Going hybrid: Adding batteries to grid-connected solar
Going off-grid may not be for everyone; a better route may be to ‘go hybrid’, by adding batteries to grid-connected solar. Andrew Reddaway explores the options.
This article was first published in Issue 132 (July–September 2015) of Renew magazine.
The solar battery industry is on the verge of disruptive change. Traditionally, large batteries were only seen in houses at off-grid locations.
For off-grid systems, reliability is crucial; failure prompts an emergency call to the solar installer, so such systems have been designed conservatively using proven lead-acid batteries.
Different batteries for hybrid
A hybrid solar system is tough on batteries. Unlike an off-grid system that may store enough energy to last multiple days, a hybrid system’s entire usable capacity will often be charged and discharged daily. This requires a battery that can handle fast discharge rates at high levels of efficiency. Lithium batteries fit the bill, and have already become dominant in consumer electronics, power tools and electric cars. Compared to lead-acid, they are also smaller, lighter, don’t require monthly maintenance and don’t emit hydrogen gas. The only things holding them back in the solar market are unfamiliarity and price.
The recently announced lithium Powerwall battery from Tesla is priced well below previous products and has a 10-year warranty. Traditional lead-acid batteries cannot compete with this new benchmark, so it’s expected that systems will start to move away from them. Hybrid systems are now expected to become viable on pure economics by 2020, at least for some households1. Early adopters are already installing lithium hybrid systems, as are some who value maintaining power during a blackout.
Option 1: Solar buffer battery
So how can a battery be added to an existing grid-connected system? The simplest concept is to connect it between the panels and the grid-interactive solar inverter, most likely wall-mounted next to the inverter. From a string of panels, current flows at, say, 400 VDC into the battery during the day. The voltage is regulated to the internal battery voltage, say 500 V. At night, DC current flows from the battery to the solar inverter and then to the house switchboard at 230 VAC. The inverter doesn’t even know that a battery is present—as far as it’s concerned the solar panels are still generating!
To work as a proper solar buffer, a sensor at the switchboard is also required. When the house is starting to import electricity from the grid the battery should discharge, and when the house is starting to export the battery should charge.
The advantage of this approach is that no new conversions from AC to DC are introduced. Costs are minimised as additional inverters aren’t required, and efficiency is high. However, blackout backup may not be available, as a typical grid-interactive inverter shuts down when the grid is not present. An issue to check is whether the battery will interfere with the inverter’s maximum power point tracker (MPPT). Also, how does panel generation bypass the battery when it’s full? If the battery has a lower power rating than the total solar array, will the panel output be clipped?
The Tesla Powerwall 7 kWh battery is designed to be installed this way. Another yet-to-be-deployed product is the Australian Sunsink. SMA already offers a grid-interactive inverter with 2 kWh of lithium batteries built-in, although its price is relatively high.
Option 2: AC-coupled inverter-charger
An alternative approach is to keep the battery separate from the existing grid-interactive inverter and wire it to the house switchboard. As the switchboard runs at 230 VAC, this is called an AC-coupled system. Batteries are DC, so an inverter-charger is required near the battery. The battery’s nominal voltage is likely to be 48 VDC.
One advantage of this method is that blackout protection can be provided by the inverter-charger. When the grid goes down, a high-quality hybrid system will step in so fast that house appliances are not disturbed by the changeover. As far as the grid-interactive inverter is concerned a blackout never occurred, so your panels can keep generating. Appliances can be powered by the total output of both the inverter-charger and the grid-interactive inverter (if the sun is shining).
To conserve the battery during a blackout, it is possible to have some high-usage household circuits switch off, such as the oven, air conditioner and pool pump. A petrol or diesel generator is relatively easy to add, with startup controlled by the inverter-charger. In fact, a good AC-coupled hybrid system has all the features to disconnect from the mains and go off-grid! However it may be under-sized to run the whole house full-time.
You can also exploit additional strategies to use the battery with such a system. For example, if you have cheap grid electricity in the middle of the night you could use it to charge the battery to cover a morning consumption peak. Another benefit of this method is that the existing solar system is not disturbed, avoiding potential remediation work where standards have changed since the original solar installation.
The main drawback of this approach is the cost of the smart inverter-charger. Also, electricity that goes through the battery requires an additional conversion from AC to DC and back again, reducing efficiency.
An issue to check is how the hybrid system regulates power from the solar panels during a blackout. Some inverter-chargers can communicate with the grid-interactive solar inverter, throttling its output when the battery approaches capacity. However, this feature may only be available with compatible models of grid-interactive inverter. If this is not possible, the system may not handle a large solar array.
If you’re installing a grid-connect solar system now and considering adding batteries later, it’s worth checking whether the inverter can communicate with any hybrid inverter-chargers.
There are many examples of AC-coupled inverter-charger hybrid systems, using, for example, Schneider Conext, SMA Sunny Island and the Australian Selectronic SP-Pro inverter-chargers.
Option 3: Self-contained appliance
This option uses an appliance with an inverter-charger in the top and lithium batteries in racks at the bottom, often called an ‘all-in-one system’. Installation is simple— wheel it in, set it on the floor and wire it into the switchboard. The solar array may connect directly via a DC cable (DC coupled) or via a grid-interactive inverter (AC coupled). Ideally, battery capacity is expandable so you can start with a small capacity to minimise costs, see the effect on your bill and add additional cells as desired. If you move house, you can take it with you! Downsides to this option are cost, and finding a suitable place for the system. Examples include the Solari Energy SolaGRID, the Bosch BPT-S and the ZEN Freedom PowerBank. These all-in-one systems were covered in a buyers guide in ReNew 128.
What about microinverters?
Currently it is difficult to add batteries to a microinverter solar system. Option 1 is not possible as electricity from the panels is AC. Options 2 or 3 are problematic as no microinverters can yet communicate with an inverter-charger. This will change when Enphase (the leading microinverter manufacturer) ships their own battery in 2016.
Predicting the future
It’s still early days for hybrid solar systems. Different concepts are competing for a small pool of early adopters and it’s not yet clear which will become mainstream.
When adding batteries to an existing solar system, I expect Option 1 will be most popular due to relatively low equipment costs. However, compatibility may be a problem for retrofits. For example, as of mid-November 2015, it appears that the Tesla Powerwall can only be easily added to existing systems if they use a Solar Edge inverter. For new hybrid installations perhaps Option 3 will dominate as it requires only a single device beyond the panels, reducing installation costs. Hopefully battery systems will settle on standard sizes as other home appliances have. A fridge shape seems optimal where space can be found in the garage or house; perhaps such spaces will be included as standard in future building plans. Longer, slimmer appliances may find a market along the inside wall of garages.
If your goal is to reduce electricity bills, it’s a good idea to hold off on adding batteries until price drops flow through to mainstream products. If you have other motivations, there are solid hybrid systems available right now, at a price. Talk to an installer experienced in hybrid and off-grid.
If you’re getting a standard grid-connected solar system, aim to keep your options open to add batteries later. When installers are quoting, ask them how batteries can be integrated.