Sun, Sun, Sun - Here It Comes

Not even the chilly London weather managed to deter the tens of thousands who flocked to the ExCeL Exhibition and Conference Centre from attending the annual EcoBuild exhibition. Over 1,300 exhibitors were on hand to present the latest innovations in sustainable design and green construction.

EcoBuild 2011 / Photo: SolarEdge

The massive presence of exhibitors from the PV market at this year’s exhibition speaks volumes of the UK PV market’s newfound growth, prompted primarily by the UK government’s generous feed-in-tariffs (FiT) – targeted at reducing carbon emissions by 35% by the year 2020 – up to 41.3p/kWh for PV generation and an extra 3p for export. At current conversion rates, that’s over 50 Euro cents and over 70 US cents for every kilowatt-hour produced.

The government’s new incentives promise a quite literal sunny new future for the UK PV market. With over 34 MW installed throughout the country since the FiT subsidization began in April 2010, the UK market became one of the world’s fastest growing PV markets. PwC’s research expects the UK PV market’s growth to continue, with a projected 1,000 Megawatts installed by 2015.

The new FiT introduced by the British government is highest for installations of up to 4kWp, which naturally makes residential-scale installations the most attractive.

Residential PV installation in the UK. Chimneys and dormer windows cast shadows over the modules in certain parts of the day / Installed by Engensa

Everyone who has been to the UK is familiar with the image of long rows of terraced houses populating British cities and towns. Aside from their unique style and appearance, the interconnected roofs dictated by this architectural style have a huge advantage over separate buildings: they allow PV systems installed on them to be easily joined together as one installation, hence minimizing the amount of inverters required and making the most out of the 4kWp FiTs by maximizing the power produced from each installation. This, in turn, means a faster return on investment and happier homeowners and installers who benefit from both clean energy and a steady income thanks to the generous tariffs.

But for traditional solar installations, the row houses’ roofs are as practically problematic as they’re theoretically attractive – which is part of the reason we still don’t commonly see sparkling PV modules lined against the brown chimneys on the UK rooflines. So, what’s the problem? The roofs of the terraced houses are often multi-faceted and feature varying tilts. And if that alone is not enough, chimneys, antennas and even the characteristic dormer windows cast shades over the modules’ potential locations during certain parts of the day.

Traditional installations require strings connected to the same inverter to be identical in length, orientation and tilt, along with other rigid design rules. The problem which the traditional approach poses here is twofold: first, to fully populate the different facets of the roof with PV modules requires multiple inverters – thus raising the cost of installation and reducing ROI, and secondly, even with multiple inverters, the partial shading on the roof creates a mismatch between the modules in the string and causes other, unshaded modules to generate less power than possible. In a traditional installation, the solution is to entirely avoid partially shaded areas – making utilized roof space even smaller.

All hopes for green solar roofs in the UK are not lost, however – as demonstrated by the following case. Last November, Mr. Bakewell of Oxford contacted leading British installer Engensa in order to receive a second opinion about his planned grid-connected PV installation. Mr. Bakewell’s intention was to install PV modules on two houses in his property, which are approximately 10 meters apart. The first PV professional he contacted recommended installing two PV installations using two inverters, one for each roof. Why the two inverters? Because of the different tilts. One house’s roof has a 40 degree tilt, while the other has a 25 degree tilt. Mr. Bakewell was also told that his pine trees could pose a problem by casting shades on his roof in the mornings.

Mr. Bakewell's solar installation in Oxford / Installed by Engensa

The surprise came when Engensa told Mr. Bakewell that it was in fact possible to incorporate the two roofs into one PV installation, using one SolarEdge solar inverter and per-module Power Optimizers. The result was a 3.3kWp system consisting of 18 185W ET Solar modules and 18 SolarEdge PB250-AOB Power Optimizers. Each power optimizer is attached to a single module and tracks its individual Maximum Power Point (MPP), resulting in optimal MPPT regardless of mismatch between modules – which is created by different orientations (in Mr. Bakewell’s case, the two roofs) as well as by partial shading (in Mr. Bakewell’s case, the pine trees). You can read more about Mr. Bakewell’s PV installation – as well as what he has to say about it - here.

Another challenge posed by traditional installations is the need to use a single type of solar module in every string. But what happens when you want to use multiple types of solar modules – from different manufacturers and with different power outputs and sizes? The ability to use different types of modules in one string is an important advantage in a residential installation, because it can help an installer utilize more roof space by using modules of varying dimensions (and ultimately, outputs). To see how this can be done, we turn to another British installation – this time in Exeter, southwest England.

Different types of modules in a single string in SunGift's test site in Exeter / Installed by SunGift SOLAR

The 3.91kWp Exeter installation was performed by another leading UK installer, SunGift Solar, as part of a test site. What makes this installation unique is the fact that SunGift Solar used five different types of modules, with five different power outputs, in the same string: six Sharp NU185 modules, four Sanyo HIT240 modules, four Conenergy PowerPlus 225 modules, four REC235 PV modules, and four DAY4 modules – a total of 22 modules in a single string. The power optimizers enabled this installation by eliminating mismatch between the PV modules. The entire string was connected to a SolarEdge SE4000 inverter.

Sounds easy, doesn’t it? That’s because it is. The FiTs that paved the way towards the UK PV market’s boom are in place and the technology that addresses the challenges facing the segment most favored by the fresh FiT is already here. All that’s left now is to sit back and enjoy the sun.