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.

Solar PV and Primary Energy in Building Regulations

The Future Homes Standard consultation has proposed that a new requirement based on Primary Energy use should be brought into the next building regulations .  In this article we look at how the Primary Energy use of a house might be calculated, and what is fair or desirable for solar.

Primary Energy is energy found in nature that has not undergone an artificial (man-made) transformation process.  Electricity generated from gas, oil, coal, nuclear or biomass is secondary energy - the original fuel found in nature has been extracted, transported and converted to electricity.  All the way along the process, energy is used or lost.  So to deliver one unit of electricity to your home, a greater number of units of primary energy is consumed.

In an earlier blog I explained the concept of Primary Energy and Primary Energy Factors, see What is Primary Energy.

The Primary Energy Factor (PEF) is a measure of how many units of primary energy are needed to get the unit of final energy to your house.

So, for example in SAP 10.1 (the draft calculation method for the next building regulations) the primary energy of natural gas from the gas grid is given as 1.13, meaning that for every kWh of gas delivered to your house, gas of energy content 1.13kWh needs to be taken out of the ground.  This figure is a weighted average of the PEF for all the different sources of natural gas that make up the UK supply - for example gas extracted from wells in the North Sea, Russia, USA and Qatar.

Primary Energy of Electricity

Electricity is even more complex.

The generation mix includes power stations that use gas, oil, coal, plutonium and wood as their feedstock, each with different Primary Energy Factors once they have been converted in a thermal power station and transmitted across the power distribution network to your consumer unit.

In addition to these thermally generated electricity sources you can add direct conversion renewables such as wind turbines and solar PV panels.  The convention is that, since the natural energy they convert is limitless, the PEF for energy generated this way is 1.0 at the point of generation.

So the electricity generation mix results in an average Primary Energy Factor that depends upon which types of electricity generation are in use at any time.  SAP 10.1 makes assumptions about what the UK's electricity generation fleet will look like in 2020-25 and estimates what the average combination will be in each month of the year.  The average figure through the year for grid electricity at the point of use is a PEF of 1.51.

A home fitted with solar PV panels will generate solar electricity.  At some times the solar electricity will exceed the electricity use of the house and electricity will flow back onto the distribution network where other buildings will use it, so called export.

Increasingly, homes with solar are also fitted with other technology that allows the building to retain more of the solar generated electricity and minimise amount exported to the grid.  Devices include PV power diverters that send excess generation to an immersion heater to heat water in a hot water cylinder, battery storage to keep the electricity generated during the day for use in the evening and smart car chargers that optimise car charging to use self-generated renewable power to the max.

Of these three technologies, SAP 10.1 includes provision for PV diverters and battery storage, but does not give credit when both are used in the same house.

The Primary Energy Use of a Dwelling

The Primary Energy used by a house is calculated by adding up the total of the different energy types used by the house, each multiplied by the Primary Energy Factor (PEF) for that energy type.

The building regulations is rightly focused on the Net Primary Energy use over the year - which takes into account both the flows of energy into and out of the building and allows for energy generation in the building.  Since the solar PV system is part of the building the solar energy flows that cross the boundary consist only of the solar generation exported to the grid.  (See image)

Solar energy generated by the building and used in the building reduces the electricity needed by the building.  So the first benefit of putting a solar system on a house for the net Primary Energy used is the solar energy kept in the building multiplied by the Primary Energy Factor of the electricity use that was avoided - the PEF of grid electricity.

The second benefit of a solar system on a house is that the solar energy exported from the house is a negative flow of energy and should reduce the net Primary Energy use of the house.  The government has suggested in the consultation that the PEF for this exported electricity should be 0.51.

Why not a PEF of 1.0?  This is the primary energy factor for solar generated electricity at the point of generation (before transmission losses).  A query to the team in charge of SAP received this explanation:

Since grid electricity has a PEF of 1.51, and solar electricity has a PEF of 1.0, the net benefit of the exported electricity is 1.51 - 1.0 = 0.51 per unit of electricity exported.

In effect what they're saying is that this unit of solar generated electricity with PEF 1.0 flows into a nearby building and saves that building from using electricity from the grid generation mix with PEF 1.51, so the net benefit (to the grid) of the exported electricity is 0.51

I can see that there is a logic to this, but on balance I think the net Primary Energy of the building can and should consider the building and not the grid - this means that the flows of energy that cross the building boundary are the ones that matter.  Unlike carbon emissions where you cannot apply a carbon saving to the exported electricity without considering the carbon emissions avoided where that energy is used, Primary Energy for solar energy has an agreed value for the PEF, and that value is 1.0.

Grid connected PV feeds into the grid at 1.0, why should microgeneration be treated any differently?

The government is keen to promote the uptake of smart technologies such as PV diverters, smart hot water tanks and battery storage, and so is the solar industry.

Does a lower PEF for exported solar energy not create a stronger driver for the uptake of these technologies in new build?  Surely the lower the PEF of exported electricity the greater the value of installing equipment to use the solar electricity in the building?

The answer to this question is yes, but only up to a point.  If the calculation is set up in such a way that solar starts to look less appealing as a technology to achieve building regulations, housebuilders may not use it at all, and then there will be zero incentive to add in smart technologies that increase solar energy utilisation.

Giving solar export a PEF of 1.0 still creates an incentive to use smart technologies, but without so significantly diminishing the benefits of solar PV.  In addition, there will be a new affordability criteria in building regulations and this will create an incentive to keep energy in the building (saving 16p/unit) rather than exporting it (yielding only 5p/unit).

BRE and DCLG should reconsider the logic behind the treatment of solar PV in the net Primary Energy calculation in building regulations as the approach being consulted upon runs the risk of unfairly under-reporting the benefits of solar electricity.