A/C Coupled Battery Back-up Test Installation Update

Back in January we told you we were gping to test a battery back up system ( https://skylit.ca/news/battery-back-up-test-project/ ). Now, our A/C coupled battery back-up system is complete and operational!

What IS an A/C coupled battery back-up system, anyway?

For many years Solar PV and batteries have gone hand-in-hand for off-grid solar applications, and of course, off-grid is where the solar PV story began. Then along came grid-tied solar, which falling costs, government subsidies and feed-in tariffs have made more and more popular. One of our most asked questions is “if the power goes out during the day, will my solar keep working?” to which we have to reply (with great regret and to homeowner disappointment) “no it won’t”.

It’s pretty frustrating to imagine that on a sunny day after a hurricane, your solar panels could be there to kick out power to keep everything running – but it can’t. This is with good reason: any power shipped to the grid would become high voltage through the transformers and could electrocute power workers trying to get the grid up and running. There are other reasons too, but this safety issue is the big one.

However, the world is changing and a new kind of solar and battery system will soon be here. This is the grid-tied, or a/c coupled, solar and battery system using very high powered lithium-ion batteries which support far higher depth-of-discharge (i.e. can deliver more power) than the older technology lead-acid batteries that were the mainstay of off-grid applications. The advent of the lithium-ion world is, of course, thanks to electric cars which have driven costs down dramatically, almost by 90%, compared to a decade ago.

Getting prepared for new battery technology

At SKYLIT we not only want to give the best service, the best products and warranties, and the most beautiful solar installations, we also want to be ahead of the curve in getting our hands on new technologies. So, backing up from this digression into the lithium story, a/c coupled batteries with on-grid solar is NEW. Like, so new that systems like this are rare to the point of almost non-existent out there – so we wanted to build one to get first hand knowledge and experience of how they go together.

So, how does it work?

For starters, it’s automatic; the grid goes down and the system automatically isolates the house from the grid and restores power to the house from the battery system. Instantly. Like 14 millisecond instantly, you don’t even see the lights flicker! Also, the power from the batteries is delivered to the house through an inverter, which also delivers power to the solar panels. This means that the solar panels “think” the grid is on so they keep producing power. So, the solar delivers power to the home and keeps the batteries charged and when it goes dark the batteries provide power to keep things running until morning when the solar starts up again. It’s kind of like the the on-grid -home becomes an off-grid home.  When the grid eventually starts up again, the system detects it (in 14 milliseconds) and the power from the grid is restored into the home and the battery system recharges to be ready for when it’s needed again.

So, it’s great! But one minor thing… no residential battery can store enough power to run everything in the house, especially if you have heat pumps. Instead, it is used to power “essential loads”. At our test installation in Bridgetown, we connected lights, fridge and freezer, some kitchen power  plugs, some living room plugs (for TV), the internet, sump pump and the oil furnace. With just 14 milliseconds before battery power kicks in, everything just keeps running smoothly, even the computer. In principle, the home can stay off-grid indefinitely, with the battery delivering power smoothly overnight and the solar providing power to recharge it and power the house during the day. If the batteries are depleted and the sun isn’t shining, a generator can be used to recharge the batteries. If there is surplus energy from the solar it needs to have somewhere to go, so it turns on some baseboard heaters to offload the excess power.

Very smart!

The A/C coupled battery back-up system
The battery back up system

This system uses gel batteries, a type of lead-acid battery, provided by the NSCC’s renewable energy lab in Dartmouth and they will be monitoring performance of the system remotely over the internet for the next few years. There will be test power outages to see how the system performs and also to see how the occupants of the house respond and learn to live with the advantages and constraints that come with the system.

Inside the Battery Box of our A/C coupled battery back-up test installation

For SKYLIT, it has been great experience. This was a particularly challenging installation presenting us with more issues to resolve than we’d normally expect – which was actually really valuable to us.

The homeowners had this to say:

First of all, we are very pleased to be selected for this project.

Right now, the system is operational, but until we experience a power outage there’s not much more to say about performance.

The installation was complicated because our panels are on the garage which is ~ 25 meters from the old farmhouse where everything else is located.  It required that Andy do lots of planning and consultation with suppliers to identify and configure the components required for our situation.We are looking forward to the trial outages that we will do with NSCC.  It will help us to learn to use the system wisely during a real outage.  They are setting up sensors to record how much energy the various Essential circuits use and we will be able to access this information during trial outages.

So What Now?

The great thing is that major companies have been working hard and will very soon be launching “Home Energy Management Systems” which are integrated purpose-built solar and a/c coupled battery back-up systems and they include AI to optimize use of the power available. They even use weather forecasts so they will prepare for a storm automatically before it arrives. We expect to be seeing them very soon and they will become much more commonplace over the next few years, particularly in new-build houses, as the cost of lithium-ion batteries continues to fall. And we are thrilled to have the first hand knowledge and experience putting this system together, and are ready for what comes next!

Amanda Brulé

Amanda is VP Sales & Residential Projects at SKYLIT.