by Eric Wesoff
April 26, 2016

First Solar is making a fundamental change in the architecture of the big solar power plants it builds and operates.

CEO Jim Hughes has made repeated claims about cost reductions at the leading integrated solar power provider, saying, "We have a technology roadmap -- by 2017, we'll be under $1.00 per watt fully installed on a tracker in the western United States."

First Solar CTO Raffi Garabedian recently outlined some of the paths the company is following in order to hit that low number -- including improved cell efficiency, new module form factors, one-axis trackers and a new solar architecture that marks a shift in the way big solar might wire and operate its plants.

That's medium-voltage DC (MVDC) -- and Garabedian detailed the new approach at First Solar's recent analyst day.


Garabedian called MVDC "a completely new, radically game-changing power plant architecture for solar."

"If you look at a conventional architecture, you've got modules in the field that are connected in what's called strings. Multiple strings of modules are combined in combiner boxes where all of that energy is aggregated in parallel, and fuse-protected, and then sent through feeder lines to power conversion stations. That transmission of energy is done at 1,500 volts today with our 1,500-volt inverter system. More commonly, it's done at 800 or 1,000 volts in the industry," he said.

"Each power conversion station is a combination of an inverter and a transformer that steps up the voltage roughly to 35 kilovolts, which goes to a PV combiner switch gear and then onto the substation and onto the grid connection. That's how power plants are built today," Garabedian continued.

"We're looking at changing the number of elements in this power plant, reducing the number of parts and improving the capacity factor of the energy generation of the plant. The way we're going to do this is what's called medium-voltage DC. By replacing the combiner boxes here with DC-to-DC converters that step up the voltage from the strings to roughly 10x the string voltage of 1,500 volts, we can dramatically improve the cost of wiring and the resistive losses in the wiring, and by using a very large-scale DC-to-AC converter, which is commonly available in the utility industry (used for grid ties globally), and by leveraging that pre-existing technology we can achieve a very good cost structure from the MVDC point forward to the grid connection," he said.

"We also bring to bear grid control and support functions that are already available in these large-scale DC-to-AC converters, which are not available in standard PV inverter systems. The system hinges on a new component, and the new component is being designed and developed by First Solar with partners. It will be a First Solar branded product, which we'll put into the market." This DC-to-DC converter "looks kind of like a transformer that you might see in your neighborhood, and it performs that voltage conversion and power-point tracking function that used to be done in the PV inverter. It also supports the same functionality as the combiner boxes and spreads some of the power conversion into the field," Garabedian added.

"What are the advantages? We can reduce the number of components in the field. We can reduce the amount of labor in the field, and we can reduce the energy losses in the power conversion systems.

"To put this in context, let's talk about power conversion stations, inverters and transformers. With a 1,000-volt system, a typical 100-megawatt power plant would have 100 power conversion stations that have to be put out in the field, installed, commissioned and brought up. [With] a 4-MVA system like our 1,500 volt architecture, that number is 25 power conversion stations.

"With MVDC, there is one power conversion station -- a single centralized power conversion station that's integrated into the substation. Fewer transformers means less dark loss, so less energy loss, better capacity factor for the plant. With the MVDC power aggregation voltage, we can reduce our overall amount of wire in the field by about 75 percent and almost cut in half the amount of trenching that's used to bury all that wire in the ground. A tremendous cost-saving impact, improved energy performance, and better economics for our customers."

According to Garabedian, "It doesn't stop there. There's one more benefit. This system is storage-ready. What that means is that we are architecting the system to accept battery storage systems integration into the DC side of the plant."

"Why are we doing that? Storage integration into the DC plant ultimately, we believe, delivers the lowest overall cost of storage-plus-solar in the market. We think this architecture will be able to greatly outperform, from a cost perspective, separate storage and PV, which are great interconnected," he said.

"We're expecting or hoping to achieve a commercial pilot in 2017 and general commercial availability to select partner customers in 2018. Putting together those technologies and a few other things we're working on, like our next-generation tracker and improvements to our wiring and wire management technologies, we expect a very dramatic continued cost-reduction curve at the system level going forward through 2020," Garabedian said.

MVDC "exponentially more ambitious than 1,500 volts"

MJ Shiao, GTM's director of solar research, sums up the MVDC approach as a "super-centralized" model with a very large medium-voltage DC-to-AC converter creating "a 100-megawatt power conversion station."

Shiao identifies all the benefits of raising DC voltages: "Less wiring, less trenching, eliminating combiner boxes (although you replace that with the DC/DC optimizer), fixed voltage output that reduces the DC-AC conversion complexity, reduced DC losses, reduced site grading/civil work, etc."

But he also has some concerns: "MVDC means a whole new ball game for construction and O&M. There's not much solar-specific equipment beyond the DC/DC optimizers in this case, but anything that's not off-the-shelf needs to be certified by UL, etc., and that could take a while."

"That also could affect O&M (requirements for who can service the system, when and how) and uptime -- that's a massive single point of failure in the power converter," Shiao said. Garabedian notes, "The large central inverters we're talking about are widely deployed on utility grids with a solid and well understood service record. We already have long historical data on availability, MTBF, costs to service, failure modes, etc. These things have been around for 50 years and have been engineered with appropriate derate factors and levels of internal redundancy. That's a big advantage to using an established product line for this part of the system."

Shiao continues, "Part of the reason why we don't see incremental medium-voltage architectures is that the jump in cost of construction, as you consider more safety, etc., likely outweighs the savings. However, if you're able to jump up so massively so as to amortize all those costs over a single 100-megawatt unit, then yes, I could see a significant net cost savings. The key factor will be the true cost for those DC/DC conversion units."

Shiao points out that the architecture isn't totally brand new. Alencon, with funding from the DOE and backing from Stephens Capital Partners, has been working on this type of architecture, albeit with a 2,500 VDC bus. Shiao suggests that SunPower is working on something along these lines (based on the DragonFly acquisition), while Ampt has also been pushing string-level DC optimization in the utility space for the past few years.  

Shiao concludes, "Much in the same way that 1,500 volts will get tons of traction this year, anyone looking at what's next for PV electrical BOS cost reductions needs to be looking at DC-DC optimization."

Scott Moskowitz, GTM Research solar analyst, notes, "This is exponentially more ambitious than [the move to] 1,500 volts," with "far more significant hurdles." Shifting to 1,500 volts only required small changes in supply chains, some R&D to reconfigure and test products, incremental changes to standards for PV components, and there was precedent from the earlier evolution to 1,000 volts. Medium-voltage DC would require significantly more technology development and would require borrowing conversion technologies from other energy industries."

Last year, First Solar CEO Jim Hughes said, "The large arrays that are built today are essentially a scaled-up rooftop system -- because it's what the code authorities in ancient years [understood]. If you were building a large-scale array from scratch on a clean sheet of paper with no prior contamination, what you would build would look different than what we build today."

"I think over the next five years, you'll see the industry transition to a fundamentally different architecture with greater use of DC bus [and] use of AC conversion at higher voltages with fewer inverters. [...] I also think we need tighter integration with the [way we] use electricity." He said there is an opportunity to do things on the DC side, as well. "If you can co-locate or locate within in a reasonable distance, you can put DC right into the data center," resulting in "a 15 percent cost savings from avoiding the whole AC conversion side of the equation," Hughes said.