Hybrid Wind-Solar Power System Optimization: A Comprehensive Design Solution for Off-Grid Applications
- Introduction and Background
1.1 Challenges of Single-Source Power Generation Systems
Traditional standalone photovoltaic (PV) or wind power generation systems have inherent drawbacks. PV power generation is affected by diurnal cycles and weather conditions, while wind power generation relies on unstable wind resources, leading to significant fluctuations in power output. To ensure a continuous power supply, large-capacity battery banks are necessary for energy storage and balance. However, batteries undergoing frequent charge-discharge cycles are prone to remaining in a state of undercharge for extended periods under harsh operating conditions, resulting in a practical service life much shorter than the theoretical value. More critically, the high cost of batteries means their total lifecycle cost may approach or even exceed the cost of the PV modules or wind turbines themselves. Therefore, extending battery life and reducing system costs have become the core challenges in optimizing standalone power systems.
1.2 Significant Advantages of Hybrid Wind-Solar Power Generation
Hybrid wind-solar power generation technology effectively overcomes the intermittency of single energy sources by organically combining PV and wind power, two renewable energy sources. Wind and solar energy exhibit a natural complementarity in time (day/night, seasons): strong sunlight during the day often coincides with potentially stronger winds at night; good solar irradiation in summer may pair with ample wind resources in winter. This complementarity enables:
- Significant extension of the effective charging time for batteries, reducing the time they spend in an undercharged state, thereby substantially prolonging battery service life.
- Reduction in the required battery capacity. Since the probability of both wind and solar being unavailable simultaneously is low, the system can often power the load directly, allowing for the use of a smaller capacity battery bank.
- Domestic and international studies confirm that hybrid wind-solar systems surpass single-source power generation systems in both power supply reliability and lifecycle cost-effectiveness.
1.3 Shortcomings of Existing Design Methods and the Proposed Solution
Current system design faces challenges. Professional simulation software from abroad is expensive, and its core models are often confidential, hindering widespread adoption. Meanwhile, most simplified design methods are inadequate—either they rely overly on meteorological averages ignoring details, or they use linear simplified models leading to limited accuracy and poor applicability.
This solution aims to propose a set of accurate and practical computer-aided design methodologies to address the above issues.
II. System Composition and Core Technical Models
2.1 System Architecture
The hybrid wind-solar power generation system designed in this solution is a completely standalone off-grid system, without backup power sources like diesel generators. The core components include:
- Power Generation Unit: Wind turbine generators, PV array.
- Energy Storage and Management Unit: Battery bank, charge controller (for managing charging and discharging).
- Protection and Conversion Unit: Diversion load (prevents battery overcharge, protects inverter), inverter (converts DC to AC to meet most load requirements).
- Power Consumption Unit: Load.
2.2 Accurate Power Generation Calculation Models
To achieve optimized design, we have established accurate hourly power generation calculation models.
- PV Array Model:
- Solar Radiation Transposition: Utilizes an advanced anisotropic sky diffuse model to accurately transpose horizontal solar radiation data measured by weather stations to the irradiance incident on the tilted surface of the PV modules, comprehensively considering direct beam radiation, sky diffuse radiation, and ground-reflected radiation.
- Module Characteristic Simulation: Employs a precise physical model to characterize the nonlinear output characteristics of PV modules, fully accounting for the effects of irradiance and ambient temperature on module output voltage and current, ensuring the accuracy of power generation calculations.
- Wind Turbine Model:
- Wind Speed Correction: Corrects the reference height wind speed from meteorological data to the actual hub height wind speed based on the exponential law governing wind speed variation with height.
- Power Curve Fitting: Uses a segmented function (different binomial equations for different wind speed intervals) to achieve high-precision fitting of the turbine's actual power output curve, enabling accurate hourly energy calculation based on wind speed data.
2.3 Battery Dynamic Characteristic Model
The battery is the core energy storage component, with dynamically changing states. The model primarily focuses on:
- State of Charge (SOC) Calculation: Dynamically simulates the battery's charge and discharge processes based on the relationship between power generation and load consumption at each time step, accurately calculating the remaining capacity, while considering practical factors like self-discharge rate, charging efficiency, and inverter efficiency.
- Charge-Discharge Management: To extend battery life, a reasonable SOC operating range is defined (e.g., limiting the maximum depth of discharge to 50%), and a model correlating float charge voltage with SOC and ambient temperature is established to accurately determine charging conditions.
III. System Optimization and Sizing Methodology
3.1 Power Supply Reliability Indicators
The design prioritizes meeting the user's specified power supply reliability requirements. Key indicators include:
- Loss of Power Supply Probability (LPSP): The ratio of system outage time to the total evaluation time, intuitively reflecting supply continuity.
- Loss of Load Probability (LLP): The ratio of the load power demand not met by the system to the total demand. This is the most critical core indicator for system optimization design.
3.2 Step-by-Step Optimization Design Process
This solution adopts a systematic optimization process, aiming to minimize the initial investment cost of equipment to find the optimal configuration.
- Step 1: Optimize PV and Battery Configuration for a Fixed Wind Turbine Capacity
- Core Task: Under the condition that the wind turbine model and quantity are fixed, find the combination of PV module and battery capacities that meets the predetermined reliability indicator (LPSP) and results in the lowest total equipment cost.
- Implementation Method: Through simulation calculations, plot the "balance curve" representing all PV and battery configurations that meet the reliability requirement. Then, using the cost tangent method or computer program screening based on equipment unit prices, determine the unique optimal combination with the lowest cost.
- Step 2: Global Optimization by Varying Wind Turbine Capacity
- Core Task: Change the wind turbine capacity or number, repeat the optimization process of Step 1, and obtain a series of optimal configurations and their corresponding costs for different wind turbine capacities.
- Final Decision: Compare the total costs of all candidate solutions and select the wind-PV-battery combination with the globally lowest cost as the final optimized system configuration.
3.3 System Performance Simulation and Output
After determining the optimal configuration, the system's annual operation can be simulated hour-by-hour, generating detailed reports including:
- Time Dimension: Hourly battery state of charge, system energy balance.
- Statistical Dimension: Daily/monthly/annual unmet load energy, reliability indicators (LPSP, LLP), wind/solar power generation share, energy surplus and deficit situations, etc.
IV. Conclusion
The optimized design method for hybrid wind-solar power generation systems proposed in this solution, based on comprehensive mathematical models and precise local meteorological data, can uniquely determine the system configuration with the minimum initial equipment investment cost while satisfying specific user electricity demands and power supply reliability requirements. This method effectively addresses the shortcomings of single-source power generation systems, overcomes the limitations of existing design approaches, and provides a powerful tool for the scientific, efficient, and economical design of hybrid wind-solar power generation systems, holding significant value for engineering applications.
