Solar electricity meets the grid
To be useful, the DC electricity from solar photo-voltaic panels needs to be converted to AC. Invertor technology is the answer, and micro-invertors offer significant benefits, as Bernd Kohlstruck explains.
What are micro-inverters and why are they becoming popular?
A micro-inverter is a device that sits on the rail behind a solar photovoltaic module (sometimes called a solar panel, or solar PV panel/module) to converts the DC electricity from the module into AC (Fig. 1).
|Fig. 1: DC from a solar PV panel can be converted to AC by a micro-inverter on the rail behind each module or panel.|
Connections to the electricity grid (the mains) must be made using AC. The outputs of the micro-inverters are connected in parallel then fed into the grid (Fig. 2). Sometimes one micro-inverter is used for each module. In other installations using a dual micro-inverter, which takes its input from two modules, can reduce costs without compromising system performance. Using micro-inverters is a completely different approach to traditional solar PV installations.
In micro-inverter architecture, the outputs of the micro-inverters are connected in parallel, so each combination of module and micro-inverter is an independent generator of AC power
The conventional method connects all the modules in series, to form a string; the modules are connected like a daisy chain (Fig. 3). The high voltage DC from this string is then fed into a single, large inverter called a string inverter, which in turn is connected to the grid.
Solar modules have been shown to have an operating life of 25 years or more. String inverters have relatively limited life and normally have only a 5-year warranty. In conventional architectures, when the string inverter fails, there is no output from the entire system.
The latest micro-inverters have a life expectancy of over 25 years and may be warranted for 20 years, so they can be expected to last as long as solar modules. Even if one were to fail, it only affects the module to which it is connected. In contrast a failed string inverter brings down the whole system.
Better reliability and the ability to get more power from every module are the main reasons why installers and consumers are now using micro-inverters, particularly in residential and commercial installations.
Micro-inverters have other advantages too. Systems with micro-inverters are much easier to plan and install, and much safer to work on. This is because no lethal high-voltage DC is created, so the specialist skills and equipment for dealing with such high voltages are not needed. An incidental benefit is that in the case of a fire, there is much less risk to fire-fighters, again because there is no high-voltage DC.
|Fig. 2: Using micro-inverter technology, the outputs of each unit are connected in parallel so that each module and its micro-inverter is an independent generator of AC electricity.|
What evidence is there to support claims of 20% improvement in energy harvest over a PV system’s lifetime when using micro-inverters?
This evidence is based on research conducted by micro-inverter manufacturers in partnership with lead customers and installers, both in the US and Europe.
There are three main factors: capital costs; power harvesting and maintenance costs.
Capital costs, particularly when using dual micro-inverters that take their input from two solar modules, are now comparable with systems that use string inverters.
When using micro-inverters, maximum power point tracking (MPPT) can be applied individually to each PV module to ensure maximum energy harvest. Further, solar PV systems based on micro-inverters do not suffer from dramatic reductions in energy harvest when one module, or part of a module, has its output reduced by shading or accumulation of surface debris.
Compared with traditional domestic and commercial solar PV systems with string inverters, Enecsys micro-inverter systems extract up to 20% more energy over the lifetime of the installation. Maintenance costs are reduced because micro-inverters are demonstrably more reliable than string inverters.
What evidence is there to support claims of high reliability?
The most credible claims are those for micro-inverters that have undergone thermal testing to IEC61215 — the same standard used to test solar modules.
Manufacturers should be able to support reliability claims with test data, and the warranty offered will give a good indication of expected reliability. It is important not to confuse product life expectancy and mean time between failures (MTBF). This is a statistical measure of failure rates, but it can be very misleading and does not predict operating life or failure mechanisms. For example, the MTBF of a 25-year old man in a developed country is about 800 years. Obviously, he is unlikely to live that long.
The other clue to reliability is the operating temperature range. Micro-inverters with an operating temperature range from -40 to 85°C are likely to be far more reliable than those that those with a smaller temperature range. This is because extremes of temperature are encountered behind PV modules on rooftops, so the inverters must be designed with this in mind.
What distinguishes the most reliable micro-inverters from others?
Components known as electrolytic capacitors are the primary causes of failure in micro-inverters and string inverters. There have been no advances in the technology of these components that is likely to change this. Electrolytic capacitors rely upon an ongoing wet chemical reaction for their operation. Over time, particularly in the harsh environment of a rooftop, they dry out, heat up and can eventually explode.
Inverters that use long-life plastic-film capacitors which are rated for four times the life of electrolytic types offer much better reliability. Plastic film capacitors do not dry out over time or contain liquids that can leak out. In fact, they exhibit a ‘self-healing’ characteristic. This means that even if an electrical voltage spike should case a capacitor fault, the components repair themselves. Choosing micro-inverters that have eliminated electrolytic capacitors from their design is important to ensure reliability.
|Fig. 3: If a string or central inverter used with solar PV panels connected in series fails the whole system produces no electricity.|
Why are micro-inverters not being used to make modules that produce AC output?
It’s probably just a matter of time before AC modules appear because module makers will want to see proven field reliability for micro-inverters. Also, they will need time to re-certify their products after they have integrated micro-inverters into them. Today’s micro-inverters are easy to install within modules or externally on mounting rails, so they address both opportunities.
Isn’t it true that micro-inverters are less efficient than string inverters?
Focusing too much on inverter efficiency can be misleading. What really matters is the cost per harvested unit of energy over the life of the system. Inverter efficiency is only one contributory factor. The latest micro-inverters have over 95% peak efficiency. You may obtain 2% greater inverter efficiency from a string inverter, but a system based on micro-inverters will still deliver up to 20% more energy over its lifetime.
If a fault develops, isn’t it harder to identify and replace a micro-inverter on a rooftop than replace a string inverter?
The opposite is true. The latest monitoring systems allow immediate identification of the exact location of a fault, should one occur. The failure of one module or micro-inverter only affects the output of that part of the system. The rest continue to work normally. Monitoring systems may also include an Internet gateway, so the installation can be remotely monitored. Also, it is not necessary to turn off the system to replace a micro-inverter; you simply unplug one and plug in another.
Will Europe come to accept micro-inverters, as seems to be happening in the US?
There are relatively few micro-inverter installations in Europe today, but that has more to do with availability than lack of acceptance of the idea. The economic case is undeniable when you use reliable micro-inverters, and product is becoming plentiful —so demand in Europe is already accelerating.
Bernd Kohlstruck, Managing Director Enecsys Europe GmbH and VP Sales & Marketing - Europe