Components of a Storage System
In my previous blog on storage, I discussed how to size a storage system. In this blog, the discussion will be on the components of a storage system. As before, we will be focused on AC coupled Sonnen systems, but the basics apply to all storage systems. AC coupled systems are integrated to solar systems that have output already converted to AC (usually another inverter involved) and are easy to add to both existing solar systems and/or install with new ones.
Most storage systems are comprised of the following components:
- Off-grid inverter
- Storage batteries
- Solar system
- Transfer switch
- Protected loads panel (PLP)
- Remote off switch
- Software controls
Other options some systems have include:
- Internet connectivity
- Generator controls
Some systems, like Sonnen, have all of these components (except the PLP) in a single bundled, warrantied, and supported package. Other storage systems cobble different products together, often job specific, making these systems non-standard and hard to support.
Let’s start with the off-grid inverter. When we hear about inverters today, it is usually the grid-tied type. Grid-tied inverters convert the DC power that solar modules make into AC power that our homes can use. When the power goes out however, grid-tied inverters stop exporting power to the grid, keeping utility lineman safe. Off-grid inverters are designed to function without utility power. They have a transfer switch, discussed below, which disconnects the utility. Thus, when the grid is down, these homeowners will have backup power.
With the exception of the batteries, the inverter is the most expensive and important component of a solar system, and they are not all designed the same. The size of the inverter (8,000 kW in the Sonnen system that Synergy sells) is the amount of power the system can provide and the amount of solar it can accept. The inverter also handles surge voltage that allows for excess voltage common when pumps and motors start. In Sonnen’s case, the inverter can handle surge voltage up to 22,000 watts. Most of the other systems on the market have a 5,000-watt inverter with a surge voltage of 7,000 watts. As discussed in my previous blog on sizing, the inverter sets the stage for how much power a system can deliver and what loads it can support.
Storage batteries allow a homeowner to store power for use at a later date. In my opinion, the most import thing to pay attention to is the battery’s chemistry and safety. Sonnen uses Lithium Ferro Phosphate (LFP). The LFP chemistry is the best for home storage because of its safety, high cycle rates, longer warranties, and you can even drive a nail into a working cell and it just fizzles, no fires! A few popular home storage companies use the same battery technology used in electric cars; Lithium Nickel Manganese Cobalt (NMC). This technology is less safe and does not cycle as well as LFP. NMC batteries can also go into “thermal run away,” meaning they catch fire when punctured or short circuit. So why would companies use NMC? It is lighter, good for cars and less expensive to produce. But when it comes to home use, LFP batteries are clearly the better choice.
When the grid is down, this stored power from batteries can be used to run appliances, lights, wells, etc. until the grid returns. See my previous blog for sizing batteries. Batteries can be charged with solar, a generator, or both when the grid is down. Batteries can also be used for financial justification; storing power in the morning when utilities charge lower rates and allowing a customer to use power during peak when it is expensive. By the end of 2019, when customers use solar to store power, they will be able to export power during peak usage periods, another financial benefit.
Solar System: While solar systems are not required with storage, almost all residential storage systems are paired with solar. With the grid down, the primary way storage systems are charged is by the sun with solar. While a generator can be used to charge a battery system instead of solar, most of the credits and rebates available for storage require them to be paired with solar. The way solar charges is different depending on whether the grid is up or down. When the grid is up and solar power exceeds the home’s usage, any extra power goes to charging the batteries. This charging can be over written by software controls to ensure peak or expensive power is not used to charge the batteries. When the grid is down, solar will continue to charge the batteries until they are full, or the sun sets.
Transfer switch: The transfer switch of a storage system is the device that disconnects the utility when power is out, allowing the storage system to power the home without back feeding the grid. The Sonnen system has an integrated 200-amp transfer switch and it works like this: When the power from the grid is up, power passes through the Sonnen system (up to 200 amps) to the loads (appliances, lights, pumps etc.) it feeds. When the grid goes down, the transfer switch disconnects the grid and feeds power directly from the Sonnen inverter/batteries to the feeds it is powering (lights, plugs, router etc.), allowing them to run even when the grid is down. When grid power is restored, the transfer switch disconnects the inverter and allows power to pass through to the Sonnen from the grid again.
Protected Loads Panel (PLP): The PLP is a sub-panel with all the loads we want backed up when the power is out. A typical PLP will have the well pump, refrigerator, kitchen plugs, lights, the router, garage door, etc. As discussed above, when the power is up, these loads pass through the storage system and are fed by the utility. When the power is out, these are the circuits that the storage system will power. With the Sonnen system design, these are also the loads that can be run off the Sonnen system directly when utility rates are high, further reducing costs.
Remote off switch: When emergency responder’s come to a home during a fire or flood, one of the first things they do is turn off the power to make it safe. Since storage systems are design to provide power when the utility is down, there needs to be an easy way to turn them off as well. The Sonnen system offers an optional remote shut off. At Synergy we install this option on all our systems, near the electrical shut off, and visibly labeled to keep our emergency responders saf
Software controls: The software controls are the brains of the system and where Sonnen systems shine. With the Sonnen system homeowners can select the primary modes, back-up, self-consumption, time of use, or a combination of them. For example, the system can be set to self-consume up to 50% of the storage battery, but to always leaves 50% for backup. When a storm is coming, a customer can use their phone or the screen on the Sonnen to stop self-consumption and store 100% for back-up. Most of our customers set the defaults and let the software controls manage the process.
Internet connectivity: Sonnen systems use internet connectivity in a few different ways. Customers can use the internet to connect to their Sonnen systems to control how it can be used in their homes. Sonnen uses the internet to remotely diagnosis problems and download firmware and software enhancements. Synergy uses this connectivity to monitor customer’s systems remotely. Internet connectivity will allow Sonnen systems to work with other battery systems and utilities in unique ways in the future.
Generator Controls: The Sonnen system has a generator control that allows an intelligent generator to work with the Sonnen system (up to 12 kW). The software allows homeowners to set the threshold for when the generator will start. Let’s say it is set for 10%. When the batteries run down to 10%, the Sonnen control sends a signal to the generator which starts up and begins to charge the batteries. When the batteries are full, the controls send a signal to the generator to turn off. A generator can be used to supplement a system when outages are followed by cloudy days with low solar output, or when large usage loads are required beyond the storage available. There can be financial benefits of using a generator with storage, but that will be covered in my next blog.
As you can tell, there are many parts that need to work together to make a storage system functional and reliable. One big benefit of the Sonnen system is their unique, integrated approach of providing a self-contained, turnkey product that is easy to install and support.
Part one of this series covered the sizing of storage systems. This blog covers the components and my next blog will discuss my favorite topic, the benefits, both financial and functional of storage. Stay tuned
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