Is RHI More Trouble than it’s Worth?

To get support from the domestic Renewable Heat Incentive (RHI), there are some hoops it’s necessary to go through, but how much do these add to the cost of a solar thermal installation?

If you install a solar thermal system in the UK you can receive financial help from the government’sDomestic Renewable Heat Incentive (RHI).  RHI payments vary depending on factors such as the size of the solar panels, their location and orientation and especially the hot water demand of the house (which is taken from the number of people who live there).  It can be worth between £1,500 and £3,500, paid out over the first seven years.  In addition to the payments householders also benefit from savings on energy bills, the value of which are much higher the RHI payments over the long life of the solar heating system.

In order to qualify for the RHI, the solar panels must be of a certain quality - achieving accreditation with the Microgeneration Certification Scheme (MCS) or SolarKeymark, the installation company must also be MCS accredited and the household needs to demonstrate that it has taken straightforward energy efficiency measures such as insulating the loft and filling cavity walls (where there are cavity walls to fill).  The way that this last requirement is proven is to produce a Green Deal Advice Report that doesn’t show loft insulation or cavity wall insulation as a recommended measure.

In recent weeks it has come to light that some solar installation companies are advising customers that there’s so much cost and bureaucracy associated with installing a solar thermal system that qualifies for the domestic RHI that they are better off avoiding the scheme.

Let’s have a look at whether this argument stacks up.

Extra Costs for the Installation

Let’s assume that the installation is of identical quality both with and without the RHI.  The installer cuts no corners on the installation standard and that the equipment that is used is registered with the MCS or Solarkeymark.

The installer must log the installation onto the online MCS database for the customer to be able to claim the RHI. There is a charge from MCS of £15 to do this.  Let’s add £20 to that to pay for the time for someone to fill out the online forms.  Total £35

In addition, the household needs to pay a Green Deal Assessor to visit and produce the Green Deal report.  You don’t need to undertake any of the recommended measures unless they include loft insulation or cavity wall insulation.  The report costs between £150 and £250. 

So the total Variable Costs (cost per installation) are between £185 and £285

Annual Costs for the Installer

For an installer to be MCS accredited, there are annual fees to pay and administrative time required.  Let’s take a look at the costs for a smaller company, as it is generally thought that the burden is highest for these.

The solar installer must pay a fee to join the scheme and be audited each year.  For a solar installer with less than 10 employees the MCS annual registration and audit fee comes in at around £470 (see NAPIT fee sheet). 

In addition there is an MCS requirement that the solar installation company must be a member of an approved renewable energy consumer protection code.  Joining RECC depends on the number of staff, but for 1-6 employees it’s £250/year. 

Let’s assume the company wouldn’t operate a formal quality system if it wasn’t going to be MCS accredited and add £1,000 of admin time to these figures to pay an office administrator to maintain the paperwork that the scheme requires each year and make sure the document handover packs and quotes remain compliant with the scheme.

Both the fees and overhead costs fall (per technology) if the company installs other MCS renewable energy technologies as well as solar thermal, but let’s assume it doesn’t.

For this small company then, the total annual Fixed Costs of maintaining an MCS solar installer registration is £1,720.   

Total Cost

The total additional cost per installation of being RHI compliant is found by dividing the Fixed Cost by the number of installations the company does each year and adding this to the Variable Cost per installation.

This is where the costs of accreditation can start to look very high – it depends enormously on how many installations the installer does each year.  See the table below.

How the admin costs of an RHI compliant solar system varies with the number of installations
the installation company does each year

If the installer does only one or two solar installations a year then, yes the costs of RHI compliance is high compared to the benefit in claiming the RHI, but even at only one system a month the extra costs start to become really quite small compared to the RHI payments. 

The more installations that the company can do each year, the more the costs trends down towards the cost of the Green Deal Report.   Nor will every customer see this as a valueless piece of paper; some may value the guidance on further measures they could take to improve their energy efficiency.

The problem for the RHI is that until the scheme starts to drive demand for a reasonable number of installations, then for small companies that perhaps combine general plumbing with a very occasional solar installation the barrier costs of being MCS registered don’t look worthwhile. 

An excellent time to encourage a customer to consider solar heating is at the same time that a hot water cylinder is being replaced, but the plumbing company standing in front of the customer won’t offer this option if it isn’t MCS registered  If they do offer solar they might encourage the customer to ignore the RHI.  This is, of course, a classic chicken/egg situation.  Unless this plumbing company starts to offer more customers solar under the RHI, they’ll never see enough demand to justify MCS accreditation.

It would be good if there was a way to encourage this plumber to promote solar thermal to customers, perhaps in cooperation with a local accredited solar installer.  For any installation company that’s doing more than a handful of solar thermal installations each year, the cost of the RHI requirements are small relative to the RHI payments.

However this is not to say that MCS couldn’t do something to reduce the burden on smaller installers to meet the ever-increasing demands of the scheme.

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The MCS Hoard

So here's an interesting thing about the Microgeneration Certification Scheme  (MCS)

The MCS is supposed to be self-funding, with fees applied on a 'per-registration' basis with these fees currently set at £15 per registration.  MCS installation companies also pay £110 annual fee, collected through the Certifying Body that accredits the installer.

Gemserv administers the scheme, and its latest annual report show that MCS has a surplus of £6.6m at the end of March 2014.

Indeed, the surplus grew in the most recent financial year by £1.3m, so this is not purely an artefact of the dash for PV of late 2011.

Figures for the number of installations the registered on the scheme database are available from the MCS website and show this:

 

 

So there were 122 thousand installations in the same period that the scheme ran a £1.3m surplus, corresponding to £10.65 of surplus per installation.

 

 

All this begs the question, why?  Why is the scheme building up such a large surplus, when its terms of reference are to be no more than self-financing?  What's all this money for?  How does MCS anticipate spending it within its terms of reference?

 

I'm personally not against accumulating some money from each installation if it's spent for the good of the whole industry, but just saving it up for a rainy day?  What use is that to anyone?

 

How would you spend it if you were in charge of the MCS?  Would you just lower the registration fee, or would you be happy to keep paying the extra, were the money to be spent on something useful.  Write in the comments below...

 

 

 

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What Has the MCS Ever Done for Us?

How to fix the Microgeneration Certification Scheme (MCS)

You pay your registration fees each year. You research each and every change to the regulations and make sure to adjust your paperwork, your working practices and the products you offer to keep in line with the rules. You engage with the annual audit of your work and put right anything the auditor finds. You go further and make changes to your processes to ensure these issues never recur. 

You are, in short, an ideal MCS installation company. The kind that the folks sitting in London running the scheme like to think that they have produced. 

The problem is that there's another installation company across town. They also have an MCS accreditation, after all it's needed for customers to access government incentive schemes. Their workmanship is shoddy, they don’t seem to be in it for the long haul and cut corners to make a quicker buck. They have accumulated a string of non-conformances on their paperwork from the annual MCS inspections but nothing ever happens, so they don't waste their time making changes. The inspector comes once a year, takes a cursory look at an installation of the company's own choosing (inspecting the roof work from the ground). So long as the company has one half-decent installation to show the inspector, they're good for another year.  This company can undercut its more diligent neighbour because it doesn't have the expense of bothering with the requirements of the MCS scheme or spending the time and care to put in the "high quality installations" the scheme claims to ensure. 

Back among the glass towers of London where industry representatives meet to oversee the MCS this company simply doesn't exist, or is at worst a 'bad apple', an isolated case. 

Unfortunately, the second company and their like are a very real in the minds of people who work for companies like the first and have to compete with them every single day of the year.  I have heard numerous stories from colleagues in the industry about companies "getting away with it", about complaints to MCS not adequately investigated and annual audits of installations that amount to little more than checking there really is a solar installation on the roof.  

This, in short, is the crisis of confidence that the MCS must recognise and urgently work to fix.  If scheme officials knows that there isn't a problem, that it really is just a very few bad apples then they should publish the evidence that has led them to reach this conclusion thereby reassuring an increasingly sceptical industry. 

Many people believe that way the scheme is managed has had the perverse effect of creating ’Natural Selection’ for the least desirable traits in registered companies.  Without meaningful audits of installations and a credible threat of expulsion from the scheme, the scheme penalises the diligent by imposing higher costs, handing a competitive advantage to those that join the scheme but don't bother trying to meet the standards. 

It is a great frustration for those of us who have worked  in the MCS technology Working Groups to hear such views.  If the enforcement really is this poor, why bother writing rules?  If the only companies that are applying the standards are those that would have worked to a good standard anyway then what’s the point of all those hours donated free of charge to the scheme.

Unless MCS changes

As I revealed in an earlier blog, the MCS is sitting on a cash pile of £6.6m, growing at the rate of £1.3m last year. That kind of money would pay for a lot of surprise audits. Perhaps industry would even be willing to pay slightly higher fees and put up with more intrusive audits if proper enforcement levelled the playing field and drove the bad apples out of the industry.

My colleague at the Solar Trade Association (STA), Chris Roberts has written a draft White Paper: 'Is the MCS Fulfilling its Potential?' to stimulate debate and comment from STA members on the MCS. Having collectively contributed more than 90% of the scheme income the views of the solar industry deserves to be heard, and I urge everyone in the industry to join the debate by reading the paper and feeding in your ideas and evidence to Chris. 

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Solar for Combi Boilers

HOW TO COMBINE SOLAR WATER HEATING WITH A COMBI BOILER

A combi boiler provides central heating and hot water.  Hot water is prepared instantaneously
and on demand as cold water flows through a heat exchanger in the boiler on its way to the outlets

Combination (combi) boilers that provide hot water on demand have become more and more prevalent in the UK.  Data from HHIC shows that 77% of all new boilers sold in the UK in the last 12 months were combis. According to the English Housing Survey 2012, 49% of all homes now have a combi boiler.

Housing developers love combis because they don't need to give up valuable space to a hot water cylinder in small (sorry, "executive starter") homes.  In addition, eliminating the cylinder means that the overall complexity and part count in the heating system is reduced which improves reliability and lowers costs.

Manufacturers of hot water cylinders argue that the flow rates that combi boilers can provide can be inadequate, especially in multi-bathroom homes, but judging by the combi’s dominance of new installations their reputation for producing only a trickle of hot water may now be undeserved.

Challenges of Combining Combi Boilers with Solar Heating

Given that combi boilers are so prevalent, you'd think that the heating industry would have sorted out how to use solar hot water with them.  However, until very recently there wasn't a good answer to the question of how to combine solar with combi boilers.  This is because combining the two raises some technical challenges.

Solar energy arrives over the course of the day, whereas domestic hot water use is intermittent and concentrated in the morning and evening.   Consequently solar heated water must be saved up for later use in a hot water cylinder or heat store.  Since the amount of solar energy that is available varies from day to day and through the year, it's also necessary to be able to bring the solar heated water to a hot enough temperature with the boiler on those days when the light levels are not high enough.

So one approach is to re-configure the combi boiler to behave like a conventional boiler.  A new heating 'zone' is added to the central heating system with separate timer control and controlled from a thermostat on the cylinder.

Solar energy (either from a solar thermal system or a PV array and excess energy diverter switch) is added to this cylinder and the cylinder thermostat tells the boiler when it's needed for a 'top up'.

(1) Reconfigure the heating system to use the combi boiler with a hot water cylinder

This approach to adding solar to a combi boiler can be difficult to implement in practice.  There's often nowhere to put the cylinder (since the house has a combi boiler).  Even when you can find a space for the cylinder, the intervention to reconfigure the heating system can be significant.

A second approach is to send solar pre-heated water to the combi boiler.  A hot water cylinder is still needed to accumulate the solar heat, but it can be a bit smaller because it doesn't need a separate boiler heated volume inside.

(2) Sending solar pre-heated water to the combi boiler

There are two issues that need to be addressed when taking this approach.

1. Combi Boiler Inlet Temperature

Some makes or models of combi boiler may not be able to accept water above a certain temperature, either because there is insufficient control of the flame or due to materials selection of components on the cold water inlet side.

A combi boiler that cannot regulate the flame down sufficiently would produce too-hot water when the inlet water arrives above a certain temperature.  This causes a safety cut out switch to activate, killing the flame.  The boiler would cycle on and off producing too-hot and then too-cool water.  Operating this way is not good for the boiler lifetime.

Some boilers have plastic components on the cold water inlet side and some of these will not be suitable for water above a certain temperature.  

A component called a combi-diverter valve is available to work with boilers that have such limitations.  One product consists of three thermostatic valves and produces the following logic:

• Inlet temperature >45C, add cold water to 45C, by-pass boiler straight to taps
• Inlet temperature 27C - 45C, add cold water to reduce to 27C, pass through boiler
• Inlet temperature <27C, pass straight through boiler

It may seem crazy to produce solar heated water and add cold water to it only to heat it up again in the boiler (for the middle temperature range), but bear in mind that doing so means you're using less of the solar heated water and leaves more heat in the solar store for later use.

The combi diverter valve ensures that the hottest inlet temperature the boiler will see is 27C, so this can be a way to make solar preheated water work with any combi boiler, so long as you can check that this temperature is OK for the model in question.

Unfortunately, boiler manufacturers do not routinely put the maximum inlet temperature of their products on their data sheets, at least in the UK.  My own experience of calling the 'help' desks of the boiler manufacturers is that it's often a struggle to get a definitive (or consistent) answer to this question.  If pushed, the safest (and default) answer is to say that the boiler should not have water above 'ambient' as an input, push again and you will get told 20C max - after all why would the manufacturer put themselves out on a limb for this?

Thankfully, an initiative by the Solar Trade Association (STA) and the Hot Water and Heating Industry Council (HHIC) has resulted in an online database of combi boiler inlet temperatures.  This list is new, and currently not comprehensive enough (special mention to Ideal for their derisory contribution - boo!).  I hope that over time more manufacturers will see the benefits of publishing this information and join in, and that those who have published will add data on more of their older models.

The STA and HHIC are to be congratulated on this initiative as it removes a significant impediment to the deployment of solar water heating.

2. Legionella Control

Legionella is a bacterium that occurs naturally in drinking water.  It is present at very low levels in drinking water but can multiply if that water is held at warm temperatures (20C to 45C).  If droplets of water containing high levels of the bacterium are inhaled it can cause Legionnaire's disease.  People with suppressed immune systems, for example the ill or the elderly, are most at risk.

Legionella bacteria can be de-activated by heating the water to above 50C, with the time taken to disinfect the water falling rapidly as the temperature increases above this level. 

A combi-boiler takes a flow of cold water, raises it to a set temperature of around 55C and then it quickly passes on to the hot water outlet.  There is very limited risk of Legionella because the water has not spent any time at warm temperatures before passing through the boiler.

Legionella risk is normally controlled in a standard hot water cylinder by setting up the controls to heat the water to 60C at least once a day (called pasteurisation), to deactivate any bacteria that could have multiplied in warm water.

Because solar energy is variable with the weather and seasons, it is not possible to guarantee that deactivation temperatures are reached every day in a cylinder heated only with solar.  Indeed, at certain times of the year a cylinder heated only with solar could spend periods at the warm temperatures in which the bacteria grow.

A twin coil solar cylinder will normally adequately control Legionella risk if the boiler heated section of the cylinder is heated daily to 60C for 1 hour, and the water is resident in the boiler heated section for 24 hours (which happens if the boiler heated volume is greater than the daily hot water use).

Putting a solar heated cylinder upstream of a combi boiler introduces a risk that the water feeding the boiler could have spent time at temperatures that allows the Legionella to multiply.  A paper by the Water Regulations Advisory Scheme (WRAS) confirms that because the combi boiler will only heat the water passing through for a short period of time, it cannot be relied upon to deactivate any Legionella that may have multiplied while the water was resident in a solar heated cylinder.

If the combi boiler cannot be relied upon to control Legionella risk, then alternative means are necessary.

For a conventional cylinder, where the water in the cylinder goes to the combi boiler, the most common approach would be a thermal pasteurisation of the water.  In practice this would mean  fitting an immersion heater to the cylinder and running it for a couple of hours each night when water is unlikely to be taken from the cylinder during the pasteurisation.

Overnight pasteurisation is a problem for solar energy for two reasons:

(1) The solar cylinder then starts the day hot.  If there isn't a significant water use in the morning, then the cylinder's capacity to take in solar heat is greatly diminished, reducing yields.

(2) The electricity used overnight to raise the cylinder to 60C is high in carbon emissions and expensive - offsetting some of the benefits of solar.

An alternative Legionella control strategy, used in products such as Viridian Solar's Pod,  is shown in (b) and (c) in the diagram below - here the volume of water in the solar cylinder is static, and instead fresh cold water is heated as it flows through a heat exchanger.  The problem of water sitting at warm temperatures for extended periods is completely avoided, and thermal pasteurisation is unlikely to be necessary.

If the stored water passes through the combi boiler, then additional immersion heating
is likely to be required for control of Legionella risk

Installing solar water heating with combi boilers has been left in the 'too hard' box by the heating industry for too long.  It is my hope that the emergence of a new class of products to make this easier in combination with greater information from the boiler manufacturers will open up solar water heating to the benefit of even more people.

Continue

Mono vs Polycrystalline Solar cells - Myths Busted

Customers often ask what's the difference, but the old certainties have gone. 

Monocrystalline have missing corners, polycrystalline cells are square : Myth

Monocrystalline solar cells are cut from a large single crystal of silicon. The process by which this crystal is grown is remarkable. It is drawn from a molten crucible of liquid silicon by dipping in a 'seed' crystal and then slowly pulling this away from the liquid surface and rotating it.  By carefully controlling the temperature gradient in the crucible and the speed of withdrawal it is possible to create a solidified single crystal with the same atomic orientation as the seed. 

If this cylindrical crystal were sliced to produce silicon wafers, they would be round and this would leave gaps when you tried to assemble them together into a solar panel.  So the cylinder is first cut along its length on four sides to make its shape closer to a square in cross-section. 

There's a compromise here. The more you slice off, the closer to a square shape you get, and the more working area you can squeeze into your monocrystalline PV panel. The less you slice off, the less material you waste and the cheaper are the cells to manufacture.  The compromise that most manufacturers have reached is to make a shape that was a square with rounded corners (pseudo-square). 

By contrast, a polycrystalline silicon wafer is made by melting the silicon feed stock, pouring it into a cube shaped mould and letting it cool and solidify.  The resulting block of silicon is sliced into pillars and these are in turn sliced into perfectly square cells. 

So one difference between mono and poly is the characteristic shape of each; Poly are square and mono have missing corners.

Not any more! 

The trimmings from cutting and slicing the silicon are no longer wasted; they are re-cycled as a material input for polycrystalline cell production. Some manufacturers now offer mono crystalline panels with full square cells.
   

Monocrystalline cells have an even black colour, polycrystalline are patterned and blue: Myth

When the polycrystalline ingots solidify in their mould, crystals start to form in many, many different places (nucleation sites) and grow until they meet up with other crystals.  The orientation of the atomic structure in each crystal is random and is different from its neighbours. When you slice though the ingot to make the wafer this creates a characteristic pattern, a kind of metal flake effect, on the surface of the solar cell because each crystal reflects the light differently. The cells also have a bluish colour. By contrast, mono crystalline cells have a homogeneous atomic structure throughout and have an even black colour. 

Not any more!

High performance solar cells are now treated during processing to create pyramidal micro structures on the surface which improves light absorption.  Anti-reflective coatings are added to reduce light reflection from the surface. Both polycrystalline and monocrystalline cells can be made to look matt black with an even colour.

Monocrystalline panels are more efficient : True - well, sort of

The boundaries between the crystals in a polycrystalline cell (grain boundaries) can impede the flow of electricity, so mono crystalline cells (which have no grain boundaries) have always had higher efficiency. However, polycrystalline  cells have been closing the gap in recent years and the point has  just about been reached where the additional active surface area from the square cell shape in a polycrystalline panel makes up for the lower efficiency in the cell itself.

Check out this table. 

It shows the product range from one of the world’s largest manufacturers.  Power is given in Watt-peak (Wp), the power output under standard test conditions.  

If you compare the standard mono and poly products (code 6/60 models), you can see the range of peak power output runs from 250 to 270Wp for the mono panel and from 245 to 265Wp  for the poly panel.  The difference is 5Wp, or 2% less power for the polycrystalline.

Monocrystalline/Polycrystalline  panels work better in low light conditions : No evidence

I have read many claims that one type of panel works better than the other in low light conditions, and writers on other websites seem to be evenly split in whether it is monocrystalline or polycrystalline that is best (presumably depending on which they sell).

I have been unable to find evidence to support these claims (in either direction…).

Until I see some evidence, I’m going to mark this one down as a myth!  Please let me know in the comments below if you know about this.

Monocrystalline panels have better high temperature performance : True – though marginal

Looking again at the table, the right hand column shows the Power Temperature Coefficient.  This is the rate at which the panel power output falls as its temperature rises.

Polycrystalline panels do indeed lose their power output more quickly, by about 0.02% more per degree C.  But what does this mean in practice?  

If, for example, a Monocrystalline solar panel were operating at 70C on a hot and sunny day, it would be producing 0.41 x (70-20) = 20.5% less power than is measured under standard test conditions (20C).  By contrast a polycrystalline solar panel could be producing 0.43 x (70-20) = 21.5% less power.

All other things being equal, polycrystalline panels would produce 1% less power at the elevated temperature.  But that is a very different thing from saying it would produce 1% less energy over a year of operation.  It’s not hot and sunny all day every day; in fact conditions to produce a 70C operating temperature are rare.  The energy penalty from choosing polycrystalline solar panels over monocrystalline would depend on climate, but will be far less than 1%.   Although there would be a penalty, it’s pretty marginal.

Polycrystalline panels are cheaper, monocrystalline are more expensive  : True, on average

The argument often goes that because the process of producing monocrystalline cells is more complex and involves more wasted material, they’re more expensive to make.

However, just because something is more expensive to make, doesn’t make it worth more to the customer.  The reason that monocrystalline panels command a price premium is that more people prefer the way they look and the panels have a higher power.  Having a higher power panel means you save money on other costs like racking and fixings for the same total energy output.  It also means that you can squeeze more energy out of situations where the area to place the panels is limited or expensive.

The PHOTON module price index report for November 2014 has average spot market prices for solar panels in Europe as follows.  (Prices are always given per watt-peak, Wp, so you can compare based on the power output).

Monocrystalline solar panels   0.65 EUR/Wp           (Range 0.48 – 0.95)
Polycrystalline solar panels     0.55 EUR/Wp           (Range 0.40 – 0.82)

So yes, on average monocrystalline solar panels are 18% more expensive on a per-watt basis, but the range of prices show that it’s perfectly possible to buy polycrystalline panels at the higher end of the market for a much higher price than the monocrystalline panels at the lower end of the market.

Conclusion

The old certainties are disappearing.  At the high end of the market, monocrystalline and polycrystalline solar panels are becoming more and more alike in aesthetics and performance.  If this trend continues, with black polycrystalline cells and square monocrystalline cells of similar performance, then average prices will converge too.

In mature solar markets, the domestic rooftop market starts to demand good looking solar panels, and has settled on solar panels with black cells and black frames with improved aesthetics.  For this market, your choice of solar panel will be far more about choosing a quality brand that you trust than worrying about whether those panels followed a polycrystalline or monocrystalline manufacturing route.

Continue

Mono vs Polycrystalline Solar cells - Myths Busted

Customers often ask what's the difference, but the old certainties have gone. 

Monocrystalline have missing corners, polycrystalline cells are square : Myth

Monocrystalline solar cells are cut from a large single crystal of silicon. The process by which this crystal is grown is remarkable. It is drawn from a molten crucible of liquid silicon by dipping in a 'seed' crystal and then slowly pulling this away from the liquid surface and rotating it.  By carefully controlling the temperature gradient in the crucible and the speed of withdrawal it is possible to create a solidified single crystal with the same atomic orientation as the seed. 

If this cylindrical crystal were sliced to produce silicon wafers, they would be round and this would leave gaps when you tried to assemble them together into a solar panel.  So the cylinder is first cut along its length on four sides to make its shape closer to a square in cross-section. 

There's a compromise here. The more you slice off, the closer to a square shape you get, and the more working area you can squeeze into your monocrystalline PV panel. The less you slice off, the less material you waste and the cheaper are the cells to manufacture.  The compromise that most manufacturers have reached is to make a shape that was a square with rounded corners (pseudo-square). 

By contrast, a polycrystalline silicon wafer is made by melting the silicon feed stock, pouring it into a cube shaped mould and letting it cool and solidify.  The resulting block of silicon is sliced into pillars and these are in turn sliced into perfectly square cells. 

So one difference between mono and poly is the characteristic shape of each; Poly are square and mono have missing corners.

Not any more! 

The trimmings from cutting and slicing the silicon are no longer wasted; they are re-cycled as a material input for polycrystalline cell production. Some manufacturers now offer mono crystalline panels with full square cells.
   

Monocrystalline cells have an even black colour, polycrystalline are patterned and blue: Myth

When the polycrystalline ingots solidify in their mould, crystals start to form in many, many different places (nucleation sites) and grow until they meet up with other crystals.  The orientation of the atomic structure in each crystal is random and is different from its neighbours. When you slice though the ingot to make the wafer this creates a characteristic pattern, a kind of metal flake effect, on the surface of the solar cell because each crystal reflects the light differently. The cells also have a bluish colour. By contrast, mono crystalline cells have a homogeneous atomic structure throughout and have an even black colour. 

Not any more!

High performance solar cells are now treated during processing to create pyramidal micro structures on the surface which improves light absorption.  Anti-reflective coatings are added to reduce light reflection from the surface. Both polycrystalline and monocrystalline cells can be made to look matt black with an even colour.

Monocrystalline panels are more efficient : True - well, sort of

The boundaries between the crystals in a polycrystalline cell (grain boundaries) can impede the flow of electricity, so mono crystalline cells (which have no grain boundaries) have always had higher efficiency. However, polycrystalline  cells have been closing the gap in recent years and the point has  just about been reached where the additional active surface area from the square cell shape in a polycrystalline panel makes up for the lower efficiency in the cell itself.

Check out this table. 

It shows the product range from one of the world’s largest manufacturers.  Power is given in Watt-peak (Wp), the power output under standard test conditions.  

If you compare the standard mono and poly products (code 6/60 models), you can see the range of peak power output runs from 250 to 270Wp for the mono panel and from 245 to 265Wp  for the poly panel.  The difference is 5Wp, or 2% less power for the polycrystalline.

Monocrystalline/Polycrystalline  panels work better in low light conditions : No evidence

I have read many claims that one type of panel works better than the other in low light conditions, and writers on other websites seem to be evenly split in whether it is monocrystalline or polycrystalline that is best (presumably depending on which they sell).

I have been unable to find evidence to support these claims (in either direction…).

Until I see some evidence, I’m going to mark this one down as a myth!  Please let me know in the comments below if you know about this.

Monocrystalline panels have better high temperature performance : True – though marginal

Looking again at the table, the right hand column shows the Power Temperature Coefficient.  This is the rate at which the panel power output falls as its temperature rises.

Polycrystalline panels do indeed lose their power output more quickly, by about 0.02% more per degree C.  But what does this mean in practice?  

If, for example, a Monocrystalline solar panel were operating at 70C on a hot and sunny day, it would be producing 0.41 x (70-20) = 20.5% less power than is measured under standard test conditions (20C).  By contrast a polycrystalline solar panel could be producing 0.43 x (70-20) = 21.5% less power.

All other things being equal, polycrystalline panels would produce 1% less power at the elevated temperature.  But that is a very different thing from saying it would produce 1% less energy over a year of operation.  It’s not hot and sunny all day every day; in fact conditions to produce a 70C operating temperature are rare.  The energy penalty from choosing polycrystalline solar panels over monocrystalline would depend on climate, but will be far less than 1%.   Although there would be a penalty, it’s pretty marginal.

Polycrystalline panels are cheaper, monocrystalline are more expensive  : True, on average

The argument often goes that because the process of producing monocrystalline cells is more complex and involves more wasted material, they’re more expensive to make.

However, just because something is more expensive to make, doesn’t make it worth more to the customer.  The reason that monocrystalline panels command a price premium is that more people prefer the way they look and the panels have a higher power.  Having a higher power panel means you save money on other costs like racking and fixings for the same total energy output.  It also means that you can squeeze more energy out of situations where the area to place the panels is limited or expensive.

The PHOTON module price index report for November 2014 has average spot market prices for solar panels in Europe as follows.  (Prices are always given per watt-peak, Wp, so you can compare based on the power output).

Monocrystalline solar panels   0.65 EUR/Wp           (Range 0.48 – 0.95)
Polycrystalline solar panels     0.55 EUR/Wp           (Range 0.40 – 0.82)

So yes, on average monocrystalline solar panels are 18% more expensive on a per-watt basis, but the range of prices show that it’s perfectly possible to buy polycrystalline panels at the higher end of the market for a much higher price than the monocrystalline panels at the lower end of the market.

Conclusion

The old certainties are disappearing.  At the high end of the market, monocrystalline and polycrystalline solar panels are becoming more and more alike in aesthetics and performance.  If this trend continues, with black polycrystalline cells and square monocrystalline cells of similar performance, then average prices will converge too.

In mature solar markets, the domestic rooftop market starts to demand good looking solar panels, and has settled on solar panels with black cells and black frames with improved aesthetics.  For this market, your choice of solar panel will be far more about choosing a quality brand that you trust than worrying about whether those panels followed a polycrystalline or monocrystalline manufacturing route.

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