Monday 20 December 2021

The Social Housing Decarbonisation Fund – A Role for Solar PV

Padiham Near Burnley where 108 electrically heated homes were improved with external wall insulation, new windows and hot water systems, and Clearline fusion roof integrated solar PV by social landlord Places for People

On 19th October, the UK government revealed its much anticipated Heat in Buildings Strategy.  Headline writers entirely focused on only one element of the announcement -  the Boiler Upgrade Scheme, a plan to give grants of £5,000 to people replacing fossil fuel heating by installing a heat pump in their own home.  (See for example coverage from Sky News, Daily Telegraph, The Guardian, The Sun, BBC News).

However the Strategy contained other initiatives which, while less-publicised, better address the barriers to the transition to electric heating - by helping mitigate both their high running costs and expensive installation. These funds aimed at social landlords and Local Authorities takes a more holistic approach since they can also be used for measures that tackle running costs by reducing heating demand and generating power on-site.

The Social Housing Decarbonisation Fund

Less reported, but with a budget many times higher than the headline-grabbing Boiler Upgrade Scheme is the funding announced for the next three years for delivery through Local Authorities and Social Landlords.  The Social Housing Decarbonisation Fund (£800m over three years) is for energy improvements to social housing and the Home Upgrade Grant (£950m) will be administered by Local Authorities and support energy efficiency improvements for low income households.

For the Social Housing Decarbonisation Fund, the approach is summarised as follows:

  • Fabric first – heat loss prevention is prioritised before other energy efficiency measures
  • Worst first – homes with lower starting energy performance attract more funding
  • Upon completion homes must achieve a minimum Energy Performance Certificate (EPC) rating of C and maximum space heating demand of 90kWh/m2.year

It is set up as a competition, with applicants scored on how well they meet the goals above, as well as for deliverability and cost-effectiveness.  The landlord contributes at least 1/3 of the cost of the upgrade with 2/3 coming from the fund.  If a landlord takes the full grant and adds the minimum contribution only, then the amount that can be spent on each type of property is given below.  Landlords can elect to spend more on the property, but the extra is then provided by the landlord.

Starting EPC

Maximum Budget Supported

(with maximum grant and minimum landlord contribution)









These are pretty chunky amounts.

Eligible measures are anything that improves the EPC, with the exception of new fossil fuel heating systems.  Low carbon heating is encouraged, but only after fabric measures have been implemented.  The tenant must be left better off - with lower energy bills.

This is where solar PV comes in -  due to the high cost of electricity compared to gas, replacing gas heating with electric heating will increase energy bills, unless the starting levels of thermal insulation are exceptionally low and can be improved by a very large amount.  (See my earlier blog – Real-World Heat Pump Running Costs)

The complementarity of solar PV and heat pumps is well-understood by social landlords, as can be seen by reviewing the successful bids in the Social Housing Decarbonisation Fund Demonstrator, which were announced March 2021 and I have put into summary form in the table below.



of homes


Aberdeen City Council




Argyll & Bute Council




Clackmannanshire Council



EWI, glazing, PV

Cornwall Council




Fenland District Council



EWI, glazing, PV

Leeds City Council




Barking and Dagenham




Manchester City Council



EWI, ASHP, glazing

Northampton Borough Council




Nottingham City Council




Nottinghamshire County Council



EWI, glazing, floor insulation

Kensington and Chelsea




Stratford-on-Avon DC




Stroud District Council




Sunderland City Council



EWI, glazing, PV

Warwick District Council



EWI, glazing, floor insulation

Wychavon District Council




Key: EWI – External Wall Insulation, ASHP – Air Source Heat Pump, PV – Solar photovoltaic panels

Fourteen out of seventeen successful bids, covering 2,103 out of 2,342 (90%) of the properties to be improved have solar PV among the measures to be installed.  In fact only one project (Manchester) is installing ASHP without PV.

With a total cost of £62.1m, the demonstrator projects grant component is an average of £26,000 per property – higher than the current ceiling, but due to solar PV being such a cost effective way to improve EPCs, it is likely to remain a feature of projects in the future waves.

This system approach to improving properties comprising significant improvements to insulation to lower heat demand, and combining low carbon heating with solar PV to keep a lid on the energy bills for residents seems far more sensible than a crude upfront grant to help cover some of the extra costs of the heat pump installation alone.

That winning combination of heat pumps and solar PV is well-understood by experienced practitioners of energy retrofit working in the social housing sector.  By contrast, owner-occupiers encouraged by generous grants to install heat pumps may find themselves on the phone to their local solar PV installer soon after their first electricity bills land on the doormat.

Thursday 16 December 2021

Where Next for Solar PV Efficiency?

Source: NREL

In the early 1990s a research team led by Andrew Blakes and Martin Green at the University of New South Wales (UNSW) in Australia was working to reclaim the world record for the most efficient monocrystalline solar cell. The group had led the way throughout the 1980s, with a series of record-breaking developments that drove the efficiency of energy conversion in laboratory-made samples of solar PV cells from 18% in 1984 to 20% by 1986 but had lost the lead to a competing team from Stanford University in 1988.

In 1989 the UNSW group reported a new type of solar cell design- called a Passive Emitter Rear Collector (PERC) Cell, with an efficiency of 22-23%, reclaiming the world record for the team.

Throughout the 1990s improvements to the PERC cell technology made by the team increased the cell efficiency to 25%, a record that would stand for 15 years.

Fast forward to 2015 and PERC solar cells had made the transition from laboratory curiosity to deployment in mass-produced solar panels.  Over the next few years, PERC has come to dominate the solar market.  In 2019 all newly installed solar cell manufacturing lines were based on PERC technology, and PERC accounted for 65% of all solar cells manufactured.

What we can see from this history is that although the laboratory development work on PERC cells was completed by 1999, it took another 16 years before products based on the technology appeared in the market.  

We can also see from the chart that between 1999 and 2014 there was no further progress in advancing monocrystalline cell efficiency in laboratories around the world.  If recent record-breaking advancements take the same length of time to graduate from research devices to volume manufacture, we’ll be waiting until 2030 before they start to appear in commercial solar panels.

It may well be that due to the massive growth of the solar industry in the intervening time, current research budgets are far greater than those in 1999 so we might not have to wait for 16 years, but undeniably the introduction of PERC could represent a plateau in the relentless march of solar cell efficiency that has been a feature of the solar PV market for many years.

How Have Panel Manufacturers Responded?

With customers that have become used to panels that increased in power output each year, manufacturers have resorted to what might look like a cheap trick.  If the cells aren't getting any more efficient, let's make the panels bigger.  A proliferation of different cell formats and panel sizes has emerged.  See my earlier blog on panel and cell format proliferation.

This not-so-subtle sleight of hand has obscured the fact that technology has stalled.  Although panel powers are increasing, the specific power (power per square metre) is rising only slightly and due to more efficient packing of the cells into the larger panels.

However there is a limit to how big you can make a module before disadvantages in handling and ease of installation begin to offset and eventually exceed the benefits, especially in rooftop solar where mechanical handling is less easy to arrange.

Where Next for Solar Cell Efficiency?

The challenge for researchers seeking solar cell efficiency gains is that cells are already getting close to a brick wall - the Shockley Queisser limit.  This theoretical efficiency limit is based on physical laws.  For a single junction p-n semiconductor monocrystalline silicon cell like those in use is solar panels today the limit is 32%.  With lab cell efficiencies of 26.1%, the current record of is already 81% of the maximum it could ever be.

Some industry participants point to HeterojunctionTechnology (HJT) Cells at the successor to PERC.  Introduced by Sanyo in the 1980s and acquired by Panasonic in 2009, HJT solar cells currently have a world record efficiency of 26.7%, a little higher than PERC cells, but these cells have a similar theoretical efficiency limit based on a single p-n junction.

One way to break free of the theoretical efficiency limit is to create a cell containing multiple p-n junctions, each tuned to different wavelengths.  A broader range of wavelengths of light can then be converted to electricity. 

Among those exploring a multi-layer cell, one approach called a tandem perovskite cell looks closest to commercialisation.  (See for example Oxford Photovoltaics).  A thin film of photovoltaic perovskite material is laid down on the surface of the silicon cell.  The perovskite skims off energy from one set of wavelengths of light and allows the rest to pass through for conversion by the silicon cell below.  Efficiencies approaching 30% have been achieved in the laboratory and importantly the rate of improvement is rapid suggesting that there may be further improvements ahead.

Source: NREL

The problem facing any challenger technology is to overcome the inertia from huge investments in existing manufacturing plants for crystalline silicon cells and to prove to customers that the next new thing will have an equal lifetime.   What is interesting about the approach of the tandem perovskite cell is that it literally builds upon the well-proven crystalline silicon cell by adding a new layer.  Existing plant could be modified rather than scrapped, and the job of proving longevity is made slightly less challenging.

The rapid emergence of a global solar industry has been driven by a reducing cost of energy generated by solar, by pushing ever lower the cost per watt-peak of PV modules.  The twin engines of technological improvements to cell energy density and scale efficiencies have worked in concert to push this cost per watt-peak down year by year.  

Now it is looking like the cell technologies that have got the industry this far are approaching their limit.

Until today's breakthrough cell technologies make the journey from lab bench to mass production like the inventions of the UNSW team, the solar industry is going to have to rely more on economies of scale and manufacturing efficiencies to drive improvements.

Monday 29 November 2021

Real-World Heat Pump Running Costs

Solar PV is a Necessary Enabler of the Transition to Electric Heating 

Much of the discussion of the transition to electric heating has focussed on the installation costs of heat pumps, but what do the running costs look like?  It’s all very well handing out £5,000 grants to make heat pump installations more affordable for consumers, but if people take this government incentive only to discover that the cost of energy bills become cripplingly expensive, the resulting negative coverage could stop the transition to clean heating before it gets going.  Conversely, if energy bills fall for houses with heat pumps, it will make it much easier to convince people to ditch their gas boilers.  To get a sense of costs, we need to know two things - how much does electricity cost compared to gas and what efficiency can we expect from heat pumps and gas boilers.

How Much do Heat Pumps Cost to Run?

Advocates of heat pumps regularly claim that a ‘well designed, well installed and properly run heat pump will cost no more to run than a gas boiler’.  A careful re-reading of this sentence will show you that three things have to go right for heat pumps to cost no more than gas heating. 

One thing we know for sure is that in the UK electricity costs much, much more per unit than mains gas.  Nottingham Energy Partnership has the average standard rate for electricity in September 2021 at 23.3p/kWh (kilowatt-hour), and mains gas at 4.39p/kWh.

The efficiency of a modern condensing gas boiler is often said to be around 90%, but since we are interested in real-world heat pump performance, we should compare like for like.  A field trial of the seasonal efficiency of 60 boilers by the Energy Saving Trust in 2009 gave a value of 82.5% for combi boilers.  

Using this efficiency one unit of gas heating costs 4.39/0.825 = 5.32p/kWh.

For heat pump heating bills to cost no more than a gas boiler, the efficiency of the heat pump would need to be higher than 100% x 23.3/5.32 = 438%, but what efficiency do heat pumps achieve in practice?

Real World Heat Pump Performance

The Energy Savings Trust and the Department of Energy and Climate Change (now called BEIS), set out to answer this question in 2008.  The first large-scale heat pump field trial in the UK aimed to determine how heat pumps perform in real-life conditions. The year-long field trial monitored technical performance and customer behaviour observed at 83 domestic properties across the UK.

The resulting report (Getting Warmer: a field trial of heat pumps), published in 2010, found that the average efficiency for an Air Source Heat Pump (ASHP) was 220% (page 16), although this was revised down to 182% by a subsequent analysis published in 2012.  This second report corrected errors and removed data provided  by ‘Manufacturer A’ which were felt to be from systems that had been hand-picked, carefully optimised and installed in the homes of the manufacturer's own staff.  (See Detailed analysis from the first phase of the Energy Saving Trust’s heat pump field trial, pages 19-25)

Note: I have focussed only on Air Source Heat Pumps because most people expect that this is the technology that will be deployed in the greatest number.  They are lower cost and more convenient to install than more efficient Ground Source Heat Pumps which require a deep bore hole to be drilled or trenches to be dug.

Image: System Efficiencies of Air Source Heat Pumps reported in “Detailed analysis from the first phase of the Energy Saving Trust’s heat pump field trial”

Despite the best efforts of the authors to put a gloss on things (“the best performing systems show that well-designed and installed heat pumps can operate well in the UK”), the results were highly disappointing.  

Real-World Heat Pump Performance - Try Again

The UK heat pump industry responded positively to the issues identified in the trial and significant changes were made to the regulatory scheme for UK heat pump installers. The Microgeneration Certification Scheme (MCS) rewrote its MIS3005 installation standard for heat pumps to better control the quality of system design, installation practices and householder training that had been shown to affect heat pump performance.

Consequently, a second phase of the study was initiated.  38 of the heat pumps in the first trial were selected for interventions to improve their performance. Interventions ranged from major (swapping an over or under-sized heat pump), medium (changing radiators, adding a buffer tank, replacing circulating pumps with variable speed DC pumps) or minor (changes to controls, refilling the ground loop, adding insulation). Householders also received improved guidance on how to operate the heat pumps properly.  Six new heat pump systems installed to the new MCS standard were added to the sample and all were monitored from April 2011 to March 2012.

The results for the second attempt were published in a summary and detailed form:

As a result of all these interventions, the average efficiency of ASHPs in the new study rose to 245% 

Note: this performance improvement Phase 2 and Phase 1 included a change of the definition of efficiency – on a like for like basis the increase was from 183% to 211%.  However the preferred efficiency measure in Phase 2 (SPF H4) is in my opinion a better comparator with boiler efficiency than the System Efficiency measure used in Phase 1.  System Efficiency includes losses between hot water tank and taps/showers, whereas the SPFH4 boundary stops at the hot water tank.   


Real World Heat Pump Performance - Third Time Lucky? 

Around 14,000 Heat Pumps were installed with funding from the RHPP, and 700 of these (around 5% of the total) were subject to a detailed monitoring study.  The study reports an average efficiency based on SPFH4 for the ASHP in the sample of 241%. 

However it also reveals that the heat meters used in the study were calibrated for water and not the antifreeze-mix with which most would be installed .  The estimated 4-7% over-statement of performance was not corrected in the published result.  Applying a mid-range 5% correction, would make the true average SPFH4 nearer 229%.

Reassuringly, this is still closer to the second EST study than the first and suggests that the changes made to the industry standards in response to the disappointing performance of systems in the first study had fed through into a higher general performance, across a reassuringly large sample of installations.

Taking efficiency from this most recent study of 229%, the annual energy costs for a house heated by a heat pump will be (23.3/5.32) x (100/229) = 1.91 times higher than the same house heated by a gas boiler.

So, even after industry steps to eliminate design errors, carefully optimising the installation and coaching the householder how to use the heat pumps, running costs are still double those of a gas heated property. 

What hope do we have when we scale up to install heat pumps in the huge numbers envisaged by UK policy makers?  If installations increase from 30,000 a year currently to the 300,000 a year called for by the government will the heat pumps perform as well as those in the second study, or is it more realistic to anticipate performance closer to the first study?

Adjusting the Price of Gas & Electricity

One approach to make heat pumps more appealing is to make gas more expensive and electricity cheaper.  Government indicated in its recently published Heat in Buildings Strategy that it would consider this approach:

we will look at options to shift or rebalance energy levies (such as the Renewables Obligation and Feed-in-Tariffs) and obligations (such as the Energy Company Obligation) away from electricity to gas over this decade” Heat in Buildings Strategy p16.

What impact might this have?  According to OFGEM  Environmental and Social Obligation Costs at 25% of the price of electricity, whereas it’s only 2.5% of the price of gas.

Infographic Bills, prices and profits, 27 Oct 2021, Source OFGEM

The cost of a unit of electricity might come down to 75% x 23.3p = 17.5p/kWh

By how much would gas need to increase to replace the lost revenue?  Again, according to OFGEM typical dual fuel domestic consumption values as of 1st April 2020 were: 12,000kWh for gas and 2,900kWh for electricity.  (Source - see footnote)

For an annual use of 2,900kWh for electricity, the social tariffs come to 25% x 2,900 x £0.233 = £169

For a gas use of 12,000kWh to replace this social levy, the price of gas would have to rise by £169/12,000 = 1.4p per kWh, taking the price of a unit of gas heating after boiler efficiency up to 6.72p/kWh

If this were to happen, we can adjust our calculation for the difference in running costs 

Under this scenario, an ASHP might have running costs (17.5/6.72) x (100/229) = 1.14 times higher than gas heating, but only in 10 years’ time as government makes clear that any transition would have to be gradual to avoid pushing people into fuel poverty.

Heat Pumps and Solar are a Perfect Combination

Even when heat pumps are ‘well installed and properly operated’, even by taking 25% off the cost of electricity and shifting it over to gas, it seems likely that consumers are going to be paying more for the shift to electric heating long after the bill for the installation cost has been settled.

For an average dual fuel bill with 12,000kWh of gas use at 4.39p/kWh, the heating cost is £527/year.  Taking the ASHP efficiency from the most recent study, with ASHP heating bills 1.91 times higher than gas, the extra cost to the householder is £479/year.

One way to make the transition to zero carbon heating cost neutral on running costs is to insulate the property and reduce its heat demand.  If heat demand could be halved, running costs would end up at the same level.  However, this might be a tall order for households that have already taken the basic steps of loft and cavity insulation and double glazing, and also taking into account that the hot water demand cannot be insulated away.

If a 3kWp solar system is installed with the heat pump, generating say 2,550 kWh a year of electricity, and if 50% of that generated electricity is used on site to offset electricity use at 23.3p/kWh (£297) and 50% is exported to the grid under the Smart Export Guarantee at 5p/kWh (£64) then we’ve saved the resident £361 a year from their energy bill.  If we combine this with battery energy storage and push the self-consumption of solar electricity up to 80%, then the corresponding saving becomes £500 a year.

The solar doesn’t need to be generating at the same time the heat pump is operating for the savings to be there – remember that any electricity use in the property can be offset (for example appliances, heating hot water and even charging electric vehicles), and every unit not bought from the grid is a saving on that household’s electricity bill.

Social Landlords, housebuilders and policy makers facing the challenge of how we are going to get our homes to zero carbon while bringing tenants, homebuyers and voters along for the ride need to start thinking of solar PV and other smart energy technologies as enabling technologies for zero carbon heating.  Otherwise the real-world running costs for heat pumps could prove to be an inconvenient barrier to mainstream adoption of electric heating.


2.12.21 - it was pointed out to me that the original version of this blog used gas boiler efficiencies of 90% (which are representative of laboratory test) and unfairly compared these with actual performance in the field for heat pumps.  The blog was updated to use field test results of combi-boilers from Final Report: In-situ monitoring of efficiencies of condensing boilers and use of secondary heating, 2009 The Energy Saving Trust, with annual efficiency of boilers re-set to 82.5% instead.

Friday 28 May 2021

Why are Solar PV Panels Getting Bigger?

Power Games

The rated power of solar PV panels has climbed steadily over time.   This has been driven in large part by innovative new processing techniques for the cells themselves, although improvements to the technology of panel assembly has also played a role.  Over the decade from 2010, customers of the panel manufacturers came to expect higher and higher module powers each year.

Because all panels were the same size, the panel power was a good shorthand measure for how advanced the cell technology was.  If a panel was rated at 320Wp then it would generate 14% more energy per square metre of space than a 280Wp module.

Squeezing more power (measured in Watt-peak - or Wp per panel) into the same footprint tended to drive down the cost per installed unit of rated power ($/Wp).  Since the cost of the glass, frame and other components of the module and all the installation materials remained the same for modules of any power.  The inexorable rise in power density of the available cells was a significant factor in helping the industry achieve the amazing feat of cost reductions we have seen over that decade.

In addition, new techniques for squeezing every drop of performance out of the cells in solar modules have also allowed the accumulation of small gains.  Using a greater number of conduction wires (bus bars) with a more slender width on the top face of the cells reduces power lost due to shading of the cell below and also reduces losses from electrical resistance.  Cutting the cells into halves, or thirds, or quarters and wiring all these fragments together into parallel circuits again reduces resistance losses as well as reducing the sensitivity of the module to shading.  The efficiency gap between measuring the cell in isolation compared to the assembled panel has reduced over the years.

However, gains from improving cell powers have reached a plateau.   See my blog on why solar cells are not getting more powerful.

So manufacturers reaching for new ways to keep the story of ever more powerful modules at ever lower cost per Wp going have found a simple answer - just make the cells and the panels bigger. 

Breathless excitement from credulous industry commentators as announcement of modules exceeding 400Wp, then 500Wp barriers misses the point.  A not-so sleight of hand is evident as soon as you look at the product behind the headline number.  Panels are not getting better, they're just getting bigger.

You can have any (size), so long as its....

When it came to solar PV panels (modules) we all used to know where we stood.  A solar PV panel was just under 1m wide and around 1.65m long.  It had each of its 60 cells were 156mm square.  A defacto standard for PV panels emerged around 2010 and manufacturers stuck to it.  

Using the Waybackmachine internet archive I accessed historic web pages of industry stalwart Trina Solar.  For each year where I could find the information, I downloaded product information for the highest power module that was being offered for sale at that time.  See the graphic at the top of the page.

You can see that from 2009 to 2018, its module powers increased from 230Wp to 315Wp while their size remained the same.  The corresponding module power density increased from 141 to 192Wp/m2.

Today Trina Solar offers modules with powers ranging from 320Wp to 600Wp by virtue of offering modules of much larger dimensions.

There were many advantages that came from working to a standard size.  

In manufacturing - everything from the sheets of glass to the width of the encapsulant film and backing sheet to the pallets and packaging could all be made to the same size.  Economies of scale for the whole industry drove down costs and raw materials are readily available from many different suppliers.

Downstream, manufacturers of so-called 'balance of materials' equipment could also standardise their products.  

The electrical characteristics of the modules were all in a relatively small range and manufacturers of module level power electronics (MLPE) such as power optimisers and microinverters could easily cover the whole market with a small range of products.

Since the physical dimensions of modules varied so little between models, manufacturers of roof mounting kits also benefitted.  The wind loading per module was the same for all modules, allowing optimised designs for fixings.  The spacing of point loads on rails was consistent for any design.  Roof integrated BIPV products emerged that worked with these standardised modules and relied on their predictable format for wide interoperability and a single wind resistance value (though fire classifications proved less simple).

When designing with standard sized modules, solar installers had confidence that the system they were designing would be available both now and into the future.  This is especially important for new build projects where the design may be priced for construction many months or even years in the future.  Even if the specific module selected with was no longer available, an alternative of the same dimensions could easily be found.

Finally customers benefitted from the knowledge that in the event that a single panel in a system was to fail, that a replacement of the same format would be available in future and could simply drop into the mounting system.

Battle of the Formats

A standards war has broken out with TrinaSolar, Risen Energy, Canadian Solar and others lining up against Hanwha Q Cells, REC Group, LG on the one hand and Longi, Jinko and others on the other  - each calling for the industry to focus on cells of the dimensions they prefer.  Will the industry settle on 158.75, 166, 182, or 210mm?  At this point nobody knows.  

(See my guide to different PV cell size formats here)

In any case the cell size no longer quite defines the module size.  Since manufacturers have started slicing up cells into smaller sub-units (half-cut , third cut cells)  there is greater freedom to choose different module sizes.  Dizzy with this new found freedom, module designers are expressing their creativity  with the results of a  a wide range of module sizes now available in the market (see graphic).

Even within single manufacturers a range of many different panel sizes are now on offer.   The graphic shows a sample of products available from four of the larger industry players.  Panel powers range from 320Wp to 800Wp, but as can be seen the power density (Wp/m2) ranges only from 193 to 212 Wp/m2.

This is because the cells are pretty much the same but the packing efficiency is ever so slightly higher in a larger panel (because the edges are a smaller proportion of the whole).

An arms race for 'plus-sized' size panels is in progress, with JA currently in the lead offering the Jumbo Blue at 2.2m x 1.8m and 800Wp.  These panels are completely impractical for rooftop applications, but are aimed instead at utility scale ground mount.

It seems that module manufacturers have decided that the cost and convenience benefits of standardisation of module sizes can go hang.  Instead they are pursuing a product strategy of  scaling up panel dimensions to increase panel power that is leaving the rest of the solar supply chain scrambling to catch up.

Where will this end?  There are clearly practical limits.  How big a module can customers handle?  The answer to this question depends on where you are trying to install it.  If you're on top of a windy roof you don't want to be humping round a 4m2 monster, but maybe that's perfectly fine with mechanised lifting for a ground mount system.

The economics of solar PV has changed.  Modules now dominate the cost structure of a solar PV installation to a much lesser extent.  While it was the case that the module was the principal cost, it made sense to work with a standard format and drive out as much cost as possible.  As panel prices have fallen, maybe it does make sense to design specialist modules for different jobs.  Naturally manufacturers are focussing on their most important markets - solar farms and hence the rush to produce the bigger formats, which reduce other costs in installation and clamps.

Could this trend extend beyond different panel sizes to panels with special design features for different applications?  Maybe in future the panels used for flat roofing installations will be different from those used for fixing to corrugated metal roofs and have special features that make installation easier?  Or maybe the industry will step back from format wars and settle on a few standardised sizes for different applications.  Some folks in the business of making mounting kits for the solar modules made by these manufacturers must certainly be hoping so!

Friday 5 March 2021

PV Cell Formats and Size Guide


Here's a handy diagram I created to help show the difference between all the new solar PV cell formats in the market right now.

Monocrystalline cells are made by slicing across a cylindrical ingot of silicon.  The least silicon waste is created by having perfectly round cells, but these don't pack very neatly into a solar panel (or module), leaving gaps between the cells which reduce the power output of the panel compared to one that fills the area more effectively.  This trade off between panel power and cell economics has typically resulted in a compromise where mono-crystalline cells have rounded corners.

As silicon prices have fallen, the economics has moved decisively in favour of cutting to make a cell with a more perfect square shape, driving up the panel power by having all of the available area as working photovoltaic material.

After a long period of standardisation on the M2 cell format of 156.75mm, manufacturers cannot agree on a standard size going forward, with each proposing a slightly different format, and of course this means that the finished solar PV modules that the cells are assembled into also differ in size.

In what may come to be seen as a collective failure , and at least for the next few years, the industry has thrown away all the downstream benefits that come from having standard module sizes, a topic I will come back to in a future blog.


Tuesday 2 March 2021

Do We Need to Talk About Fugitive Emissions from Heat Pumps?

 Could the Benefits of Future Building Standards be Affected?

It’s not a recent study, but its contents may have become newly relevant.  In March 2014, the then Department for Energy and Climate Change (DECC) published the results of a study commissioned from Eunomia Research and London Southbank University - “Impacts of Leakage from Refrigerants in Heat Pumps”.  

Governments are looking for a huge increase in the number of heat pump installations in an attempt to decarbonize the heating of buildings.  Leading the charge will be developers of new homes, since these are the easiest to legislate for.  The UK government has unveiled its Future Homes Standard and Scottish Government is consulting on a New Build Heat Standard.  Neither of these regulations would force developers to install heat pumps in the homes they build, but the standards will be set in such a way that fossil-fuel powered boilers will not comply - the clear intention is to use regulation of the new-build sector to scale-up the heat pump industry.

The approach being considered by Scottish Government is to require heating systems to emit no carbon dioxide at the point of use.  Technologies considered to meet this requirement are direct electric heating (panel heaters, underfloor electric, electric boilers), heat pumps and also heat networks (where you could be burning coal at the far end of the pipe but this doesn't count as point of use).

But since the goal of all of this is to reduce global warming, then surely the emissions of other gases that cause global warming should also be considered.  It turns out that the refrigerant gases used inside some heat pumps are greenhouse gases that are thousands of times more powerful than carbon dioxide.  So long as they stay safely inside the heat pump during its life and are collected up at end of life, there is no problem, but the question the report set out to answer was how much of this refrigerant gas is lost to leaks, and how big an issue could that be?

The researchers examined the logbooks of heat pumps to see how often they had to be re-charged with refrigerant, and also conducted experiments where heat pumps were emptied and then recharged with refrigerant gases and the amount lost in the process was measured.

How Much Refrigerant is Lost?

The report found that:

  • 10% of domestic heat pumps experience a leak of refrigerant in any given year
  • The median amount lost was 35% of  charge (the total refrigerant gas in the system)
  • So the equivalent annual leakage rate was 3.48% of the charge
  • The mean refrigerant charge for a domestic heat pump was 3.3kg
  • So the mean annual leakage for a domestic heat pump is 0.114kg of refrigerant
  • Decommissioning losses at end of life were 15% of charge 
  • Commissioning and recharging losses were around 0.06kg
So if we assume a 20 year life for the heat pump, a charge at commissioning, one recharge and decommissioning at the end of life then the total losses are:

Filling: 0.06kg 
Recharge: 0.06kg
Leakage: 10% x 35% x 3.3kg x 20 years = 2.31kg
Decommissioning: 15% x 3.3kg = 0.495kg

Total Refrigerant Losses over 20 year life: 2.925kg

Refrigerant loss per year: 0.146kg

Does it Matter?

At the time of the report, R410A was the most used refrigerant.   It has a Global Warming Potential (or GWP) of 2,088, which means that 1kg of R410A released into the atmosphere has the same effect on global warming as if 2.088 tonnes of carbon dioxide were emitted.

So, a refrigerant leakage of 0.146kg per year from a heat pump is like it emitted 305kg of CO2 per year.  This is expressed as 305kgCO2e (CO2 equivalent)

The authors of the report compared the impact of refrigerant leakage to the benefits of the low-carbon energy delivered by the heat pump.  The annual heating load for a domestic property was assumed to be between 10 and 25 thousand kWh and it was concluded that the impact of leakages was small. 
However, the leakage losses from the heat pump does not vary with the amount of heat it supplies.  The more heat the heat pump delivers, the less of a penalty the refrigerant leakage is per unit of heat delivered.  Conversely the more energy efficient is the building and the lower its heat demand, the more of a penalty the leakage becomes per unit of heat.

Since the regulations are aimed at new homes, perhaps we should instead consider refrigerant emissions in relation to the heat demand of these.

Homes that are built today have very much higher levels of airtightness and insulation than the average building stock.  Based on energy calculations we see in our design team, an average sized new build house (say 85m2 total floor area) will have space heating demand around 3,300kWh per year to which you can add 1,600kWh per year for domestic hot water.  The proposed standards envisage tightening rules on insulation too, so by then buildings will have an even higher level of thermal efficiency than this.

Many houses are now being built to ‘passivhaus’ standards which remain at a comfortable temperature most of the year without heating, so the annual heating demand approaches a minimum which is that needed for hot water for showers and baths.

If we divide our 305kgCO2e figure by these lower figures for annual energy demand, the contribution to global warming per unit of heat delivered looks very different.

Table 1: Release of refrigerant gases per  kWh heat delivered (CO2e)

Annual Heating Budget (kWh)






Gas Heating CO2 emissions for comparison

Annual Emissions from Refrigerant Leakage per kWh of heat delivered (kgCO2e)








The table shows the global warming impact of refrigerant leakage for each unit of heat provided by a heat pump running R410A for different levels of annual heat demand.

It can be seen that for buildings with high heating demand (such as the levels assumed by the report authors), the CO2e figure is very low compared to delivering the same heat with gas heating.  However for very highly energy efficient properties the penalty from the refrigerant leakage rates is very significant – approaching that of gas heating.

Current Refrigerants in Use

Of course, we would expect heat pump technology to have moved on in the last six years and with regulations aiming to phase out the use of the refrigerants with the highest GWP, maybe R410A is no longer in use?

I did a quick straw-poll of the products available today from a range of heat pump manufacturers.  As you can see from the table below, there are clearly now models available that work with refrigerant with much lower GWP than the R410A used in the calculation above.  Notable leaders are products from Vaillant using R290 (which is propane gas and has a GWP of 3) and from Mitsubishi using R744 (which is carbon dioxide and so has GWP 1) - although both these manufacturers still offer models with the much higher GWP refrigerant for some reason, perhaps cost or efficiency?

It is also clear that there are also many, many models still being sold today that are using refrigerants with GWP above 1,000 and that R410A is still very popular.









Altherma 3 H HT - EPRA014-018DW




Altherma 3 R - ERGA04-08EVA




Daikin Altherma - EDLQ-CV3




Altherma R - ERLQ-CV3












Ecodan R744




Ecodan R32




Ecodan R410A




Aquarea HT Bi-bloc F Generation




Aquarea T-CAP Bi-bloc H Generation




Aquarea High Performance All in One Compact J Generation




Arotherm plus




Arotherm Split









Based on the report, it appears to me that the level of “Fugitive emissions” from heat pumps is significant enough that heat pumps with refrigerant of high GWP  should not be considered as zero carbon (equivalent) at the point of use.  

This is especially the case when the heat pump is providing heat to a highly energy efficient new building and the emissions ‘cost’ is spread over a much smaller heat ‘benefit’.

Perhaps legislators should consider the global warming potential of refrigerant in heating systems when they create new building standards?  Since heat pump models are available with improved refrigerants, perhaps they should consider applying a ceiling GWP value to the refrigerant in use?

I would be interested to hear from colleagues in the heat pump industry about whether this issue has been much discussed, and what the future direction of travel in heat pump technology is to address the likelihood or impact of leakage of refrigerant - please comment below!