The flexibility pathways for integrating renewable energy into China’s coal dominated power system: The case of Beijing-Tianjin-Hebei Region

Journal Pre-proof The flexibility pathways for integrating renewable energy into China's coal dominated power system: The case of Beijing-Tianjin-Hebei Region Jian Zhang, Yanan Zheng PII:

S0959-6526(19)33795-3

DOI:

https://doi.org/10.1016/j.jclepro.2019.118925

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JCLP 118925

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Journal of Cleaner Production

Received Date: 21 June 2019 Revised Date:

16 October 2019

Accepted Date: 16 October 2019

Please cite this article as: Zhang J, Zheng Y, The flexibility pathways for integrating renewable energy into China's coal dominated power system: The case of Beijing-Tianjin-Hebei Region, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.118925. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

The Flexibility Pathways for Integrating Renewable Energy into China’s Coal Dominated Power System: the Case of Beijing-Tianjin-Hebei Region Jian Zhang 1, Yanan Zheng 2, * 1 Department of Electrical Engineering, Tsinghua University, Beijing 100084, China 2 Energy Research Institute, Chinese Academy of Macroeconomic Research, Beijing 100038, China Corresponding author: Yanan Zheng, Energy Research Institute, Chinese Academy of Macroeconomic Research, Beijing 100038, China E-mail: [email protected]

Abstract The Beijing-Tianjin-Hebei (BTH) Coordinated Development Strategy marks the beginning of China's endeavor to build a world-class Capital Circle. Facing a slew of challenges, the BTH Region has an urgent need for identifying a green, low-carbon and circular development path and building a clean, low-carbon, safe and efficient modern energy system. The Region’s coal-dominated energy consumption style has brought about serious environmental and ecological impacts on itself. Vigorous efforts to develop renewable energy have resulted in an explosive growth of wind and solar in the BTH region and huge integration challenges to the power networks as well. Sufficient operational flexibility is crucial for promoting the renewable penetration into the power system. However, there is still a lack of effective means to systematically evaluate the flexibility performance. In this paper, we conducted a first-of-kind in-depth renewable integration study for the BTH Region. First, we summarized the challenges of renewable energy power generation in the BTH region from the perspective of "source-grid-load". Second, we proposed a balanced renewable energy integration analytical framework, which can systematically evaluate the technical and economic performance of various flexible resources. Then we conducted a detailed evaluation of the technical and economic performance of main flexible resources with the analytical framework. Finally, we explored the medium- and long-term flexibility development pathways and proposed policy recommendations for the BTH Region to enhance the flexibility of power system and thereby promote the integration of renewable energy. Based on the proposed development pathways for improving the Region’s flexibility of "source-grid-load", the BTH Region is expected to have a significant flexibility to support the accommodation of 125.93 GW of wind power and 44.30 GW of PV power by 2035.

Key words: Beijing-Tianjin-Hebei; Power system flexibility; Renewable energy integration; Coal power flexibility retrofit; Power grid coordination

Highlights Development challenges of renewable energy in the BTH Region from the perspective of "source-grid-load" A balanced renewable energy integration analytical framework Detailed evaluation of the technical and economic performance of main flexible resources Development pathways and policy recommendations for the region to enhance the flexibility of power system The BTH Region is expected to have a flexible capacity to support the accommodation of 125.93 GW of wind power and 44.30 GW of PV power by 2035

1. Introduction The Beijing-Tianjin-Hebei (BTH) Region, also known as China's "Capital Circle” located in the heart of the Bohai Rim, is one of the largest and most dynamic regions in northern China (Zhan et al., 2019). It is composed of Beijing, Tianjin, and 11 prefecture-level cities, including Shijiazhuang, Tangshan, Langfang, Baoding, Qinhuangdao, Zhangjiakou, Chengde, Quzhou, Handan, Xingtai and Hengshui of Hebei Province, with a total population of over 120 million, a land area of approx. 216,000 square kilometers and a coastline of 640 kilometers long. In February 2014, China unveiled a national strategy for the coordinated development of the BTH Region, which aims to build a world-class circle of the economy around the country’s capital. Raising a clear goal of “adhering to harmonious coexistence between man and nature”, China has made continuous efforts over the past decade (Ma et al., 2019), including adjusting its economic development mode (Song et al., 2019), paying increased attention to green chemistry and green engineering (Matus et al., 2012), and putting forward the strategic adjustment of economic restructure and green development (Zhen et al., 2015). However, the Region is still facing a slew of challenges, such as growing eco-efficiency disparity among different areas, extreme water resource shortages, and worsening environmental damages and pollution, amongst others. To identify a green, low-carbon and circular development path and build a clean, low-carbon, safe and efficient modern energy system has never been more urgent and important for the BTH Region. The BTH Region as a key regional energy consumption center in China accounts for over 10% of the country's total energy consumption (National Bureau of Statistics of China, 2018). The Region’s extensive energy consumption style, which has long been dominated by coal, thereupon, has brought about serious environmental and ecological impacts on itself. The environmental issues in the Region,

such as carbon emission, has attracted increasing attention (Chen et al., 2019; Wang et al., 2019). Since 2013, China has released a series of documents, including “Detailed Rules for Implementing the Air Pollution Prevention and Control Action Plan in Beijing-Tianjin-Hebei and Surrounding Areas”, “The 2017 Work Plan for Preventing and Controlling Air Pollution in Beijing-Tianjin-Hebei and Surrounding Areas”, and “Coordinated Energy Development Action Plan of the Beijing-Tianjin-Hebei Region (2017-2020)”, amongst others, highlighting the need for controlling the Region’s total coal consumption and promoting clean energy uses. With significant advances in wind and solar photovoltaic (PV) power generation technologies, renewable energy development has become a key direction for energy transition of the Region (Liu et al., 2012; Tan et al., 2013). There are two 10-million kilowatts level wind power bases and one million kilowatts level PV power base currently under construction in the Region. Nevertheless, the rapid development of wind and PV power has brought about great uncertainty to the Region’s power system operations (Atwa et al., 2009; Tuohy et al., 2009). The issue of insufficient power system flexibility keeps worsening (Denholm and Hand, 2011). Wind and solar energy curtailment have turned into serious matters in some localities, such as Zhangjiakou, which witnessed a wind curtailment rate of over 10% in certain years. Sufficient operational flexibility is crucial for enabling the power system to accommodate elevated renewable penetration and thus, achieving the carbon emission reduction target (Yang et al., 2018; Zhang and Zhang, 2019). Kehler and Hu (2011) presented the concepts and practice of the power system flexibility in different stages of development and in different system operation process. Luo et al. (2019) discussed the relationship between the demand for the flexibility of thermal power units and the accommodation of wind power. Yin et al. (2017) proposed an effective short-term flexibility solution which could use China’s dominant coal-fired power capacity to provide peak regulation

services for the priority dispatch of wind power. Tan et al. (2019) proposed a dispatching optimization model embedding a carbon emissions trading mechanism which could increase the share of renewable energy in the dispatching system. Dong et al. (2018, 2019) studied the implication of the flexible operation of coal power units, and further examined the energy efficiency, CO2 and pollutant emissions characteristics of China's generic wind-coal combined power generation system. In addition to focusing on the technological implications, China has put in place many support policies in order to promote wind power development. Zhao et al. (2016) analyzed the impacts of the price policy and non-price policy on the increase of installed capacity in areas with different wind resources. Further, Zhao et al. (2017) discussed the substitution capacities of wind power for coal-fired power to realize the CO2 emission reduction target in 2020 and 2030. Tan et al. (2013) discussed the prevailing policy and potential issues in respect of the wind power development in China with a macroscopic view. Although the flexibility for promoting renewable energy integration has been studied extensively, to the scope of our knowledge, there is still a lack of effective means to systematically evaluate the flexibility performance. In addition, there is little literature on the subject of flexibility development and related policies in the BTH Region. Therefore, this paper, in such a context, will be mainly focused on the topic of evaluating the flexibility performance and promoting the integration of renewable energies, such as wind and PV, in the BTH Region. This study fills the research gap from the following aspects. First, based on the status quo of renewable energy development in the BTH region, we summarized the challenges of renewable energy power generation in the region from the perspective of "source-grid-load". Second, we developed a balanced renewable energy integration analytical framework, which can systematically evaluate the technical and economic performance of various flexible resources. Third, with the analytical

framework, we conducted a first-of-kind in-depth renewable integration study for the BTH Region. Moreover, we proposed policy recommendations for the Region to enhance the flexibility of power system and thereby promote the integration of renewable energy. The remainder of the paper is as follows: Section 2 briefly describes the status of renewable energy development in the BTH Region. Section 3 introduces the definition of power system flexibility and the technical and economic assessment methods used. Section 4 conducts a detailed evaluation of the technical and economic performance of main flexible resources with the Region’s “source- gridload”. Section 5 proposes, in view of renewable energy development targets, a development path for improving the Region’s flexible resources. Section 6 puts forward relevant policy recommendations for the development of flexibility measures in the BTH Region. Finally, Section 7 summarizes and concludes. 2. The status quo of renewable energy development in the BTH region 2.1. Renewable energy resources 2.1.1 Wind energy Tianjin's coastal areas and Hebei Province are abundant in wind power resources, and Hebei's wind power development is at the forefront of the country. The Region is located on the eastern shore of the Eurasian continent at the mid-latitudes and on China’s eastern coastline. It belongs to a temperate zone semi-humid semi-arid continental monsoon climate. According to China Meteorological Administration’s 2014 onshore wind power resource evaluation results, the potential technical available capacity of wind energy in an area with a wind power density greater than or equal to 200 watts per square meter and at the height of 70 meters above the ground reaches 3.13, 0.92 and 110.78 million kW, respectively, in Beijing, Tianjin and Hebei. The potential technical available

capacity of wind energy at the height of 100 meters above the ground reaches 3.42, 1.15 and 130.46 million kW, respectively, in Beijing, Tianjin and Hebei (National Meteorological Information Center, 2015). Possessing superb resource conditions and a vast land area, Hebei has become a key area for wind energy exploitation among the three, whose wind power is developing at an accelerated pace. Some localities, such as Zhangjiakou, emerged as important national wind power bases. In the meanwhile, Tianjin has also made great efforts to accelerate coastal wind power base construction and promote its non-grid-connected wind power desalination industry. 2.1.2. Solar energy Whilst Beijing, Tianjin, and Hebei all have good solar energy resource conditions, Beijing and Tianjin focus more on distributed utilization, and Hebei makes centralized photovoltaic power generation its top priority. Located in a region with rich solar energy resources – second only to Northwest regions in the country, Hebei receives annual radiation of 4981-5966 MJ/m2, with an average of 3000~3200 hours of sunshine hours in northern Zhangjiakou and Chengde and 2200-3000 hours in the eastern and central parts of the Province. Beijing, which falls into a region with relatively rich solar energy resources, has more favorable conditions than Shanghai, Yunnan, Jiangsu and Zhejiang, with its annual radiation amounting to approx. 5061 MJ/ m2 and annual sunshine time reaching up to 2761 hours. Tianjin, located in an area with abundant solar energy resources in general, receives an average total solar radiation of 5966 MJ/m2 over a 30-year period, with its annual sunshine time staying in the range of 2471~2769 hours (National Meteorological Information Center, 2015). Beijing, Tianjin and Hebei are all national leaders in solar energy utilization. Beijing and Tianjin vigorously encourage power consumers to set up distributed solar PV stations employing the method of “self-generated electricity retained for self-consumption, surplus electricity sent to power grids, and

power grid playing a regulating role”, and actively promote building-integrated, grid-connected distributed solar PV systems; Hebei, on the other hand, makes solar PV power generation one of the key parts of its new energy development strategy. To date, Hebei has already formed a complete industrial chain, extending from silicon material, wafer, battery through a component. Since the start of the 12th Five-Year Plan, Hebei has successfully built a large number of concentrated solar PV power plants. 2.1.3. Biomass energy Hebei, as a large agricultural province, possesses a certain amount of biomass resources and had an old history in the development of certain industries. The amount of resources available for energy utilization is around 20 million tons per year. The annual output of crop straws is over 61.76 million tons, of which there are 10.46 million tons available for energy processing and utilization, except those utilized as firewood or for farmland reclamation, aquaculture and papermaking purposes; the annual amounts of three forest residues (logging residue, bucking residual and processing residual) and edible fungi available for utilization are 5.7 and 1.3 million tons, respectively (National Bureau of Statistics of China, 2018). For Beijing, its biomass resources mainly consist of crop straws, agricultural processing residues, urban wood residues and livestock and poultry manures. Their theoretical reserves are 1.88, 0.23, 0.9 and 7.42 million tons, respectively. Since the 1970s of the last century, different types of biomass industries have been developed in the BTH Region. Boosted by large amounts of investment, some biomass projects in the fields of biomass power generation and biogas utilization witnessed continuous growth. However, due to the collection, storage and transportation constraints, biomass utilization scale is still very limited. 2.2. Development and challenges of renewable energy power generation

2.2.1. Source side While the Region’s installed renewable energy capacity shows explosive growth, there is not enough power supply-side flexibility to match it. As shown in Fig. 1, the installed wind power capacity of the BTH Region rose from 3.86 GW in 2010 to 12.35 GW in 2016, at an average annual growth rate of 21.4% (China Electricity Council, 2017). At the end of 2016, the installed wind power capacity of Hebei, which possesses better wind power resources, exceeded 11.88 GW, accounting for over 96% of the total installed wind power capacity of the BTH Region. Owing to rich solar energy resources, the Region saw explosive growth of installed solar power capacity starting from 2013. At the end of 2016, its cumulative installed capacity reached 5.18 GW, as shown in Fig. 2. A majority of the projects are centralized solar PV power plants, followed by distributed systems; the installed solar power capacity of Beijing, Tianjin and Hebei is around 0.15, 0.6 and 4.43 GW, respectively. Affected by a slew of complex factors such as weather and seasons, wind and solar energy demonstrate significant volatility and intermittence. As a result, renewable energy (e.g. wind and solar) generation entails great uncertainty and rely heavily on the power system’s regulation capacity to ensure effective uses. This is especially true with large-scale integration of wind and solar power into the power system, which raises higher and higher requirements on key indicators of the power supply side, i.e. peak shaving potential, ramping capacity and start-stop time, amongst others. However, the BTH Region is seriously deficient in power supply-side flexibility. At the end of 2016, the installed hydropower capacity of the BTH Region was 2.81 GW, representing a mere 3.2% share of its total installed capacity; the Region’s installed capacity of thermal power was 68.59 GW, representing a 77.1% share of its total installed capacity (China Electricity Council, 2017). Thermal power plants are the main power sources providing regulation services in the BTH Region, with more than 19 GW of combined heat and power

units. However, the Region’s coal-fired power plants have not yet undergone a flexible retrofit and are burdened by the task of ensuring heating services. The Region has been increasingly challenged by insufficient power source regulation capacity, which results in ever-worsening renewable power curtailment and rationing incidents.

56.2

14.00

60

12.00

50 40

8.00 6.00

30

22.2

18.2

15.9

15.5

4.00 5.9

2.00

%

GW

10.00

20 10

0.00

0 2010

2011

2012

2013

2014

Installed Wind Power Capacity

2015

2016

Growth Rate

Fig. 1 Installed Wind Power Capacity of the BTH Region During 2010-2016 6 5.18 5

GW

4 3

2.42

2 1.17 1 0

0

0

0.07

2010

2011

2012

2013

0 2014

2015

2016

Fig. 2 Installed Solar Power Capacity of the BTH Region During 2010-2016 2.2.2. Grid side The power grid continues to grow rapidly, but the grid-side flexibility lags badly. At the end of 2016, the length of power transmission lines at or above 220 kV in the BTH Region reached 43,642 km, of which that of Beijing, Tianjin and Hebei were approx. 4,354, 4 ,417and 34,871 km, respectively

(China Electricity Council, 2017). The ratio of installed capacity to the length of power transmission lines at or above 220 kV of the BTH Region, however, was only approx. 2,027 kV/km, which was merely 57.0% of the European power grid (2015), 53.3% of the US power grid (2012) and 27.1 of the Japanese power grid (2014); judged by power supply, the ratio of total power consumption to length of power transmission lines at or above 220 kV of the BTH Region was 8.05 million kWh/km, which was less than 70% of Europe and the US, and far below 22.91 million kWh/km level of Japan and 15.22 kWh/km of South Korea. It can be seen that currently, the apparently modest grid resource allocation efficiency and insufficient grid-side flexibility of the BTH Region has exerted a huge impact on renewable energy integration both within and without the region. 2.2.3. Load side The electricity consumption structure develops continuously with the industrial transformation, raising a higher demand for the flexibility of power system. With the BTH Region successively entering the post-industrialization stage, their industrial load proportion has seen sharp declines, while commercial and residential load proportions rise rapidly. In 2016, the proportions of primary industry, secondary industry, tertiary industry and residential electricity consumption of the BTH Region were 2.6%, 63.4%, 20.5% and 13.5%, respectively (China Electricity Council, 2017). Compared to the 2011 levels, the proportions of primary and secondary industry electricity consumption decreased by 0.8% and 6.7%, respectively, while those of tertiary industry and residential electricity consumption increased by 5.7% and 1.8%, respectively. As a result of electricity consumption structure adjustments, the difference between peak and valley load of the electric power system in the BTH Region continues widening, while the load rate keeps declining. In 2016, the rates of maximum daily peak-valley load difference of Beijing, Tianjin and Hebei were 63.0%, 40.1% and 43.1%, respectively, representing

9.0%, 0.1% and 1.4% increases compared to their 2011 levels; the average daily load rates of the three places were 83.0%, 86.2% and 75.5%, respectively, which was basically flat for Beijing and represented 1.7% and 6.9% declines for Tianjin and Hebei, when compared to their 2011 levels. Deteriorating load characteristics not only raise higher and higher requirements on power system flexibility but also reduce the room for renewable energy integration in the BTH Region. 2.2.4. Integration of renewable energy Due to the lack of “source-grid-load” flexibility, renewable energy generation curtailment and rationing situation are getting worse. Development of the power system of the BTH Region puts forward higher requirements on the system flexibility, but the release of flexibility in BTH Region is seriously lagging behind. Insufficient power supply regulation capacity, insufficient power grid mutual aid capacity, and continuous deterioration of user side load characteristics, all seriously hinder the penetration of wind and solar energy in the BTH Region. With rapid renewable energy development, renewable energy curtailment and rationing incidents keep emerging in the BTH Region, as shown in Fig. 3. The wind curtailment situation, in particular, shows no sign of abating. In the Zhangjiakou region, where renewable energy is concentrated, the wind curtailment rate exceeded 10% in some years, and the renewable energy curtailment has demonstrated a tendency of getting worse over time.

Wind Curtailment Electricity (GWh)

2500

Wind Curtail ment Rate 9.0%

Wind Curtail ment Rate 10.4%

2000 1500 1000 500

Wind Wind Curtail Curtail ment Rate 3.6%

0 2015

Beijing

Tianjin

2016

Fig. 3 Wind Curtailment Situation in Beijing, Tianjin and Hebei during 2015-2016 Hence, enhancing “source-grid-load” flexibility has become an essential prerequisite for furthering the development and effective use of renewable energy in the BTH Region. In this sense, to conduct an in-depth analysis of the Region’s flexible resources for power supply, grid and load, assess their technical and economic performance and explore development paths of flexibility measures for “source-grid-load” in the medium and long term has never been so important. 3. Methodology for flexibility resource assessment 3.1. Definition of power system flexibility Flexibility, being a new requirement on operations of power system with an increasing share of electricity generation from renewable sources, will be one of the indispensable indicators for measuring the operational characteristics of power systems in the future. Current studies on power system flexibility both at home and abroad are mostly still in an early stage, coming up with very different definitions on power system flexibility, such as the general characteristic of the ability of the aggregated set of generators to respond to the variation and uncertainty in net load (Denholm and Hand, 2011), the ability to adjust the net power flow onto the grid (Dong et al., 2018), and the capability of the system to respond to external uncertainties (Luo et al., 2019). Flexibility as defined by North American Electric Reliability Corporation (NERC) is as follows: power system flexibility is the ability

of a power system to use related resources to meet load changes and is mainly reflected in operational flexibility; a focal area for research is on methods for improving power system flexibility (North American Electric Reliability Corporation, 2009). International Energy Agency (IEA), on the other hand, defines flexibility as: ability of a power system, under certain economic operating conditions, to quickly respond to big fluctuations in supply or load (e.g. foreseeable or unforeseeable changes and events), in a way to allow it to reduce supply when load demand decreases, and vice versa (Chandler, 2011). From NERC’s and IEA’s definitions of power system flexibility, it can be seen that their definitions are quite similar, that is, the coping capacity of the power system to maintain supply-demand balance at times when supply or demand changes, in a bid to ensure safe and stable power grid operations. A “flexibility service” provided by power system may either be "upward” or “downward” regulation. The former refers to the method of providing additional power to the system. Increasing output of a generating unit or reducing the load achieves the same effect. The latter refers to the method of reducing redundant power of the system. Reducing output of a generating unit or increasing load achieves the same effect. 3.2. Method for assessing renewable energy integration 3.2.1. Analytical Method for Balanced Integration of Renewable Energy As wind and solar PV power generation is normally associated with high uncertainty, effective renewable energy utilization relies heavily on system flexibility. To better understand the roles of various types of flexible resources in promoting renewable energy integration, this paper develops an analytical method for balanced integration of renewable energy to study the influences of different types of flexible resources, along the “source-grid-load” chain, upon the Region’s power system reliability and on their roles in enhancing renewable energy integration. Fig. 4 shows the basic

principles of the proposed method, combining both electric power and energy balance estimation and random production simulation based on universal generation functions. Taking full account of all major power system resources, including different types of “source-grid-load” sources and energy storage, the method is capable not only of conducting a comprehensive review of the effects of various types of flexibility resources on renewable energy integration, but also of assessing the roles of various factors in improving system reliability, based on observation of the operation status of different “source-grid-load” resources through a random production simulation, hence thoroughly analyzing the whole process from planning through operation. The characteristics of this method make it adopted by many provincial power grid companies in China, such as Jilin, Shandong, Henan, Beijing, Tianjin, Hebei, etc. The practical application in the power grid companies shows the effectiveness and applicability of this method. The proposed analytical method adopts a random production simulation (Li et al., 2017; Guo et al., 2019) based on universal generating functions (UGF) (Ding and Lisnianski, 2008). The method discretizes power generating capacities of various types of generators to obtain the probability distribution of capacities, and then to derive the combined probability distribution of all generators’ total capacity through combination operation of the generating capacities of individual generators. Finally, based on a supply-demand matching simulation, it computes the expected power output of each generator and reliability indicators for system operations. Its basic principles are illustrated in Fig. 5 below. The main process of simulation includes the following steps. First, the joint probability distribution of power system can be calculated by multiplying the UGF of various components. Second, the expected generating energy of various units and reliability indices in this period can be obtained through matching the joint probability distribution with load in corresponding period. And last, for

multiple periods, these indices are able to be obtained by repeating the above two steps. For example, in period t , the UGF of system containing the first m units u ( m) ( z, t ) can be expressed as follows:

u

( m)

( z, t ) = ⊗ {u1 ( z, t ), u2 ( z, t ),L, um ( z, t )} =

n( m ) (t )

∑

k pro ,( m ) (t ) ⋅ z

P(km ) (t )

(1)

k =1

k where ui ( z , t ) is the UGF of unit i ; pro ,( m ) (t ) is the corresponding probability of k th state in

period t ; P(km ) (t ) is the output of k th state in period t . By comparing each output state P(km ) (t ) with load L (t ) , and counting these state whose output is less than load, the reliability indices such as loss of load probability (LOLP) become available. For example, LOLP can be calculated as follows:

LOLP(m, t ) =

∑

pro(km ) (t )

P(km ) ( t )< L (t )

Fig. 4 Analytical Method for Balanced Integration of Renewable Energy

(2)

Fig. 5 Diagram of Basic Principles for Random Production Simulation Based on Universal Generating Functions 3.2.2 Method for Assessing Renewable Energy Integration This paper uses five technical indicators, i.e. potential for LOLP reduction, resource economy, technology readiness level (TRL), potential for boosting additional wind power integration, and potential for boosting additional solar power integration, to evaluate the “source-grid-load” flexible resources of the BTH Region. With a normalization method, the indicators comprehensively assesse various types of resources and makes a horizontal comparison of such resources, in a bid to provide support for identifying possible development paths for promoting medium- and long-term flexible resources in the Region.

•Potential for LOLP reduction This technical indicator reflects the capacity of response flexibility resources for improving the level of system operation reliability in power generation and transmission. This paper uses the proposed analytical method to measure system’s LOLP for reaching a maximum load of a typical day in summer or winter, of various flexible resources after being connected to the current system. As

expressed in Formula (3) below, the minimum value of LOLP reduction of a typical day in summer or winter, relative to that under the current baseline scenario, is used as an evaluation indicator to be then processed with a benefit-based normalization method.

I iR =

I iR − min I iR i

(3)

max I iR − min I iR i

i

•Economy It reflects the costs associated with, and fees spent on, flexible resources from the planning, survey, design and construction stages through their entire product life cycles. For the purpose of this paper, we mainly concern the sum of design, construction and use costs of various flexible resources, measured in investment per kilowatt. To assess the economy of power grid mutual assistance, inter alia, both the existing retrofitting and grid expansion costs are considered; the costs of power demand-side management include both promotional costs and those spent on related smart equipment and management platforms; given the vigorous efforts to develop electric vehicles in the BTH Region, energy storage cost in the context of this paper mainly refers to that of lithium-ion battery energy storage. Table 1 below shows the investment costs per unit of flexible resources of “source-grid-load” in the BTH Region. The data is then processed using a cost-based normalization method, as expressed in Formula (4).

I = C i

max I iC − I iC i

(4)

max I iC − min I iC i

i

Table 1 Economic Evaluation Table

Indicators

Pumped storage power stations

Flexibility retrofitting for coal-fired plants

Power grid mutual assistance

Demand-side management

Energy storage

Investment per kilowatt/RMB

5000

300

2000

500

8000

•Technology readiness level It refers to the industrialization and practicality level of response flexible resources in terms of technological maturity, technological process, supporting resource and technology life cycle. In view of the Energy Research Center’s relevant studies and suggestions from experts in related fields, this paper evaluates the TRL of various “source-grid-load” flexible resources with a 5-point rating scale. Table 2 below shows the rating criteria and evaluation results using a benefit-based normalization method, as expressed in Formula (5).

I = T i

I iT − min I iT i

(5)

max I iT − min I iT i

i

Table 2 Technical Readiness Level Evaluation Table

Indicators

The development of technology involves great uncertainty

The technology will mature within 10 years

The technology will mature within 5 years

The technology will mature within 3 years

The technology is mature now

Readiness level

1

2

3

4

5

•Potential for boosting additional wind power integration We use the proposed analytical method to measure the impacts of various flexible resources on boosting wind power integration in the BTH Region, the data of which is then processed using an efficiency-based normalization method, as expressed in Formula (6) below.

I iW =

I iW − min I iW i

max I iW − min I iW i

(6)

i

•Potential for boosting additional solar power integration We use the proposed analytical method to measure the impacts of various flexible resources on boosting solar power integration in the BTH Region, the data of which is then processed using an efficiency-based normalization method, as expressed in Formula (7) below.

I = S i

I iS − min I iS i

(7)

max I iS − min I iS i

i

4. System-wide flexibility options in the BTH Region 4.1. Power source 4.1.1. Hydropower As of the end of 2016, the Region’s total installed hydropower capacity reached approx. 2.81 million kW, among which small hydropower stations - mostly supporting facilities built for farmland irrigation and water diversion purposes with basically no flexibility regulation capacity - contributed 0.71 million kW; and pumped storage power stations contributed 2.1 million kW. As of now, the installed capacity of hydropower stations with regulation capacity in the BTH Region is around 2.1 million kW. Shangyi Pumped Storage Station, a major construction project listed in China’s 13th Hydropower Development Five Year Plan, has an installed capacity of 1.2 million kW. In consideration of pumped storage projects currently in operation or under planning, e.g. in Fengyun, Funing and Yixian, the combined installed capacity of pumped-storage power stations in the BTH Region is expected to reach 9.3 million kW by 2030, as shown in Table 3, although there could presumably be no big capacity addition afterwards due to restrictions in site resource conditions.

Table 3 Pumped Storage Development in the BTH Region

Project Planning

Project

Location

Fengning-20 23

Fengnign, Hebei Funing, Hebei Shangyi, Hebei Yixian, Hebei

Funing-2030 Shangyi-202 0 Yixian-2023

Annual power generation (GWh) 3424

Commissioni ng date

Installed capacity (MW)

Number of generators

Maximum power (MW)

2023

3600

12

300

2030

1200

4

300

1140

2020

1200

4

300

1140

2023

1200

4

300

1140

Setting various types of the Region’s installations in 2016 as benchmarks, this part of the paper assesses the effects of the 9.3-million-kW pumped-storage capacity on the Region after its integration. Table 6 shows the assessment results. Analysis suggests that the TRL of pumped-storage plant development is the highest among all types of flexible resources, despite the fact that there are only limited pumped-storage resources in the BTH Region; in consideration of existing projects in the neighboring regions, currently investment per kilowatt of pumped-storage power plant built in the Region is about 5,000 yuan. With rising labor costs, for instance, investment costs are likely to keep climbing over time; integration of the 9.3-million-kW pumped-storage capacity contributes positively to increased power system reliability across the entire Region. Whilst contributing to reduced system LOLP, due to operating characteristics of pumped-storage power plants, it is still very hard to reduce the vast non-regulatable capacity of existing thermal and external power at a large scale; hence its effects on boosting additional wind and solar installations is virtually zero.

4.1.2 Thermal Power As of the end of 2016, the Region’s total installed thermal power capacity reached 68.59 million kW, of which coal-fired generating units contributed 54.11 million kW, or 78.9% of the total; gas-fired generating units contributed 11.27 million kW, or 16.4 of the total; oil-fired generating units contributed 0.24 million kW, or 0.4% of the total; waste heat and residual pressure units contributed 2.19 million kW, or 3.2% of the total; and biomass-based generating units contributed 0.78 million kW, or 1.1% of the total. By region, Beijing’s total installed thermal power capacity was approx..9.709 million kW, mainly composed of gas-fired generating units, taking up an over 87% share of its total installed thermal power capacity; Tianjin’s total installed thermal power capacity was 13.783 million kW, mainly composed of coal-fired generating units, taking up a 78.1% share; in addition, it also had

2.771 million kW of gas-fired power generating capacity; Hebei’s total installed thermal power capacity was 45.10 million kW, mainly composed of coal-fired generating units, which took an over 94% share, in addition to a certain number of captive power plants. Despite a large number of thermal power installations in the BTH Region, in order to ensure heating needs in winter, cogeneration units take up a considerable share of thermal generation, with its installed capacity amounting to approx. 19 million kW; in the meantime, the waste heat and residual pressure capacity of captive power plants do not actually participate in system regulation. All these reasons have greatly affected the release of the flexibility of thermal power generation. At present, China has not paid adequate attention to tapping the full potential of the flexibility of coal-fired power generation units from design to operation. This, on the one hand, has caused the coal-fired power plant's peak-shaving and frequency-modulation capabilities not being fully released; on the other hand, to ensure stable operation of generating units, it has also resulted in the reduced room for grid connection of wind, solar and other renewable power generating units. The release of thermal power flexibility in the BTH Region will mainly come from the retrofitting of its large number of coal-fired power generating units, including both pure condensing coal-fired units and coal-fired heat-power cogeneration units. Reference can be made to the experience in flexibility retrofitting of various types of conventional coal-fired power generation units both at home and abroad. Relevant suggestions on improving the flexibility of coal-fired units in the BTH Region are shown in Table 4. We estimate that flexibility retrofitting of coal-fired units in the Region may help release over 20 million kW in additional peak-shaving capacity to the Region.

Table 4 Suggestions on Improving the flexibility of Conventional Coal-fired Units in Beijing, Tianjin and Hebei Unit type

Flexibility parameter

Conventional coal-fired unit

Start-up and shutdown time (h) Minimum output (%)

Current level 72 50%

Recommended level 36 20%

CHP unit

Self-provided power unit

Unit ramp rate (%/h) Start-up and shutdown time (h) Minimum output (%) Unit ramp rate (%/h) Start-up and shutdown time (h) Minimum output (%) Unit ramp rate (%/h)

60% 72 80%-90% 30% 100% -

60% 36 40% 60% 50% -

Using the Region’s various types of installations in 2016 as benchmarks, with a reference made to the suggested flexibility retrofitting levels of coal-fired units as shown in Table 4, this part of the paper evaluates the effects of retrofitting of coal-fired units on the Region. Evaluation results are shown in Table 6. Analysis reveals that flexibility retrofitting technology for coal-fired units is relatively mature at present, and the cost per kW of investment remains at a level lower than most other flexible resources; the retrofitting of pure condensing units, CHP units and captive power plants will not only provide the Region with a solution to problems arising from the integration of renewable power generation capacity, and contribute to increased reliability of the power system, but also boost additional wind and solar power capacity by 10.08 and 22.83 million kW, respectively, in the BTH Region. It will become an important way to increase the renewable energy share of the Region’s total energy consumption.

4.2. Power Grid A reliable grid network system has taken shape in the BTH Region, which creates basic conditions for flexible electricity interconnection and interoperability among different provinces and cities across the Region. As of the end of 2016, the Region’s total 220 kV (or above) substation capacity reached approx. 329.24 million KVA, of which Beijing, Tianjin and Hebei contributed 69.02, 54.47 and 205.75 million KVA, respectively; its total 110 kV (or below) substation capacity was approx. 254.7 million KVA, of which Beijing, Tianjin and Hebei contributed 48.8, 41.72 and 164.18 million KVA, respectively.

A total of 11 EHV and UHV transmission lines, with a total electricity exchange capacity of 64 million kW, have been set up to connect the Region with neighboring provinces, including Shanxi, Henan and Shandong. In the meantime, the Region is also vigorously promoting flexible transmission grid application. Zhangjiakou, for instance, is under way to build the world’s first ± 500 kV four-terminal DC power grid, which is expected to have a 3000 MW transmission capacity. Flexible DC, which has independent control of active and reactive power, improves support for reactive voltage and significantly enhances the safety of local large-scale renewable energy grid connection. Since there are no synchronization stability issues, unstable renewable energy supplies can be collected from multiple sources to form a stable and controllable power supply, thus solving the issue of renewable energy delivery; it also helps to form a highly controllable system that makes full use of the complementary characteristics of large-scale wind and solar energy within the region, to ensure effective renewable energy integration, aided by flexible peak shaving capability of pumped-storage stations. However, due to current scheduling, management and settlement mechanisms, compared with European countries such as Denmark and Germany, mutual assistance and regulation capabilities of power grids between the Region and neighboring provinces have not been fully tapped. And there is still room for improvement with respect to grid interconnection between the Region and neighboring provinces. Setting the Region’s various types of installations in 2016 as benchmarks and taking into account possible medium- and long-term development opportunities of power grids, this part evaluates the effects of 100 million kW outbound switching capacity on the BTH Region. Evaluation results are shown in Table 6. Analysis reveals that, from a technical point of view, the TRL (technology readiness level) of power grid mutual assistance is relatively high, requiring little improvement on existing lines;

however, some upgrades are still needed for the scheduling management system; when new lines are considered, power grid investment is about 2,000 yuan/kW, which compared with other flexible resources, remains at a moderate level; the growing flexible grid applications are almost certain to bring about a certain degree of system reliability losses, but reduction in reliability will be limited within a very small range; in the meanwhile, power grid flexibility and mutual assistance will be conducive to effectively solving the problem of uneven resource and demand distribution within the province, and be capable of mobilizing flexible resources of neighboring provinces to help add 6.72 million kW wind capacity and 32.57 million kW solar capacity in the BTH Region. Hence, it will become one of the most important ways to increase its renewable energy share of total energy consumption.

4.3. Demand Side Management As of the end of 2016, the Region’s total electricity consumption exceeded 509.3 billion kWh. The amounts of electricity consumed by primary industry, secondary industry, tertiary industry, urban residents and rural residents were 1.33, 32.43, 10.33, 4.0 and 2.84 million kW, respectively. As the economy continues to grow, the Region’s power demand structure has also undergone continuous changes, as reflected in a continuous decrease in the share of electricity consumed by the secondary industry and significant rises in the shares of electricity consumed by the tertiary industry and residents. The BTH Region is one of the most important load centers in China. Within this region, besides huge industrial loads of Hebei and Tianjin, mega- or super-sized cities like Beijing, Tianjin, Shijiazhuang and Baoding all have vast commercial and residential electricity demands. This has created the basic conditions for carrying out power demand side management (DSM) in the Region. However, due to technical and institutional constraints that currently exist in the Region, DSM is primarily carried out

by way of administrative mandate, like “orderly use of electricity”, which has so far achieved very limited effects. In consideration of domestic and international experiences and the current status of DSM implementation in China, we calculate the ratios of different industries in implementing power DSM in the Region over the medium to long term. Table 5 shows the results. Based on the current power demand structure, the BTH Region is expected to provide over 10 million kW in maximum load curtailment and over 0.9 million kW in interruptible load. Power DSM is likely to become an important flexible resource for the Region in the future.

Table 5 Ratios of Different Loads in Power Demand Side Management in the BHT Region

Befo re 2025 Befo re 2035

Industrial interruptibl e load

Industrial reducible load

Commercial interruptible load

Commerci al reducible load

Urban residential interruptible load

Rural residential interruptible load

0.25%

10.00%

0.20%

15.00%

5.00%

2.00%

1.00%

25.00%

0.60%

25.00%

10.00%

6.00%

Setting the Region’s various types of installations in 2016 as benchmarks, and taking into account the current electricity demand structure, and the power DSM ratios of Table 5, this part of the paper evaluates the direct effects of 10.69 million kW peak shaving and 0.96 million kW load response transfer on the BTH Region. Evaluation results are shown in Table 6. Analysis reveals that due to complexity in power load, the TRL of power DSM is currently quite low nationwide, regardless of it being in the BTH Region or elsewhere in China. In such areas as user-side load control tools, management platform construction and incentives for institutional mechanisms, huge investment and innovation are required; that being said, it should be noted that power DSM brings about huge economic benefits, next only to flexibility retrofitting for coal-fired power plants; likewise, power DSM also helps to boost the reliability of the Region’s power systems and lower system LOLP; however, constrained by development scale, additional wind and solar capacity may be difficult to

achieve as a result of power DSM activities.

4.4. Energy storage Except pumped storage, other major energy storage methods still do not offer much of an advantage from either a technological or cost perspective. Judged by cost decline trends of various energy storage methods especially in the past few years, however, the cost of battery energy storage has demonstrated a faster pace in cost reduction. Per Wh production cost of various types of batteries is expected to drop below 1 yuan/Wh, close to that of pumped storage, after their cumulative output reaches 1TWh. Affected by the current scale expansion of the domestic electric vehicle industry, the cost of the lithium-ion battery will drop even more rapidly. Considering the Region’s electric vehicle development speed and power system flexibility demands, there will be a lot of space for lithium-ion battery storage development in the BTH Region. And its economic advantages will emerge gradually in the future. Setting the Region’s various types of installations in 2016 as benchmarks, this part of the paper evaluates the effects of 5 million kW li-ion battery energy storage capacity on the BTH Region. Evaluation results are shown in Table 6. Analysis reveals that due to its low TSL, the investment cost, amounting to around 8000 yuan per kW, of li-ion battery energy storage exceeds that of all other flexible resources. This also constitutes an important obstacle for its application at a large scale; like pumped storage, the addition of 5 million kW battery storage capacity greatly improves the reliability of the Region’s entire power system. Nevertheless, it will do very little to change the uncontrollable output of the system’s power generation at a large scale, and to boost new wind and solar installations.

Table 6 Technical and Economic Assessment of Various Flexible Resources in the BTH Region Resource type

Technology readiness level

Economy

Potential for LOLP reduction

Potential for boosting additional wind power

Potential for boosting additional solar power

Comprehensi ve score

Pumped storage power stations Flexibility retrofitting for coal-fired plants Power grid mutual assistance Demand-side management Energy storage

integration

integration

1.0

0.4

0.5

0.0

0.0

1.9

0.8

1.0

1.0

1.0

1.0

4.8

1.0

0.8

0.0

0.7

1.0

3.4

0.6

0.9

1.0

0.0

0.0

2.5

0.6

0.0

0.3

0.0

0.0

0.9

5. Flexibility development pathways in the BTH region Under current environmental and resource constraints, the Chinese government emphasizes efforts to promote supply-side structural reforms. The guiding principle for any energy development strategy is to develop clean energy as quickly as possible and to use fossil fuels prudently. The BTH Region, being a national center, will rely on electricity as a primary form of energy supply in the future, as its end-use energy consumption getting increasingly dependent on electricity. After fully considering the technical and economic benefits of various flexible resources with regard to the Region’s “source-grid-load”, in combination with findings of the series of studies carried out by the Energy Research Institute and the State Grid Corporation on the Region’s electricity demand and supply, we propose the following development paths of flexibility measures for the BTH Region in the near, medium and long term, as shown in Fig. 6 below.

Fig. 6 The Flexibility development pathways in the BTH region 5.1. Prior to 2020 Hebei and Tianjin need to complete, in total, 5 million kW or above of coal power capacity in flexibility retrofits and transform the generating units to possess a 70% deep peak load regulation capacity; the Hebei Shangyi 1.2-million kW pumped storage station will be completed and ready to play a part in easing the peak regulation difficulties with Hebei’s Southern Grid.

5.2. Prior to 2025 Hebei and Tianjin need to complete, in total, 8 million kW or above of coal power capacity in flexibility retrofits; the Hebei Fengning Pumped Storage Station Phase I and II projects, and Yixian Pumped Storage Stations, will be completed to make the Region's total pumped storage hydropower capacity reach the level of 8 million kW or above; given the Region’s load characteristics, power DSM will be promoted at a certain level across the Region. The aim is to reduce 5.32 million kW or above in industrial and commercial loads, and achieve over 0.68 million kW of industrial, commercial and residential interruptible loads.

5.3. Prior to 2030 the BTH Region will see its coal power flexibility retrofits fully completed. In addition to realization of 80% rated capacity of pure condensing units for deep peak load regulation, the minimum stable output of cogeneration units during heating season is expected to drop to 40% of their rated capacity; through software and hardware upgrades, inter-provincial regulation capacity of the power grid will be significantly improved, and mutual assistance flexibility of power grids between the BTH Region and neighboring provinces increased by 30%; and construction of the 1.2 million-kW Hebei Funing Pumped Storage Station will be completed .

5.4. Prior to 2035 A 36 million kW, or greater, power grid mutual assistance capacity will be added for the BTH Region and neighboring provinces, with the power grid flexibility further improved; power DSM will be further strengthened; a reduction of 14.32 million kW or more in industrial and commercial loads will be realized, and over 1.75 million kW or more of industrial, commercial and residential interruptible loads retained; while economical affordability is guaranteed, our main efforts will be put on the development of over 5 million battery storage capacity in Beijing and Tianjin.

6. Policy recommendations Energy transition is an urgent task for the sustainable development of the BTH region, while improving the flexibility of the power system is a key measure on facilitating the energy transition. This paper has made an in-depth analysis on the challenges faced by the BTH region in renewable energy development, established analytical methods on balanced integration of renewable energies, and explored the development paths of flexible resources in the medium and long term in the BTH region. A series of policy proposals have been put forward in light of the current development situation in the

BTH region, in the aim of providing advices and suggestions on green and low-carbon development of the BTH region.

(1) The BTH Region shall promote the flexibility retrofitting for coal-fired plants and development of power grid mutual assistance as a priority, advance the construction of pumped storage power stations in an orderly manner, carry out power demand-side management actively and explore the development of energy storage power stations. The promotion of flexibility retrofitting of coal-fired units and the improvement in flexibility provincial transmission channels will be the primary choices for enhancing the flexibility of the power system in the BTH region. The preparatory work of the construction of pumped-storage power stations in Shangyi, Yixian and Funing of Hebei Province will be accelerated to ensure the construction of each station is completed on schedule. Efforts shall be put on the strict control of the energy efficiency of electrical equipment, enhance of awareness of energy conservation for all, and establishment of a demand-side management implementation system that is led by the government, with grid companies as main players and the whole society's participation. We shall timely track the development trend of the energy storage industry, reduce the influences of future uncertainties of energy storage, develop a number of pilot applications for renewable energy supporting technologies of different technology types and at different application scenarios, in order to explore an affordable path in the development of energy storage with mature technologies.

(2) The principle of ecological priority shall be adhered to in the development of pumped storage power stations. We shall coordinate the development of pumped-storage power stations and environmental protection, strengthen pre-development research and environmental demonstration, strengthen planning environmental impact assessment (EIA) and project EIA work to scientifically

demonstrate the environmental rationality of projects, enhance researches on key technologies to promote the construction of demonstration projects of seawater pumped storage power stations, and make an overall consideration on comprehensive utilization requirements, optimize operation scheduling, enhance utilization efficiency of pumped-storage power stations, and give full play to the overall benefits of pumped-storage power stations in the BTH region.

(3) Combination of coal power conversion and capacity elimination shall be promoted. As a key area to dissolve excess capacity of coal-fired power plants, the BTH region faces the situation of “excess electricity and power shortage” in the future. We shall explore and establish a “storage” mechanism to maintain the flexibility resources in the BTH region in response to changes in the power supply and demand situation, accelerate the promotion of flexibility retrofit of coal-fired units in the BTH region in accordance with international advanced standards, encourage coal-fired generating units to adopt efficient heat energy storage devices, in order to continuously improve coal-fired power regulation capacity, gradually guide primary baseload undertakers of coal-fired power to convert to main providers of flexibility capabilities of the system and resolve the risk of overcapacity in coal power generation step by step.

(4) Interest barriers shall be broken down to promote power grid openness and a win-win situation. We shall make full use of the characteristics of difference in time between power consumption load and renewable energy power generation in the BTH region, tap the potential of inter-provincial power regulation and enhance renewable energy integration; establish a unified mechanism for resource and backup sharing in grid adjustment in the BTH region to promote resource coordination adjusted between the sending and receiving end areas; implement low-carbon scheduling methods to ensure priority grid connection of renewable energy, and establish a safe dispatch operation

system that adapts to the characteristics of renewable energy power.

(5) Technological innovation shall be accelerated and intelligent demand-side management vigorously developed. We shall promote research and development of key technologies of equipment, information and communication technologies, and interactive technologies on regulation relevant to monitoring and control in power demand side management, strengthen integration of data on system operations, market transactions, and power consumption by users, enhance load-side big data analysis capabilities, and guide users of various kinds to actively participate in the power demand side response; carry out demonstrations on intelligent power demand response and user interaction engineering such as smart communities and smart parks; encourage all types of energy-intensive enterprises to improve techniques and production processes, to provide auxiliary services such as interruptible load and controllable load to the system. and launch pilot projects of energy-efficient power plants to drive them to become an important resource for power planning and operation regulation.

(6) Energy storage costs shall be driven for further reduction and diversified utilization. We shall vigorously promote the application of new energy storage technologies, actively explore commercialized energy storage methods, and further reduce the cost of energy storage, carry out large-capacity energy storage pilot projects in Chongli County and Zhangbei County with an abundance of renewable energies, conduct distributed energy storage pilots at the users’ side in large-scale cities as Beijing, Tianjin, and Shijiazhuang, etc. to explore possible future development models on energy storage; explore the role of electric vehicles in energy storage, actively promote two-way interaction on energy and information between electric vehicles and smart grids, and vigorously tap the regulation role of energy storage on power grid by electric vehicles.

7. Conclusions

Face with the urgent demand for identifying a green, low-carbon and circular development path and building a clean, low-carbon, safe and efficient modern energy system, the BTH Region needs to complete the energy transition through the rapid development of renewable energy. Meanwhile, it is crucial for the power system to have sufficient operational flexibility to cope with the great uncertainty that wind and PV power bring to the Region’s power system operations. To systematically evaluate the flexibility performance and thus promote the penetration of renewable energy, this paper proposed a detailed evaluation method of the technical and economic performance of main flexible resources and conducted a systematic analysis of the flexible resources in the BTH Region. The systematical evaluation of five flexible resources shows that in the BTH Region different resource exerts flexibility at different technical and economic level. In the case taking 2016 as benchmark, the flexibility retrofitting for coal-fired plants has the highest comprehensive score, meaning it is the first choice and important way to promote cleaner production in the BTH Region, followed by the power grid mutual assistance, the demand-side management, and the pumped storage power stations. The energy storage has the lowest comprehensive score due to its low technology readiness level and high investment cost, so it is difficult for its application at a large scale to promote the accommodation of wind and PV power. With the systematical evaluation and in-depth study of the flexibility, we proposed development pathways for improving the Region’s flexibility of "source-grid-load" under the guiding principle of developing clean energy. To ease the peak regulation difficulties, the BTH Region need to complete 5 million kW of flexibility retrofitting for coal-fired plants and 1.2 million kW of pumped storage stations by 2020. The significant flexibility of power system of the BTH Region is expected to support 25.77 GW of wind power and 26.83 GW of PV power by 2025, and 125.93 GW of wind power and

44.30 GW of PV power by 2035. This paper fills the research gap on the flexibility of the BTH Region from the above aspects. Whereas we investigated in detail the flexibility performance and proposed the comprehensive and detailed development path, future research can further develop such insights and assess their transferability and relevance to other contexts. There could also be valuable research done on the electricity market mechanism compatible with flexibility and the flexibility performance of other types of sources such as electric vehicles.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: