By Hanan Fishman, president, Alencon Systems
Solar power plants are often referred to as “solar farms.” Who exactly coined the term “solar farm” first is unclear, but the reality is PV projects are highly akin to traditional agriculture where crops are planted and harvested as they ripen. The difference with solar farms is that the crop is clean, renewable energy.
But what happens to that energy harvest as a PV project ages? This article will unpack how solar is harvested and how changes to the project over time can impact the need to alter those approaches.
MPPT: How energy is harvested from solar farms
The sun provides the fuel for solar panels via the photovoltaic effect as it does to plants via photosynthesis. Just like traditional farms, a solar farm’s bounty must be harvested. The technical means by which solar energy is harvested from PV panels is maximum power point tracking, or MPPT for short.
MPPT is the algorithm by which the inverter connected to a single PV panel, a string of PV panels or an entire array extracts the maximum amount of power from those PV panels to send to the grid. (Learn more about how these algorithms work in this article.)
In commercial- and utility-scale PV plants, MPPT can vary between centralized approaches via central inverters to more granular strategies using string inverters. Central inverters generally have one MPPT for an entire PV array, while string inverters harvest energy from each string of solar panels and have multiple MPPT.
Over the past decade of solar power’s meteoric growth, large-scale PV plants have evolved from using 600-volt PV strings, to 1,000-volt strings to today’s 1,500-volt plants. The voltage rating of a PV plant refers to the length of a PV string, or the number of solar panels placed in series to create a string. As PV panel technology has evolved, the maximum length of PV strings has continued to increase, which has decreased the cost of building a solar plant.
What happens to plants with centralized MPPT functions over time?
There is a significant level of debate as to which approach to building large scale plants — the centralized approach of central inverters or the distributed approach of string inverters — makes more sense. This article will not weigh on that topic, but will simply address the fact that the vast majority of older, larger-scale solar farms built upwards of five to ten years ago in North America use central inverters. Those plants are typically 600-volt plants.
To understand what can happen to energy harvest over time as a PV plant ages, let’s return to our farming analogy. Imagine that you are a property owner and you decided to start growing corn on your land. To do so, you hired someone to initially build the field for you, turning an unused plot into one with neatly planted rows of corn. The folks who did the work for you even delivered a very clever, automated harvesting system to you as part of their scope of work. Once a day, that system harvested the corn from all the rows all at once in a way to maximize your bounty. In this analogy, the rows are strings of solar panels and that clever harvesting system is the MPPT function of the central inverter.
Over the first few years, this approach to running your farm worked great and the crop yield helped you achieve a great return on your initial investment in the farm. But as the years went by, some challenges arose. For a few years, it was really dry and the rows of corn started to grow unevenly. Then there were a couple of years of excessive rains and flooding (isn’t climate change the worst?). After almost a decade, those once neatly and uniformly growing rows of crops are no longer growing and ripening evenly. The centralized harvesting system you installed is yielding a lot less because the ripening patterns of the crops from row to row has significantly changed.
In the context of a PV plant, that change in yield rates is called mismatch, or the variance in the energy yield characteristics between strings of solar panels. The worse mismatch gets, the less energy a centralized MPPT approach yields. The degree of mismatch can actually be measured by checking the health of your PV strings by taking IV curve traces, measurements of each PV string’s energy generation characteristics. An IV curve is a plot of voltage (V) and current (I) against the amount of power generated. Power generated over time is, of course, energy. The point on the IV curve that yields the highest level of energy production is the maximum power point (MPP).
When a PV plant is new, the IV curves of each string are essentially identical. Over time, due to issues such as module failure and damage, shading and soiling or uneven panel degradation, those IV curves can significantly diverge.
The relative effectiveness of the centralized approach to MPPT used by a central inverter relies on the fact that all the IV curves are the same. However, as those curves diverge over time due to the reasons explained above, the effectiveness of the centralized MPPT approach can significantly decrease.
String-level optimizers to the rescue
Here at Alencon Systems, we build devices known as string-level DC-DC optimizers. Devices such as Alencon’s SPOT (String Power Optimizer and Transmitter) can be installed in an older PV array to help address the decreased power production in the PV array over time. The Alencon SPOT applies a single MPPT to each PV string, so the power production characteristics are addressed in a more granular way.
In the context of string-level MPPT and aging PV plants suffering from PV panel mismatch, the sum of the parts can truly be greater than the whole when it comes to energy power product. By this, we mean that the sum of the MPPT of each string can be significantly greater than the energy harvested by a single MPPT applied to an entire array.
Just how much more energy can be generated from this approach is directly correlated to the degree of PV panel mismatch. At Alencon, we have developed a mathematical model to assess just how much yield can be increased based on the difference in PV string power generation characteristics from IV curve traces. The results of such modeling exercises help customers understand just what sort of return on investment they could get from installing string-level optimizers in their PV systems.
As PV projects age, solar developers would be wise to consider adding more MPPs to their investments to harvest more power, and in turn, more profits.
Hanan Fishman is the president of Alencon Systems. Hanan has over 20 years of experience in technology development and commercialization. In his role at Alencon, he is responsible for managing all aspects of the company’s operations including business development, product sales, marketing, engineering and production teams.