In a recent webinar Mara White, product manager for Outback Power discussed how inverters and storage work together. Here’s a recap of a Q&A with listeners. You can watch the full webinar here.
Solar Power World: A lot of people are asking about net metering and solar storage. They’re wondering about its impact on the solar and storage market.
White: Net metering is going to be very region specific—program by program and utility by utility. Even within the larger state of California, we’re going to see three or four different programs along the coast. It really depends on what the utility requirements are and whether that grid connected with the battery chargers are going to comply. Quite often, the majority of them absolutely do.
SPW: Why aren’t nickel metal batteries usually considered for renewable energy?
White: Nickel metal hydride batteries are not a mainstream technology. The vast majority of the form factors are for smaller systems. Nickel isn’t within the capability of most of the inverters, it would fall under custom battery settings. The place you see it used most right now is in electric vehicles, like the Prius. The issue with the nickel iron chemistry is that it has a low efficiency factor for recharging. It does have a very long life, but it’s not very efficient. It takes a lot more energy to keep the battery topped off than it would a more common lead-based or lithium system.
SPW: People also have some questions about using different battery technologies with different types of inverters. Do you have to check if the inverter will work with specific types of batteries or are they universally compatible?
White: You can use different storage technologies with one inverter. Generally you’ll pick one to go with because the inverter does not like to work from two. For instance, you wouldn’t have a lead-acid system paralleled with a lithium system. One thing to keep in mind when using lithium batteries is that they require battery management systems. Most lithium battery management systems are part of the battery pack, so many inverter manufacturers will include that component as well, because of the communication that has to happen in those systems. Looking at pairing programability and voltage windows is also important when matching inverters and battery technologies.
SPW: Are you able to add storage to microinverter systems?
White: The primary way of adding storage to microinverters is AC coupling; it’s the least difficult way to do it. Becasue the microinverters are aggregated on the roof, and you’ve got a pair of wires usually coming down from the roof that feed into a sub panel, it’s pretty easy to back it up with an AC coupling system.
SPW: Are there any design constraints with AC coupling, most particularly with respect to sizing batteries and back-up inverters?
White: It really depends on the original grid-tie system is. The AC coupled inverter should be oversized to compensate for efficiency loss; you normally would not take an AC coupled inverter to it’s nameplate when your only source of energy coming into it is from a PV array. Specifically, the multiplier is 1.25 over capacity on the inverter.
If you’re doing a hybrid type system, whatever loads you want to operate when the power has failed need to be within the capacity rating of the inverter. There’s factors that we use that calculate that. There’re standard NEC factors that are used in the size of the circuit breaker ratings. Typically if your have normal residential loads, you’d follow similar NEC guidelines. If you have a lot of big well pumps or things like that, then generally we recommend you stay within the surge rating of the inverter so you size that into your calculation. It really comes down to how much energy storage you’re willing to put in those kinds of systems. This determines what size inverter you’re going to need.
SPW: People are asking a little bit more about how to calculate loads. How is this done?
White: There’re quite a few online load calculation software programs available for free online. We usually recommend that a customer looks at those first. Then they can also look at their current energy usage. Their power bills are really important as well, as you can also get devices to measure what each load is drawing. You can plug them into your outlet and then plug the device into it. This will tell what’s going on with those loads. Calculating loads is definitely a crucial part of the design process.
SPW: One question about fire safety and battery back up installation. I heard a firefighter speak on this in one of our past webinars, but if you could also touch on this topic. Are there any best practices for helping out fire safety professionals when you’re dealing with a solar storage system maybe that’s different than just a solar array in general?
White: This can be fire code specific. There are parts of the fire codes that can specify space around panels for accessing for if personnel have to go on the roof to make any holes for smoke ventilation. There can be differening battery bank compliance codes as well. Many of the charge controllers and other systems that would be directly interfacing with the batteries all contain disconnects, which would be the first point at which the firefighter would interact with the system. Disconnects are usually included for the PV coming from the roof, the AC disconnect on the house and the DC disconnect. Most systems, specifically battery backed-up systems carry labeling that would let the firefighter know what part of the system is active.
SPW: Reducing battery prices is reliant on refining storage technology and increasing volume. Is there a way to reduce the cost of storage on solar systems from the soft-cost side, such as on installation?
White: Probably the largest single contributor to lowering your cost of your backed-up system is reducing the number of loads that you’re trying to back up. That has the largest impact on the system costs. Everything you’re trying to operate and you’re expecting to work properly in the backed-up state, you’ve got to provide sufficient energy for to run. Probably the first thing you do in any system that you’re considering either running off grid backing up, is to consider conservation first and look at ways of reducing the load that you’re going to run in the back-up interval. You typically don’t see electric heating or any of the loads that are more easily operated by any other source of fuel. These are generally taken off the electrical system. When we talk about critical load panel, critical is the key word. What’s absolute necessary to run? The freezer and fridge? Some lighting and entertainment? Although we do have customers who want to run the whole place, but it’s going to cost a lot of money and involve a huge system. They want to run their whole house, they have lots of money and they have huge systems.