String optimizers, like this one from Ampt, put voltage and current limits on each string to achieve 100 percent longer strings without exceeding 1,000 volts.
The need to lower PV system costs is driving large-scale solar developers to consider changing from 1,000- to 1,500-volt DC systems. The United States PV market was able to lower the cost of PV systems by increasing the maximum system voltage from 600 to 1,000 volts a few years ago. A similar outcome is expected by raising the maximum system voltage again to 1,500 volts.
However, according to a recent report by GTM Research, the lower cost promise of 1,500-volt systems cannot be fully realized today due to the limited availability and higher cost of 1,500-volt components. In addition to the economic barriers, there are delays associated with the implementation of newly defined codes and standards. As the industry works through these challenges and the actual value of 1,500-volt systems is understood, there is a lower cost alternative using DC string optimizers. In fact, string optimizers complement both today’s 1,000-volt systems and tomorrow’s 1,500-volt systems to deliver more value than can be achieved by simply raising the maximum system voltage.
The 1,500-volt promise
Higher system voltages drive down costs by reducing the amount of electrical balance of system (EBOS) components and increasing inverter power density. Raising the system voltage by 50 percent allows string lengths to be increased by 50 percent. Longer strings mean fewer strings for a given power level, which means 33 percent fewer combiners and cables per kW. This reduces the amount of associated labor as well.
Raising the maximum system voltage increases the operational voltage of the system. Increasing the system’s DC voltage capability allows the inverter to deliver a higher output voltage at the same current to increase the rated output power of the inverter. Typically, 1,500-volt inverters can increase their rated output power by 10 to 40 percent to use fewer inverters per system.
While 1,500-volt systems use fewer EBOS components and inverters at a system level to lower cost, there are barriers preventing the full value from being realized.
Barriers to cost reductions
The promise of 1,500-volt systems is worth pursuing; however, getting the full value of 1,500-volt systems will be delayed due to equipment cost and availability as well as the recognition and adoption of new standards and codes.
Equipment for 1,500-volt systems is more expensive since components must be redesigned to withstand higher voltages. As a result, the unit cost of PV modules, combiners and inverters goes up, which lessens the cost per watt savings. In addition, some manufacturers will initially be able to charge a premium due to the lack of availability.
According to GTM Research, the materials cost per watt of 1,500-volt systems is actually higher than 1,000-volt systems and today’s savings come from direct labor. The cost per watt of materials will remain higher for 1,500-volt systems until production scales and competition grows to squeeze the prices down over time.
While many of the codes and component standards have been defined for 1,500-volt systems, they still need to be recognized and adopted. Equipment manufacturers need to engineer products that meet the new standards. In addition, PV system designers, authorities having jurisdiction (AHJ) and installers need to learn how these new requirements impact their areas of expertise and develop new practices to operate accordingly.
Instead of navigating higher component costs, limited product availability and the complexities of new codes and standards associated with 1,500 volts, developers and EPCs can use string optimizers in 1,000-volt systems to realize more value.optimizer savings
DC string optimizers in 1,000-volt systems deliver greater cost savings than can be achieved today by 1,500-volt systems. String optimizers are DC/DC converters that manage power out in the array and more effectively address the same value drivers as 1,500-volt systems.
Like 1,500-volt systems, string optimizers in 1,000-volt systems increase string lengths, but instead of raising the maximum system voltage, string optimizers put voltage and current limits on each string to achieve 100 percent longer strings without exceeding 1,000 volts. Compared to traditional 1,000-volt systems, string optimizers eliminate 50 percent of the combiners, cables and associated labor. This is greater than the 33 percent reduction offered by 1,500-volt systems, which have the added disadvantage of using combiners that cost more per unit as well.
Also, like 1,500-volt systems, string optimizers increase the inverter power density. The increase in rated output power for 1,500-volt inverters is limited because they must operate over a wide voltage range to accommodate temperature extremes, whereas string optimizers allow the inverter to set a high and narrow DC operating voltage — even on hot days — to deliver a higher AC voltage at the same current. This typically translates to a 40 to 70 percent increase in inverter rated output power with little or no increase in unit cost. Again, 1,500-volt inverters typically increase rated output power by only 10 to 40 percent at the expense of higher unit costs and often with premium pricing.
By operating in 1,000-volt systems, string optimizers are used in a familiar environment where the codes and standards are well established, making this cost savings immediately available to markets world wide.
String optimizers in 1,000-volt systems enable longer strings and higher power density inverters while using 1,000-volt components that are readily available and already offered at volume pricing. As a result, string optimizers uniquely enable both materials and labor cost savings to lower total upfront system cost. Plainly stated, string optimizers deliver greater EBOS savings in terms of both quantity and unit cost. And the net savings from day one, after paying for the optimizers, is greater than the savings of moving to 1,500 volts.
In addition to the cost savings, a 1,000-volt system using string optimizers has a performance advantage over a 1,500-volt system without string optimizers. As the design block sizes increase, the maximum power point tracking (MPPT) resolution can decrease — that is, the MPPT per kW goes down. Lower MPPT resolutions diminish the system’s ability to recover energy loss due to mismatch. This problem is exacerbated as the system ages and degradation becomes more of a revenue inhibitor.
String optimizers allow for larger design blocks while putting MPP trackers on each string to track the maximum power point across 10s of modules compared to 1,000s of modules with central inverters alone. So, while the design block size increases in both 1,000-volt systems with string optimizers and 1,500-volt systems, only string optimizers simultaneously increase MPP resolution by orders of magnitude to deliver more energy over the lifetime of the system.
The value of placing MPP tracking closer to the source of energy generation is more desirable as long as the cost doesn’t offset the benefits. Using string optimizers in 1,000-volt systems actually lowers the total upfront system costs while producing more lifetime energy. String optimizers offer a true spend-less-get-more value proposition.
Enhancing 1,500-volt systems
DC string optimizers add more value to 1,000-volt systems by allowing longer string lengths,
like the SolarEdge P700 power optimizers in this 1-MW Monticello, Fla., installation by Region Solar.
Once the barriers to 1,500-volt systems come down, developers and EPCs can shift from using string optimizers in 1,000-volt systems to using them in 1,500-volt systems to achieve even more value.
String optimizers in 1,500-volt systems achieve cost savings by doubling string lengths to eliminate 50 percent of the EBOS components and labor compared to traditional 1,500-volt designs without string optimizers. The operating voltage for 1,500-volt systems using string optimizers increases, which enables 1,500-volt inverters to have an even higher rated output power and a lower cost per watt. In addition to the cost savings, string optimizers distribute MPP tracking throughout the 1,500-volt array to improve lifetime system performance.
While the move to 1,500 volts is ultimately good for the PV industry, these cost savings are difficult to realize today. Developers and EPCs can avoid the higher material costs of today’s 1,500-volt systems by using string optimizers to bridge this value gap. DC string optimizers allow 1,000-volt systems to deliver more value than 1,500-volt systems without string optimizers by allowing longer string lengths, fewer EBOS components and higher power inverters to achieve an overall lower cost per watt. Moreover, string optimizers increase the MPPT resolution of the system to deliver more lifetime energy. As the cost and other barriers associated with 1,500-volt systems come down over time, string optimizers can be deployed in 1,500-volt applications to realize even greater cost savings.
Mark Kanjorski is the director of marketing at Ampt.
— Solar Builder magazine