How can photovoltaic cells be integrated with energy storage systems?

The combination of photovoltaic cells and energy storage systems is a key technical approach to addressing the intermittency of photovoltaic power generation and enhancing energy utilization efficiency. The integration of the two can be carried out from multiple dimensions such as system architecture, technical logic, and application scenarios. The following is a specific analysis:

I. System Integration Architecture: From energy flow to device connection

1. Physical connection method

Direct Current Coupling

Technical logic: The direct current (DC) generated by photovoltaic cells is directly connected to the battery management system (BMS) of the energy storage system, without the need for an inverter to convert it into alternating current (AC), resulting in higher energy conversion efficiency (reducing AC/DC conversion losses and increasing efficiency by approximately 5% to 10%).

Equipment composition: Photovoltaic array →DC-DC converter → energy storage battery → bidirectional DC-AC inverter (used when connected to the grid).

Advantages: Suitable for off-grid scenarios (such as power supply in remote areas) or systems requiring high reliability, reducing equipment costs and energy consumption.

Alternating Current Coupling

Technical logic: Photovoltaic cells are first converted into alternating current through an inverter, and then connected to the energy storage system through a bidirectional inverter. The energy storage system can be independently connected to the power grid or load.

Equipment composition: Photovoltaic array → Photovoltaic inverter (DC-AC) → AC bus → Bidirectional energy storage inverter (AC-DC) → energy storage battery.

Advantages: High flexibility, compatible with the renovation of existing photovoltaic systems, suitable for grid-connected scenarios (such as household and industrial and commercial energy storage projects).

2. The core role of the Energy Management System (EMS)

Real-time monitoring and dispatching: Through sensors and algorithms, the output of photovoltaic power, battery status (SOC), load demand and grid electricity price signals are monitored in real time, and the energy flow direction is dynamically adjusted.

Typical strategy:

When the photovoltaic power generation exceeds the load demand, the excess electricity is stored in the energy storage battery.

When the photovoltaic power generation is less than the load demand or at night, the energy storage battery discharges to make up for it.

When the grid electricity price is at a low point (such as at night), the energy storage system can charge from the grid (in combination with the peak-valley electricity price strategy), and discharge during the peak electricity price, achieving "peak-valley arbitrage".

Ii. Key Links in Technology Integration

1. Photovoltaic side: Optimization to meet energy storage demands

Power smooth control: By absorbing or releasing electrical energy through the energy storage system, it smoothed out the fluctuations in photovoltaic output (such as power sudden changes caused by cloud cover), meeting the stability requirements for grid connection (for example, the "Guidelines for Grid Connection of Photovoltaic Power Generation" in China requires that the fluctuation range be ≤10% of the rated power).

Maximum Power Point Tracking (MPPT) collaboration: Energy storage systems can assist photovoltaic arrays in optimizing MPPT strategies. For instance, when light conditions change, energy storage can buffer energy fluctuations to prevent efficiency losses caused by MPPT misjudgment.

2. Energy storage side: Design that matches the characteristics of photovoltaic

Battery type selection:

Lithium batteries (such as lithium iron phosphate) : Suitable for household and industrial and commercial scenarios, they have high energy density, long cycle life (≥3000 times), fast response speed (millisecond level), and can quickly compensate for photovoltaic fluctuations.

Flow batteries (such as all-vanadium flow batteries) : Suitable for grid-level energy storage, with large capacity (hundreds of megawatts), high safety, and extremely long cycle life (≥ 10,000 times), but with low energy density, they are more suitable for long-term energy storage (4-8 hours).

Charge and discharge strategy:

Threshold control based on SOC: Prioritize charging when SOC < 20%, and limit charging when SOC > 80% to prevent overcharging and overdischarging, thereby extending battery life.

Meteorological prediction linkage: In combination with weather forecasts (such as predicting overcast weather the next day), adjust the energy storage discharge strategy in advance to ensure power supply reliability.

3. Synergy of power electronics technology

Compatibility of bidirectional inverters: They need to support bidirectional energy conversion between photovoltaic grid connection and energy storage charging and discharging. For instance, a three-phase grid-connected inverter can be used to simultaneously meet the control requirements of photovoltaic MPPT and energy storage BMS.

Microgrid technology: In off-grid scenarios, photovoltaic and energy storage can form a microgrid. Through inverters, the frequency and voltage of the power grid can be simulated to provide stable power to loads (such as power supply to islands and communication base stations).

Iii. Typical Application Scenarios and Integration Models

Household and industrial and commercial distributed systems

Objective: Increase the rate of self-generation and self-consumption and reduce electricity expenses.

Combination mode:

Photovoltaic + energy storage + load: Household rooftop photovoltaic systems are paired with household energy storage batteries (such as Tesla Powerwall). During the day, electricity is generated for household use, and the excess electricity is stored for energy. At night, it is discharged, reducing the need to purchase electricity from the power grid.

Peak-valley arbitrage: Industrial and commercial users charge with surplus photovoltaic power or low-priced electricity from the grid during the off-peak electricity price period (such as 23:00-7:00), and discharge during the peak electricity price period (such as 10:00-15:00), making profits by taking advantage of the electricity price difference.

2. Grid-level centralized photovoltaic power stations

Objective: Enhance the power grid's absorption capacity and assist in peak shaving and frequency regulation.

Combination mode:

Photovoltaic power station supporting energy storage: Build an energy storage power station within or near the photovoltaic power station site (such as 100MW photovoltaic + 20MW/40MWh energy storage), smooth out the photovoltaic output through energy storage to avoid impact on the power grid, and at the same time participate in peak shaving of the power grid (such as charging during the day and discharging at night).

Virtual Power Plant (VPP) : It aggregates scattered photovoltaic-energy storage systems and conducts unified dispatching through cloud EMS to participate in grid ancillary services (such as frequency regulation and reserve capacity) and generate additional revenue.

3. Power supply in off-grid and remote areas

Objective: Independent power supply to address the electricity demands in areas without grid coverage.

Combined mode: photovoltaic array + energy storage battery + diesel generator (backup). During the day, photovoltaic power supplies and charges, and at night, energy storage discharges. In case of continuous rainy weather, the diesel generator starts to replenish energy to ensure power supply reliability (such as in remote villages and field base stations).

Iv. Economic Benefits and Policy Drivers

Cost and benefit balance

Cost reduction path: The price of photovoltaic modules drops (the cost per watt in 2023 is approximately 1.5 yuan) + the cost of energy storage batteries decreases (the cost per kilowatt-hour of lithium batteries is less than 0.5 yuan /kWh), promoting the investment payback period of "photovoltaic + energy storage" projects to shorten to 5-8 years (for household scenarios).

Source of income:

Electricity cost savings (self-generated and self-consumed)

Peak-valley electricity price arbitrage (economically viable when the price difference is ≥0.7 yuan /kWh);

Revenue from power grid ancillary services (such as frequency regulation compensation).

2. Policy and market mechanism support

Mandatory energy storage policy: In some regions, photovoltaic power stations are required to be equipped with energy storage (for instance, in Gansu and Qinghai, 10% to 20% of energy storage is required, with a duration of 2 hours), promoting the integration of energy storage and photovoltaic power.

Subsidies and incentives: Provide subsidies per kilowatt-hour for "photovoltaic + energy storage" projects (such as ITC subsidies in the United States and local special subsidies in China), or allow participation in power market transactions.

V. Technical Challenges and Future Trends

Current challenges

Costs still need to be reduced: Energy storage systems account for 40% to 60% of the investment in "photovoltaic + energy storage" projects, especially grid-level energy storage, which is under great cost pressure.

System efficiency optimization: Multiple energy conversions (photovoltaic DC→AC→ energy storage DC) result in an overall efficiency of less than 80%, and it is necessary to enhance the efficiency through topology optimization (such as DC coupling).

2. Future Trends

Integrated photovoltaic and energy storage design: Photovoltaic modules and energy storage batteries are integrated at the manufacturing end (such as the combination of photovoltaic roofs and energy storage walls), reducing installation costs.

Intelligent dispatching algorithm upgrade: Introduce artificial intelligence (AI) to predict photovoltaic output and load demand, optimize energy storage charging and discharging strategies, and enhance system economy.

Breakthroughs in new energy storage technologies: such as solid-state batteries and compressed air energy storage, which reduce costs and enhance safety.

Summary

The combination of photovoltaic cells and energy storage systems, through the synergy of "generation - storage - consumption", has achieved the transformation of solar energy from an "intermittent energy source" to a "stable power source". The core lies in solving the randomness problem of photovoltaic power generation through power electronics technology, energy management systems and compatible energy storage technologies. From electricity cost savings in household scenarios to peak shaving assistance at the power grid level, this combined model is driving the energy system to upgrade towards low-carbon, flexible and efficient directions.