Despite the rapid introduction rate of new systems, it is equally as important to maximise the value of the existing systems. Therefore, on an industry level, it is of crucial value to make use of repowering tools in systems, which either need or could benefit from it. Particularly, considering the high Japanese feed-in tariffs, repowering is certainly a prudent choice to gain the most out of these tariffs. In an interview with Solarplaza in the run-up to Solar Asset Management Asia 2018, Mark Kanjorski, the director of marketing at Ampt, discussed the various repowering approaches and how they affect performance.
Repowering and its impact on performance
“At Ampt, we typically break down repowering approaches in three ways,” said Kanjorski. The most direct approach, often referred to as repowering, involves adding String Optimizers in order to increase production by recovering losses due to system aging and degrading. String Optimizers are DC/DC power converters that do maximum power point tracking (MPPT) on each string of PV modules. The calculation to identify the optimum time to repower an existing PV system with String Optimizers differs depending on the market and is heavily driven by one variable; the value of energy, which has been historically high in Japan due to solar feed-in-tariffs (FIT). Adding String Optimizers not only enhances performance but it also changes the slope of the future degradation curve, according to Kanjorski. For instance, in lieu of degrading 1% per annum, the system may degrade annually at 0.66% over its remaining life. This small difference adds up. A 10-year-old system with 15 years of remaining life would generate a 7% increase in lifetime production. In Ampt’s experience, thus far, this direct approach of repowering PV systems is in highest demand.
High DC-AC ratio
The second approach to repowering PV systems is to increase the amount of DC power relative to the AC power that can be supplied by the inverter (or PCS). In other words, adding more PV modules to the system. It is now common to build systems with DC-AC ratios of 1.2 to 1.4 balancing the relative cost of DC & AC components against the value of energy produced. The decline of PV module costs financially justifies higher ratios today than were commonplace in the past. In some markets, Ampt’s customers are now deploying 2-to-1 DC-AC ratios. Such high ratios are made possible using String Optimizers. Typical inverters cannot exceed a 1.5 DC-AC ratio. However, by using String Optimizers, those inverters do not have to because the optimizers manage the high DC ratio. In Japan, that enables the systems originally built at comparatively low DC-AC ratio to add more PV modules on the existing inverter. Prior to 2018, the regulations in Japan allowed systems to raise the DC-AC ratio while still generating revenue under the historically high feed-in-tariff. Many system owners received approval and will now expand those systems.
A third way to repower an existing PV system encompasses replacing the inverter. Inverters start to fail as they age and may have to be replaced depending on the type of the failure or cost of ongoing maintenance and downtime. Replacing legacy inverters can be complicated in any market depending the unique requirements of each system design and the specifications of the modern inverters available. In Japan, replacing inverters is becoming particularly challenging as most systems were built with a 600-volt DC maximum system voltage. Replacement inverters for 600-volt DC systems are less and less available from global manufacturers, and they are often less efficient and more expensive than modern inverters built for 1000-volt DC systems. In this context, Ampt’s String Optimizers act as adaptors by enabling system owners to buy 1000-volt DC inverters and deploy them at full rated power in systems that are 600-volts DC. “With our String Optimizers in the system, system owners can buy modern inverters that have all the latest functions, are more efficient and cost less on a per watt basis,” elaborated Kanjorski.
Most recently, PV plant owners and operators are repowering existing systems by adding DC-coupled energy storage. In this application, Ampt string optimizers enable a lower-cost energy storage solution (ESS). With Ampt, mixing strings of old and new PV modules is possible without creating electrical imbalances that would otherwise harm performance. Existing systems can be expanded with more DC power by adding more modules, or by replacing existing modules with modern, higher power modules. Because systems with Ampt allow High DC-AC ratios of greater than 2-to-1, the system expansion may be done without replacing the inverter, and the existing electrical components (wiring and combiners) are used at a higher capacity to minimize costs. Since the batteries are DC-coupled, the extra energy generated by the added PV is stored at the highest efficiency (fewer power conversions), and without involving the existing AC grid interconnection agreement.
All in all, whether it is by providing string-level MPPT to improve performance, upgrading inverters to more modern designs, increasing the DC-to-AC loading ratios to produce more energy, or adding DC-coupled energy storage, String Optimizers increase the value of solar assets.