Distributed Power Optimization
Companies are working together to lower the cost of PV even further.
The PV industry is faced with increasing pressure to lower the cost of PV systems without sacrificing performance in order to achieve a lower LCOE. As much progress as the industry has made to date, cost reduction efforts continue to be constrained by both technical and market dynamics.
From a technology perspective, traditional architectures that rely on the inverter to manage both DC and AC power are self-limiting by requiring system and equipment builders to make tradeoffs between performance and cost. Meanwhile, like most “early” markets, the solar industry has a highly fractured supply chain. The combination of these two forces hampers system-level innovation.
However, by considering reasonable architecture changes and collaborating across the value chain, the industry can capitalize on an opportunity to create significant system cost reduction while enhancing PV system performance — a true “spend less, get more” value proposition.
Limitations of Traditional Systems
Traditional PV power plants use a series-parallel architecture with centralized power management controlled by an inverter. In this architecture, the inverter accommodates a wide range of input voltage and current fluctuations to convert DC power to AC power. The limitations of this traditional architecture include:
- Under-Utilized Inverters: The rated output power of traditional inverters is limited by the minimum input voltage of the inverter. Since PV module output varies greatly with irradiance and temperature, the inverter’s minimum input voltage must be low enough to accommodate the hottest day of the year. The fact is that traditional inverters are capable of delivering a lot more power, but the lower input voltage requirement constrains the inverter utilization and drives up inverter cost.
- Cabling Design and Cost Issues: String lengths in traditional systems are hampered by the open circuit voltage of the PV module on the coldest day of the year. Strings must be kept short so the system voltage does not exceed code limits. This increases the number of strings and combiner boxes needed. Traditional systems also require an over-sun multiplier when determining wire size which further amplifies costs.
- MPPT-Related Energy Losses: Traditional PV systems use an inverter with one or several maximum power point (MPP) trackers. These MPP trackers adjust the DC bus voltage while looking for the highest power possible aggregated across the array. The farther the MPPT is from the source of generation, the more compromises are made between module and string performance which results in less energy being delivered over the life of the power plant.
Spend Less, Get More
The industry has already started to migrate away from traditional systems with the increase in adoption of distributed architectures such as microinverters, string inverters and DC optimizers. The problem with many of these alternatives is that they come with a spend-more-get-more value proposition. In other words, spend more on capital costs to get more energy production. However, distributed power optimization (DPO) uses DC optimizers to put MPP tracking as well as voltage and current limits on each string to deliver higher performing systems at a lower cost.
The higher resolution of MPP tracking and the voltage and current limits provide the following advantages to lower system costs and improve performance:
- Lower Cost Inverter – Inverters can operate with a higher and narrower input voltage range since they no longer have to accommodate the low input voltage at higher temperatures. The narrow input voltage range allows the inverter to always operate close to its maximum MPP voltage, which allows the inverter to deliver up to twice as much power from the same hardware. Doubling the output power is equivalent to cutting the inverter cost in half.
- Lower Cost Cabling – The voltage and current limits of the DC optimizers allow for longer strings. For example, system designers can put up to twice the number of modules per string compared to traditional systems. Increasing the array voltage means that the same or more power can be generated with less current flow. This reduces the gauge of wire, size of conduit, number of combiners and associated labor costs. This also can decrease wire losses as well. When considering these together, the electrical BOS costs are reduced by up to 50%.
- More Energy – DPO architectures have higher MPP tracking resolution than other string or system level solutions which minimizes or eliminates the effects of real world energy production inhibitors like non-ideal landscapes, PV module mismatch, uneven soiling and non-uniform degradation. Putting MPP tracking closer to the source of generation increases PV system energy output. The more granular and efficient the power management is, the higher the system production will be under changing environmental and system conditions over the lifetime of the power plant.
Collaborating Across the Value Chain
Achieving these architecture changes and improving on them over time requires a new level of industry collaboration. This need has resulted in the formation of industry alliances focused on bringing companies together around a set of defined open standards, shared best practices and unified product labeling. An example of such collaboration is the HDPV Alliance.
The open standards ensure product compatibility which provides the industry with choice. Since customers can choose from a variety of interoperable products, competition remains intact throughout the collaborative innovation process. Sharing best practices allows the industry to accelerate the adoption of lower cost and higher performing systems. And unified product labeling serves the industry by making it easy to identify and source components that conform to the open standards.
Historically, distributed power optimization that puts DC power electronics out into the PV array have presented a “spend more, get more” value proposition in which overall system costs go up, but increased production of energy and monitoring features are intended to justify the expense. For the most part, this has limited the adoption of DPO to residential and small-scale installations where the presence of significant amounts of non-uniform shading can sometimes justify the incremental cost of such solutions.
Today there is a new reality. Companies across the value chain are collaborating to deliver a DPO architecture that puts either module- or string-level DC optimizers in the PV field which can lower both the cost and risk of PV projects to create significant value. This “spend less, get more” approach enables distributed power optimization to thrive even in large commercial and utility-scale applications.
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