Economic feasibility of large-scale hydro-solar hybrid power including long distance transmission

1 Introduction

At present, large-scale clean energy increasingly plays an important role in energy security and planning owing to advances in low-carbon and clean energy sources.In recent years, solar PV generation has significantly increased, while its cost has rapidly decreased.Although it is expected to become the most competitive renewable energy power, solar PV still faces several challenges, such as large-capacity power accommodation, long-distance transmission, and low power quality.On the other hand, hydropower is a type of clean renewable energy with instant output adjustment and flexibility in storage and discharge.It can be used as compensation for solar PV to effectively address the abovementioned challenges.

In term of global distribution, Africa, South America,Central Asia, and Southeast Asia have abundant hydropower resources and excellent solar conditions with enormous potential for power development.However, barriers remain in these regions where the energy resources are usually far from load centers, requiring long-distance power transmission, thereby undermining the economic feasibility of such energy sources.Hydro-solar hybrid power development with complementary operation and bundled delivery can be adopted to overcome these barriers by improving the power quality, transmission channel efficiency, and economic efficiency.Therefore, a hydrosolar hybrid power system will probably encourage clean energy development in these regions.

Some establish a technical economic model to analyze the technical and economic feasibility of multienergy complementarity.In order to solve the problem of continuous energy supply in photovoltaic (PV) power plants, Jure Margeta, Ioannis Kougias and Alexandre Beluco et al.have proposed the theory of hydro and solar power complementation, which can solve the intermittent and unstable solar power generation, while avoiding the curtailment [2] -[5].In the application of hydro-solar complementation, in order to maximize the system energy production and guarantee rate while ensuring the stability of the system, it is often necessary to establish a longterm multi-objective optimization model [6-8, 10].At the same time, the potential of new energy sources has been continuously explored.Relevant scholars have combined solar photovoltaic energy with wind energy for power generation [1, 9, 11].However, because wind and solar PV are greatly affected by climate, system is unstable.Therefore, many countries use energy storage to cooperate, thereby increasing system profits and reducing risks [20, 23].Due to the instability of wind resources, relevant scholars have combined pumped storage power stations with wind farms [13-15], seeking the most modest operation mode between the two, so as to maximize the benefits.Li et al describe the simulation of economic performances of further PV integration considering the technical constraints [16].A unique combination of Solar PV, Wind, Biomass and Vanadium Redox Flow Battery (VRFB) storage integrated hybrid Microgrid has been modeled and implemented practically by Sarkar et al [17].Ryan et al.show that levelized cost of energy(LCOE) analysis can be used to confidently prioritize cost-effective solutions [18].Benasla et al.put out the specific HVDC links via which North Africa’s solar resources could play a key role in European sustainable energy system [19].Some analyze two or more combined operations of wind power, photovoltaic power, hydropower, energy storage power, etc.for example, winddominated systems or balanced mixes of wind and solar [20], “virtual battery” facilitated by hydro reservoir based power plants and their current connection to grids [21], distributed grid-connected systems with and without battery [22], the least cost system consisted of batteries, pumped hydro storage, adiabatic compressed air energy storage, thermal energy storage, and power-to-gas [23], the integrated operation of a novel hybrid involving wind and solar power and a hydroelectric power station with pumping installation [24], and the hybridization of CSP and PV systems [25].Some take longyangxia, the largest hydro-photovoltaic complementary station, as an example to calculate the complementary of hydropower and photovoltaic power, and put forward the scheduling mode, calculation theory and method of hydro-photovoltaic complementary [26-29].Wei et al.propose three operating strategies of hydro-solar PV complementary power generation and establish the model of optimal operation strategy, then analyze the project of complementary photovoltaic-hydro power generation system in Golmud finding that the operation strategy has good application effects in operation of the complementary photovoltaic-hydro power generation system [30].Pang et al.put out that a photovoltaic power plant is used as an additional unit of a hydropower station that rapidly adjusts its generating units and compensates for fluctuations in the photovoltaic power output so as to achieve an optimized total power output [31].

In this study, an economic analysis model of hydrophotovoltaic complementarity in data-lacked or non-data regions is proposed to use for macro-planning research of large-scale hydro-photovoltaic complementarity.The research contents are as follows: explore an economical and efficient way of hydro-photovoltaic complementation joint development, to the end of the floor price lowest as the goal, aiming at the lowest landing price at receiving end, the operation mode of hydropower and photovoltaic complementation is optimized to calculate the economical installation ratio of them, take some typical hydropower projects in Africa as cases to carry out the technical and economic analysis of hydro-photovoltaic complementary delivery and verify the relevant conclusions.

2 Research approach and quantification model

2.1 Research approach

The goal of the optimization in this study is to minimize the bundled LCOE in order to develop an economical approach for large-scale transmission of hydro-solar power.In this study, the following approach was used.

(1) Establish a quantification model to optimize thehydro-solar hybrid operation and calculate the bundled LCOE, including hydro-solar power and transmission cost, taking the requirements of hydropower and solar PV operation into consideration.

(2) Classify hydropower according to the reservoir regulating capacity.Allocate different installed capacities of PV systems to hydropower stations with given installed capacities; perform techno-economic calculations on a number of long-distance hydro-solar power package transmission schemes through quantitative modeling to determine the most economical hydro-solar installed capacity ratio.

(3) Calculate the economic indicators of bundled transmission of hydro-solar hybrid power system with the most economical hydro-solar power ratio using typical hydropower stations as case studies.

2.2 Quantification model

The quantification model can be used to simulate the hydro-solar power output process based on the resource characteristics of hydropower and solar PV stations as well as the power transmission characteristics to calculate the on-grid energy, curtailment power, transmission loss, off-grid energy, transmission line utilization, and other parameters of hydropower and solar PV.In addition, the off-grid electricity cost can be calculated in addition to the investment and operating costs of the power plant and transmission lines.

2.2.1 Requirements for Hydro-solar hybrid power Operation

(1) In peak hours of solar output, the hydro power output is reduced and solar PV output is fully absorbed.In the period of low solar output, however, the hydropower output is regulated based on the electrical loads at the receiving end to meet the electricity needs of consumers.

(2) In order to meet the downstream water needs, the minimum hydropower output must not be lower than the forced output and the constraints of the reservoir volume and the storage capacity should be met.

(3) The characteristics of the bundled transmission output should be matched as much as possible with the electrical loads at the receiving end.That is, the output of the combined power source should fit the standard load curve of the receiving end as much as possible.

2.2.2 Calculation of Electricity Quantity Indicator

The hydro-solar hybrid power output process was optimized step by step to minimize losses in the combined power system and maximize the transmission channel utilization.The hydropower output was adjusted based on full utilization of the maximum capacity of solar PV.That is, the solar PV station was regarded as the base station.

(1) Calculation of the on-grid energy:

where Esum is the annual on-grid energy of the hydro-solar hybrid power system, Pt(i) is the maximum transmission capacity of the transmission line in the period i, P(i) is the maximum output of solar PV in the period i, f(h, q) is the dispatch criteria of the hydropower plant operating separately, and Phy(f(h, q), i) is the hydropower output in the period i based on the scheduling operation daily rule.

(2) Calculation of the power curtailment:

2.2.3 Calculation of Electricity Price Indicator

The economic indicator is expressed in terms of the LCOE.The LCOE of hydropower, solar PV, and transmission lines is calculated as given in Eq.(3):

where i is the type of energy component, that is, hydropower, solar PV, and transmission line in this study; It,i is the investment expenditure of energy i in the year t; Mt,i is the operating expense of energy i in the year t; Et, i is the quantity of grid-connected electricity or quantity of transmitted power of energy i in the year t; r is the discount rate; and n is the number of operating years.

The bundled levelized cost of power from the power plant to the demand side is:

wheres the wteighted average of the levelized cost of hydro-solar power; LCOEt is the levelized cost of power transmitted by ultra high-voltage direct current (UHVDC).

3 Study on installed capacity ratio of hydrosolar power system

The economically optimal ratio of hydropower installed capacity to solar PV installed capacity is deduced by the lowest bundled LCOE obtained for various cases.

The reservoir regulating capacity of hydropower determines the capability of compensation regulation and load tracking of the daily solar PV output.It is generally considered that hydropower with seasonal regulation or above capacity reservoir can be employed for daily tracking regulation of solar PV throughout the year withoutadditional curtailment (compared with similar hydropower operated separately), whereas hydropower with daily/weekly regulating capacity reservoir can only be combined with solar PV in dry season without additional curtailment.Therefore, this study investigated hydropower based on its regulating capacity reservoir.

(1) Hydropower with yearly regulating capacity reservoir: A hydropower station A, with yearly regulation reservoir was selected with installed capacity of 6 000 MW and capacity factor of 0.46.The maximum developable capacity of the supporting PV base station B is 12 GW and the capacity factor is 0.23.The transmission distance is 2 000 km and the transmission capacity is equal to the larger of the installed capacity of hydropower and solar PV stations.Table 1 presents the main parameters of the power stations and the transmission lines.

In the calculation cases, hydropower station A was bundled with solar PV stations of different installed capacities to measure the bundled LCOE.The installed capacity of the hydropower station was kept constant at 6 000 MW, whereas that of the solar PV station was varied from 0 to 12 000 MW in steps of 600 MW.The bundled LCOE is minimum (4.71 cents per kWh) when the installed capacity ratio is 1:1, which is 16% and 28% lower than when hydropower and solar PV are separately transmitted, respectively.Therefore, an installed capacity ratio of 1:1 gives significant competitive advantages.The economic indicators of each case are shown in Fig.2.

Technical and economic calculations for the 21 cases were carried out using the quantification model.The results show that with hydropower output regulation, the curtailment rate of solar PV in each case is lower than 3%.Therefore, the LCOE of solar PV and hydropower are essentially the same in all the cases.The load factor of the transmission line first increased, then decreased with the increase in the installed capacity of solar PV.When the installed capacity of the solar PV station is 6 000 MW, the load factor of the transmission channel reached the maximum (0.683) and the LCOE of the transmitted power is minimum.

Fig.1 Operating indicators for each case

Fig.2 Economic indicators of each hydro-solar hybrid power case

(2) Hydropower with daily regulating capacity reservoir: The representative hydropower station B with only daily regulating reservoir was selected (see Table 1); the parameters of the solar PV generation and transmission line remain unchanged.In general, the capacity factor of a hydropower plant with low regulating capacity is large.The capability of the bundled solar PV is relatively limited as the load factor of the transmission line is large.In the calculation cases, the hydropower station B was bundled with solar PV of different installed capacities to calculate the bundled LCOE at the receiving side.The capacity of the hydropower station is 6 000 MW, whereas that of solar PV is verified from 0 to 3 000 MW in steps of 300 MW.In total, 11 calculation plans were established.

Table 1 Main characteristic parameters of power supply and transmission line

S/N Item Value Hydropower station A Installed capacity(MW) 6000 Capacity factor 0.46 Regulating capacity(MW) Reservoir with yearly regulating capacity Total investment(Bil.USD) 10362

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S/N Item Value Hydropower station B Installed capacity(MW) 6000 Capacity factor 0.722 Regulating capacity(MW) Reservoir with daily regulating capacity Total investment(Bil.USD) 9452 Solar PV base station Irradiation intensity(kWh/m2) 2300 Capacity factor 0.23 Investment per kW(USD/kW) 900 Transmission line Transmission capacity The larger of the installed capacity of hydropower and solar PV stations Transmission distance(km) 2000 Total investment(USD/kW) 650

Technical and economic calculations of the 11 cases were carried out using the quantification model.The calculation results show that when the installed capacity of solar PV increased, the load factor of the transmission line increased from 0.72 to 0.82, the LCOE of the transmitted power decreased from 2.10 cents per kWh to 1.93 cents per kWh, the curtailment power of the solar PV station increased from 120 GWh to 2 010 GWh, the LCOE of solar PV increased from 2.8 cents per kWh to 4.21 cents per kWh, and the bundled LCOE increased from 4.95 cents per kWh to 5.10 cents per kWh.When the installed capacity of the solar PV is 1 800 MW, the bundled LCOE reaches the minimum (4.95 cents per kWh).The case with hydro-solar installed capacity ratio of 1:0.3 is economically optimal; the price is 3% and 15% lower than when hydropower and solar PV are separately transmitted, respectively.The technical and economic indicators of the above-mentioned plans are shown in Fig.3 and Fig.4.

4 Case analysis

The large-scale hydropower station C in East Africa with annual regulation reservoir and the hydropower station D in West Africa with daily regulation reservoir were bundled with a large-scale PV base station to calculate and analyze the economic indicators of transmission.The characteristics of the plan are shown in Fig.5.

Hydropower station E is the planned hydropower project which has the largest installed capacity in Africa.It will ease the current situation of power shortage in southern Africa and in the meantime, it can be delivered distantly to Europe for market consumption.

Hydropower station C is the largest hydropower project in Africa which is currently under construction.The electricity station is built to solve the problem of local power shortage and poverty.It is also considered to transmit part of the power to North Africa and Europe to promote local economic development.

Fig.3 Main technical indicators of each plan

Fig.4 Economic indicators of each hydro-solar hybrid power plan

In the above two cases, the economy of long-distance delivery is the key factor in determining the effect of the project.

Case 1: The power generated by the bundled hydropower station C and solar PV station D is transmitted to Greece and Italy in southern Europe subject to hydrosolar hybrid operation.

Case 2: The power generated by the bundled hydropower station E and solar PV station F is transmitted to Spain.

Fig.5 Schematic diagram of a typical hydro-solar hybrid power project in Africa

4.1 Case 1

4.1.1 Hydropower Station C

In the hydropower station C with annual regulation reservoir, the reservoir storage capacity is approximately 60 billion m3, the installed capacity is 6.45 GW, the annual average electricity generation capacity is 16.145 TWh, the number of full-load utilization hours is 2,502, the construction period is 6 years, the total investment is USD 4.8 billion, and the LCOE is 3.06 cents per kWh.

4.1.2 Solar PV Base Station D

In the PV base station D, the irradiation intensity is 2 300 kWh/m2, and the technically developable installed capacity is 280 GW.According to the solar energy in this location, the number of power generation hours is 2 000.When this power station is operated separately, the electricity price is 2.8 cents per kWh.

The calculation results (Table 5) of the economic indicators show that the bundled LCOE is 6.72 cents per kWh after hydro-solar hybrid operation, which is respectively 1.94 cents per kWh and 1.27 cents per kWh lower than those of hydropower and solar PV transmitted separately, thereby significantly improving the economic performance of the project.

4.1.3 Hydro-solar hybrid power

The number of utilization hours of hydropower station C and solar PV station D is small.If the power generated by the two stations is transmitted separately, the utilization rate of the transmission channel is low, and the price of transmitted power is high.In addition, the solar PV station does not match the load characteristics at the receiving end due to lack of regulating capacity and large power curtailment, leading to increase in the bundled LCOE and reduction in the market competitiveness.Considering the high regulating capacity of the hydropower and the location of the solar power station on the external transmission channel of the hydropower station, the hydro-solar hybrid power bundle was adopted to transmit power (6.4 GW) from power station C to PV base station D to be bundled with solar power (6.4 GW), followed by transmission to southern Europe.

The hydro-solar hybrid power operation was simulated as shown in Fig.6.When the solar power output increases, the hydropower output decreases.For extremely small power curtailment ratio (less than 1%), the number of channel utilization hours increased to 4 421, and the price of transmitted power after bundling is more than 50% lower than those of hydropower and PV power transmitted separately.In addition, the quality of transmitted power improved, and the transmission capacity during peak hours increased.The hydro-solar hybrid power results in economic benefits considering the price difference between the peak and valley hours at the receiving end.

The calculation results (Table 5) of the economic indicators show that the bundled LCOE is 6.72 cents per kWh after hydro-solar hybrid power operation, which is respectively 1.94 cents per kWh and 1.27 cents per kWh lower than those of hydropower and solar PV transmitted separately, thereby significantly improving the economic performance of the project.

Fig.6 Daily output of hydro-solar hybrid power

Table 5 Economic and technical indicators of case 1

Hydro-solar hybrid power Installed capacity of hydropower station (GW) 6.4 - 6.4 Installed capacity of solar PV station (GW) - 6.4 6.4 Hydro-solar power ratio of installed capacity only hydropower only Solar PV 1:1 Transmission capacity of hydropower (GW) 6.4 - 6.4 Transmission capacity of hydro-solar hybrid power (GW) - 6.4 6.4 Price of hydropower (GW) 3.06 - 3.06 Price of solar PV (GW) - 2.8 2.84 Power curtailment (GWh) 0 0 113 Transmitted power (GWh) 16134 12000 28021 Transmission price (Section 1) (cents per kWh) 5.6 - 2.45 Load factor of transmission (Section 1) 0.286 - 0.286 Transmission price (Section 2) (cents per kWh) - 5.19 2.15 Load factor of transmission (Section 2) (cents per kWh) - 2000 4421 Bundled LCOE (cents per kWh) 8.66 7.99 6.72 Item Hydropower transmitted separately Solar PV transmitted separately

4.2 Case 2

4.2.1 Hydropower Station E

In the hydropower station E, the installed capacity is 8,000 MW, and the capacity factor is 0.72.Therefore, this hydropower station essentially has no regulating capacity.The total investment is 112 billion USD, and the LCOE is 2.8 cents per kWh.

4.2.2 PV Base Station F

In the solar PV base station F, the solar irradiation intensity is 2,300 kWh/m2, the technically developable installed capacity is 110 GW, and the capacity factor is 0.23.The LCOE is 2.8 cents per kWh.

4.2.3 Transmission Plan

Hydropower station E with low regulating capacity reservoir runs largely as the base station.If its output significantly decreases, the curtailment power will increase rapidly.When the hydropower is bundled with solar PV (2.4 GW) at a ratio of 1:0.3, the utilization rate of the transmission channel and the economic performance can be improved.

The power (8 GW) of hydropower station E is transmitted to Morocco in North Africa and bundled with solar PV (2.4 GW), followed by further transmission to Spain and Portugal.

The typical daily operation of the hydro-solar hybrid power is shown in Fig.3.When the solar PV output increases, water is stored in the reservoir in the hydropower station to minimize hydropower output and absorb solar PV.The hydropower output is increased during peak load hours at night.

The calculation results of the economic indicators (Table 6) show that the bundled LCOE is 6.56 cents per kWh in the case of hydro-solar hybrid power, which is 0.1 cent per kWh and 0.65 cent per kWh lower than those of hydropower and solar PV transmitted separately, respectively.This can improve the economic performance of the project.

Table 6 Economic and technical indicators of case 2

Item Hydropower transmitted separately Solar PV transmitted separately Hydro-solar hybrid power Installed capacity of hydropower station (GW) 8 - 8.0 Installed capacity of solar PV station (GW) - 8 2.4 Hydro-solar power ratio of installed capacity Full hydropower Full solar PV 1:0.3 Transmission capacity of hydropower (GW) 8 - 2.4

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Item Hydropower transmitted separately Solar PV transmitted separately Hydro-solar hybrid power Transmission capacity of hydro-solar hybrid power (GW) - - 8.0 LCOE of hydropower (cents per kWh) 3 - 3 LCOE of solar PV (cents per kWh) - 2.8 2.8 Power curtailment (GWh) 0 0 156 Transmitted power (GWh) 51992 16000 55152 LCOE of transmission (Section 1) (cents per kWh) 3.66 - 1.95 Transmission load factor (Section 1) 0.721 - 0.721 LCOE of transmission (Section 2) (cents per kWh) - 4.41 1.79 Transmission load factor (Section2) - 0.228 0.789 Bundled LCOE (cents per kWh) 6.66 7.21 6.56

5 Conclusions

Africa, South America, Central Asia, and Southeast Asia have abundant hydropower resources and excellent solar conditions.Hydro-solar hybrid power system can improve the utilization rate of the transmission channels and reduce the power curtailment of the bundled power system owing to the regulating capacity of the reservoir.This can reduce the bundled LCOE, improve the overall economic performance, and promote the development of large-scale clean energy bases.

Hydropower with yearly regulating capacity reservoir is very important in hydro-solar hybrid power system, and should be given priority in the planning and development of large-scale clean energy power centers.Hydropower station with seasonal or above regulating capacity reservoir bundled with solar PV capacity at the ratio of 1:1 for long distance transmission will significantly improve the overall economic performance and the transmitted power quality.Taking the hydropower station C (6.4 GW) in East Africa and solar PV station D as case studies, the bundled LOCE was 6.72 cents per kWh when the two stations were bundled, which is 1.92 cents per kWh lower than that of the hydropower transmitted separately.

For hydropower with daily regulating capacity reservoir, a hydro-solar hybrid power system can also be implemented under appropriate conditions to improve the overall economic performance.

When the solar PV LCOE is lower than that of the hydropower and investment in additional transmission channels is low, hydropower with daily regulating capacity can be properly bundled with solar PV.The overall economic performance was optimal when the hydropower station with daily regulation reservoir was bundled with the solar PV station with 0.3 times the equivalent capacity of the hydropower station for long distance transmission.The optimal ratio of the installed capacity of a specific project should be calculated according to the hydropower capacity factor and the bundled LCOE of the hydro-solar hybrid power.The hydropower station E (8 GW) in West Africa was used as case study.When this station was bundled with the solar PV station F (2.4 GW), the bundled LOCE in Europe was 0.1 cent per kWh lower than that of the hydropower transmitted separately, which slightly improved the economic feasibility.

Acknowledgements

This work was supported by the Global Energy Interconnection Group’s Science & Technology Project “Global Clean Energy Potential Estimating Model: Methodology and Application” (524500180011).