficiency rates of both power production capacity and power production quantity (A5)
On the basis of the technological progress scenario (A2), the power supply security is further enhanced, and the self-sufficiency rate of power supply quantity is increased to 50%.Other parameters remain the same as those in A2.

Setting up scenario parameters
Based on the characteristics of each scenario and its specific description, the parameters of different scenarios in 2023-2030 are set up from the perspectives of installed capacity, energy efficiency improvement, energy saving and emission reduction measures, etc.The settings and the reference basis are shown in Table 3.

Business as Usual (BAU)
(1) Installed capacity: All types of installed power-generation capacity are constructed in accordance with the planning of the government of Guangzhou (2) Energy efficiency of power generation: same as that in 2022.
(3) Measures for energy saving and reduction of emissions: same as that in 2022.

Renewable energy promotion (A1)
(1) Installed capacity: the solar photovoltaic installation will be increased by three times from 2022.
(2) Energy efficiency of power generation: same as that in 2022.
(3) Measures for energy saving and reduction of emissions: same as that in 2022.

Technological progress (A2)
(1) Installed capacity: same as that in BAU scenario (2) Energy efficiency of power generation: 1) Energy consumption of power generation: The unit consumption of coalfired power units will be improved to the average national benchmark level of 303.83 grams of standard coal per kilowatt hour in 2025.By 2030, it will be improved to the average national benchmark level of 292.5 grams of standard coal per kilowatt hour.
2) Power generation hours: The power generation hours are assumed to be increased by 5% in 2025 on the basis of that in 2022 for various types of power generation and by 10% by 2030.
(3) The measures of energy saving and reduction of carbon emissions: Carbon emissions will be reduced by 4% from the original emissions by 2025, and by 15% by 2030.The annual reduction of carbon emissions is shown in Table 4.

Combination of the scenarios of technological progress and renewable energy promotion (A3)
(1) Installed capacity: same as that in A1 Other parameters are the same as that in A2.

Phased development of power supply security and low-carbon emissions (A4)
(1) The installed capacity of gas-fired power generation planned by the government for 2028, that is, 12.74 million kW under the BAU scenario will be completed by 2025.By 2027, all the installed capacity of gas-fired power generation planned for 2030, that is, 14.15 million kW, will be completed.The planned installation of renewable energy is postponed to the "15th Five-Year Plan", and the new additional installation is PV power generation.
(2) Energy efficiency of power generation: same as that in scenario A2 (3) The measures of energy saving and reduction of carbon emissions: same as that in scenario A2.

Self-sufficiency rates of both power production capacity and power production quantity (A5)
(1) Installed capacity: In order to increase the self-sufficiency rate of power supply quantity to 50%, the installed capacity of various types of power generation will increase by 16.38% on the basis of the BAU scenario.It is assumed that through the efforts in the period of the 14th Five Year Plan, 16.38% of the new increase of coal-fired power generation capacity will be installed and put into operation after 2025.We assume that through the efforts in the period of the 14th Five Year Plan, 16.38% of the new increase in coalfired power will be completed and put into operation after 2025.
(2) Energy efficiency of power generation: same as that in scenario A2.
(3) The measures of energy saving and reduction of carbon emissions: same as that in scenario A2.

Calculation of CO2 emissions
The CO2 emissions from local power generation are calculated using the conversion coefficient between standard coal and CO2 (2.69 tons of CO2/ton of standard coal)."Notice of Guangdong Provincial Development and Reform Commission on Issuing the Accounting Methods for Carbon Dioxide Emissions from the Cities Above Prefecture Level in Guang-dong Province" was adopted as the carbon emission factors of outsourcing power from 2022 to 2030 (Table 5).

Calculation of power generation cost
The costs used in this paper were mainly referenced from the following domestic research literature, and the average values were adopted for calculation (Table 6).

Data Source
The data in this paper were adopted from public documents or statistical materials such as the national, Guangdong Province, Guangzhou Statistical Yearbook, and government planning.Some of the data (mostly power generation costs) were adopted from the summary of the research achievements of the domestic researchers.

Results and discussion
3.1.Results

Power supply and its structure.
(1) Power supply The power supply of Guangzhou comes from the local power generation and the electricity purchased.In terms of the power supply from the local generation, the power supply from different scenarios increases year by year (Figure 2).The power generation is in the order of A5>A4>A3>A2>A1>BAU from most to least.By 2030, the power generation under scenario A5 will be up to 78.6 billion kWh.A self-sufficiency rate of 50.3% of the power supply quantity, and the power generation under the BAU scenario will be 61 billion kWh.Despite the year-by-year increase in local power generation, driven by a faster increase in electricity demand, the demand for outsourcing power is also showing an upward trend (Figure 2).The demand for outsourcing power is in the order of A5<A4<A3<A2<A1<BAU, exactly reverse to that of the local power generation.(2) Local power supply structure From the perspective of the structure of local power generation (Figure 3), although the installed capacity of gas-fired power exceeded that of coal-fired power in 2020, the quantity of generated gasfired power has not overtaken that of coal-fired power to become the main source of power supply until 2024.Under the six scenarios, the coal-fired power generation remains basically unchanged, but with the ratio decreasing year by year, from 52% in 2022 to about 30% in 2030.The ratio of clean energy generation other than coal-fired power is increasing every year, and the structure of the local power generation is also getting cleaner.(3) Power supply security According to the governmental plan of Guangzhou, the self-sufficiency rate of power supply capacity will reach 60% in 2025.The self-sufficiency rates of power supply capacity of all six scenarios are above 60% (Figure 4), indicating that the infrastructure construction of Guangzhou for the self-sufficiency target of power supply capacity can be overfulfilled against the planning.Especially in scenario A5, by 2030, the self-sufficiency rate of power supply capacity will reach nearly 80%, while the self-sufficiency rate of power supply quantity will increase from 31.63% in 2022 to 50.3%.Compared with the other scenarios, A5 will become the safest power supply mode.

Carbon emissions from power generation
(1) Total carbon emissions from the power sector The total carbon emissions from the power sector include those from local power generation and from outsourcing power (Figure 5).In general, the total carbon emissions (Figure 6) in scenario BAU are the highest from 2022 to 2030 and show an increasing trend year by year, followed by A1.In both scenarios, carbon emissions are rising every year, showing no signs of reaching peak values.In the other four scenarios, peak values will be reached, but with different peaking times and peak values.Among them, the carbon emissions of scenarios A2, A3, and A5 will reach peak values in 2028, with peak values of 59.92, 59.55, and 62.17 million tons of CO2, respectively.The carbon emissions of scenario A4 will reach a peak value one year earlier, with a peak value of 60.91 million tons of CO2.(2) Potential of carbon emission reduction Compared with scenario BAU, scenario A3 has the greatest potential for carbon reduction and shows an increasing trend year by year, with a reduction of carbon emissions of 7.07 million tons of CO2 by 2030 (Figure 7), accounting for 10.7% of the carbon emissions.The next is scenario A2, with a reduction ratio of 9.74% and a reduction of 6.43 million tons of CO2 by 2030.As scenario A4 develops in stages, the carbon emissions of scenario A4 are higher than scenario BAU from 2022 to 2025.After that, it will decrease rapidly.By 2030, the emission reduction and the reduction ratio will be in line with scenario A2. Figure 7. Emission reduction potential in all scenarios from 2022 to 2030.

Cost of power generation.
The total power supply cost consists of the cost of local power generation and the cost of outsourcing power.In terms of the cost of local power generation (Figure 8), the power generation costs of scenarios A2, A3, and A5 increase every year, while the costs of the other scenarios increase first and then decrease, in the order of A5>A4>A3>A2>A1>BAU.In terms of total power supply cost, the power generation cost in all scenarios from 2023 to 2030 will increase year by year, in the order of A2>A3>A4>A5>BAU>A1.

Characteristics of power supply in different scenarios.
By comprehensively comparing the power supply capacity, power supply security, carbon emissions, and power supply economy under the above six scenarios (Figure 9), we have the following findings.
The local power generation is the lowest, and the dependence on the external power supply is the highest in scenario BAU, with characteristics of "low power generation, high carbon emissions, low security, and low generation cost".The power generation capacity of scenario A1 is slightly higher than that of scenario BAU, with characteristics of "low power generation, high carbon emissions, low security, and low generation cost".Both scenarios A2 and A3 focus on improving power generation quantity and power generation efficiency through technological progress while adopting terminal carbon emission reduction technology.The general characteristics of these two scenarios can be summarized as "moderate power generation, low carbon emissions, medium security, and high cost".Both scenarios A4 and A5 pay attention to policy factors, laying stress on the realization of carbon reduction targets and power supply security, the improvement of power generation quantity and efficiency, and the adoption of terminal carbon emission reduction technologies.In the period from 2022 to 2025, scenario A4 has the characteristics of "high power generation, high carbon emissions, high power supply security, and high cost".In the period from 2026 to 2030, the general characteristics of this stage are "high power generation, moderate carbon emissions, high power supply security, and high cost".Whereas scenario A5 still keeps the characteristics of "high power generation, moderate carbon emissions, high power supply security, and moderate cost".Note: The larger the value of the power generation and power supply security index, the better, while the smaller the carbon emissions and the cost index, the better.

The construction path of new power systems in energy-deficient cities.
Scenarios A3, A4, and A5 are the optimal transition paths of the best power supply mode we have selected.In these three scenarios, the installed capacities of the power supply have gone through the following evolutionary path (Figure 10): In scenario A3, it is necessary for the city government to make greater efforts to develop renewable energy.PV power generation has the greatest potential, with a threefold increase in installed capacities as compared with that in 2022.In addition, 160, 000 kW of the installed capacity of wind power is under construction as planned by the city government, which will increase the ratio of renewable energy capacity from 19.9% in 2022 to 23.2% in 2030.The installed capacity of coal-fired power will remain unchanged at 3.68 million kW.The installed capacity of gas-fired power will increase from 4.63 million kW in 2022 to 14.16 million kW by 2030.
In scenario A4, following the principle of "ensuring power supply security in the 14th Five Year Plan period and achieving carbon peaking in the 15th Five Year Plan period", the installed capacity of gas-fired power planned by the city government for 2028 under scenario BAU will be completed by 2025, which is 12.7419 million kW.Before 2027, all the 14.1619 million kW installed capacity of gasfired power planned for 2030 will be completed.The planned installation capacity of renewable energy will be postponed and will be completed in the "15th Five Year Plan" period.The newly installed capacity of renewable power is mainly for photovoltaic power generation.
Scenario A5 requires an increase of 16.38% of the installed capacity for various types of power generation on the basis of scenario BAU.This means that the total installed capacity will increase from 10.62 million kW in 2022 to 24.78 million kW in 2030.Due to the strict approval requirements for a coal-fired power project in China, in order to successfully pass the approval process, the new facilities for coal-fired power generation have to be mainly the most advanced supercritical and ultrasupercritical power generation units equipped with CCS technology.

Conclusions
(1) The current installed power capacity of Guangzhou can ensure that the self-sufficiency rate of power supply capacity reaches 60% of the planned target and the self-sufficiency rate of power supply quantity reaches about 40%.In order to increase the self-sufficiency rate of local power supply quantity to 50%, Guangzhou needs to increase the installed power capacity by 16.38% on the basis of the current installation plan.The annual cost increase is only 0.2-1% compared with that of scenario BAU, which is a safe and economical power supply mode.
(2) Guangzhou is in shortage of renewable energy, and the development potential of other renewable energy is close to saturation except for solar energy resources.It is impossible for Guangzhou to construct a new power supply system by developing a high proportion of local renewable energy.This means that Guangzhou itself does not have the basic conditions to establish a new power supply system.Local renewable energy plus clean thermal power is the critical path for the development and evolution of power sources in the future.The combination with increasing the proportion of purchased clean electricity is an inevitable choice for building a new power supply system in the future.
(3) The self-sufficiency rates of power supply capacities are high in scenarios A3, A4, and A5, and the power supply security is also high.The carbon emissions and power supply costs are at low and medium levels.The matching results for cities with different characteristics show that scenario A4 is the optimal power supply mode for the cities that have less consideration of power supply costs.For the cities that pay attention to the economy of power supply, the development mode of scenarios A3 and A4 can be taken into account.After considering the purchase of electricity, the A5 scenario can better balance the dual carbon goals, energy security, and power supply economy.

Figure 1 .
Figure 1.Geographic location of the Guangzhou city.

Figure 2 .
Figure 2. Changes in local power generation and outsourcing power in all scenarios from 2022 to 2030.

Figure 3 .
Figure 3. Changes of structure of local power generation in all scenarios from 2022 to 2030.

Figure 4 .
Figure 4. Changes of self-sufficiency rates of power supply capacity/quantity under all scenarios from 2022 to 2030.

Figure 5 .
Figure 5. Changes in carbon emissions from local power and outsourcing power from 2022 to 2030.

Figure 6 .
Figure 6.Changes in total carbon emissions from the power sector from 2022 to 2030.

Figure 8 .
Figure 8. Changes in trends of power generation cost in all scenarios from 2022 to 2030.

Figure 9 .
Figure 9. Radar map of four key indicators in the six scenarios.

Figure 10 .
Figure 10.Evolutionary path of the installed power capacities in scenarios A3, A4 and A5

Table 3 .
Setting of scenario parameters.

Table 4 .
Setting of the indicators related to energy efficiency improvement, energy conservation and reduction of carbon emissions.

Table 5 .
Average carbon emission factors of power supply in Guangdong Province in the period of 2022-2030.

Table 6 .
Costs of different types of power generation.