Planning of wind-photovoltaic-storage-hydrogen-water for a zero-carbon microgrid on an independent island

Due to low daily demand and the remote location from the shoreline, diesel generators dominate the energy supply on these islands, resulting in significant emissions of harmful gases. In this paper, a collaborative planning approach is proposed for a zero-carbon microgrid incorporating wind turbines (WTs), photovoltaic modules (PVs), electrochemical energy storages(EESs), hydrogen energy storages(HESs), and sea water desalination on an independent island. Long-term and short-term energy storage mechanisms, represented by HESs and EESs, respectively, are coordinated to harness the abundant wind and solar energy resources. Additionally, consideration is given to the requirement for sea water desalination to achieve a self-sustained equilibrium of zero-carbon water and electricity for the island’s inhabitants. Illustrative analysis demonstrates that the proposed planning method accounts for the stability and environmental preservation in terms of the planning and operation. This has profound implications for advancing the transformation of the island’s microgrid energy infrastructure, enhancing the utilization of renewable energy, and bolstering the self-sustaining capacity of hydropower resources.


Introduction
The development of the marine economy and island resources, coupled with the improved living conditions of island residents, has spurred a growing demand for more environmentally friendly and reliable water and power supply systems on islands.For most islands, the sustainable provision of energy resources has posed a significant challenge, impeding their development and utilization potential.As hydrogen generation and storage technologies mature, leveraging the abundant wind and other renewable energy resources unique to these islands becomes essential for establishing a dependable power and fresh water supply system.The introduction of hydrogen storage systems and electrochemical energy storage for long-term and short-term energy regulation is pivotal in enhancing the quality of life for island residents, fostering the growth of tourism and fisheries, and safeguarding coastal interests.
Currently, research is underway to address planning issues related to island microgrids.A method was introduced for calculating the installed capacity of pure renewable energy in large-scale grids and determining the optimal combination of WTs and PVs for island configurations [1].Miao et al. [2] used China's Yongxing Island as a case study, conducting a comprehensive uncertainty analysis and specifying the optimal allocation for its microgrid in 2030.Du et al. [3] presented a bi-level multiobjective planning model for offshore wind power microgrids, striving to design island microgrid solutions that comply with both economic and energy efficiency requirements.While these methods have all established planning models for island multi-energy systems, the planned networks are IOP Publishing doi:10.1088/1742-6596/2771/1/012004 2 interconnected with the mainland.In this networking mode, the large grid is responsible for providing stable voltage and frequency support to the system.In this scenario, the microgrid can be regarded as a load or power generation unit within the large power grid, which plays a supportive and safeguarding role in the operation of the island microgrid.
However, independent islands encountered challenges when connecting to mainland power grids, making complete electricity self-sufficiency difficult to achieve.Often, these independent islands rely on diesel generators for off-grid power generation.Stenclik and Richwine [4] introduced a novel plan to revamp the infrastructure of Puerto Rico's islands, focusing on rooftop solar power and increased investments in distributed renewable energy generation.Wang et al. [5] proposed a methodology for planning offshore microgrids that minimize investment costs while adapting robustly to fluctuations in marine renewable energy sources.Zhang et al. [6] introduced a life-cycle planning approach for battery energy storage systems in off-grid microgrids, considering factors such as demand growth, battery capacity degradation, and component contingencies within a multi-timescale decision-making framework.While these methods offer valuable solutions, they still rely on diesel generators to ensure a continuous power supply to independent islands.The noise and emissions from diesel generators conflict with the pristine ecological environments characterized by clear skies and waters on these islands.
Furthermore, the supply of fresh water to independent islands often involved the expensive and challenging task of shipping water from outside the island.Significant resources were consumed by this method, and it was difficult to maintain an uninterrupted supply of fresh water.Maximizing the use of local renewable resources to achieve self-sustained fresh water production is critical for developing microgrids on oceanic islands and optimizing the allocation of microgrid resources, loads, and storage capacity.Lin et al. [7] systematically analyzed the state of industrialized desalination technology in China and explored potential solutions to overcome constraints on further development.Wang et al. [8] proposed a 100% renewable energy supply system based on WTs and the integration of concentrating solar power plants and sea water desalination units, utilizing the dispatchability of CSP power plants to complement wind power generation.Song et al. [9] introduced a technological solution for a small-scale sea water desalination system with an energy recovery device, investigating the adaptability of plungertype high-pressure pumps with an integrated energy recovery device through numerical simulations for year-round round-island sea water desalination.
In view of the above analysis, a collaborative planning approach for zero-carbon microgrids comprising WTs, PVs, EESs, HESs, and sea water desalination is presented in this paper for independent islands.It establishes a distributed power generation model based on the islands' wind energy, solar energy, and water resources and sizes renewable energy units and energy storage equipment with the aim of minimizing investment and operational costs.To achieve fresh water self-sufficiency for island residents, a desalination project is integrated into the power grid as an adjustable load, enhancing the flexibility of the island microgrid.

Sea water desalination system model
Sea island fresh water can be mainly divided into four steps, respectively: water extraction, sea water pretreatment, reverse osmosis treatment, and subsequent treatment, as shown in Figure 1.Sea water desalination equipment utilizes a pressurized system to extract high-salinity sea water directly from sea water wells.Then it is transported to the pre-treatment device, where precision instruments are used to filter out particulate matter, ensuring that the sea water is prepared for the reverse osmosis treatment of the raw liquid.The reverse osmosis membrane is introduced and pressure is applied externally by a high-pressure pump to pass the pre-treated sea water through the reverse osmosis membrane to complete the reverse osmosis treatment.Finally, after the follow-up treatment, fresh water and wastewater are separated, the fresh water is used for the daily life of the islanders, and the concentrated salt water released by the energy recovery equipment is discharged to the sea.
The high-pressure pump represents the principal contributor to energy consumption within the reverse osmosis treatment process.The relationship between the active power of each high-pressure pump L_SWRO P and its speed r ϖ is outlined as shown in Formulas (1) -( 2): where e T and m T for the electromagnetic torque e and mechanical torque m of the high-pressure pump, respectively; J stands for the high-pressure pump moment of inertia; F represents the friction coefficient.
As can be seen from Formulas (1)-( 2), when the motor is operating steadily, the relationship between the power reference value for the outer ring of the high-pressure pump * L_SWRO P and the speed reference value * r ϖ is depicted in Formula (3): In summary, the regulation of water pressure in front of the membrane can be achieved by adjusting the regulating valve.This, in turn, results in the alteration of the torque applied to the high-pressure pump, thereby controlling the energy consumption of the high-pressure pump.It facilitates the continuous adaptation of energy consumption within the desalination process.Subsequently, this process becomes integrated into the operation of the isolated microgrid through flexible demand-side responses, enhancing the utilization of clean energy sources such as wind and solar power and improving the stability of the microgrid system.
For the sake of simplicity and without accounting for the time required by the fresh water preparation equipment for water production, we can express the physical model of the fresh water preparation system based on the moment of completed electricity consumption and fresh water production as shown in Formulas ( 4) -( 8): where , ft S is the water level capacity state of fresh water pool f at time t; f δ is the water security coefficient of users; t W is the water demand of users on the whole island at time t; pre , ft P is the electric power used for water production of fresh water pool f at time t and with a negative value; f,rated f P is the rated electric power used for water production of the pool f; α is fresh water power conversion coefficient, which indicates the number of tons of fresh water that can be prepared by consuming a unit of energy; pre f γ is the charging efficiency of fresh water pool f; θ is the fresh water demand coefficient, which indicates the energy consumed by hydrogen energy storage to electrolyze 1 ton of water; ET , ht P is the electric power used by the electrolyzer in hydrogen storage h at time t with a negative value.

Distributed resource modeling 1) Wind turbines
The output of the fan depends on the actual wind speed at the hub and the operating characteristics of the fan, and its output constraint can be expressed as: where wind , mt P is the output power of wind turbine m at time t; wind,rated m P is the rated active power output of wind turbine m.
2) Photovoltaic modules The output of photovoltaic modules depends on the actual irradiance and photovoltaic array area, as shown in Formula ( 10): solar , nt P SG γ <√√ (10) solar solar,rated , , 0 where solar , nt P is the output power of photovoltaic module n at time t.γ is the energy conversion efficiency, S is the area of the photovoltaic array, unit in square meters, G is the light intensity, kW/m 2 ; solar,rated n P is the rated active power output of photovoltaic module n.
3) Electrochemical energy storage Electrochemical energy storage operation has been studied in detail by many scholars, and the model adopted in this paper refers to Li et al.'s work [10].
4) Hydrogen energy storage: Hydrogen energy storage is a long-term energy storage technology that uses hydrogen for energy storage and release, which mainly includes three parts: water electrolysis, fuel cells, and hydrogen storage tanks.Hydrogen production by water electrolysis represents the charging process of hydrogen energy storage, fuel cell power generation represents the discharge process of hydrogen energy storage, and a hydrogen storage tank represents the hydrogen energy storage capacity state, then the physical model can be expressed as: γ ET are the working efficiencies of fuel cells and electrolyzers in hydrogen energy storage, respectively; t Χ is the interval time; κ is the hydrogen production coefficient, that is, the amount of hydrogen that can be produced by consuming a unit of energy.

Optimization model of island microgrid planning
On the premise of stable operation of the key load throughout the year, and with the goal of minimizing the comprehensive investment and operation cost of the microgrid system, a planning model for the zero-carbon energy supply of independent islands is built.

Objective function
The objective function is to minimize the comprehensive cost, including the power supply investment cost and the system operation cost, which can be expressed as:

C
is the equivalent annual cost to express the construction cost of each power source, including the investment, operation, and maintenance costs of WTs and PVs, and the specific expression is as shown in Formula (18):

2) cut sys
C represents the load cost other than the key load in the whole life cycle of the system, and for the convenience of expression, it is converted into an equivalent annual cost of 1 year, and the specific expression is as shown in Formula (19): where T N and D N are the planned hours and days, respectively; voll c is the unit load reduction cost; cur t L is the amount of load shed at time t.
3) storage C is expressed as the investment and operation and maintenance cost of energy storage and desalination system at the same annual cost, which is specifically expressed as:

Constraints
In addition to the constraints on the physical models of wind turbines, photovoltaic modules, energy storage equipment, and fresh water equipment (Formulas (1)-( 16)), the following constraints need to be considered: 1) System installed capacity constraints: To ensure that wind, solar, electrochemical energy storage and hydrogen energy storage combined zero-carbon energy supply meet the installed capacity of the system power supply in the whole life cycle, the capacity constraints are as shown in Formula ( 21):

G
are the upper limit of the planned and constructed installed capacity of wind turbine m, photovoltaic module n, electrochemical energy storage e, electrolyzers h, fuel cell h, and fresh water pool f, respectively.
2) System power balance constraint: where cur  is the power supply adequacy coefficient to ensure the stability of the power supply of the primary load of the system.
Considering the above constraints (Formulas ( 21)-( 23)), we can plan and obtain the configuration of each unit with the minimum comprehensive investment and operation cost of the microgrid system, coordinate the economic operation and reliability requirements of the island microgrid, and optimize the utilization of resources.

Case data
An actual island microgrid project in Fujian Province is presented as a case study, appropriately reconfigured for analysis.Fujian Province, situated on the southeastern coast of China, is home to numerous sea islands.The specific oceanic island under consideration is located approximately 45 kilometers away from the mainland, lacking any water and power supply pipelines connecting it to the mainland.Water supply primarily relies on the collection of natural precipitation and the transportation of fresh water by ships from outside the island and power supply is primarily generated by a number of diesel generators.The island's power grid infrastructure is outdated, and local residents receive electricity through a poorly maintained 380 V line.The electricity consumption of the island in 2022 is approximately 315,200 kWh.
The island experiences notable fluctuations in its monthly and daily load characteristics.The peak load period occurs annually from June to October, with the island's overall power consumption averaging around 35,000 kWh per month.The maximum load is typically maintained at around 200 kW, primarily fulfilling the electricity needs of residents, refrigeration, and tourist hotels upon their return from fishing trips.During the off-season, from January to April, the island's electricity consumption primarily revolves around residential needs, averaging about 15,000 kWh per month.
The island boasts abundant wind and solar energy resources.It features an annual average wind speed of 7.479 m/s at a height of 10 meters, signifying favorable wind conditions.Furthermore, the area experiences an annual average total solar radiation of 4283.78MJ/m 2 , classifying it as a region rich in solar energy resources, offering substantial value for utilization.
The following data is derived from actual engineering construction for similar islands.
1) The parameters of the renewable energy units are listed in Table 1.2) Electrochemical energy storage unit power cost is 0.5 million RMB/kW, unit capacity cost is 0.5 million RMB/kWh, and SOC upper and lower limits is [0.1, 0.9]; hydrogen storage electrolyzer unit power cost is 50,000 RMB/kW, and fuel cell unit power cost is 52,000 RMB/kW; hydrogen storage tank unit capacity cost is 0.1325 million RMB/L.The electrochemical energy storage and hydrogen storage remaining parameters are listed in Table 2. 3) The desalination system infrastructure cost is 2 million RMB, the O&M cost coefficient is 0.02, the maximum operating life is 20 years, the operating desalination efficiency is 0.8, the fresh water demand coefficient is 0.2, and the unit capacity cost of desalination cisterns is 0.1 million RMB/t.4) Load data refers to the load of an island in Fujian for the past years, and the load data and wind data are selected for 4 consecutive weeks in a year, in which there exists a lack of wind and solar resources on the island for 48 hours, which cannot support power generation, as shown in Figures 2-3.The discount rate is 6%, the spinning reserve rate is 5%, the unit load cutting cost is 350 RMB/kWh, and the key load proportion is 30%.This approach exploits the capability of long-term energy storage and spans a planning duration of 672 hours.Based on the above-mentioned historical data, a 4-week, hour-by-hour (672 hours) time-series simulation is employed to investigate key technologies for achieving the synergistic and stable operation of zero-carbon microgrids on independent islands, relying on wind, solar, storage, hydrogen, and water resources.

Results and discussion
Four scenarios have been devised and the capacities of WT s, PVs, EESs, HESs and se a wate r desalination under different scenarios have been cooperatively optimized to evaluate the feasibility of operating a zero-carbon microgrid on the island and to validate the effectiveness of the model proposed in this paper.
Scenario 1: In this scenario, long-term and short-term energy storage are integrated into the microgrid.Scenario 2: Under the same load-shedding ratio as that of Scenario 1, only long-term energy storage is connected to the microgrid without the introduction of short-term energy storage, and the maximum allowed installed capacities of WTs, PVs, and HES are augmented.
Scenario 3: Under the same load-shedding ratio as that of Scenario 1, long-term and short-term energy storage are incorporated into the microgrid, while the maximum allowed installed capacity of WTs is increased without PVs.
Scenario 4: Under the same load-shedding ratio as that of Scenario 1, long-term and short-term energy storage are incorporated into the microgrid, while the maximum allowed installed capacity of PVs is increased without WTs.The planning results are presented in Table 3. Analyzing the above experimental results, in Scenario 2, the installed capacity of WTs is increased by 100 kW, PV capacity is raised by 63.7 kW, and fuel cell capacity is augmented by 18.17 kW.However, this change results in an increase in the investment cost of power supply by approximately 2.12 million RMB.The energy storage investment rises by about 9.88 million RMB, leading to a significant rise in the cost per kWh, reaching 8.03 RMB/kWh.With an abandonment rate exceeding 50% for solar energy, the cost of water production is basically unchanged.This situation arises due to the system relying on releasing stored energy during periods of scarce wind and solar resources to meet the island's electricity demand.During these times, short-term energy storage is not utilized due to economic considerations, and the maximum fuel cell capacity is limited.Consequently, the capacity of fuel cells and hydrogen storage is necessary to release stored energy during resource scarcity.
In Scenario 3, as compared to Scenario 1, the system exclusively relies on wind power for renewable energy generation.The installed wind turbine capacity is increased by 154.3 kW, the installed capacity of the electrolyzer is reduced by 4.09 kW, and the fuel cell capacity is extended by 0.06 kW.While the investment cost of power supply increases by 1.77 million RMB, the energy storage investment decreases by 0.25 million RMB, resulting in a higher cost per kWh of 4.77 RMB/kWh.In this case, the island's abundant PV resources, distinctive in their nature, are not thoroughly leveraged.It becomes necessary to introduce large-scale, long-term energy storage solutions to mitigate the power generation deficit.Nevertheless, the associated expenses with long-term energy storage are substantial, leading to a notable escalation in kWh costs.From a long-term development perspective, this approach is not conducive to the establishment of zero-carbon microgrids on coastal islands.In Scenario 4, as compared to Scenario 1, the system exclusively relies on PV generation.The installed photovoltaic capacity is increased by 9107 kW, the installed capacity of the electrolyzer is expanded by 45 kW, and the fuel cell is extended by 24.66 kW.The investment cost of power supply increases by 40.14 million RMB, while the energy storage investment increases by 8.3 million RMB, resulting in a high cost per kWh of 15.50 RMB/kWh.In this scenario, electricity generation depends solely on PV modules, which are highly concentrated and unable to generate power at night.Consequently, significant investments in energy storage devices are required to maintain a continuous power supply during the night and periods of low PV resource availability, making this scenario unfeasible.
The desalination system is considered in all four scenarios as a part of the island microgrid's operational response.During periods of surplus wind and solar energy, the desalination equipment consumes electricity to produce fresh water, which is stored in a reservoir, reducing energy waste.During resource scarcity, the desalination equipment ceases operation, and stored fresh water is released to meet residents' water needs.The investment cost of the desalination system includes basic and reservoir costs, with the desalination cost per ton of sea water totaling approximately 6.2 RMB.This cost is significantly lower than that of long-distance water transportation from the mainland.Integrating the desalination system into the island microgrid not only conserves substantial human and material resources but also enables the system to benefit from flexible demand-side responses.
In summary, Scenario 1 presents the most favorable planning outcomes, underscoring the effectiveness of zero-carbon microgrid planning for independent islands.This scheme incorporates wind, solar, storage, hydrogen, and water resources, effectively reducing carbon emissions from diesel power generation.It facilitates carbon neutrality and peak carbon reduction while addressing issues related to wind and solar resource utilization and consumption challenges, ultimately contributing to an overall reduction in system costs.The integration of long-term and short-term energy storage significantly enhances wind power consumption and enhances economic viability.

Conclusion
This paper presents a planning method for zero-carbon microgrids on independent islands, incorporating wind, photovoltaic, storage, hydrogen, and water resources.The method accounts for the coordinated operation of electrochemical and hydrogen energy storage while utilizing WT and PV resources for power generation, which maximizes the roles of both long-term and short-term energy storage in renewable energy utilization.The optimization model not only prioritizes the economic aspects of grid planning but also ensures a reliable fresh water supply for island residents.The following conclusions can be drawn: 1) The zero-carbon microgrid wind-solar-storage-hydrogen-water synergistic planning for independent islands effectively mitigates the environmental impact of traditional diesel generators, enabling year-round zero-carbon microgrid operation.Simultaneously, configuring long-term and shortterm energy storage reduces renewable energy waste, and improves economic viability.
2) Relying solely on a single renewable energy source cannot fully exploit the potential of the island's resource advantages.It necessitates the introduction of a significant amount of energy storage to satisfy power supply needs, which can be costly and may not yield economic benefits.
3) Long-term energy storage effectively addresses the issue of unstable renewable resources, such as WTs and PVs on the island, which ensures a stable power supply during nighttime and peak electricity consumption periods.Additionally, the coordination with the operation of sea water desalination equipment enables demand-side response and active participation in the operation of the isolated island microgrid.
f,rated f S is the rated water level capacity state of pool f; min fsoc,f e and max fsoc,f e are the upper and lower limits of the operation of the capacity state of pool, the discount rate; w n and s n are the design service life of WTs and PVs; w R and s R are the operational and maintenance cost factor for WTs and PVs, respectively; wind per unit capacity of WTs and PVs, respectively.
life of electrochemical energy storage, hydrogen energy storage, and fresh water pools, respectively; e R , h R and f R are the operation and maintenance cost coefficients of electrochemical energy storage, hydrogen energy storage, and fresh water pools, respectively; ess N , hes N and fresh N are the number of electrochemical energy storage, hydrogen energy storage, and fresh water pools, respectively; per unit power of electrolyzers and fuel cells, respectively; cap hes c is the cost per unit capacity of hydrogen energy storage; 0 C is the cost of infrastructure construction of desalination systems; cap fresh c is the cost per unit capacity of desalination reservoirs.

Figure 2 .
Figure 2. Wind speed and solar intensity time series data.

Figure 3 .
Figure 3. Load and water consumption timing data.

Table 1 .
Parameters for renewable energy units.

Table 3 .
Optimization results for 4 planning scenarios.