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.

A report released last week announced that the solar industry has grown worldwide despite the recession which has gripped economies. The report stated that the photovoltaic industry saw a 89 per cent increase through 2008, something which has been reflected through the first quarter of this year with the rise of investment in green technologies.

2008 marked an important watershed for the solar industry and photovoltaic technology in general, particularly in the UK as the British government passed legislation designed to promote green energy. The Energy Act of November and the establishment of the Department of Energy and Climate Change was seen to mark a shifting of gear in British political circles as the UK government sought to establish environmental legislation, emulating the success stories of California, Spain and Germany. The solar successes, particularly in Western Europe have been based largely on the establishment of coherent feed-in tariffs which have proved to be effective mechanisms at incentivising investment in the green sector.

In spite of the global photovoltaic revolution breaking out, the UK government has been slow to get behind the solar industry with enough weight to encourage green investment en masse. The recent political rhetoric of the prime minister Gordon Brown in which he espoused the need for a ‘Green New Deal’ in order to revitalise the economy through ‘greentech’ investment has not been immediately followed up by action. The feed-in tariffs which were established in principle at the end of last year will not come into effect until 2010 and until then, there are no other government schemes in place to make solar investment viable since the government terminated its grant program without warning at the beginning of the year.

Leading members of the solar industry, along with representatives from the construction industry have lobbied the government in order to ensure that the government’s rhetoric on solar and climate change is matched by action which will allow the solar industry to reach its potential in the UK, just as it is doing in Germany with outstanding commercial results.

Sharp Solar announced last week that they will begin to match their photovoltaic (PV) products to individual customer needs in a move designed to meet the needs of the housing trade. Sharp Solar, part of the Japanese electronics company Sharp spoke last week during the Ecobuild exhibition in London and reaffirmed the massive popularity of Solar roofing systems. Sharp Solar announced that interest in PV systems has grown dramatically and that Ecobuild has also highlighted a growing desire from businesses to reduce their carbon footprint and of course hopefully benefit from government schemes which will come in to lay in 2010

The new PV systems will be part of a bespoke service which will allow a choice of colours and more importantly, can be installed on roofs more easily making the possibility of future retrofitting much more convenient as the systems can be locked in to place on brackets. The new ‘Slot and Play’ system which was exhibited in Earls Court last week will offer value to new low carbon projects across the UK where buildings are fitted with the latest PV technology.

Key to the success of companies such as Sharp Solar is the highly popular feed-in tariff, due to be introduced next year by the UK government. Members of the industry have, since November’s Energy legislation and the creation of the Energy and Climate Change department been highly supportive of the tariff system as they believe it will give a much needed kick start to renewable investment as it has done for example in Germany.

Solar Sharp have given their backing to the ‘We Support Solar Campaign’ along with other leading members of the industry, providing a lobby which is seeking to push solar to the forefront of the energy debate and seek key government legislation and funding which will be paramount to the initial success of investment in Photovoltaic technology in this country.

The crucial factor in the success or failure of the solar industry will of course be the feed-in tariff, designed to spur the growth of investment in the renewable sector by guaranteeing long-term, healthy yields to investors. The long-term contracts set a fixed, above market rate for megawatts fed in to the national grid by small (systems under 5MW), green energy producers. The fixed rate for the renewable energy is paid by the power companies, the additional costs of which are absorbed by all consumers adding a small amount to monthly utility bills. Certainly, in order to keep apace with the infrastructure of areas such as Germany and California many believe that the government should set a rate of around 50p/kWh unit generated and the ‘We support Solar Campaign’ will make this clear when they produce their research findings to the government at the end of March.

Regarding the possible findings of the report, Andrew Lee of Sharp Solar commented,

“I can’t pre-empt what it is going to say, but there’s a lot of work being done on feed-in tariffs. There are a number of different options how feed-in tariffs would work, and certain job creation scenarios. There could be 300,000 to 400,000 people in this market if the feed-in tariff is fit for purpose.”

The renewable energy industry has warned that the renewable energy sector risks failing in its infancy if the proposed closure of the micro-renewables fund, worth £50m goes ahead. The possible loss of funds along with the news that feed-in tariffs will not be introduced until 2010 has concerned some members of the industry who have lobbied the government for essential funds, crucial to the development and investment in small, renewable installations.

Ed Milliband, Secretary of the newly formed Department of Energy and Climate Change announced that all funding for low carbon public sector buildings will be withdrawn from June 2009. The scheme, called the Low Carbon Buildings Program (LCBP) has, until now paid up to half of all costs incurred in the installation of micro-renewable facilities in public sector buildings and has been absolutely essential for covering the inevitable cost of installing renewable energy plant.

Philip Wolfe, Director of REA criticised the government proposal stating, “The government rightly talks about a green jobs revolution, but these initiatives will be strangled at birth if the companies that deliver them have no market in the meantime.”

Lobbyists are arguing that the government should continue the funding program at least until the introduction of feed-in tariffs in order that the industry, in particular manufacturers of heating units do not collapse. The proposed feed-in tariff will help renewable investors by guaranteeing a fixed, premium rate for power fed-into the national grid. The premium rate paid for the green megawatts will be paid for by existing power companies and will offset the expense of generating power by renewable means. In places such as California, Germany and Australia this scheme has been extremely successful as a way of attracting investment.

The importance of the LCBP and the vitality it provides to the industry was highlighted by Ray Noble, ex UK head of BP Solar, “Before, there was about £8-10m worth of funding per year. The LCBP nearly tripled the amount.”