Solar PV inverter technology has witnessed limited progress in terms of design, size, efficiency improvement and manufacturing cost reductions as compared to other electronic devices like computers and TV sets. The maximum power ratio (per kg) for inverters has improved by only five times over a period of about 25 years, whereas for the computer industry, the processing power almost doubles every 18-24 months. The main reasons behind this slow progress are limited advancement in the fundamental conversion design and the use of large magnetics and cooling elements, which makes solar inverters expensive to manufacture and install.
However, with the solar segment becoming more competitive with other power generating sources and reaching grid parity, efforts are being increased to reduce the cost and improve the efficiency of inverters, which account for a significant part of the entire cost of solar projects. Innovations are also taking place to make solar inverters more compact with an improved power ratio, as well as expand its role in solar PV systems. Companies are designing inverters to work as the brain of the PV system, with capabilities of managing communications, monitoring, smart energy management, storage and grid interaction. This is likely to expand further in the years to come.
The various technological innovations introduced in the solar inverter segment over the past two years are discussed in the following sections…
The latest technology in this space is high definition (HD)-Wave technology, which was introduced by Israel-based power electronics company, SolarEdge. The company, in fact, won the Intersolar Award 2016 in the PV category for this technology. Its key features are distributed multi-level switching elements to create a sine wave, the requirement of lesser magnetics to create the alternating current (AC) sine wave, and a highly efficient design with minimal heat loss to reduce cooling requirements. The distributed switching elements are highly efficient, and thus help in reducing heat losses and eliminating the need for large and heavy aluminium heat sinks.
With this technology, the company has developed solar inverters that are less than half the weight and considerably more efficient than standard inverters. In a conventional solar inverter, the bulk of the cost is that of the electronics and heavy metal parts, whereas in the new inverter, most of the chassis is of plastic. This new technology has increased the potential of cost and size reduction of solar inverters.
HD-Wave technology uses algorithms to better synthesise the sine wave inside the inverter, resulting in the use of smaller cooling elements, and much smaller magnetics for generating the same amount of power. The HD-Wave technology uses thin-film capacitors that are more reliable, smaller and lighter. This technology also allows the use of very small, efficient and standard silicon switches. The increase in efficiency from 97.5 per cent to 99 per cent observed with HD-Wave technology also means that the inverter dissipates less than half the heat of the current models. In the ramp-up phase as of now, SolarEdge’s long-term plan is to eventually phase out its current inverters and replace its entire product range with these HD-Wave technology-powered models, which also include 1.5 kW backup home power systems.
With the rising number of grid-connected large-scale solar projects, the demand for grid-tie inverters is also rising significantly. A solar grid-tie inverter converts the direct current (DC) output of PV modules into AC power, while matching phase with a utility-supplied sine wave. Grid-tie inverters are designed to shut down automatically at the time of loss of utility supply for safety reasons. However, they do not provide backup power during utility outages.
The grid-tie inverters that are available in the market today use a number of different technologies. They may use the newer high-frequency transformers, conventional low-frequency transformers, or no transformer at all. Instead of converting DC straight to 120 V or 240 V (AC), high-frequency transformers employ a computerised multi-step process that involves converting the power to high-frequency AC and then back to DC and then to the final AC output voltage. It must optimise the power output via maximum power point tracking (MPPT), and additionally monitor the system and grid connection.
An on-grid solar inverter is composed of a DC-DC module, a DC-AC module and a control module. The DC-DC module is built with a metal–oxide–semiconductor field-effect transistor (MOSFET), an inductor and a transformer, and functions to provide a stable DC output through rectifying and filtering the unstable DC power produced by PV modules. The DC-AC module includes an insulated-gate bipolar transistor (IGBT) array and an output filter circuit to convert the DC output of the DC-DC module into an AC output suitable for the grid voltage level. The control module is the core of the whole system. It has a datalogging and supervisory control (DSC) system, a voltage sensor, a current sensor and a driver that drives the MOSFET and IGBT module. The DSC system calculates the maximum power point of the PV module array based on the signals collected by sensors including the PV modules’ voltage and current, the grid’s voltage and current, as well as the phase. Accordingly, it sends instructions to the driver that drives the DC-DC and DC-AC modules.
Furthermore, the DSC is able to find out abnormal conditions such as grid failure and take measures such as disconnecting inverters from the grid. Additionally, the control module has an interface for external display showing the PV modules’ status and input/output voltage and current, and integrates a RS-232/RS-485 communication interface to connect with the control centres of solar power plants so that real-time monitoring of solar panels and inverters can be implemented.
Some of the most advanced grid-tie inverters available in the market include the Ecopower 1,000 W New Solar Grid Tie Inverter 2016. This newly developed grid-tie inverter (second generation) with a different design structure, enables a better and stable operation quality, with a much better output pure sine wave form, a power factor of about 95 per cent and an MPPT efficiency of 99 per cent. Some of the others are the Missyee 1,000 W Grid Tie Solar Power Inverter Converter DC 20 V-45 V for solar panel systems of 24 V to 36 V capacity, the Solinba 1,000 W Pure Sine Wave Grid Tie Power Inverter DC22 V-56 V to AC90 V-130 V, and the IP65 600 W Micro Grid Tie Inverter with power line communication.
In India, Su-Kam has developed a grid-tie solar inverter with a maximum power point range of 150 V to 550 V DC with a maximum efficiency of more than 97.5 per cent. This domestically manufactured inverter provides an attractive solution to developers. It works on a wide voltage range, which enables it to generate solar power even during periods of low sunshine.
In September 2016, SunCulture Solar Inc. announced the launch of SolPad™, a new series of disruptive energy products designed to be the most advanced integrated home or off-grid solar energy products in the world. The SolPad’s integrated solar panel combines multiple patented technologies into a single device, including battery storage, an innovative inverter system and an intelligent software that engages and interacts with users. The SolPad units also incorporate intelligent software that communicates with the users and their home systems, allowing for granular control over power consumption.
SolPad’s built-in flexgrid inverter can automatically detect when to charge from the sun and from the local utility grid, adjusting for cloudy or rainy days, as well as changing local electricity rates. During the most expensive daytime hours, SolPad Home switches to stored battery power and then switches back to grid power when rates are low. Flexgrid is also capable of detecting power outages or blackouts, and safely disconnecting itself from the grid. Once off the grid, SolPad
automatically forms a personal solar micro-grid that will keep delivering power to specific lights and appliances.
General Electric has also launched its LV5+ series of solar inverters, which contain Silicon Carbide (SiC) technology and have higher efficiency levels of 99 per cent for utility-scale solar plants. SiC is a synthetically produced crystalline compound of silicon and carbon that is resistant to high temperatures and high electrical
conductivity. All these features make the material the ideal substitute for traditional semiconductors and completely transforms the power conversion methods used conventionally.
During Intersolar 2016, SMA also presented its innovative battery inverter, which allows high-voltage storage systems for new and existing PV systems. The new Sunny Boy inverter family from SMA has a new design and an ultra-modern user interface as well as the latest communication standards, capable of informing end-customers or the installer directly about faults and the required service.
ABB also launched two ranges of inverters, the ABB TRIO-50.0 and the outdoor central inverter PVS980, at the Intersolar event. The ABB TRIO-50.0 transformer-less inverter combines the performance and price advantage of a central inverter with the flexibility and ease of installation of a string inverter. The PVS980 inverter has a power rating of up to 2,000 kVA, high efficiency, increased DC input voltage of up to 1,500 V and a robust enclosure with a self-contained cooling system to ensure performance during tough environmental conditions.
With the introduction of these innovative technologies, the future of solar inverters seems positive. The Indian as well as the global solar inverter market is expected to witness significant growth given the research and development activities being carried out by companies, the emergence of new and compact-sized solar inverters, and the competition in prices.