Impact of supporting power source on a high-proportion renewable energy generation base

In recent years, China has organized three batches of high-proportion renewable energy generation bases, with the aim of resolving the geographical imbalance between renewable energy supply and consumption. Renewable electricity is transported from the resource-abundant western regions to the electricity-demanding eastern regions. Given the inherent variability and unpredictability of wind power and photovoltaic power generation, there is a pressing need for additional support from more reliable energy generation sources, including coal-fired power and concentrated solar power (CSP). This paper presents a power system optimization planning model that incorporates the internal energy transfer of CSP. It proposes strategies for establishing high-proportion renewable energy generation bases supported by both coal-fired power and CSP. Furthermore, it investigates the primary constraints affecting the energy generation base. The study also compares the peak shaving depth and output characteristics between coal-fired power and CSP. Moreover, this paper introduces the method of natural gas supplemental combustion to enhance the power supply stability of CSP, enabling the renewable energy generation base to supply electricity through conventional DC transmission.


Introduction
Renewable energy generation, exemplified by photovoltaic and wind energy generation, has experienced rapid growth in recent years owing to its clean and sustainable advantages [1].However, they exhibit inherent uncertainties and temporal-spatial mismatches.Large-scale access to the grid can have a substantial impact on power quality and grid stability [2].Consequently, as wind and photovoltaic power generation continues to expand in scale, it becomes imperative to conduct research into the complementary generation of stable power supply through diverse power source categories.
China boasts abundant wind, solar, and land resources primarily concentrated in the western region, while the substantial renewable energy deficit is prevalent in the eastern region.To bridge this gap and enable the transportation of renewable power from the western to the eastern region, China has undertaken the establishment of three successive waves of high-proportion renewable energy generation bases and hopes to use ultra-high voltage direct current (UHVDC) power transmission to solve the problem [3].The power supply within the UHVDC channel presents a quintessential case of multisource energy complementarity, necessitating additional support from power sources such as coal-fired power to complement the variable output of wind power and photovoltaic power generation.In this context, concentrated solar power (CSP) emerges as a formidable contender for replacing coal-fired power among renewable energy.Through the utilization of thermal storage tanks, CSP captures and stores the heat generated from solar energy.This approach enables the provision of a relatively controllable renewable energy output, thereby offering valuable support to an energy power generation base with higher proportion renewable.

Model
This study autonomously develops an extensive optimization planning model for a high-proportion renewable energy power system, including wind power, PV power, coal-fired power, and CSP. Figure 1 illustrates the simplified structure of the model, showcasing various configurations of wind power, PV power, coal-fired power, and CSP on the generation side.On the demand side, the study formulates load curves.The study conducts integrated operational simulations for different energy source combinations on an hourly basis throughout the year.Wind power and PV power constitute the primary output sources in the renewable energy base.The model engages in simulations using predicted output curves derived from historical solar and wind resource data.The simulation employs the resource conditions of Jiuquan City in Gansu Province as an illustrative example.In accordance with the renewable energy base construction criteria, the utilization rate for wind power and PV power generation should exceed 90%: where pre, ,, tt Cap PP are the predicted output at moment t, the actual output at t, and the installed size of the power source, respectively.
Coal-fired power serves as a supportive energy source in new energy bases, ensuring power output stability during fluctuations in new energy production.This study does not consider aspects such as heat supply, maintenance, or unit failures.The model sets constraints on the minimum technical output and ramping rates for coal-fired power sets.
The CSP system is more intricate than photovoltaic power generation and comprises three primary components: the solar field (SF), the thermal storage system (TSS), and the power generation system (TPS).Energy is exchanged between these systems using a heat transfer fluid (HTF), such as molten salt or heat transfer oil.For instance, in a tower CSP station, sunlight converges at the receiver, which is located on the tower's upper end through mirror reflection.Low-temperature molten salt absorbs the heat and heats up to around 565 degrees Celsius, after which it is stored in high-temperature molten salt storage tanks.During electricity generation, the high-temperature molten salt enters the heat exchanger with water to produce high-temperature, high-pressure steam, which drives the turbine unit to generate electricity.After releasing heat in the heat exchanger, the molten salt returns to the low-temperature molten salt storage tank.As the power generation of CSP is also influenced by fluctuating solar resources, some plants consider supplementing combustion with natural gas heating boilers to ensure power generation stability during insufficient light.The internal power balance equation of the CSP is as follows: where SF , t E C are the stored energy and the daily heat loss coefficient.Moreover, the thermal storage system incorporates boundary constraints for both charging and discharging power, boundary conditions for the system's energy state, and constraints related to the thermal state during charging and discharging processes.The modeling of the power generation system encompasses considerations for thermal power balance, output boundary constraints, ramping constraints, and start-stop coupling constraints.
The load curve employed in the simulation (as depicted in Figure 2) is formulated based on the characteristics of renewable energy production and the demand patterns of domestic electricity usage.Domestic electricity demand exhibits two peaks during summer and winter seasons, with a daily peak.Consequently, the overall power output curve of the entire base exhibits higher loads during July, August, December, and January.This load curve aligns with the new energy production characteristics, which feature a high output from 10:00 to 18:00 daily.Also, the base possesses a rated power of 8GW and an annual power generation capacity of 5,000 hours.

Boundary conditions
The annual utilization hours for wind power amount to approximately 2,250 hours, while photovoltaic power achieves an annual utilization of around 1,850 hours.Each coal-fired power unit has a capacity of 1 GW, with a minimum technical output set at 25% of the rated power, and a ramp rate of less than 5% per minute.On the other hand, each CSP unit has a capacity of 100 MW, with a minimum technical output of 15% of the rated power and a ramp rate of less than 10% per minute.The SF has an optical efficiency of 50%, and the TPS operates at an efficiency of 42%.Renewable energy sources (including wind power, PV power, and CSP) constitute more than 50% of the base's total power generation capacity.

Results
The study, with the goal of minimizing total power from the grid, conducts a time-series production simulation for a high-proportion renewable energy base, utilizing coal-fired power plant and CSP generation as supporting power sources.Both coal-fired power and CSP have a capacity of 4 GW, and the CSP has a heat storage duration of 10 hours.

New Energy Scale and Power Output
When coal-fired power serves as the supporting power source, there exists a mutual influence between the requirements for the renewable energy proportion and renewable energy utilization within the UHV channel.Under the specified boundary conditions utilized in this study, it is observed that these two requirements cannot be simultaneously satisfied.Specifically, Working Conditions 1-3 shown in Table 1 represent the results in terms of power output and characteristic parameters when the renewable energy scale is set at 11 GW, 11.5 GW, and 12 GW, respectively.As the renewable energy scale increases, the utilization rate of new energy gradually decreases, while the proportion of renewable energy in the channel steadily rises.Furthermore, Case 2, Case 4, and Case 5 illustrate simulation results for different wind and PV power generation ratios, with a total renewable energy scale of 11.5 GW.In these cases, the utilization rate of new energy does not attain the 90% target, and it is observed that the proportion of renewable energy increases as the scale of wind power generation increases.
Table 1.Results of power output and characteristic parameters (Coal-fired power).The results presented above indicate that even when coal-fired power is utilized as a supporting energy source, combined with wind and PV power generation to establish an 8 GW UHVDC channel, there remains a deficiency in peaking resources.Case 6 represents the results when an additional 10% electrochemical energy storage (0.8 GW*2h) is integrated based on the conditions in Case 2. By introducing electrochemical energy storage to peak shaving, the utilization rate of renewable energy rises to 90%, concurrently reducing the proportion of the total power from the grid to 0.9%.When CSP is employed as the supporting power source, the share of renewable energy within the channel exceeds 90% (shown in Table 2).The annual utilization hours of CSP within the channel are lower than those of coal-fired power.This difference enables the allocation of new energy on a larger scale, meeting the demand for wind and PV energy utilization.As the scale of wind and PV energy increases in Case 7-9, the proportion of renewable energy gradually grows.

Autonomous operational capability
The greater the reduction in power from the grid at the UHVDC channel's sending end, the reduced reliance on the main grid, and the enhanced capacity for independent operation.In channels supported by coal-fired power, the proportion of power from the grid is less than 1.5%, indicative of a superior autonomous operational capability.When CSP serves as the supporting power source, it is susceptible to fluctuations due to varying solar resources, leading to output instability.To enhance the channel's autonomous operational capability, supplementary natural gas combustion heating boilers are considered.Case 10 represents the results of this approach in addition to the conditions in Case 9, demonstrating its effectiveness in reducing the power demand from the grid and the channel's dependence on the main grid.
The simulations above underscore that a combination of coal-fired power and energy storage, as well as CSP coupled with natural gas supplemental combustion, can successfully support a conventional UHVDC transmission channel for 8GW, 5500 hours.However, relying solely on CSP as the channel's support is more suitable for flexible DC transmission.

Power Output on a Typical Day
As shown in Figures 3-5, results of Case 2 (a) and Case 10 (b) in three typical days were selected for analyzing power output patterns.The analysis results reveal that the primary distinction between coalfired power and CSP output is most apparent during days with rich wind and solar resources.
During peak wind and PV output periods, both PV and wind outputs are high throughout the day.Coal-fired power generation maintains a minimal technical output during this time.After sunset, PV output diminishes, along with a reduction in load demand, primarily met by wind output.In the latter half of the night, wind power generation decreases, prompting coal-fired power to increase its output to support the channel.On the other hand, when supported by CSP, it mainly stores heat during the day without electricity generation and operates at near-full capacity during the early morning hours.In situations with low wind resources, there is a high daytime output from PV, and coal-fired power operates at maximum technical output.The daily output curve for CSP under these typical conditions resembles that of coal-fired power.In instances where both wind and solar resources are insufficient, coal-fired power operates at its maximum technical output but is still unable to meet the demand, necessitating the purchase of power from the main grid.The power output characteristics of CSP are akin to those of coal-fired power.

Conclusion
The following conclusions have been drawn from the study conducted through time-series production simulations: • Besides coal-fired power, CSP generation can also serve as a viable option for supporting and providing peak shaving in a renewable energy base.In this study, the appropriate scale of energy source within the research boundaries consists of 4 GW of coal-fired power matched with 11.5 GW of new energy, and 4 GW of CSP matched with 13 GW of new energy (Wind and PV).• When coal-fired power is employed as the supportive energy source, striking a balance between renewable energy utilization constraints and renewable energy share constraints becomes crucial, which essentially is the deficiency in peaking capacity.Continuing to expand the scale of renewable energy after experiencing curtailment results in decreased new energy utilization and an increased share of renewable energy in the channel.• For channels primarily supported by CSP, which exhibit a high proportion of electricity from the grid, they are suitable for transmission through UHV flexible DC systems.The addition of natural gas supplemental combustion can reduce this proportion, enhance independent operational capability, and meet the requirements for conventional UHVDC transmission.• The primary distinction between coal-fired power and CSP as supporting power sources lies in their differing depth of peak shaving.Consequently, the difference in output is more pronounced during periods of high renewable energy generation, while other typical daily outputs remain similar.

Figure 1 .
Figure 1.Simplified structure of the power system model.
represents the efficiency and the operating state of supplementing combustion, respectively.The solar energy capture equation of the solar field is: t Ax are the area of solar field and its DNI value.The energy state equation of thermal storage system is: ( )

Table 2 .
Results of power output and characteristic parameters (CSP).