A New Solar Calculation for Building Regulations and EPCs

 An Assessment of the Solar Energy Calculation in the Home Energy Model

The HEM introduces a new variable - the degree of ventilation of the solar panels

As part of its consultation on the Future Homes Standard, UK Government has revealed details of its proposed replacement of the associated energy calculator.  You can read more about the new 'Home Energy Model' in my earlier blog on the subject.

Alongside the consultation on the Home Energy Model (HEM) a paper was published describing how solar PV generation and the proportion of energy used in the property (self-consumption) would be calculated.

Modelling PV electricity generation and calculating self-consumption within the Home Energy Model - A technical explanation of the methodology

Also available for the consultation is a prototype of the calculator implemented as a web page which can be played with.

Home Energy Model: Future Homes Standard assessment consultation tool

The solarblogger has been busy checking how this new tool will treat solar energy and in this article I'll be sharing my findings.

The Method

The energy output of the solar PV system is calculated according to BS EN 15316-4-3:2017 using the hourly procedure described in the standard.

Inputs to the calculation are:

• rated peak power (kWp) of the solar array under standard test conditions
• location of the house (which selects a climate file with irradiation data)
• orientation  
• tilt angle 
• the area of the solar array, and its height above the ground
• shading (captured as part of the general shading of the building)
• 'ventilation strategy' of the solar panel

This last input captures the difference between above-roof (rack mounted) solar that is rear-surface free, classed as Moderately Ventilated and in-roof (roof integrated) solar which is classed as Unventilated.

Since the HEM is modelled on a half-hour time slice, it can account for real-time variation in the PV generation and the energy demand in the property to estimate how much solar energy is used in the home, or available to charge a battery, divert to a hot water cylinder or export to the grid, as appropriate.  Generated electricity is assumed to be allocated in this order of priority 

1. To meet household demand
2. Into battery storage (until full)
3. To a PV diverter (until the hot water reaches maximum set temperature)
4. Exported to the grid

Consumed electricity is assumed to be taken in this order of priority:

1. From solar PV generation 
2. From battery storage
3. From the grid

Testing

The online tool provided with the consultation helpfully comes with two case studies - a detached house with two bedrooms, 82m2, with a heat pump serving both hot water and space heating and a bungalow with one bedroom, 40.5m2, direct electric heating and hot water.

The detached two bedroom house was selected as the base model and features of the solar PV system were varied and the annual energy generated was derived for each case.  This figure was compared with:

• the solar calculation in SAP 10.2, the predecessor to the HEM
• the solar calculation used for the Microgeneration Certification Scheme (MCS)

Panel Ventilation

The HEM introduces a new variable ignored in both the SAP10 and MCS calculations - the degree of ventilation of the rear of the panel.  Solar PV panel power output decreases with increasing temperature of the panel, so a panel installed with an open back side should produce more energy than the same panel with less ventilation to the rear.

Choosing 'Moderately Ventilated' produced around 2% more energy than SAP10, whereas 'Unventilated' produced 3% less (see graph at top of article).  The difference between Moderately Ventilated and Unventilated - 5% - is in broad agreement with this study by Viridian Solar / Cambridge University into the difference in yield between roof integrated solar and above roof solar which found a difference of 3%.

All the following comparisons are made with the ventilation set at Moderately Ventilated.

Location

The HEM solar yield prediction was compared with SAP10 and MCS at five different locations in England (rest of UK is not offered in the consultation version, which is for English regulations).

The HEM follows SAP10 closely.

Tilt Angle

The annual energy yield from a solar panel in the UK is optimal at around 35 degrees tilt angle from horizontal.  The HEM model follows the shape of the MCS prediction albeit at a lower predicted energy, closer to SAP 10.

Orientation

A solar panel facing south will generate the most energy yield each year in the UK, with progressively less energy the further from south it is facing, though the effect is less pronounced than most people expect due to the very high level  (around 40%) of diffuse light - that reflected from clouds, sky, surroundings - in the UK.

The HEM deviates very significantly from both the MCS and SAP10 predictions as the panel orientation moves further from south.  It starts matching SAP 10 closely when facing due south, but by north facing SAP 10 predicts 64% more energy yield.

This aspect of the HEM model is very concerning and warrants further investigation to check for a bug.

Self Consumption

The output from the software also shows the amount of solar energy used in the property and the amount exported to the grid, so it was possible to derive a scatter plot from all of the results generated in the above analyses and take a look at how the model predicts self consumption.

The plot below shows how the predicted proportion of solar generation that would be self-consumed changes as the size of the solar installation increases.  In this scenario, there is no battery storage or solar PV diverter in the house.

Comparing with some work done previously on SAP10 self consumption prediction shows that the HES predicted self consumption ratio drops more quickly than was the case in SAP 10.  It is worth noting that SAP 10 was based on a very small data set and it is possible that there is more and better data available against which to test the HEM both with and without battery storage.

Conclusion

The testing given to the HEM on its solar energy prediction has only raised one serious red flag - that the modelling of panel orientation looks off and should be checked.

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!