Options, Options - The Building Regulations Review & the Notional House

I have read commentary in recent weeks on the 2020 Building Regulations Review that suggests an alarming level of ignorance about the way the building regulations work.  It would be a real shame if the organisations behind these comments were to base their response to the consultation on such a fundamental misunderstanding.

The government consultation is proposing two options for new 2020 building regulations - one that it estimates would deliver a 20% reduction in carbon emissions compared to current regulations and another expected to deliver a 30% reduction.

So you would expect that groups interested in energy efficiency would support the second option - producing a 30% reduction.  But no, some seem to prefer Option 1, because they wrongly think it will result in homes with higher levels of thermal insulation.

It won't.

Let me explain.

The Building Regulations for Energy - How it Works

To comply with the Building Regulations for energy efficiency, housebuilders must use a calculation called the Standard Assessment Procedure (SAP) to demonstrate that the house they plan to build will meet requirements to limit carbon emissions and (new in the upcoming version of building regulations) primary energy consumption and affordable energy bills.

Related article: What is Primary Energy?

Focusing on carbon emissions and primary energy, the way the calculation works is as follows.  (See also the figure above).

1. You decide the geometry of the house you want to build (it's dimensions, shape and openings - number and size of windows and doors)

2. You calculate a Target Emissions Rate (TER) and Target Primary Energy (TPE) for a "Notional House".  The Notional House is the same shape as the actual house you want to build but has a technical specification based on Reference Values defined in Appendix R of SAP.  The Reference Values include insulation performance (U-values) for all the building elements (walls, windows, roof, floor), a maximum allowable amount of openings, as well as air change rates, a heating system and renewable technologies.

3. You then choose the technical specification you actually want to build the house to.  These can differ from the Reference Values - you are free to choose a different heating system, to build to higher or lower insulation levels, to aim for higher or lower air-tightness and whether to include more or less renewable or energy saving measures.  The only constraint is that insulation levels must be higher than so-called backstop values, which are also defined in the regulations.  You calculate the Dwelling Emission Rate (DER) and Dwelling Primary Energy (DPE) based on this house design.

4. So long as the carbon emissions and primary energy for the actual house are lower than the target figures generated by the Notional House, you're good to go, the design is compliant.

This elegant system defines a level of performance for the energy efficiency of new homes while giving developers a free hand in how they want to build.

Option 1 in the consultation sets the Reference Values for the notional house to have highly insulated walls, floor roof  and openings.  The Reference Values given for Option 2 come with slightly lower insulation levels, but add in solar PV and waste water heat recovery to the specification, resulting in lower overall energy use and carbon emissions than Option 1.

Just because Option 1 has higher insulation in the reference values it does not mean that houses will be built with this level of insulation.  As mentioned earlier, developers have complete freedom to choose a specification so long as it meets the target emissions and primary energy levels.  If it is a lower cost option, they are just as likely to reduce the insulation levels and add solar PV to meet Option 1.

If you are interested in lobbying for a 'Fabric First' approach, then you should focus on arguing for more ambitious backstop values for insulation and airtightness, but please don't argue for Option 1 Reference Values.  Option 2 will deliver higher-performing homes and will force housebuilders to push energy efficiency further and faster.  It will also likely result in higher levels of insulation in as-built homes.



 

The Future Homes Standard Consultation

Where next for Building Regulations?

In the week where Extinction Rebellion activists were arrested for hosing the Treasury in 'blood' in protest at the lack of progress on tackling a climate emergency, the consultation on the Future Homes Standard came out.  There's talk of solar panels for all new homes - so let's take a look under the hood of the consultation.

The consultation itself consists of two main parts - consideration of the Future Homes Standard due to come into force in 2025 which is intended to deliver "world-leading levels of energy efficiency" for new homes and  an update to the Building Regulations Part L (energy efficiency) and Part F (ventilation) in 2020 to provide a "meaningful but achievable" uplift in energy efficiency as a first step towards the 2025 vision.

There's also a raft of supporting documentation

The Standard Assessment Procedure (SAP) calculation version 10.1
An Impact Assessment, which includes details of cost assumptions
Approved Documents L and F 

2020 Part L - a Stepping Stone to Future Homes 2025

There's a lot to talk about here.  This is no 'tweak' but a significant revision, at least in part forced by the significant changes to the carbon intensity of grid electricity, but also by the Grand Challenge Mission for Buildings, announced by Theresa May about a year ago.

1. Primary Energy Use is the new Gold Standard

Until today, Part L has always used carbon dioxide emissions as its measure of compliance with regulations.  Buildings had to achieve a certain Dwelling Emissions Rate (DER) in kgCO2/m2.

DCLG has rightly concluded that as the electricity provided by the grid comes with a lower and lower carbon intensity, developers could switch to electric heating and hit a carbon target without improving the energy efficiency of buildings.  If energy efficiency of buildings is not improved, then decarbonising the grid becomes more challenging and costly.  So a new measure is required and primary energy, which has the benefit of aligning UK regulations with the measures chosen in the EU Energy Performance of Buildings Directive, is added as a new metric.

(See this article on the rapid progress made in decarbonising the grid.)

The latest revision to the government's Standard Assessment Procedure (SAP) version 10.1 has been published alongside the consultation.  This is the calculation used to demonstrate a house complies with the building regulations.  In this version of SAP the carbon intensity of electricity is set to 136gCO2/kWh, a projection of the average from 2020-2025, and a massive reduction from the value of 519gCO2/kWh in the current version of SAP 2012.  Electricity now produces less than 65% of the carbon emissions of mains gas (which is at 210gCO2/kWh).

By contrast, the primary energy content of a unit of electricity is 1.501 compared to gas at 1.130.

This document explains primary energy and how the values were arrived at

Fitting solar PV to a property reduces the grid electricity that is needed by the house, solar PV generation used in the building (self-consumption) reduces both the carbon emissions and primary energy by the same factor as grid electricity.

Electricity sold to grid also reduces both the carbon and primary energy use of the dwelling but it's primary energy factor is only 0.501.

The impact of this is that a unit of electricity generated by PV and used in the building would save 1.501 kWh of primary energy use, but a unit of PV generated electricity exported to the grid would only save 0.501 kWh of primary energy use in the calculation.

Since the benefits of battery storage (SAP Appendix M) and PV diverters (SAP Appendix G4)  have also been added to this update to SAP, the combination of using primary energy as the main regulatory target and the low primary energy factor for PV export has the effect of incentivising measures such as these to use as much PV-generated electricity within the building.

The trouble with this is that

(a) developers prefer combi boilers so there's no hot water cylinder in most new homes for a PV diverter to divert excess electricity into.
(b) batteries are approaching cost effectiveness but are likely to be seen by developers as an additional cost and not a sellable benefit.

We understand that the logic for choosing this value for exported energy is that the exported energy has a primary energy factor of 1.0 (renewable energy), and displaces a unit of energy from being fed into the grid at the grid average of 1.501, so the net benefit to primary energy added to the grid is 0.501.

The solar industry might argue that considering things from the point of view of the building produces a different logic (and after all what we're supposed to be modelling is the energy performance of the building).  The net primary energy consumption of the building is the electricity imported at a primary energy factor of 1.501 less the PV generated electricity exported which should have a primary energy factor of 1.0. 

A minimum carbon emissions requirement is retained in addition to the primary energy requirement as this remains an important consideration for government and there is concern that certain solutions could produce low primary energy figures with high carbon emissions - for example heating oil and coal both have low primary energy but  high associated carbon emissions.

Finally, the current fabric efficiency requirement is dropped to make way for a new householder affordability target, with fabric efficiency now considered adequately protected by tougher minimum heat loss standards for building elements.  As discussed, electricity has low and falling primary energy and carbon emissions factors, and government is concerned that direct electric heating would be a viable option for meeting both the carbon and primary energy targets, but with the side-effect of saddling occupants with too-high energy bills.  To guard against this the new affordability rating is likely to be set at a level that means direct electric heating would only be an option when combined to other measures to reduce electricity bills such as increased thermal insulation, PV panels or battery storage.

2. Uplift of the Minimum Standard

The minimum performance standard is defined by publishing a build specification (insulation levels, heating system, light fittings, microgeneration technologies) to be used by the developer to model a 'notional house'.  The developer then has to design the house they plan to build to produce modelled carbon emissions and primary energy lower than that of the notional house.  It's an elegant way to allow the developer complete freedom in design but control the outcome.

 The consultation proposes two options for the minimum performance standard:

Option 1 - "Future Homes Fabric"

This specification would produce a 20% reduction in CO2 emissions when compared against the specification in current building regulations .  The standard is based on a notional home with improved insulation measures (including triple glazing) plus a gas boiler and waste water heat recovery.

The estimate given in the consultation is that this option adds £2557 to the build cost of a semi-detached house and saves households £59 a year in energy bills.  (Payback 43 years)

Given that by 2025 the Future Homes Standard needs to be at a 75% of the carbon emissions of 2013 regulations, 20% does not seem like a big enough step - it only brings England roughly to the level that  Scotland's developers have been achieving since 2015.  DCLG appears to agree, stating that it's preferred option is Option 2.

Option 2 - "Fabric plus Technology"

In this option, the specification of the notional house is set at a level to produce a 30% reduction in carbon dioxide emissions across the build-mix.  The specification has slightly lower insulation than Option 1 plus waste water heat recovery and a solar PV system.

SAP 10.1 Appendix R outlines the specification for the notional house.  The size of the PV system in kWp for the notional house is 40% of the building foundation area divided by 6.5.  So for example for a typical two-storey semi-detached house of total floor area 85m2, this would be

[40% x (85/2) ] / 6.5 = 2.6kWp (around 9 or 10 panels)

DCLG's modelling estimates that building to this new notional home adds £4847 to the building costs and saves £257 a year in energy bills.  (Payback 19 years).

The costs used in the accompanying impact assessment for solar PV are £1,100 fixed costs plus £800 variable per kWp installed.  This implies the following installed costs:

1kWp  £1,900  £1.90/kWp
2kWp  £2,700  £1.35/kWp
3kWp  £3,500  £1.17/kWp

Solar is a fast-paced technology and it would be unusual if a government consultation were to use up-to-date cost information.  My understanding is that solar installers operating in the new-build sector are typically charging an installed price the range of £1.10-£1.20/kWp for four or five panel systems (1 -1.25 kWp).  So it is likely that the costs of Option 2 are over-stated relative to Option 1.

If the solar industry can provide evidence that costs in Option 2 are over-stated, it will make it easier for government to hold the line on its preferred option.

DCLG reckons that Option 2 might result in developers moving away from gas boilers to air-sourced heat pumps.  A specification based on ASHP alone over-shoots the Option 2 target at a lower cost than the notional house (£3,134), which would allow some relaxation of the fabric for further cost savings.  The experience in Scotland suggests that housebuilders will avoid ASHP for as long as possible because customers neither like nor understand them.

3. Heat Pumps - "Lord Make Me Chaste - but not yet!"

The consultation steps away from banning gas heating in 2020, this change is timetabled for 2025.  However it does impose extra conditions on wet space heating systems to ensure that they are 'future proof'.  In practice this will mean that 'emitters' (normal people call them radiators) will be increased to a size that would work at lower temperature, and so the house would be suitable for later conversion to a heat pump heating system without the cost of replacing all the radiators.

A side effect of this requirement is that increasing the cost and space requirements for wet systems could push developers towards direct electric heating with panel heaters, simple underfloor electric or radiant heat panels.  The removal of the entire cost of the wet heating system would offset a considerable chunk of the costs for the additional measures (PV solar, more insulation) needed to stay within the householder affordability target.  A house without a wet heating system would be low on maintenance and low cost to build, coupled with better insulation plus lower cost PV and battery storage to keep bills down this could become a favoured option for new homes.

4. Transitional Arrangements

This proposed change is likely to cause significant concerns at housebuilding companies.

The current situation is that as new Building Regulations come into force, they apply only to whole developments as new planning applications are lodged with local authority planning offices and work has started on site.

The practical outcome of this rule is that new homes are still being built to versions of building regulations in force many years ago, because:

(a) Developers rush to submit planning applications in the run up to new regulations coming into force, banking large numbers of homes to be built under the old regulations
(b) Large sites of many hundreds of homes are built out over many years, but there is a site-wide application of the regulations.

This was clearly demonstrated by the 2015 Scottish building regulations change, where it is only now (nearly 4 years later) that pretty much all new sites coming forward for tender require solar.

The consultation proposes moving from a site-based application of building regulations to one based on specific buildings.  Large developments spanning many years would have to redesign to meet new building regulations that apply as the building is being built.

Housebuilders will be alarmed by this proposal because all developments still under construction under 2013 regulations will be caught in this net.  The land for these sites would have been bought at a price based on the construction costs expected under those 2013 regulations and the housebuilders will argue that this measure is a retrospective action that will harm their profitability.  How much sympathy there is for the housebuilders having to shoulder the extra costs remains to be seen, when government has been subsidising the housing market through the Help to Buy scheme and the chief executives of some companies have been given bonuses amounting to £10,000 per house built .

 5. Other Stuff

Solar PV on Apartment Blocks

 In the original SAP10, PV on apartment blocks connected to the landlords' supply did not improve the DER of the individual apartment, whereas in SAP 2012 the carbon savings were apportioned across apartments by floor area.  The Solar Trade Association argued that connection to Landlord's supply was often by far the most cost-effective and practical way to install solar on apartment blocks, that the changes would force systems to be split into mini-systems serving each apartment at great cost, and that the carbon savings were real.  It seems that this argument has prevailed as SAP 10.1 has changed the treatment of solar PV in apartment blocks back to as it was in SAP 2012. 

Heat Networks Get a Free Pass

SAP 10 introduced punitive heat losses on district heating networks, based on evidence that large amounts of heat are lost in the underground pipework of these systems (40-50% even for best practice new ones).  It seems that government thinks that heat networks will be an important part of the energy future, and that their drawbacks should be ignored.  So a fudge-factor (they call it a 'technology factor') is applied to buildings that use a heat network.  These are allowed to emit 45% more carbon for heating and 5% more primary energy.

The Government's enthusiasm for heat networks is baffling considering that there is a perfectly good electricity network that loses far lower energy in transmission and is already connected to every single property.  A heat network is not of itself low carbon - it depends what you're doing to make the heat.

The Future Homes Standard - for 2025

The second part of the consultation is some early range-finding questions for the Future Homes Standard due to come into force in 2025.

The government reckons a 70-80% reduction in carbon emissions compared to current housing is possible.  This will be achieved by adding low carbon heating (heat pump or district heating) to the Option 1 fabric proposed in the 2020 regulations, and relying on further decarbonisation of grid electricity to do the rest.  Government is seeking views on whether this is achievable.

Local authorities which have been using planning powers under the Planning and Energy Act 2008 to require developers in their region to build to standards above those of the current building regulations.  This role for local authorities has been crucial for pushing forward on energy efficiency during a period of inaction from Westminster.  The consultation considers whether these powers should be removed alongside the 2020 regulations, the 2025 Future Homes Standard or not at all.

Summary

This change is significant and there's still some modelling to be done to figure out which packages of technology developers are likely to favour, but given the simplicity and popularity of solar it seems unlikely that the technology will not be a big winner from these changes to building regulations.

The Smart Export Guarantee Scheme (SEG)

Why is central government continually surprised that when the big energy companies are asked to ‘do the right thing’, they instead do what is right for them?

Central government seems to love handing responsibility for delivering energy reduction targets over to the big energy suppliers.  The scheme names come and go -  CESP, CERT and ECO – but the common factor has been to require energy companies to invest in energy efficiency measures such as loft insulation, and cavity wall insulation for homes.

Pause for a moment to think about it.  You’re asking a business to do things to reduce demand for its own product – energy.   How surprised should be we be that that foot-dragging, missed targets and ineffective measures have been the result?

In 2014, many of the energy suppliers were fined for failing to meet their targets to install insulation. British Gas was fined £11million, a development which their PR department brazened-out as a charitable donation.  One is left wondering if the energy companies see these fines a small price to pay instead of helping people spend less on energy.

With the government's new proposals for a Smart Export Guarantee (SEG) are we again about the make the same mistake by asking the big energy companies to decide what the ‘market price’ for electricity exported by householders and businesses with solar panels?

Why we Need a Smart Export Guarantee

Many people in the solar industry that I speak to have pretty mixed feelings about the Feed in Tariff.  They recognize the transformative effect of 19 years of subsidy on the industry, helping it to achieve scale and cost-competitiveness with fossil energy.  At the same time, they regret the reckless way that the scheme has been managed.  Successive ministers at DECC and then BEIS have inflicted real pain on many good people who had invested their time, energy and money in solar businesses an effort to be part of the solution.

As a consequence, the industry is genuinely looking forward to a future where it no longer needs ‘help’ like that from government and the technology can stand on its own feet as a significant contributor (maybe the dominant contributor worldwide) to the clean energy revolution.

It remains crucially important for the sector that householders and businesses that invest in solar are able to sell generated solar energy that they cannot use themselves.  This makes possible efficient and cost-effective solar systems that minimize the cost of energy rather than being sized to just meet demands in the building at times of peak output.

So, as the Feed in Tariff (FIT) draws to a close on March 31st, government is consulting on a new scheme, the Smart Export Guarantee (SEG) – that requires larger energy suppliers to purchase excess solar energy from small generators at a fair market price.

There is much to welcome in the proposals for SEG

• the Microgeneration Certification Scheme is thrown a life-line as the only way to qualify,
• there is to be no requirement for the building to achieve a certain energy efficiency level (EPC), a requirement in FIT that excludes many older and listed properties
• installations that occur after the FIT closes but before the SEG is available will be able to join the SEG as soon as it opens
• export will be metered and not estimated (as in the FIT), rewarding people that install larger systems
• a central database of solar installations will be maintained beyond the FITs
• the high price for bought in electricity compared to the low value of exported will encourage the deployment of battery storage and electric vehicle charging (when compared with other arrangements, for example net metering)

Concerns About the Detail

However, there are two big concerns with the proposals as they currently stand:

1. Smart metering IT systems are not up to the job at present
2. The reliance on conflicted businesses to set a market price

Smart Metering Systems

At recent Solar Trade Association meetings we were astonished to hear that the SMETS1 smart meters that have so far been installed ‘go dumb’ as soon as you change supplier.  Although second generation SMETS2 smart meters fix this problem, the IT infrastructure that collects the data is not yet ready to a point where this data can be shared between an energy supplier and a separate company that you have signed your SEG deal with.

It would be just like government to say ‘well, we’ve done our bit’ as they launch a completely theoretical SEG scheme, which nobody can use in practice because the billing arrangements are not ready.

That’s why we need something - dare I call it a ‘backstop’ - that makes the SEG work from day one and creates an incentive for energy companies to sort out the IT, rather than having a strong incentive to drag their feet and take as long as possible to prevent the SEG ever happening.

A backstop could look a lot like the export tariff part of the current Feed in Tariff:

• A fixed value, for example £0.04 /kWh
• A deemed export 50% of generation 

This would create a strong incentive for the energy companies to pull out their fingers because they are likely to be over-paying for generation where they cannot meter it.

Setting  a Market Price

Electricity costs vary during the day as supply and demand varies.  The industry would be absolutely delighted if export was paid a fair market price at the time of export – that is a price set between a willing buyer and a willing seller.

The preferred option in the SEG consultation is to simply leave it to the energy suppliers to set the price, with the only control being that the price is higher than £0.00

My concern is that the proposed mechanism will not result in a fair market price, because the companies that are being relied upon have every incentive to keep the amount of solar installed as low as possible.  They are conflicted because every time a household or business installs solar it will buy less power from the energy suppliers.  Setting a higher price for exported energy would make solar a more appealing investment and harm the business models of the energy suppliers.

The energy companies do not meet the requirement of being a ‘willing buyer’ for the power and a fair market price will not result. There is a market failure and government cannot leave pricing the invisible hand of the market – except that it can, it just needs another way.

The ‘market’ already sets a price for electricity – and one that is free from the conflicts set above.  For example market exchange Nordpool publishes day ahead pricing for wholesale electricity on an hourly basis.  These prices could be better taken as the ‘market price’ for electricity between a willing buyer and a willing seller.  Energy companies should be required to purchase from microgenerators at the wholesale market price.

SAP 10 - Big Changes Afoot for Solar

Image: Viridian Solar

A new version of the Government's Standard Assessment Procedure (SAP) for the calculation of energy use in dwellings has been published and it contains a number of changes to the way the impact of solar technologies is assessed. 

The key outputs of the calculation described in SAP are:

Dwelling Emission Rate (DER) - the carbon emissions from energy use to heat the house, provide hot water and power lighting, pumps and ventilation. It is expressed in kgCO2 per square metre of floor area per year.

SAP Score - a figure rating the energy costs normalised by floor area to heat the house , provide hot water and power lighting, pumps and ventilation. A house with a score of 100 has energy cost of zero each year, a house with a score of 0 has huge energy costs. The scores from 0-100 are divided into bands corresponding to EPC ratings from 'A' to 'G'

Fabric Energy Efficiency (FEE) - the space heating requirements for the dwelling in kWh/m2

Energy Consumption per Unit Floor Area - which can exclude plug-in appliances (as the above measures do), or include an allowance for appliances and electrical equipment.

Not Just for New Build

SAP is used to calculate the energy efficiency of newly built homes to meet Building Regulations. New homes must currently have a Fabric Energy Efficiency and Dwelling Emission Rate below a mandatory maximum.

Through Reduced Dataset SAP (or RdSAP), the calculation is also used to generate Energy Performance Certificates for existing properties. Over time the SAP rating of homes has become embedded in a range of government initiatives and incentives, for example EESSH in Scotland requires that all social rented homes in Scotland achieve a minimum SAP score by a certain date, and access to preferential Feed in Tariffs are linked to the house having an EPC rating higher than D.

What's Changed?

Carbon Intensity of Grid Electricity

The electricity grid has decarbonised with the move away from coal burning power stations and greater input from gas fired generation and from renewables (see my blog on this subject here). The proposal is to reduce the carbon intensity of electricity from 0.519kgCO2/kWh in SAP 2012 to 0.233 kgCO2/kWh in SAP10.

Discussion

This is a huge (55%) reduction compared to the current version of SAP, and lower than the figure consulted upon (0.398 gCO2/kWh). However, it is only a reflection of the huge progress that has been made in decarbonising the grid.

The impact for solar photovoltaics is that solar systems will need to be more than double the size of current systems to produce the same carbon benefit in the calculation, which could reduce the competitiveness of solar PV as a means of meeting building regulations. SAP10 will only be brought into use for the next update to the Building Regulations, and government will need to carefully consider whether it is now time to change the primary focus of the regulations away from emissions and towards energy consumption (like for appliances).

For example, emissions from mains gas will be 0.210kgCO2/kWh in SAP10. When you take into account efficiency losses from burning gas in a boiler to heat a house, developers will be able to achieve the same dwelling emissions rate using simple electric heating instead of gas - for example panel heaters and a hot water tank with immersion heater. It may be possible to remove the whole wet heating system and gas supply from new homes, yielding considerable construction and maintenance savings, but possibly saddling house buyers with unaffordable energy bills (unless, perhaps, solar is also fitted).

Export of Solar Generated Electricity

The value of exported electricity in SAP10 is 3.8p/kWh, whereas the cost of grid electricity is 16.6p/kWh on standard tariff. In SAP2012, exported solar electricity is assumed to be of the same value as electricity bought by the householder (which was dubiously justified by the existence of Feed in Tariffs - despite that in solar schemes for social tenants the tariffs went to financiers).

SAP2012 also assumed that 50% of generated electricity was used in the house (called the beta-factor) and 50% exported.  While this generally accepted assumption has started to look rather shaky as installed solar systems got larger, it didn't really matter because the value of exported electricity was the same as the saving made for energy not bought from the grid.

For SAP10, a more sophisticated treatment of the beta factor is used. The proportion of energy used in the house is now a function of the size of the solar system's energy output as a proportion of the energy demand. Larger solar systems attached to small energy demands will have a smaller beta factor and smaller solar systems attched to a large energy demand will have a higher beta factor. Adding a battery into the property will increase the beta factor.

PV diverters can also contribute towards energy for hot water in SAP10, so long as a battery is not present and the hot water cylinder has a sufficient volume (more than daily demand). 80% of generation, less the beta factor is available for input to the hot water cylinder, and the benefit is further diminished by a factor of 0.9 to take into account increased cylinder losses due to higher average storage temperatures.

Discussion

None of these changes affect the Dwelling Emissions Rate used for current building regulations. Solar PV saves carbon whether the electricity is used in the house or not.

These changes do, however, impact the SAP score and EPC band, as they impact on the calculation of the energy bills associated with the house.

The calculation of the beta factor was derived from a relatively small data-set, some of which was provided to BRE by the Solar Trade Association. An industry group is working to develop a much more comprehensive set of data to improve confidence in the value that SAP produces and to feed into the Microgeneration Certification Scheme guidance to solar / battery installers.

Shading

The PV shading penalty has been increased, that for solar thermal remains unchanged.

SAP2012 applies the following penalties to energy production - Modest shade 0.8, Significant shade 0.65, Heavy shade 0.5

SAP10 modifies as follows - Modest shade 0.5, Significant shade 0.35, Heavy shade 0.2
As an alternative the MCS overshading figure can be used.

Discussion

Industry were concerned about a complex two-step shading calculation process that was proposed in the consultation, and it seems that these concerns were noted, albeit with what look like penalty default values for systems with shading.

Hot Water Demand

A new, more complex calculation for domestic hot water demand reflects the growing importance of this area of energy consumption as increased insulation levels drive down space heating requirements. This is an area that the solar industry has been lobbying for change.

The new calculation takes into account the higher flow rates and lower inlet temperatures associated with the now more common mains hot water showers (either from pressurised hot water cylinders or combi boilers), when compared with header-tank fed systems.

Inlet temperatures have also been reduced for both header tank and mains fed systems as a result of input from the solar industry (by 2-3 degrees).

Discussion

I calculated a 10% increase in hot water demand for mains pressure fed systems compared to SAP2012. The decrease in inlet temperature will add a further 5% or so to the energy required to heat the water.

An increase in the assumed hot water energy will be welcomed by the solar thermal industry in particular, but higher general energy consumption will aid all energy producing technologies.

Solar Thermal Space Heating

In previous versions of SAP, solar thermal could only be applied to meeting hot water demand, which created an restriction on its potential contribution to household energy demands.

The Solar Trade Association proposed EN 15316-4-3 as a potential route to the inclusion of solar space heating in addition to solar water heating, and provided BRE with guidance and assistance in assessing the new method.

The published version of SAP 10 did not include details of the new method as testing was not complete at the time of publication, so the solar thermal appendix currently has holding text. I will be able to discuss more about how the new calculation works and the results it gives once the final version is revealed.

Discussion

The solar industry will welcome that SAP includes solar space heating. Less for the opportunities it brings in new build (where space heating demands are limited due to high levels of insulation), rather for the possibilities it opens up to improve the EPC ratings of existing properties with high space heating demand. The domestic Renewable Heat Incentive only supports solar water heating at present due to there being no approved method of 'deeming' (calculating) the expectd savings. The new SAP methodology will open up the enticing prospect of solar themal payments under dRHI linked to heat generated for both water and space heating.

Solar By Others

How to Get What you Want and not Get What You're Given

Architects, developers and planning officers often go to exacting lengths to make absolutely sure that they get the look that they're aiming for in a building. For houses that can mean specifying the type of brick, the tile on the roof, and specific styles of windows and doors. Even soffits and guttering do not escape careful scrutiny, selection and specification.

Damn right, too! These materials have a huge impact on the overall appearance of the house and should be defined carefully to preserve the integrity of the design, and the quality and sale-ability of the finished product.

Which makes it all the more surprising to find in developers' design packages a great big rectangle drawn on the roof labelled "solar by others" or "solar by specialist installer".

Surely you know that once you hand over your beautiful, carefully considered design to a Quantity Surveyor, if your specification does not nail down the materials you're looking for, then the words "solar by others" might as well say: "solar - the cheapest you can find, no I honestly don't care what it looks like - yes, I know I was really fussy about the exact make and model of cavity closer, but really, just get what you want for the solar - it's not like anyone will notice it's there."

It doesn't all look this good.  Image credit: ARPower

Solar PV is becoming more and more common on roofs. Incentivised by Feed in Tariffs, more than 800,000 households have now chosen to install solar as a retrofit. Building regulations in Scotland have made solar the norm on new homes and planning conditions in many local authorities (including zero carbon homes in London) also mean new homes are more likely than ever to need solar.

With the coming shift towards electric transport - (the speed of which I predict will take policy makers and energy companies completely by surprise), economies of scale for battery manufacture will drive the availability of cost effective electricity storage, and make solar an even more compelling feature of a mainstream home.

What you Need to Know

The cheapest panels have silver frames, a white backing sheet and polycrystalline cells. Sticking them on a framing system above the roof covering is still (only a little) cheaper than going inline with the roof. If you don't specify what solar you want, this is what you're likely to get.

Here are the choices you face, starting with those that have the greatest impact on 'kerb appeal'.

1. Panel Layout

The number one impact on the overall look of the building is the layout of panels on the roof.  Early design engagement with solar specialists means that cluttered designs fitted around other roofing features can be avoided.  Higher power panels can be selected to achieve energy goals in the most aesthetically balanced way.  See also this guidance on panel design by the  Campaign to Protect Rural England.

2. Frame colour. 

Solar panel frames are most commonly either silver or black. Both have a protective anodised surface finish, but a silver (natural) colour avoids the dyeing process needed to make a black frame so is slightly cheaper.  In most (but not all) situations black frames are considered the most discreet and harmonious choice.


3. Mounting System. 

Panels can be mounted on metal racking above the tiles or slates or conventional roof covering, or they can be sunk into the roof covering (roof integrated), replacing the conventional roof covering and looking more like an intended part of the building design and less like a 'bolt-on'. Many systems use a 'top clamp' arrangement to hold down the panels to the framing, but some systems have hidden fixings, resulting in a less cluttered finish above the plane of the panels.

Roof integrated systems with visible clamps (top) and invisible fixings (bottom) 

4. Backsheet.

A white backing sheet means you can use ever-so-slightly lower power cells in your panel for the same overall panel output (the white sheet reflects light and keeps temperatures a little lower so the same cells perform better). When combined with mono crystalline cells (which are not quite square and have missing corners), a white backing sheet will produce a characteristic pattern of diamonds running up the panel in columns.

Monocrystalline cells (left) and polycrystalline cells (right) in combination with a white backsheet showing the characteristic diamond pattern of a monocrystalline panel

5. Cell Type. 

Polycrystalline cells are sometimes a similar price to mono crystalline cells, but in times of over-supply often seem to fall further and faster. Right now modules based on polycrystalline cells are around 10% lower in cost than those based on mono crystalline cells. In general poly cells will look a bit bluer than mono, and may have little more colour variation across and between panels , but modern cell production technologies can mean that nowadays they rarely show the crystalline pattern that used to be so characteristic of this type of panel.

(More information on the differences between polycrystalline and monocrystalline cells can be found in this blog).

6. Cell Interconnections. 

Some manufacturers hide the bus-bars (silver strips at the top and bottom edges of the panel that electrically connect the cells together, but obviously this also adds cost. Some panels have cells with rear face connections so there's no silver lines visible on the top face of the panel.

How About Just Asking For Roof Integrated Solar?

For sure there are some great looking roof integrated solar systems available. But specifying roof integrated can still result in a wide range of outcomes when you hand it over to the commercial team. This is particularly the case for roof integration systems that give freedom to use any old panel. 

I took the pictures below at the same site and they show two phases of the same development.  The specification called only for "in-roof solar", opening the door to the silver-framed installations in the lower image which meet the letter, if perhaps not the spirit, of the specification. 

Both are roof integrated solar

For something that has such a big impact on the way a building looks, surely it's time for designers to take control of the solar they get, rather than giving the commercial team carte-blanche to go with the cheapest option offered.  

Unfortunately there’s no substitute for carefully choosing and specifying the product you want, just like you do for other building materials.

BIPV

What is BIPV and what are the advantages of this approach to using solar on our buildings?

Building Integrated Photovoltaics (BIPV) are photovoltaic materials that are used to replace conventional materials to form part of a building envelope.  Buildings come in all sorts of shapes and sizes from the smallest homes to huge tower blocks, but photovoltaics is such a versatile technology that as we'll see the potential applications for BIPV are very wide.

Roofing

The most obvious location to put BIPV is the places that get the most sunshine.  This is normally high up the building (to avoid shade) and (in the northern hemisphere) tilted towards the south.  Roofing, roof windows and sloping patent glazing are ideal parts of building envelopes where photovoltaic materials can be used.

As part of a refurbishment of Kings Cross railway station, London, the historic barrel-vaulted roofing was renewed in 2014.  The patent glazing at the top of each arch was fitted with 1,400 glass-glass BIPV laminates by Sundog Energy (now part of Photon Energy).  In glass-glass laminates the silicon solar cells are encapsulated in clear plastic and sandwiched between two clear sheets of glass.  The gaps between cells allow light transmission into the building through the BIPV laminates.  Both sides of the apex were treated in this way, facing east and west.

Kings Cross Station 

240kWp BIPV patent glazing, generating 175,000kWh/year ( 730kWh/kWp)

At the Ziekenhuis (hospital) in Aalst, Belgium, an eye-shaped atrium is formed from sloping curtain walling system with glass-glass BIPV laminates supplied by SAPA building systems forming the entire cladding.  Cells are spaced further apart than in a standard PV panel to create the desired light transmission into the atrium behind, resulting in a beautiful dappled light in the enclosed space.

Ziekenhuis, Aalst, Belgium 

46kWp sloping curtain walling, generating 31,000kWh/year (675 kWh/kWp)

It's not only new build projects that can benefit from BIPV.  The renovation of the Appleton Tower at Edinburgh University included 80 solar PV modules attached to the building with the Schletter Efa facade mounting system by installers Absolute Solar and Wind.

Appleton Tower, Edinburgh University
26kWp facade cladding, generating 19,000kWh/year (703 kWh/kWp)

At a more modest scale, buildings such as homes and offices often have a sloping roof covered with tiles or slate.  Here, more standardised BIPV products are available that replace the tiles and slates.

This house in Cambridgeshire has a patch of BIPV solar interlocking tiles, each tile replacing a row of standard concrete tiles.

Tile format BIPV integrated in concrete interlocking tiles

This roof is on the set of "Desperate Housewives" where Elon Musk launched his BIPV glass slates and tiles.  Details of these products are still emerging as (aside from those of Tesla executives), only one or two homes have been completed with this product.  Early indications are that this is a premium product with a very high price - one of the first customers said the product was "not for financially sensitive people", describing an installation process that took 10-15 people 2 weeks and cost him $100,000.

Tesla glass slates for launch event, Desperate Housewives set, California

A more cost effective approach is to take advantage of the huge economies of scale in solar PV panels that are mass produced in standard formats and figure out a clever way to make them part of the roof covering.  This house near York has Clearline fusion roof integrated solar panels covering the whole roof.  The installer, The Phoenix Works were involved in the design of the new build eco home from an early stage so could work closely with the architect, with stunning results.

Whole roof BIPV roof on a new eco home

6kWp Clearline Fusion roof integrated solar from Viridian Solar

Walls and Facades

Of course as building become taller, the available roof area becomes ever smaller in proportion to the building size.  However, an unshaded south facing vertical wall will still get 70% as much incident light each year as an unshaded south facing pitched roof at optimum angle, and a much larger available surface area on the walls can easily makes up for any shortfall

The very first large scale BIPV project in the UK was a facade system on the Northumberland Building at the University of Northumbria in Newcastle upon Tyne.  BIPV solar was installed to a south facing wall as part of a building renovation in 1994.   The 85Wp BP Solar modules were mounted on frames on a south facing facade, tilted to catch more light and to partially shade the windows below from the high summer sun, so helping the building avoid over-heating in summer.

Northumberland Building, University of Northumbria, Newcastle upon Tyne
40kWp facade and solar shade, generating 25,000kWh/year (625 kWp/kWh)

Fast forward to the present day and the 230m high Heron Tower in London, completed in 2011 has 153kWp of glass-glass laminates built into the south elevation of curtain walling in two great stripes running from street level to the top of the tower in front of the two lift shafts.

Heron Tower, London
153kWp BIPV curtain walling  generating 92,000kWh/year (620kWh/kWp)

As well as being a stunning example of what is possible with BIPV, Heron Tower is, unfortunately something of a cautionary tale too.  For across the road  from the south facing elevation at [[110]]] Broadgate, another tower that will top out at 181m is rising from the ground.  When complete this tower will obscure the entire south elevation from direct sunlight for large parts of the day (see image).

Windows

We've already seen how BIPV curtain walling and patent glazing can be configured to allow light through, but from time to time reports come out of new materials that will enable the creation of BIPV windows that you can see through but which generate electricity too.  It's normally accompanied by a picture of a hand holding a clear piece of glass.  Sometimes the hand has a latex glove on it.  Most of these remain lab curiosities for the present, however thin film solar panels are already available that are partially transparent.  They tend to finish up with a finish that is either orange in colour or smoked.

Bus station at Bournemouth University has two power generating BIPV canopies
20% transparent CdTe thin film modules from Polysolar

Advantages of BIPV

The principal advantage of BIPV is an aesthetic one.  At its best, BIPV alooks like a considered part of the building, rather than a bolt-on.

New home in Sussex with Clearline fusion roof integrated solar replacing slates

BIPV also produces offset costs - the cost of the materials that you would have used if the BIPV was not there plus the cost of fitting it. Sometimes these costs can be substantial, for example in this project where Welsh slate was substituted for integrated solar PV and the saved costs for slate were equivalent to the cost of the PV roofing.


There are also advantages in the ease of ongoing maintenance compared to bolt-on PV. For example access to tiles on roofs. If a tile fails and needs replacement, the task of doing so is made very much more complex and costly if it is behind an above-roof solar PV system. The system must be decommissioned, removed and the tile replaced before the system is reassembled and recommissioned. A job that could have been completed from a ladder now involves scaffolding and electrical works.


It is becoming evident that birds nesting behind bolt-on solar is an issue, especially for domestic installations where the noise nuisance is disturbing. A mini-industry has sprung up to bird proof above-roof solar by fitting wire mesh around the system.

Conclusion

When you take the time to include solar as part of your design for a new building or refurbishment, BIPV means that your solar can be beautiful as well as functional.

The MCS Pricing Mess and How to Fix it

New homes often have smaller solar installations.  Image: Viridian Solar

A government sponsored monopoly raises its fees by 233% .  Cue outrage from the industry, not only from the fact of the raise itself - most people accept that the Microgeneration Certification Scheme (MCS) must live within its means - but mostly from the way it was implemented.  There was no consultation, all was decided by the small, self-elected group who run the scheme.  Little thought had apparently been given to how the change would affect the diverse businesses that rely on certifying their installations to the MCS, and have nowhere else to go for this service.  The transition arrangements were wholly inappropriate.

It's not like they didn't know this was coming.  The consultation to slash the Feed in Tariff was announced in August 2015, at which point it was obvious to everyone that the MCS was facing an existential threat to its income streams, 90% of which derive from solar PV.  This could have been implemented with a lead-in time if the managers of the scheme had acted sooner.

The worst affected are  those that do a large number of low value installations, they are hit disproportionately hard by the £20 increase per certificate.  Businesses providing solar installations to house builders are right at the sharp end.  Solar installations can be as modest at one or two panels - representing only a few hundred pounds' worth of business per house - and when you're in the business of doing hundreds of these each month, those extra £20 sure add up.  To compound their situation, they are installing based on quotations accepted and ordered many months ago, and the contract may be expected to run for many months more.  One business owner estimates that this change has taken more than £100k a year from his bottom line.  Oh, and if you were about to suggest that they should just ask for more money from their housebuilding clients - forget it - that is not how it works in construction.

The other reason for the outrage is that the increase throws into stark relief the many ways that the scheme has failed the industry it purports to be there to benefit.  The purpose of the scheme was to  increase consumer confidence in the new clean heating and electricity generating technologies.  Time and again the scheme has shown itself to be incapable of tackling abuses by the small number of bad apples that have the potential to drag down the reputation of the industry.  People would be more supportive if the scheme had ever bared its teeth and kicked a few companies off the list.

So how to fix this?

If you accept that the MCS needs more income, then you have to accept that prices must rise.  But why must they be the same for every single installation?  The scheme covers 'micro generation' which means systems right up to 50kW in size.

A £35 certificate is a vanishingly small cost for a 50kWp solar installation, which might have a contract value of £50,000.  0.07% to be precise.  On the other hand £35 is a much, much larger proportion of the cost of a small 0.5kWp system on a new home.

To those that say "but the certificate costs the same for the large and the small installation" I say "so what?"

Does my seat on a plane cost the airline the same as my neighbours?  You bet!  Did I pay the same price as they did?  Almost certainly not - especially if I bought mine in a big rush last night and they are more organised and planned ahead.  Does a Gucci handbag cost 1,000 times more to make than an unbranded one.  No chance.  I could go on.

Businesses left cost-plus pricing behind years ago - you price your product at the value someone attaches to it.

A fairer way to apportion the cost of running the scheme is to charge a different amount for a certificate based on the size of the system that is being certified.  By way of example, I'm going to propose how it could work for solar PV - similar approaches could be applied to the other technologies covered by the scheme.  I don't have access to the MCS figures on installation size and number, so I'll use the Feed in Tariff (FIT) statistics to illustrate the concept.

The table shows the number of installations registered with the Feed in Tariff in the 12 months to July 2017, and the number of MWp installed, split by the FIT tariff bands.  If the MCS had been charging £35 per installation, it would have netted £1.25m of income from the 35,815 installations.

If, instead, a certificate had cost £10 per kWp installed, the scheme would have netted £1.315m - a very similar number.

I've just used a straight £10/kWp formula - as I'm working with average values.  A formula that had a minimum of say £20, for installations below 2kWp would collect more from the smaller installations, meaning that the increase for the large scale installations could be kept smaller.

Could something like this work better for industry?  What do you think?

Thin Film Solar PV vs Silicon Wafer - Which is Better?

A guest article by Dr KT Tan cuts through the marketing to find out



Figure 1 (Source: Jethro Betcke, Oldenburg University, Germany)

Thin film solar PV was hailed as the next big thing in solar nearly a decade ago. Then, crystalline silicon wafer (c-Si) cells occupied more than 80% of the market share compared to thin film PV (1). There was a high anticipation in the industry for thin film PV to position itself for a run at c-Si and dominate the market for the near future. However, 10 years on, history shows that not only did thin film fail to conquer the market, but its market share has subsequently declined to only 7% (2).

Obviously, one major factor was due to the collapse of the price for c-Si cells, which quickly wiped off the cost advantages of thin film technologies. This blog is not going to discuss the reasons for this distorted market competition, caused mainly by the exponential expansion of production in c-Si cells, but to question and compare the technical merits of thin film PV versus c-Si.

Do thin film PV technologies have an arsenal of special features to outperform c-Si cells? 



Low Light Performance

The first common belief is that thin film solar PV performs better in low light conditions or diffuse sunlight (for example on a cloudy day). But is this true? The fact that this has been heavily promoted by the marketing guys is because these two technologies do have different spectrum responses to solar light. In other words, their ability to convert solar energy to electricity varies at different wavelengths. In general, the average wavelength in diffuse sunlight is shorter (i.e. more blue) that of direct sunlight – so if you have a spectral response peaking at short wavelengths, e.g. thin film amorphous silicon (a-Si), then you would perform better under diffuse conditions than clear sky conditions.

Figure 1 for shows the different spectrum responses of different solar technologies against the power of sunlight of different wavelengths at sea level at mid-lattitudes of Earth (called AM1.5).  Crystalline monocrystalline silicon (labelled m-Si) is compared against different thin film solar technologies based on amorphous silicon (a-Si), Copper Indium Gallium Selenide (CIGS) and Cadmium Telluride (CdTe).

If you look at Figure 1, you probably would have noticed that not all thin film technologies have the same performance response to differing light wavelengths. Thin film CIGS solar panels, for example, have a broad spectrum response akin to mono-crystalline wafer cells (m-Si), so based on this their performance in diffuse lighting conditions would be little different to m-Si.

Amorphous Silicon has a quite significantly different spectral response to crystalline silicon, with a greater response to low wavelength light.  So how do they compare in field trials? Figure 2 illustrates the results of a comparative study between a-Si and c-Si on a cloudy day. On average, the tests show an increase in energy generated of 15% for a-Si at low irradiance levels below 260 W/m2.  (Note: the tests were published by NexPower, a manufacturer of amorphous Silicon panels)

Figure 2 (Source: NexPower In-house test report)

 

However, performing better on cloudy days is of little benefit if it is combined with performing less well on sunny days (when more energy can be collected). If this were the case then the advantage of thin film PV under diffuse conditions might be a complete red-herring created by the marketing gurus.

A recent research project (3) supported by the Deutsche Bunderstiftung Umwelt (German Federal Foundation for the Environment) compared several solar module types (including thin film and c-Si) under North German Climatological conditions in a side by side trial for a year, and it turned out that no significant difference between the performances of the different type of modules could be found .

Shading

Let’s move on to the second common claim, that thin film PV are more immune to shading effects. There is no magic physics in thin film technologies that make them less tolerant to PV’s number one enemy – partial shading, except that the cells in thin film panels are usually very long and narrow (5 to 10mm wide and the whole length of the panel). In this case, the likelihood of total cell shading is diminished, provided that the installer has correctly oriented the solar modules. Most modern thin film solar modules have further split the narrow cell into multiple sections and incorporated by-pass diodes (4). Nevertheless, if they are not oriented wisely to avoid potential shadows, then it is back to square one (See figure 3).

Figure 3a: Correct orientation to shading           Figure 3b: Incorrect orientation to shading
(Source: Technical Note – Optimising Thin-Film Module PV Systems by SolarEdge)

High Temperature

Finally, how about the claims for superior heat resistance of thin film PV? This is perhaps the only undisputable advantage of thin film technologies – intrinsically, they all have a better temperature coefficient compared to s-Ci (5). In other words, their performance does not degrade as quickly as s-Ci when cell temperatures increase above 25oC.  However, as figure 4 shows, different thin film technologies display a wide variation in temperature response.  Amorphous Silicon (a-Si) is least affected by temperature, whereas CIGS solar panels are very similar in performance to crystalline Silicon.

Fig. 4 Variation of Power Output with Temperature for Different Solar Technologies
 Source: Virtuani. A, Pavanello. D and Friesen. G. Overview of Temperature Coefficient of Different Thin Film Photovoltaic Technologies, 25th European Photovoltaic Solar Energy Conference and Exhibition. 2010, Spain.

A comparative study between amorphous silicon and crystalline silicon suggests the benefit can be up to 20% more output on a hot day with an average ambient temperature of 34oC. See Figure 5. (Note: the tests were published by NexPower, a manufacturer of amorphous Silicon panels).

Figure 4 (Source: NexPower In-house test report)

Although the above result may sound impressive, you may be wondering which parts of the world regularly has an average ambient temperature above 30oC. Unsurprisingly, some research bodies in countries likes, Thailand (6) and India (7), have recommended thin film PV for precisely this reason.

In Summary

Bringing all these factors together, a collaborative research project carried out by Universities of Stuttgart and Cyprus compared thin film PV and c-Si by measuring actual performance over many years in Cyprus (8). The data has obviously taken into account all the differences in spectrum responses and temperature coefficients, the results are summarised in Figure 5.  Data for four years is presented from 2007 (labelled a) to 2010 (labelled d). The clear conclusion from this multi-year side by side test is that thin film modules do not outperform crystalline silicon modules.

Figure 5 Muli-Year Comparison of Solar Energy Yield from Different Technologies
(Source: Reference 8 – page 222)

There appears to be no clear technological advantage for thin-film PV against c-Si at present. In order for thin-film PV to experience a revival, there must be other factors involved which would make thin film PV more attractive than crystalline silicon solar PV. 

For example the homogenous appearance of thin film panels may make them look more appealing.
Thin film solar can be printed on any thickness of substrate and combine with other materials to form see-through graphics, stained glass, company logos, and blinds. With the ability of being semi-transparent, they could even mimic the appearance of natural materials, for example wood or marble.

Needless to say, apart from such niche applications, thin film PV also needs to gain more headroom in cost advantage against c-Si to offset a lower overall efficiency. Until then, it seems like c-Si will stay on top for now.

References:

(1) http://www.marketsandmarkets.com/Market-Reports/thin-film-pv-31.html
Renewable Energy Sources and Climate Change Mitigation: Special Report of the Intergovernmental Panel on Climate Change
(2) Photovoltaics Report by Fraunhoher Institute for Solar Energy System. 6 June 2016www.ise.fraunhofer.de
(3) FLINS Project (www.flins-projekt.de) hosted by Universitat Oldenburg, Germany (http://www.uni-oldenburg.de/en/physics/research/ehf/energiemeteorology/research/former-projects/flins/).
(4) Correspondence with NexPower (www.nexpw.com )
(5) Overview of Temperature Coefficients of Different Thin Film Photovoltaics Technologies by Alessandro Virtuani, Diego Pavanello, Gabi Friesen at 5th World Conefrence on Photovoltaic Energy Conversion, Spain (https://www.researchgate.net/publication/256080289)
(6) Investigation on Temperature Coefficients of three types Photovoltaic Module Technologies under Thailand Operating Condition by P. Kamkird, N. Ketjoy, W. Rakwichian and S. Sukchai. Published on Procedia Engineering 32 (2012) 376 – 383.
(7) Variation of Temperature Coefficient of different technology Photovoltaic modules with respect to irradiance by P. Dash and N. Gupta. Published on International Journal of Current Engineering and Technology, Vol. 5, No. 1 (Feb 2015).
(8) Performance of Photovoltaics under Actual Operating Conditions by G. Makrides, B. Zinsser, M. Norton and G. Georghiou (pages 201 to 232). Third Generation Photovoltaics ISBN 978-953-51-0304-2. March 2012.