Comparison Analysis of AC-Coupled and DC-Coupled Architectures for PV Energy Storage
Preface
“ In the construction of distributed integrated photovoltaic-energy storage projects, AC coupling and DC coupling are the two most widely adopted system architectures. Both architectures achieve local consumption of photovoltaic power, storage of surplus electricity, and power supplementation by energy storage during power shortages. Nevertheless, they differ remarkably in power transmission paths, equipment configurations and operating efficiency. These two topologies are frequently confused in engineering design, resulting in irrational system configuration. A thorough understanding of the current flow, operating logic, merits and demerits of the two architectures serves as an essential prerequisite for photovoltaic-energy storage system design and project type selection.”
🎥Schematic Video of Current Flow for AC Coupling and DC Coupling
I. Current Flow of AC Coupling (Two Independent Inverters, Energy Converges at AC Bus with Bidirectional Energy Flow)
AC Coupling Scenario Diagram
▶ Working Condition 1: Adequate PV output, surplus electricity charges the batteries PV panels (DC power) → PV inverter (DC to AC) → AC bus → Power Conversion System (PCS, AC to DC) → charge the energy storage battery cabinet.
⚠️Electric energy conversion process: DC → AC → DC, two conversions leading to higher power loss.
▶ Operating condition 2: Insufficient PV output, batteries discharge to supplement load power Energy storage battery cabinet (DC) → Power Conversion System (PCS, DC to AC) → AC bus, converging with AC power generated by the PV inverter to jointly supply the load. ▶ Operating condition 3: No photovoltaic power generation at night, batteries supply power independently Energy storage batteries → inverted into alternating current via PCS → AC bus → loads.
Core features: Photovoltaic and energy storage systems are connected in parallel on the AC side. Batteries must be connected to the AC bus via PCS, marked as "bidirectional energy flow" in the diagram.
II. Current Flow of DC Coupling (All-in-one PV & Storage Machine with Shared DC Bus)
DC Coupling Scenario Diagram
▶ Working Condition 1: Sufficient PV power, supply loads first and charge batteries with surplus electricity Direct current generated by solar panels flows to the DC bus and splits into two branches: ① DC power is transmitted to the inverter (DC to AC) to produce alternating current for loads; ② excess DC power is directly stored in energy storage batteries without secondary AC-DC conversion.
▶ Working Condition 2: Insufficient PV generation, batteries discharge to supplement power PV DC power and battery DC power converge at the DC bus, then flow into the inverter collectively for inversion before supplying AC power to loads.
▶ Working Condition 3: No solar irradiation at night DC power from storage batteries flows through the DC bus and is inverted by the inverter to supply power to loads.
Core features: PV and batteries converge on the DC side with only one inversion process, resulting in lower conversion loss, which is marked in the figure as "higher centralized inversion efficiency".
III. Concise Comparative Summary
Charging path of AC coupling: PV DC → [PV Inverter] → AC bus → [Energy Storage PCS] → DC → Battery
Charging path of DC coupling: PV DC → DC bus; part of the power charges the battery directly, and the rest flows into the inverter and is converted into alternating current for loads.
Comparison Diagram of AC-Coupled and DC-Coupled Topologies
Intuitive Distinction Supplement
✅ AC Coupling: All power is converted into alternating current before convergence. Energy storage devices operate independently, ideal for retrofitting energy storage onto existing photovoltaic power stations.
✅ DC Coupling: PV modules and batteries converge at the DC side and share an inverter, suitable for newly-built integrated PV-storage projects.
