A key trend in the solar segment, which has led to technology advancements in solar inverters, has been the increased focus on balance of systems to maintain high returns on investment. To this end, inverters play a significant role, given that they generate a lot of data.
Inverter capacities have been on an upward trend. Technological advancements in the semiconductor space have prompted this change. In a decade, the maximum central inverter capacity has increased from 100 kW to roughly 5 MW. In the same time period, the maximum string inverter capacity has increased from 10-15 kW to more than 100 kW for the commercial and industrial segment.
Upgradation of the size of solar modules has been another key trend that has led to technology improvements in solar inverters, as inverters need to have the capability to synchronise with high watt peak solar panels. The maximum operating current of a PV panel has increased from 9 ampere to 11-11.5 ampere due to the launch of bifacial technology. In addition, new modules are being built with 210 mm wafers and are thus able to generate 17 ampere of current. Inverter capacities need to be upgraded because of this trend.
Moreover, as inverters become smarter, their efficient utilisation assists in reduction of total operations and maintenance (O&M) costs. While inverters are becoming smarter in a bid to become the key information generators for solar projects, they will need to become safer as well. To this end, enclosures are being given greater importance in the industry to ensure that inverters can operate in harsh climatic conditions.
This article summarises the different kinds of inverters that are available in the market, and their pros and cons…
Central and string inverters
While central inverters have traditionally been used to convert the direct current (DC) generated by solar power plants, string inverters are now being increasingly deployed across the world. Central inverters differ from string inverters in their basic design. While a central inverter is centrally connected to all solar power module arrays, a string inverter is a smaller inverter connected to and controlling only a single array or string of solar modules. Central inverters have lower per unit power cost but higher installation costs. String inverters, on the contrary, help in lowering balance of system costs despite having a higher per unit cost. Moreover, a central inverter has simpler connections due to fewer components, whereas a string inverter has more interconnections but requires less ongoing maintenance. String inverters offer greater modularity, which helps in case of untoward incidents, allowing only the affected array to be turned off instead of the whole system.
String inverters have evolved over time. The new inverter systems are equipped with advanced grid features to accommodate smart grids as and when introduced into the system. Further, string inverters are modifying themselves into enhanced energy management centres with the use of sensors and weather monitoring tools.
Furthermore, string inverters generate data on the amount of maximum current a string is able to draw from an array. This assists in deciding the necessity of O&M activity, considering the weather conditions in that particular area. With this, the optimum frequency of module cleaning can also be decided, resulting in savings on O&M costs.
There has been a greater uptake of string inverters due to the chronic challenge of inverter breakdowns leading to significant generation losses for the developer. With competitive tariffs being discovered, there is no room left for developers to lose out on this crucial revenue. In addition, their ease of design and O&M are the other key advantages of string inverters. With this technology, even breakdown time can be minimised, resulting in greater revenues with enhanced power generation. According to industry experts, another concern with central inverters is that they mostly need to be upgraded or repaired, and not replaced. Old inverters become obsolete as the raw material used for making them keeps changing. String inverters, again, are a preferred alternative, as they are more flexible and easily replaceable. In sum, the technology advancements are usually designed to make inverters more adaptable to the available technologies in the market.
According to experts, for a central inverter of higher capacity, the per watt price is always lower compared to that of a string inverter. The capital expenditure (capex) of string inverters is also around 2-3 per cent higher than that of central inverters. However, given the higher generation yield and lower O&M costs possible with string inverters, the capex gap can be recovered in the long run.
The grid is posing complex challenges for inverter operations with the rising uptake of distributed energy resources. To solve these issues applicable to traditional inverters, smart inverters are entering the picture.
As smart inverters work autonomously, they are convenient for residential rooftop solar owners. Furthermore, being more flexible, they are beneficial for the grid. Voltage and frequency sensors in smart inverters allow them to detect grid abnormalities and send feedback to utility operators. This two-way communication is of immense help. However, the key issue with smart inverters is certification, given the stringent grid codes. Although there are several available standards, there are not enough facilities for testing and certification. Therefore, not-so-smart inverters enter the market that are dubbed as “smart”. This is a key challenge for developers, O&M players and inverter manufacturers.
Another key focus area for smart inverters would be the introduction of arc-fault circuit interrupters which will be able to determine whether an arc fault is at the inverter or the module level. This functionality can trigger an alarm in less than a second, thus allowing the inverter to disconnect the circuit. This technology feature is considered useful for rooftop solar projects in particular, as they have the most trouble addressing arcs at the DC end.
Microinverters are also becoming a popular choice for residential and commercial installations. As compared to conventional string and central solar inverters, which are connected to multiple solar modules or panels of a PV system, a microinverter converts DC generated by a single module into AC. Microinverters offer several advantages over conventional inverters. Key among these is that small amounts of shading, debris or snow lines on any one solar module, or even a complete module failure owing to panel-level conversion, do not reduce the output of the entire array. Since microinverters monitor the performance of individual panels, they are suitable for installations that have panels on multiple planes and facing various directions. However, since each inverter needs to be installed adjacent to a panel, installing microinverters entails a higher initial equipment cost per peak watt as compared to a central inverter. This also makes their maintenance more difficult, and their replacement costlier. Some manufacturers have attempted to address these issues by deploying panels with inbuilt microinverters.
With the growing importance of storage systems in solar installations, battery-based inverters have also started gaining traction. These are hybrid inverters that convert DC from batteries into AC at the appropriate voltage and frequency, and also supply backup power during a grid outage. They require a battery bank to function and can be used both in off-grid and grid-connected systems. The battery bank is charged by a PV array connected through a charge controller or a battery inverter via AC coupling. Under normal conditions, battery-based inverters export surplus power produced by the PV array. During a grid outage, these inverters automatically disconnect from the grid and supply AC power to the protected load subpanel by drawing energy from the battery bank and the solar array. When the outage is over, the inverter automatically switches back to grid-connected operations and recharges the batteries.
All in all, interesting times are ahead in the solar inverter space, given the different technology trends that are emerging to keep pace with the changing dynamics of the solar energy segment.