The solar PV blog

DECC has published some FAQ’s on the Feed in tariff …  FIT FAQs

This includes an answer to the question that many folk have asked over the last few days regarding the requirement to have a band D Energy Performance Cerificate (EPC) …

“Can I install the solar PV system to get a band D – and then apply for the FIT?”

The answer is, broadly, yes.

Timing will be everything – the DECC FAQ answer goes on to say that a generator must be able to provide as part of their FIT application an EPC certificate showing band D or above

I have recently Just come across PV module that’s marketed as 235Wp +7% to +12%

To me that’s a 251Wp module with 0-5% variation in output

At first I was thinking – perhaps this is just marketing hype – but on reflection I am not so sure it isn’t more significant …

The module rating ultimately effects what FIT band a completed system falls into

• An array of 16 of these modules – according to the manufacturer – is rated at 3.76kWp >>> the highest FIT tariff band
• An array of 16 of these modules – at 251kWp each – is rated at 4.016kWp >> a lower tariff band

Obviously the larger the system – the bigger the impact. And if this principle is acceptable … what’s to stop a manufacturer rating a module at +20% to +25%

One thing is certain about the Feed in Tariff at the moment – there is currently a lot of uncertainty!

FIT rates were not originally scheduled to be changed before 1st April 2012, yet at the end of October, in launching a consultation on the FIT scheme, the Government announced significant cuts to the FIT rates to come into effect on the 12th December 2011. This date was before the December 23rd closing date of the consultation period.

In a case heard in the High Court on 21st December – the judge agreed that the proposals to cut FIT payments before the official consultation closed were unlawful. DECC lodged an appeal against this decision. The Court of Appeal heard this case on January 13th and came back rejecting the appeal on January 25th. Following this failure, DECC announced: “The Court of Appeal has upheld the High Court ruling on FITs albeit on different grounds – we disagree and are seeking permission to appeal to the Supreme Court.” No formal appeal has yet been lodged.

While all this was happening, on January 19th DECC laid before Parliament an order which, regardless of the outcome of the appeal process, will set FIT rates for PV systems with an eligibility date after 3rd March 2012 to the lower levels as published in the consultation document.

So …

•  if the Government wins on appeal, we are where we were before legal action began, i.e. the Government will continue with the consultation process, DECC will respond to the consultation, and FIT rates are very likely to drop to the rates listed in the consultation document.
• If the Government does not win on appeal, then the 43p tariff looks set to remain in place for all registered installations until 3rd March.

Given all this, about the only thing we can state for certain is that any PV system installed from 12th December will get a FIT payment of at least those values published in the consultation document (e.g. 21p/kWh for 0 – 4kWp retrofit installations).

The Standard Assessment Procedure (SAP) is used to assess the energy performance of dwellings and includes a section to estimate the annual performance of a solar PV system. PV installers should be very familiar with the PV – SAP calculation as it is the method currently proscribed under the Microgeneration Certification Scheme (MCS).

A proposed revision to SAP – SAP 2012 – is out for comment at the moment >> SAP 2012

The most significant change to the solar PV performance calculation procedure in the draft of SAP 2012 is a wholly new means to obtain the annual solar radiation for the site in question. Previously, SAP made no allowance for geographic location and a simple look-up table gave the values as shown below:

 SAP 2012 provides 21 different zones around the UK and proscribes a series of steps to generate the solar radiation value for each of these areas. As such it provides a much more region specific value for solar radiation. Examples for a couple of these zones in the same tabular format as the present SAP-2009 table are shown below (calculated from the draft SAP2012 procedure):

While this change should provide more accurate results, it is in my opinion spoilt by the retention of my least favourite table – table H2 – the Overshading factor. Solar PV systems are significantly affected by shade, so shade considerations form a big part in the assessment of any PV site. The SAP table simply allows for the selection of 4 options:

• None or very little shade (<20% of sky blocked by obstacles)
• Modest shade (20-60% of sky blocked by obstacles)
• Significant shade (>60% -80% of sky blocked by obstacles)
• Heavy shade (<80% of sky blocked by obstacles)

The “percentage of sky blocked by obstacles” is a very poorly defined term and in our experience is both very hard to determine and also very open to miss-use or variations in interpretation. Retaining this simplistic assessment of shade for PV systems seems at odds with the now improved methodology for assessing the solar radiation.

Solutions are of course difficult. We are currently trying to define a procedure for the new PV guide – see my earlier post.

The SAP 2012 consultation closes on 28 March.

A low afternoon sun is shining on our office PV system just now – and the back row has a small amount of shading on it  (was predicted in our modelling).

The photo below was taken 10min ago – it shows partial shading on 4 of the 10 modules in the string.

This part of the office system has two identical arrays – each feeding two identical inverters. One system is currently totally unshaded … and the other has the small amount of shade shown in the photo.

• Output of unshaded array – 805W
• Output of shaded array – 168W

Looking at comments on the draft of Edition 3 of the PV guide … bit surprised we didn’t get more comments on the AC module part of section 2.1.13 – d.c. isolation and switching

Edition-3 of the Solar PV guide was issued as a draft for Public comment in October … this has a section on AC modules and DC isolators as follows:

++++++++++++

Systems using micro-inverters may omit the d.c. switch disconnector where all of the following requirements are met:

1. The micro inverter is located immediately to the rear of the PV modules
2. The micro inverter is plugged directly into the flying leads provided by the module manufacturer (no extensions to the flying leads may be used)
3. The micro inverter and dc cables are generally inaccessible or only accessible to trained or authorised personnel
4. The d.c. conductors between the module and micro inverter are adequately protected against mechanical damage

NOTE: A roof mounted PV installation where the micro-inverters are located on the rear of the PV modules and where access is only possible by first removing some of the PV modules is a typical scenario where switch disconnectors can be omitted. Where the same inverters are brought into a loft void, it is expected that a switch disconnector would be required.

Engineering Recommendation G83/1-1 is a key document for the solar PV industry, as it sets the requirements for the connection of PV systems (of up to 16A per phase), to the distribution network. G83/1 addresses all technical aspects of the connection process from system functionality to site commissioning and was first published in 2002, with a revised version issued in 2008. Last week, the Distribution Code Review Panel (DCRP) approved the draft of ER G83/2 for public consultation.

The draft includes a number of significant changes from G83/1-1, including:

Inverter requirements

• Two-stage protection settings – aligned with G59/2 (based on 230Vac nom)

• New short circuit current contribution test

• New frequency drift and step change stability test

• 180s start up delay shortened to “20 – 60 seconds”

• DC injection limit adjusted (20mA limit for generators up to 2kW; Larger systems – 0.25% of the AC current rating of the device)

• Power quality tests (including DC injection) to be done with a minimum system of 2kW. Where an inverter is less than 2kW – inverters to be tested in a group

• Adoption of BS EN 62116  anti-islanding test procedure

Connection requirements

• Clarification and renaming of 2 stage application process … to make it clearer and easier

• Clarification on definition of “close geographical region”

• Significantly simplified application forms

• Requirement for AC isolator next to consumer unit to change to isolator located “in an accessible location”

The public consultation runs until 3rd February 2012. The consultation paper, the draft of G83/2 and the DCRP comments form can be downloaded here >> http://www.energynetworks.info/consultations/

Arc Faults are once again on the agenda at the IEC PV meeting this week. Arc faults in a PV system have the potential to cause a fire – and, sadly, there are a number of documented cases around the world where this has occurred. In considering the issue, three categories of arcs need to be addressed:

Series arcs

Series arcs have the potential to occur at any poor connection within the DC circuit. These can occur at any point in the circuit … from poor soldered joints within a module to damaged or faulty plug and socket connectors.

Parallel arcs

Parallel arcs occur where there is a direct short between two conductors – eg arcs between positive and negative string cables.

Ground fault arcs

In floating systems where no conductor is tied to earth, two ground faults are needed before a current can flow and an arc form. While on first glance this seems unlikely, it is often said that “where one ground fault has occurred, a second is likely to follow”. An example of such a scenario is poor site-work that has led to a number of PV string cables getting damaged by a sharp edge on a metallic cable tray or conduit.

Arc fault detectors are not particularly new for AC systems, but devices designed for PV DC circuits are only now beginning to appear on the market. The devices work by detecting the characteristic frequency an arc makes and imposes on the PV circuit.

One challenge with arc fault prevention in a PV circuit is what to do when you detect an arc. Series arcs can be broken by opening the circuit. However, opening a PV circuit with a parallel arc may actually increase the fault current flowing into the arc.

Obviously the best option is to prevent arcs happening in the first place – by using good products, careful installation and thorough testing. However, a system for detecting arcs is also an important consideration. In the US, the 2011 National Electrical Code requires new PV systems on or penetrating a building to include a “listed arc fault protection device”. While there is a US standard for PV arc fault detectors, there is not currently any IEC standard for them or requiring them … hence our discussion.

Having spent the last few weeks on roofs installing, I am now swapping screwdriver for laptop and heading to an IEC PV standards meeting in Switzerland.

One of the things I love about my job is this mix of work (though spending such a constant time working on site is unusual these days – and thanks only to the FIT deadline). I hope this mix enables me to contribute one of the vital things of standards development – the requirement to make sure that they are grounded in the real world.

One of the main items on the agenda for next week is a review of the 27 pages of comments on IEC62548 – Design requirements for photovoltaic (PV) arrays. The document is approaching its final version, but it will still be a while until it is issued as a BS standard in the UK. Within the MCS PV working group, we have been working to make sure that  MIS3002 and the Guide to the installation of PV systems are aligned as far as possible with IEC62548. Work on the PV guide was put on hold due to the FIT changes – but on my return from the IEC meeting there are 158 comments on the Guide to go through. The plan is to release the Guide early in the new year.

Any ideas of properly determining how quick the TrinaMount system really is has been dashed by the weather this week. Thanks to very high winds, it’s been a real stop-start operation … just too dangerous to handle the PV modules at times.

Losing a day to the weather is pretty commonplace in the PV business … and a day to play catch-up on inside jobs the normal activity. However with the Dec 12th FIT deadline only 16 days away, it’s been a very frustrating week. I just hope that it’s been a safe couple of days for all the other solar PV installers working away up and down the country. An inevitable consequence of the unexpected FIT deadline is that corners will get cut on some installations – with potential consequences for quality and safety.

Some good news from Good Energy however. Being the excellent folks they are, they have announced that their office will be open for business over the final weekend – to make sure last minute FIT applications can get processed. The Good Energy office will be open from 10am to 4pm on Saturday 10th and from midday till 6pm on Sunday 11th . More information can be found here.