What do you get when you combine the benefits of microinverter and string inverter technology? Silicon Valley-based HiQ Solar and its veteran team used their background in the precision communications test and measurement field to produce an inverter that is like none other, and they manufacture it right in California. We reached out to HiQ’s Craig Hartwig to find out more.
SPW: Tell us about your background and how you got into solar.
CH: I began my career in the electric utility industry working with A.B. Chance. More recently I was at a start-up that made test equipment used for precise measurements on telecom equipment and semiconductors. Our customers needed to know that the integrated circuits being measured were talking to each other correctly without making mistakes. It’s all about making sure that when your internet purchase at Amazon is flying across the ether, your credit card number isn’t confused with someone else’s because the infrastructure underneath made a mistake along the way. In that industry I saw innovation being used to reach higher performance and to reach lower cost.
Later on, when one of my colleagues started HiQ Solar, I decided to join, along with many of my former teammates. Again we focus on innovation to reach higher performance and lower costs, but now in the power industry – so it’s kind of like coming home for me.
SPW: What issues did you see with inverters currently on the market and wanted to solve or improve?
CH: There are many, but one concerns the size of string inverters. The market is pushing for lower cost per Watt, and the primary way inverter manufacturers respond is to make the string inverters bigger–almost small central inverters you bolt to a wall. These 40 to 60-kW solar inverters may be cheaper to buy, but that isn’t the end of the story. When you parallel many strings together, it becomes very difficult to tell what’s going on. These large string inverters parallel several strings, say 16 modules each. Paralleling has several issues. Firstly, if you detect an issue, you are narrowing the issue down to perhaps a group of 64 modules, which makes troubleshooting harder. Secondly, if you parallel strings it can have the effect of “averaging” out effects, so you might not even know there is a problem. Lastly, arc-detection is almost impossible to do reliably when many strings are paralleled; you either don’t see the issue, or you make the detection so sensitive it trips accidentally all the time.
There are many other problems with larger string inverters that go beyond the brief benefit of low upfront-cost, including an increase in other BOS costs, increased risk of DC wiring mistakes, under-optimizing PV in a given area, complicated layout and design and a reduced energy harvest.
SPW: Can you talk about how this lead you to develop the TrueString inverter?
CH: Our team really liked microinverters, but we knew it’s hard to design them at low cost. This is because each inverter needs it’s own enclosure, communications and power supplies. So we designed the 8-kW TrueString to use much of the microinverter technology but apply it to a small string inverter. This makes the cost very attractive and still retains powerful features.
We have power optimization per string, monitoring per string and arc detection per string. The inverter takes in two strings and handles them completely separately. Strings might be somewhere from 10 to 20 PV modules depending on type. It’s not the per-module resolution that microinverters have, but the appropriate resolution for commercial rooftops and carports, and much better than the large string inverters.
Also, 2014 NEC Rapid Shutdown says that the voltages outside the footprint of the array must be safe if a firefighter throws a switch. The most cost-effective approach of meeting this is to mount the inverter next to, or under the array. For us, that’s easy. For large string inverters it is difficult because they were designed for mounting on a wall, not on their backs on a roof. They require extra mounting carriages. They are also really heavy, hard to get onto the roof, and even harder to replace later if there’s a problem. Our model is a lot lighter and provides the benefits that come from not needing an extra mounting carriage. The NEC says that the AC and DC sides of the inverter must have a means of disconnect. Sometimes these are extra-cost options on string inverters, sometimes the customer has to put their own in, at a cost to them. Like some microinverters, our connectors are the certified means of disconnect, which means the customer saves money by not needing to buy them separately.
SPW: You say you have the “most power dense solution on the market” can you briefly explain how you were able to accomplish this, while maintaining high reliability?
CH: The traditional approach for inverters is to build them like battleships so that they will be resistant to corner-case issues like grid spikes that occur a small percentage of the time. This approach uses lot of steel but adds to the size, weight and cost. However, we learned in telecom that computing power is cheap. We accurately size the components for the power conversion job at hand, and use intelligence in the micro-processors to react quickly during rare events. Inside the inverter, we avoid using electrolytic capacitors, cooling fans and make sure all of the components are automotive-grade. They’re packed inside a NEMA6 enclosure protecting the electronics in any environment, making the inverter very robust. The end result is highly reliable, but a smaller and lighter unit compared with convectional approaches.
SPW: You say your inverter is ideal for commercial applications?
CH: Yes. The TrueString is a 480-V, 3-phase product, so it doesn’t apply to residential installs. It works really well in roof mounts and carports cause it’s small enough, at 24 lb, to tuck under the PV array out of site on a roof, or be concealed with the PV modules safely away from the public on a carport. It solves the Rapid Shutdown requirement simply, and as the connectors are the certified means of AC and DC disconnect it saves on the cost of those as well. Then, with the inverter’s NEMA6 enclosure and no fans, it is also great for hostile environments, where others models tend to fail such as in high altitudes, deserts and coastal regions with corrosive salt. Some of our inverters have been working well in the Mojave Desert for several years, so we’re confident in the technology.
SPW: What’s industry response to the model been, and where has it been deployed?
CH: Initially we were primarily working with local commercial installers in California. However, we have been getting a lot of interest from installers across the U.S. and abroad such as Mexico and South America. Aside from many installations in California, we also have the inverter deployed in many other states around the U.S. Because of our simple and unique solution for Rapid Shutdown, our pipeline in states where this is a requirement is growing quickly, especially in the northeast. The response to the TrueString inverter at tradeshows has been overwhelming, not just from potential installers, but from competitors as well; most of them are skeptical it’s even real. I think we’re a bit of a shock for them. We’re building a solid track record, having recently shipped 100+ units for a 1.4-MW rooftop install in the Bay area, and several more large projects are in the works.
SPW: Why should solar contractors consider using this inverter?
CH: It’s a problem solver, and price competitive with much larger string inverters. Its 8-kW size is nice because it makes laying out a roof easy. It has two independent MPPT inputs, one for each string, so you can use both or only one and have different string lengths on each. This makes it nice for those difficult corners on a roof. It fits in any orientation, so it can be tucked away wherever convenient, and it doesn’t take up space in the parking lot. One size really does fit all. Individual string level visibility makes it fast to commission and correct wiring issues, and you don’t need a hoist to get it on the roof. It’s simple–try one out.