Ingeniero

AC Coupling in Renewable Energy Systems
Most renewable energy systems are “DC coupled” Most renewable energy systems are based on DC current, yet these have certain scalability limitations. Higher currents and associated wiring of lower-voltage DC components’ limits are particularly evident in micro-grid and single residences with multiple buildings. DC coupled systems use either an off-grid ora grid interactive inverter/charger depending on whether the user is connected to a local utility or is completely selfsustaining. Off-grid users typically use battery banks to store energy for the times a renewable source isn’t available. Grid-tie systems are designed with and without battery back-up. Those without batteries primarily sell power back to the utility to lower a user’s power costs.Basic DC Coupled off-grid system with solar and backup AC generator

Main Panel

MPPT DC Charge Controller

Battery-based Off-Grid Inverter/Charger

Generator

Size matters

Increasing the size of any DC system component–the solar array, battery bank, or the loads–often calls for an increase in the size of the system’s electrical conductors. This can be unexpectedly expensive giventhe escalating cost of copper. An alternative to DC coupled systems based on AC coupling offers renewable energy users an alternative without compromising the quality or quantity of their electrical power. Simply put, AC coupling combines batteryless and battery-based inverter/chargers in the same off-grid or grid-tied system, resulting in a system that is more easily upgraded and expanded than aDC coupled system. AC coupling accommodates multiple charging sources on the AC side of the system and does not require a charge controller to regulate DC power. For some applications, AC coupling has advantages over DC. In a DC coupled battery-based system, current from a renewable energy source is used to either recharge batteries, run loads or sell back to the utility grid. A grid-tied systemwithout a battery bank offers no back-up or stored power in the event of a utility outage. In a typical system with batteries, renewable energy passes through a charge controller and is then either stored in the batteries or passes through an inverter/ charger to power loads or sell back. AC coupling allows direct powering of AC loads through batteryless inverters and/or battery recharging througha battery-based inverter/charger. Backup generators can also be used to charge the batteries or power the loads through the battery based inverter/charger.

AC coupling basics

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Basics continued

Although an AC coupled system requires the addition of a battery-based inverter/charger, some of this cost is offset by smaller conductor sizes and the absence of a DC charge controller. Notevery system design–for instance, small off-grid applications–will benefit from AC coupling. Those that will benefit include: • A system whose solar panels, wind turbine, or other power generation source are a long distance from the loads it’s powering • Large grid-tie system owners wanting battery back-up not proportional to their power generating source (i.e. a 4 kW PV array with a 2 kW batterysystem) • Multiple buildings and power generating locations off-grid • Retrofitting battery back-up to an existing grid-tie system • Systems using positively grounded modules, but desiring battery back-up

BASIC SYSTEM DESIGN USING AC COUPLING
Power from the solar array powers the loads through a batteryless grid-tie inverter. Current passes through the load center to the battery-basedinverter/charger to both recharge the battery bank with the remaining power selling back to the grid. It is important to note that power is only passing through the battery inverter/charger. The batteryless grid tie inverter is selling back.

Sub Panel

Batterylyess Batteryless Grid-Tie Grid-Tie Inverter Inverter

Main Panel

KWH

KWH

Battery-based Off-Grid Inverter/Charger

The above...