Archive for the ‘Climate Change’ Category

Climate Communications

July 27, 2022

Friends, as you may know I am frustrated at the inappropriately slow response of our government to the climate emergency we all face.

And I have had a dawning realisation that no superbly written blog article, no gem of a Tweet, and no YouTube presentation is going to change things.

So this week, I took a step inspired by a (completely fictional) scene in the movie The Darkest Hour. In this scene, Winston Churchill ventures onto the London Underground in search of ‘the mind of the people’.

Similarly, in search of grasping ‘the mind of the people’ I spent a few hours this week standing by a small table on Teddington High Street asking passers by what they thought about about our Climate Crisis.

Unsurprisingly, a solid 99% of people politely ignored me. But a few people did stop by and I took pains to note down what they told me. And below is a non-fictionalised account of what happened.

In search of ‘the mind of the people’ on Teddington High Street. 

Monday 25th July

10:45 I set up in front of the sorting office at the end of Elmfield Road.

11:10. A lady stopped by to ask “What can we do?” The lady lived locally and came back a few minutes later and gave me a Bakewell slice she had just bought from the bakers. She said I looked lonely! I was touched by her kindness.

11:50 A DHL driver asked where Nando’s was?

12:00 A South African gentleman stopped by to tell me he didn’t believe climate change is caused by humans but that it was a natural process caused by volcanoes. I told him that humans emitted more CO2 than volcanoes and asked him where he thought the balance was between volcanoes and people in terms of emissions. As he left he told me solemnly that the “World will end with fire”.

12:10 A lady approached me to tell me about a neighbour of hers who was an architect with multiple cars. She also spoke of the Beckhams and their celebrity lifestyles with multiple flights, the Cambridge’s and their palaces that they use helicopters move between, the evil of people keeping dogs, and the many flights and helicopters associated with racing horses. 

13:00. A single lady aged 73 stopped by with lots of positive ideas. She had looked up installing a heat pump but thought she needed to change absolutely everything in her flat. At the same time a lady I had seen earlier with a Fortnum and Mason’s bag wanted to complain about the tactics of extinction rebellion which she thought just put people off. She had herself been delayed by an Extinction Rebellion protest on the way to a funeral

13:30 Stopped

Tuesday 26th of July.

10:51 Set up outside CarpetRight

11:09 A man passed by and said he didn’t want to talk about the climate crisis because it was upsetting

11:21 A nice lady stopped for a chat. She was very sympathetic, acknowledged that this was really a crisis of capitalism and consumption, and was sympathetic to extinction rebellion. She had been off meat for many years and had avoided flying for eight years. She had no real idea what we could do collectively.

11:30 A lady stopped by who seemed know everyone, E.g., the Pope, and Greta Thunberg, and she spoke to me for a long time.

11:40 My neighbour and one-time colleague at NPL, Gordon Edwards stopped by and said hello.

11:54 A nice lady was concerned and knew all the regular things one could do, but agreed that it didn’t quite match the scale of what was required. She was depressed by the previous night‘s Tory party debate in which nobody paid any attention to the climate crisis. Or indeed the crisis in health and social care.

11:58 A young man with a rucksack, nodded his head and said good morning. But didn’t stop

12:19 Interesting talk with lady from a nearby road. She was very worried and very concerned but didn’t know what to do. She lived a very frugal life, and she and her husband had no children but said that if she had children she would be very concerned for what they would inherit

12:31 Another one-time colleague stopped by. He said that it seemed like there was a cloud hanging over the world in the form of climate change, politics, and geopolitics. We agreed that it was important just to keep raising the subject when one had friends who are sceptics: Silence was an enemy.

12:44 A lady who works at TearFund, (a Christian International Development Charity with its headquarters in Teddington) stopped and said they have a kit to allow local churches to declare climate emergencies. She spoke about how Tear Fund saw action on a Climate Change as a theological issue (I said I think I would call it moral issue), because it involved justice and fairness and inequality.

13:07 A young man stopped by who is doing marketing and PR for a company called PATCH which is looking to market direct carbon removal for different companies. He suggested many things including that I focus on the monetary aspect of going green. He was enthused by the fact that private companies were becoming involved in the face of government inaction.

13:20 Stopped.


Friends, I don’t know what to make of this experience, but I felt it was valuable and I will keep doing it.

For the 99% of people who passed by, I feel they saw a person raising the issue on the street. Many would be embarrassed or fearful to stop and talk, but they saw the words ‘Our Climate Crisis’.

Of the 1% of people who stopped, several were profoundly concerned and were doing what they could in their personal lives, but they had more or less abandoned hope of coherent and serious government action on this.

In Winston Churchill’s fictionalised visit to the London Underground, the ‘mind of the people’ became apparent to him in just a few minutes, and with dramatic clarity. I am finding it a little more difficult to decipher their message, but I intend to keep trying.

Domestic Thermal Storage 3: Concrete

July 23, 2022

Friends, this is the third and final article comparing different types of thermal storage.

In previous articles I looked at the humble domestic hot water (DHW) cylinder and the use of a phase-change materials (PCMs) to store heat.

In this article I will look at the use of ‘very big lumps of something very hot‘ as a thermal store.

This brute force technology is implemented in a so-called Zero Emission Boiler (ZEB) from the Tepeo company, and in a behemoth of a storage device akin to a miniature version of the ‘Sand Battery’ that got me started on this, the Warmstone store from the Caldera company.

Zero Emission Boiler (ZEB)

A ZEB thermal store is a device about the size of washing machine that is conceived as replacement for a gas boiler in settings where a heat pump cannot be installed.

It is generally placed wherever the gas boiler was, but being essentially a large lump of concrete, it is extremely heavy – 370 kg – and must be placed only on the ground floor of a house.

Click on the image for a larger version. A ZEB thermal store in an extraordinarily uncluttered kitchen as pictured on the Tepeo website.

The difference between a ZEB and the other thermal storage devices I have described is in the amount of energy which can be stored – a ZEB can store 40 kWh of thermal energy – around 5 times more than a typical DHW cylinder. This is enough thermal energy to not just provide hot water to a house (typically 5 to 10 kWh/day) but also to heat an entire home via its central heating system.

One can imagine a ZEB as being a centralised version of old-fashioned electrical storage heaters. It stores thermal energy by electrically heating a block of ‘high-density concrete’ to an astonishing 800 °C.

At these high temperatures, heat loss is significant, but my estimates suggest that ~150 mm of insulation around a 40 cm cubical block, should limit heat losses to ~ 5.6 kWh/day or around 14%/day.

Extracting thermal energy from a ZEB at 800 °C into water flowing at (say) 50 °C is tricky. Slightly to my surprise, energy cannot be extracted rapidly enough for this to instantly heat water and so it cannot be used to replace a combination boiler. It must still be used with a DHW cylinder as an intermediate store of hot water. However it seems likely to me that Tepeo will solve this problem in the next few years.

Below is a YouTube video  in which Robert Llewelyn discusses the ZEB he has had installed in his own home.


A Warmstone thermal store is something like a miniature version of the ‘Sand Battery‘ that claimed to store heat inter-seasonally. But instead of storing 8 MWh like the ‘sand battery’ – it stores ‘only’ 0.1 MWh or 100 kWh.

It achieves this large capacity by heating a large mass of material – probably concrete of some description – to 500 °C. But it uses vacuum installation to reduce the heat losses to just 4.8 kWh/day – or 5% per day – half the fractional losses of a DHW cylinder or PCM material.

Unsurprisingly the device is large and heavy, weighing 1.8 tonnes and standing 1.8 metres tall with a diameter of 1 metre, so this too large for it to pretend to be a domestic installation. The company imagine it being used in large homes which have outhouses or large gardens.


In the first article I looked at thermal storage in a DHW cylinder. This is the default thermal storage that many people still have in their homes – holding about 8 kWh of thermal energy.

In the second article I looked at Sunamp’s PCM storage which can operate in practice like a DHW cylinder – but is typically less than half the physical size while storing similar amounts of thermal energy.

In this last article I looked at two companies looking to ‘go large’ and store one to two days use of thermal energy for a home. To achieve this they have used large masses of stored material electrically-heated to high temperatures. Surprisingly, despite the large masses and high temperatures, the rate at which water can be heated by these devices is still (currently) limited and so they must both still be used with a DHW cylinder.

All the technologies beyond the basic DHW cylinder all feature computer technology which allows Apps to control the storage and allow integration with smart home controllers – something which is apparently very important, but is an area in which I have absolutely no interest: sorry.

What I learned in writing these articles is the very simple lesson that there is no magic to thermal storage technology. There are no magic materials and there is no magical insulation. To store more energy one simply needs a large mass of material, heated to a high temperature, and kept as well insulated as possible.

Domestic Thermal Storage 2: Phase Change Material

July 23, 2022

Friends, this is the second of three articles in which I am comparing three different types of thermal storage.

In the last article I looked at the humble domestic hot water (DHW) cylinder, and in the next article I will look at large thermal stores. Here we will look at the use of a phase-change material (PCM) to store heat.

In practice a PCM thermal store looks like a regular ‘white goods’ metal box and is typically placed wherever the domestic hot water (DHW) cylinder would have been in a dwelling.

Click on the image for a larger version. Publicity images from the Sunamp web site demonstrating the small physical size of their PCM thermal stores.

But a PCM thermal store has a big advantage over a DHW cylinder: it is typically one third to one half the size for the same amount of thermal storage. Dimensions are typically 1 metre high, 60 cm deep and 40 cm wide.

Click on the image for a larger version. On the left-hand side is a commercial PCM thermal store. On the right-hand side is a schematic explanation of how it works. In this versions of the device, the PCM material is charged using an electrical immersion heater. In other versions it can be charged using a heat pump. In operation, cold water flowed into the device is rapidly heated and discharged.

Additionally a PCM store is cubical, and so makes use of the corners of spaces that DHW cylinders – being cylindrical – can’t use.

Functionally it works like a DHW cylinder. When a tap is opened, cold water flows into the device and is heated as it flows through pipes embedded in the hot PCM material – and hot water flows out.

However, the PCM thermal store has a trick up its sleeve. If the PCM stored heat in a substance at high temperature, then the temperature of the substance would have to be high initially – with high losses – and the storage medium would cool as heat was withdrawn.

PCM thermal stores get around this by using a material which melts – typically at around 55 °C to 60 °C.

  • Charging the PCM involves heating it up to its melting temperature, then supplying the so-called ‘latent heat’ required to change it from one ‘phase’ (a solid) to another ‘phase’ (a liquid). It is then heated further as a liquid.
  • When cold water flows through pipes embedded in the PCM, the PCM cools and freezes around the pipes. When it freezes it stays at its freezing temperature releasing its so-called ‘latent heat’ until the entire charge of of PCM has solidified.

In practice this means that one gets the benefits of a DHW cylinder in a smaller space. PCM thermal stores are particularly well-suited to smaller single-person dwellings.

PCM’s: Home experimentation

A common PCM with which you can experiment at home is candle wax.

While my wife was out at work, I put two candles into a glass container and melted them (one at a time) by putting the container in a jug of boiling water.

Click on the image for a larger version. Top-Left: Melting a candle in a jug of hot water. Right: A partially melted candle.Bottom-left: Measuring the temperature as the molten wax cooled.

When both candles were melted, I put a thermocouple into the wax, wrapped insulation around the glass vessel and then measured the temperature as the molten wax froze – i.e. changed ‘phase’ from liquid to solid (to use the technical terms). The data are shown below:

Click on the image for a larger version. The graph shows the temperature of a thermocouple embedded in 55 g of wax as it froze. Note that there is a sharp change in cooling rate when the wax starts to freeze due to the release of so-called ‘latent heat’. This allows the wax to stay above 50 °C for almost 3 hours, while if it had continued cooling at the initial rate, it would have fallen below 50 °C in under 1 hour.

What one sees is that as the molten wax cools, it looks like it will fall below 50 °C after about 50 minutes. However, once the wax starts to freeze (at about 57 °C), the cooling rate is reduced to roughly one tenth of its previous rate, and the liquid/solid mixture stays above 50 °C for around 160 minutes.

Using a very rudimentary analysis based on googled data:

  • Heat Capacity of wax ~2.5 J/g/°C – assumed the same in liquid or solid state;
  • Latent Heat of wax ~176 J/g;

…one can roughly estimate how much heat is released at temperatures above 50 °C.

Click on the image for a larger version. Analysis of cooling curve in the previous graph allows an estimate of the amount of heat released at temperatures above 50 °C. The latent heat of 55 g of wax amounts to just under 10,000 joules.

Although I had followed the golden rule of experimental physics, I still failed to anticipate just how long it would take the wax to solidify – the experiment took 4 hours and I was almost late preparing my wife’s dinner!

This extended experiment indicates just how much ‘latent’ heat a material can store compared with ‘sensible’ (i.e. sense-able: which can be detected with a thermometer) heat storage.

Based on the latent heat alone, 100 kg of wax – which would occupy a cube with a side of 50 cm – could store 5 kWh of thermal energy – the equivalent of a small DHW cylinder.

Commercial PCM Devices

I don’t know, but I am pretty sure that commercial PCM devices do not use wax as a storage medium.

Update: A Twitter Source tells me that the Sunamp uses “Sodium acetate tryhydrate (plus a few secret additives).”

Sunamp’s list of patents includes a variety of chemicals which can be used, but the particular chemical used and the way it is prepared is likely a trade secret. Nonetheless, I suspect their basic properties are not so different from wax.

They will have a phase change temperature ideally around 55 °C. If the phase change temperature is much higher than this, then the store will operate at too high a temperature and lose more energy. If the phase change temperature is much lower than this, then water will not be sufficiently hot when discharged.

Early models of the PCM stores were designed to be ‘charged’ electrically with a heater immersed in the PCM material. This could be powered either from the grid – ideally using off-peak electricity – or from solar PV panels. However recent versions can also be charged using a heat pump.


PCM thermal stores  represent a clever way to incorporate thermal storage in dwellings where space is at a premium. They are particularly useful in flats and households with just one or two people.

However, like all thermal storage devices, they are not perfect.

One disadvantage is that unlike a DHW cylinder, the storage medium has to ship with the device – it can’t be shipped empty. This makes the devices heavy: A PCM store equivalent to a 200 litre cylinder weighs ~ 172 kg. Of course a DHW water cylinder holding 200 litres of water would weigh more – but it can be filled and emptied in place!

Heating losses are similar to DHW cylinders – with roughly 10% of the stored energy being lost each day – and like DHW cylinders, it can be tricky to know how ‘full’ the store is because it can be difficult to work out what fraction of the PCM material is liquid or solid.

But all-in-all, the PCM thermal stores devices seem to have found a niche where they can make themselves genuinely useful.

Domestic Thermal Storage: Part 1: Hot Water

July 23, 2022

Friends, writing about the ‘Sand Battery’ fiasco the other day brought to mind smaller thermal stores that are used domestically. And so I thought it would be interesting to write about the physics of thermal storage.

But it has all got out of hand and now this this is the first of three articles over which I will compare three different types of thermal storage, one most people are familiar with, and two that are less familiar:

  • A domestic hot water tank.
    • This stores thermal energy in water which is then used directly within a household.
    • A typical Domestic Hot Water (DHW) cylinder stores between 7 kWh and 10 kWh of thermal energy.
  • A phase-change thermal storage device.
    • This stores thermal energy in the so-called ‘latent heat’ of a material which absorbs thermal energy when it is melted, and releases it at a constant temperature as the material freezes.
    • A typical Phase Change Thermal Store stores between 4 kWh and 8 kWh of thermal energy, comparable with a DHW cylinder, but requiring only approximately half the volume.
  • A Zero Emission ‘Boiler’.
    • This stores thermal energy in the heat capacity of a ‘thermal core’ – a cylinder of concrete weighing ~300 kg – which is heated to an astonishing 800 °C.
    • This can store up to 40 kWh of thermal energy.
  • A ‘big thermal store’.
    • Like a Zero Emission Boiler, but heavier – and ‘only’ heated to 500 °C.
    • This can store up to 100 kWh of thermal energy.

Click on the image for a larger version. Schematic illustration of four different types of thermal storage devices and a human being for scale.

The key role of all these devices is to separate two events:

  • The time when energy is consumed from a central resource – such as the electricity grid,
  • The time when energy is used domestically – such as when you take a shower.

Separating these events has two benefits:

  • It allows users to store thermal energy slowly but to release large amounts of thermal energy quickly – such as when you need a flow of hot water ‘instantly’.
  • It allows users to store thermal energy when it is cheap or convenient .

For each device we need to consider how it is heated (‘charged’) and how it passes on its stored heat (‘discharged’).

In this article we will look at how a domestic hot water (DHW) cylinder works and in the following articles we will look at how Phase Change Material Stores works and how Zero Emission Boilers and big thermal stores work.

Domestic Hot Water (DHW) Cylinder

When I first heard a DHW cylinder described a ‘thermal store’, I was initially confused. I had always considered them as storing water!

In the other thermal stores, heat is first stored in a material, and subsequently transferred to circulating water or DHW only when it is required. In a DHW cylinder the storage material is the water which will itself later emerge from a tap.

The amount of stored thermal energy can be estimated as the product of:

  • The volume of water in the tank
  • The heat capacity of water in the tank (4,200 J/°C/litre)
  • The difference between the storage temperature and the charging temperature.

For a 200 litre tank storing water at 55 °C which has been heated from 20 °C this amounts to ~29 MJ or 8.2 kWh.

One can store more energy in a cylinder of a given size by storing water at a higher temperature: at 75 °C the stored energy in the cylinder above would be 12.8 kWh. To prevent discharge of scaldingly hot water, a blending valve would be used on the top of the cylinder and set to (say) 50 °C.

Click on the image for a larger version. Schematic illustration of the structure of a DHW cylinder showing the internal coil for heating the stored water. On the right is a manufacturer’s illustration of the coils within their cylinder.

A DHW cylinder can be charged in one of several ways.

  • In the simplest way, an electrical heater immersed in the water heats the water directly. A 3 kW heater can charge a 200 litre cylinder to 55 °C in just under 3 hours. The heater could be powered by either grid or from excess solar PV.
  • Alternatively, hot water heated by a gas boiler or a heat pump can be flowed through a coil inside the cylinder, passing on its heat to the stored water. The rate of heating in this method will generally be slower than using an immersion heater.

Discharging the cylinder is simple: one opens a tap and the mains water pressure forces water out of the top of the cylinder replacing it with cold water at the bottom.

The rate of discharge of thermal energy is given by the product of:

  • The discharge flow rate (litres/second)
  • The heat capacity of water in the tank (4,200 J/°C/litre)
  • The difference between the storage temperature and the charging temperature.

So if 10 litres of water at 50 °C is discharged per minute, thermal energy is being released at a rate of 21 kW. This is a very high rate of energy use.

‘Combination boilers’ can provide this very high heating rate, but only at the cost of releasing (at the specified flow rate) around 0.1 kg of CO2 for each minute of operation.


One of the difficulties with a DHW cylinder is that natural convection within the cylinder causes the hot water to rise to the top. And the stratification within the cylinder can be very dramatic.

Since most cylinders have only a single thermometer somewhere in the middle of the cylinder, even after reading the thermometer it is difficult to know how much heat is currently stored in the cylinder.

Additionally since the heating coil or immersion heater is typically in the lower third of the cylinder, practically the whole cylinder must be re-heated before any sufficiently hot water is available at the top of the cylinder – which typically takes several hours.

Some modern cylinders made by the Mixergy company exploit the stratification by heating the water from the top and then carefully mixing it with the colder water below.

These computer-controlled cylinders can give a reasonable estimate of the state of charge of the cylinder, and also allow rapid heating of small volumes of water at the top of the cylinder. However, I don’t understand precisely how the technology works.

Update: This video gives a clear – if rather glossy – explanation of how the system works. It turns out that Robert Llewelyn was given one as part of a research study!

Click on the image for a larger version. The water in a conventional DHW cylinder is hotter at the top but the temperature gradient from top to bottom is not well-defined. More modern computer-controlled cylinders from the Mixergy company can precisely control the location of the temperature gradient.

Heat Losses 

A DHW cylinder holding 200 litres is typically 1 metre high with a diameter of 50 cm and insulated with 50 mm thick layer of polyurethane foam with a typical thermal conductivity of 0.025 W/°C/m.

Click on the image for a larger version. Heat losses from a DHW cylinder are typically 10% of the stored energy per day.

For a cylinder at 55 °C, this leads to a heat loss of roughly 35 watts, or 0.85 kWh/day. i.e. the cylinder loses about 10% of its stored energy per day.

This loss rate increases if the water is heated to a higher temperature. For a cylinder at 75 °C the loss rate is ~ 55 watts or 1.32 kWh/day – again, about 10% of its stored energy per day.

The only way to reduce the heat loss is to apply either better insulation (which is expensive) or to apply a thicker layer, which makes the cylinder larger.


A DHW cylinder holding 200 litres is a simple way to store hot water for use around the house.

In the context of renewable energy, it allows a heat pump with a COP of 2.5 to use perhaps 1.5 kW of electricity for 2 hours (3 kWh) to fully charge a cylinder with ~8.5 kWh of thermal energy. This can then be discharged at 10 litres per minute i.e. releasing stored energy at a rate of 21 kW.

The downsides of a DHW cylinder, (large size, 10% leakage per day, unknown temperature gradient within the tank) are generally considered acceptable.

But there are alternatives and we will look at one of these in the next article.

A Sand Battery: Not obviously a great idea.

July 21, 2022

Friends, a few weeks ago I was asked by several people whether I had heard about the latest ‘sand battery’? I had never heard of any such thing.

Most people asking me had come across this article on the BBC which gushingly described a heat storage device – not a battery – that had been built in Finland.

I quickly checked that I correctly understood the meaning of the word ‘battery’:

And I then concluded that this was a deliberate ruse to make a thermal storage system sound more interesting. No need – I love thermal storage systems!

Genuine Thermal Storage ‘Batteries’ do exist and Rosemary Barnes visited one on her Engineering with Rosie Channel a year or so ago.

In the Stiesdal company’s system that Rosie visited, excess electrical energy is stored as heat in an insulated container – similar to the sand battery. But instead of just using this stored thermal energy to directly heat homes (as the ‘Sand Battery’ does) the heat is used to drive a generator and make electricity. So it functionally, it operates like a conventional chemical battery – storing and releasing electrical energy.

In the video, Stiesdal claim overall efficiency is expected to be ~60% but the discharge function produces ‘waste heat’ that could be used for district heating, bringing overall storage efficiency to ~90%. In other words it can do what the sand battery does AND generate electricity.

But as will become clear, I suspect that efficiency is only for short term storage – a day or two – rather than genuinely seasonal storage. Anyway: back to the sand ‘battery’.

Sand Battery Details: How long can it store energy for?

The BBC report that:

Sand is a very effective medium for storing heat and loses little over time. The developers say that their device could keep sand at 500 °C for several months.[my emphasis]

Colloquially this sounds plausible: we are familiar with ceramic materials retaining heat for a long time. But I am sceptical.

Why? Because there are no perfect thermal insulators, and the rate of heat loss from an object is proportional to the difference between the object and its environment. So for an object at 500 °C, heat loss will likely be a serious problem.

I wondered if there might be a better way to do this, for example by choosing a material with a larger heat capacity. Then the same thermal energy could be stored in a medium at lower temperatures – and hence have lower heat losses. The storage material would also need to be cheap so I wondered about something really cheap: water.

Water? Yes. A given volume of water stores three times as much heat as sand heated to the same temperature. However (without pressurisation) water is limited to being heated to 100 °C.

Alternatively, one could say that sand is so poor at storing energy that it HAS to be heated to high temperatures to make it even half-way useful. But at high temperature it will lose heat faster. And storage times of months seem unlikely to me.


Friends, I made a calculation!

I calculated the storage time of two storage vessels both storing 8 MWh of thermal energy (like the Finnish design), one storing sand at 500 °C and the other storing water at 100 °C.

To understand the scale of this system:

  • A typical domestic hot water cylinder at 60 °C stores around 7 kWh of thermal energy so we are envisaging systems roughly 1,000 times larger than a domestic ‘thermal storage’ system.
  • My house requires approximately 4,500 kWh (4.5 MWh) of heating over a winter, so 8 MWh of thermal storage could store enough energy to heat perhaps two homes over winter.
  • Yes, I said two (2).

I fixed the height of the vessels at 7 m (as in the Finnish design) and varied the diameter of the vessels to store enough substance to store 8 MWh (as in the Finnish design). The water vessel was 3.8 m in diameter and the sand vessel was 2.8 m in diameter.

I assumed both vessels were covered with 300 mm of mineral wool insulation with a thermal conductivity of 0.03 W/m/°C, and then calculated the time constant over which the vessels would lose 50% of their stored heat.

Click on image for a larger version. Graph showing the rate at which two vessels (see text for details) would lose their stored energy.

  • The sand vessel would lose 50% of its heat in 2.1 months after which the water would still have retained 84% of its stored energy.
  • The water vessel would lose 50% of its stored energy after 8.4 months after which time the sand would have lost 94% of its stored energy.


Friends, I hate this kind of news story. 

This ‘sand battery’ can store enough heating over winter for 2 houses – neglecting losses – or more likely one house accounting for losses.

Community Thermal Storage – may well make sense in some contexts. Indeed I have seen other implementations of similar ideas (can’t find the links at the moment) that don’t require very high temperatures and hence high heat losses.

But this story reads like a BBC correspondent swallowed a PR story from Finland and then regurgitated it all over the BBC web site.

Inter-seasonal storage of renewable energy is an important technology that will be required if we want to build a system capable of supplying year-round energy from sustainable, but intermittent, sources.

But there are many alternatives. The Stiesdal implementation returns energy as electricity rather than heat, which is much more useful. The electricity could then be used to run heat pumps which would boost the overall efficiency of the system.

Alternatively energy could be stored as green hydrogen, or as pressurised gas. It is still not clear to me which technology will prove optimal: in the end it will come down to cost.

So it’s a complex situation, but facile stories like this do not help anyone.


40 degrees: reflections

July 20, 2022

Friends, Wow! So that was it. I have lived through temperatures probably never experienced in these islands since human beings lived on them.

I am finding it hard to grasp the significance of what has just passed, so these notes are mainly for myself, but I hope you find them relevant.


I wrote about this weather event last Friday because it had been predicted with high confidence by many computer models.

And as it turned out, the models did pretty well.

Click image for larger version. The left-hand panel shows the difference from ‘normal’ temperatures’ (aka ‘temperature anomaly) predicted one week in advance. The right-hand panel shows the same quantity as it actually happened. See text for details.

The images above show the forecast temperatures and the actual temperatures across Europe. My reference for this is a slightly obscure resource which Karsten Haustein  kindly tweeted about.

Considering the complexities of this forecast – think about the different landscapes across the continent for example – this is really impressive.

Note that the ‘actual’ temperatures are a ‘re-analysis product’ – this means they are based on weather station data but the gaps between the weather station locations are interpolated.

From this we can conclude something which is not obvious – this was not a ‘freak’ occurrence. It can be understood using the same basic physics that underpins all weather forecasts and climate models.

The experience

Click image for larger version. The temperature recorded by the weather station in my back garden over 18th and 19th July. The peak temperature was 39.9 °C.

I deliberately went out in the hot weather – 35 °C by 10 a.m. – just to experience it. It felt like being in another country.

And I was reminded that once air temperature exceeds body temperature (~37 °C) a breeze no longer cools the skin by aiding evaporation of sweat – it simply heats the body more effectively. And in the afternoon, with the air temperature around 39 °C, a breeze blew up and it felt like I was walking into a giant hairdryer. It felt shocking.

‘Popping down’ to the local supermarket for salad ingredients I found that the central refrigeration system had failed and all the food was being thrown out.

Click image for larger version. Total refrigeration failure at Teddington Tesco.

I felt a big relief in the evening when temperatures cooled down to (still hot!) 24 °C at 9 p.m.


Aside from the experience of walking in what felt like the output of a giant hairdryer, three things shocked me.

The first was the image of wildfires burning homes on the outskirts of London. The pictures below (stolen from a DW News Report entitled “Hot Weather Brings UK to a Halt“) are truly appalling, and reports indicate that the Fire Service was massively overstretched with similar wildfires across the UK.

The major parks near me – Bushy park and Richmond Park – are both extremely dry and these wildfires made me reflect that this danger was not restricted to the outskirts of London.

The second was an appreciation of the pan-continental scale of the heatwave.

Click image for larger version. The heatwave was pan-continental – see text for details . Image courtesy of NASA.

This image is from the NASA Earth Observatory and shows the surface air temperatures across most of the Eastern Hemisphere on July 13, 2022. It was produced by combining observations with a version of the Goddard Earth Observing System (GEOS) global model, which uses mathematical equations to represent physical processes in the atmosphere.

Of course it’s summer in the Northern Hemisphere, so one expects the map to be red. But as the article explains, temperature records are being broken right around the world.

And the heatwave has not gone away. The hot air has returned to the mainland of Europe, but it could of course return: August is usually just as hot as July in the UK.

And the finally the most shocking thing of all is that this event is still not enough to persuade politicians that this needs to be the focus of all their actions.

It seems they are simply unable to grasp that ‘business as usual’ is the problem, not the solution.

40 degrees Celsius

July 15, 2022

Click image for a larger version. Met Office forecast on Friday 15th July 2022 for temperatures on Tuesday 19th July 2022.

Friends, I am sitting down to write on Friday 15th July 2022, having just read warnings that next week the temperature somewhere in the UK is likely to exceed 40 °C for the first time since… well almost certainly several thousand years.

Despite the fact that this is hardly a surprise, it is still a shock. I feel sick.

Rising Tide

I am reminded of the situation of visiting an unfamiliar beach, and wanting to know if the tide is coming in or going out. Sometimes it’s obvious. But at other times things are not so clear. The waves breaking can obscure the slow steady rise or fall of the tide.

So typically one would pick a ‘fiducial marker’ – perhaps a rock that is very definitely not wet. And one then watches this for a few minutes to see if the tide moves towards it or away from it.

Passing 40 °C is like the tide reaching the fiducial marker on the beach. It confirms that the tide of rising temperatures is still rising with a sickening and merciless inevitability.


Click for a larger version. This is the summary of part 1 of a recent talk I gave to teenagers.

As I mentioned in a recent talk to teenagers (link) the climate in which their parents grew up is gone forever. It will never return. And to most of my readers – that’s the climate that you and I grew up in.

As I wrote the slides for that talk I again felt sick at confronting these children with the magnitude of the misfortune they will face.

It’s a misfortune that was once avoidable, but which is now inevitable.

And yet as I write, leading UK politicians are still competing with each other to reduce our response to this challenge.

I am bewildered at their madness.

What else can I do?

July 14, 2022

Friends, while writing the slides for my recent talk to teenagers, I became very aware of the awfulness of the future facing the children I was addressing – and my own children who are only around 8 years older.

This awareness took the form of something between panic and gloom. And it caused me to reflect on my own efforts to reduce carbon dioxide emissions, and to ask myself “What else could I do?”.

In case you are short of time I will summarise my conclusion here: I have eliminated most of carbon emissions from my life that can be cut by simply spending money! To go further, requires significant lifestyle changes. And maybe trying harder to influence other people.

What am I doing already?

As summarised on the ‘My House‘ pages of this blog and this video, I have spent most of my life-savings (aka my ‘Pension Tax-Free Lump Sum’ of around £60,000) on steps to reduce carbon dioxide emissions from my house in Teddington.

Briefly, the money has been spent on:

  • Triple Glazing & External Wall Insulation reducing the heating demand by half.
  • 12 Solar Panels generating ~3,500 kWh/year of electricity, Together with a battery, this is enough to take us off-grid for roughly 90 days a year and to substantially reduce the use of grid electricity in all but the darkest months.
  • A heat pump eliminating the use of gas for heating.

Altogether, these steps have reduced CO2 emissions associated with the house by around 80%, from 3.7 tonnes per year to about 0.8 tonnes per year.

And our lifestyle has not been impacted at all: in fact the house is more comfortable: warmer in winter and cooler in summer – cooled with solar-powered air conditioning.

These carbon emissions are real reductions of emissions – actions that result in no CO2 being emitted into the atmosphere when compared with the alternative of not taking these actions. And after the carbon payback period, emissions associated with the house will be around 3 tonnes less per year than they would have been.

But in this calculation I have not included three other things that could notionally be included.

  • Exports: Each year the house exports ~900 kWh of electricity to the grid. One can argue about how much this reduces emissions from other people’s use of electricity.  But this probably reduces emissions by between 0.2 to 0.4 tonnes per year.
  • Direct Air Capture: Each month I pay the Climeworks foundation £40 and in return they promise to directly capture 50 kgCO2 from the air and turn it into carbonate rock deep beneath the surface of Iceland. I really don’t know how well-validated this process is, but Climeworks promise that within 6 years of my payment, they will permanently remove 600 kgCO2/year from the atmosphere ‘in my name’. Notice this is not ‘offsetting’ which I believe to be tantamount to fraud.
  • Wind Farm: Earlier this year my wife & I paid Ripple £2,000 to buy a share of an 8-turbine wind farm they plan to build in Scotland. This share is sufficient to generate roughly 3,500 kWh/year of electricity – 100% of the electricity the house draws from the grid each year. It’s scheduled for completion in November 2023 and from that point onwards, the carbon emissions nominally associated with the house will fall by very roughly 600 kgCO2/year.

In accounting terms, this means that – as the graph below shows – the household could possibly be classified as ‘carbon negative’.

Click on image for a larger version. The graph shows cumulative carbon dioxide emissions from the house up to the year 2040 based on several different assumptions. The red line shows expected emissions if I had not done any work on the house. The green line shows the effect of those works. The dotted black line shows the effect of my paying for Direct Air Capture of CO2. And the dotted blue line shows the effect of my purchase of a fraction of a ‘Ripple’ Wind Farm. The carbon embodied in the modifications to the house is now (July 2022) just about paid off.

What else could I do?


There is only limited scope for further work on the house. Installing underfloor insulation could reduce the heating requirements by perhaps another 20%. And there is room for a few more solar panels and more batteries. However neither change would alter the graph above dramatically.

Additionally I am keen not to adopt a techno-utopian stance – forever consuming more of the latest tech to enable me to humble-brag about some sexy piece of equipment.

So what about emissions from other aspects of my life – Transport, Consumption and Pensions.


My wife and I still own and drive a car and we drive around 3,500 miles/year which corresponds to just over 0.8 tonnes of CO2 emissions.

It pains me every time I drive – wasting 70% of the energy in the petrol and emitting CO2 directly. However, although my wife might be able to afford an electric car, with our low annual mileage, the 10 tonnes of CO2 emitted during the manufacture of an EV is hard to justify.

Probably our the best strategy is just to reduce the amount we drive – cycling and using public transport even more than we do.

I guess at some time I will be obliged to travel by air again – but until that becomes necessary, I will simply try to avoid this. Although the idea of international travel is intermittently attractive (particularly to my wife), to me it seems too appalling to emit tonnes of CO2 in an afternoon on nothing more than a whim!


I have given up taking milk in my tea (and coffee) and this has resulted in our using 1 litre of milk less each week – saving 50 litres per year. Using figures from Our World in Data, this – apparently – reduces our annual carbon footprint by ~ 0.15 tonnes! Other sources suggest that UK milk does not have such high emissions, and the avoided emissions are more like 0.075 tonnes (75 kg). Whatever the actual answer is, I have made the switch and I don’t intend to switch back. And even 75 kg per year up to my planned date of death is 18 x 75 = 1,350 kg CO2.

My wife and I have both changed our diet significantly in recent years: reducing beef purchases to zero, and only eating other meats perhaps once a week. We eat vegetables and salads much more than we used to, including food from my wife’s allotment which has low associated ‘food miles’. However, we still eat eggs and cheese.

I am trying to buy fewer ‘things’. And as my perspective has slowly shifted, I have realised I really need very few new objects.


As I lamented in an earlier article, the money invested on my behalf by Legal and General and USS probably generates many tonnes of CO2 – probably more than all the other categories combined.

After writing that article, a correspondent suggested to me that considering emissions from my pension savings was double-counting. In other words, I was counting emissions from say petrol purchases already under the ‘travel’ category, so counting those again as part of a share portfolio that probably includes oil companies was not consistent. This is a fair point. The emissions associated with an oil company’s activities can either be assigned to the consumers of their products, or the shareholders, but not both.

However, my money is still invested in ways I would not personally choose. But I feel so unsure of myself as an investor that I am not confident that switching investment funds would result in an improvement.


Aside from emitting as little CO2 as I can, I also want to live a life which includes joyful activities and is not a relentless drudge.

But it seems that I am approaching the limits of what I can do personally to live a life with minimal emissions of carbon dioxide.

There are still actions I can take, – and I will – but at this point it seems that probably the most significant thing I can do is to try to influence other people to take action to reduce their carbon dioxide emissions.

Mmmmm. I will have a think about how best to do that.

Our Climate Crisis: Bringing it all back home

July 12, 2022

Friends, I have just returned from a holiday on the South Coast in the ancient town of Winchelsea.

This slideshow requires JavaScript.

It was a lovely break, but it was not all crazy golf and castles. I also took a day off to come back to London to give a talk to some 6th formers about climate change.

I thought that possibly some other people might be interested in the presentation and so the other day I sat down in front of my television and talked my way through the slides.

The school presentation was 45 minutes long, but given the liberty of time, the video version is  slightly longer.

In the first half of the talk – video 1 (23 minutes long) – I look at the nature of our climate crisis and the urgency of the need for action. I thought the presentation of the effect of carbon dioxide in the atmosphere was quite novel – unusual when covering something I have talked about so many times. But writing the slides I was shocked again at just how bleak our situation is.

In the second half of the talk – video 2 (35 minutes long) – I explain how the way in which we use energy in our homes gives rise to carbon dioxide emissions – typically more than 3 tonnes per household. I then point out that thankfully we have the technology to reduce emissions dramatically without any loss in quality of life. And I conclude by pointing out that throughout their lives there will be many career opportunities for these teenagers to take part in the grand challenge of limiting the damage from Climate Change.

Powerpoint File

Powerpoint File  (50 Mb!)

Video 1

Video 2



Carbon Emissions from my Pension

June 8, 2022

Friends, in recent years, I have spent a good deal of time and money trying to reduce ‘my’ emissions of carbon dioxide (CO2).

I have considered my CO2 emissions in four categories.

  • House
  • Travel
  • Consumption
  • Savings

I have written extensively on this blog about about reducing CO2 emissions from the house. So far annual emissions are down from ~3.7 tonnes CO2 per year to ~0.7 tonnes of CO2 per year. I will write more about how I am working to eliminate those residual emissions in due course.

Regarding travel, emissions are probably ~ 1 tonne of CO2 per year, mainly associated with the 3,500 miles a year we drive our family car. But I am traveling less, using public transport when possible, and avoiding flying.

Regarding consumption emissions are probably ~ 1 tonne of CO2 per year, but I am eating less meat, I have stopped drinking milk and I am eating more vegetables. I am also trying hard to stop just buying “new things”.

But I have also been thinking hard about my savings, which in my case is mainly invested in my pension. And this article is about trying to work out the carbon emissions associated with my pension investments – and what I can best do about them.

If you don’t want to read to the end, then my conclusion is that my pension savings are now – by a large measure – likely to be responsible for more CO2 emissions than any other aspect of my life.

My Pension Savings

I am fortunate enough to have two pensions.

I have no control over the first pension from my 13-year University career which ended in 2000. This is a final-salary scheme pension and I cannot influence how the money is invested.

However, I do have some control over my second pension from my 20 year ‘career’ at NPL. This is basically just a pile of money invested by Legal and General (L&G) on my behalf.  I withdraw money from it each month to allow me to continue my – frankly decadent – lifestyle.

The pension is invested in all kinds of ‘assets’ and companies all over the world via a fund called the “L&G PMC Multi-Asset 3” fund.

Click for a larger version. This is how my pensions savings are invested. Basically – I am into everything! In investing circles this is known as a ‘diversified portfolio’.

I can switch my pension savings with L&G from one pension fund to another, but they all do broadly similar things. And how can I tell whether one fund is less damaging to the climate than another?

Estimating CO2 emissions from Investments

In order to estimate CO2 emissions from my pension investments I would need to track down the hundreds of companies in which it is invested, research their carbon dioxide emissions, and then work out how much of those emissions are mine by considering what fraction of their share capital I owned.

That is a lot of work to do for even a single UK company, and it isn’t feasible to do that for my pension savings where I don’t even know the names of the individual companies involved!

And frankly, I don’t need that level of detail. I would be happy to know the order of magnitude of emissions. Is it tonnes per year? Or tens of tonnes per year? Or even hundreds of tonnes per year?

The web has not been helpful, but I did find this Canadian article which listed the CO2 emissions of different companies per CAN$100 of their market capitalisation – the combined value of all their shares.

Click on image for a larger version. Based on a Canadian investment average, CAN$100 yield 14.35 kg of CO2 emissions per year. I have converted this to show the emissions per £1000 based on an exchange rate of 0.63 CAN$ to the pound.

They also helpfully showed the average emissions for a ‘diversified portfolio’ of shares – similar to my pension savings – which (when converted to pounds) comes out to 228 kgCO2 per year £1000 of savings.

We can also estimate this very approximately in other ways.

For example one could take the value of the FTSE100 Share Index (~2.0 trillion pounds [£2.0 x 10^12]) and a divide by the UK’s annual CO2 emissions (~450 million tonnes CO2 [0.45 x 10^12 kgCO2/year]) to yield 225 kgCO2 per year £1000 of savings. This estimate is based on the idea that the companies in this index are responsible for the majority of the UK’s CO2 emissions.

Alternatively one could take the value of the Dow Jones Share Index (~27 trillion dollars [£27 x 10^12]) and a divide by the USA’s annual CO2 emissions (~4500 million tonnes CO2 [4.5 x 10^12 kgCO2/year]) to yield (after converting from dollars) 133 kgCO2 per year £1000 of savings.

All of these estimates are inaccurate, but – slightly to my surprise – they are all of the same order of magnitude. I will take this as an indication that this is correct order of magnitude.

Given the ‘diversified portfolio’ used by my pension fund, it seems likely that every £1000 of my pension investments is responsible for on the order of 100 kgCO2 emissions per year. Let’s use that figure and bear in mind that it might be easily twice as large, or possibly a bit smaller.

When I looked today, my L&G pension fund stood at £175,000 and so based on this sort of analysis, my savings are responsible for ~17.5 tonnes of CO2 emissions per year. My university pension is probably associated with similar emissions.

So my pension savings are likely associated with ‘a few tens of tonnes’ of CO2 emissions per year, much greater than emissions associated with the house, my travel or my consumption.

What to do? Follow the carbon dioxide…

Friends, I have put off doing this calculation because I feared the answer, which I confess I did not find at all surprising.

The fact that my pension – which I am relying on to keep me comfortable in my decaying years – is my largest source of carbon emissions is deeply troubling.

It’s troubling because all my options are problematic.

In order to decide what to do, I think it is important to focus on the carbon dioxide. So for example, at the moment, because of the battery and solar panels, I have been off-grid for 33 days. This allows me to know for certain that this house is emitting no carbon dioxide when it otherwise would have been.

But changing my pension investments from one fund to another? I think that this will result in no changes in carbon dioxide emissions, even if the great carbon accountant in the sky no longer associates those emissions with my pension.  I think that is just accountancy.

I could probably move my pension savings to a fund marked as ESG (Environmental and Social Governance). Companies in these funds are deemed to be operating ‘ethically’. Presumably the implication is that all the other companies are unethical! But having read a little bit about ESG certification, it seems to be a tickbox process, and there is likely to be widespread fraud and greenwashing.

And even if I did move my savings, the ‘unethical companies’ producing steel and oil and weapons – things which society collectively still needs – would not cease to exist. I think this would again be mere accounting.

Indeed, almost any diversified portfolio – the kind I would choose to retain the value of my savings – by definition is invested in the roots and branches of our western society which is collectively failing to address Climate Change.

So I am unsure what to do.

The only thing that I feel clearer about after this analysis is that if I had a chance to use this money to invest in projects which genuinely reduced CO2 emissions – wind farms or solar farms or energy storage projects – then devoting a fraction of my savings to that would be likely to genuinely reduce the emissions associated with my pension savings.

In conclusion, and I hope you don’t feel this is a cop-out – I feel this analysis shows the limited applicability of the concept of a ‘personal’ carbon footprint.

In truth, our society’s carbon dioxide emissions are all ‘ours’ whether we own shares in the companies or not. Ultimately, we need to act collectively to eliminate them.

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