PV Cell Formats and Size Guide

 

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

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

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

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

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

Sources:

https://www.dsneg.com/info/solar-wafer-m12-m10-m9-m6-g1-m4-m-43376010.html 

https://www.dsneg.com/info/new-m10-182mm-x-182mm-monocrystalline-silico-50983227.html 

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

 Could the Benefits of Future Building Standards be Affected?

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

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

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

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

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

How Much Refrigerant is Lost?

The report found that:

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

So if we assume a 20 year life for the heat pump, a charge at commissioning, one recharge and decommissioning at the end of life then the total losses are:

Filling: 0.06kg 

Recharge: 0.06kg

Leakage: 10% x 35% x 3.3kg x 20 years = 2.31kg

Decommissioning: 15% x 3.3kg = 0.495kg

Total Refrigerant Losses over 20 year life: 2.925kg

Refrigerant loss per year: 0.146kg

Does it Matter?

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

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

The authors of the report compared the impact of refrigerant leakage to the benefits of the low-carbon energy delivered by the heat pump.  The annual heating load for a domestic property was assumed to be between 10 and 25 thousand kWh and it was concluded that the impact of leakages was small. 

 

However, the leakage losses from the heat pump does not vary with the amount of heat it supplies.  The more heat the heat pump delivers, the less of a penalty the refrigerant leakage is per unit of heat delivered.  Conversely the more energy efficient is the building and the lower its heat demand, the more of a penalty the leakage becomes per unit of heat.

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

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

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

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

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

Annual Heating Budget (kWh)	2,000	3,000	5,000	10,000	25,000	Gas Heating CO2 emissions for comparison
Annual Emissions from Refrigerant Leakage per kWh of heat delivered (kgCO2e)	0.152	0.102	0.061	0.031	0.012	0.208

 

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

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

Current Refrigerants in Use

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

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

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

 

Company	Model	Refrigerant	GWP	Source
Daikin	Altherma 3 H HT - EPRA014-018DW	R32	675	https://www.daikin.co.uk/en_gb/products/epra014-018dw.html
Daikin	Altherma 3 R - ERGA04-08EVA	R32	675	https://www.daikin.co.uk/en_gb/products/erga04-08eva.html
Daikin	Daikin Altherma - EDLQ-CV3	R410A	2,088	https://www.daikin.eu/en_us/products/edlq-cv3.html
Daikin	Altherma R - ERLQ-CV3	R410A	2,088	https://www.daikin.eu/en_us/products/erlq-cv3.html
Kensa	Shoebox	R134A	1,430	https://uh8ex3jph2xqg0pb4bs7if12-wpengine.netdna-ssl.com/wp-content/uploads/2014/03/TI-Shoebox-heat-pump-v6.3.pdf
Kensa	Evo	R407C	1,774	https://uh8ex3jph2xqg0pb4bs7if12-wpengine.netdna-ssl.com/wp-content/uploads/2017/02/TI-EVO-v6.0.pdf
Mitsubishi	Ecodan R744	R744	1	https://les.mitsubishielectric.co.uk/products/heating/domestic/outdoor/ecodan-quhz-monobloc-air-source-heat-pump
Mitsubishi	Ecodan R32	R32	675	https://les.mitsubishielectric.co.uk/products/heating/domestic/outdoor/ecodan-r32-ultra-quiet-puz-monobloc-air-source-heat-pump
Mitsubishi	Ecodan R410A	R410A	2,088	https://les.mitsubishielectric.co.uk/products/heating/domestic/outdoor/ecodan-puhz-ultra-quiet-monobloc-air-source-heat-pump
Panasonic	Aquarea HT Bi-bloc F Generation	R407C	1,774	https://www.aircon.panasonic.eu/GB_en/product/aquarea-f-generation-ht-bi-bloc-single-phase-three-phase-heating-only-shf/
Panasonic	Aquarea T-CAP Bi-bloc H Generation	R410A	2,088	https://www.aircon.panasonic.eu/GB_en/ranges/aquarea/t-cap/
Panasonic	Aquarea High Performance All in One Compact J Generation	R32	675	https://www.aircon.panasonic.eu/GB_en/product/aquarea-high-performance-all-in-one-compact-j-generation-1-phase-r32/
Vaillant	Arotherm plus	R290	3	https://www.vaillant.co.uk/downloads/aproducts/renewables-1/arotherm-plus/arotherm-plus-spec-sheet-1892564.pdf
Valillant	Arotherm Split	R410A	2,088	https://www.vaillant.co.uk/for-installers/products/arotherm-split-heat-pump-58752.html
Valillant	Arotherm	R410A	2,088	https://www.vaillant.co.uk/for-installers/products/arotherm-air-source-heat-pump-2944.html

 

Conclusions

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

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

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

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

Comparing Building Regulations for Energy - Scotland and England

 

Development of new homes in Glasgow by CCG Homes, Installation by Arc-Tech

New regulations have been published for Part L building regulations in England.

Let's take a quick look at how they compare to Scotland, where almost all new homes now come with solar panels.

The building regulations work by defining a specification for a 'Notional Dwelling' - read this article for more detail of how it works

In the table below I have shown a selection of the more significant requirements for each.  It is evident that not only do both Notional Houses include solar PV, but that the England requirement is much higher.

Housebuilders do not have to build the Notional House specification, but they must equal or exceed its performance.  The scope for reducing the solar provision by increasing the insulation performance of building elements such as walls, roofs and windows (collectively often referred to as fabric measures)  is limited in the new English regulations, which equal or exceed the fabric requirements in Scotland.

‘Notional Dwelling’ Summary Specification Selected Requirements
	Scotland 2015 (Gas Package)	England 2021
U Values in W/m2K
External Walls	0.17	0.18
Floors	0.15	0.13
Roofs	0.11	0.11
Doors	1.40	1.00
Windows	1.40	1.20
Air Permeability	7 m3/h.m2 at 50Pa	5 m3/h.m2 at 50Pa
Heating	Gas boiler 89% efficiency	Gas boiler 89.5% efficiency
Heat Emitter Type	Regular radiators	Low temperature heating 55°C flow
Heat Recovery	Instantaneous Waste Water 45% efficient	Instantaneous Waste Water 36% efficient
Solar PV in kWp	Smaller of (a) Total floor area (m2) x 0.01   (b) 0.3 x roof area based on 30 degree roof pitch x 0.12	40% of dwelling floor area/6.5
Example solar PV for 85m2 house with two floors	0.85kWp	2.6kWp

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.