Study on integrated development and hybrid operation mode of nuclear power plant and pumped-storage power station

1 Necessity and superiority of integrated development and hybrid operation mode of nuclear power plant (NPP) and pumpedstorage power station (PSP)

The NPP is characterized by large front-end investment, less environmental pollution, low fuel cost, and deadly high safety requirements.Because of low fuel cost, convenient transportation, and theoretical inexhaustibility, the nuclear fuel resource is of high economic benefit.Stringent requirements are imposed for managing the nuclear power reactor and certain technical restrictions on frequent changes in the base load electricity generation of the unit.The main disadvantages of nuclear power participating in peak regulation include the following: difficulty in operation resulting in a high risk of human error and breach of technical specifications; metal fatigue arising from numerous temperature variations and transient states which shorten equipment life; numerous boronizing attenuation operations which result in more gas waste, liquid waste, and solid waste, thus increasing the environmental burden and social risks; and frequent power change which has an impact on burn-up distribution, causing difficulty in fuel design and after treatment.

The PSP has unique, flexible, and diversified operation modes that not only allow for peak load shaving, they also provide many dynamic services such as load following, frequency control, spinning reserve, and phase modulation, which can both improve the economic operational efficiency of the entire electric power system and increase security, stability, and reliability of such system.In general, the benefits arising from peak load shaving by pumpedstorage are referred to as static benefits, while those arisingfrom providing various dynamic services are referred to as dynamic benefits.Theoretically, in a competitive electricity market, the PSP can obtain a relevant profit by participating in peak electricity bidding; providing auxiliary services including quick response, frequency modulation, standby, phase modulation, and black-start; and undertaking load-shaving operation to pump water for load shaving during low load periods at night, thus reducing start and stop frequency of thermal power and other units.At present, electricity market-oriented reform in China is still at the initial stage; the market mechanism is immature, and the auxiliary service exchange mechanism has not been established.If the PSP only participates in electricity market bidding, it is likely to obtain the expected profit level on the conditions that peak and low-load tariffs differ greatly, and the operational hours of electric power generation reach a certain level.Therefore, the PSP faces a huge financial risk.

The NPP is suitable for operation with a base load, while the pumped-storage unit mainly applies to the capacity benefit in an electric power system; hence, the hybrid operation comprising the two can better meet the needs of an electric power system.Compared with a conventional single business mode, the superiority of the hybrid operation is mainly embodied in guaranteeing safe and stable operation of the electric power system; helping to realize optimization and scheduling of such system; and helping pumped-storage gain a proper role in the electricity market.This study conducted a thorough analysis of the operating characteristics of the PSP and NPP and performed systematic research on the economic efficiency and hybrid operation of the two modes to facilitate their close coordination, aimed at their playing a role in peak and frequency regulation in the power grid, and conducted an analysis of the economic benefits from joint operation of PSP and NPP to address their respective status and role in the electric power system, guarantee their respective financial income, and enhance their market adaptability.The hybrid operation mode allows full development of their characteristics, enhances total power generation benefits, reduces the cost, improves the financial yield rate of both parties effectively, and achieves safe and stable operation of the power grid.

2 Meaning of integrated development and hybrid operation mode of NPP and PSP

2.1 Integrated development mode of NPP and PSP

The development modes for various types of PSPs are power grid wholly-owned, independent, and integrated.

In view of the new situation, to improve their market competitiveness, reduce the operational cost, increase the equipment utilization hours, and enhance market flexibility, integrated development of the PSP in collaboration with the NPP is performed based on the principle that it is mutually complementary and beneficial.This mode, illustrated in Fig.1, is beneficial to the operation of the PSP and it can increase the annual energy output of the NPP.[1][2][4][8][10]

Fig.1 Integrated development mode of NPP and PSP

2.2 Hybrid operation of NPP and PSP

The basic aims for the hybrid operation are to move the NPP off peak regulation, but give full play to PSP, and enable the hybrid energy output curve closely tracking the daily load curve.The different curves illustrating the NPP and PSP units tracking the typical daily load curve after hybrid operation are described in the following three categories: full tracking mode, sectional tracking mode, and partial tracking mode.

(1) Full tracking mode

The hybrid operation units will fully track the typical daily load curve; its output curve is illustrated in Fig.2 We know the utilization hours of NPP units and the cooperative PSP units, thus we can calculate the utilization hours of hybrid operation units.In addition, we can get daily electric energy production of the hybrid operation PSP units andpumping energy provided by NPP; in case of insufficient water storage, an additional independent pumped-storage unit is required for pumping, which shall be solved by weekly regulation; in case of sufficient water storage, such unit can provide other services for the system.[1][2][4]

Fig.2 Operation of full tracking mode

(2) Sectional tracking mode

According to a local typical daily load curve, a day is divided into three periods including peak time, offpeak time, and low-load time, and the output curves of the hybrid operation units are correspondingly divided into three periods during which the hybrid operation units show different output status resulting in a different effective output.The output curves under the three-section tracking mode are illustrated in Fig.3.Under this mode, the utilization hours of NPP units and hybrid operation PSP units can be determined, from which the utilization hours of hybrid operation units can be calculated.In addition, we can obtain the daily electric energy production of the hybrid operation PSP units and pumping energy provided by NPP; in case of insufficient water storage, an additional independent pumped-storage unit is required for pumping, which shall be solved by weekly regulation; in case of sufficient water storage, such unit could provide other services for the system.[1][2][4][7][11][12]

Fig.3 Operation of three-section tracking mode

(3) Partial tracking mode

Based on local typical daily load curves and pumping hours of the PSP units, the daily energy production hours and the operating capacity of the units can be obtained.The corresponding output curves are illustrated in Fig.4.Under this mode, we determine the utilization hours of the NPP units and cooperative PSP units; we can calculate the utilization hours of hybrid operation units, and independent PSP units can be calculated.The pumping energy required for PSP units will entirely come from NPP.[1][2][4][5]

Fig.4 Operation of partial tracking mode

2.3 Integrated business mode of NPP and PSP

The integrated business mode of NPP and PSP means that the two are operated by the same operation company, wherein the PSP acts as the supporting facility of the NPP, undertaking peak regulation, peak load shaving, frequency modulation, and phase modulation required by the power grid system to enable a stable operation of NPP at rated power.The pumping energy of the PSP comes from the NPP with which its electric energy production is bound.The operator schedules the electricity generation output of PSP and NPP, and manages the cost, profit, and repayment of capital and interest related to the two stations in a unified manner.This mode is illustrated in Fig.5.[3][6][9][13][14][15][16]

Fig.5 Integrated business mode of NPP and PSP

3 Working routes of integrated business mode of NPP and PSP

The study on the economic efficiency of hybrid NPP and PSP operation shall take into account the planning and dispatching mode at the present stage and potential electricity market mode in future.Under different market circumstances, the two types of stations will play different roles in the electricity market, resulting in diverse integrated business modes.Currently, China is aggressively promotingelectricity market-oriented reform; step by step, their economic operation period will surpass the existing planned scheduling mode, transit to an incomplete market mode, and finally to a complete market-oriented mode.Hence, it is necessary to conduct an analysis of the current planned scheduling mode and the electricity market circumstances tending toward completeness through reform, then study the hybrid operation mode of NPP and PSP under the following four systems and mechanisms: present planned scheduling mode, near-term electricity market reform mode, interim incomplete market mode, and long-term complete market mode.

4 Current working routes of joint leasing business mode

Currently, the COD PSPs and COD PSPs without tariffs approved will execute a two-part tariff mechanism, namely fixed charge and running tariffs, while those having gone through tariff appraisal will do likewise, step by step.As the PSPs are under a unified scheduling and arrangement by power grid, their pumping energy is provided by the power grid enterprises, and a pumping tariff is collected as a 75% benchmark on-grid tariff on coal-fired units; it is difficult to achieve their economic benefits because peak and lowload tariffs differ inconspicuously, an auxiliary service compensation mechanism has not yet been established, and a low pumping tariff and reasonable fixed charge are unavailable.Similarly, owing to a currently large difference in peak and low loads in the power grid and the increasing installed capacity ratio of clean energy, such as wind power and solar power, the peak regulation in the power grid becomes more severe, and annual utilization hours of NPPs as base load operation are downgraded year by year.This imposes increasing pressure on NPPs in terms of participating in peak regulation, greatly reducing their operational economic benefit and endangering their safety.Therefore, when PSPs and NPPs are operating independently, NPPs can “rent” the capacity of PSPs to increase their load factors and also improve overall profitability.

5 Low-tariff pumping electricity for PSPs and hybrid operation with NPPs in the near term

As the reform of the electricity market goes forward, PSPs now adopt two-part tariff.Owing to the fact that to generate 1 kWh of electricity, a PSP needs to consume kWh to pump the water (i.e., consume 4 kWh to generate 3 kWh), while the tariff for pumping is of the feed-in tariff, the revenue from the running charge almost offsets the cost of pumping.Therefore, most PSP revenue comes from the fixed charges.In the near term, low fixed charges bring limited revenue and cannot truly reflect the capacity service provided by the PSP units.Therefore, PSPs could buy pumping electricity from hybrid operation NPPs in a low tariff (lower than the of the feed-in-tariff) to realize joint operation of PSP and NPP.In such a hybrid pattern, a PSP could obtain power cheaper than of the feed-in tariff, thus generating more revenue from power generation after the pumping cost.The resulting net earnings will enhance the overall profitability of PSPs.In return, NPPs need to generate surplus power; although at a tariff, the utilization hours are increased and generating cost is lowered, higher commercial profits are obtained.

6 PSP plus NPP jointly participating in market competition in the mid-term

In the electric power system, if a nuclear power unit frequently involves peaking operation with fluctuating output, the annual utilization hours and economic benefits of the plant will be degraded.Such operation conditions will speed up material fatigue and thus require more maintenance works, which will shorten the service life of the unit.Furthermore, the increase or decrease of unit output is realized by the insertion or extraction of nuclear rods, which is likely to cause axial power imbalance and endanger the entire power plant.During low-load time, PSPs pump water and increase the output of NPPs.During peak time, PSPs generate power and relieve the pressure of peak load regulating of the power system, contributing to the stable operation of NPP units and relieving the regulating pressure of the grid brought by the base load operation of the NPP.

As electric system reform goes deeper in the mid-term, the electricity market will be further improved, thus PSPs can jointly operate with NPPs.During low-load time, PSPs utilize surplus nuclear power to pump water, and during peak time, they transform the low-price electricity (cheaper than that bought from the grid) to peak-price power to gain an ideal return on investment.NPP participates in the peak load regulating in conjunction with the PSP by providing additional pumping power.This mode reduces the regulating pressure of the power system to a certain extent and greatly increases the power output and utilization hours of NPP, such that the load rate enables the nuclear unit to operate at the design utilization hours, thereby reducing the cost of power generation.While obtaining better economic benefits, this mode also ensures the safe and stable operationof the NPP unit, and realizes a win-win for PSPs and NPPs.In general, with the reform continuing, if PSP can obtain low- tariff power from NPP, joint operation will be a better approach from both safety and economic perspectives in the mid-term.

In the power system, as the load position continues to drop, the marginal feed-in tariff also decreases, and vice versa.By generating electricity during peak load periods and storing water during low load periods, PSPs play a positive role in peak-shaving and low load-filling, which can effectively reduce the peak regulating pressure and increase the acceptance of nuclear power.The electricity market will be more complete in the mid-term.Wheeling charges should be paid when the NPP, operating in high annual utilization hours, provides excess electricity directly for the joint PSP for water pumping.Once the PSP starts to shift the NPP power and hybrid operation is realized, the annual utilization hours of the nuclear power plant will be higher, and the internal rate of return on capital funds that NPP and PSP earn will be higher.Hence, both parties will gain substantial economic benefits.

7 Shifting nuclear electricity as primary and serving power system when needed

In the long term, when the market is fully mature, NPPs and PSPs can engage in market activities side by side.During market competition, owing to the introduction of low-price surplus electricity, PSPs may follow certain quotation strategies and gain price advantages.Owing to surplus power generation and low cost, NPPs can operate in safer and more stable conditions while enjoying a sound economic return.The combined investment and hybrid operation can effectively improve the utilization rate of PSP while ensuring the base load position of NPP.The output curve of hybrid operation can meet the load needs of the power system and improve the quality of power outputs.In addition, PSP also provides build-in auxiliary functions such as frequency regulation, phase modulation, and black start for the power system.

When the electricity tariff is fully marketized, NPP and PSP can engage in market activities side by side.NPP can reach high profitability by ensuring safe operation and maintaining long annual utilization hours.PSP can make profits from a reasonable spread between peak and low-load tariffs and from services provided for the power system.In this mode of operation, PSP and NPP are operated by the same operator, yet quoting as two individuals, i.e., independent quotation and operation.The general feature of the mode is that the PSP generates profits from the spread between peak and low-load tariffs on the market.

8 Adopting suitable mode of hybrid operation for different market environments

To study the economics of hybrid operation, it is necessary to consider the current planning and dispatching mode as well as the potential electricity market mode in the future.In different market environments, NPP and PSP will play different roles in the electricity market.Hence the mode of joint operation should be different.

In the current planning and dispatching mode, NPP and PSP can operate jointly; “leasing” the capacity of PSP can upgrade the load factor of the NPP, and overall profitability can be enhanced to some extent; however, PSP and NPP still cannot gain suited return.Based on the electricity market reform in the near term, hybrid operation can produce ideal economic profits for both parties.With the progress of the reform in the mid-term, all, or part of, the pumping power are coming from NPP units.The capital return rate can exceed the hurdle cut-off rate, and NPP and PSP can obtain ideal profits from hybrid operation.In the long term, when the market is fully mature, NPP and PSP can engage in the market activities side by side.During market competition, owing to the introduction of low-price surplus electricity, PSPs may follow certain quotation strategies and gain price advantages.

Acknowledgements

This paper is funded by the Project “Resource Characteristics of Main Watersheds and Key Issues in Development and Utilization of Hydroelectricity in South America and Africa” and the National Science Foundation of China (U1766201).