Riding the Wave: How Morningstar designed its software-first solar inverter for the lithium battery age

Morningstar Multiwave

Solar inverters have improved so much over the years that they are all starting to seem comparable in power and performance these days, but a new entry hitting the market from Morningstar stands out because it wasn’t being built for “these days.”

For starters, Morningstar isn’t a solar inverter company. It is an employee-owned business that has specialized in passively cooled charge controllers for 25 years. When it finally decided to develop its own solar inverter, the team started with a different goal in mind: Build the ultimate off-grid solar inverter for the lithium battery storage era. The tangible result, the 4-kW Multiwave solar+storage inverter, will be ready to ship this summer.

The Multiwave is best suited for markets where there is no grid or sell back is discouraged or not incentivized and operates in one of three ways: off-grid solar with generator backup (its primary segment right now); solar priority with grid support (a.k.a. self-consumption operation); and backup and uninterrupted power supply (UPS). Let’s look into how it works.

High(brid) frequency

At a base level, what makes Multiwave ideal for lithium batteries is (surprise) its charging capabilities. What goes into the battery is what comes out, with nary a conversion loss. To hit that level of efficiency, the inverter needed to use high-frequency topology.

“We came at this analytically: How can we design the next gen inverters for the post-lead-acid age?” says Mark Cerasuolo, marketing director for Morningstar. “We looked at where wanted to go and realized high frequency was the way to get there.”

/*** Advertisement ***/

Solar inverters are usually low frequency because it is a great way to generate grid-quality electricity while providing adequate protection for sensitive internal circuitry. This is all due to using/needing large transformers. High-frequency topology on the other hand can achieve switching at much faster rates and remove the need for large transformers and magnetics, but for years the componentry just wasn’t reliable enough to protect sensitivity circuitry.

Morningstar says they were able to smoosh together the advantages of high and low frequency by choosing hardware components for their performance instead of economics (over-specing in some cases), by relying on their thermal engineering expertise and by building it around the platform’s software capabilities.

The cool factor

OK, so I lied earlier, Morningstar built a solar inverter previously. They were involved in a big rural electrification project in Brazil that called for 20,000 300-W solar inverters that all had to be quiet, sealed and tamper proof. This led to the development of the SureSine.

“It will keep its cool all day at full tilt and surge up to 600 watts,” Cerasulo says. “They were even designed to take a dead short. You can’t hurt them. We cut our teeth on that.”

The Multiwave borrows from the SureSine template, mainly its quiet ruggedness and power. Like every product in the Morningstar lineup, the Multiwave doesn’t need a cooling fan, which is a common point of failure for chargers and inverters. They are also parasitic, noisey and capable of sucking in debris. They can burn out and are often warrantied for fewer years than the rest of the inverter. Morningstar prides itself on its thermal engineering, and Cerasuolo says the Multiwave is the only inverter in its power class that is passively cooled.

Software-driven results

As solar system installations increase, more PV systems will be installed by less skilled people and will require more of a focus on ongoing customer service and serviceability. This is why Morningstar spent more time on the software and interface than is typical.

“We used a bit of reverse thinking,” Cerasulo says. “The majority of inverters are hardware-focused first with the control software adapted to that. In this case, we wanted a solution that was more software driven and then a hardware platform that makes it possible.”

Morningstar Multiwave

With no transformer or cooling fans and fewer magnetics, the Multiwave is less cramped on the inside, making O&M simple.

The added design complexity on their end leaves less complexity on the installer and user’s end. Plus, with no transformer, no cooling fans, fewer magnetics and a software-first design, the Multiwave is lightweight and less cramped on the inside, making O&M (stuff like switching out the control board) simple for people with regular human hands. To me, this simplicity is another “future-proof” element of the Multiwave. Some examples:

  • It comes pre-programmed
  • One person can lift it and put it on a wall.
  • Much of the main circuitry is in one large board. So if there is a failure, you can swap it out pretty easily.
  • It is expandable. A built-in DIN “Ready Rail” allows you to add on and sync up generator control, system monitoring alert capabilities and battery management systems as simply as inserting a USB drive in a laptop.
  • To help achieve its No. 1 design goal — building it for the lithium battery era — the company can also provide specific programming instructions for any battery submitted to its lithium battery Partner Program.

Energy Storage Partner Program explained

Lithium batteries, specifically the safer lithium-iron phosphate (LiFePo) types, have a lot of potential, but they are higher priced than lead-acid and require different usage strategies and settings. The Energy Storage Partner program is purpose-built to address these nuances.

“It’s not as easy as just dropping this stuff in, so we said, well, let’s make it as easy as just dropping this stuff in,” Cerasuolo says.

On the product side, the MultiWave inverter/charger and selected Morningstar charge controllers have a lithium “foldback” feature. Lithium batteries can be damaged when charged under cold conditions. So, if it drops below freezing, the system will cut back on charging and even turn it off.

As for battery specific set points, that’s where the Energy Storage Partner Program comes in.

“Increasingly, we have installers asking for complete instructions on how to set them up with certain lithium batteries,” Cerasuolo notes. “So, for batteries in our Partner Program, we make sure they’re compatible and then we come up with the guide to use that battery with our product. Basically, what we’ve done is demonstrate our compatibility with the leading brands of advanced battery chemistries (mostly LiFePO4) to provide new and existing customers peace of mind that Morningstar products are able to adapt to the requirements of new battery technologies.”

“Through direct cooperation with each manufacturer, we have derived the appropriate setting values required for successful integration with each compatible Morningstar product,” says Jacob Sherry, applications engineer for Morningstar. “These settings correspond with industry-standard terminology and conventional charging algorithms found in battery-based power electronics.”

For example, many of the critical settings detailed in each document pertain to adaptation of the standard multi-stage charging profile used to charge lead-acid and other battery chemistries. Therefore, it is required that each brand provide specific recommendations for absorption voltage, absorption time and float voltage (if needed). Battery High Voltage Disconnect (HVD) and Load Low Voltage Disconnect (LVD) are also important settings to consider, since many controllers can be damaged due to a battery overvoltage disconnect during charging. Similarly, many lithium batteries may require manual site intervention at uncontrolled low Depth of Discharge (DoD).

“All of these values can vary quite significantly between manufacturers, depending on their specific cell characteristics and the tolerances of their particular Battery Management System,” Sherry says. “Having these settings and information properly documented provides a wonderful convenience for our customers who may be interested in exploring the benefits of newer battery technologies, without losing the quality and reliability of their favorite charging solution.”

The Multiwave isn’t shipping yet, but you can check it out at the NABCEP Continuing Education Conference March 26-27. If you’re interested in adding lithium batteries to a new or existing system, check out Morningstar’s Energy Partner Program here.

Leave a Reply

Your email address will not be published. Required fields are marked *

*

This site uses Akismet to reduce spam. Learn how your comment data is processed.