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.

The role of the inverter is often overlooked in a photovoltaic system. Kept inside in the attic or in a closet, it is not the most visible part of a system but it performs a critical role and makes up large component of the equipment costs. The inverter is the hub that converts the direct current produced by the solar panels into alternating current suitable for the UK grid.

In a typical residential photovoltaic system, solar panels are connected in a ‘string,’ which means they are connected together in series so that the voltage of each module adds up. The positive and negative ends of the string are connected to the inverter which then does two main things:

Firstly, the inverter applies the optimum voltage across all the solar panels in the string. In order to extract the maximum energy from a solar panel you need to apply to certain voltage across it. The easy way to understand this is by remembering that power equals current times voltage. Current will still flow out of the solar panel if there is no voltage across it, but it won’t be able to provide energy. If too much voltage is applied to the solar panel then you lose current coming out of the solar panel, so the optimum voltage is somewhere in between. It is the inverter’s job to keep the solar panels at this optimum voltage. This is quite tricky since the optimum voltage changes with the temperature of the solar panels. To cope with this there is a special algorithm built into the inverter called ‘maximum power point tracking’, which makes continual adjustments to the voltage to ensure the most energy is got out of the system.

The second important job of the inverter is to convert the direct current produced by the solar panels into alternating current suitable for the mains electricity grid. In the UK, the mains frequency is 50Hz so the inverter must make sure that the electricity it supplies is matched to this frequency so that it can be used by other appliances in your house or be sold to your energy supplier.

Inverters are very common, for example your laptop charger uses an inverter to convert mains 50Hz electricity into direct current for your computer (this partly explains why laptop chargers are so expensive though I still think it’s a rip-off), and there are some very good solar inverters already out there. The largest manufacturer of solar inverters is called SMA, which enjoys a +30% market share worldwide (their line of residential solar inverters is called the ‘SunnyBoy’). Other big manufacturers are Kaco, Xantrex, Danfoss and Mastervolt to name a few. These inverters work well, so what are the developments on the horizon that make inverters interesting?

One issue is efficiency. Most commercial inverters are around 97% efficient, which is pretty good, but it still means that you lose 3% of all the energy you produce converting it from DC to AC. Increasing efficiency to 99% would increase the return on investment of your solar system and give a real competitive advantage. Several manufacturers claim to be close to offering new, super-high efficiency products.
The next issue is reliability. Most inverters are guaranteed for 10 years, which although is not bad, its only half the guaranteed lifetime of solar panels. This means consumers must allow for replacing the inverter at least once when financing a solar project. If inverters could be guaranteed for 20 years, it would mean consumers could feel comfortable knowing that the system will operate under guarantee for its entire lifetime until the whole thing needs replacing. Inverter manufacturers have been striving to improve reliability of their systems and products guaranteed for 20 years should be on the market soon. As a side point; proving 20 year reliability is very hard to do without actually waiting 20 years, and there is an entire field of study devoted to ‘accelerated stress testing’ of these products.

Another set of new features is how information is displayed. Many inverters come with an optional WebBox that allows you to view the performance of your solar system online. Some inverters now even come with iPhone apps so you can watch your solar energy production on-the-go, importantly show your friends in the pub. These types of innovations will keep coming so keep an eye out if this is something that interests you.

Perhaps the most radical development for inverters is the ‘micro-inverter’. Basically this means having not one big inverter but lots of smaller ones attached to each solar panel. This has several advantages. Firstly, it can improve the performance of the system significantly. Going back to the maximum power point tracking feature mentioned above, a normal inverter has trouble if not all the solar panels are performing the same. Solar panels could be at different temperatures to each other or just have different performance from factory errors. By using micro-inverters you can ensure that each solar panel is being operated at its own optimum voltage. Another issue is to do with shading. If one solar panel in a string is shaded or performing badly, it acts like a big resistor and dramatically reduces the performance of the whole system. Using micro-inverters isolates the performance of each solar panel so that power loss from shading is minimized. Enphase Energy, a leading manufacturer of micro-inverters in California claims that these features can lead to an improvement of up to 25% better energy output.

Other benefits of micro-inverters include the elimination of dangerous high voltage DC cabling on the roof, which can reduce fire and electrocution risks. (Another side point; some micro-inverter products are not actually micro-inverters, but they perform maximum power point tracking at each solar panel and then the AC:DC conversion at a central point.)

Currently, there is not a single micro-inverter product available in the UK. This is because it is still a new technology and the UK is such an insignificant market that it is not of interest to manufacturers rushing to bring their products to commercialization. That being said, the success of companies like Enphase in California, and the spate of companies following in their footsteps, means that it won’t be long before they become a real option, even in the UK.

If you’ve ever carried a solar panel you’ll know that they’re pretty heavy (about 25kg for a 1.5sqm panel), and if you add on the racking that’s required it makes things even heavier. This is a bit of a problem for roofs that can’t support large weights, and for the installers who have to get the stuff up there.

As with many things in life however, technology has a solution on the way. In this case the solution comes in the form of flexible solar panels. This new type of solar panel doesn’t use glass as the supporting material; it uses transparent, flexible plastic sheets. They can be rolled up like carpets and unfurled across a low-sloping roof. This process is much quicker and easier than normal solar panel installation. The solar panels just need to be tacked down at the edges, rather than have heavy metal racking bolted into the frame of the roof. The material is also light enough so that any roof can support its weight.

This technology is spreading quickly but has yet to win dominance in the market. This is for several reasons. Firstly – only one company in the world is making flexible solar panels in large volumes. That company is UniSolar, based in Michigan, USA. UniSolar have developed their own proprietary process for depositing thin-film solar cells (see discussion “REF TO previous article”) on flexible plastic sheets.

In order to increase efficiency of the panels, their design in fact uses three solar cells stacked one on top of the other. Each solar cell responds to a different part of the sun’s spectrum so it maximizes the amount of energy converted to electricity. Despite this compmexity, these solar panels are significantly less efficient than traditional, crystalline silicon solar panels. They are made from ‘amorphous’ silicon and are currently around 6-8 percent efficient, compared to 16 percent for crystalline silicon panels. This means you have to cover a larger area of the roof.

A number of companies claim to have more efficient versions of the technology on the way. Companies such as US based Advent Solar, claim to have flexible solar panels that will soon reach over 10 percent efficiency while other companies, such as G24 Innovations in Wales claim to have lower manufacturing costs for this technology.

Given the success of UniSolar with their low efficiency and complex design, any company that can make an improvement is likely to have success with flexible solar panels. Let’s wait and see…

Good question. There is a huge amount of innovation happening everywhere in renewable energy and although solar technology has evolved rapidly in the last few years there is still a long way to go. There are lots of different aspects of a photovoltaic system that can be improved, and I will cover as many of them as I can on these pages.

First of all though, what is the basis on which we can judge these improvements? What is the ultimate goal here? Everyone can have their own opinion, but my ambition is to see solar energy compete economically with conventional energy sources, and for that to happen requires just one thing, lower cost of energy. Now you can get to lower cost of energy either by reducing the cost of the solar energy system or by increasing the amount of energy you get out of it. As we shall see, people everywhere are coming up with a lot of cool technology to go down both of these routes, but lets start with a company that I like called Nanosolar, who may eventually make the key step that makes solar power cheaper than coal.

First a bit of background: Solar panels are the most expensive part of a Solar electricity system, making up between half to two thirds of all the upfront costs. Most solar panels (sometimes called photovoltaic panels) are made from 50 or so ‘solar cells’ which are thin slices of silicon crystals specially treated so that they can turn sunlight into electricity. Its basically the same process that’s used to make electronic chips, which is fine for making tiny things that go inside your computer, but quite expensive for covering a small fraction of the earth’s surface with. Therefore a phenomenal amount of research has and is being done to find cheaper alternatives. So far the leading candidate for a replacement is the called ‘thin-film’ solar cell. In this case you start with flat panel of material such as glass, and coat the whole thing in a series of super-thin electronic layers that convert the sunlight into electricity. This process is cheaper than making normal silicon panels, however they are not as efficient at producing electricity.

Nanosolar are a frontrunner in developing thin film solar panels and are taking the technology to the next level. Most makers of thin-film solar panels need to use big vacuum chambers to deposit the semiconductor and can only process one panel at a time. Not so Nanosolar; they’ve cleverly developed a special electronic ink that they can literally ‘print’ onto big rolls of flexible metal sheets. Their factory in Silicon Valley looks very similar to a newspaper printing press – it’s much more suited to covering large areas.

When running at full speed the printing press should be able to cover XX football fields a day. Once the electronic layers have been printed on the foil they are cut into 6-inch squares and flown to another, newly opened factory in Germany where they are laid out into modules sandwiched in glass. Nanosolar claim their process is much cheaper than existing manufacturers out there, so cheap that it doesn’t matter that their panels are less efficient than traditional silicon solar panels. If this is really true be good news for consumers in the future, as Nanosolar could significantly bring down the price of solar panels.

Its not all plain-sailing for the US company however. They’ve been working on their process for nearly ten years and so far spent around half a billion dollars and have very little in the way of earnings. They’ll have to sell a lot of solar panels before their investors can start to relax. As with all new technologies, it takes time for customers to overcome reliability concerns, so getting to high sales volumes may take a bit of time.

Whether it’s Nanosolar that succeeds or one of the few dozen other firms pursuing similar strategies is not so important. What is important is that technology makes solar power economically viable without subsidies, and as we shall discuss on this blog, there are a lot of people out there dedicated to making that happen.