Solar battery system types - AC Vs DC coupled

Quick Summary

DC-coupling using solar charge controllers is often favored for small mobile systems in RVs and caravans, as well as smaller residential off-grid systems. On the other hand, AC-coupling using solar inverters is considered more efficient for grid-tie energy storage systems and larger off-grid installations, particularly when daytime loads are high. The advantages and disadvantages of each system type are discussed in detail below.

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The Four Main Solar System Types

1. DC-coupled systems - Off-grid
2. AC-coupled systems - Off-grid
3. AC-coupled Battery Systems - Grid-connected
4. DC-coupled Hybrid Systems - Grid-connected

Typically, DC or AC-coupled systems are most commonly used for off-grid solar installations. The reasons are explained below, along with a comparison of AC vs DC-coupled solar for off-grid power systems.

Understanding AC-coupling and the Factor 1.0 Rule

1. Introduction to the AC-Coupling Concept

1.1 What is AC-coupling?

In an AC-coupled system, a grid-tied PV inverter connects to the output of an inverter like the Multi or Quattro. The PV power first powers the loads, then charges the battery, with any excess potentially fed back to the grid. When the system is disconnected from the grid, it creates a micro-grid allowing the PV inverter to operate during blackouts.

• AC-coupling is available in single-phase, split-phase, and three-phase systems.
• Systems with only a grid-tied PV inverter will fail during a grid blackout. However, a micro-grid system will continue operating and utilize solar power.
• A micro-grid can also run on a generator.

1.2 What is Frequency Shifting?

Frequency shifting regulates the output power of a Grid-tie PV Inverter by altering the AC frequency. This method prevents overcharging the battery or overloading the inverter/charger.

2. The Factor 1.0 Rule

2.1 Rule Definition

The maximum PV power must match or be less than the VA rating of the inverter/charger. For instance, a 3000 VA inverter should not have more than 3000 W of installed solar power.

2.2 Example and Background

Consider a PV inverter at full power supplying a load, and then that load suddenly switches off. The inverter continues operating at full power until the AC frequency increases, leading to excessive charge directed into the batteries, causing voltage spikes and potential damage.

2.3 Charge Current Limit

A common question is why the factor is 1.0 when the charger capacity is less. The charger adjusts the output frequency to match the limit and regulate power.

1. When connected to the grid, all excess power can be fed back.
2. When off-grid, the system increases frequency to reduce output and match the charger's capacity.

2.4 Should You Look at the Total PV Array or the PV Inverter Rating?

The calculation should use the smaller value between total installed PV power and the inverter’s capacity.

Minimum Battery Capacity

A sufficient battery size is crucial for system efficiency and economic operation. The recommendations vary for lead and lithium batteries.

3.1 Lead Batteries

1 kWp installed PV requires approximately 5 kWh of lead acid battery storage.

• 100 Ah at 48 Vdc
• 200 Ah at 24 Vdc
• 400 Ah at 12 Vdc

3.2 Lithium Batteries

1.5 kWp installed PV requires about 4.8 kWh of battery storage.

• 100 Ah at 48 Vdc
• 200 Ah at 24 Vdc
• 400 Ah at 12 Vdc

Requirement of Adding DC-coupling - MPPT Solar Chargers

Additional DC-coupling is not required for energy storage systems in reliable grid situations but is necessary for off-grid applications or areas prone to extended grid failures.

Software Configuration

For AC Coupled systems, install either the ESS Assistant (for grid-connected systems) or the PV Inverter Support Assistant (for off-grid systems). The Inverter RS will automatically shift frequency when needed without additional configuration.

Monitoring

Refer to the GX manual's 'Connecting a PV Inverter' section for detailed instructions.

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