I love Greta Thunberg

Click image for a larger version. My son gave me a Christmas Tree decoration in the likeness of Greta Thunberg.

Friends, love is a strong word.

Back in 2012 I wrote that I loved James Hansen. If you haven’t heard it, I strongly recommend his TED talk.

I wrote:

When I hear him speak I feel I am listening to a human being who understands enough to feel compelled to shout ‘Fire’ in the ‘cinema’ of the modern world. He feels that no matter what the consequences, we must face up to the climate challenge ahead. Being prepared to be arrested for his insistence that the US government should listen to what the science (they have paid for!) has to say seems like an act of great bravery to me.

And today I would like to declare a similar – but different – admiration for Greta Thunberg. Greta is not after all, a world-leading scientist.

The fact that my son gave me a Christmas Tree decoration in the likeness of Greta Thunberg – but not James Hansen – is testament to their different roles. He also gave me a book of Greta’s speeches and not a copy of James Hansen’s papers (e.g. this one from 1981).

Greta Thunberg has unintentionally become a global cultural phenomenon. But having read her book, I can assure you it is not because of her oratory. It is because of her unflinching honesty.

Reading her words addressed to old people like me, I do not feel inspired: I feel shamed.

I will leave you with a quote from the book: Greta’s speech to the UK Parliament in 2019. I hope you too will be as affected by her honesty as I have been.

UK Parliament 2019

23 April 2019:

“Is my microphone on? Can you hear me?

Around the year 2030, 10 years 252 days and 10 hours away from now, we will be in a position where we set off an irreversible chain reaction beyond human control, that will most likely lead to the end of our civilisation as we know it. That is unless in that time, permanent and unprecedented changes in all aspects of society have taken place, including a reduction of CO2 emissions by at least 50%.

And please note that these calculations are depending on inventions that have not yet been invented at scale, inventions that are supposed to clear the atmosphere of astronomical amounts of carbon dioxide.

Furthermore, these calculations do not include unforeseen tipping points and feedback loops like the extremely powerful methane gas escaping from rapidly thawing arctic permafrost.

Nor do these scientific calculations include already locked-in warming hidden by toxic air pollution. Nor the aspect of equity – or climate justice – clearly stated throughout the Paris agreement, which is absolutely necessary to make it work on a global scale.

We must also bear in mind that these are just calculations. Estimations. That means that these “points of no return” may occur a bit sooner or later than 2030. No one can know for sure. We can, however, be certain that they will occur approximately in these timeframes, because these calculations are not opinions or wild guesses.

These projections are backed up by scientific facts, concluded by all nations through the IPCC. Nearly every single major national scientific body around the world unreservedly supports the work and findings of the IPCC.

Did you hear what I just said? Is my English OK? Is the microphone on? Because I’m beginning to wonder.

During the last six months I have travelled around Europe for hundreds of hours in trains, electric cars and buses, repeating these life-changing words over and over again. But no one seems to be talking about it, and nothing has changed. In fact, the emissions are still rising.

When I have been travelling around to speak in different countries, I am always offered help to write about the specific climate policies in specific countries. But that is not really necessary. Because the basic problem is the same everywhere. And the basic problem is that basically nothing is being done to halt – or even slow – climate and ecological breakdown, despite all the beautiful words and promises.

The UK is, however, very special. Not only for its mind-blowing historical carbon debt, but also for its current, very creative, carbon accounting.

Since 1990 the UK has achieved a 37% reduction of its territorial CO2 emissions, according to the Global Carbon Project. And that does sound very impressive. But these numbers do not include emissions from aviation, shipping and those associated with imports and exports. If these numbers are included the reduction is around 10% since 1990 – or an an average of 0.4% a year, according to Tyndall Manchester.

And the main reason for this reduction is not a consequence of climate policies, but rather a 2001 EU directive on air quality that essentially forced the UK to close down its very old and extremely dirty coal power plants and replace them with less dirty gas power stations. And switching from one disastrous energy source to a slightly less disastrous one will of course result in a lowering of emissions.

But perhaps the most dangerous misconception about the climate crisis is that we have to “lower” our emissions. Because that is far from enough. Our emissions have to stop if we are to stay below 1.5-2 °C of warming. The “lowering of emissions” is of course necessary but it is only the beginning of a fast process that must lead to a stop within a couple of decades, or less. And by “stop” I mean net zero – and then quickly on to negative figures. That rules out most of today’s politics.

The fact that we are speaking of “lowering” instead of “stopping” emissions is perhaps the greatest force behind the continuing business as usual. The UK’s active current support of new exploitation of fossil fuels – for example, the UK shale gas fracking industry, the expansion of its North Sea oil and gas fields, the expansion of airports as well as the planning permission for a brand new coal mine – is beyond absurd

This ongoing irresponsible behaviour will no doubt be remembered in history as one of the greatest failures of humankind.

People always tell me and the other millions of school strikers that we should be proud of ourselves for what we have accomplished. But the only thing that we need to look at is the emission curve. And I’m sorry, but it’s still rising. That curve is the only thing we should look at.

Every time we make a decision we should ask ourselves; how will this decision affect that curve? We should no longer measure our wealth and success in the graph that shows economic growth, but in the curve that shows the emissions of greenhouse gases. We should no longer only ask: “Have we got enough money to go through with this?” but also: “Have we got enough of the carbon budget to spare to go through with this?” That should and must become the centre of our new currency.

Many people say that we don’t have any solutions to the climate crisis. And they are right. Because how could we? How do you “solve” the greatest crisis that humanity has ever faced? How do you “solve” a war? How do you “solve” going to the moon for the first time? How do you “solve” inventing new inventions?

The climate crisis is both the easiest and the hardest issue we have ever faced. The easiest because we know what we must do. We must stop the emissions of greenhouse gases. The hardest because our current economics are still totally dependent on burning fossil fuels, and thereby destroying ecosystems in order to create everlasting economic growth.

“So, exactly how do we solve that?” you ask us – the schoolchildren striking for the climate.

And we say: “No one knows for sure. But we have to stop burning fossil fuels and restore nature and many other things that we may not have quite figured out yet.”

Then you say: “That’s not an answer!”

So we say: “We have to start treating the crisis like a crisis – and act even if we don’t have all the solutions.”

“That’s still not an answer,” you say.

Then we start talking about circular economy and rewilding nature and the need for a just transition. Then you don’t understand what we are talking about.

We say that all those solutions needed are not known to anyone and therefore we must unite behind the science and find them together along the way. But you do not listen to that. Because those answers are for solving a crisis that most of you don’t even fully understand. Or don’t want to understand.

You don’t listen to the science because you are only interested in solutions that will enable you to carry on like before. Like now. And those answers don’t exist any more. Because you did not act in time.

Avoiding climate breakdown will require cathedral thinking. We must lay the foundation while we may not know exactly how to build the ceiling.

Sometimes we just simply have to find a way. The moment we decide to fulfil something, we can do anything. And I’m sure that the moment we start behaving as if we were in an emergency, we can avoid climate and ecological catastrophe. Humans are very adaptable: we can still fix this. But the opportunity to do so will not last for long. We must start today. We have no more excuses.

We children are not sacrificing our education and our childhood for you to tell us what you consider is politically possible in the society that you have created.

We have not taken to the streets for you to take selfies with us, and tell us that you really admire what we do.

We children are doing this to wake the adults up.

We children are doing this for you to put your differences aside and start acting as you would in a crisis.

We children are doing this because we want our hopes and dreams back.

I hope my microphone was on. I hope you could all hear me.

The James Webb Space Telescope

Friends, a gift to humanity!

On Christmas Day at 12:20 GMT/UTC, the James Webb Space Telescope will finally be launched.

You can follow the countdown here and watch the launch live via NASA or on YouTube – below.

In May 2018 I was fortunate enough to visit the telescope at the Northrop Grumman facility where it was built, and to speak with the project’s former engineering director Jon Arenberg.

Everything about this telescope is extraordinary, and so as the launch approaches I thought that it might be an idea to re-post the article I wrote back in those pre-pandemical days.

As a bonus, if you read to the end you can find out what I was doing in California back in 2018!

Happy Christmas and all that.

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

Last week I was on holiday in Southern California. Lucky me.

Lucky me indeed. During my visit I had – by extreme good fortune – the opportunity to meet with Jon Arenberg – former engineering director of the James Webb Space Telescope (JWST).

And by even more extreme good fortune I had the opportunity to speak with him while overlooking the JWST itself – held upright in a clean room at the Northrop Grumman campus in Redondo Beach, California.

[Sadly, photography was not allowed, so I will have to paint you a picture in words and use some stock images.]

The JWST

In case you don’t know, the JWST will be the successor to the Hubble Space Telescope (HST), and has been designed to exceed the operational performance of the HST in two key areas.

• Firstly, it is designed to gather more light than the HST. This will allow the JWST to see very faint objects.
• Secondly, it is designed to work better with infrared light than the HST. This will allow the JWST to see objects whose light has been extremely red-shifted from the visible.

A full-size model of the JWST is shown below and it is clear that the design is extraordinary, and at first sight, rather odd-looking. But the structure – and much else besides – is driven by these two requirements.

Requirement#1: Gather more light.

To gather more light, the main light-gathering mirror in the JWST is 6.5 metres across rather than just 2.5 metres in the HST. That means it gathers around 7 times more light than the HST and so can see fainter objects and produce sharper images.

Image courtesy of Wikipedia

But in order to launch a mirror this size from Earth on a rocket, it is necessary to use a  mirror which can be folded for launch. This is why the mirror is made in hexagonal segments.

To cope with the alignment requirements of a folding mirror, the mirror segments have actuators to enable fine-tuning of the shape of the mirror.

To reduce the weight of such a large mirror it had to be made of beryllium – a highly toxic metal which is difficult to machine. It is however 30% less dense than aluminium and also has a much lower coefficient of thermal expansion.

The ‘deployment’ or ‘unfolding’ sequence of the JWST is shown below.

Requirement#2: Improved imaging of infrared light.

The wavelength of visible light varies from roughly 0.000 4 mm for light which elicits the sensation we call violet, to 0.000 7 mm for light which elicits the sensation we call red.

Light with a wavelength longer than 0.000 7 mm does not elicit any visible sensation in humans and is called ‘infrared’ light.

Imaging so-called ‘near’ infrared light (with wavelengths from 0.000 7 mm to 0.005 mm) is relatively easy.

Hubble can ‘see’ at wavelengths as long as 0.002 5 mm. To achieve this, the detector in HST was cooled. But to work at longer wavelengths the entire telescope needs to be cold.

This is because every object emits infrared light and the amount of infrared light it emits is related to its temperature. So a warm telescope ‘glows’ and offers no chance to image dim infrared light from the edge of the universe!

The JWST is designed to ‘see’ at wavelengths as long as 0.029 mm – 10 times longer wavelengths than the HST – and that means that typically the telescope needs to be on the order of 10 times colder.

To cool the entire telescope requires a breathtaking – but logical – design. There were two parts to the solution.

• The first part involved the design of the satellite itself.
• The second part involved the positioning the satellite.

Cooling the telescope part#1: design

The telescope and detectors were separated from the rest of the satellite that contains elements such as the thrusters, cryo-coolers, data transmission equipment and solar cells. These parts need to be warm to operate correctly.

The telescope is separated from the ‘operational’ part of the satellite with a sun-shield roughly the size of a tennis court. When shielded from the Sun, the telescope is exposed to the chilly universe, and cooled gas from the cryo-coolers cools some of the detectors to just a few degrees above absolute zero.

Cooling the telescope part#2: location

The HST is only 300 miles or so from Earth, and orbits every 97 minutes. It travels in-to and out-of full sunshine on each orbit. This type of orbit is not compatible with keeping a gigantic telescope cold.

So the second part of the cooling strategy is to position the JWST approximately 1 million miles from Earth at a location beyond the orbit of the moon at a location known as the second Lagrange point L2. But JWST does not orbit the Earth like Hubble: it orbits the Sun.

Normally the period of orbits around the Sun get longer as satellites orbit at greater distances from the Sun. But at the L2 position, the gravitational attraction of the Earth and Moon add to the gravitational attraction of the Sun and speed up the orbit of the JWST so that it orbits the Sun with a period of one Earth year – and so JWST stays in the same position relative to the Earth.

• The advantage of orbiting at L2 is that the satellite can maintain the same orientation with respect to the Sun for long periods. And so the sun-shade can shield the telescope very effectively, allowing it to stay cool.
• The disadvantage of orbiting at L2 is that it is beyond the orbit of the moon and no manned space-craft has ever travelled so far from Earth. So once launched, there is absolutely no possibility of a rescue mission.

The most expensive object on Earth?

I love the concept of the JWST. At an estimated cost of $8 billion $10 billion, if this is not the most expensive single object on Earth, then I would be interested to know what is.

But it has not been created to make money or as an act of aggression.

Instead, it has been created to answer the simple question

“I wonder what we would see if we looked into deep space at infrared wavelengths.”. 

Ultimately, we just don’t know until we look.

In a year or two, engineers will place the JWST on top of an Ariane rocket and fire it into space. And the most expensive object on Earth will then – hopefully – become the most expensive object in space.

Personally I find the mere existence of such an enterprise a bastion of hope in a world full of worry.

Thanks

Many thanks to Jon Arenberg  and Stephanie Sandor-Leahy for the opportunity to see this apogee of science and engineering.

Resources

Breathtaking photographs are available in galleries linked to from this page

Christmas Bonus

Re-posting this article, I remembered why I was in Southern California back in May 2018 – I was attending Dylanfest – a marathon celebration of Bob Dylan’s music as performed by people who are not Bob Dylan.

The pandemic hit Dylanfest like a Hard Rain, but in 2020 they went on-line and produced a superb cover of Subterranean Homesick Blues which I gift to you this Christmas. Look out for the fantastic guitar solo at 1’18” into the video.

And since I am randomly posting performances inspired by Dylan songs, I can’t quite leave without reminding you of the entirely palindromic (!) version of the song by Wierd Al Yankovic.

Happy Christmas and Best Wishes for 2022

Click image for a larger version

Friends, it’s the end of the year and there is still so much to write about. But for the next couple of weeks, it won’t be me doing the writing.

I feel the need for a break and so I will be hunkering down in Podesta Towers and dreaming of spring sunshine on my solar panels.

My aim is to stay warm, catch up on some other projects, and try to avoid catching Omicron!

I wish you all the best for the Christmas Season and a splendidly low-carbon 2022.

COVID 19: Wave#3: Omicron Dawn

Friends, I have been keeping quiet about pandemical developments recently because – frankly – I couldn’t think of anything to say.

Looking back at my most recent comments, I realise that I have felt this way for a while.

In October I wrote:

“The UK seems to be living out a form of collective cognitive dissonance, with the scale of infection and death very high from a global perspective. And yet there is widespread behaviour as if it were all over.”

“My guess is that if the death rate and hospitalisation rates remain similar to current rates, then ‘people’ will accept almost any level of infection rates – no matter what the eventual harm from Long COVID, or the risk of generating further variants.

And in November I added:

As we stare into the coming winter, I find it very hard to see how the epidemic will evolve; how the government will respond or how people will respond.

Since then, the disease has continued to kill just over 100 people per day and the third-wave death toll is currently approximately 18,431.

My latest data collation is shown below:

Click the image for a larger version. Logarithmic graph showing positive cases, hospital admissions and deaths since the start of the pandemic. The blue arrows show the dates of 2021’s ‘opening’ events. The green dotted line shows an extrapolation from the first week of June. The blue dotted lines show an extrapolation of trends, doubling every 41 days. Also highlighted in purple are the Euro finals, and the dates of returns to school and university in 2020 and 2021. Growth with a doubling time of ~3 days is shown by dotted arrows on the right-hand side of the graph.

The last few months

Very roughly since August we have experienced (in rounded numbers):

• Roughly 40,000 cases per day
• Just less than 1000 admissions per day (~0.25 % of cases)
• More than 100 deaths per day(~15 % of admissions and ~0.04% of reported cases)

There are periods of 2 to 3 weeks where the epidemic grows exponentially, but at a relatively slow rate – doubling every 6 weeks or so. But these growth periods, while concerning, are not sustained.

And we seemed to be set for a long and miserable COVID winter. But a winter in which the NHS was not overwhelmed and in which a tolerable level of social activities could take place.

Omicron Dawn

Although current data show trends continuing as before, genomic analysis indicates that a rapidly increasing fraction of the cases are caused by the so-called omicron variant of COVID-19.

Omicron appears to spreading rapidly with an estimated doubling time of just 3 days or so. This is similar to the doubling rate when the original viral strain hit the UK back in March 2020 – before we had any mitigations in place.

The fact that omicron can spread at this rate amongst a population in which the vast majority have immunity from either vaccination or prior infection is worrying.

The significance of the short doubling time is hard to over-emphasise.

Over a period of 2 weeks during which prevalence of the current delta variant might hardly change, prevalence of the omicron variant might grow by a factor ~16 – all other things remaining constant.

So there is the potential for significant harm.

• Positive cases could exceed 100,000/day – off the top of the graph at the head of the page. Many people would be ill and isolating. And mixing at Christmas could easily lead to super-spreading events.
• Even if omicron is less harmful than delta, and even if the growth is not quite so explosive as anticipated, hospital admissions – particularly in certain areas – could rise rapidly to high levels.
• And if cases and admissions rise, then an increased rate of death would inevitably follow a couple of weeks later.

Please notice the words in blue above. Among European countries, omicron seems to be hitting the UK first, and so there is still considerable uncertainty about how the epidemic will evolve.

So…

We are expecting:

• A sharp rise in cases in the couple of weeks leading up to Christmas.
• A delayed rise in admissions to hospital around the start of the New Year.
• And a rise in the rate of death in January 2022.

But the magnitude of all these effects is very hard to anticipate.

Please take care.

I’m dreaming of 2026…

Friends, a curious thought occurred to me yesterday:

“What would happen if the Hinckley C nuclear Power Station was completed on time in 2025?”

I know it’s unlikely but it is conceivable.

But whenever it does start operating, it will produce an extra 3.26 GW of electricity 24 hours a day, 7 days a week. This is enough to change the UK electricity generation “market” at a stroke.

 January 2020

To illustrate the effect that Hinkley C will have, I downloaded hourly generation figures for 10 days at the start of 2020.

I picked this range of dates because the data was free – I would need to have paid to obtain more data! But it is sufficient to make my point. Take a look at Gridwatch for monthly and yearly summaries.

I have not plotted all the generating sources or interconnectors bringing us electricity, but instead I have just drawn three curves from the data set.

• Total Demand
• Nuclear 
• Nuclear + Wind

Click Image for a larger version. Generation from the first 10 days of 20 showing electricity from nuclear power stations, the sum of nuclear + wind generation, and total demand. The gap between the black and green curves is filled with electricity from a variety of sources, but mainly with electricity from gas-fired generators.

I have picked these data because of the way the UK grid is run. In simplified form, it works like this:

• First the grid accepts all the nuclear electricity available. This is because (for technical reasons) nuclear power stations cannot easily change their output.
• Then the grid accepts whatever renewable energy (solar or wind) that is available. This can be well-predicted a day or two ahead of time.
• Then, through a complex system of contracts, and “market”-mechanisms, the grid adds electricity from a variety of sources to meet demand. Most of this is usually met by gas-fired generation which emits ~ 450 g CO2/kWh of electricity supplied.

But what would the situation look like if Hinkley C were operating, and wind generation were twice it’s value in 2020?

This situation could hypothetically occur as soon as 2026.

January 2026?

• IF demand in January 2026 were by chance the same as in 2020, and…
• IF Hinkley C were generating at full power and…
• IF wind power was twice what it was in 2020…

…then we would find ourselves in the extraordinary situation depicted in the graph below.

Click Image for a larger version. A hypothetical situation in January 2026 when nuclear supply is supplemented 3.26 GW of generation by Hinkley C, and wind generation is twice what it was in 2020. The gap between the black and green curves is now routinely reversed – indicating a regular ‘oversupply’ of green electricity.

The graph shows that the combination of ‘nuclear + wind would’ regularly exceed demand before considering any other sources of generation.

During these periods demand would be met entirely with low carbon sources and the carbon intensity of UK electricity would fall to pleasingly low values (~50 gCO2/kWh).

And during these periods – shown in blue on the graph above – there would be typically 10 GWh a day of ‘surplus electricity’.

The marginal cost of this ‘surplus’ electricity is debatable, but it is close to zero.

Could this really happen?

Yes. It has already happened briefly this year over the late May bank holiday weekend. And as renewable electricity generation grows, such situations will occur ever more frequently.

And whenever Hinkley C comes on stream – in 2025 or later – such events will inevitably become commonplace.

What are the consequences?

I don’t know!

The electricity “market” operates by complex rules, and as this year’s ‘odd’ May weekend event showed, prices cannot just fall to zero, but actually become negative: i.e. companies will pay you to use their electricity!

But however it is dealt with by the “market” rules, the reality is that the UK will be routinely generating renewable electricity in excess of demand at a cost close to zero. This has consequences at many levels.

• First of all: from the point of view of investors looking to build renewable generation – solar or wind – they will no longer be able to ‘dump’ electricity onto  “the market” whenever the Sun happens to shine and the wind happens to blow. This will make life more difficult for these investors.
• Secondly: anyone who can store energy for later re-sale or re-use will have access to very low cost electricity. This represents an opportunity for many nascent industries and technologies.

However the biggest consequence could be a change in conception of how the “market” operates. So far, we have almost always assumed that supply will adjust to meet demand. We have had some incentives to use ‘off-peak’ electricity, but not many.

With an ‘oversupply’ of electricity, there will be opportunities for industries which can adjust demand to meet available supply. If this is implemented well, then the economic singularities arising from a zero price will be avoided – but the energy should still be cheap and it will still be green.

Opportunities

If green hydrogen is ever to play a role in the UK, then this could present an initial opportunity. 10 GWh/night of electricity is sufficient to generate 200 tonnes of hydrogen (@50 kWh/kg).

If EV use grows at the rate that many anticipate, then smart charging at night could help sustain that growth. 10 GWh/night would be enough to deliver 25 kWh (roughly a 50% charge) to 400,000 EVs.

I am sure many more ideas will emerge about how to reap this low-cost, low-carbon harvest.

Conclusion

So the conclusion of my whimsy is that whenever Hinkley C starts generating it will transform the UK electricity supply “market” overnight. And this could happen sooner than I had anticipated.

Whenever it happens, it will significantly reduce the average CO2 emissions from electricity in the UK. But there will also be costs associated with this ‘cheap’ electricity.

You might consider it ironic – and perhaps not a little unfair – that the introduction of some of the most expensive electricity ever put onto the UK grid – EDF are guaranteed ~10.6 p/kWh for all the electricity that Hinkley C produces – will put pressure on generators using wind turbines and solar PV stations – the suppliers of the cheapest energy ever supplied to the grid.

You might consider that this is not how “markets” are supposed to work. That is why I have put every instance of the word “market” in quotation marks. If you know a better word to use, I would love to know what it!

November 2021: Heating and Carbon Emissions

Friends, it is December already and I am preparing to hibernate.

But before I curl up and doze, I just thought I would summarise some of the energy statistics for the house in November.

They are actually pretty remarkable: the low-carbon solar-powered world really is here already…

Weather 

The average November temperature in Teddington was a very typical 8 °C, but the end of the month included several colder days with average temperatures of ~1 °C, and minimum temperatures down to -3 °C.

Click image for a larger version. Daily averages of the internal and external temperatures this November 2021. Also shown is the monthly average (dotted blue line) and the internal temperature measured every 2 minutes (black line connecting the red dots).

However, as the graph above shows, internally our house remained at an extremely stable temperature, barely changing day or night.

Electricity

Over the month, the house used 609.4 kWh (~20.3 kWh/day) of electricity. This is roughly double our typical use of electricity without heating.

Click image for a larger version. Remarkably, ~21% of the electricity used in November 2021 came from the solar panels. The bulk (~70%) was purchased off-peak (00:30 to 04:30) and stored in the Tesla Powerwall for use during the day. Just 9% was purchased at peak rates after the battery ran out of charge.

This electricity demand was met as follows:

• 129 kWh (~21%) came from the solar panels.
• 428.9 kWh (70%) was off-peak electricity from the grid @5p/kWh = £24.02
• 51.6 kWh (9%) was peak-rate electricity from the grid @16.26p/kWh =£8.39

The shifting of our time-of-use by using the Tesla Powerwall resulted in the average cost of a unit of grid electricity being ~ 6.2p/kWh.

Added to those costs is the standing charge of 25p/day, or £7.50/month.

Gas

We are still using gas for cooking and through the month we used:

• 41.84 kWh (~1.4 kWh/day) @3.83 p/kWh = £1.60

Added to this is the standing charge of 23.85/day, or £7.16/month.

Heating & Domestic Hot Water (DHW)

Over the month, the heat pump used 312.1kWh (~10.4 kWh/day) of electricity – about half of the total 609.4 kWh used through the month.

At an average cost of 6.2 p/kWh of electricity, this cost £19.35.

Using this electricity, the heat pump delivered 1128.6 kWh of heat at an average cost of (£19.35/1128.6) = 1.7 p/kWh.

If a 90% efficient gas boiler had delivered this energy it would have used (1128.6/0.9)= 1254 kWh of gas which would have cost £48.03.

So the heat pump delivered savings of approximately 60% over using a gas boiler.

If we had not used the battery to allow the use of cheap-rate electricity, then 312.1 kWh of electricity would have cost approximately £44.76 – roughly a 7% saving over using gas.

Heat Pump Performance

As the graph at the head of the page makes clear, the 5 kW Vaillant Arotherm plus heat pump performed well, providing heating and DHW uncomplainingly even in the cold weather.

Click image for a larger version. COP data from last 12 days of November 2021. The blue dots are hourly averages and the large yellow dots show daily averages. The data include the domestic hot water and anti-legionalla cycles which heat water above 55 °C. The trend line indicates that COP is typically between 3 and 4 for external temperatures between 0 °C and 11 °C.

The key measure of the performance of a heat pump is its coefficient of performance (COP). This specifies the ratio of the heating energy delivered divided by the electrical energy consumed.

The graph above shows how the COP varied hour-by-hour and day-by-day through the last 12 days of November.

It is clear that the COP falls at lower external temperatures. This is because the heat pump has to work harder to deliver the heat across a bigger temperature difference.

• Outside the external temperatures are lower
• Inside the heat pump increases the temperature of the water flowing through the radiators to deliver more heat.

More specifically,

• When the external temperature is ~ 10 °C, the water flowing through the radiators is at ~ 32 °C, a temperature difference of ~22 °C. With this small temperature difference the heat pump can operate with a COP in excess of 4.
• When the external temperature is ~ 0 °C, the water flowing through the radiators is at ~ 42 °C, a temperature difference of ~42 °C. With this temperature difference the heat pump can only achieve a COP of ~ 3.

Some hourly readings show COP values much less than the trend line. These are hours in which ‘odd’ events occurred, such as de-frost cycles on the heat pump heat exchanger, or re-heating the hot water tank at 55 °C.

Carbon Dioxide Emissions

Since this is just a monthly analysis, I will consider only the fuel costs and neglecting the embodied carbon in the heat pump etc.. But please be assured, in the fuller analysis this is fully accounted for.

During November MyGrid GB reported the average carbon intensity to be 235 gCO2/kWh whereas Carbon Intensity reported the average to be 191 gCO2/kWh. The graph below shows the hour-by-hour data and the curves look similar but appear to be offset by ~ 45 gCO2/kWh. I don’t know which one is correct but I am going to calculate with the Carbon Intensity figures because they allow me to download half-hourly data for the whole month. The difference between the estimates can be added as a constant. Apologies for the confusion.

Click image for a larger version. Hour-by-hour Carbon intensity data from Carbon Intensity and MyGridGB They appear to differ by a constant additive number of ~ 45 gCO2/kWh. I do not know which one is right.

Analysing the data, I find that the 4 hours of off-peak electricity had an average carbon intensity of 141 gCO2/kWh versus 191 gCO2/kWh for the other 20 hours. (This would be ~ 186 gCO2/kWh versus 246 gCO2/kWh for the MyGridGB estimates.)

Using Carbon Intensity figures I estimate that:

• 428.9 kWh of off-peak electricity @141 gCO2/kWh released = 60.47 kg CO2
• 51.6 kWh of off-peak electricity @191 gCO2/kWh released = 9.86 kg CO2

for a total of 70.33 kgCO2 released from electrical use through the month. (92.46 kgCO2 using the MyGridGB data)

The effective carbon intensity would be 146 gCO2/kWh (or 191 gCO2/kWh if the MyGridGB figures were correct)

The heat pump used 312.1kWh of electricity and so released 45.57 kgCO2. Dividing this by the 1128.6 kWh of heat delivered by the heat pump (£45.57/1128.6) = 40 gCO2/kWh. Using the MyGridGB estimate this would increase to 53 gCO2/kWh.

Whichever estimate is correct, this is truly low-carbon heating.

If a 90% efficient gas boiler had delivered this heating it would have released 251 kgCO2.

So the heat pump emitted around 20% of the CO2 which would have been emitted by using a gas boiler.

Even if we had not used the battery to allow the use of cheap-rate electricity, the carbon savings would still be dramatic.

Summary

As I wrote the other day, I am relieved to find that all the investments I have made in my home are working – the house is emitting just a small fraction of the CO2 it emitted previously, with absolutely no loss of quality of life.

• The triple-glazing and external wall insulation reduce heating demand by half
• The solar panels are still delivering 20% of our electricity demand in November!
• The battery allowed me to time-shift the use of low-carbon off-peak electricity.
• And the air source heat pump operated with more than 300% efficiency even on the coldest says.

Time to snuggle up…