The cost of photovoltaic (PV) solar energy has fallen dramatically over the past few years. In fact, the unsubsidized cost of solar is now competitive with fossil fuels in some markets. In a race to expand its reach, the solar industry continues to look for innovative ways to lower LCOE.
LCOE, or levelized cost of energy, is the metric of merit when evaluating the economics of solar against other forms of energy generation. LCOE quantifies the cost of generating electricity by comparing the expected lifetime costs of a power plant with its expected lifetime energy generation and is measured in cents per kilowatt-hour (¢/kWh).
Many approaches to lower LCOE tend to focus either on lowering costs or on improving performance. It is rare to find one that addresses both. Three emerging trends in large-scale solar exemplify each of these approaches. One lowers cost by raising the system voltage to 1500 volts. Another improves performance by using string inverters. The last trend uses string optimizers to both lower cost and improve performance.
Raising systems to 1500 volts
An emerging trend in solar that focuses primarily on cost reduction is increasing the maximum system voltage from 1000 to 1500 volts. Higher system voltages eliminate electrical components from the system and lower inverter costs. However, it also introduces challenges.
Increasing the system voltage to 1500 volts allows 50% more modules to be connected in series strings. This reduces the number of parallel connections and associated hardware by 33%.
In addition, moving to 1500 volts allows for new inverters that operate at higher input voltages, and therefore, higher output voltages while maintaining the same output current. This promises a 10 to 40% increase in rated output power and a potential savings on a cost per watt basis.
While this is a good move for the industry, it is difficult to realize these cost savings today with the limited availability of components rated at 1500 volts. There are also unknowns such as the impact of higher voltages on the reliability of modules and inverter components as well as the adoption of new electrical codes. While these challenges will be worked out over time, upgrading the entire bill of materials to 1500 volt components is a significant undertaking to lower cost but does little or nothing to improve performance.
Using string inverters
An example where efforts to lower LCOE are primarily focused on improving performance can be seen in the recent trend of using string inverters in large-scale PV power plants approaching the 10MW range. This runs counter to historical practices which favored lower cost central inverter stations.
String inverters increase the resolution of maximum power point tracking (MPPT) in the field compared to central inverters. A higher resolution of MPPT allows the system to recover more energy that would otherwise be lost due to electrical balances – also known as mismatch.
Although systems using string inverters cost more per watt than those using central inverters, the difference is now reaching the point where the performance advantage can be worth the added expense – even on large systems. This trend highlights a spend more to get more approach to lowering LCOE that puts a premium on performance.
Using string optimizers
A third trend in the industry uses string optimizers in combination with central inverters to both lower cost and improve performance of large-scale PV plants. This preferred approach avoids the cost-performance tradeoffs inherent with the other two trends described above while delivering more value.
String optimizers track MPP every 10 modules
String optimizers are DC converters that put voltage and current limits as well as multiple MPPTs on each string of PV modules.
The voltage and current limits allow 100% more modules per string in a 1000 volt system to reduce the amount of electrical hardware such as combiner boxes and homerun cabling by 50%. The central inverter’s rated output power is also increased by 40 to 70%. Therefore, string optimizers reduce total system costs even more than moving to a 1500 volt system.
String optimizers track MPP every 10 modules compared to 100s of modules with string inverters or 1000s of modules with central inverters alone. The higher the MPPT resolution is, the greater the system’s ability to recover mismatch losses and maximize system performance.
String optimizers are uniquely suited to improve the economics of large-scale PV plants by addressing both sides of the LCOE equation.
Focusing on both cost and performance
Lowering LCOE is the goal; however, we may short change ourselves when we focus on either cost or performance instead of both. As illustrated by the string optimizer example above, it is possible to find innovative solutions that deliver a true spend less, get more value proposition. In doing so, we accelerate the adoption of solar.
Written by Levent Gun, Chief Executive Officer of Ampt