By: Adam Baker
Solar Owners Don’t Know Their Site’s Effective DC/AC Ratio.
The entire reason to install a solar farm is to get paid for energy. The more energy you produce, the more revenue you collect. On a clear sky day, some potential energy is wasted, due to clipping. The ability to maximize energy depends on the area under the clipping curve for a fixed tilt site, and I plan to discuss the topic in some detail. Note that for sites with trackers, the same issues exist, but the potential output curve looks MUCH different. I’ll discuss that difference in an upcoming blog.
The idea of clipping, and lost potential for energy is actually an expected part of every site’s design. However, once in operation, it’s nearly impossible to determine if inverters are clipping when they’re EXPECTED to, and lack of visualization tools to report on DC performance could cost solar plant owners some serious revenue.
The DC/AC Ratio
A solar power plant is required to deliver full power for the life of its power purchase agreement, which can be up to 25 years. It’s expected the performance of solar modules will slowly degrade over time, and issues like soiling will periodically reduce the amount of DC available to inverters. However, reaching peak power in good weather continues to be a requirement.
That’s part of the reason why utility-scale solar sites are always built with more DC than AC inverters can convert.
A typical site ratio is 1.3 to 1, meaning each 1MW inverter has 1.3MW of DC (modules) behind it. However, geography, cost of land, modules, and other factors affect that ratio on an array by array basis. For example, wetlands and limits of disturbance make it difficult to optimize the layout of each array on the site, and as a result, I’ve seen the ratio below 1.1 in some instances…which might push the ratio of arrays in other areas of that plant as high as 1.6 or 1.7 (where inverter tripping can be a problem).
Take a look at this irradiance bell curve for a fixed tilt system on a clear sky day. Since inverters can only convert 1 MW of DC to AC, solar owners lose everything produced above the 1000kw line. They make more energy than they can cash out on.
Wasted energy is all part of being a healthy solar plant. What isn’t healthy, however, is that the original plan to decide the ratio doesn’t take into account some common site maintenance issues.
The Problem with PVsyst’s DC/AC Ratio Calculation
In the sample graph, the orange line shows energy at a 1.0 DC/AC ratio, the blue line shows a 1.2 DC/AC ratio, and the green line shows a 1.4 DC/AC ratio. In short, there’s a lot more wasted energy the higher your DC/AC ratio is above that 1000kw line.
On the other hand...
Take a look at the “shoulders” of each curve. As the ratio increases, there’s more value under the 1000kw line in shoulder area than energy lost up top. That’s why a 1.3 ratio is pretty typical.
If you get paid a lot for energy, the steeper curve, the better. If you could imagine an ‘infinitely tall’ DC/AC ratio, the energy output would be nearly rectangular, but the incremental cost of the DC will FAR outweigh the small incremental energy output the site will be able to deliver ‘on the shoulders’. So how do you determine the optimum balance between amount of DC and the amount of energy?
The industry standard design analysis tool is a software package called PVsyst. During initial development, and later detailed engineering phase of the project, it is used to automatically run through all scenarios, taking into account every variable (ratio, how much you get paid for energy, module cost, PPA bonuses, tilt angle, time of day, land use, number of inverters, geography, etc.). PVsyst does the math for optimal site DC/AC configuration given the inputs for site design, and priorities for the project lifecycle.
So, you’ve done the math, determined the appropriate DC/AC ratio for your specific project and priorities, and commenced construction. The DC/AC ratio is now fixed forever, right? Not exactly. Installation and maintenance issues will affect how effective the installed DC is over the site’s lifetime.
The only ‘maintenance’ issue PVsyst calculates for is module degradation over the length of your PPA, and some assumptions about the amount of soiling you estimate will occur over different times of year. Because the engineers using PVsyst and who ultimately design the site aren’t the ones performing O&M duties, they don’t take into account any delays in the identification and correction of the DC infrastructure (modules, harnesses, whips, combiners, associated fusing, all the field crimping, etc.). Little do they understand that maintenance issues on a solar site are extremely difficult to locate, and never fix themselves. They affect performance immediately, and persist until corrected.
That means solar owners don’t know what the effective DC/AC ratio is for their site in any given month, quarter, or year. I can tell you it won’t be more than your original specification, but the odds are very good it’s less. Perhaps much less.
Maintenance Issues Change DC/AC Ratio and Result in Revenue Loss
How would you know if a 1.4 DC/AC site is running at 1.2 DC/AC? Technically, you’re still reaching maximum power at the peak of the day, so no output alarms should sound. Unfortunately, if you were built for a 1.4 ratio and are only producing a 1.2 ratio, you’re losing that extra revenue produced from the steeper shoulders of your bell curve.
There are plenty of reasons why your ratio would decrease, but they all boil down to maintenance issues. For example:
- A bad MC4 connector
- A blown fuse
- Cracked cells / defective modules
Yes, these maintenance issues are small, but by improving energy by just 1% on a 10MW site, an owner can recover $50 on a good generation day. Probably $12,000 a year. That’s about $250,000 over the life of a plant.
How to Increase DC Health and Avoid Early/Late Clipping
Because the elevation of the sun changes with the season, the amount of DC energy available will change through the year, thus the clipping window should be expected to be shortest on December 21, and widest on June 21 (Winter and Summer solstices).
By analyzing each inverter’s AC output, I can determine effective DC/AC ratio on an ongoing basis. From this metric, owners can compare the ongoing ratio to the ratios in their engineering design.
The trick is, where do you go from there?
The easiest and most rarely-used path in utility-scale solar installations, is to check string monitoring values. Affinity Energy’s Solar String Analysis system works on any age site or size, independent of typical solar SCADA or other monitoring solutions. It goes beyond typical site commissioning to analyze combiner box data inputs and calculate the energy lost from underperforming strings.
Ultimately, it allows owners to understand their effective DC/AC ratio, and can provide the data needed to make critical decisions about maintenance to get that DC/AC ratio back to where it needs to be.
Battery Storage – The Long-Term Clipping Solution
One of the unfortunate things about what happens during clipping is lost energy opportunity. The most effective battery storage solution would be smart enough to collect the energy that exists above the clipping curve, harvest it, and store it so it could be delivered later in the day.
But tight regulations and lack of commercially available technology make it impossible to store clipped energy...at least for now.
Because the inverter is the only piece of equipment that interfaces with DC and knows when it goes into clipping, an optimal battery storage solution would be tied to an inverter. Ideally, the inverter would shunt the clipped energy to battery storage, for it to be injected back through the inverter later when it would otherwise be under its clipping limit.
The problem is, inverters would need a provision for a utility-connected energy and a provision for a battery-connected energy. At this time, there are no inverters that have this capability, and it’s not clear that utilities would be quick to embrace this approach.
Energy storage could not only solve utility problems with uneven renewable energy, it could also start a new revenue source from clipped energy for solar plant owners.
What Solar Owners Should Do to Maintain DC Health Today
Even if battery storage were approved, existing sites are limited by their decades-long interconnect agreements. What are you supposed to do about the lack of insight into current clipping trends?
Get your site analyzed by Affinity Energy to find your effective DC/AC ratio, and if you find the ratio has changed, work with an integrator to locate the maintenance issues that are decreasing your ratio.
Adam Baker is Senior Sales Executive at Affinity Energy with responsibility for providing subject matter expertise in utility-scale solar plant controls, instrumentation, and data acquisition. With 23 years of experience in automation and control, Adam’s previous companies include Rockwell Automation (Allen-Bradley), First Solar, DEPCOM Power, and GE Fanuc Automation.
Adam was instrumental in the development and deployment of three of the largest PV solar power plants in the United States, including 550 MW Topaz Solar in California, 290 MW Agua Caliente Solar in Arizona, and 550 MW Desert Sunlight in the Mojave Desert.
After a 6-year stint in controls design and architecture for the PV solar market, Adam joined Affinity Energy in 2016 and returned to sales leadership, where he has spent most of his career. Adam has a B.S. in Electrical Engineering from the University of Massachusetts, and has been active in environmental and good food movements for several years.