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.

The head of the newly formed New and Renewable Energy Centre (Narec), Tim Bruton, has made the claim that if every south facing home in the United Kingdom fitted solar panels, they would generate enough electricity to meet the country’s energy needs.

Speaking ahead of Solar Flair 2009, a conference to be held in Northumberland designed to highlight key issues regarding the take up of photovoltaic (PV) energy, Bruton gave his full backing to solar energy as a way of combating climate change.

With the north-east trying to put itself forward as a future leading light in solar PV expertise, Bruton is one of many academics from the region hoping to put the north of England on the PV map. As a fellow of the Institute of Physics and a reputation for insightful publications of articles relevant to the field of solar PV, Bruton asserted that the UK is on the ‘verge of something exciting’, commenting,

“The University of Northumbria carried out a study for the Department of Trade and Industry looking at the existing south-facing buildings in the UK”, adding,

“All we have to do is take the things we have already built and put solar panels on them and we can generate all the electricity we need.”

The claims made by Bruton have been made all the more possible with the announcement by the government that 2010 will see the introduction of the Clean Energy Cash Back Scheme, essentially a solar feed-in tariff (FIT) designed to attract investment in the new industry. The scheme would work by offering small scale solar energy producers guaranteed, premium rates for energy fed back in to the national grid.

The mechanism is designed as a way of off-setting the obvious initial costs of solar panel installation and where such FITs have been introduced elsewhere, they have proved to be very effective ways of nurturing fledgling renewable sectors offering returns on investments to investors which would otherwise have been impossible. Regarding a UK solar FIT, Bruton stated.

“If you look at what has happened in Germany, Spain and California where you have the right subsidy structure from the government, the market has taken off.”

Certainly, all involved in the UK solar industry will be hopeful that the government’s controversial tariff will be sufficient to see the fulfillment of Bruton’s prophecy in the coming years.

The government has recently introduced a new certification program for sustainable energy products. Called the ‘Micro-generation Certification Scheme’ (or MCS), the program is designed to protect customers by ensuring good quality in both products and installation. The scheme works by putting manufacturers and installers through an inspection process in which the applicant has to demonstrate a certain level of competency in the technology they offer, provide a documented, quality management process and show an example of a finished product or installation. In order to incentivize the industry to sign up to the certification scheme, the government has declared that customers may only apply for grants or feed-in-tariffs if their system is entirely covered by MCS which means there is little point in buying an installation without MCS accreditation.

Preventing cowboys from entering green-industry and exploiting customers trying to do their bit for the environment is essential. However, concerns have been raised regarding the real impact of the scheme on customers and regarding the credibility of the certification process itself.

Since this is solarfeedintariff.co.uk, let’s look at the certification requirements for solar energy as an example. In order to claim the UK solar feed-in-tariff arriving next April, you have to install solar panels that have been through the MCS process. At this stage however, few manufacturers have obtained MCS accreditation for their products. In many cases it is simply because they haven’t heard of the UK’s MCS program yet. In other cases, manufacturers who have been told about the scheme may not immediately decide to go for it. To get accredited, you have to pay a private certification center (that in turn has been ‘accredited’ by the MCS administrators) to inspect its product and facilities. This is a costly and time-consuming process. In many cases, someone from the accreditation center has to be flown (at the expense of the manufacturer) to a factory in Europe or Asia so that it can be ‘inspected’ often by someone with little or no experience of the photovoltaic industry. Once you have MCS, the rewards for manufacturers are not clear. In the world of solar energy the UK barely even registers. I am often met with surprise (or worse still, laughter) when I say I’m from the UK at solar conferences. The German market will be over 200 times greater than the UK market this year. There is not a single ground mounted PV power plant in the UK. This means there is no guarantee that investment in MCS will be worthwhile.

The second issue is that certification processes for solar modules already exist. The most prominent is the IEC certification process that can be administered only by a handful or institutions worldwide. This is a rigorous performance and reliability procedure that tests the energy output of a solar panel under controlled conditions, and puts it through a large number of stress tests. These include the damp-heat test (thermal cycling in a humid chamber) and the hail-test (bombarding the module with pellets of ice fired from a canon). IEC tests are designed and continually improved by a committee of international solar energy experts. Wisely, the MCS recognizes IEC accreditation and requires it as part of its inspection. This does lead to the amusing situation where a UK inspector will visit mulit-billion dollar factory that has been supplying solar panels to the rest of the world under the IEC has ‘approve’ that everything in order.

In a photovoltaic system, there are many different components besides the solar panel, however MCS applies only to the solar panel. This is hard to explain. The other expensive part of a PV system is the inverter, which converts the direct current produced by the solar panels into mains 50Hz alternating current so it can be used by most appliances in a building or sold to the grid. The inverter is therefore critical to the good functioning of a system and is known to be significantly less reliable than solar panels which are normally guaranteed for 20 years compared to just 10 for the inverter. Why then is the inverter not covered by MCS?

The MCS for installers in the UK has a clearer role. Many people are familiar with cowboy builders or decorators providing shoddy service, and MCS could be an excellent way to reduce this. Questions remain about the implications for accessibility of micro-generation however. Does MCS mean that a competent DIYer interested in building their own micro-generation system is denied access to government support because they haven’t forked out for an MCS installer?

The main concern is that MCS reduces the amount of competition in the UK, limiting the choice consumers have when it comes to products and installers. Prices of Solar PV systems in the UK are already shockingly high compared to Germany (up to twice the price) and MCS risks being a barrier to entry so that certain manufacturers can now charge even higher prices to UK customers. The MCS could be hurting those it is designed to protect, to the benefit of the manufacturers and installers already within its program.

Certainly the intentions of the MCS are good and with time it could play a key role. The MCS needs to be very careful however not to stunt the growth of the UK solar industry from its current insignificant size. The biggest barrier to that growth is cost, so any measures that may increase costs to the end customer must be rigorously justified.

Labour MP and advisor to the Department of Energy and Climate Change (DECC) Alan Simpson has warned of the presence of an cartel acting against the interests of renewable energy in the UK. At an event organised by Solar Century to promote the government’s proposal of a feed-in tariff system, Simpson announced that there is currently a lobby opposing the renewable campaign headed by the big utility companies keen to protect their own commercial interests at the expense of the development of green energy in the UK.

With the government’s announcement regarding the introduction of the Clean Energy Cash Back system (essentially a feed-in tariff system) in April 2010 much debate has raged regarding the tariff rate which will be required in order to optimise investment in the fledgling UK renewable energy industry.

The feed-in tariff works on the principle that small, renewable energy producers are guaranteed a fixed, premium rate for all units of energy they feed back into the national grid. The renewable energy units are purchased by the utility companies, something which they are obliged to do by the tariff legislation. In actual fact, the government has set a rate of 5p/unit with a subsidy of 36.5p for units of energy generated by small scale solar and wind installations, something which Simpson has controversially asserted will not be sufficient to spark the must needed investment in the industry.

Simpson claims that with the current rate set at 5p, the ROI for solar investors will only be around 5-7 per cent, yields which would possibly not be generous enough to turn the heads of investors who would potentially be attracted by more generous tariff rates elsewhere in the world. With a tariff rate of 10p, Simpson believes that returns could be a more healthy 10 per cent, rendering the UK as a highly competitive market in the world for attracting renewable investment in the long term.

For the UK to finally become one of the major players in the world of solar drastic changes will need to occur within the coming years to catch up with established markets such as Spain and Germany who are currently generating 2,511 MW and 1,500 MW of renewable energy annually respectively compared to the UK’s peak 6MW. Simpson certainly believes that this shortfall can only be remedied with the introduction of comprehensive tariff systems. Speaking at the Solar Century event, Simpson announced,

“Current energy policy in the UK is dominated by the vested interests of “Big Power”. The national grid is monumentally inefficient as an energy system. It was a half-decent idea for the middle of the last century, but 70%-80% of energy put into the grid disappears before you or I even switch the light on. We need not an energy, but a power revolution that takes control from the centre and literally puts power back into the hands of the people”.

Those within the industry back the words of Alan Simpson and are well aware that the future of the UK renewable energy industry is completely reliant on a strong tariff rate. Come April, it will be there to be seen if the government’s rhetoric on tackling climate change can be matched by a determination to take on the big utility companies and drive through a system which will see the UK become a leading light in the green energy revolution.