Overview of hydro-wind-solar power complementation development in China

1 Introduction

Hydropower generation in China started over a century ago, greatly contributing to their economic and social development.Wind power and photovoltaic (PV) power generation began on a large scale in the 21st century, and both developed rapidly.The continuous development of economy and society as well as the improvement of people’s living standards lead to growing demand for energy.To reduce air pollution and greenhouse gas (GHG) emission and to realize the goals of the percentage of nonfossil energy as primary energy consumption being 15% and20% in 2020 and 2030, respectively, China proposed the strategy of vigorous development of renewable energy that makes use of renewable energy such as hydro energy, wind energy, solar energy, among others, in order to guarantee energy security, improve energy configuration and promote the construction of an ecological civilization.

The output of wind and PV power is featured with volatility, intermittence, and randomness with no selfregulating ability, and the swelling grid-connected scale of wind and solar power requires compensatory regulation.With flexible and controllable power sources in the power system, both hydropower and pumped-storage power stations have good regulating ability and energy storage capacity.The mutual complementation of such power stations and wind and solar power under a coordinated operation mode of hydro-wind-solar power can protect the safe grid connection of new energy on a large scale.

2 Hydro-wind-solar multi-energy complementation

Hydro-wind-solar multi-energy complementation is not a simply numerical sum, but it takes full advantage of the output complementary feature of wind, solar, hydropower and pumped-storage hydropower to make the final output more stable, friendly, and beneficial to grid dispatching and operation.There are diversified types of multi-energy complementation and listed below are the types of mutual complementation of wind, solar and hydropower generation:

2.1 Wind-wind complementation

Different wind farms in the same region or wind farms in different regions are more or less complementary, which helps mitigate a dramatic variation in wind power output, suppress the volatility of daily output, stabilize the output curve, and reduce power transmission capacity.For example, in Qinghai Province, provided that 95% output is guaranteed, the simultaneity factor of all regions of the Haixi Prefecture is 0.67 while that of Hainan Prefecture is approximately 0.83.The different geographic locations result in conspicuous time and space disparity of wind energy resources of these two prefectures such that the simultaneity factor of the entire Qinghai Province is lowered to 0.57.

2.2 Solar-solar complementation

Solar farms of different sizes in the same region or those in different regions are more or less mutually complementary, which mitigates the dramatic volatility of solar power output.The bundled transmission improves PV utilization and reduces power transmission capacity.In northern China, solar complementation is mainly concerned with time allocation in the latitude direction while in southern China, it is mainly PV output complementation due to local climate change.For example, for the aforesaid Haixi and Hainan prefectures in Qinghai Province, provided that 95% output is guaranteed, their simultaneity factors are both around 0.8.For the entire province, such a factor can be lowered to around 0.7% together with less random volatility of the PV output.

2.3 Wind-PV complementation

Although having the fairly random, wind and solar power can be mutually complementary to some extent.However in most regions in China, wind power has a larger output during the night and a smaller output during the day and is thus more or less complementary with PV power.The wind-solar complementation in the same region may use the same power transmission lines so that the same grid-connected capacity can transmit more power that, to some extent, increases the transmission hours and makes it more cost-efficient.The successful grid connection of a 54-MW/100-kWp wind-solar complementary power plant in Nan’ao, Guangdong Province, in 2004 was the first wind-solar complementary power generation system officially launched for commercialization in China.Later, in 2012, a 9-MW wind-solar complementation demonstration project in Changma, Yumen, Gansu Province, was officially connected to the power grid.

2.4 Hydro-solar complementation (or hydrowind complementation)

A hydropower station or pumped-storage hydropower with daily and above regulating capacity may properly store water to reduce output when the grid has a valley load and the wind/solar power output is considerable, and it may enlarge the output during peak load times when the wind/solar power output is small.This is done so that the power generation output is convertible with water storage in the reservoirs and allocable with respect to time.Owing to the quick regulating speed, the hydroturbine units are able to accommodate a certain extent of volatility of wind and solar output.The Longyangxia hydro-solar complementation power station in Qinghai Province, China, is connected with the Longyangxia hydropower station by one circuit of 330-kV lines and the existing transmission lines of the hydropower station are utilized for grid connection, achieving suppression of the fluctuation of PV power generation curve and optimization of PV power quality.

2.5 Hydro-wind-solar complementation

Wind and solar power is complementary.The quick start/stop of hydro-turbine units can accommodate certain volatility of wind and solar power output where the hydropower station supports the wind-solar complementary output operation so that the overall output is stabilized and involves less fluctuations.The hydropower station with daily and above regulating capacity makes use of reservoir regulation to convert the wind/solar output into water storage and makes reallocation in due time.At present, most hydro-wind-PV complementation in China is achieved by compensating wind power and PV power generation by regulating power sources, such as a unified dispatch of hydropower and pumped-storage power stations on the grid side.The prophase planning of hydro-wind-solar complementary clean energy bases has been conducted in Sichuan, Qinghai, and some other provinces of China.

3 Coordinated operation technology

3.1 Build suitable multi-energy gathering platform and power transmission channels

If the wind and solar power stations are directly connected to nearby power grid substations, it leaves almost no space of complementation among different renewable energy power stations, thereby causing excessive built capacity of substations and transmission lines and huge investments on connected grids.If the grid connection plans and construction schedules can be unified and coordinated in the project planning stage to build suitable multi-energy gathering platforms and transmission channels and then transmit the gathered power through these unified channels, such a practice can fully provide the complementary benefits of different power stations and types of new energy, lower the overall simultaneity factor and volatility of renewable energy, realize grid friendliness, and reduce the investment required in grid connections.

3.2 Make wind-solar forecast more precise

Owing to the randomness of wind speed and solar radiation, wind and solar power output is random, intermittent, and uncontrollable.However, since wind speed and solar radiation have a certain regularity of variation throughout a year, wind and solar power generation is more or less foreseeable across different seasons.The hydropower output does not follow the change of natural runoff due to reservoir regulation, while the natural runoff forecast is highly precise.Therefore, hydropower output can be controlled manually.

The wind and solar power generation forecast serves as the foundation of hydro-wind-solar complementation and its accuracy directly influences the implementation effect.Therefore, multiple approaches should be applied to forecast the outputs of wind power and PV power, instead of any single approach, thus combining their strength and avoiding the weakness of any single forecast in order to obtain satisfactory forecast result and provide fundamental data to optimize system dispatching.

3.3 Clarify the principle of multi-energy complementation evaluation

First, multi-energy complementation should spur the consumption of wind and solar power.Wind-solar-hydro complementation may be conducted only after thorough research on wind-wind, solar-solar and wind-solar complementation.After hydropower or pumped-storage hydropower regulation, the total output of wind-solar-hydro complementation should have the least volatility, that is, in turn, beneficial to the consumption of wind and solar power in the grid.The hydropower station works with wind and solar power stations to balance the wind-solar-hydro output for better consumption of wind and solar power in the grid.The pumped-storage power station has dual purposes of both power generation and pumped-storage ability that converts lower-quality random wind and solar energy into stable peak load power supply of higher quality.The pumped-storage power station usually has limited reservoir capacity.Compared with a conventional hydropower station having large reservoir capacity, the pumped-storage power station may only function for dispatching for a short time.Under the power generation condition, idle capacity of a power station is used to enlarge output in a short time to make up for the gap of wind and solar power output; under the stoppage condition, it may also be quickly started in the case of an emergency in order to respond to the dispatching for sudden variation of wind and solar power output.

Second, it is of benefit to peak regulation as well as a safe and stable operation of the power grid.The increasing development and grid connection of wind and solar power brings about a considerable challenge to the real-time balance and operation safety and stability of the grid.Both conventional hydropower stations and pumped-storage power stations feature a quick startup and climbing speed, which is fit for the regulation of drastic volatility of wind and solar power in a short run and capable of system frequency regulation as well as other auxiliary tasks such as load fast tracking, standby, reactive regulation, among others.In a hydropower system, the power generator is the supplier of peak load power; meanwhile, the pumped-storage power station is also a consumer of the system’s valley energy.The well-coordinated energy storage system and renewable energy system can effectively reduce the impact of renewable energy sources upon the system, mitigating the negative impact of grid connection.The involvement of hydropower stations helps ensure the safe and stable operation of regional grids and reduces system blackout, thus generating considerable economic and social benefits.

3.4 Optimize the operating and dispatching strategy

The operating and dispatching strategy of wind-solarhydro multi-energy complementary operation mainly include the annual dispatching plan, day-ahead dispatching mode, real-time dispatching control strategy, and power system stability control strategy.The mode of wind-solarhydro multi-energy complementary operation is of utmost importance.Under conditions such as characteristics of hydro-wind-solar complementary operation, power transmission channels, limitation of market consumption, compensation benefit of cascade reservoirs and flood prevention requirements, research on dispatching policy optimization of wind-solar-hydro complementation power station should be conducted to determine the comprehensive dispatching of different dispatching levels and the priority levels of wind, solar, and hydro power so as to improve the operation management level of multi-energy complementation power stations.It plays an important role in practically increasing the power generation benefit of multi-energy complementation power stations, securing the grid safe operation, and improving the hydro-wind-solar energy utilization rate.

With the extra connection of wind/solar new energy, the dispatching of hydro-wind-solar complementation system becomes more complicated than that of conventional hydropower systems, which should satisfy not only the demand of power load, flood prevention and control, urban living water supply, ecological water of riverway, among others, but also the matching and complementary demand among different types of energy.In addition, when the total hydro-wind-solar output exceeds the load demand, it is critical for the system to reasonably allocate the extra wind, solar, and hydro power that would otherwise be abandoned.Therefore, in accordance with the characteristics of multienergy complementation systems, establishing a multi-layer, multi-dimensional and multi-goal intelligent mathematical model of wind-solar-hydro multi-energy complementary operation dispatching optimization is necessary.The hydro-wind-solar multi-energy complementary operation relates to both the power system and various resource systems.Therefore, based on the electric load demand and generation characteristics of hydro, wind, and solar power sources, systems engineering methodologies should be applied to study the balanced allocation of electric load to different power sources and to reasonably develop corresponding long-term, short-term, and in-plant dispatching policies with the aim of guiding the operation of all parts of multi-energy complementation system and lay a solid foundation of safe, economical and environmentfriendly operation of the entire power system.

4 Future hybrid development of hydro-windsolar complementation

Clean and low carbon power generation will be an inevitable trend of global energy development in the future.Renewable energy is an inevitable means to achieve clean and low carbon development.In the future, China’s power demand and power configuration adjustment still have large potential.High penetration of renewable energy in China requires a large-scale increase in hydropower, pumpedstorage hydropower, wind power, and PV power in China.To meet the requirement of large-scale renewable energy for grid connection and to achieve more efficient hydrowind-solar complementation, improving the operation management and further optimizing the hybrid development mode is necessary.

4.1 Improvement of hydro-wind-solar complementary system intellectualization

Improvements in intelligent monitoring systems forms the basis of the intelligent dispatching of the hybrid system, with considerable data being acquired on the same system platform for the integrated and correlated monitoring of the hybrid outputs and provision of reliable data for energy management and big data analysis systems.The energy management system and control strategy should be optimized in combination with the hybrid outputs, load demand, environmental constraints, among others, to automatically control the hybrid outputs and maintain the real-time balance between power generation and load for higher efficiency of hybrid operation.A big data analysis system should be set up to discover the in-depth value of data using data mining and then realize intelligent early warning, intelligent operation and maintenance, and intelligent dispatching based on big data.The construction of decision support systems should be promoted to improve the prototype structure design and integration methods, generalized template design and development, systemdevelopment mode and development platform selection, and communications and network environment selection, for improving the practicability, interactivity, versatility, expansion, of the systems and realizing centralized, automated, intelligent, visualized dispatching operation.

4.2 Improvement of the hydro-wind-solar complementary operating and dispatching methodology

As hybrid development primarily involves hydropower adjustment, coordination among the dispatching authorities of hydropower, wind power, and solar power is necessary to achieve the goal of unified planning and dispatching.For separate connections of hydropower, wind power, and solar power with the grid, the dispatching center should conduct hybrid operations according to the dispatching strategy.

For a hybrid connection with the grid, a grid dispatching system may assign power generation tasks to the hybrid dispatching system, which then plans the power generations for hydropower, wind power, and solar power according to the hydropower adjustment demand, wind power forecast, and solar power forecast.It then adjusts the real-time output of cascade hydropower stations, wind farms, and PV power plants through coordinated control strategies, to meet the power demand of the system.

4.3 Establishment of hydro-wind-solar complementary clean energy base

China has abundant hydropower sources, mainly distributed in the main streams of great rivers.These regions are also rich in wind and solar energy sources; thus, the generation of hydropower from these rivers can help develop a hydro-wind-solar complementary base.In view of the position and function of hydropower and hybrid operation and management, hydropower stations at rivers will play a significantly important role in revealing its adjusting ability.Wind and PV in regions with rivers are also being developed to establish a renewable energy platform and to connect it to the grid.Preliminary planning works regarding the hydro-wind-solar complementary clean energy base are currently being conducted in China.

5 Conclusion

To address climate change, China is positively adjusting the configuration of energy generation and consumption as well as developing renewable energy sources.China has made considerable efforts with respect to hydrowind-solar complementary development.It has abundant resources of hydropower, wind power, and solar power and shows promising potential for future development.It is still necessary to conduct research on this hydro-windsolar complementary base so as to establish a clean energy system.Hydro-wind-solar complementary energy system development, as an important means of power supply-side reform, will further promote the development of renewable energy and the construction of a clean, low-carbon, safe, and efficient modern energy system.