Engensa, a UK based solar company recently installed the UK’s first micro-converter system – a radical new technology that enables millions of otherwise unsuitable roofs to be used for solar PV by eliminating many of the problems associated with shading.

As thousands of families each month install solar PV systems, the crucial role of the inverter – the nerve centre of any system – is coming under increasing scrutiny.

A standard inverter has two distinct roles: the first is to convert the DC current produced by the panels into AC current that can be used in the home.  The second, crucially important, role is to manage the output of each of the panels and this is where traditional inverters can struggle.  The problems come about when even a small part of the installation is shaded by a tree or a nearby building.  Solar cells are essentially large semi-conductor diodes (similar to computer chips) which convert sunlight into electricity and are connected together to make a panel.  When even a single cell within a panel is shaded it limits the current that can flow in the whole system, because with a normal inverter the solar panels are connected in series.  This means that with a regular inverter the entire system performs to the standard of the weakest panel.

As the snows fell before Christmas, Engensa installed the UK’s first ever micro-converter – a radically different kind of solution manufactured by SolarEdge, an Israeli based leader in PV power optimization.

Instead of having a single inverter, the SolarEdge system is made up of multiple PowerBoxes, which sit under each solar panel and maximise the power of each individual panel and communicating this to a central inverter across the existing power lines.  In addition, PowerBoxes maintain a fixed DC string voltage, allowing optimal efficiency of the SolarEdge PV inverter at all times and enabling a significant increase in the amount of electricity generated over the lifetime of the system.

According to Dr. Toby Ferenczi, Engensa CTO, it is ‘the UK’s first of a kind installation with distributed power harvesting.  In plain English that means you get more energy output and greater PV monitoring capability compared with a conventional solar PV system because each panel is controlled separately.’  According to experts at Engensa, this new technology means that the impact of shadows falling on the panels is greatly reduced because only the output of the shaded panels are affected, rather than the whole system.  It also means you can install panels in different orientations giving much greater flexibility when designing the system.  A third benefit is that system owners have much greater insight into how their system is performing since it allows the output of each individual panel to be monitored in real-time, even from an iPhone.

‘SolarEdge’s product is a breakthrough that we have been waiting for,’ says Dr Ferenczi.    ‘Our focus, in the increasingly competitive solar market, is to provide our customers with the best technological innovations from around the world and as part of this commitment we are delighted to have installed the first SolarEdge system in the UK.’

Monitoring

How do I know if my PV system is working properly? This is a very good question and something that many customers of solar energy systems wonder. The answer is to spend some time and perhaps some money in monitoring the performance of your system over an extended period of time.

Perhaps the first point to mention is that a good installer should do this for you. They should care deeply whether or not their installations perform as well as they will have predicted. Before choosing your installer try to determine what they offer in terms of after sales support and care.

If you like to take matters into your own hands there are a number of ways to monitor your system. If you are not inclined to pay extra for monitoring hardware, the simplest solution is to use information from the displays of the generation meter or the inverter (or both). Both these instruments will be present in all PV systems installed in the UK by a microgeneration certification scheme installer and will be able to tell you the number units of electricity generated since the system was put in. Get into the habit of checking this number at the end of each day, along with a record of what the weather was like – clear sky, partial cloud, heavily overcast etc…

Your installer will have provided you with an estimate of the annual energy production in kWh – in southern England this should be in the region of 850 kWh per kWp). This means that over the course of the year you should expect an average of two and a half kWh of energy each day for every kWp you have installed. This is heavily dependent on the weather of course. On a clear day you could expect 8 kWh, and on a very dark day as little as 0.2 kWh. After several days of checking you should be able to have an idea of whether your system is significantly under-performing.

Over time, your measurements will become more reliable. Each month of the year has an expected solar energy output. Ask your installer for a chart showing the average monthly variation of solar energy for your location. After one month you can compare the energy you got with what you would expect. Be warned however, monthly solar energy output can vary widely, some months can be particularly good or bad for solar compared to normal, so comparing with the average is not necessarily accurate. Still, you should be able to tell if you are getting 30% less energy than you should be.

Getting a more accurate picture of your system’s performance is challenging. For instance, if your system is underperforming by 10%, how do you prove this?

A good method is to compare it to a nearby reference system which is known to perform well, and compare daily production to that. This can be challenging to find however, especially in the UK where there are still very few PV installations.

Another technique is to try to determine if there is something wrong with your system. Your inverter should also tell you the DC voltage and current coming from your solar panels. Under a clear sky, you can check if these values are in line with what they should be from the datasheet. Whether the voltage or current is lower than expected can provide information on what might be wrong.

If your system is underperforming, it could well be because of shading. If shadows are passing across the solar panels during the course of the day that weren’t accounted for in the system design, then this can really contribute to underperformance. Try to get in a position where you can see the solar panels at different times of the day. Any shadows on the module during the middle of the day (when they should be producing the most energy) can be serious. Many systems will be shadowed in the morning or evening, but this is generally less severe. Make sure to also check for dirt or muck on the panels, even small markings can cause big performance losses for solar systems.

It is also worth looking out for long term degradation. Whilst rare, it has been known for systems to get significantly worse over time.

If you would like buy a monitoring solution yourself then the simplest product is a power meter. Examples are the ‘OWL’ meter or the ‘Wattson’ made by DIY Kyoto. These are both simple meters that can easily be installed by clipping a sensor onto the AC output cable of your inverter. What’s useful is that the data can be transmitted wirelessly and viewed in real time and even stored to show you how energy production varies over the course of the day.

Depending on your installer, many will offer to monitor your system for you. This can be advantageous since they will have access to data from a large number of systems on which to benchmark performance. Make sure to ask them exactly how they plan to do this however.

On the upside, most systems should perform fine and are unlikely to go wrong but when you’re investing such a large amount of money in a PV system, its nice to know how to check its working.

In the UK’s fledgling PV industry there are some unexpected issues emerging.
One of those is related to the grid or ‘mains’ voltage. This should be, as
in the rest of Europe, fixed at 230V. However it can vary, and in reality
the legal limit is 230V plus or minus 10 percent, generally being higher
voltage close to the nearest transformer, and getting lower voltage as you
move away.

This is important for PV systems because the inverter must convert direct
current into mains compatible AC with an acceptable voltage level. Inverters
are also designed to shut down if there is a problem with the grid for
safety reasons.

In Germany, the voltage level is very precise (perhaps as you might expect)
and since Germany is the world’s largest solar market by far, most inverters
in Europe have their settings with Germany in mind. This means that when the
grid strays slightly away from 230V, the inverter temporarily shuts down. In
the UK, the grid is much more likely to deviate from 230V, meaning that with
German settings, an inverter could well spend more time off than on.

Luckily the problem is generally easily fixed by changing the inverter to
new settings which make it tolerant to a wider voltage range. The key point
to remember is that with the inverter shortage, products are being sold
which are completely unchanged from their German settings. This means you
need to be extra vigilent when buying an inverter to ensure compatibility.

A related topic, that I will soon cover, is to do with how lots of solar
energy connected to the grid can actually affect the grid voltage and
frequency – but that’s another issue.

If an inverter is used in the UK without any change in settings then chances
are, with our fluctuating grid, you will have peaks or dips in the voltage
that shut off the inverter from time to time. The solution is simply to
change the settings via the firmware to allow the inverter to carry on
working in a wider range of voltages.

As a follow up to the recent article on inverters, I thought it would be a good idea to warn you that if you wanted to buy an inverter in time for Christmas I’m afraid you’re out of luck. As result of the huge demand for residential PV systems Germany described on this site below, it is virtually impossible to find an inverter of any size or manufacturer in the whole of Europe at the moment.

The big manufacturers; SMA, Fronius, Mastervolt and the like are all completely sold out and are unclear when their next shipments can be made. Wholesellers are operating on a first come first serve basis and taking orders three months in advance. This means that if you were thinking of installing a PV system in the next few months but haven’t ordered your equipment yet you’d better get a move on. We’ve even heard reports of SMA shipping inverters to German customers without LCD displays in efforts to meet demand. (apparently the displays will be delivered in a few months time when they can be clipped on).

I guess the shortage is a good sign that the solar industry is alive and kicking. However if the feed-in-tariff is announced in the UK soon and it turns out to be a good one, there’s going to be a lot of frustrated people out there unable get involved for the lack of an inverter.