The Impact of Solar PV and Solar Thermal on EPC Ratings

The EPC Rating is a score between 1 and 100

Introduction

The Energy Performance Certificate (EPC) is a fundamental plank of the government’s strategy to improve the energy performance of the UK’s building stock.

• Since 2007 it has been a legal requirement that homes for sale have a report of their energy performance for potential buyers.
• From 2018 it will be a requirement that rental properties have a rating higher than E.
• In Scotland, all social housing will have to achieve an energy rating of C or D (depending on house type) from 2020
• To access the Feed in Tariff for solar PV, it is necessary that the building achieves an EPC D rating
• To access the domestic Renewable Heat Incentive, it is necessary to undergo a Green Deal Assessment, which is essentially an EPC with added extras to factor in the way you use energy.

The certificate rates buildings with a score from 1 (least efficient) to 100 (most efficient), with the scores divided into bands A through G as shown.

How it is Worked Out?

An approved calculation called the Standard Assessment Procedure (SAP) is used to calculate the energy used to heat the home, provide hot water to occupants together with electricity for lighting, pumps and fans.  Electricity used by other appliances is not considered.

The actual number of people in the house and the way it is heated is ignored.  A standard occupancy and a fixed indoor temperature is assumed – the idea is to compare the building with other buildings, not compare one set of occupants with another.

The energy used (gas, electricity oil) is then multiplied by fuel cost factors to produce a calculated energy cost for the property.  The energy cost is normalised by the floor area of the property to give a score between 1 and 100, the higher the score, the lower the building costs to run (with a house with a score of 100 nominally costing nothing)

For new homes the current calculation is SAP 2012 and the calculations are updated every time the building regulations change.

For existing homes, assessors use RdSAP 2009 (the Rd stands for Reduced Data).  This is an extra appendix to the SAP 2009 calculation which provides guidance on what assumptions to make when you don’t know or can’t see the exact specification of insulation and equipment.

Solar PV

SAP 2009 gives an unshaded, south-facing solar PV installation around 858kWh/year per kWp installed, irrespective of its the location in the UK.

Its calculation of energy costs assumes that 50% of the energy from a solar PV system is used in the dwelling and 50% exported (and this assumption does not change with the size of the PV installation).

The saving on the energy bills is then:

50% x energy generated x cost of purchased electricity

+

50% x energy generated x payment for exported electricity

In SAP 2009 the cost of purchased electricity is set to 11.46p/kWh for homes on a standard tariff, and the payment for exported electricity is also set at 11.46p/kWh.

Solar Thermal

A solar thermal installation could benefit the household energy bills in two ways.  First the solar system will generate heat that the boiler or electric immersion heater no longer needs to supply.  Second, a new solar hot water cylinder with better insulation will result in a reduced escape of heat from the stored water.  Although this heat loss contributes towards space heating in winter, but is wasted energy in summer.

SAP 2009 calculates out a solar energy input of 1,316 kWh per year for an unshaded 4 square metre flat plate installation, facing south and heating a 250 litre cylinder in an 85m2 house.  This translates into a fuel saving of 1,586kWh when the replaced heating system is a gas or oil boiler (taking into account their lower efficiency in summer months).

In addition the replacement of an old cylinder (where there is one) with a new would reduce heat losses.  For example, replacing a 50mm jacket insulated 180litre cylinder with a new 210 litre solar cylinder with 105 litre auxiliary heated volume and declared loss of 1.8kWh/day will save 444kWh/year based on a boiler winter efficiency of 90%.

Impact on EPC Rating

The impact on EPC rating of solar PV and solar thermal was calculated as follows.  For every SAP 2009 rating from 1 to 100, the implied Energy Cost Factor (ECF) was calculated by rearranging equations (10), (11) and (12) –  Section 12, page 33.  A house of 85 m2 floor area was considered, being the UK average size.

The energy cost that had resulted in that SAP rating could then be calculated from equation (357).

The saving on the energy cost was calculated by multiplying the energy savings from solar (discussed above) by the fuel prices in table 12.  The reduced energy cost was then converted back to a SAP rating.  The table shows the improvement in energy performance score for solar thermal and PV.

The impact of solar on a home's EPC energy score

NOTE: Figures shown are for a starting EPC score of 40 or higher (beginning of EPC band E), below this the size of the improvement decreases a little.

Conclusion

There are a growing number of drivers that are pushing building owners to improve the energy performance of their buildings.

Until recently, much of the focus has been on insulation measures to achieve these goals, but as more and more of the available cavity walls and lofts have been treated, the remaining insulation options such as external wall, internal wall and under-floor become disruptive and costly.

Solar thermal and solar PV are low hassle – high impact measures that can help increase the energy performance of homes.

Solar PV can give a significant boost to the energy rating of homes, particularly those with a clear roof of adequate size.

Solar thermal can be extremely cost-effective when combined with other heating system works such as boiler or hot water cylinder replacement and is more suitable for smaller roofs and partial shading.

Is This As Good as it Gets?

The Case for Good Looking Solar

Is that really the best you've got?

Do you like your coffee regular, large, super size or in a 6-litre bucket?  Americano, double espresso, flat, cappuccino?  Perhaps chocca-mocha or the icy one (whatever that’s called).  With skim milk, full fat or a drizzle of lard?  Do you want them to sprinkle brown dust onto the foamy topping for you?

Henry Ford famously said that you could have his Model T car “in any colour so long as it’s black”.  From the vantage point of our highly developed consumer market how amusing it is to imagine giving customers such limited choice.

Well, dear solar industry reader, why not take a fresh look at what are we offering our customers.

“You can have any solar installation so long as the panels are 2m x 1m modules fixed onto a rack above the roof, the array size matches a standard kit from a merchant and the panels are arranged in a nice easy rectangular shape.”

Yes, we talk about black-on-black modules or silver frames, poly or mono, micro inverters, optimisers or string inverters and all the rest, but are these technical issues really of interest to any but the earliest of early adopters?

Your home is your single biggest purchase and the roof is a very visible part of its kerb appeal.  If, when you come to sell it, a proportion of your potential buyers are put off by an insensitive solar installation on the roof it could cost you a lot of money.

Image: Viridian Solar Clearline PV30 and PV15 roof integrated solar panels

Forward thinking solar installation companies are already positioning themselves for a ‘post Henry Ford’ solar market.  They realise that as we move past early adopters chasing lucrative Feed in Tariffs and on to convincing the early majority to go solar we need to listen to people’s needs and offer them more choice, for example

• Downsizing the installation to avoid over-crowding the roof
• Grouping panels to create a more balanced, symmetrical installation
• Roof integrated systems where panels look more intended and less like an afterthought
• Large format modules for less clutter on the roof
• Solar tiles and slates
• Complete solar roofs
• Offering solar thermal for roofs with limited space

As the costs of the equipment for solar installations have fallen, the extra cost of roof integration has fallen to the point where it can no longer be ignored as an option for customers.


But  the industry needs to overcome some prejudices.

First, on ease of installation.

Removing a patch of tiles adds very little extra work.  This is especially true for large format concrete interlocking tiles, which are quickly lifted.  Integrated solar panels are not just for new build.

Second, on maintenance.

A PV system should last more than 20 or 30 years so the chance is high that some maintenance of a roof covering will be necessary.  Even new homes may suffer a single cracked tile from a manufacturing defect or mishandling during roofing.

Replacing a broken tile is a simple and easy job for a roofing contractor working from a roof ladder.  If the cracked tile is behind a solar array, then it’s a very different situation.  Scaffolding is required, an electrician is needed to disconnect the solar and then remove panels to hunt for the source of the leak below.  A simple job costing £100 has become a costly exercise that could comfortably exceed £1000.  Looked at this way, roof integrated systems are preferable.

Third, on energy performance.

Everyone knows that integrated systems are going to run hotter than systems that have more open to ventilation, but how big is the actual effect on energy yield?  Viridian Solar recently published research on this subject in collaboration with the University of Cambridge Department of Engineering.  The answer turns out to be only 3%.

A new, sophisticated and more demanding breed of customers is emerging for the solar industry.  Less interested in details of the technology and less accepting of “one-size-fits-all” solutions.

More and more solar installation companies are seeking to escape the race to the bottom by differentiating their offer.  Building integrated solar panels are a way to broaden the appeal of solar to more customers and add value to your business.


This article first appeared in Solar Business Focus UK Magazine

Solar Power in the CRC Energy Efficiency Scheme

The solarblogger spoke at a recent Solar Trade Association event on the commercial solar market.  In my presentation I discussed two drivers that were encouraging commercial customers to install solar power on their rooftops.

The first case study highlighted the importance of local planning requirements.  Many local authorities require that a percentage of energy consumption in new commercial buildings should come from renewable energy.  The calculation of the energy consumption of the building is woefully low because it doesn’t take into account heating or lighting in warehouse spaces or the use of the building.  However, the very fact of the planning requirement created the opportunity for the new warehouse owner to find out about the financial benefits of installing a solar PV system.  In the end they opted to install a system more than ten times larger than that required to discharge the planning requirement.

However, it was my second case study which raised an issue that this blog will clarify.

This case study was a company that operates a business and leisure park.  They have a very high annual power use of 5,500 MWh and an energy bill over £600,000 per year.

Any business with a combined electricity consumption across all sites and subsidiaries greater than 6,000 MWh per year must join the CRC scheme.

Once on the scheme, the business needs to inform government of details of all energy consumption - electricity and gas (where this is used for heating purposes).  A carbon emission level for consumed energy is calculated and the business must pay a carbon tax for every tonne it emits each year.

For 2014-15, the emissions factor for grid electricity is 0.5331 kgCO2/MWh.  The fees in this year were around £16/tonne depending on when you buy your allowances.

So any business breaching the 6,000MWh limit and qualifying for the scheme is hit with a bill for

6,000,000 kWh x 0.5331 x £16 / 1,000 =  £50,000

The asymmetric nature of the tax creates an enormous incentive for businesses that are approaching the cut off to reduce their use of energy.

A member of the audience questioned whether solar electricity counted towards energy use or not:

"We've been advised that because it's an efficiency scheme you have to total up all your electricity use, whether that comes from on-site solar panels, a diesel generator or grid electricity."

Further research has clarified the situation.  PV generated electricity should be reported, but is excluded from the CRC so long as the installation is eligible for, but has never received, Feed in Tariff (FITs) or Renewables Obligation (RO) payments.

See page 40, section 4.3.2 of The CRC Energy Efficiency Scheme guidance for participants in phase 2, version 2.  November 2014.

So there you have it.  An on-site PV solar installation is a valuable way for businesses that are approaching the CRC qualification limit to avoid the significant costs and administrative burden that come from having to join the scheme.

For businesses already on the CRC, the carbon tax rate of £16 a tonne translates into a saving of just under 1p for every kWh of electricity the PV offsets (to which you can add savings of around 10p of grid electricity not bought).  With the Feed in Tariff yielding 10p/kWh currently, to which can be added payments for export and savings on grid electricity, it seems like most businesses already on the CRC scheme would opt to take the FIT unless the PV installation could drive them below the qualifying limit for the scheme.

The Commercial Solar Rooftop Market - Fitting A Quart into a Pint Pot

The solarblogger examines the opportunities and challenges for PV on commercial rooftops.

Commercial Rooftop Solar was only 8% of the Market in 2014

It seems to be accepted wisdom that the commercial rooftop sector is set to be the next big thing in solar. In recent weeks companies more commonly associated with the solar farm sector such as Lightsource, Conergy and Lark have announced their intention to develop commercial rooftop divisions.

Indeed, there are good reasons to expect the commercial rooftop segment to take off.  As feed in tariff support has been gradually withdrawn, the savings on electricity bills become a greater and greater proportion of the financial justification for solar PV.  Unlike the domestic sector the daytime energy use in office buildings and factories is well matched to solar energy production.  

Unlike ground mounted solar farms, the aesthetics of solar on factory, warehouse and barn roofs isn’t the least bit controversial.  Even the most virulent strain of Daily Mail journalist couldn’t really complain about the ‘industrialisation’ of an industrial estate, could they?  Actually, don’t answer that question, but you get the point.

Commercial rooftops offer installations at a scale to drive low installation costs. Industrial estates often already have chunky electricity supplies.

This combination of scale, self-consumption, and relaxed attitude to aesthetics has created a real sweet spot for solar.  Commercial rooftop seems to be the sector that’s closest to a subsidy-free market. 

Despite this, mid-size commercial rooftops (which I've taken as >50kWp FIT non-standalone) represented only 8% of the PV capacity installed in 2014, far behind solar farms (assumed to be RO funded) with 64% and domestic scale FIT with 19% of the market. Given all this, it’s hardly surprising that many people have identified this sector as providing a real opportunity for growth.

The Challenges

For sure, the sector faces challenges.  Demanding hurdle rates or payback times are common for investment decisions in commercial businesses.  Making an offer that works for both a landlord and their tenants and the limitations of our creaky electricity grid could all be potential barriers to deployment.  

However, these challenges are as nothing compared to the emerging risk to the Feed in Tariff (FIT). 

The level of quarterly deployment that triggers a 3.5% degression to the FIT has been set at 50MWp for systems above 50kWp.  This band was recently split into standalone systems and systems supplying buildings.  The trigger level for systems supplying buildings has reduced to 32.5 MWp going forward, an annual deployment level of 130MWp.

The graph shows deployment for the combined segment, which has risen from 2012 to breach the degression trigger twice in the last four quarters.

The FIT depression cap has already been breached in 2 of the last 4 quarters

The companies piling into the commercial rooftop sector are refugees from a solar farm sector brutally cut back by the government.  In 2014 solar farms accounted for nearly two thirds of all solar PV installed in the UK.  1,100 MWp was installed in farms larger than 5MWp and a further 200MWp in farms smaller than 5MWp.  

In October 2014, the government shut down the RO subsidy scheme for solar farms larger than 5MWp, citing reasons of affordability due to the high levels of deployment.  The funds provided by city institutions to invest in solar farms quickly needed to find a new home.

Expect lots of 4.99MWp solar farm projects to immediately put pressure on the RO budget and don’t be surprised if these also lose RO support relatively quickly.

But imagine what the entry of these businesses could mean for the commercial rooftop sector.  It’s like Shane MacGowan, Gerard Depardieu and George Best crashing a genteel party where the host has already been struggling to keep the punch bowl topped up.

Lightsource alone has allocated £125m fund to the rooftop sector, perhaps 150MWp of anticipated deployment, and on its own sufficient to trigger a FIT tariff degression every quarter.  Without changes to its structure of the FIT, this sector could become victim of its own success.  The graph shows what would happen to the level of the FIT if deployment regularly exceeded the different degression bands.

If deployment exceeds 130MWp per quarter (a fraction of the hole left in solar farm deployment), the FIT for this tariff band could have reduced by 80% before the end of 2016, so called 'hyper-degression'.

Government promised to 'put rocket boosters' under the mid-scale commercial rooftop sector.  Industry has responded, but changes are urgently required to the Feed in Tariff structure to increase the degression limits for the >50kWp band.  If we can't get this fixed, then we risk yet another cycle of boom-bust for the industry.

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.

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.

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.