The Future of Solar System, The Era of Smart Inverter Technology is Coming

Tomorrow’s Smart Solar Technology for Today!

Solar panels are installed on the rooftop or at the mid to large remote site, but it’s the inverter that does all the real work. Choosing an inverter technology will always be the most critical decision you’ll make when going solar, unfortunately, the conventional (central/string inverter) solar systems now we use still not yet compliance with future smart grid requirements, such as reactive power control, automatic safety disconnect on solar module level, rapid shutdown protection, smart module level monitoring, cloud updating & upgrading, and real-time issue communication/reporting…etc. Though the current solar string inverter technology and topology has been introduced for several decades, it still cannot solve many existing problems like module level mismatch, single point of failure, high voltage, high system stress & failure rate, system communication issues…

Like smart phone we love to use today and change our lives completely when Apple first introduced its iPhone in 2007, a new smart inverter technology is now evolving and will shortly make a great impact to current solar energy landscape, further to change our ways of producing, storing, & consuming energy, the smart inverter technology will simply enable every solar module to operate with its ultimate efficiency which digitalize controlled by semiconductor based ASIC topology. This would deliver much greater energy harvest, enhance system overall reliability and more intelligent monitoring, and to reduce the O&M cost over time.

Comparing Inverter Technology

Conventional, Central (string) invertersIn a solar panel installation with a conventional, central inverter, solar panels are connected in series to form an array, which may be considered as a large PV panel, with a nominal rating, say, of about 600–1,000 VDC. The solar panels array is connected to a single central inverter in order to convert the generated solar electricity to alternating current (AC). The usual voltage power for North America is 120–180VAC at 60Hz whereas for Europe is 220–240VAC at 50Hz.

String Inverter pros and cons

With a central (string) inverter arrangement, solar panels are connected in a series (string connection) with a big disadvantage being that the overall system performance is defined by the performance of the system’s bottleneck. That is, the maximum output performance of the string is defined by the minimum (poorest) performing solar panel — see solar panel efficiency. This is a main disadvantage as solar panel installations may face several sources of performance reduction. For example, if a solar panel has a manufacturing defect reducing its performance level, in string connection with a central inverter, it will affect negatively the performance of the whole string, even if the other solar panels are performing to their maximum power output. The same applies in situations of partial shading onto a solar panel, from debris, or snow, or bird droppings etc., affecting the output of the whole string connection. That is why solar panel cleaning is of ultimate importance, especially if central inverters are used. This is extremely important since solar panels have dramatically lower outputs when shaded. Reduced performance in solar electricity generation results in reduced earnings for on-grid installations, especially with FIT’s, leading to longer payback periods and poor financial performance of a solar panel investment. From conducted research it has been found that a shade of as little as 9% on the solar panel surface in a module can lead to power reduction of the whole system as much as 54%. [1]

When solar panels are connected together in a series string, shading just one of them can dramatically affect the entire array

Another disadvantage of string inverters is that they normally come at a limited selection of power ratings; this means that the power rating of the solar panels has to be matched to the power rating of the string inverter. For example, an array of 20 solar panels with a total power of 4900 Wp will need an inverter rated at 4900 Wp+, say 5000 Wp or 5300 Wp. This leads to system un-utilized inverter capacity that you actually pay for but you never use. In addition, it places limitations on the option of expansion of the solar panel system. For example, in order to add more panels one need to consider keeping the match between the power rating of solar panels and the central inverter used.

Some additional disadvantages of string inverters include their increased need for cooling and their need of large diameter wire to handle the low voltage of DC in a string connection. Due to larger power ratings, central inverters have integrated cooling fans making them more bulky and noisy.

So why should anyone undergo the disadvantages of string inverters if there is not benefit? The main reason for anyone using a string inverter is leveraged cost. Provided a system’s power rating is optimized between solar panels and inverter, string inverters may have the cost advantage. The other main issue is location and size of the solar photovoltaic system. When dealing with large scale photovoltaic power plants, especially in rural areas with no surrounding buildings, string inverters are a preferable solution.

Smart Inverter pros and cons

Smart inverters’ (micro-inverters) smaller power rating also results in smaller size than of central inverters which has several technical advantages. For example, smart inverters are less prone to failure from heat as they do not have the same need for cooling as, bigger, central inverters do.

Apart from these technical improvements, the main advantage of smart inverters, compared to central inverters, is that they manage to isolate each solar panel, or pair of 2 or 4 solar panels, from the whole system without affecting the overall performance of the photovoltaic system. For example, in a solar panel installation with 20 solar panels and a total power of 4900 Wp, if one of the solar panels is shaded from snow, or bird droppings or other debris, the reduction in the performance of the affected solar panel will not influence the performance of the other panels, thus will isolate the ‘damage’ to a single panel. Considering manufacturing defects in solar panels and performance factors such as, shading, temperature issues etc., manufacturers claim that an array of solar panels with smart inverters can produce as much as 5% to 25% more than the equivalent array with a string inverter, depending on custom specific circumstances.[2]

Having a Micro Inverter on each solar panel, the outputs are completely independent of each other, according to the US National Renewable Energy Laboratory, this effect can yield as much as 12% more energy.

Furthermore, smart inverters, unlike central (string) inverters are very flexible offering many system expansion options without significant limitations. Because they tend to become an integrated part of the solar panel itself, or of two panels in dual-inverters, they offer the option of adding as many solar panels with a smart inverter to an existing system as one wants. In addition, extended panels are not limited to have the same power rating of existing panels.

Another main advantage of smart inverters is their ability to produce grid-matching power at the level of a single solar panel. Each solar panel has its own smart inverter; solar panel arrays are connected in a parallel connection configuration and then connected to the power grid. The parallel structure with smart inverters, opposed to string connections with central string-inverters, has the crucial advantages of isolating each panel, thus isolating problems of low performance from the entire system. The sources for such problems may include shading on panels, malfunction of the solar panel or the micro-inverters themselves etc. Considering that micro-inverters operate at lower power levels than central (string) inverters, they offer additional advantages such as lower heat-loads, improved Mean Time Between Failures (MTBF), smaller diameter connection cables which all conclude to better reliability the overall system.

The main disadvantage of smart inverters has, up until recently, been their increased initial cost. Because a system with micro-inverters requires one smart inverter for every panel (or every two panels in cases of dual-micro-inverters) the overall cost is marginally higher than that of a system with central, string-inverters. However, with the introduction of dual smart inverters and technological advancement in manufacturing integrated smart inverters in solar panels the cost difference is continually reducing.

Next Generation Smart Inverter Technology

The next generation smart inverter will comprise of a two stage inverter plus additional capability to alter the output VAR (Reactive Power Control) component of power. There are no current solar inverter solutions to this and requires an additional VAR control box external to the system to meet the requirements in EU(current) and future on North America(by 2018).

A Single Chip ASIC Based Smart Inverter

A single chip ASIC based solution cost/performance as key driver. The simple design architecture supports <$0.20/Wp with less than 200 parts count and will be as cost competitive to any small to mid sized string inverter today, the technology can deliver up to 96.5% CEC efficiency @300Wp, and designed to meet the latest worldwide grid tied standards, the product is fully programmable through our long range LoRa gateway IoT products.

Inverter plays a key role in the whole solar system infrastructure. It works like the CPU in a computer. For optimizing the solar power generation, smart inverters are required for the future smart grid and smart energy homes. It is a purely semiconductor based design topology. It comes with great benefits which the traditional string-inverters cannot compete.

Comparison between string-inverter and next generation smart inverter

So the answer to the question of which inverter is best; smart inverter or string inverter is rather dependable on custom specific circumstances. If you are dealing with a small system such as residential solar panels perhaps it would be justifiable to undergo the extra initial cost of smart inverters in order to avoid evident risks of underperformance. In residential areas the chances of underperformance, either from shading or from technical faults, are usually high. On the other hand, if you have a roof higher than any other building, with low chances of shading on your solar panels and in an open area with cooling breezes, the cheaper option of string inverters might be a better solution.