Archive for September, 2021

Energy Storage with Bricks – a Really Bad Idea

September 22, 2021

Friends, the intermittent nature of many renewable energy resources makes energy storage critical for any future renewable electricity network.

But the amounts of energy that need to be stored are immense.

In round numbers, to store one day of electricity for the UK requires 1 terawatt-hour of storage.

  • 1 kWh is the unit of electrical energy used by electricity companies on our domestic electricity meters – typically it costs about 20 p for us to buy and about 5 p for the companies to buy.
  • 10 kWh is roughly the amount of electricity my wife and I use in a day.
  • 1000 kWh is 1 Megawatt hour (MWh)
  • 1000 MWh is 1 Gigawatt hour (GWh)
  • 1000 GWh is 1 Terawatt hour (TWh)

And multi-day storage is just a multiple of that.

This requirement – and the business opportunities available to those who can compete in this market – have driven people to consider all kinds of off-beat ideas.

This article is about one idea which is really stupid, which will never work, but which is apparently worth over a billion dollars.

Existing Energy Storage 

The hot money in energy storage is in electrical batteries of all kinds.

Tesla (for example) can supply a collection of batteries that occupies a football field or so with the following (rounded) specifications

  • 130 MWh of storage (0.01 % of 1 TWh)
  • 100 MW charge and discharge rate
  • Efficiency ~ 90% – some energy is lost in the charge-discharge process.
  • 100 million US dollars in 2020
  • $0.8M is the cost per MWh

Over the course of time I would expect this kind of facility to get cheaper and better.

The biggest storage facility in the UK is the Dinorwig Power Station in North Wales.

  • 9.1 GWh of storage (0.9 % of 1 TWh)
  • 1.7 GW discharge rate – charges more slowly
  • Efficiency ~ 75% – energy is lost in the charge-discharge process.
  • 500 million US dollars in 1984 – about 2 billion US dollars now
  • $0.2M is the cost per MWh

Dinorwig works by pumping water between two lakes with a height different of 500 m. The mathematics is easy to do.

The stored energy (in joules) is the calculated by multiplying three numbers which school students learn as mgh

  • m is the mass of water stored in the upper lake (in kilograms)
  • g is strength of gravity – roughly 10 newtons of force for each kilogram of mass
  • h is height difference (in metres)

The discharge rate (in watts) is also calculated by multiplying three numbers

  • The mass of water per second flowing through the turbines (kilograms per second)
  • g is strength of gravity – roughly 10 newtons of force for each kilogram of mass
  • h is height difference (in metres)

Compare and contrast 

Dinorwig is massive – storing almost 1% of the UK’s daily electricity requirements and and the storage is cheap per unit of energy stored.

But there are – as far as I know – no other sites in the UK with similar potential.

Tesla batteries can be placed anywhere but they are relatively expensive.

We can foresee that there will be technological innovation, and mass-production effects that will reduce the costs and improve the performance of batteries in coming decades.

However there is nothing we can do to substantially improve the performance of Dinorwig. The simple formula mgh limits all gravity-based storage systems.

To get good performance, one needs a big mass (m) lifted up, and then dropped from, a great height (h).

No technological innovations can beat mgh.

  • But is there a gravity-based storage system which doesn’t need a unique geography?
  • Something which could be built out in modular form like Tesla’s battery farms?

Here’s the stupid idea: Project Jenga

The idea – from a company called Energy Vault –  is to store energy by building a pile of bricks.

Their videos make it seem a superficially clever idea, but I can’t get the visions of Jenga out of my head.

Basically a robot crane system uses electricity to build a tower out of very large bricks. This is equivalent to ‘charging’ a battery.

To ‘discharge’ the tower, the crane lets the bricks down onto a lower tower, and as the bricks fall they turn a generator.

Fortunately, because all gravity-storage systems are limited by the mgh equation I mentioned above, it’s possible to work out its performance parameters.

Based on information gleaned from their videos and web site I conclude that:

  • Brick size ~ 6 m x 1 m x 2.5 m – mass ~36 tonnes
  • Tower in charged state – 40 layers tall with ~ 100 bricks per layer
  • Tower in discharged state has ~ 500 bricks per layer and so is ~ 8 layers tall

Still from an Energy Vault video showing their concept for their Jenga-like tower. Notice they imagine this free-standing pile of bricks being built near wind-turbines

The total stored energy is mgh where:

  • m is the total mass of the tower, and
  • h is difference between the heights of the centres of mass in the two configurations, which must have the same basic volume and number of bricks.

Calculation of stored energy in the tower system. Each state (charged and discharged) is modelled as a hollow cylinder, with the discharged cylinder being 1 metre outside the charged cylinder. The volume is conserved between the two shapes. The stored potential energy is mgh where h is the difference in height between the centres of mass in the two configurations

So my estimate for the system they describe is:

  • 37 MWh of storage
  • 6.8 MW charge and discharge rate (assuming (optimistically) it takes 10 seconds to move 2 bricks simultaneously)
  • Efficiency ~ 85% is claimed.
  • Energy vault claim $18M, but I find it hard to believe it will cost less than 100 million US dollars: The bricks alone will cost around $5M in raw materials.
  • ~$3M is the cost per MWh

So the system costs more per MWh than a battery-based system, with no potential for future technological improvements.

Why it won’t work

For a 37 MWh Energy Vault device, charging and discharging requires building, and then dismantling, a structure the height of Canary Wharf Tower, at 240 metres tall, the UK’s third tallest building.

A 37 MWh Energy Vault store would be 240 m tall: the height of Canary Wharf Tower. Charging would involve building such a tower from free-standing bricks in about 6 hours. Mmmmm.

The 37 MWh of stored energy in such a structure, when sold as electricity at 20p/kWh, would be worth – optimistically – around £10,000. The company profits would then be the difference between the sale and the purchase price of the electricity – let’s guess £5,000 per dis-assembly/assembly cycle.

  • Nominally the system would take a few hours to build (charge) and dismantle (discharge)
  • Can you imagine building anything the size of Canary Wharf in a few hours for £5000?

The charged structure would be free-standing with no reinforced concrete or steel beams to hold it together.

But this tower is envisaged to be deployed in open country, perhaps  near wind turbines – i.e. where its often windy!

Later versions of the Energy Vault concept have a different format – with mass movements taking place using some clever un-revealed geometry inside a building which looks like it is only about 40 m tall, but spread out over a much larger area.

A still from another video showing a newer version of EnergyVault enclosed in a frame inside a building. It appears to be only – maybe – 40 metres tall.

But no matter how clever they are, they can’t escape mgh.

If the building is 40 m tall, then the centre of mass is at most 20 m off the ground.

For the same 4000 bricks they used in the ‘Jenga’ design, the uncharged area of all bricks on the ground would be 10,000 m^2 i.e. 100 m x 100 m.

If this whole 144,000 tonne structure were raised by 20 m (a likely overestimate) then the stored energy would now be just 8 MWh, storing only £1,600 worth of energy in the charged state.

But in the charged state this would now be supported by an immense (= expensive) reinforced concrete frame capable of lifting and moving these large loads.

8 MWh of storage is tiny: the equivalent of 600 Tesla PowerWall batteries (like I have in my house) which would cost around £6M but which could be bought ‘off the shelf’ with no risk.

What’s going on?

It’s not just me that has noticed that this idea is a non-starter. The video above calls out the project for its ridiculousness at great length.

But if you look at the Energy Vault website you will see story after story about investment by banks and grand plans to establish a company worth billions of pounds.

What’s going on? I have no idea: it is simply madness.

Passionate about Insulation!

September 19, 2021

Protesters last week blocked access to the M25 orbital motorway (BBC Story) causing widespread traffic disruption.

Campaigners blocking access to the M25. Picture from the BBC.

The protestors were from the campaign group Insulate Britain and they demanded…

1. That the UK government immediately promises to fully fund and take responsibility for the insulation of all social housing in Britain by 2025;

2. That the UK government immediately promises to produce within four months a legally binding national plan to fully fund and take responsibility for the full low-energy and low-carbon whole-house retrofit , with no externalised costs, of all homes in Britain by 2030 as part of a just transition to full decarbonisation of all parts of society and the economy.

I sympathise with these goals, and with their assessment of the importance of this aspect of de-carbonisation.

But I deplore their actions which will – I expect – have achieved less than nothing.


In the last couple of years I have spent £25,790 on External Wall Insulation and £10,280 on Triple-Glazing. I mention this as evidence that I personally understand that this stuff really matters.

I should also mention that although I am comfortably off by UK standards, these were significantly large sums to me – more than half my NPL Pension ‘lump sum’.

Together these two steps have reduced the heating required in my house by about 50%.

If something similar were done nationally it would reduce the amount of heating (and cooling) required dramatically.

But having done this personally, and having discussed it with many people, I have learned a thing or two about insulation.

Primarily I have learned that insulation is a fraught business and that people are quite fussy about it.

Things I have learned about Insulation

1. Loft Insulation is a no brainer. It’s really cheap and effective. But most people already have some insulation – though adding would generally better.

But do we just give it away? Or offer it free to installers? Or offer people a grant if work is done by a registered and trained installer? How do we make sure it’s installed well – or at all?

And people with lofts to insulate generally already own houses or flats – and are generally not the least well off.

2. Improving Glazing is another easy choice. Windows need regular maintenance and replacing or refurbishment. And everyone hates draughty or cold windows and replacing windows might reduce heat losses by between 10% and 15%.

But despite that people are very attached to the visual impact of their windows and many consider standard standard uPVC windows ugly.

In areas with Edwardian or Victorian houses, people value their old draughty windows as ‘original features’.

Should we be paying for artisan double-glazing refurbishment for such people – or obliging them to have ‘Government Windows’?

3. External Wall Insulation (EWI also known as ‘cladding’) is one of the few ways to substantially reduce the need to heat a property. It is expensive – costing roughly £100 per square metre of wall – but if external work is being done on a property it becomes very cost effective to add insulation at the same time.

I think the EWI on my house looks great, but when I tell people I have put cladding on my house they think:_ _ _ _ _ _ _: I won’t even mention the name but you know what it is. They look upon me with pity as though I am bonkers!

Again in areas with Edwardian or Victorian houses, people cling on to their ‘original brickwork’ with pride. To my eyes the houses all look naked! Especially the sides of houses which could be cheaply covered. But people would never put cladding on them.

And good luck if you are trying to persuade – or even compel – people in high-rise homes to have insulation added. Poor regulation has meant that this will now never happen.

4. Under Floor Insulation is another way to substantially reduce heat losses in properties with a ground floor. But it is one of the major steps that I avoided because of the need to lift up the entire ground floor. It would be hard to think of a more disruptive intervention.

5. Draught-proofing is another easy win – it’s cheap, and it’s easy to train people to install it either for themselves or for others. But while it’s a great idea – it’s not going to make a massive difference.

Do Insulate Britain‘s demands make sense?

Insulate Britian’s first ‘demand’ is:

1. That the UK government immediately promises to fully fund and take responsibility for the insulation of all social housing in Britain by 2025;

This ‘demand’ focuses on generally poorer people, and I am sympathetic to it. But I feel it would likely be characterised by a large amount of shoddy work. And I think some people would refuse to live in any house with External Wall Insulation.

The Government’s Energy Performance Certificates (EPCs) are currently little more than a guess at the Energy Performance of a house – and so it would be difficult to assess whether such policy had actually worked.

Insulate Britian’s second ‘demand’ is:

2. That the UK government immediately promises to produce within four months a legally binding national plan to fully fund and take responsibility for the full low-energy and low-carbon whole-house retrofit , with no externalised costs, of all homes in Britain by 2030 as part of a just transition to full decarbonisation of all parts of society and the economy.

This ‘demand’ focuses on “everyone”, and seems wildly unrealistic. A plan would be a great thing, and insulation should surely be a part of that plan. But there are other issues too.

Off the top of my head, there is: security of energy supply; the cost of energy; driving further de-carbonisation; and energy storage. All of these are as important as insulation.

And achieving any of these things equitably will be hard.

So what would I do?

Well, I wouldn’t block the roads and irritate the people I hoped to persuade.

Regarding existing housing, I would suggest:

  • That the Energy Performance Certificates be improved to more properly reflect the energy performance of dwelling.
  • And that subsequently, an element of council tax should be linked to EPC rating. Dwellings with better EPC ratings would pay lower taxes.
  • Money raised from this would be used on a program of works to improve the insulation and heating in social housing.

Broadly speaking this gives better-off people an incentive to do things to their homes as they see fit – and helps people who can’t afford the investment in home improvements.

Regarding new housing, I would suggest:

  • That all new homes be carbon neutral and built to very high energy standards starting as soon as could be arranged.

Who will do all this work?

In fact, there are thousands of valuable things that could be done – and many are already being done.

And the people on the front line of the battle against Climate Change are plumbers installing heat pumps, builders adding insulation, and triple-glazing installers.

Whatever the Government does should make sure it helps these key front-line workers to grow their business, train new installers, and thrive.

Carbon and Debt

September 12, 2021
There are parallels between the 'debt crisis and the carbon emissions crisis?

There are parallels between the ‘debt’ crisis and the carbon emissions crisis?


This article is about the similarities between financial debt and ‘carbon’ debt. 

I wrote it almost 10 years ago back in December 2011, but the subject has been on my mind again recently. 

  • The financial situation was very different back then, but also very much the same! 
  • The carbon situation is now much worse: we have had 10 wasted years and emitted another 360 billion tonnes of carbon dioxide.


I have been struck recently by profound similarities between the debt crisis and the carbon crisis. Here are seven points: see what you think:

1. Both these crises arise from a choice to consume now and pay later.  

With the debt issue, this is true both at a personal and a national level. Politicians have shied away from making people aware of the true cost of their policies for fear of unpopularity. Similarly, because of the potential unpopularity of the policies required to address carbon emissions, politicians have held back from policies that would dramatically cut carbon emissions.

2. Both these crises require us to address intergenerational morality.  

In the same way that it is unfair to spend money now and expect our children to pay it back, so it is unfair to emit carbon now, and expect our children to deal with the consequences.

3. Both these crises require international solutions.  

National politicians have failed us: they are unable to resist spending and borrowing more in order to stay popular. And so the Eurozone have now called for central oversight of national budgets to make sure countries do not surreptitiously borrow too much. Similarly, the nations of the world require external limits to be imposed upon them. Only in this way can politicians tell their people: ‘its not our fault’.

4. Neither of these crises will ever be ‘solved’.

Rather, they are perpetual struggles not isolated events.  Recent events in Europe may make it seem that a particular path to a solution has been found. But it hasn’t. The forces which drove Europe into its difficulties and which created spectacular indebtedness in the UK are still all in play. Similarly, the outcome of the Durban conference is neither a cause for celebration or depression: it is just another step on the path, and we really don’t know what lies ahead. [Note: I have no idea now what that conference was about!]

5. Accounting is difficult and dull and boring. 

But accounting is essential. This has been true of financial accounting for many years, and it will be equally true of carbon accounting when the concept becomes established. But what appears to be a constraint on freedoms or our growth, is simply a way of staying honest.

6. Spending money you have borrowed is like burning carbon.

Why? makes us feel good now. Building a hospital we can’t afford brings benefits and so does burning carbon – we get improved lifestyles today – and much cheaper than the sustainable lifestyles we might aspire to. But eventually we will have to pay the cost. In financial terms this can involve reduced incomes which will the harm people’s health as well as their wealth. In carbon terms, we really don’t know what the costs will be, or who will be required to pay them.

7. Paying back debt is really hard.

If you have ever had to pay back any significant amount of debt, then you know how hard it is. This is as true for nations as it is for people – with the additional unfairness in that the people who borrowed the money and benefitted are not the people who have to pay it back. If we are to ever get back to some kind of carbon neutral economy then it will involve real pain as we wean ourselves off carbon emitting technologies. Real pain – and most probably real reductions in quality of life.

I could go on because I think the parallels are quite deep, but I think I have made the point.

However, I do want to add that in both cases, there is no need to despair. The world is very beautiful, and very resilient. And we have each other. If we can learn a lesson, and teach our children, then we can still make things better than they otherwise would have been.

Using Radiators with Heat Pumps

September 8, 2021

Click for a larger image. A typical dual-panel radiator transfers heat from the hot water flowing through it to the room in two quite distinct ways. By direct heating of the air in contact with the metal surfaces, and by radiation from the outer metal surface.

Switching to heating a house with a heat pump rather than a gas boiler is not entirely straightforward. But it is much easier if one can keep using one’s existing radiators.

But heat pumps operate most efficiently when circulating water at lower temperatures – ideally 40 °C or so. However radiators don’t work so well at these lower temperatures, so in the worst case it might be possible that not enough heat will be transferred to the house to keep it (and you!) warm.

In this article I thought I would explain how radiators work and how one can estimate how well they will work when the water flowing through them is at lower temperatures.

This article is a little bit technical and involves tables of data and mathematical formula: sorry.

The key to understanding radiators is that radiators transfer heat to the room using two quite distinct physical mechanisms:

  • radiation
  • convection.

And in fact, convection is generally more important that radiation. Let’s look at each mechanism in turn.

How radiators work: Radiation

The heat transferred by radiation occurs mainly from the outer panel facing the room and the amount of heat transferred (in watts) is given by a fancy formula.

Click for a larger version.

The power radiated into to the room depends on:

  • The Stefan-Boltzmann constant 5.67 x 10^-8 W/m^2/K^4
  • The front surface area of radiator in m^2 i.e. height (m) x width (m)
  • The physical property of the surface known as emissivity – typically 0.9 for many painted surfaces.
  • The difference between the temperature of the radiator surface and room temperature. But the it is not just the simple difference between the temperatures. It depends on the difference between the 4th power of absolute values of the temperatures.
  • To find the absolute temperature one adds 273.15 K to the temperature in degrees Celsius. So a room temperature of 20 °C corresponds to 293.15 K (kelvin) and a flow temperature of 50 °C corresponds to 323.15 K

The graph below shows the amount of power radiated from the front surface of a radiator at various temperatures

Click for larger version. The heat radiated by a typical radiator with a surface area just over one square metre. Warming the temperature of the surface from 30 °C to 40 °C results in 57 W of additional heat transfer to the room. Further warming the temperature of the surface from 40 °C to 50 °C results in 63 W of additional heat transfer to the room.

The emissivity of the radiator has a maximum value of one – and so can’t be increased very much from it’s typical value of 0.9.

So to radiate more heat from a radiator one must either increase its area, or its flow temperature.

How radiators work: Convection

The heat transferred by convection occurs at the all the vertical heated surfaces of the radiator.

Click for a larger version. For a radiator with 2 heated panels, convection is induced on 4 vertical surfaces.

Heat is transferred by direct contact between the air and the painted surface. Since the heated air has lower density, it become buoyant and a self-sustaining upward air flow is developed.

It is difficult to develop an exact formula that describes the heat transfer process, but most simple analyses assume that heat transfer is proportional to the temperature difference between the radiator and the room.

However, at higher temperature differences, the moving air speed increases and this further improves heat transfer to the air. This leads to a slight non-linear dependence on the radiator temperature.

Convective and radiative heat transfer can be calculated using complex mathematics at this web site.

The graph below shows the amount of power transferred by convection from the front surface of a radiator at various temperatures.

Click for larger version. The heat transferred by convection from just the front surface of a radiator is compared with the heat radiated the front surface of the same radiator as in the previous figure. Warming the temperature of the surface from 30 °C to 40 °C results in 36 W of additional convective heat transfer to the room. Further warming the temperature of the surface from 40 °C to 50 °C results in 39 W of additional convective heat transfer to the room.

However even a single-panel radiator can transfer heat convectively from two surfaces (front and back). And a double-panel radiator can transfer heat from 4 surfaces (the front and back of each panel).

And we can increase the convective heat transfer further from a radiator by more adding vertical surfaces for air to flow past. For example, for example, the figure below shows the design of several Stelrad Radiators. There are several additional ‘corrugated’ fins with a length which exceeds the basic width of the radiator.

Click for a larger view. These are cross-sections of radiators showing different numbers of panels and fins. All the radiators have roughly the same radiated output 317 W: this is proportional to the frontal area. But the overall power outputs are 1568 W for the K1 model, 2155 W for the P+model, 2770 W for the K2 model. This extra power is achieved by additional convective heat transfer from the panels and the fins which can have a much larger surface area than the panels. All figures assume 70 °C water flow.


For a single panel radiator, with no fins, radiation and convection contribute roughly equally to heat transfer.

But for more complex radiators with additional fins and panels, convection is much more important for heat transfer. For the K2 radiator in the figure above, convective heat transfer is 8 times larger than radiative heat transfer.

The physical models of heat transfer are too complicated to calculate for every variety of radiator. So there is a standard curve adopted for calculating overall (radiative and convective) heat transfer for water flow at lower temperatures.

This standard curve is shown as a dotted line in the figure below. It matches the physical models reasonably well, but predicts a slightly lower heat output.

Click for larger version. The heat transferred by convection from the four vertical surfaces of double panel radiator, and the heat radiated the front surface of the same radiator. Their sum is shown in black and the standard de-rating curve is shown as a dotted line. Operating the radiator at 70 °C (dangerously hot) results in a total heat output of 1072 W. Cooling the temperature of the surface to in (roughly) almost a 50% reduction in heat transfer to the room. Cooling further to 40 °C the de-rating is close to 70%. And using a flow temperature of 30 °C will result in an 85% reduction in heat out put compared with the nominal radiator specification.

But the summary is simple. The nominal heat output of a radiator is specified assuming that the room is at 20 °C and the water flowing through the radiator is at an average temperature of 70 °C.

  • The estimated heat output with a flow temperature of 50 °C is reduced to ~50% of the standard output.
  • The estimated heat output with a flow temperature of 40 °C is reduced to ~30% of the standard output.
  • The estimated heat output with a flow temperature of 30 °C is reduced to ~15% of the standard output.

The standard de-rating factor F is given within 1% by this formula:

where both temperatures are expressed in degrees Celsius.

So what temperature should I set my hot water flow? 

This is difficult to work out. But I think the procedure work like this.

  • First work out how much heat is required to heat a home on a cold winter day. In the south of England where I live this typically corresponds to an outside temperature of about -2 °C. Based on my weekly readings of the gas meter on the coldest week last winter (average temperature 0.2 °C) the peak heating required for the house was around 72 kWh/day – or around 3000 W.
  • Next one considers all the radiators and measures their height and width. Analysing the Stelrad data for about 40 different radiator sizes I saw that:
    • K1 type radiators are rated at about 1600 watts per square metre,
    • K2 type are rated at about 2800 watts per square metre.
    • I then guessed that my old single-panel no-fin radiators will give roughly 700 watts per square metre.
  • Collating all the data I arrived at a table like that below.

Click for a larger version. Analysis of all the radiators in the house estimating first their ‘standard output’ and then their output with a flow temperature of 40 °C.

  • This table suggests that a flow temperature of 40 °C, the radiators should output 3214 watts of heating – which just about matches the 3000 watts required in the coldest weather.

So I am hopeful that my existing radiators will work fine with the new heat hump at the reasonably low flow temperature of 40 °C.

According to the specifications of my 5 kW Vaillant Arotherm plus (excerpt below) with a flow temperature of 40 °C through the radiators, the seasonal coefficient of performance should be over 4.

If most of the electricity is purchased at night using the Octopus Go rate of 5p/kWh, this means that the cost per kWh of heating will be around 1.25 p/kWh i.e. around 30% of the cost of heating with gas.

Click for a larger version. Excerpt from the operating specification of the Vaillant Arotherm Plus heat pumps. The 5 kW model is highlighted in blue. At 40 °C flow it claims to be able to deliver 6 kW of heat when the external temperature is – 5 °C, with a seasonal coefficient of performance of 4.13.

One final issue is whether the heating is in the right places in the house. The bedrooms are often very warm, and our kitchen is the coldest room, having only a single old single-panel radiator and this may need to be upgraded.


Assessment of Heat Pump heating water to 50 °C and 70 °C

September 7, 2021

Friends, our Air Source Heat Pump (ASHP) (a 5 kW Vaillant Arotherm Plus) has been installed for over a month now and I am beginning to get a feel for how it is working.

At this time of year (early September) we have no space heating requirements so the work load for the heat pump is low.

Most of the day it sits in the garden admiring itself, and consuming 12 W of electrical power (0.29 kWh/day)

Our heat pump idling away the late summer days in the back garden. It only works for an hour a day!

Each night it wakes itself at 3:00 a.m. and if the hot water tank requires a top up, it operates for about an hour, heating the tank to roughly 50 °C.

  • Typically it uses ~1 kWh of electricity and delivers ~3 kWh of heat.

On Wednesday mornings it additionally heats the water in the tank to 70 °C in a so-called Anti-Legionella cycle.

  • This typically uses ~3 kWh of electricity and delivers ~7 kWh of heat.

Later in the year I expect that the heat pump will begin to be required to heat the house, and I’ll write about that in a little while.

But for now let me just describe how the system is working at present.

A Normal Cycle

Two typical water-heating cycles from the 5th and 6th September are shown below. The external air temperature in each case was about 15 °C.

Click for a larger version. Typical performance of the heat pump when heating domestic hot water. The two upper panels show data from 5th September and two lower panels show data from 6th September. In each case the left-hand panel shows electrical power consumed (watts), the heat delivered to the cylinder (watts) and the water temperature (°C). The right-hand panel shows instantaneous COP and dotted lines show two estimates of the average COP. 

The key measure of how well a heat pump works is its coefficient of performance (COP) which measures the ratio of thermal energy delivered, to electrical energy consumed.

The graphs on the right above show how the COP varies from minute to minute through the heating cycle.

Also shown as dotted lines are two estimates of the average COP.

  • The blue estimate includes all the electrical energy which the heat pump uses during the 23 hours when it is not ‘working’.
  • The purple estimate includes only the electrical energy which the heat pump uses during the heating cycle’.

Depending on which measure one uses, the COP is between 2.5 and 3 i.e. the heat pump delivers between 2.5 and 3 times as much as heat as the electrical energy it uses

An Anti-Legionella Cycle

Legionella bacteria, which can cause Legionnaires Disease, are capable of lurking in hot water systems at temperatures below 60 °C.

To counteract this, every Wednesday morning the heat pump system additionally executes an Anti-Legionella cycle which heats the water to 70 °C. It should be noted that it is very unusual for heat pumps to operate at all at such high temperatures.

Click for a larger version. Typical performance of the heat pump during an anti-legionella heating cycle on 1st September. The left-hand panel shows electrical power consumed (watts), the heat delivered to the cylinder (watts) and the water temperature (°C). The right-hand panel shows instantaneous COP and dotted lines show two estimates of the average COP. 

From the graphs above one can see that heating to higher temperatures is hard work for the heat pump and the average COP falls from the range 2.5 to 3.0 when heating to 50 °C, to just around 2.1 when heating to 70 °C.

Hot Water Temperatures

Click for a larger version. The measured temperature of hot water at three hand-basins in the house over a period of 20 days. After the anti-legionella cycle in the early hours of Wednesday morning, the flow temperature of water at the taps can reach almost 70 °C, a potential scalding hazard. At other times, the hot water is delivered at just under 50 °C

One unanticipated feature of the Anti-Legionella cycle is that on Wednesday mornings, the temperature of water delivered from the hot water taps is very high – almost 70 °C.

With our level of water use, the system typically skips the Thursday heating cycle because the water is still hot from Wednesday’s ‘super’ heating. Indeed, the water does not return to ‘normal’ temperatures until Saturday!

Delivering water at almost 70 °C is a significant hazard and so I will shortly have anti-scalding valves fitted to the outlets which will limit the maximum temperature of hot water to about 45 °C.

Once I have finished with my tests, I will also reduce the normal hot water temperature by a few degrees.


Overall the system is doing well.

Click for a larger version. COP performance of the heat pump during normal heating cycles and during anti-legionella heating cycles. Heating the water to 70 °C degrades the performance of the heat pump.

Looking at the performance during normal heating cycles, the heat pump heats water from around 15 °C to 50 °C with a COP of typically 3.4

Looking at the performance during anti-legionella heating cycles it heats water from around 15 °C to 70 °C with a COP of typically 2.4

These COPs do not include the electrical energy consumed during the 23 hours when the heat pump is on ‘stand by’. This better indicates the operating performance of the pump, but of course this ‘stand by’ energy still has to be paid for.

Overall (including the ‘stand by’ consumption) the heat pump is delivering on average 4.5 kWh/day of hot water heating at the expense of about 1.77 kWh of electricity/day.

At this time of year, all this electricity comes from solar energy stored in the battery and so costs nothing.

But as the winter season draws in, we will eventually operate this using mains electricity on the Octopus Go tariff. This provides electricity at 5p per kWh between 00:30 and 4:30 a.m. each day.

So the cost of 4.5 kWh of hot water in winter will be about 1.77 kWh x 5 p/kWh = 8.85 p per day.

This is equivalent to just under 2p/kWh (thermal) – which is about 40% cheaper than gas heating which costs about 3.3p /kWh (thermal)

Things will be a little harder in winter as the average external temperature falls, but I am very curious to see how the Vaillant ASHP performs.



No: You are not using 100% renewable electricity

September 2, 2021

Everyone’s favourite energy company EDF recently wrote to tell me…

And by ‘changing’ they meant ‘increasing’. Roughly, the price of electricity is going up by about 10% apparently because “The wholesale cost of energy has gone up more than 50% in the last six months.”

But later in the e-mail they assured me that:

I’ll come to the asterisk in a minute, but it struck me that these two statements didn’t sit well together.

First of all, no electricity source has ‘zero’ carbon emissions: they just mean: ‘low’ carbon emissions.

Secondly, low-carbon electricity comes from either nuclear, wind, solar, biomass or hydroelectric sources, none of which have had cost increases: the increases have been mainly in the price of gas.

So one might think that EDF would be under no-obligation whatsoever to raise their prices.

Could the asterisk hold an explanation? The full asterisk text is at the end of the article but the key part is this:

All our residential tariffs are backed by 100% zero carbon nuclear electricity

Since the costs of nuclear electricity have not risen at all, one might feel further emboldened in the idea that EDF might not be obliged to raise prices after all.


Electricity doesn’t work like that

The way we supply electricity in the UK is complicated, and includes several ‘market’ elements. Here are two ‘explanatory sites’

At its simplest,

  • ‘Wholesale’ suppliers offer to supply electricity and ‘Retail’ companies buy from a variety of suppliers.
  • The ‘Wholesale’ companies try to sell as much electricity as they can, and make money from the difference between what it costs them to produce electricity and the ‘market price’ of electricity – which varies through the day.
  • The ‘Retail’ companies buy on behalf of their customers, and make money from the difference between what they pay and what they charge you.

The market structure is complex but aims to make sure that demand is met, and that there is a contingency against plant failures or surprise demand.

Some contracts are long-term – signed months or years in advance. And others are short-term signed only a few days before the required date of delivery. But if the price of electricity from gas-fired stations goes up – then because that electricity is essential to ‘keep the lights on’ – the market structure results in a general price increase.

But all the electricity which is supplied – from solar or nuclear or gas-fired plants or interconnectors – is ‘pooled’ to meet our collective national needs and supplied over The National Grid.

National Grid

The National Grid infrastructure is owned by a multinational for-profit company called National Grid plc. It’s major shareholders are banks.

Click for a larger version. These are the top 10 shareholders in National Grid plc as of June 2021 (source: StockZoa)

If EDF delivered their low-carbon electricity to you over EDF’s own wires, then it could potentially be low-carbon.

Similarly, if you use electricity generated on your own rooftop without calling on the National Grid at all, it can be genuinely low-carbon.

But if you have your ‘green’ electricity delivered over the grid, then it is pooled with electricity from all other sources and what you draw from the grid can – in my opinion – no longer be considered ‘low-carbon’.

Engineering not Accounting

Now you might think that if you paid for ‘green’ electricity to be ‘poured’ into the grid on your behalf, then using clever accounting you can consider the electricity you ‘withdrew’ from the grid to be ‘green’.

Unfortunately, although National Grid plc is run by accountants, the network itself operates on the principles of basic physics and engineering.

And the plain fact is, the grid doesn’t work without ALL the suppliers contributing. So in order to supply you with ‘your’ ‘green’ electricity, it is necessary to have gas-fired stations operating pretty much all the time.

And if those gas-fired power stations were switched off, the demand on the grid would exceed supply and the grid would shut down, and none of us would get ANY electricity.

So if the delivery of ‘your’ green electricity requires other people to have ‘grey’ electricity, then I don’t think that ‘your’ electricity should really be considered ‘green’.

And there’s more!

Although we can choose to use nominally ‘green electricity’ in our own home, we rely on electricity being used by lots of other people. For example:

  • In shops, and their supply chain, particularly for refrigerated products.
  • In factories that manufacture stuff we need.
  • On roads, to operate street light, traffic lights and speed cameras.
  • In hospitals.
  • In internet service centres.

And much more. This ‘other electricity’ which may be ‘grey’ or ‘green’ in terms of carbon-accounting, is in part ‘ours’ too, even when we personally are not using it.

This is the nature and power of the grid. Just like the electricity that flows in it, it is shared by us all.


When I am working out how much carbon dioxide I am personally responsible for, I don’t assume that my electricity is carbon free, despite being told that it is by my electricity company!

Instead I use figures from web sites such as MyGridGB or Carbon Intensity who add up the actual sources of electricity contributed to the grid and calculate the overall carbon dioxide emissions.

  • For each kWh I draw from the grid emissions are, (in 2021) about 240 gCO2 per kWh in 2021
  • For each kWh I draw from my solar panels, after accounting for their embodied carbon dioxide, 0 grams of CO2 are emitted.

Also for each kWh of solar electricity I export back to the grid, I count the CO2 that was not emitted by a gas station because of my contribution which is about 450 gCO2 per kWh.

When I am trying to convince myself that net-zero living is achievable, I subtract these emissions from my total. But I am not sure even that is fair.


The asterisk text in full

* EDF home customers get energy tariffs backed annually by zero carbon electricity as standard.

All our residential tariffs are backed by 100% zero carbon nuclear electricity, with the exception of our EV tariffs which are backed with 100% zero carbon renewable electricity.

Electricity for our GoElectric tariffs come from renewable sources such as wind, solar, biomass, tidal and hydroelectric. At the end of each fuel mix reporting year we’ll make sure we’ve purchased enough renewable electricity from EDF owned, renewable generation to match the total volume of electricity supplied to all of our customers on the GoElectric tariffs. A fuel mix reporting year begins on 1 April and ends on 31 March the following year. UK fuel mix disclosure information, published by the Government (BEIS), recognises electricity generated from wind, solar and nuclear fuel produces zero carbon dioxide emissions at the point of generation. See our tariff table for more information.

Other environmental benefits:
Other suppliers include the funding of other carbon reducing initiatives such as tree planting in the price of their tariffs. Whilst our GoElectric tariffs don’t directly fund or offer any additional environmental benefits beyond being sourced from renewable generators, EDF is Britain’s biggest generator of zero carbon electricity and as part of the EDF Group (which, in 2017, was the largest generator of renewable electricity in Europe) is committed to going beyond the requirements of 2°C trajectory set by COP21 by drastically reducing our CO2 emissions.

Spreading the word

September 1, 2021

Click for a larger version. I have put a sign outside my house!

One of the aims of my mission to reduce carbon dioxide emissions from my house was to make sure that, in the end, the house looked normal.

And with annual carbon dioxide emissions reduced by an estimated 80%, I feel I have succeeded: the house still looks very ordinary.

I felt that if the house looked futuristic or weird, it might deter people from doing something similar.

But one flaw in that strategy is that as people walk past – they don’t notice the house at all!

So I have put up an A4-sized notice board in the front garden to tell people how amazing the house is.

A notice board? 

I am aware that the 21st Century offers opportunities for communication other than noticeboards.

I have heard that entirely visual apps such as Tickety Tok and Instantgram are very popular with the under fifties.

But there are also a lot of people filling those channels of communication with a tsunami of… stuff.

I am targeting the pensioners and families of Teddington, many of whom – but by no means all – are in a position to do something similar to their own homes.

Frankly, I am not optimistic – but I thought I would give it a go.

I’ll let you know how it goes.


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