For Installers, Renewables, Solar, Storage

Tech Tips: Know your battery-inverter systems, impress your customers

SolaX Australia chief engineer Edwin Cotter talks about what’s been bothering installers.

The solar industry is expanding rapidly. This is not only due to technological advances in panels and inverters, batteries are now playing a larger role in the industry. It may not make much sense at the moment to use lithium batteries in cars, but it certainly makes sense using them for home energy storage.

Due to marketing and online information resources, consumers are asking more and more questions about adding batteries to their solar systems.

Installers are dealing with issues like battery compatibility between inverter companies and challenges such as wiring or finding an ideal location for the system. There are several ways to build these systems but the most popular for residential grid-connected systems would be to either use a full hybrid system, hybrid-ready system, DC-coupled battery system or an AC-coupled battery system.

Full hybrid systems

If you are dealing with a consumer who has no existing solar and is ready to go for both solar power and battery power right away, a full hybrid system makes sense. Full hybrid systems are in control of both the solar panels and batteries and can make intelligent decisions about when to charge/discharge the batteries or to power loads in the home.

Having a system with a single common point of control is great for efficiency and for installers and makes life very easy in terms of commissioning. Rather than requesting several systems to communicate, plug in the PV, plug in the battery, plug in the grid and press play.

Hybrid-ready systems

A hybrid-ready system is for the consumer who wants solar power but is not yet ready to pull the trigger on a battery system, although they’re conscious about purchasing batteries in the future.

This is where quite a few installers can trip over. There are a number installers who have installed IP65 hybrid inverters right next to the meter box of the consumer’s home to reduce system complexity. Most dwellings in Australia have the meter box on the side of the home where space is limited and the wall is generally not protected. So where are the batteries going to be placed?

Many, if not all, of these hybrid systems operate on 40-60V batteries. With this battery voltage, it gets very difficult and expensive to extend battery cable length due to severe voltage drop. The greater the distance between battery and inverter, the bigger the battery cable must be. Not to mention the potential load on the cable being taken into account.

A hybrid-ready system must be planned from the beginning otherwise the consumer will be left with several issues to correct before a battery can be installed. This system type will cause the most headaches for installers in future.

DC-coupled battery systems

A DC-coupled battery system refers to a system that interfaces with a solar PV inverter via its MPPT input. This is most commonly used for high-voltage batteries but there are some products on the market which use 40-60V.

With the system operating without any DC-AC conversions, system efficiency is good and installation can be quite simple. High voltage batteries do not suffer from the limitations of 40-60V batteries and can be located some distance from the inverter, however this type of system is not without complication.

The main cause for concern is that we have systems in series. In order for this battery to operate, the connected inverter must be able to operate. If we are connecting a DC-coupled system to a legacy inverter, the battery is bound by the efficiency of that inverter.

A common request from the consumer is the ability to use their batteries to power the home when there is a blackout. In today’s market, several inverter companies can provide this service but a DC-coupled system would find it very difficult to deliver on that promise. PV inverters in most cases will de-activate when the grid goes down. Even with the DC input from either the PV or the batteries, without the grid it cannot provide power.

A DC-coupled system can be either very easy to complete or very difficult depending on the system purpose.

AC-coupled battery systems

The AC-coupled battery system is making its rise to glory. Everyone is talking about it due to marketing and system compatibility. The great advantage of this system is that it is not dependant on the PV inverter and normally comes with its own monitoring and control.

An AC-coupled battery is not just a battery, it has a DC/AC inverter and hence can be classed as an additional inverter. This can be troublesome in some areas with inverter limitations for grid-connected system.

There are losses when considering round-trip efficiency in AC-coupled system, the most basic being power conversions from AC to DC and DC to AC. For example, a PV inverter converts DC from PV to AC, AC-coupled battery converts AC to DC to charge the battery, there is normally another DC-DC conversion to charge the battery at the correct voltage and to use the battery we must convert the DC back to AC for use in the home.

If your thing is system efficiency this system may cause you to flinch, but for a basic system install with very few issues regarding compatibility, it is definitely the system to watch.

Easy when you know how

Installers deal with many types of hybrid solutions, knowing which to use on a case by case basis is a very valuable skill. A system that you use due to price or industry partnerships may cause you more issues than it is worth. All the aforementioned system types are valid solutions, but full hybrid and AC-coupled solutions seem to be the future of our industry.

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