Talking to the Council

Friends, I’ve written a couple of articles in recent months about issues to do with my local Council.

• Notes from Paradise
• What a Council Web Site should say.

My interactions with local Government have led me to sympathise with Winston Churchill’s comment:

‘Many forms of Government have been tried, and will be tried in this world of sin and woe. No one pretends that democracy is perfect or all-wise. Indeed it has been said that democracy is the worst form of Government except for all those other forms that have been tried from time to time.…’

But nonetheless, last Tuesday (20 February 2024) I gave up my evening for the opportunity to address Richmond Borough’s Environment, Sustainability, Culture and Sports Committee on the issue of how hostile they are to people who want to install heat pumps.

This page contains links to all the relevant documents and also to a recording of the webcast of the event.

The Previous Item on the Agenda

The previous item on the agenda concerned the outsourcing of the contract to run the Borough’s leisure centres, most particularly the swimming pools.

[Aside: Personally, my sole New Year’s resolution for 2024 had been to break through the borough’s extensive security and go for a swim. They don’t actually have a sign saying “Just go away!” outside the swimming pool, but that’s only because they probably haven’t thought of it yet. In Richmond, the days of just ‘going for a swim’ are long gone. It now requires registration, booking days ahead, with on-line card payment in advance. I have now been swimming several times and all it requires is for me to catch my breath and allow the hostility of the booking system to wash over me.]

The Council discussion was extensive with perhaps six or so members of the public registered to speak (for 3 minutes) along with councillors. They were concerned that things would get worse when out-sourced – while simultaneously expressing dissatisfaction with the current state of affairs. In the end it seemed to come down to a conflict between council officers (who spoke of ‘the market’ and ‘business opportunities”) and everyone else (who basically didn’t trust the council officers). Historically the Borough grossly mismanaged development of Richmond Ice Rink and the open air pool on Twickenham Riverside – both now sadly gone. I found it curious to see democracy in action – it’s very slooooowww.

The Report on The Borough’s Climate Emergency Strategy

So we finally reached the agenda item to which I would speak: Item 7: the report on the Borough’s Climate Emergency Strategy – Annual Report 2024.

When I read through the document I wondered how – in the face of the dire emergency in which we find ourselves – anyone could write such self-congratulatory bullshit. There is not a hint of self-criticism in the whole document. This is despite such gross errors as claiming that the council’s use of electricity now has zero associated carbon dioxide emissions, something which is just not physically possible. Even if the council had bought their own wind farm or solar park, there would still be associated carbon dioxide emissions! The inclusion of this kind of gross error in the report indicates to me that no one at the council has a clue about the physical reality of climate change.

I spoke fourth and last after:

• A worker for the Habitats and Heritage Scheme
• An architect asking for a lifting of the requirement of prior acoustic reports on heat pump installations.
• A person who thought that Absolute Zero (not the temperature – but absolute zero emissions) would mark the end of civilisation as we know it.

I spoke on the issue of increasing the rate of heat pump installations in the Borough. My spiel starts at 2h:04m:30s on the webcast. And this is what I meant to say.

“My name is Michael de Podesta and I am resident of the borough. I am Fellow of the Institute of Physics and a Chartered Physicist. From 2000 to 2020, I was senior scientist at the National Physical Laboratory and on behalf of NPL, and in a private capacity, I have been speaking in public on the issue of Climate Change since 2004.

• The idea of ‘net zero’ is Physics not politics.
• The Earth is currently warming at approximately 0.2 °C every decade.
• If we do not achieve ‘net zero’ emissions of carbon dioxide, the Earth will continue to warm indefinitely.
• If we do achieve ‘net zero’ emissions of carbon dioxide, the Earth will stop warming.
• That’s the choice: Continued warming? Or not.

However…

• The Climate in which we all grew up, is already gone, and nothing we do will ever bring it back or cool the Earth.
• Crucially, the longer we take to achieve ‘Net zero’ emissions of carbon dioxide the hotter the Earth will get in the meantime.

Net Zero in this Borough

• Achieving ‘net zero’ requires action in every borough of every city in every country on Earth. Including this one.
• Richmond Borough has a declared goal of achieving ‘net zero’ by 2043 which is just over 4,400 working days away. To achieve this goal, every gas boiler in the borough must be decommissioned – that’s about 10 per day – and most will be replaced with a heat pump.

Heat Pumps and Net Zero

• For most borough residents, the single-most important action they can take to reduce carbon dioxide emissions is to remove their gas boiler and to replace it with a heat pump.
• The Council has an essential role to play in facilitating heat pump installations at the most rapid rate possible. But currently the Council’s attitude is actively hostile to those who are trying to install a heat pump.
• I would like to urge the committee to end this hostility and instead work with local architects and heat pump installers to create ‘template installations’ for the most common types of property in the borough. These should spread best practice, facilitate planning approval, and accelerate the borough’s progress to ‘net zero’.

Discussion

After a short clarification, councillors and council officers then proceeded to congratulate themselves on their great report. I think my contribution to proceedings made close to ‘net zero’ difference to The Council’s mindset. I had felt very nervous as I waited to speak, and afterwards I felt rather deflated.

A little later, Councillor John Coombs spoke (at 2h:21m:16s on the webcast) to say that he – like many people – was mortgage free and elderly – and that installing a heat pump would result in no “return on investment” in his likely lifetime.

I found this attitude despicable. This person – likely amongst the richest 1% of people of Earth – was stating that he would not do a single thing to reduce the harm from carbon dioxide pollution unless he could personally make a profit from it.

His comments put me in mind of the comment from Kurt Vonnegut that “We’ll go down in history as the first society which wouldn’t save itself because it wasn’t cost-effective”

After Councillor Coombs spoke, I left the meeting, and I don’t think I’ll be returning. The room was full of well-meaning people – but the Council processes have no mechanism for negative feedback. And negative feedback is the essential process that any system requires in order to reliably steer itself towards its goals.

5 reasons heat pump installations have problems

 

Friends, it is a tragic fact that although heat pumps are a tremendous way to heat a home, not every installation is a happy one. In this article I thought I would look at the 5 main causes of unhappiness:

1. The MCS heat pump survey overestimates the heat pump size required.
2. Heat pump siting.
3. The primary pipework is too narrow.
4. The secondary pipework is too narrow.
5. The radiators are too small.

I have written about all these issues previously, but I thought it would be helpful to bring them all together under one heading.

There are, of course, other reasons that heat pump installations go wrong. There can be a fault with a heat pump itself, just as there can be with any piece of technology. Or the controls can be programmed incorrectly. But, the five faults above are systemic – and recovering from them can be expensive, so it’s good to look out for them in advance.

Click on the image for a larger version. Schematic showing the 5 causes of poorly performing heat pump installations. #1 Over-sizing on the MCS heat loss survey. #2 Poor siting of the heat pump. #3 and #4 Undersizing of the Primary and Secondary pipework. #5 Undersizing of the radiators.

1. The MCS Survey

The Microgeneration Certification Scheme (MCS) are the de facto gateway to the government’s £7,500 grant towards the installation of a heat pump. An MCS standard heat loss survey is required and the grant can only be awarded for an installation in line with the recommendations of this survey. Unfortunately the survey is capable of misinterpretation that leads to widespread overestimation of the heat pump size required.

In one recent case, a family of my acquaintance in Twickenham made daily measurements of their gas usage during the cold weather, and found that the maximum gas usage was 192 kWh/day, which corresponds to 8 kW of continuous usage. The MCS surveyor ignored this data and stated that the property required at least 16 kW of space heating, which they recommended should be met by two 12 kW heat pumps. Their recommended installation was certainly between two to three times too large. They were charged £450 for the privilege of receiving this piece of nonsense.

My – admittedly-limited – experience is that this is very common. It arises for two reasons. Firstly the MCS guidelines make unrealistically pessimistic assumptions about a property. Specifically:

• The U-values assumed for brick walls can be unrealistic – by a factor as large as 1.6 (U = 1.3 W/m^2/°C or U = 2.1 W/m^2/°C.)
• The number of air changes per hour is never actually measured, but usually instead overestimated – often by a factor of 3 or more.
• The temperature assumed for the underfloor cavity and for party walls can be unrealistically cold.

Click on image for a larger version. Excerpt from a standard heat loss assessment. Making different defendable estimates for the fabric of the building can result in a factor 3 difference  in the estimated heat loss. If no check is made against actual energy consumption then there is no way to know which one is correct. (Source The Suburban Pirate)

Surveyors are free to use whatever figures they deem reasonable – but they need to be able to defend their choice.  But many surveyors instinctively ‘oversize ‘just to be safe’. But if they make all these mistakes – which MCS permits them to – they will come up with the wrong answer.

Click on the image for a larger version. Screenshot from a calculator showing the effect of different assumptions about the number of air changes per hour (ACPH). For a household with a volume of 200 cubic metres assuming 1.5 ACPH adds an additional heat loss of 1 kW compared with a more likely estimate of 0.5 ACPH. Spreadsheet is downloadable here. Spreadsheet downloadable from here . (Typo in figure and caption corrected on 10/2/2024)

The second reason MCS surveys get it badly wrong is that surveyors are not required to check the outcome of the survey. If the survey is correct then it ought to directly predict the client’s existing gas consumption. By failing to check their own calculations surveyors make wild errors that cost their clients money.

There are worse things in life than an oversized heat pump, but a customer of such an installation is unlikely to be happy. Firstly they will have spent a lot more money: in the example above this could be as much as £10,000 too much. And since most of the year the heat pump will be operating well below its maximum heat output, the heat pump is unlikely to be able to reduce its output sufficiently to match the client’s need for heat, and will instead simply switch on-and-off all the time.

Click on image for a larger graph. Chart showing in the number of days per year (in Teddington) that the given percentage of full heating power is required. Typically the power required is less than 60% of peak for for more than 300 days per year. Data from this article.

In the Twickenham example above, the family nearly found themselves with 24 kW of heating power when they would only need more than 5 kW on 50 days each year.

You can guard against these errors by (a) Using the Rule of Thumb to estimate the heat pump size required or (b) measuring gas consumption on the coldest day (or group of days) each year. Using these safeguards you can check the workings of your MCS surveyor. You shouldn’t need to, but institutions like MCS are locked-in to the existing procedure, despite the widely-known basic errors in the methodology.

Click on image for a larger graph. Chart based on a survey of heat pump installations on Heat Pump Monitor.org The chart shows the ratio of the MCS estimated coldest day heat loss, to the actual heat load on the coldest day of the year so far. About 20% to 30% of the heat pumps are ‘about the right size’. But over half are more than 50% too large and some are almost 300% too large. In short: oversizing is a widespread problem. Data from this article.

2. Heat Pump Siting

Heat pumps ‘feed’ off fresh air and they need a continuous supply. They draw in air at the ambient temperature, extract energy from the air, cooling it by typically 3 °C, and then expel the air.

Click on image for a larger version. Left: A well-sited heat pump. The cooled air (shown as blue) expelled from the heat pump is not drawn back into the heat pump inlet. Right: A poorly-sited heat pump. The cooled air (shown as blue) expelled from the heat pump can be drawn back into the heat pump inlet.

If the heat pump is sited in a confined space then it is possible for some of the cooled, expelled air to be drawn back to the intake. This means the heat pump has to cool it further in order to extract energy. If this happens to a small extent, it will not be critical. But in very confined spaces, a large fraction of the expelled air may be drawn back into the heat pump and a viscious cycle may then ensue with the ‘frost pocket’ around the heat pump getting colder and colder.

This should never happen, but it does occasionally. In the sketch above, I’ve drawn a 2-D view of a heat pump. If this were situated in a passage way between houses – a common location for heat pumps – then there would nearly always be a flow of air along the passage way – into or out of the plane of the screen – and the expelled air would be taken away.

It is only in dead ends – such as my own installation(!) – that one needs to look out for this. In my case, I have done smoke tests that shows that (in my case) this is not too bad a problem.

3. Primary Pipework

One of the most basic errors of a heat pump installation is failing to use large enough pipes to connect the heat pump to the radiator network. This connection is the so-called primary pipework. If this pipework is too long or too narrow, then the hydraulic pump (i.e. the pump that moves water through the pump) may not be able to build up sufficient pressure to drive enough water into your home to heat it.

Click on image for a larger graph: Graph showing the relationship between the flow rate of water from the heat pump to the house when the returning water is 5 °C cooler than the out-flowing water. The required the flow rate (in litres per minute) is roughly three times the heating power required (in kilowatts). (Spreadsheet)

The rate at which water must flow through your heat pump – and house – is directly proportional to the amount of heating required. For example, if the water returning to the heat pump from your house is 5 °C cooler than the water leaving the heat pump (i.e. dT = 5 °C), then:

• A heat pump delivering 4 kW of heating requires a flow rate of roughly 11.5 litres/minute
• A heat pump delivering 6 kW of heating requires a flow rate of roughly 17 litres/minute
• A heat pump delivering 8 kW of heating requires a flow rate of roughly 23 litres/minute.
• A heat pump delivering 10 kW of heating requires a flow rate of roughly 29 litres/minute.

If the primary pipework is 28 mm copper tube (inner diameter 26.2 mm) then to put 29 litres/minute through 10 metres of pipe (5 metres each way) this handy calculator suggests the pressure required is ~4,000 Pa. This amounts to just 0.6 p.s.i or 0.04 bar. However if one tried to deliver water at the same rate through 15 mm tube (inner diameter 13.6 mm) then that would require a pressure of ~92,000 Pa (13.5 p.s.i. or 0.9 bar). This would require almost 50 W of power just to move the water. So the lesson is simple: use large diameter pipework for primary pipework. For smaller heat loads, 22 mm diameter pipes may suffice, but for heat loads above (say) 5 kW, use 28 mm pipework.

Click on image for a larger graph: Graph showing the way the pressure required to pump water through a circuit (the so-called ‘pressure head‘) varies with flow rate for different pipe diameters.  The data are calculated for 10 metres of pipework and water returning  5 °C cooler than the out-flowing water. Also shown as vertical lines are the flow rates required for different heating powers. A pressure of 10,000 Pa corresponds roughly to 0.1 bar or 1.5 p.s.i.

In my own installation, the heat pump was installed near the wall outside the place where the gas boiler was previously located. Without understanding this issue, I asked the installer to re-use the existing 22 mm pipework inside the house: this seemed economical and neat to me. The installer agreed and it works OK, but only because the heat load in the house is so small. The heat load is 3.5 kW when it is -5 °C outside so the flow rate required is just 10 litres/minute (600 litres/hour) at dT = 5 °C. Without the external wall insulation, a larger (7 kW) heat pump would have been required and the flow rates would have been proportionately higher, and the primary pipework would definitely have needed upgrading.

4. Secondary Pipework

When installing a heat pump, the installer can choose the size of the primary pipework. However it may not be possible to replace all – or indeed any – of the internal (secondary) pipework. This pipework connects the primary pipework to the radiators (and hot water tank) and is typically of unknown diameter and structure, although an experienced installer will be able to make an educated guess.

In an ideal world, during an installation, the floorboards would raised and pipework checked and possibly altered. The occupants of the dwelling would stay away either at their pied-a-terre in Mayfair, or their cottage in Devon. But – and this may shock many readers – some people installing a heat pump do not have a second home. Yes! Really! And they are extraordinarily reluctant to let installers lift up their floorboards and bring chaos to their lives – even for just a day or two. Consequently, installations are often made ‘blind’ without knowing for sure the length and diameter of the secondary pipework. How should an installer cope with ‘blind installation problem’?

The least complicated solution is to simply connect the primary pipework to the secondary pipework. In this case the installer is relying on the hydraulic (water) pump within the heat pump to move the water through the secondary pipework. If all is well, then the pressure ‘head’ required will not be too large and the system will work well. The installer can check there is sufficient flow before completing the installation.

A second common tactic is to install a so-called low-loss header: this is illustrated below. A low-loss header (LLH) consists of a short length of very large diameter (~15 cm) tube connected across the primary pipework. This presents a very low flow impedance to the water flowing from the heat pump, and this ensures that (primary pipework permitting) the hydraulic pump in the heat pump will be happy. Side ‘ports’ allow a secondary circulation pump to draw off water from the heat pump and circulate it around the central heating circuit.

Click on image for a larger version: Illustration the operation of a low-loss header (LLH). The heat pump delivers water to the LLH at 50 °C and after flowing through the LLH it returns at a temperature of about 47 °C – corresponding to delivery of heat at a rate of roughly 6.5 kW. The secondary pump draws water from the hot end of the LLH, and increases the water pressure to drive water through the radiator system – in this example it pumps 20 litres/minute with the return temperature being 5 °C colder than the flow – corresponding to heating rate of roughly 6.5 kW. This is close to a balanced system. If the flow rates and dT’s in the primary circuit and secondary circuits are not well-matched (or balanced) then this leads to inefficient operation and a lower COP.

Installing an LLH separates the flow problem into two parts:

• The low flow impedance of the LLH ensures that the heat pump can easily supply high flow rates and high heating power. So the heat pump is more or less guaranteed to work well.
• The use of secondary pump allows the pumping power to be tailored to the flow impedance of the secondary pipework and radiators.

So installing a LLH is a kind of insurance policy – it makes a successful installation more likely, but it comes at a cost of extra components (an LLH and a pump), extra work, and it is likely to not reach quite as good a COP as a system without an LLH.

In my own installation an LLH was installed and I wondered about its effect for a couple of years and then in the summer of 2023 I plucked up the courage to have heat pump ninja Szymon Czaban remove the low-loss header so I could see what effect it would have. It seems to have made very little difference.

Click on image for a larger graph: This complicated graph shows how two quantities have varied week-by-week through the year. In red, shown against the left-hand axis you can see how the COP varies through the year. In blue, shown against the right-hand axis you can see how the heat pumped per day (averaged weekly) varies. 50 kWh/day corresponds to about 2 kW continuous usage so the heating power is very low. The numbers in red boxes and the red dotted line show the seasonally averaged COP. The removal of the low-loss header appears to have made almost no difference.

This does not mean that the original installer did a bad job: on the contrary. Prior to the installation and during the installation things were still very COVID-y. So when trying to get an installation done with poor prior knowledge of the home, using a LLH and adjusting the secondary pump correctly was a smart move. And it resulted in an installation that worked straight away, and was no worse than an installation without an LLH.

5. Radiator size.

Many people are aware that one of the consequences of switching from heating with a gas boiler to heating with a heat pump is that “you have to have all your radiators replaced“. In fact, it is much more common that you may need one or two radiators replaced. But getting the right size radiators is really important for getting the best out of a heat pump installation, and it really isn’t that expensive.

Heat pumps capture heat from outside your home and deliver it into your home, but you only have to pay for the electrical energy used to operate the heat pump. The ratio of the heat delivered to your home (kWh) to the electrical energy used to operate the heat pump is called the Coefficient of Performance (COP). A good heat pump installation can operate with a COP of 4. Operating with a high COP means that you get all the heat you need, but it costs you less. And the main thing which determines the COP is difference between

• The outside temperature: there’s nothing you can do about that!
• The temperature to which the water flowing through your radiators must be heated to deliver sufficient heat to home. This is where having more radiators or bigger radiators or radiators with more fins makes a difference.

At an efficiency of 85%, a boiler can easily heat water to as hot as 70 °C, and water flowing through a radiator at 70 °C, might cool to 50 °C as it radiates (and convects) at a rate of (say) 1 kW, warming the room. A modern heat pump could do the same, and with an efficiency of ~200% (COP ~ 2). But if the heat pump flowed water continuously through the same radiator at a much lower temperature – perhaps just 40 °C, then it might radiate (and convect) only (say) 500 W into the room – but it might do so at an efficiency of maybe 350% – saving you money – and reducing emissions of carbon dioxide.If the radiator size – by which I mean its frontal area facing the room – were doubled, or the number of fins or panels attached to the radiator were increased, it might once again radiate 1 kW – but at a lower flow temperature, and a higher COP. The physics of radiators is discussed in this article.

A proper heat loss survey will work out the maximum required heating power in each room, and for a given maximum flow temperature – and hence COP –  work out whether the existing radiators need to be replaced. The spreadsheet accompanying this article does a calculation for a whole dwelling assuming all the radiators are of the same type – this is illustrated below.

Click on image for a larger version: Three screenshots from the radiator calculator in the accompanying spreadsheet. In the first screen shot, the calculator shows that even with 20 m^2 of Type 1 (Single panel radiators) it is not possible to provide 7 kW of heating with a flow temperature of 40 °C. However 7 kW could be provided by 8 m^2 of Type 22 radiators or 6 m^s of type 33 radiators. If only 5 kW of heating was required, then 17 m^2 of Type 1 radiators could provide that output. If the flow temperature were increased to 50 °C, then only 11 m^2 of type 1 radiator would be required, or just 3 m^2 of type 33 radiators.

In my own installation, I opted to retain my existing radiators, and this has more or less worked. I did this against the recommendation of the installer because I just wanted to see if it would work. The graphs below show how the flow temperature varied through the month of January 2024 – and how that change affected the COP.

Click on graph for a larger version: Graph showing how my heat pump installation responded to changes in external temperature during the 30 days following 19th December 2023. Notice that as the external temperature falls (to a minimum of – 5 °C) the flow temperature in the radiators rose to 45 °C – maintaining an internal temperature around 20 °C.

Click on graph for a larger version: Graph shows the flow temperature in the radiators as in the previous graph, but also shows how the COP falls as the flow temperature rises. The grey dots are the COP averaged hourly and the green line is the COP averaged daily. The average COP for the above month was 3.3 including daily heating of DHW and weekly Legionella cycles.

In the coming summer I plan to change a few of the remaining radiators that have no convective fins, and I hope this will allow me to lower the flow temperature further, this increasing the average annual COP.

Summary

Friends, I have (as seems to be the case rather often these days) written too much!  But as I read this now I think I have more-or-less covered the 5 main causes of unhappiness with heat pump installations, including my own experience of all them, and made notes on how you can avoid them.

1. The MCS heat pump survey overestimates the heat pump size required.
2. Heat pump siting.
3. The primary pipework is too narrow.
4. The secondary pipework is too narrow.
5. The radiators are too small.

Somehow having a heat pump installed is still much more of a palaver than it ought to be: But I hope you have found this helpful, and I wish you good luck with your endeavours.

If you think I have missed something or mis-stated something, please do let me know.

Acknowledgement

Friends. I would just like to acknowledge the help of Andrew Cunningham in developing my ideas for this article, particularly as they relate to dodgy MCS heat loss survey guidelines. Thanks Andrew.

 

Local Councils and Climate Change

Friends, I love the phrase “Think Global, Act Local“. In the UK, it perfectly describes the important role of local councils in facing up to climate change.

I had cause to reflect on this role earlier this week when I spoke at an event organised by Kingston Council: a “bite-size” retrofit event  at which members of the public could meet with ‘retrofit’ installers and advisers. It seemed to be a happy affair and for anyone who was (or wasn’t) there, you can download the Powerpoint slides from talk here. There will be a larger Kingston’s Efficient Homes Show on 18th May 2024 but the booking arrangements don’t seem to have been published yet.

It was good to see suppliers and ‘punters’ coming together, but nothing about the event – except possibly my talk – implied that there was any urgency at all about getting things done. This is despite the fact that Kingston Council declared a climate emergency in 2019 and set a target of 2038 for the borough as a whole to reach net-zero emissions of carbon dioxide. Richmond Council gave themselves 5 more years. But as I wrote previously, this requires installing heat pumps and solar panels at scale right now. But – in my opinion – the attitude of the councils towards the people they serve is completely inappropriate. It’s as if they just don’t know how to be helpful.

So rather than just moan, I thought I would follow on from my previous words on this subject by describing what councils could do. Everything in green below this paragraph is what I think should be on the council’s web sites.

Please don’t concentrate on the details: these are just ideas off the top of my head of things which councils could do: a daydream. But do please notice the general tone. It’s a tone which shows awareness of the urgency of our situation, and the need for a radically different approach.

We need our Councils to imagine what a sustainable future might look like rather than trying to preserve the past. In the case of Richmond upon Thames and Kingston upon Thames, these boroughs will be regularly and inevitably flooded in the coming centuries unless we collectively take drastic action now. And then the ‘character’ they are trying to preserve with their restrictions on heat pumps and solar panels will be utterly worthless.

====================

Principles

Following on from the borough’s declaration of a climate emergency in 2019, a review of progress in 2024 revealed that our current policies are not on track to meet our goal – or even get close to it. In fact, the review identified the council as a major hindrance to progress on climate goals. Despite our public proclamations, we were guilty of what the Review Team called Institutional Passivism through which we failed to respond to the magnitude of the problem we face. We are now resolved to become “part of the solution” rather than continuing to be “part of the problem.” 

The review team reminded us that until the world as a whole reaches net zero emissions of carbon dioxide, Earth’s temperature will keep rising. And even after we reach net-zero, Earth will not cool for thousands of years. So we have already lost our stable climate, but by achieving net zero as rapidly as possible, we can still minimise the damage caused, and the chances of catastrophic changes. This is a global challenge but it requires a local response. Changing climate will affect every one of us, even in our comfortable borough. And so this Borough – like all towns and cities across the UK and the world – needs to play its part.

The task that we face is immense. Familiar habits will need to change and that will be uncomfortable for all of us. The following policies have been arrived in conjunction with the Scientific Review team, and represent a cross-party response to this threat. 

1. Methane (Natural Gas)

The burning of methane gas for heating and cooking is not compatible with net zero emissions and so the council have banned the installation of any new gas boilers in council-maintained buildings, including schools.

Cooking with gas will be stopped in council-maintained properties. Borough engineers will be retrained to install electric only replacements for boilers and cookers.

Because we anticipate that the gas network will not be needed beyond 2043, the council have withdrawn permission for all but emergency renewal of gas mains in the borough. This will result in reduced congestion and the road maintenance required after gas works.

2. Heat Pumps.

The carbon dioxide emissions from heating homes are currently the largest single source of carbon dioxide emissions in the borough. The task of changing heating from gas to clean electrical heating is enormous and we have been slow to start: it is imperative that we now begin to make rapid headway on this task.

It is clear that in order to meet our net-zero target, by 2040 the majority of homes in the borough will need to be heated with heat humps. As a council we cannot compel people to install heat pumps, but we can do everything we can to make it as easy as possible for people to install them. 

As part of this, the council has removed all restrictions on the installation of heat pumps with a rated heating power less than 12 kW. Any heat pumps with an accredited sound power output (EN 12102, EN 14511 LWA, A7/W55 of dB(A) 63 dB or below requires no further proof of suitability. This applies for both air-to-water and air-to-air heat pumps. We understand that residents may be unsure about installing heat pumps, so we have established a regular Saturday ‘heat pump clinic’ at which residents can ask questions of installers and council experts.

Additionally, in order to minimise duplication of effort in solving common problems, the Council have identified the 10 most common dwelling types in the borough and worked with local architects and installers to identify solutions to problems that people often encounter when installing heat pumps in these dwellings. These are:

• Terraced houses (Small)
• Terraced houses (Large)
• End-of-terrace house
• 1930’s style semi-detached homes
• 1930’s style Detached homes
• Edwardian town houses
• Basement Flats
• Ground Floor Flats
• First floor flats
• Second floor flats – and above

We have begun testing these ‘template’ solutions and if you would like to be part of our test program, please contact the Council. 

3. Solar Panels & Batteries.

The use of solar photovoltaic panels (Solar PV) is one of the technologies which will gives us a chance to create a sustainable way of living in the coming century. We are aware that some may consider solar panels unsightly, in this emergency we feel it essential to minimise carbon dioxide emissions as rapidly as possible. Adding solar panels does not harm a building and if in 50 years time the climate crisis has been solved, the panels can be removed and the original look and feel of the building restored.

Consequently, all  restrictions and guidance on the visual appearance of dwellings fitted with panels have been withdrawn. Residents are at liberty to fit as many solar panels as they choose – subject to District Network Operator (DNO) approval. The council have begun work with the local DNOs to establish the optimum location of neighbourhood batteries. These batteries will reduce the borough’s net draw on the UK grid network, and enable a higher fraction of solar generation to be used locally.

Starting immediately, all new housing in the borough will be required to generate at least 10 kWh/year of PV electricity for each square metre of floor space in a dwelling. Designs which cannot meet this criterion will not be considered.

Similarly, any development of a dwelling in the Borough that requires planning permission for any reason, must include a solar PV installation.

Any retail developments that include car parking for more than 50 cars, must incorporate a solar canopy for solar generation.

4. Insulation, draught-proofing, and secondary glazing.

Insulation and draught-proofing are among the simplest and most cost-effective was to reduce heating demand, and thus carbon dioxide emissions. Many draught-proofing solutions can be fitted by home-owners themselves. The Council have produced videos showing how to fit draught-proofing to the most common types of doors and windows in the borough.

No matter the architectural or historic merit of a dwelling, it’s residents are entitled to live comfortably. So the council is lifting all restrictions on the use of External Wall Insulation and secondary glazing in all buildings in the borough. These developments will be at the discretion of the owners. Examples of good practice have been developed for the application of EWI in typical homes in the borough. 

5. Public Transport and Electric Vehicles

Vehicles powered by combustion engines of any type are not compatible with Net-Zero. The Council’s own fleet of vehicles will electrified within the next three years. Following on from this, all Council suppliers and contractors will be required to use electric vehicles when working on Council contracts.

The borough will continue to develop safe cycling and walking infrastructure, and together with our already excellent public transport, we anticipate this will form an increasing part of resident’s travel within the Borough. But some use of personal cars will still be required. In line with Government targets, we anticipate that an increasing fraction of resident’s vehicles will be electric vehicles (EVs).

To meet the charging needs of residents who do not have access to an EV charging point, we will expand further our lamp-post chargers, and residents will be able to use a council-approved cross-pavement cable slot to charge cars near to their homes. Additionally from 2025, all businesses with a car park for more than 20 vehicles will be required to provide EV charging facilities for customers.

6. Schools

The burning of methane gas for heating and cooking will be banned in schools. Schools will transition to the use of heat pumps for heating, and school roofs and canopies will be used for solar PV installations. 

7. Drains

The more intense rainfall events predicted are already with us, and poor drain maintenance in the Borough has led to widespread temporary flooding. In order to minimise this, an improved regimen of drain clearing and street cleaning will be implemented to maximise the capacity of existing drains to avoid local flooding. 

Summary

The Borough Council exists to serve its residents. As the Review Team identified, the Climate Crisis will deepen in coming years in ways we cannot yet fully anticipate. The Council’s resources are limited and most of the steps required to transform the Borough into a zero-carbon Borough will need to be taken by residents themselves using their own money. But the Council is fully resolved to use all its powers to enable the Climate Actions that our residents are eager to undertake. 

Estimating the rate of air changes in a dwelling

Friends, Happy New Year! Over the Christmas break I have managed to get back to doing some experiments – playing with candles, weighing scales, and gas cylinders. And in doing experiments I have remembered the pleasure of experimental “play”, free from the tyranny of having to produce results.

I have been experimenting with estimating the rate of air change in my home, by use of a carbon dioxide concentration monitor. I wrote about this previously, but was stimulated to re-visit the topic by a comment from a Twitter friend: (Peter Miller) who suggested using candles as a quantifiable source of CO2.

Regular readers, may know that I love candles, and it seemed delightfully quixotic to be able to do some actual measurements by ‘dosing’ CO2 into the home at a known and measurable rate by burning candles. Someone else then suggested using cartridges of CO2 that are available for rapid re-inflation of bicycle tyres.

So I began a series of experiments using candles and CO2 canisters and a CO2 meter which records readings every 5 seconds onto a micro-SD card. But 10 days in, I have realised that I have proverbially  “bitten off more than I chew“. And so rather than plough on, I am going to pause my experiments, and summarise what I have learned so far. And hopefully by writing the summary, the future of the experimental program at Podesta Towers will become clearer to me.

Why Air Changes per Hour (ACPH) matters

Everyone knows that a draughty home is not a comfortable home. The number of Air Changes per Hour (ACPH) quantifies this effect. And knowing the ACPH for a dwelling is useful for making a retrofit plan.

Let’s suppose a dwelling has a volume of (say) 200 cubic metres and has 1 ACPH. This means that every hour, 200 m^3 of air from outside (at maybe 0 °C) enters the house and has to be heated up to (say) 20 °C. The heat capacity of air is around 840 joules per cubic metre, so this represents a heating load of 200 m^3 × 20 °C × 840 J/m^3 = 3.36 MJ per hour or about 930 W – just under a kilowatt.  With 3 ACPH this becomes 2.8 kW which is a very significant heat load – and financial burden.

At the same time, if the number of air changes per hour is low, then the concentration of CO2 can build up to levels around 2,000 ppm at which level people will likely feel drowsy. And air quality – in terms of particulates and unpleasant smells – will likely suffer.

I designed a spreadsheet to show the balance of these two effects.

• ACPH calculation spreadsheet

Click on the image for larger version. This is a screenshot from the spreadsheet for calculating CO2 concentrations in homes of different volumes, different occupancy and different numbers of air changes per hour (ACPH).

But despite being very important, ACPH is a difficult quantity to measure, and consequently in practice it is never measured! Instead, in the few cases where people make any measurements, they measure a different quantity which is related to ACPH by an unknown factor, which may be between 20 and 50.

Really? Yes. The most common measurement is a so-called door blower test in which people attach a giant fan to the door of a dwelling which sucks (or blows) air out of (or into) the dwelling, recording the rate at which the pressure inside the dwelling falls (or rises). From the air flow at 50 pascals of pressure difference between the inside and outside of a dwelling, the number of ACPH is inferred by dividing by a factor 20. [Aside:In case you are not familiar with the pascal unit, 50 Pa is a very low pressure difference – the difference between the air pressure at the ground and the air pressure at a height of 5 m].

The door-blower test is a good way of comparing the air-tightness of one dwelling with another, but simply does not measure the thing one wants to know! A more modern so-called ‘Pulse’ technique achieves a similar measurement more quickly by exploding an “air bomb” inside a house and measuring the transient changes in pressure. Once again this is a good way of comparing the air-tightness of one dwelling with another, but simply does not measure the thing one wants to know!

As Persily & de Jonge point out in their landmark paper, simply measuring the concentration of carbon dioxide in the air has been recognised since the time of Lavoisier as an indicator of indoor air quality. The idea is simple: as people go about their usual business in a dwelling, they breathe out carbon dioxide, increasing the concentration of carbon dioxide in the indoor air. As the air is exchanged with outside air, the concentration of carbon dioxide in the indoor air decreases. If the rate at which people produce CO2 is known (the subject of Persily & de Jonge’s paper) then the ACPH can be estimated.

This was the subject of my last article on this subject and I thought I had said all I had to say: but then Peter Miller suggested using candles.

Candles and Canisters

Persily & de Jonge’s paper addresses the variability of the rate of human carbon dioxide emissions with age, gender and activity level. So unless one characterises the age, gender and activity level of all the people in one’s home, it can be hard to estimate the ACPH.

However, as this excellent paper on the physics of candles makes clear, a candle burns steadily for many hours and produces carbon dioxide at a rate of 1.71 litres of pure CO2 per gram of wax burned. So a candle can become a standardised source which releases CO2 at a known rate – around 10 litres per hour for a typical candle – not so different from the roughly 14 litres per hour that a lightly-active adult creates.

Alternatively, a canister of CO2 containing 16 g of carbon dioxide (stored in liquid form at almost 60 atmospheres!) can be used to rapidly inject about 8.7 litres of CO2 into a dwelling.

So using a combination of candles and canisters, might it be possible to characterise the ACPH of a dwelling by measuring the carbon dioxide levels, but with no uncertainty from the number, age, gender and activity level of the occupants? The answer is definitely “Yes”, but the optimum procedure is not obvious. Let me show you results from some of my experiments and then I’ll discuss possible sources of uncertainty, and my difficulty in working out what a workable procedure might look like.

Click on image for a larger version: The tools of the trade: candles, CO2 cylinders, discharge device, weighing scales, and a CO2 meter.

#1: Candles in a room

One of the first tests involved burning a candle in a room with dimensions 4.0 × 3.1 × 2.9 m i.e. a volume of 28.9 cubic metres or 28,900 litres. I positioned the CO2 meter on the other side of the room from the candle and set it to record overnight. The next day I lit the candle, let it burn for a few hours and then extinguished it. The data looked like I anticipated it might – but rather better than I had hoped for!

Click on image for a larger version. Graph showing the measured concentration of CO2 (ppm CO2) in a room versus time (hours). The concentration of CO2 fell overnight, rose when the candle was lit, and fell when it was extinguished.

The overnight fall in CO2 concentration arises from air changes with the outside air and with air in the rest of the house.

After igniting the candle, one expects the CO2 concentration to at first rise linearly. Then, as the concentration rises, one expects the natural air changes with the rest of the house and the outside to replace some of the CO2-rich air with air at the background concentration – causing the initial linear rise to slow down. Eventually, the concentration will stabilise when the rate of emission of CO2 from the candles is just matched by the rate of removal of CO2-rich air. The expected trajectory of the concentration has a well-known form. By matching the standard form to the data one can estimate the rate of emission of CO2 (litres/hour) and the rate of exchange of air (ACPH). I’ve described the mathematics at the end of the article.

Click on image for a larger version. Detail from the previous graph showing the time around the ignition and extinguishing of the candle. The graph shows the measured concentration of CO2 (ppm CO2) versus time (hours).

Based on the rate of loss of wax (established by weighing the candles) I anticipated that the concentration of CO2 would initially rise at about 350 ppmCO2/hour. When I analysed the CO2 concentration data I found that the initial rate of rise of CO2 concentration was 346 ppmCO2/hour. This level of agreement (within ±1%) is remarkable given that one prediction is based on weighing and analysis of the chemistry of candles, and the other is based on readings from a device measuring the transmission of infrared light through the air. It suggests to me that CO2 meter is reading at least roughly correctly, and that CO2 from the candle is mixing reasonably well in the room. It suggests that the technique might be capable of giving estimates of ACPH with low uncertainty.

The data looked like I expected it to: a linear rise, curving over, peaking when I extinguished the candle, and then falling in a similar fashion. Based on the analysis, it seemed that the number of ACPH in our front room was around 0.28. But I noticed that the initial rise had a small delay: I thought this might be because it took a few minutes for the CO2 from the candle to mix in the room. And I noticed that after I extinguished the candle, the CO2 concentration fell immediately – perhaps I left the door open too long?

Click on image for a larger version. Graph showing the time around the ignition and extinguishing of two candles in our front room. The graph shows the measured concentration of CO2 (ppm CO2) versus time (hours).

The next day I tried the same experiment with two candles and obtained similar results. This time the data suggested that the number of ACPH was 0.22, slightly different from the value of 0.28 from the previous day.

The following day I tried using a single night-light candle which I knew would self-extinguish after about 3 hours. There was good agreement between predicted and measured initial rates of rise of CO2 concentration, but now the ACPH appeared to be 0.60 – much higher than estimated previously. Additionally, I could also estimate ACPH from the nearly steady state concentration being 244 ppm higher than background just before the night-light self-extinguished. This analysis suggested ACPH of around 0.75.

I thought these larger estimates might be an artefact of the fitting because I took the measurements at what were initially higher CO2 concentrations than the background level – and so the baseline concentration was probably falling at the same time as the candle was burning.

Click on image for a larger version. Graph showing the time around the ignition and extinguishing of a single “night-light” candle in our front room. The graph shows the measured concentration of CO2 (ppm CO2) versus time (hours).

At this point I thought that the analysis was valuable – but it was taking a long time to make the measurements, I was leaving a lighted flame unattended, and the results showed a higher variability (between 0.22 and 0.75) than I thought was really the case.

#2: Canister in a room

So after a suggestion on Twitter, I tried some experiments discharging 16 g CO2 capsules. These capsules are remarkable, storing the CO2 as a liquid under approximately 60 atmospheres of pressure! Devices are available which can pierce the sealed canister and discharge the gas – which emerges rapidly and is very cold – with relative safety.

In the first experiment I discharged a single CO2 canister in the front room. By weighing the canister before and after discharge, I calculated that the cylinder had contained 15.1 g of gas (rather than the nominal 16g), which corresponds to 8.25 litres of  pure CO2.

Click on image for a larger version. Graph showing the time around the injection of CO2 from small gas canister into the front room. The graph shows the measured concentration of CO2 (ppm CO2) versus time (hours).

Based on the volume of the room (28,900 litres), I expected the CO2 concentration to rise almost immediately by 285 ppm to about 835 ppm. But in fact the concentration increased to 890 ppm – which suggests that the ‘cloud’ of CO2 rich gas reached the sensor before fully mixing with the total volume of air in the room. Analysis of the decay curve suggested that there were 0.76 ACPH.

#3: The whole house?

In general, people want to know the ACPH for an entire dwelling, and not just a single room. But how to distribute a CO2 dose uniformly to an entire dwelling? I tried an experiment in which I lit 6 candles for a period of 2 hours and distributed the CO2 around the house by using an array of 4 fans positioned to make sure ‘dead ends’ of the house were exposed to the CO2.

However I didn’t have enough fans to distribute the CO2-enriched air into and out-of the 3 bedrooms and the loft. So I closed all the upstairs doors to roughly exclude them from the experiment. The resulting volume was approximately 200 m^3 and I lit 6 candles to produce a sizeable signal in this larger volume.

Click on image for a larger version. Graph showing the time around the ignition and extinguishing of six candles with the resulting CO2 ‘plume’ being distributed around the whole of the ground floor by four fans. The graph shows the measured concentration of CO2 (ppm CO2) versus time (hours).

The data looked more or less as I expected. Based on weighing the candles, I expected that the CO2 concentration would rise at approximately 293 ppmCO2/hour, but the CO2 meter registered an initial rate of rise about 20% higher than this (348 ppmCO2/hour). This suggests that the volume estimate and the CO2-mixing strategy are almost, but not quite, right.

The fit to the data suggested 0.53 ACPH and extrapolation to the limiting value suggested 0.45 ACPH, a pleasingly low discrepancy.

In the graph above, the ACPH is estimated by analysing data in just the rising part of the curve. But the time constant of that fit also describes well the first part of the declining curve after the candles were extinguished. But after four hours, my biggest fan (my wife 🙂 ) returned to the house and the background level of CO2 increased.

I then thought I would try recording data over a long period, and try to model all of the changes – due to people entering and leaving the house – and any candle-burning or canister-bursting experiments.

Click on image for a larger version. Graph showing the 20 hours of CO2 concentration measurements (ppm CO2)  in the house. The red dotted lines show attempts to analyse the data. The sharp peak at 14 hours is due to the release of 2 CO2 cartridges – this is shown in detail in the next figure.

The results from Test 7 show a recording of CO2 concentration on the ground floor of Podesta Towers over 20 hours starting roughly at midnight. They show the decline in CO2 concentration on the ground floor as my wife & I slept upstairs. Then there is a rise as first my wife and then I got up, and then a decline after my wife left for work. Then at 12-ish I discharged a CO2 cylinder, and then just before 14 elapsed hours, I discharged two CO2 cylinders in quick succession. After 16 hours, I left the house and returned later.

My aim had been to model all these changes – as shown by a dotted red line – assuming a single value of ACPH. But I became overwhelmed by the complexity of the process. For example, the first cylinder discharge barely shows up on the data. The second double-discharge (see below) shows up, but the back ground levels before and after the discharge are clearly not the same.

Click on image for a larger version. Detail from the graph above. Notice that the CO2 levels change before and after the pulse for reasons that are not obvious.

Summary

My experiments have convinced me that there must be a way to use measurements of CO2 concentration in homes to meaningfully assess ACPH. But I am not sure of the best way to proceed: these are the things on my mind at the moment.

• The number of actual ACPH will likely change with external factors such as wind speed and direction. It may also vary with internal factors such as the use of fans.
• Ideally a measurement would not inconvenience people living in a house. This favours measurements made either (1) quickly, or (2) overnight when living areas of a dwelling are likely unoccupied or (3) while people are present. Each option has advantages and disadvantages.
  1. This might involve a canister discharge and CO2 measurement at multiple locations to ensure the dose of CO2 is uniformly distributed.
  2. This technique uses the occupants to ‘dose’ the living areas of the house, and the overnight decline in CO2 could be used to estimate ACPH if the sleeping areas are separated from the living areas.
  3. Perhaps the statistical parameters of CO2 measurements: Average, Peak, Minimum, Maximum and minimum rate of change – could be used to estimate ACPH.
• Candles should really be inside a fire-proof candle holder.
• The technique is limited by the need to have CO2 uniformly distributed around the volume being assessed. Usefully, this can be restricted to just a single room, but it is difficult to extend the technique over multiple floors.

So I have lots of things on my mind – but I have written 3,000 words already, and so for now, I just need to stop! Please do share any suggestions you have!

Mathematics

The image below shows the functional form used to model the transient changes in CO2 concentration.

Solar PV: Review of 2023

Friends, the long northern hemisphere nights give one a chance to reflect on the wonder of technologies which can ‘bottle the Sun’. Each dark day I dream idly of the Sun’s return in the coming spring, but in more practical moments, I have been collating the statistics describing another year of PV generation and battery storage.

The System

For those readers who have not been paying attention, our solar PV system consists of:

• 6 × 340 W-peak roof panels facing 20° south of West.
• 6 × 340 W-peak roof panels facing 20° south of West.
• 5 × 390 W-peak roof panels facing 20° north of East.
• 3 × 390 W-peak roof panels on a flat roof ’tilted’ to the South East.

Click on image for a larger version.

System#1 (comprising the first 12 panels) was installed in November 2020 and so this is its third complete year of service, and System #2 (comprising the last 8 panels) was installed just one year ago in November 2022. The 5 roof-mounted panels in System #2 face 20° north of east, and so only generate significantly in summer: recall that in the UK the Sun rises 40° north of east at midsummer. The panels are connected via two Solis 3.6 kW inverters to a 13.5 kWh capacity Tesla Powerwall 2 which was installed in March 2021.

Powerwall Battery

The first graph shows the average amount of charging and discharging from the battery through the months of the year – the data are averaged monthly, but expressed as average kWh per day. It’s clear that the battery is worked harder in winter where it is basically fully-charged and fully-discharged once on each winter day.

Click on image for a larger version. Graph showing the amount of electricity stored in and then discharged from the Tesla Powerwall 2 each month since March 2021. 

Over a year the battery is used to store and discharge around 3,500 kWh of energy. If we consider the battery cost (about £10,500 in March 2021) and imagine a lifetime of about 10 years, then we can imagine that every unit of electricity drawn from the battery – even apparently “free” electricity from the solar PV system – actually costs about 30 p/kWh in terms of battery “wear and tear”. If we more realistically assume that only 30% of the battery capacity will be lost after 10 years (see below) and that the battery as a whole will remain otherwise viable, then the cost per kWh falls to about 10 p/kWh.

It’s also interesting to note that the amount of energy discharged from the battery is always less than the amount of energy with which it is charged. The ratio of these two quantities is known as the “round trip efficiency”.  Inefficiencies arise because the AC electricity in the mains must be converted to DC electricity for storage in batteries, and the reverse process takes place when the battery discharges. Also, the basic physical processes of charging and discharging are never 100% efficient.

The Powerwall specifications suggest that the round trip efficiency is 90%, and if we evaluate this for the 30 months for which we have measurements, we see that in the best cases, 90% is just exceeded. It’s interesting to notice that the round trip efficiency has a seasonal variation – being close to 90% in winter, but falling to about 85% in summer.

Click on image for a larger version. Round Trip Efficiency. Graph showing the ratio of the energy discharged from the Tesla Powerwall 2 to the amount of energy with which it was charged. The data are evaluated each monthly since March 2021. The Round Trip Efficiency appears to be between 85% and 90%.

I think this seasonal variation occurs because – as shown in the figure below – in winter, the battery does not hold the charge for long: almost as soon as the battery has been charged overnight, it is immediately discharged. In contrast, in summer the battery is often fully-charged by midday, and the charge is stored for perhaps 8 hours before the discharge process begins. As I have written previously, this storage actually consumes energy, and I think this is showing up as an apparent loss in round trip efficiency.

Click on image for a larger version. Screen shots from the Tesla App for a typical winter day (left) and a typical summer day (right). In winter the battery charges at night and discharges through the day, running flat late in the evening. In summer the battery charges when the sun shines and the household runs on solar energy most of the day, only drawing energy from the battery at night.

Finally we should look again at the inevitable degradation of battery capacity from which all batteries suffer. I assess battery capacity by recording the amount of energy discharged on days when (a) there is very little or no solar charging of the battery and (b) the battery fully discharges from “full” to “empty”: these conditions occur mainly in December and January.

Click on image for a larger version. Estimates of the practical capacity of Tesla Powerwall 2 battery over the last three winters. The decline in capacity from 21/22 to 22/23 was 3.5%, but the decline in capacity from year from 22/23 to 23/24 has been only 1.9% (so far).

The decline in capacity from 21/22 to 22/23 was 3.5%, but the decline in capacity from 22/23 to 23/24 has been only 1.9% (so far). Extrapolating – and assuming linear trends! – this suggests that battery capacity may fall to under 11 kWh in 2030. If the other parts of the battery system continued to work in 2030, this would still represent a useful battery capacity.

Click on image for a larger version. The same data as in the previous graph but plotted on a larger scale. Linearly extrapolating the last two years of data suggests that battery capacity may be less than 11 kWh in 2030.

Solar PV

Below are several graphs showing solar PV generation through the year. Of particular interest this year is the extra generation from the System#2 panels installed in November 2022. The conventional month-by-month chart shows the expected summer boost to generation. As I remarked back in April, March 2023 was exceptionally dull – with generation down on March 2022 despite the addition of 8 extra panels! December 2023 likewise appears to have been similarly grey. Happily, June 2023 was exceptionally sunny.

Click on image for a larger version. Solar PV generation averaged monthly and expressed as average kWh/day.  See text for details.

From a performance point of view, I find it more useful to plot the cumulative generation through the year. The graph below shows cumulative generation throughout each of the last three years. The 2021 and 2022 curves show the typical variability between years – alongside the forecast generation based on the MCS recommended procedure. The 2023 curve shows the boost of 1.8 MWh arising from the extra System#2 panels. If we divide the cost of the system by the anticipated generation (assuming (optimistically) 20 years of flawless performance) the price of the solar electricity comes out at about 8 p/kWh.

Click on image for a larger version. Cumulative solar PV generation throughout each of the last 3 years.

In the graph below I compare this year’s cumulative generation (5.7 MWh) with cumulative household consumption (6.2 MWh) showing that on a whole-year-basis, the solar installation is not quite large enough to produce enough generation to match consumption. If the year had been sunnier, and if since August we had not been charging an EV at home (800 miles at 4 kWh/mile ~ 0.2 MWh) we might just have broken even.

But of course the timing of consumption and generation are quite out of phase. The graph below shows that for the middle portion of the year we consume no grid electricity, and were net exporters from August until late November.

Click on image for a larger version. Cumulative solar PV generation for 2023 compared with cumulative consumption, grid exports and imports.

Summary

Friends, the Solar PV and battery systems are performing as well as I might reasonably have hoped. So my current plan is to just monitor the performance of the system as they and I grow old together.

There are options, but without really shocking the neighbours, I can’t see a way to generate any more electricity locally. I might consider investing in more batteries, but our annual bills are only around £400, and so there is not much more money that we can save. But if prices were low enough, having extra battery capacity might allow us to avoid emptying the batteries in winter, and this would probably prolong their life. Some more entrepreneurial people might begin to try to export electricity for profit, and I wish them well – but that’s not my goal.

Click on image for a larger version. Estimated monthly costs for heating, cooking, and electricity for the last two years. 

From April 2023 the Ripple wind farm at Kirk Hill should begin generating and our share of that generation amounts to about 3.5 MWh/year which should cover all our grid consumption with low-carbon generation, and be more in-phase with our consumption than our solar generation. I am not sure quite how I will account for the generation, but it’s good have some problems to look forward to in the New Year.

2024: Reasons to be cheerful

Friends, I often feel deeply anxious about the future – most particularly with regard to global warming and the legacy of a changed climate that we are leaving to our children. Perhaps you feel that way too.

But there is also good news, and occasionally it is necessary to expose oneself to this good news in order to lighten the darkness within. Please allow me to share some of the good things which are happening.

UK Electricity

In the future we will electrify almost every aspect our lives, and so greening UK electricity is a critical process. And as we manufacture and deliver more and more items using green electricity, so the carbon dioxide emissions associated with almost every thing we do will fall.

Progress is reasonably good: the graph below shows the average carbon intensity of UK electricity since 1998. For the last few years emissions have been stuck at around 0.23 kgCO2/kWh, which is 50% of the historic coal-dominated value of 0.46 kgCO2/kWh. But this year, with new wind farms and solar parks and battery installations, the value has fallen to about 0.20 kgCO2/kWh.

Click on image for a larger version. The chart shows the average carbon intensity of UK electricity since 1998. Data is from MyGridGB and includes the embodied carbon of the generating plant.

The values on this chart include the embodied carbon dioxide in the infrastructure: the more widely-reported marginal carbon intensity is lower than these figures by approximately 0.045 kgCO2/kWh. It’s clear that we are making reasonable progress.

And here are a couple of highlights: In 2023 according to the Grid Analysis Web site, in 2023, cumulatively we generated 3.4% more electricity from renewables than from fossil fuels.

Click on image for a larger version. The chart shows the average generating power (GW) of each category of renewable or fossil fuel generation. Data is from UK Grid Live.

Did the grid collapse? Were there power cuts? Was  there panic in the streets? No. It all passed off smoothly. And I am sure that this process will continue for several more years, with the grid just getting greener and greener.

And another aspect of our greening grid is that on 21st December 2023 we achieved a new record for a peak in wind generation of 21.8 GW. But we have almost 100 GW of generation in the construction “pipeline”, so what is exceptional today will become an everyday event in a few years time.

There is no shortage of problems to address as we continue to reduce the carbon intensity of UK electricity, and as we approach 100% renewable contribution these problems will only become more difficult. But the changes that have happened already have happened with – to me – shocking speed.

UK Heating Policy

Friends, I genuinely do not know why right-wing politicians hate heat pumps. But this year has a seen a concerted campaign in The Telegraph to suggest everything from the laughable idea that Heat pumps don’t work in the UK to the idea that Heat Pumps are somehow part of a Big Government plan to increase Tax.

Click on image for a larger version. A few stories from The Telegraph this year – just search Google on “The Telegraph Heat Pumps”.

But despite the bluster, what has actually happened on the ground is that the heat pump grant (the so-called BUS or Boiler Upgrade Scheme) has been increased to £7,500 from £5,000. For many homes, companies (such as Octopus) can install a heat pump which can replace a gas boiler for less than the cost of the upgraded grant making the installation practically free, or certainly cheaper than a gas boiler. So I think 2024 is likely to a good year for heat pump installations even though we are a long way from the installation rate we really need.

And critically, the idea of using Hydrogen for home heating has finally been defeated.

There is no shortage of problems to address as we continue to reduce the carbon emissions associated with heating the UK, but the changes that have happened this year – despite appearances – have actually been positive.

Globally

I don’t want to write a comprehensive review of the state of ‘climate action’ world wide, but there are signs that epochal changes are underway.

Click on image for a larger version. A story from The Guardian in June this year reporting record renewable production in China.

In China, production of wind turbines and solar panels is expanding at a rate such that it would be hard to imagine that it would be physically possible for them to grow any faster.

And world-wide production of batteries for energy storage and electric vehicles (EVs) is increasing at a breathtaking rate, as is progress on both improving battery performance and re-cycling them when they reach their eventual end of life.

The production of Electric Vehicles is accelerating in every vehicle category and the average selling price of an EV is falling. This growth is often sneered at by many ultra-greens who consider use of any transport technology other than bicycles to be tantamount to advocating for the destruction of the Amazon rainforest.

But such ultra-greens are mistaken: massive growth of EVs is critical to the success of our global energy transformation. Firstly EV production displaces ICEV production which is pushing companies that make ICEVs to the edge of collapse. Even marginal falls in ICEV sales push conventional manufacturers close to bankruptcy as they struggle to repay their massive debt. This makes it clear to investors and governments that the ICE age is over.

Click on image for a larger version. The graph on the left shows the change in vehicle sales (in millions) between 2022 and 2015 for a range of vehicle manufacturers. ICEV sales are generally falling. The chart on the right shows the Altman Z score for manufacturers showing imminence of bankruptcy. Data is from Clean Technica

And as EV sales grow, petrol sales will eventually peak and begin to decline year-on-year reducing emissions. And as EV use for deliveries increases, the carbon dioxide emissions associated with every thing we use and eat will fall also.

Summary

Friends, not one thing I have said should be taken as suggesting that we are not in a serious situation: the increase in global temperature this year has been especially shocking. But focussing only on the problems we face can bring on despair which both feels terrible and helps nobody.

And there are genuinely many positive developments both here in the UK, and worldwide. These are changes that would have been scarcely believable in 2004 when I first began speaking in public about Climate Change. If you are – like me – inclined to occasional despair, can I urge you to occasionally take a look at these positive developments.

Anyway: I wish you and your families best wishes for the New Year to come. I am hoping for more good news!

Michael

Variability of heating demand throughout a year.

Friends, as I drift off to sleep at night, I often reflect on the subtle wonder of heat pumps, and last night I imagined this article in my head. However writing the article has proved more difficult than I had envisaged in my sleepy reverie.

There are lots of technical details, so in case you need to leave early, the question I wanted to answer was this:

• How many days a year does a heat pump (or indeed any other kind of heater) need to work at full power to keep a dwelling warm? And what fraction of the time does it need work at, say, half-power?

The answer for the region around Heathrow Airport is shown in the graph below.

Click on image for a larger version. Based on 3 years of daily data, the graph shows the typical number of days per year that the given fraction of full heating power is required. Full power is the power required to keep the dwelling at 20 °C on the coldest day in the last three years. Adding up the individual data points, one can see, for example, that for typically 69 days per year, the required heating power is between 40% and 60% of the maximum. Heating power between 80% and 100% is only required on 12 days per year.

This graph is useful because it allows one to estimate the costs of running a gas boiler or a heat pump throughout the whole year. And it also one to assess how a battery can be used with a heat pump to reduce running costs during the winter.

So let’s see how to calculate the graph above.

1. To begin

The first step is to estimate how the heating demand varies through the year. To do this I downloaded daily heating degree-day (HDD) data from degree days.net for my neighbourhood airport, Heathrow, for the last 3 years. I used the 16.5 °C HDD data because this corresponds roughly with the heating demand for a dwelling kept at 20 °C.

Click on image for a larger version. 3 years of daily heating demand data based on the meteorological station at Heathrow Airport UK, just 10 km from my home. You can see the two cold spells we had last winter (22/23) in December and January.

I then divided each data point by the maximum heating demand – which was 18.8 °C. I then expressed the heating demand as a fraction of this maximum.

Click on image for a larger version. 3 years of daily heating demand data based on the meteorological station at Heathrow Airport UK, just 10 km from my home. Data are expressed as a fraction of the maximum heating demand which occurred in January 2023.

In this graph the data are just the same as in the first graph, but the vertical axis is now labelled by the fraction of maximum heating demand. Looking at this graph we can see that there are:

• Lots days in which heating demand was less than 10% of maximum,
• Just a few days in which heating demand was greater than 90% of maximum
• Lots of days where heating was in the range between 10% and 90% of maximum.

This graph applies to any kind of heater, gas boiler or heat pump – it’s just describing the variation in heating demand through the year that must be met in order to keep a dwelling at the same temperature.

2. Let’s make a histogram

Fascinating as the above graph is, it does not tell us what we want to know! We want to be able to estimate the fraction of the time that the heat pump operates in the range between (say) 10% and 20%, or in the range between (say) 50% and 75%. To work this out we need to re-structure the data. Here are some graphs of the re-structured data with the heating power divided into twenty 5% bands: 0-5%, 5.1% to 10%, …..95.1% to 100%.

Click on image for a larger version. Histogram of 3 years of daily heating demand data based on the meteorological station at Heathrow Airport. See text for explanation.

Each point on the graph shows the percentage of the heat demand in each band. So for example the above graph tells us that:

• 22.4% of the time the heating demand is less than  5% of full power.

Or considering several bands together,

• The heating required is between 40% and 60% of full power for 5.1% + 5.9% + 4.2% + 3.8% = 19.0% percent of the year
• The heating required is above 80% of full power for 0.9% + 0.9% + 1.0% + 0.5% = 3.3% percent of the year.

We can also usefully re-draw the graph expressing the frequency of each level of heat demand as a likely number of days per year on which that heating demand will be required.

Click on image for a larger version. The graph shows the typical number of days per year that the given fraction of full heating power is required. Adding up the individual data points, one can see, for example, that for typically 69 days per year, the required heating power is between 40% and 60% of the maximum. Heating power between 80% and 100% is only required on 12 days per year.

Grouping the data into 20% bands we see that there are around 160 days – generally known as “summer” – with very low heating demand. There are a further (roughly) 160 days with medium heating demand (between 20% and 60%) and finally (roughly) 45 days with high heating demand (between 60% and 100%).

3. Let’s think about a dwelling with a particular heating demand

I find the above analysis fascinating, but it becomes even more interesting if one considers a specific dwelling with (say) 5 kW of maximum heating demand. We can now re-draw the above graph in a number of ways.

Click on image for a larger version. For a dwelling with a maximum heating demand of 5 kW, the graph shows the typical number of days per year that a particular average heating power per day was required to keep the dwelling at 20 °C. For example, for typically 69 days per year, the required heating power is between 2 kW and 3 kW.

The graph above now has the heat pump power in kilowatts (of heat) showing that – for example – the average daily heating power is between 3 and 4 kilowatts (of heat) for typically 36 days per year.

If a heater delivers on average 3 kW of heating power for a day it will deliver 3 kW × 24 hours = 72 kWh/day of heat energy into the dwelling. So we can replot the graph again but this time labelling the horizontal axis with the heat energy delivered per day (kWh/day).

Click on image for a larger version. For a dwelling near Heathrow with a maximum heating demand of 5 kW, the graph shows the typical number of days per year that a particular number of kWh of heating required to keep the dwelling at 20 °C. For example, for typically 12 days per year, the required heating energy exceeded 96 kWh/day.

And now we can begin to see something useful. The graph tells us that – on average – for 23.8 days a year, the dwelling requires ~48 kWh/day of heating. So if we multiply 23.8  days × 48 kWh/day we get 1,142 kWh. So we can now work how much heat is delivered in each operating band. Remember that although there are not many days in the high-power band, lots of heat is delivered on each of those cold days. And likewise, although there are lots of low-power days, not much heat is delivered on those days.

Click on image for a larger version. For a dwelling near Heathrow with a maximum heating demand of 5 kW, the graph shows the typical amount of heat (kWh) delivered at each power level. For example, Of the 13,900 kWh required for the whole year, 1,300 kWh were delivered at between 80% and 100% power, and 4,300 kWh were delivered at between 40% and 60% power.

This graph yields lots of information:

• Adding up all the data we see that throughout the year the heat delivered amounts to 13,900 kWh.
• If this had been delivered by a 90% efficient gas boiler then the boiler would have consumed 15,400 kWh of gas per year. And the Rule of Thumb would have suggested that “right size” of heat pump would be 5.3 kW – quite close to the actual ‘perfect’ heat pump size.
• We see that the coldest 12 days of the year require 1,300 kWh of heating – around 9% of the annual heat load – and that the bulk of the heating (10,700 kW or 77%) is delivered on milder days when the average heating power is between 1 kW and 4 kW.

4. Cost

The discussions so-far has just centred on the heat delivered to the dwelling. Nothing so far has been about costs, or the specific advantages of using a heat pump. The above discussion could apply to electrical heaters or a gas boiler. The graph below shows the distribution of costs across the different power levels expressed as cost per day, assuming heating at 8p/kWh. As I write this typical gas tariff.

Click on image for a larger version. For a dwelling near Heathrow with a maximum heating demand of 5 kW, the graph shows the typical distribution of cost per day (£) if each unit of heating delivered costs £0.08 £/kWh. For example, 9% of the annual bill would be incurred on the coldest days, each day costing more than £8/day.

The wonder of heat pumps is that they can deliver more than one unit of heat energy for each unit of electrical energy consumed. The ratio of the two is called the Coefficient of Performance or COP, and it changes with the operating parameters of the heat pump. In particular, at lower external temperatures when the heat pump is working hardest, the COP is lowest. The graph below shows a guess for how the COP of a heat pump might change with heating demand – a proxy for outside temperature.

Click on image for a larger version. The graph below shows a guess for how the COP of a heat pump might change with heating demand – a proxy for outside temperature. At 100% heating demand the outside temperature was around – 2 °C.

Assuming this variation we can now work out how much electrical power – which we need to pay for – was required by the heat pump to deliver the necessary thermal power.

Click on image for a larger version. For a dwelling near Heathrow with a maximum heating demand of 5 kW, the graph shows the typical amount of heat (kWh) delivered at each power level, and the amount of electricity (kWh) required to deliver that heat. For example, on the coldest days when the heat pump was operating above 80% capacity, 1,300 kWh of heat were delivered using only 484 kWh of electricity.

This analysis tells us that over the entire year 3,800 kWh of electricity would be required to pump the 13,900 kWh of heat – a seasonally-averaged COP of 3.6.

• If one paid 28 p/kWh for electricity this would cost £1,064 to deliver the heat.
• If one paid 32 p/kWh for electricity this would cost £1,216 to deliver the heat.
• If we were to use gas a 90% efficient gas boiler to deliver the same 13,900 kWh of heat at 7p/kWh this would cost £1,081.

To the accuracy of this calculation, the costs of using gas at £0.07/kWh or a heat pump at £0.28p/kWh are similar. Of course the carbon costs are dramatically different. Using a gas boiler would emit 3,500 kg of CO2 compared with emissions of only 874 kg when using a heat pump: a 75% reduction.

5. Making the heat pump option cheaper

Friends. we now reach the point where I explain why I have taken you on this long journey. I didn’t want to emphasise it at the start of the article because it seemed so complicated – but if you have made it this far, I think will now get the point!

Wouldn’t it be great if the heat pump could not only supply low carbon electricity, but could also do it at significantly lower cost – significant enough to justify the capital expenditure of a heat pump?

To achieve lower cost, one needs to spend even more money and use a heat pump with a battery to allow off-peak electricity purchases: But what size battery should one buy? And how much will it save? Now we are getting to the serious questions!

So first we need to re-jig the graphs above to show how electricity use is distributed across the different amounts of heat delivered per day.

Click on image for a larger version. For a dwelling near Heathrow with a maximum heating demand of 5 kW, the graph shows the typical number of days in which a particular amount of electricity (kWh) is  to deliver heating to maintain 20 °C.

So now I will consider my current tariff with Octopus: they keep changing its name so I won’t confuse you, but it means I can buy electricity at 7.5 p/kWh for 6 hours at night: the rest of the time electricity costs 32p/kWh. I use this to run the house and charge up the 13.5 kWh Powerwall battery.  The battery then has to run the house for 18 hours until the next cheap period. If the capacity is not enough then I have to buy some full price electricity!

Considering only space heating – not hot water or other appliances – and assuming 100% battery efficiency, we see that for the days where the heating demand in that 18 hours amounts to less than 13.5 kWh, we could run the house entirely on cheap electricity. So when daily demand is less than 18 kWh of electricity, the heating demand could be met entirely with cheap-rate electricity.

If we had more batteries, then we could store more cheap electricity. The graph below shows an estimate for the impact that different sizes of batteries would have on the annual heating bill.

Click on image for a larger version. For a dwelling near Heathrow with a maximum heating demand of 5 kW, the graph shows the typical annual cost (excluding standing charges) of using a heat pump  to deliver heating to maintain 20 °C.

Please note: this calculation covers just the cost of the consumed units of electricity: it ignores many relevant factors such as:

• Standing charges.
• The use of electricity to do other essential activities.
• Inefficiencies charging and discharging the battery.
• The capital costs of the heat pump and the battery.
• Other more complex tariff structures.
• Use of the batteries to export electricity.
• Any use of solar panels.

Reflections

Friends, I conceived of this article in a reverie, and I am having difficulty finishing it – it’s like a dream from which I cannot wake up! But here’s the bottom line:

• Using a battery with a heat pump, but without Solar PV is an unusual combination. However I thought I would investigate to see if it might possibly be a useful combination.
• Using a heat pump with a battery can reduce annual running costs – but even for a modest 5 kW of peak heat loss (i.e. 72 kWh of heat/day), one needs a large battery – around 10 kWh to make a significant dent in the annual bill.

If you have read this far, I would just like to personally say “Thank you, and Congratulations” – and I hope you don’t feel short-changed at the conclusion.

 

Notes from Paradise

Friends, I live in Teddington in the London Borough of Richmond upon Thames which has been reported by The Guardian to be the “Happiest place to live in Great Britain“. Lucky me.

If I ever do make it to paradise, then surely each day there would include a leisurely stroll to a relaxed and friendly café where I would partake of a coffee and perhaps a Lebanese pastry. But since that is already part of my daily routine, perhaps I am living in Paradise already?

But lovely though the borough is, it cannot escape Climate Change. And being only 10 metres above sea level with a large tidal river flowing through it, there are real reasons to be concerned. The river already floods the local roads at high tide – the video below shows an exceptional occasion earlier this year.

And indeed the borough council has a climate target of reaching net-zero itself in 2030, and making the borough as a whole net-zero by 2043. And at the end of November they held a “Sustainability Forum” to consult with residents on these matters.

Sustainability Forum

Around 100 ‘concerned’ residents came along to the forum which consisted of:

• A talk by some council officers (from their “net-zero team” I think) who told us how wonderfully they were doing.
• A set of consultation exercises such as writing ideas on a whiteboard or a map.
• A short Question & Answer session at the end.

It was heartwarming to be amongst so many concerned people, and the council officers seemed very well-meaning. But overall, I found the event depressing.

Firstly, the council officers were ill-informed. They boasted that they had reduced carbon emissions from electricity by 72% simply by changing supplier.  Of course they hadn’t reduced their emissions at all: they are still using exactly the same electricity as they were before and their carbon dioxide emissions are identical.

Secondly, they seemed to have no plan about how they would affect the borough’s overall carbon dioxide emissions, the largest part of which  (I would guess) arises from the use of gas for domestic heating.

Thirdly, they had no sense of urgency. When I commented on this the ‘head honcho’ (whose name I did not catch) appeared to be affronted – looking almost tearful.

When I raised one specific issue – the Council’s contradictory policy on solar panels – the ‘head honcho’ responded immediately that the Council had to consider all kinds of things that I didn’t appreciate. They made no effort to listen or to consider that maybe they could improve their guidance.

Well-meaning as the Council team were, they came across as self-satisfied and resistant to even the tiniest change.

Net-Zero by 2043

To reduce carbon dioxide emissions in the borough over 20 years means reducing emissions by 5% each year starting now. The rate of reduction required to reach zero by 2043 increases with every year of delay. In fact, with every day, week and month of delay.

Richmond’s population is around 200,000 souls and so I guess there are maybe 70,000 dwellings – and perhaps 30,000 houses. If we consider just the houses, an essential component of net-zero means removing gas from 1,500 homes a year, or 30 homes a week.

We will also need to generate renewable electricity which means (probably) using solar PV panels in as many locations as possible.

The council could play a major positive role in achieving both these ends if it chose to. But it deliberately chooses not to. In fact the council is itself a significant impediment to achieving net zero. It is currently part of the problem rather than part of the solution.

I have spoken to many local residents keen to install solar PV and heat pumps in their homes, but the council guidance is confusing and restrictive. If the council want to actually help they need to change their attitude completely.

Regarding heat pumps

The only solution for low-carbon heating for homes is heat pumps. If the council really wants to achieve net-zero by 2043 then it needs to acknowledge the reality that in 2043 every home will have a heat pump. And the process of installing them needs to start right now.

The Council’s answer to the question “Can I have a heat pump in my home?” has to be a simple “Yes” in every case. Ideally the response would be “Yes, please allow us to help you.”

The Council could usefully work with heat pump installers and architects to design off-the-shelf solutions to every planning problem. Heat pumps can be mounted on walls, on roofs, in front of houses, in back gardens or they can be camouflaged. Solutions exist to every problem: but the Council needs to begin by just saying “Yes”.

For example, the Council could work out solutions for all the types of property in the borough: terraced houses, fancy Edwardian villas, listed-buildings. All these buildings will need to be heated with heat pumps by 2043 – and the Council need to get busy now helping people spend their own money to achieve this.

At the moment the Council just says: “No”: it puts responsibility on residents to endlessly re-invent solutions. For example, the Council could specify particular models of heat pumps that meet whatever noise requirements they have invented, instead of putting the responsibility on residents to commission sound-level surveys.

The Council’s current negativity has a real impact. At the talks I give, there is a fair overlap between “residents affluent enough to install heat pumps” and “residents that live in a conservation area”. And many people start a conversation by telling me they can’t have a heat pump because they are in a conservation area. People assume this because of the Council’s negativity: this needs to stop!

And the council could measure how they are doing, not year-by-year, but week-by-week. Have they helped with the installation of 30 heat pumps this week? If not, how will they increase the number next week to make up for the fact that they are already missing their targets?

Regarding Solar PV

The main way the council can reduce carbon dioxide emissions from electricity use is to generate low-carbon electricity using solar PV panels. So as with heat pumps, the Council’s answer to the question “Can I have a solar PV on my home?” has to be a simple “Yes” in every case. Ideally the response would be “Yes, please allow us to help you.”

The Council might respond that it has to consider the aesthetics of its buildings. My response is that Solar PV (or indeed heat pumps) causes no permanent damage to buildings. And if in 20 years our climate crisis is over, then the panels can be removed in a single day if required. But currently, we are in a climate crisis and the Council have themselves resolved to achieve net-zero in 2043 [Typo edited on 12/12/23].

And as with heat pumps, the council could measure how they are doing. It would not be difficult to estimate the amount of Solar PV generating resource in the borough – and set a target for this to (say) match consumption entirely in summer. Richmond borough’s roof tops could be come a substantial generating resource. The council could work with the local District Network Operator to (say) add local battery storage to ensure the stability of the local network.

Money

All UK councils operate under tight financial control. But Richmond is a “happy borough” because Richmond is an affluent borough. The council does not need to spend any more than it already does on its “Net-Zero team”: it just needs to let people spend their own money on their own homes.

Currently the Council sees itself as an institution whose aim is to hinder residents, even as they are trying to achieve the borough’s own goal of reducing carbon dioxide emissions. If the Council genuinely intends to meet its goals then this attitude needs to change.

 

Powerwall Battery Degradation: Winter#3

Friends, as you are no doubt all aware: everything is getting worse. But the other day I noticed that one thing in particular was not getting worse quite as quickly as I had expected: the nominal capacity of my Tesla Powerwall 2 battery.

The Tesla Powerwall 2: Capacity

The Tesla Powerwall 2 is a home battery with a nominal capacity of 13.5 kWh when new. However the capacity of all rechargeable batteries declines over time, and it can be quite difficult to assess the actual capacity of the battery in use. I wrote about this at the end of the winter of 2022/23 where I estimated that since the previous winter the battery capacity had fallen by about 3.5%. It’s only the start of the winter of 2023/24 but so far it looks like capacity has fallen by less than a further 1%.

Click on image for a larger version. Measurements over the last three winters showing the amount of electricity discharged from the battery as it goes from 100% full to empty within a single day. See the text for more details.

What do we mean by Battery Capacity?

Assessing the capacity of a battery while it is in use in a home is tricky. And one reason for that is that it is hard to define even what one means by ‘capacity’. Really? Allow me to explain.

The nominal capacity of the Tesla Powerwall 2 is 13.5 kWh, but the battery can only be charged from AC power with an efficiency of about 95%. And it can only be discharged with an efficiency of about 95%.

• So to ‘fill’ the battery requires 14.2 kWh of AC electricity, 95% of which will be stored in the 13.5 kWh of battery cells, with the additional energy ending up as heat.
• Similarly, when the battery is discharged at 95% efficiency, only 12.8 kWh of useful AC electricity will be produced.

In practice, I can only assess the capacity of the battery in winter on days when the battery is re-charged to 100% capacity overnight and then we run from the battery until it is drained again. In this way I can obtain a figure for the total energy discharged from the battery.

Click on image for a larger version. Four screenshots from the Tesla ‘App’ showing the complete discharge of the battery during the day. The lower section of each screen shows the battery state of charge. The upper section of each screen shows charging from the grid, discharging to meet the household load, and a few brief episodes of solar recharging.

One small complication arises from small amounts of solar generation during these winter days. If solar generation exceeds the household demand then the battery will start to re-charge (at 95% efficiency) and this extra charge will then discharge at 95% efficiency.  I only consider days in which this solar re-charging is small, and simply subtract it from the nominal capacity, but the simplest measurements to interpret are on those on dull days when there is no solar charging.

Click on image for a larger version. Measurements over the last three winters showing the amount of electricity discharged from the battery as it goes from 100% full to empty within a single day. See the text for more details. Each blue dot represents a single full-to-empty measurement. The large black circles show yearly averages and trends are shows as dotted lines.

The graph above (the same as the one at the head of the article) shows all the results since 2021. Each blue dot represents a day of full discharge. And each blue dot with a pink outer circle is day of full discharge in which there was no solar re-charging.

• Based on the specification we might hope for a discharge of around 95% of the 13.5 kWh nominal capacity i.e. 12.8 kWh.
• During the winter of 2021/22, the average daily discharge was 13.1 kWh – rather better than we might have hoped for.
• During the winter of 2022/23, the average daily discharge was 12.7 kWh – a decline of 3.4% in one year.

This article is simply to mention the good news that:

• So far, during the winter of 2023/24, the average daily discharge has been  12.6 ± 0.2 kWh – a decline of less than 1% since last year. Which is pleasing.

What should I expect?

If we extrapolate two trend-lines, one based on the decline in Years 1 & 2, and the other based on the decline in Years 2 & 3 (so far), then one trend line indicates a 20% decline in capacity over 6 years, and the other suggests a 20% loss in capacity after 20 years. My guess is that the answer will be somewhere in between the two.

Click on image for a larger version. Measurements over the last three winters showing the amount of electricity discharged from the battery as it goes from 100% full to empty within a single day. See the text for more details. Each blue dot represents a single full-to-empty measurement. The large black circles show yearly averages and trends are shows as dotted lines.

When I bought the battery I guessed that the battery degradation might be similar to that seen in early Tesla cars (Model S and X). This data (now 5 years old) is plotted versus kilometres travelled below.

Click on image for a larger version. 2018 data from Electrek showing battery capacity (%) for Tesla Model S and X cars versus kilometres travel. The trend indicates about 10% loss of capacity after 250,000 kilometres. Both graphs show the same data but the right-hand side ‘zooms in’ on the data.

The data shows two interesting things.

• Firstly  it shows a relatively rapid decline in retained capacity in the first 20,000 km of life – perhaps the first year of typical use, followed by a lower rate of decline out to 250,000 km.
• Secondly, there is a lot of variability in the data. This data is from 2018 and so would feature battery packs installed in perhaps 2012. Some batteries don’t seem to perform well at all. The cells in my Powerwall are still of this design type (so-called 2170 cells), but I suspect manufacturing quality has improved substantially since 2012.

However the battery packs (i.e. collections of battery cells) in a car battery and a domestic battery are subject to quite different duty cycles. Car batteries are only rarely filled to 100% or drained to 0 % and avoiding these extremes inhibits many of the physical processes which degrade the battery. In contrast, domestic batteries are frequently filled to 100% and emptied to 0%: this probably happens about 100 times each winter.

So we might think that a domestic battery pack will have a much tougher time than a car battery pack. However, the temperature at which charging and discharging take place is also important, and the Powerwall includes a heating and cooling system and with the battery pack in a semi-sheltered location in the UK, I would guess the cells experience less extreme temperatures during charging and discharging than an EV battery pack.

Summary

So to summarise,  the battery degradation observed so far this winter is less than I expected – and that is a good thing. I’ll be sure to write an update at the end of the winter.

Our Fragile Moment: A Review

Friends, recently I have been writing less because I have been reading and reflecting on “Our Fragile Moment” by Michael Mann.

Click on image for a larger version. The colourful geological graphic is from the NOAA web site.

The book takes a look at Earth’s climate from a geological perspective, highlighting climatic changes in Earth’s history and asking whether what we learn from these changes might be relevant to our current situation.

The answer is “Yes“: we can learn a lot about our current situation by looking at previous non-anthropogenic episodes of Climate Change.

An Epic – but difficult – story

Our Fragile Moment takes us on a rip-roaring tale through Earth’s history in which total global glaciation is followed by a hot-house Earth, and mass extinction ‘events’ appear to be rare, but inevitable occurrences.  But despite the epic scale of the drama, and our compelling motivation to understand such changes, this is not an easy read.

Firstly, the geological naming conventions are arcane and arbitrary. I had a similar sensation to reading novels with long and unfamiliar names (think Tolkien or Dostoevsky) and realising I mixed up two characters with similar names (e.g. Paleocene and Paleozoic).

Secondly, the time scale of Earth’s history is unimaginably long. Human history and prehistory – perhaps the last 3 million years – is less than 0.1% of the age of the Earth. And the rate of change of geological processes is so slow it almost hurts to think about them. It is easy to be unsure about whether ‘snowball Earth’ last millions of years, or tens of millions of years.

Thirdly, the way we infer past climates is not straightforward. Except for ‘recent’ changes (i.e. the last few hundred thousand years) all we have left are rocks, and we have to infer what has happened by evidence left – or not left – in rocks. It’s a kind of ultimate Crime Scene Investigation of the coldest of cold cases. And so there is inevitable uncertainty in working out what has happened.

All this being said, as we evaluate our current situation, the geological perspective is especially valuable, and as an overview of that perspective, the book is valuable. For most of time, no being has been able to understand the present and predict future events. Humanity’s ability to predict climate change is only decades old and is still imperfect – despite using the most detailed and complex computer models. Having historical systems to ‘calibrate’ the models is invaluable.

Lesson #1: The Players 

Some of the ‘characters’ with which have become familiar in discussing our current predicament, come up time and time again as Michael Mann describes what we know of Earth’s Climate Saga:

• water – in the oceans, as rainfall, as vapour, and as clouds.
• carbon dioxide – in the oceans and the atmosphere.
• methane – captured in the biosphere and free in the atmosphere.
• the Sun – it’s variability and stability.
• the Land – its motion around the Earth over geological time.
• life – and it’s influence on atmospheric composition.

As each episode of climate change is described – it eventually becomes clear that it is the same characters as play each time – but each time with a different starting position, and with interactions that are similar, but distinctly different, from previous episodes.

Lesson #2: Feedback 

Whatever event ‘initiated’ an episode of climate change, the evolution of the climate that results depends on the strengths of various responses to the initial change.

Some responses are ‘rapid’ on a geological timescale, and some are slow. For example, if I have remembered correctly, the creation of the Himalayas resulted in enhanced weathering of rock which slowly reduced carbon dioxide concentrations over millions of years, and thus reduced global temperature on a similar timescale.

Some responses re-inforce the initial change – positive feedbacks – and some responses act to reduce the effect of the initial change. In periods where the climate is reasonably stable, the stability is the result of negative feedbacks being larger than positive feedbacks. So in a stable climate small changes to say – the amount of sunlight reaching the Earth – result in changes in (say) ice albedo or cloud cover that negate the initial change.

But this stability only exists for a small range of initial perturbations. Larger perturbations can cause the stability to be lost and positive feedbacks can drive the climate into an entirely different state, which is generally not predictable.

The interplay between these feedbacks plays out time and time and time again through the episodes described in the book.

Lesson #3: The scale of human intervention

When we started emitting carbon dioxide on an industrial scale, humanity was unaware of climatic consequences of the emissions. Now, almost two centuries later, the energy we have released – and are continuing to release – has transformed the way we live – and altered Earth on a geological scale. Our emissions have changed the composition of the atmosphere – increasing the atmospheric concentration of carbon dioxide by about 50% about ten times faster than even the most rapid changes in the geological record.

Lesson #4: What’s going to happen next? 

Fascinating as the story of Earth’s Climate is, what I really wanted to know was whether my doomiest thoughts were justified. Are we already outside the range of change where negative feedbacks will resist us sliding into a new climate paradigm? Unfortunately, it’s still hard to tell. But Michael’s Mann’s interpretation is that the doomiest outlooks are probably not justified.

In the doomiest outlooks, the consequence of the CO2 we have already emitted is that we are already committed to warming way beyond the initial likely warming of 3 °C by 2100. In these outlooks we are already committed to losing the Greenland Ice Sheet and parts of the Antarctic Ice Sheet. This will result in unknown climatic consequences, but will raise global sea levels over the next few centuries by more than 10 metres. Teddington where I live will – along with most of London – be submerged.

Michael Mann’s view is that this outcome – while possible – is not the most likely result. “Our situation is urgent, but we have agency“. He considers that if we act to reduce CO2 emissions now, reaching zero emissions in the coming decades, then the lesson from the relatively-recent Eemian period (about 130,000 years ago) is that we are unlikely to suffer a ‘methane-runaway’ – because the Earth was warmer then and that did not occur then. Sea levels were also higher – but we did not totally lose the ice sheets.

Précis

Michael Mann’s summary is that the reality of the climate change that we are facing right now – this year and in the coming years – is bad enough. We don’t need to motivate ourselves with doomsday scenarios. But we do need to act urgently, because the lesson from Earth’s history is that if we do push the complex, interlinked climate system too far from its stable state – then doomsday scenarios can ensue. And then Our Fragile Moment could be over.