Prefeasibility analysis of the pumped hydro storage (PHS) system in Türkiye: A case study on a hybrid system

: Pumped hydro storage (PHS) power plants aim to exploit the price difference between storing and generating electricity. These power plants operate by pumping water from the lower reservoir to the upper reservoir, consuming energy and generating electricity by transferring water from the upper to the lower reservoir. There is no pumped storage power plant in Türkiye yet and it is in the planning stage. This study aims to provide a preliminary feasibility analysis of this investment from an economic and technical point of view and to contribute to this issue through the recently announced feed-in tariff for PHS. The planned PHS at Gökçekaya Dam was considered as a proposal in this study and was carried out using a developed algorithm. The algorithm determines the optimal installed capacity of hybrid energy. This feasibility analysis is based on two scenarios. The difference between the first and the second scenario is due to the investment cost of the PHS system. Additionally, the second scenario considers integrated hybrid Solar Hydroelectric (SHE) system. Each scenario is evaluated in terms of base price, average price, maximum feed-in price, and market peak price. The result of the study is that only the market price represents a remarkable payback period for pumped storage power plants. As a result of the study, it was found that it’s possible to support the pumped storage power plant with hybrid solar power system and market price if only the storage volume should be increased. The feed-in tariff should be set to cover the demand. In the first scenario, only the PHS was evaluated, and after completing the economic analysis, the investment has a payback period of 28.39 years for the market peak price. If the PHS facility is supported by a hybrid solar energy system for internal energy needs, the payback periods can be reduced. In the first scenario, the investment has a payback period of 18.05 years supported by integrated hybrid solar energy. In the second scenario, the PHS investment has a payback period of 9.63 years for the highest price on the market. The investment has a payback period of 8.66 years, which is supported by the integrated hybrid solar energy. Due to the high self-consumption of energy, an integrated hybrid solar energy is suitable for the PHS projects.


Introduction
Pumped hydro storage power plants are hydroelectric power plants aimed generating additional electricity.The concept of such plants is to pump the reservoir from a lower level to a higher level and then, when needed, release that volume of water back into the lower reservoir.The water is pumped into the upper reservoir during off-peak hours when electricity prices are low and released into the lower reservoir during peak hours when electricity prices are high, resulting in an economic gain.A typical pumped hydro storage (PHS) plant is shown in Figure 1.
The largest share among renewable energy sources is accounted for by hydropower plants.The dams or storage facilities are used not only for electricity generation, but also for irrigation and flood control.Recently, hydropower plants with pumped storage are on the agenda in all countries.These plants are considered in the context of physical storage.The aim is to increase the volume of conventional water storage or to make profits by exploiting the price differential between pumping and power generation.Hydroelectric Power Plant (HEPP) systems with pumped storage are used as an alternative to other storage systems.Since battery technology is more expensive to implement and the annual operating cost/renewal cost/battery life isn't what is expected, pumped storage has been chosen, which can be a physical storage method.The table below provides a cost comparison between other storage and pumped storage HEPP facilities.This table is prepared with 2021 indicators by Pumped Hydro Storage International Forum (PHSIF).According to the pumped hydro storage capabilities and costs study of this forum, storage costs are still much higher.Information on storage types and unit costs is provided in Table 1 below [2] .
Table 1.Comparison of energy storage technologies [2] .Here PHS stands for pumped storage power plants, LFP for lithium-ion battery storage, LAB for lead-acid batteries, Vanad.RF for vanadium battery storage RF, CAES for compressed air storage, and Hydrogen for hydrogen combined with fuel cells.All the technologies mentioned in the table are methods of energy storage.Pumped Hydro Storage plants are one type of mechanical energy storage, other general methods are electrochemical, thermal, electrical, and hydrogen storage.Each of these types of storage has its own characteristics.When we compare these energy models with each other, we find that they differ in terms of charging/discharging time and size specification.Thanks to these features, they can be used in different areas.Basically, we can divide the common usage areas into three sections that clarify reserve & response services, transmission & distribution support grid, and bulk power management.Pumped Hydro Storage is the energy storage method that requires the highest charge-discharge time and maximum size of installed capacity.In the Figure 2 below, all energy storage methods used in detail and their comparisons between themselves and Pumped Hydro Storage are given [3] .
Figure 2. Comparison of the energy storage systems in terms of storage capacity and discharge time [3] .
According to the IHA 2022 Status Report, there are pumped storage power plants worldwide with an installed capacity of about 162 GW.The five countries with the highest installed capacity are listed in Table 2 below [4] .Most countries prepare feasibility studies and analyze studies to determine the above pumped storage power plant.In Türkiye, feasibility studies for the development of pumped storage power plants have been carried out for a long time and there is no plant in operation yet.Under the legislation published in the Official Gazette on February 12, 2020, the first step was taken for HEPP with pumped storage.The Gökçekaya PHS, to be completed between 2020 and 2032, will have a total installed capacity of 1.400 MW and is to be completed in Turkish-Japanese cooperation with a value of TL 6.3 billion, according to the legislation that came into force.Türkiye Elektrik İletişim A.Ş. under the coordination of the abolished Electricity Works Survey Administration.(TEİAŞ) and Japan International Cooperation Agency (JICA) under the "Optimal Power Generation Project for Meeting the Peak Demand in Türkiye" which started on February 02, 2010 and was completed in February 2011, HEPP projects were also developed by Tokyo Power Company (TEPCO) experts [5] .
Although pumped storage plants are a new challenge for the world, the plants need to be developed in a short time, especially in terms of the supply of energy and water resources, and many studies have been conducted on this mechanism.Rehman et al. evaluated pumped storage technology, the suitability of hybrids, and studies on islanding of Pumped Hydro Storage systems [6] .Blakers et al. emphasized that pumped hydro energy offers a longer storage time than other battery storage methods [7] .Steffen investigated the application areas of pumped hydro storage for Germany [8] .Ma et al. analyzed the use of pumped storage power plants and battery storage for islands that meet their energy needs from renewable sources [9] .This study is a kind of research article that consists of energy storage technologies, like electrochemical (battery) and mechanical (pumped hydro storage-(PHS)) facilities, for the system in Hong Kong.A case study is performed to clarify the relationship between energy storage systems and the grid.Within the scope of this study, the authors aim to reveal the potential of energy storage systems and lead to project sponsors.A comparison of energy storage systems is examined by the levelized cost of energy (LCOE), life-cycle costs (LCC), and the LCC ratios methodologies.At the end of this comparison, pumped hydro storage is defined as the optimal solution for the system, with the lowest LCC ratios.Additionally, due to the charge/discharge period of the pumped hydro storage facilities, the system which is connected to the grid with PHS, is more feasible than the battery option.Yang and Jackson conducted a SWOT analysis of pumped storage power plant use for the United States [10] .Hunt et al. evaluated existing and proposed pumped storage power plants [11] .They emphasized that integrated hybrid energy is so trend, combined with renewable sources like wind and solar energy.The main problem of this synergy is defined as instability of the renewable energy.So, they examined energy storage options in their study.They exhibited the positive and negative sides of the pumped hydro storage facilities.They examined the cost of water storage.Sivakumar et al. studied and projected the long-term use of pumped storage power plants for India [12] .Foley et al. evaluated pumped hydro storage (PHS) that can be operated in the long term with respect to wind energy [13] .They stated that pumped hydro storage facilities are so important for the grid connection.The most compelling circumstances part of this investment is the financial structure.So, economic analysis is the vital evaluation of this project.In general, economic indicators like the payback period are taken into account by project sponsors or decision-makers.In this study, they offer to combine pumped hydro storage with wind energy.The pumped hydro storage can be used as mechanical energy storage in this mix generation facility.Additionally, a better economic analysis can be obtained by this combined energy system.Javed et al. studied the interoperability of renewable energy sources such as wind, solar, and pumped storage power plants in a hybrid structure [14] .They emphasized that energy storage is the future of energy.They attracted attention to renewable energy penetration.The most optimal solution is offered as solar-wind-pumped hydro storage in their study.This combined energy facility structure is considered on a preferential basis in terms of economic, environmental, and technical.Within the scope of this penetration, they offered an optimization.Renewable hybrid storage facilities can lead to ongoingness alternatives to subsidiary the flabbiness of each other and will be up-and-coming areas for the next generation of investigation.Ma et al. studied the feasibility of a hybrid renewable energy structure for an island in a city, including a pumped storage power plant park order [15] .Kusakana studied the optimization of distributed energy systems using pumped storage power [16] .He stated that the electrification system of the rural area is still a challenge.The renewable energy source is the most promising technology.However, the flexile generation profile of solar and wind resources as well as the flexile electricity consumption restrain these penetrated energy systems from being trustworthy without applicable energy storage systems.In general, solar and wind energy are good candidates in the grid structure of a country field, however, energy storage systems are not taken into account in this penetration.In this study, a combined structure is designed with wind energy, solar energy, pumped hydro storage, and diesel generators for the satisfaction of the electricity demand.This study aims to reduce the operational expenditure of the system.The demonstration of this structure has been constituted by using MATLAB software program.The proposed simulation model provides a balanced energy structure for the rural area.Barbour et al. studied the international energy value of pumped storage power plants [17] .Lin et al. have worked on a small system of photovoltaic and residential pumped storage [18] .Ding et al. have studied the energy management of a system of wind and pumped storage power [19] .They designed an energy structure combined with wind energy and pumped hydro structure for the reliable prediction of wind energy.In the first step, the mixed integer programming (MIP) formula is used to determine the limitations of the unit total on and off frequencies, as well as the unit among of pumping and generating.The designed simulation offers more reliable energy management of wind energy.Javed et al. analyzed a hybrid structure consisting of a battery and pumped storage [20] .Kocaman and Modi conducted a performance analysis for pumped-storage hydro in a hybrid system [21] .Kim et al. studied the operation of air and pumped storage in a hybrid structure [22] .Kapsali et al. focused on an economic analysis of a pumped storage power plant using solar energy [23] .Stocks et al. studied closed-loop systems that provide no outflow, one of the types of pumped storage power plants [24] .Fan et al. prepared a pre-feasibility study for the use of pumped storage power plants at a currently abandoned mine site [25] .Bredeson and Cicilio, reviewed the Pumped Hydro Storage for Alaska region [26] .Baniya et al., stated that Himalaya is so suitable for the pumped storage facilities because of the geographical advantages [27] .Soucek et al., analyzed computational fluid dynamics by using numerical modelling, standards and scientific literature for Pumped Hydro Storage plant [28] .Hu et al., performed a quantitative study for liquid behaviour of the manifold in seawater pumped storage facility [29] .Wang et al., organized energy management of a hydropower plant as Pumped Hydro Storage by using other renewable sources [30] .Ghanjati and Tnani, analized the optimum installed capacity of a hybrid energy facility, including Photovoltaic (PV)/Battery/Pumped Hydro Storage structure by using artificial intelligence methodology [31] .Lei et al., focused on operating conditions of Pumped Hydro Storage [32] .
They analyzed adaptability of the vane under complicated conjuncture of the Pumped Hydro Storage.Lan et al., stated that Pumped Hydro Storage is so important for renewable energy integration.They focused on transient process of facility and performed a case study in China [33] .Huang et al., reviewed the selected project site of the Pumped Hydro Storage such as abandoned mine site.They tried to reveal heavy metal impact on water and environment [34] .Liu et al., optimized a strategy for operation conjuncture of Pumped Hydro Storage [35] .Yi et al., indicated that supercapacitors/batteries can be used in biomedical equipment, aerospace, electric vehicles, military industry, transportation industry and portable electronic equipments [36] .This energy storage systems are used in quick response technologies.The charge and discharge time of this systems are so short.These technologies can be defined as electronic equipment.Pumped Hydro Storage has so different project characteristic.The size of Pumped Hydro Storage can be more than these technologies as a mechanical energy storage.Liu et al., clarified the recent development in the lithium-ion batteries [37] .This technology is a kind of electrochemical energy storage system.In recent years, lithium-ion battery technology has become the most popular technology in the energy storage system.It can be use as a supportive vehicle to the Pumped Hydro Storage facilities.Ma et al., reviewed the economic life of the supercapacitors at various conditions [38] .These technologies are used in quick response electronic devices.They have short time charge and discharge period.Cycle of this tools are shorter than other energy storage systems.These tools are used in high-tech applications.Previous studies have focused primarily on battery technologies, which are electrochemical energy storage methods, and pumped storage has generally been considered an example of mechanical energy storage.In previous case studies, it was assumed that these mechanical energy storage systems will contribute to the environment and the economy if they are built.In contrast to previous studies, this study evaluated the investment period for pumped storage power plants and examined the conditions required for the construction of this structure.This study demonstrated the conditions under which such a structure can be economically evaluated.In contrast to previous studies, the feasibility of a pumped storage power plant was demonstrated, taking into account the contribution of the integrated hybrid Solar Hydroelectric (SHE) system.
In this study, PHS and the suitability of the solar system for pumped storage power utilization was analyzed.The designed algorithm was used in determining the appropriate size of the solar power plant.The algorithm works with the support of benefit-cost methodology.The algorithm uses the Matlab database for a data center.The yield resulting from the amount of energy generated in each cycle step is compared to the cost of the system.The purpose with the best proportional result gives the main applicable installed energy amount.It was found that the obtained result of solar hydropower is a support for the proposed pumped storage power plant.There are many methods of energy storage that can be used given current technologies.Pumped storage power is one of them as mechanical energy storage.Although the charging and discharging times of mechanical energy storage systems are much longer than other energy storage systems, these systems have advantages in terms of the size of installed capacity, durability, number of cycles, and maintenance and repair requirements.In this study, it is shown that the planned pumped storage power plant will be more successful if it is supported by an integrated hybrid renewable energy.The economic analysis was carried out in assumed scenarios, alone and with integrated hybrid SHE systems.The main objective of this study is to provide an economic analysis of the conditions under which a meaningful pumped storage power can be established.The second objective is to demonstrate the effect of integrated hybrid solar-hydroelectric (SHE) systems in energy management and process optimization.

Materials and methods
An algorithm is designed for hybrid power optimization.Definitions of algorithm are given in below.

퐺ℎ푦푑 = 푎푐푡푢푎푙 푒푙푒푐푡푟푖푐푖푡푦 푔푒푛푒푟푎푡푖표푛 표푓 푡ℎ푒 푃퐻푆
(1) 퐶ℎ푦푑 = 푎푐푡푢푎푙 푐표푠푡 표푓 푡ℎ푒 푃푆퐻 푒푛푒푟푔푦 (4) where 퐺ℎ푦푑 is the realized generation of hydro energy, 퐺푠표푙 is the predicted generation of solar energy, 퐺 is the total generation of hybrid facility, 푥 is the repetitive step number, 퐶ℎ푦푑 is the realized cost of hydropower, 퐶푠표푙 is the estimated investment cost of solar energy, 퐶 is the total cost, 푃 is the feed-intariff, 퐵 is the benefit value of hybrid system, Loop is the ratio of benefit/cost, 푓표푝푡푖푚푢푚 is the optimum point of algorithm function.The optimum point determines the optimal installed capacity of hybrid Solar Power Plant (SPP) part.Designed algorithm is given in Figure 3 below.This algorithm is not an embedded program, designed by the author for the optimization of the solar energy installed capacity.The algorithm can be clarified as four parts;  Hydro part of the system (1.
Step)  Solar part of the system (2.
Step)  Cost of the system (3.
Step)  Benefit/Cost cycle of the system-Iteration (4. Step) The developed algorithm evaluates based on the benefit-cost methodology.Currently, energy generated by hydropower is supplemented by solar energy, taking into account grid-connected transformer capacity.Algorithm evaluates the solar energy that can be generated without idle capacity brings the value to a more reasonable level.The goal of the algorithm is to maximize the benefit achieved per unit.Optimal installed capacity is determined by applying this algorithm to existing hydropower/PHS facilities.PHS consumes power to pump volume from a lower to an upper reservoir.The installed power of the pump that provides the specified amount of consumption is calculated using the following equation.
where 푃, is the transferred power of pump (kW), 푄, is the flow (m 3 /h), 퐻, is the height (m), 휌, is the density (kg/m 3 ), 367 is the conversion coefficient, 휂ℎ, is the hydraulic efficiency (%), 푒푓, is the security factor.In general, 휂ℎ, is a value between 40-80 % proportions.Security factors can be change according to required power.

Results and discussion
A case study examines the economic analysis of pumped storage power plants.The existing Gökçekaya Dam and HEPP plant were used for the case study.Although storage has gained importance in recent years, physical storage is still the most commonly used form of energy storage.Two scenarios are discussed as part of the case study.The results are compared when pumped storage is supplemented with stand-alone and hybrid solar hydropower.When evaluating hydropower with pumped storage alone, the energy needed for the pump is purchased from the grid and at lower unit prices.When the same structure is supported by a solar plant, the energy needed for pumping is obtained from the SHE plant.The optimal size of the SHE plant was found considering the prevailing HEPP.oject characteristics is given in the Table 3 below.Using the data of Gökçekaya reservoir, the designed algorithm is executed.In the study, the actual power generation for the year 2021 is obtained from the transparency platform of İstanbul Energy Exchange (EXIST).The algorithm is used to determine the optimal hybrid system SPP as a hybrid structure.In addition, two scenarios are carried out for this study.The scenarios are the pumped storage power plant and the pumped storage power plant supported by the hybrid structure.The designed algorithm was applied to the selected Gökçekaya HEPP plant.The assumptions made for the evaluation are listed below:


The power generation amount of Gökçekaya Dam in 2021 was taken from the transparency platform EXIST.This website is a platform on which the amount of electricity generated is tracked and disclosure to the public by the government [40] . Grid connection is limited to 278.4 MW for the energy generation (278.4MW is the installed capacity of Gökçekaya Dam and HEPP).This is the legal grid restriction of the facility.Energy generation above this limit value cannot be supplied to the grid.The excess generated energy is interrupted by the facility control management, before being given to the grid.


The economic life of the SPP plant is assumed to be 25 years.The stated economic lifetime is the period specified in the datasheets for the First Solar brand PV panels used in the study.The electromechanical equipment of turbines and generators in hydropower plants has a similar economic lifetime.The economic lifetime is a factor that affects the levelized cost of energy (LCOE).
The longer this period is, the lower the LCOE [41] . Installed capacity of the PHS is accepted as 1,400 MW.This installed capacity is the announced by the government officially [42] .


The unit cost is assumed as 857 USD/kW for the SPP facility.The cost of SPP effects LCOE and payback period (PP) calculations.The higher cost is, the higher the LCOE [43] .


Within the scope of the study, a performance comparison was carried out by analyzing two scenarios.


In the first scenario, feed-in tariff is used for the revenue calculation of the PHS  In the second scenario, the energy consumption of the PHS is met by Hybrid SPP facility and this scenario considers integrated hybrid SHE facility  The cost of installed PHS capacity is from the 2021 Pumped Hydro Storage Forum report (2,202 USD/kW) (PHSIF 2021).Under the legislation published in the Official Gazette on February 12, 2020, the Gökçekaya PHS, to be completed between 2020 and 2032, will have a total installed capacity of 1,400 MW and is to be completed in Turkish-Japanese cooperation with a value of TL 6.3 billion.So, 1,046,250,000 USD (equivalent of the TL 6.3 billion according to the February 12, 2020 Central Bank forex buying value) is used as alternative PHS's construction cost.The cost of facilities effect LCOE and payback period (PP) calculations.The higher cost is, the higher the LCOE and PP [44] .


The amount of power generation of combined hybrid energy determined by the algorithm.This electricity generation of facilities effect LCOE and payback period (PP) calculations.The higher generation is, the lower the LCOE and PP.


The payback period calculations assume that the investment is completed in one year and can be commissioned within the next year according to the assumptions.The longer this period is, the higher the PP.


The calculation of the payback period was made very roughly, aspects such as Value Added Tax (VAT), taxes, maintenance investments and depreciation weren't taken into account in the calculation.The additional cost of this expenditure has adverse impact on LCOE and PP.


Only revenue and expense differences are considered in the payback period calculation.In this study, nominal payback period is taken into account.The discount rate has adverse impact on PP.  Feed-in tariff is used as price of the electricity generation.This tariff is announced on 1st May of 2023 by the government.If feed-in tariff is low, this situation will affect LCOE and PP calculations adversely [45] .


Electricity sales prices are supported night, day, and peak unit prices valid for 3 months as of 1.4.2023 announced by Energy Market Regulatory Authority (EMRA).Related prices are given in the table below [46] . 1 USD equivalent is accepted as 19.7607 TL (18.05.2023Central Bank Forex Buying).Possible changes in the exchange rate affect in particular the prices per unit of goods/price in Turkish lira [47] .


The investment cost of SPP is predicated on the 2021 IRENA renewable energy costs report [42] .


The designed algorithm was run with 45,000 cycles, each step size was taken as 25 kW. 2019 IRENA renewable energy costs report relies on the expense assumptions of renewable energy sources.Within the IRENA renewable energy costs report, it's stated that the annual disbursement for solar energy plants varies between 9.5-18.3USD/kW.For the solar energy plant, 14 USD/kW is accepted because of the unit disbursement.it's stated within the report that the fixed disbursement for hydroelectric power plants is 0.06% of the whole investment cost and therefore the variable disbursal is 0.003 USD/kWh.In PHS, the identical figures, excluding electricity consumption expenses, are accepted as operating expenses [43] .The higher cost of this operational expenditure has adverse impact on LCOE and PP.Announced feed-in tariff is given in Table 4 below.Announced energy consumption unit price is given in Table 5 below.In the second step of algorithm, the program contains a section on the generation of solar energy.The amount of solar energy is using for the internal hybrid energy is determined.The solar radiation and the meteorological data provided by NASA for any point on the Earth.A smaple year of original solar radiation data obtained from official website of NASA is given in Figure 5 below [48] .
Figure 5.The daily solar radiation obtained from the NASA website (365 days) [48] .
The NASA database provides data daily, and solar radiation must be determined on an hourly basis to ensure energy management and optimization of installed power.The two models used when obtaining solar radiation amounts on an hourly basis are the econometric model and the empirical model.While conducting the study, it was decided to use an empirical model for the reasons stated below.


Ability of the conversion of daily data  Availability and usage of public data  The high correlation coefficient  Prediction of the long term data The study, which includes an empirical model, was used to obtain the amount of solar radiation on an hourly basis using MATLAB gui.In the Figure 6 below, the results obtained for sample days and the amount of solar radiation obtained for the whole year are given.While obtaining the amount of solar energy, the amount of solar radiation on an hourly basis and the current-voltage response of the PV panel to temperature and radiation were used.The daily radiation amounts were taken from the NASA website for the solar energy plant located within the Gökçekaya HEPP.This original hourly based dataset is given within the Figure 7 below.After determining the hourly solar radiation in the selected region, the amount of solar energy is calculated using the designed algorithm.When calculating the solar energy generation, information obtained from the PV data sheet is used.PV data sheet information includes current-voltage information and variations that can be obtained under nominal conditions (1000 W/m 2 , AM 1.5, 25℃) and radiation.The fluctuations are mainly due to temperature fluctuations.First Solar brand PV panels, model FS-6450, with an installed capacity of 450 Watts were selected for the case study.The Table 6 below contains PV specifications taken from First Solar's official website [41] .Once all the steps specified in the algorithm had been completed, the installed output of the SHE system, which can be implemented as an integrated hybrid system, was determined.Once the contribution of solar energy had been determined, the economic analysis studies began.Taking into account the conditions previously discussed in the "Acceptance" section, the LCOE and payback period were determined as the key indicators.The revenues and costs were calculated according to the specified scenarios.Results are given in Table 7 below.
where 퐿퐶푂퐸, is the levelized cost of energy (USD/kWh), 푃푃, is the nominal payback period of the facility (year).퐼푁푉퐶표푠푡, is the investment cost (USD), 퐸퐿, is the economic life of the facility (years), 푂푃퐸푋, is the annual operational expenditure (USD/y), GEN is the electricity generation of the facility (kWh/y), 푅푒푣푒푛푢푒, is the revenue of the facility (USD/y).
The energy generation of Gökçekaya Dam is about 215 million kWh in 2021.During the construction of the pumped storage power plant, it was planned to pump the discharge from the lower basin to the upper basin.The pumped storage power plant will be built in the future.Since the pumped storage power plant has the same water flow as the existing Gökçekaya power plant, it can produce 1,075 million kWh with an analogous generation profile.It was calculated that a pump with an installed capacity of 1,400 MW.In the 2021 calculations, it was determined that the Gökçekaya HEPP would require an energy consumption of approximately 1,397 million kWh to pump this reservoir from the lower level to the upper level.In the economic evaluation, it was assumed that the required energy consumption would be covered with the relatively low prices for the unit of demand (at night) and that it would be sold during the peak period, which implies a high price for the unit of measurement when selling energy.In addition to this evaluation, feed-in tariff base/average/cap situations are teaken into account.In the first scenario, only the PHS was evaluated, and after completing the economic analysis, the investment has a payback period of 28.39 years for market peak price.Other feed-in tariff options cause meaningless results or long-term payback periods.If the PHS facility is supported by a hybrid solar energy system for internal energy requirement, payback periods can be shortened.In the first scenario, the investment has a payback period of 18.05 years which is supported by integrated hybrid solar energy.In the second scenario, the PHS investment has a payback period of 9.63 years for market peak price.The investment has a payback period of 8.66 years which is supported by integrated hybrid solar energy.The difference in the first and second scenarios is due to the investment cost of the PHS facility.In the first scenario, the size of the plant SPP, which can be installed as a hybrid plant within the existing Gökçekaya dam and the HEPP plant, is optimized using the developed algorithm in addition to the HEPP pumped storage plant.It is shown that a plant SPP with an installed capacity of 904.5 MW (36,180th stage × 25 kW) can be constructed.In the second scenario, it is shown that a plant SPP with an installed capacity of 548.775MW (21,951th stage × 25 kW) can be constructed.The cost-benefit analysis is shown in Figure 8 below.When an evaluation is created to incorporate the pumped-storage HEPP with the contribution of the hybrid SPP structure, it's observed that the second scenario pays back the investment in 8.66 years.During this study, it's assumed that the number of the energy required by the pumped-storage HEPP facility is met by the SPP facility, while the increased energy production is sold during the market demand.The daytime demand unit prices supported sales are the unit prices announced by EMRA and valid for three months as of 01.04.2023.The increases in energy costs experienced in recent months are reflected in unit prices.The best benefit/cost optimization point, SHE system energy amount, and current HEPP/SHE system comparison are given within the graphics Figure 9 below.As can be seen in Figure 9, the energy generation of the existing HEPP plant is sensitive to climatic and meteorological changes.So, the energy management is quite difficult.It also goes without saying that the existing HEPP plant has a low capacity utilization rate for the energy of the facility shown in the graph.A low utilization rate is an indicator of inefficient operation of the plant.In the other graphic, the integrated hybrid structure, it is observed that solar and hydro energy complement each other in terms of energy generation.The generation of the HEPP facility is high in the spring months when the amount of solar energy is relatively low, and the generation of the SPP facility is high during the period when rainfall is low, and these two energy sources complement each other perfectly.This situation leads to more efficient operation of the facility and higher capacity utilization rates.Additionally, the energy efficiency of the transformer is ensured.

Conclusion
Pumped storage power plants can provide additional water volume and profits by taking advantage of the energy sales differential between pumping and power generation.Solar hydropower (SHE) is a hybrid structure of solar and hydropower that uses the same electrical infrastructure.In this study, a PHS system with the new feed-in tariff is investigated.In addition, an integrated hybrid solar power system uses an algorithm to achieve optimal installed power.Solar energy is an option for hybrid energy because of its applicability.Two scenarios were discussed in the case study.The difference between the first and the second scenario is due to the investment cost of the PHS facility.Each scenario evaluates in terms of the base, average, and maximum feed-in price, as well as the market peak price.In the first scenario, only the PHS examine, and after completing the economic analysis, the investment has a payback period of 28.39 years at a market peak price.In the first scenario, the investment has a payback period of 18.05 years, supported by integrated hybrid solar energy.The economic analysis performed in this assessment assumes that the electricity used for pumping consumes at low unit prices and the electricity generated is offered in the market at high prices.The algorithm developed is based on the benefit-cost method.The solar generation profile, compatible with and complementary to the installed profile of the existing hydropower plant when it's low, was determined using the Matlab database.The optimal installed capacity was determined by comparing it with the amount of electricity generated by the sun and the associated investment costs.In the second scenario, it's envisioned that the electricity requirement of pumping will be generated from hybrid solar energy.The electricity consumption required by the pump was covered by solar energy, and the increased solar energy was also fed into the grid at average unit prices (07:00-18:00).Other feed-in tariff options cause meaningless results or long-term payback periods.If the PHS facility is supported by a hybrid solar energy system for internal energy requirements, payback periods can be shortened.In the first scenario, the investment has a pay back period of 18.05 years which is supported by integrated hybrid solar energy.In the second scenario, the PHS investment has a payback period of 9.63 years for the market peak price.The investment has a payback period of 8.66 years which is supported by integrated hybrid solar energy.
The Covid 19 pandemic and, consequently, the recent and ongoing tensions between Russia and Ukraine have led to an increase in commodity, goods, oil, and energy prices.Accordingly, feed-in tariff, which consists only of pumped hydroelectric storage or supported by an integrated hybrid solar energy, no economically viable result can be achieved.The only market price can be applicable for this investment.If the PHS facility is supported by integrated hybrid solar energy for its internal energy consumption, results will be more effective.As a result, storage remains a more expensive technology today.It's expected that the investment and operating costs per unit will decrease with technological development.However, physical storage is expected to be more sustainable to meet current large-capacity needs.In addition, pumped storage is believed to be a response to warming and water scarcity expected in the future.As a result of the study, it was found that it's possible to support the pumped storage power plant with a hybrid solar system and market prices if only the storage volume is to be increased.In addition to this, the feed-in tariff should be determined as a price sufficient to meet the requirement.

Figure 6 .
Figure 6.Estimation of the hourly based solar radiation by using the empirical model in Matlab GUI (a) 31st March, (b) 30th June, (c) 30th September and (d) 31st December.

Figure 7 .
Figure 7.The hourly based solar irradiance obtained from the daily solar irradiance dataset (8760 hours = 24 hours/day × 365 day).

Figure 9 .
Figure 9. Energy generation comparison of existing HEPP (a) and Hybrid SHE (b) facilities.

Table 3 .
The hydropower plant project's characteristics.

Table 5 .
Energy consumption unit price.

Table 6 .
Nominal values of PV panel.