Monthly archives: November 2009

The whole idea of this feed-in-tariff business is that you earn money by selling units of energy produced by your solar panels. So much so that after 25 years of operation you’ve made your money back and have even turned a tidy profit. This means that in order to know whether putting up some solar panels makes any sense, you need to know exactly how much energy they’re going to produce over the 25-year guarantee period.

Easy, you might say – the calculation is pretty straightforward. You find out the average annual irradiation (sunnyness level) from your local weather station, and multiply by the efficiency of your solar panels and the number of square metres you have. This will give you a nice number and away we go. The only problem is you might be more than 50% wrong because we’ve missed out a couple of variables. Variables such as temperature coefficient, tilt angle, diffuse-light fraction, solar cell type, shading losses, inverter losses, cable losses, degradation, module de-rate factor, mismatch losses, anti-reflective coatings, snow and lightning strikes, to name a few.

Of course there are an infinite number of effects that can influence the output of your photovoltaic system (solar eclipse, anyone?). The question is whether you have considered the important ones or not.

Knowledgeable installers use one of a number computer programs designed specifically to take these factors into account. You type in what type of solar panel you’re using, how many, where they are, what angle they’re tilted at, what direction they’re facing and then press ‘go’. It then calculates the amount of energy you’ll produce each month and even the return on investment if you want it to. Behind these models is actually some physics that describes the behaviour of solar cells under different light intensities and correctly.

The most commonly used model in Europe is called PVSyst, developed at the University of Geneva. This software package contains information on a large number of different solar panel types and is capable of taking into account many of the above listed factors. Installers across Europe use this software package to predict the energy yield of residential solar systems, as do many banks pondering whether to provide multi-million euro loans to super-large PV power projects. Even with this advanced software package however, some of these factors are very complex, and improving these models is an active area of research.

Here, I’ll deal with a couple of these complications as examples. When you buy a solar panel, it invariably comes with a power rating. Full size modules are generally around 200W. What does this mean though? In principle, the power rating indicates what you get when the panel is illuminated by full-sunlight. ‘Full sunlight’ is not very specific, so the international community has defined what is known as Standard Test Conditions (STC), which corresponds to an irradiation of 1000 W/m2 and a cell temperature of 25oC, when the light has a specific spectrum (or colour) known as Air Mass Index 1.5. So the power of your solar panel comes from its performance under exactly these conditions. In general this is measured using special type of lamp called a ‘solar simulator’ that tries to reproduce the AM1.5 spectrum as closely as possible. Calibrating these lamps precisely is notoriously difficult and there are very few testing centers around the world that are truly trusted. The National Renewable Energy Laboratory (NREL) in Colorado, USA uses at least two different lamps and one outdoor measurement to record STC performance, after a long period of calibration.

Because measuring the STC performance is so tricky, the power rating you get has a plus or minus 5 percent error margin. This is hard to include in your simulation. In addition, manufacturers will often deliberately under-rate the power of their solar panels to be sure they don’t fall below the warranty. This means you may well get considerably more power than you expect.

Another factor that adds to uncertainty is the degradation factor. When you buy solar panels they are normally guaranteed for 20 years, but only to 80% of the initial power output. This means the manufacturer expects them to degrade 1% per year on average. When calculating performance in the models, people also tend to use a 1% degradation rate per year. This is only a rough estimate however. During the certification process, solar panels are given all sports of nasty treatment to test their reliability to breaking point. This doesn’t tell you much about the rate of degradation when the solar panels are outside under normal operation though. The only reliable way to test degradation over 20 years is to wait 20 years, but this is complicated by the fact that technology improves reliability much faster than that. So the degradation of solar panels made in 2008 has only been tested since, well, 2008.

What these issues highlight is that understanding the energy yield output of your solar panels is not as straightforward as it may at first seem. When having your system designed, make sure who-ever you’re dealing with has some experience, and if possible, get a second opinion.

The other critical piece of information for understanding the financial viability of a solar installation is how much you will get paid per kWh under the feed-in-tariff. Unfortunately, the UK government has not released the final figures yet, which means no-one in the UK can make a reliable financial plan for getting solar panels, even when the launch date for the feed-in-tariff is just 4 months away.

Hopefully I will be able to update you on this in the near future. For now though, it’s better to be more conservative with your numbers than too ambitious….

With the UK government’s announcement of the introduction of the Clean Energy Cash Back system, essentially a feed-in tariff designed to attract investment in the British renewable industry, controversy has raged with solar industry insiders believing tariff rates to be too low.

It therefore comes as no surprise that the Federation of Master Builders (FMB) has also announced that they believe the tariff rate which has been set (5p/unit with a subsidy of 36.5p for units of energy generated by small scale solar and wind installations) will be too low to make the UK market competitive and have suggested a rate increase of 10p.

Speaking under the banner of the widely publicised ‘We support solar’ campaign the FMB’s announcement comes in the light of a number of criticisms aimed recently at the Department of Energy and Climate Change (DECC) legislation to be introduced in the April of next year. The FMB is being given the full backing of the National Federation of Roofing Contractors (NFRC), and Electrical Contractors’ Association (ECA) with around 16,000 building firms adding their weight to the ‘We support solar’ demands.

Feed-in tariffs are designed to offer premium, guaranteed rates to small scale producers for renewable energy which is fed in to the national grid and bought by the utility companies. In markets where they have been introduced elsewhere they have proved successful at attracting investment in new solar markets. In Germany and Spain, solar sectors have experienced booms thanks to the attractiveness of solar stocks in those countries with high returns on investment made possible by the feed-in tariff mechanism.

It is certainly considered that while the UK does not enjoy Iberian sunshine levels a strong tariff would enable the sector in the UK to take off and of course attempt to catch up with other mature markets. Some critics have argued that a strong anti-solar lobby in Westminster led by the utility companies has influenced the government’s decision to go forward with legislation which is generally accepted to be insufficient. With this in mind Liberal Democrat MP Simon Hughes stated,

“The proposed “cash back” payments are designed to dampen solar PV demand over the next three years rather than to encourage it. This mindset needs to change. Solar power can play a significant role in the “greening” of our towns and cities, while providing tens of thousands of new construction sector jobs.”

Indeed, with support among certain power brokers and pro-solar lobbies acting to add 10p to the current tariff it may well be possible to tweak the legislation, making it workable in the long term. If not, the ‘We support solar’ campaign may fail to see the fledgling UK PV sector take off.

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.