Archive for the ‘My House’ Category

Would you like milk with your tea?

January 9, 2022

Every blog article starts with a mug of tea.

Friends, I am addicted to tea.

I like all kinds of tea, but my favourite is a basic brew, with milk.

The ritual of settling down with a mug of hot tea is an essential pre-requisite for any kind of concentration – such as writing this article.

This the main use of milk in the household and each week my wife and I consume around 2 litres.

So per year we use roughly 100 litres of milk.

Looking online, I find this corresponds to emissions (mainly of methane) which are equivalent to 315 kg of carbon dioxide per year: almost a third of a tonne!

I think there is a lot of uncertainty in that estimate, and it probably varies from country to country. But taking it at face value, it is a truly colossal impact from a very mundane activity.

Click image for a larger version. The graph shows the global warming impact of emissions associated with production of milk and cheese in terms of the equivalent amount of CO2 emissions which would have the same impact. The data is from Our World in Data

After all the work I have had done on the house, annual heating and electrical emissions have fallen from 3.7 tonnes to about 0.7 tonnes.

So emissions from drinking tea alone have become a significant fraction (∼50%) of general household emissions!

What to do?

As far as I know, the only way to avoid these emissions, is to stop drinking milk –  or to reduce the amount I drink substantially.

The problem

The problem with this solution is that I – like millions of other people – really like having milk in my tea.

I have tried using milk alternatives derived from oats, almonds, and soya. These products look like milk and come in packaging which suggests they are in some way similar to milk.

But they do not taste even remotely like milk.

Additionally, I am emotionally attached to the idea that milk comes from cows that live on farms. For someone who is basically a city-dweller, this connection feels meaningful.

So at the start of this new year I am facing a dilemma.

What to do?

As far as I know, the only way to avoid these emissions, is to stop drinking milk –  or to reduce the amount I drink substantially.

Switching to the plant-based alternatives is just not acceptable, which leaves me with just two options:

  • Abandoning milk in my tea altogether. This is an extreme option, but one I am keeping under review.
  • Currently I am experimenting with a 50:50 mixture of milk with Oat ‘derivative’ product. It is predictably, not as nice as just milk, but it is borderline acceptable. But the emissions are still substantial.

I will let you know how it goes when I have a few more weeks under my belt.

The wider problem

The wider problem with this solution is that I haven’t even mentioned butter or cheese, other dairy-based staples of my diet.

Because of the large amount of raw milk used, each kilogram of cheese is apparently is associated with 24 kg of CO2 equivalent emissions (mostly as methane).

My wife and I eat – and enjoy prodigiously – about 0.5 kg of Davidstow Cheddar each week. This corresponds to around 25 kg per year, and emissions with the equivalent impact of 600 kg of carbon dioxide per year.

Basically the emissions associated with our cheese consumption have an impact roughly equivalent to all the electricity we use to heat and run the house for a year!

Fortunately Our World in Data does not have information about butter. I say ‘fortunately’ because I feel sure it will be bad.

The wider truth is that in regards to my house, all the changes I have made to reduce carbon dioxide emissions have been expensive, but they have not really affected my quality of life.

But it seems that emissions from some of the basics of my diet, foods I love and have eaten all my life, are apparently responsible for more annual carbon dioxide emissions than my entire house!

Reducing these emissions is going to be much tougher and feel much more like a personal sacrifice with a very direct and (at least initially) negative impact on my quality of life.

I guess nobody said it would be easy.

I am going to sit down now with a nice cup of tea to think about this…

 

 

 

 

Carbon accounts 2021: looking back and looking ahead

January 2, 2022

Click the image for a larger version. Representation of the reduction in domestic carbon dioxide emissions from gas and electricity over the last four years. Also shown are the steps I have taken to achieve these reductions.

[Article Summary: it is actually pretty difficult to estimate carbon dioxide emissions – but it is important to try.]

Friends, it’s New Year’s Day.

And at this still point in the ever-rolling cycle of the years, it seemed like a good time to summarise progress on my project to reduce carbon dioxide emissions from the house.

It may seem like a good time, but actually this is not a good time at all to summarise annual emissions. The best time to do this is 6 months away in the summer. This is because household emissions peak in the winter and fall to practically zero in the summer.

You can see this on the graphs below which show cumulative electricity and gas consumption through the last few years.

Gas

To estimate the emissions from gas I have estimated the meter reading which I would have had in the summer of 2018 and used this as a baseline.

I then subtract weekly meter readings from this baseline and convert them to kWh of thermal energy, and then multiply the number of kWh of gas by 200 gCO2 per kWh.

Burning methane gas in a boiler releases around 183 gCO2 per kWh of gas – as documented in this official spreadsheet. (Look up the ‘Fuels’ tab and use cell F42) But some additional CO2 emissions are associated with delivering the gas to my home: compressors drive the gas along pipelines and ships deliver gas across the oceans.

The actual value of these ‘upstream’ emissions is difficult to know precisely, but actual experts suggest it amounts to roughly 24 gCO2 per kWh of gas delivered. So in principle the best estimate of CO2 emissions from gas delivered would be 183 + 24 = 207 gCO2 per kWh. This figure is 4% larger than the figure I used.

However, it is likely that direct methane leaks at wells and in handling plants are underestimated (example). Evaluated over a decade after leakage, methane is 84 times more powerful as a greenhouse gas than CO2. So if even 0.1% of methane leaked on its way to my home, the CO2 equivalent emission would be increased by 17 gCO2/kWh. However some people estimate that actual leakage is more than 1%. If that were so that would practically double the climate impact of using gas to nearly 400 gCO2 per kWh. The unknown magnitude of leaks is just one more reason to stop using methane gas.

Given these uncertainties I have used a figure of 200 gCO2 per kWh as a likely underestimate of true emissions which is not obviously wrong, but which is a convenient round number.

Click the image for a larger version. The graph shows the cumulative emissions of carbon dioxide emissions from domestic gas use over the last three years.

The graph above is based on weekly gas meter readings.

The data form a series of ‘steps’ and it is clear that measuring from one step level to the next gives a better estimate of the yearly emissions than choosing an arbitrary point on the ‘riser’ of the staircase. This implies measuring from summer to summer

The reason is that if the winter is mild before the New Year but cold after New Year, the emissions fall in different years even though they arise from the same winter.

But however one analyses the data, it is clear that there has been no step this winter of 2021/22. We now use gas only for cooking and I hope shortly to stop even this use and make that curve go entirely flat. For ever!

Electricity

To estimate the emissions from electricity I have multiplied meter readings in kWh by 230 gCO2 per kWh.

As with gas, it is not obvious how much carbon dioxide is emitted for each kWh of electricity consumed from the grid. Depending on the generation source, the so-called carbon intensity of the electricity can vary significantly. For example, as I write – with low demand and high winds – the carbon intensity of the electricity is just 111 gCO2 per kWh.

The MyGridGB web site maintains a live monitor of carbon intensity, and shows an annual summary of average carbon intensity through the year.

Over the last 3 years the average carbon intensity in the UK has been 245, 222, and 235 gCO2 per kWh. Since these figures are within a few percent of each other I have used a rounded value of 230 gCO2 per kWh for the entire range of the measurements.

Click the image for a larger version. The graph shows the average carbon intensity for each year. The values for eth last three years have been 245, 222, and 235 gCO2 per kWh. The red line at 100 gCO2 per kWh is the target carbon intensity for the year 2030.

Multiplying the number of kWh used by 230 gCO2 per kWh tells me the emissions associated with my use of grid electricity. This is shown below on the same vertical scale as on the gas graph above.

Click the image for a larger version. The graph shows the cumulative emissions of carbon dioxide emissions from domestic electricity use over the last three years. The scale is the same as in the previous graph showing carbon dioxide emissions from gas use.

The graph above is based on weekly electricity meter readings.

During 2019 and 2020 carbon emissions occurred at a regular rate with no seasonal steps. Even, looking closely, I cannot detect the point in November 2020 when solar panels were installed.

But in March 2021 when our Powerwall battery was installed, the curve goes flat as the combination of solar panels and battery was sufficient to take us practically off-grid for the summer.

In September 2021, as solar generation weakened, we began to draw electricity from the grid again, and also began heating with electricity using our air-source heat pump. Currently we are using 20 to 25 kWh/day – more than twice the previous rate. This will probably continue until March

It is hard to estimate precisely, but I think – with colder months ahead – the summer-to-summer emissions will be similar or slightly less than last year.

Gas and Electricity

To estimate the emissions from both gas and electricity use, I have added the data from the two previous graphs together.

Click the image for a larger version. The graph shows the cumulative emissions of carbon dioxide emissions from domestic gas and electricity use over the last three years. The scale is the same as in the previous graphs.

Anticipating data from the spring of 2022, it looks like emissions will have fallen from about 3.6 tonnes in 2018/2019 to (hopefully) only 0.7 tonnes in 2021/22. This is an 80% reduction.

But even though this has already been a tedious article, this is not quite the end of the story.

Embodied Carbon

To achieve that 80% cut in annual emissions, I had to buy things which involved the emission of carbon dioxide – so called embodied carbon.

It is difficult to estimate the amount of embodied carbon in a particular object, but after quite some effort I have come up with the following estimates.

Intervention

Embodied tonnes of CO2

EWI PU Boards

1.6

EWI Mortar

1

Argon Triple Glazing

1.9

Solar Panels

1.6

Battery

1.4

Heat Pump

1.5

Air Conditioning

1.5

Total

10.5

They amount to 10.5 tonnes of embodied carbon. To find out when this embodied carbon has been ‘paid for’ I need to compare the CO2 emissions described above with the so-called counter-factual: the emissions which would have occurred if I had done nothing.

If I had done nothing then my guess is that emissions over the last 4 years would be simply 4 x 3.6 tonnes of CO2 – or 14.4 tonnes.

Actually, CO2 emissions over the last 4 years have been 3.6 + 2.9 + 2.0 + 0.7 = 9.2 tonnes.

So my ‘investment’ of 10.5 tonnes of embodied carbon will have saved 5.2 tonnes of emissions by summer 2022, and should continue to save (3.6 – 0.7) = 2.9 tonnes per year for several years to come. So I should ‘break even’ during the year 2024. Everything beyond that will be pure emissions savings.

Looking ahead

Click the image for a larger version. The graph shows the estimated household emissions from 2018 to 2040. The red line shows the emissions which would have occurred if I had done nothing. The green line shows the emissions according to the current plan. The dotted line shows emissions if the money I pay to Climeworks is not a scam.

Looking further ahead, the tonnes of carbon ‘debt’ I have incurred seems less significant. And carbon dioxide emissions avoided by 2040 amount to 60 tonnes.

Additionally since March 2021, I have been paying Climeworks £40/month to permanently remove 50 kg/month (0.6 tonnes/year) of carbon dioxide. If they are actually doing this – and I have no real way of knowing! – then our household is very nearly carbon neutral.

However none of what I have discussed accounts for emissions arising from consumption, or travel, or from my pension investments – all of which are likely to be quite significant.

So there is still lots to do in the new year.

November 2021: Heating and Carbon Emissions

December 1, 2021

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…

The Role of a Battery in Meeting Winter Electricity Demand

November 26, 2021

Click Image for a larger version. The 23rd November 2021 was a glorious day in West London.

Friends, these last few days of November have finally been cold enough for all the parts of my plan for a low-emissions home to come together.

  • The triple glazing and external wall insulation are reducing the heating required to keep the house comfortable.
  • The air source heat pump is reducing the amount of energy required to produce that heat.
  • The battery is charging itself at night on cheap electricity and running the house during the day, only running out of charge in the evening.
  • And on sunny days, the solar panels are topping up the battery during the day!

And so far, nothing major has gone wrong!

In this article I will outline the role of the battery in all of this by looking at how the battery operated during two recent quite chilly days, one sunny and one dull.

Battery: Capacity

The battery is a Tesla Powerwall 2, with 13.5 kWh of storage. This is large for a domestic battery but still only 25% the capacity of a typical mid-size EV battery.

We use about 10 kWh/day of electricity for day-to-day living, and in summer the battery and solar PV allow us to operate off-grid for almost 6 months.

But now we are heating our home electrically, and expect to have to provide around 50 kWh/day of heating – something achieved using around 15 kWh/day of electricity using an air source heat pump.

So winter demand is expected to be around 25 kWh/day of electricity – roughly twice as much electricity as the battery can store.

So in winter the main role of the battery is to allow us to buy cheap rate electricity, and then use it later in the day, minimising the cost of the extra electricity we are using for heating.

Battery: Losses

Intrinsically, battery cells can only store DC electricity, but the Powerwall needs to store and discharge AC electricity.

So the Powerwall, includes an AC to DC converter on its charging input and a DC to AC converter (called an inverter) on its output. Overall, it promises to deliver back 90% of the electric energy stored in it.  Tesla call this ‘90% round trip efficiency’.

Additionally the battery requires power to maintain itself: it needs to keep its internal controllers going, and to operate a heating and cooling system to maintain the battery at a suitable temperature during charging and discharging in order to ensure the longevity of the battery. Tesla guarantee that the battery will retain 80% capacity after 10 years.

These two effects – round trip efficiency and self-consumption – make it difficult to estimate the so-called ‘state of charge’ of the battery (SOC) i.e. how ‘full’ it is. This is because energy stored in the battery seems to slowly disappear and it is not clear quite how to account for that.

So I have crudely approximated both effects by a simple average power loss in the battery, typically between 100 W (2.4 kWh/day) and 150 W (3.6 kWh/day). I have then adjusted this rate to make sure the Powerwall is ’empty’ at approximately the observed time. The Powerwall also reports what ‘it’ thinks it’s internal state of charge is.

Household Demand

The graph below shows that household demand on Tuesday 23rd November 2021 and Wednesday 24th November 2021 was broadly similar.

Click Image for a larger version. Household demand on the 23rd and 24th November. Most of the roughly 1 kW of demand is from the heat pump (~0.6kW).

Normal non-heating household demand is typically ~10 kWh/day but on these days, the house used ~24 kWh of electricity with ~15 kWh being used to operate the heat pump to provide hot water and space heating:

The battery itself seemed to consume about 3 kWh with probably ~1.3 kWh of that being round-trip losses, and 1.7 kWh (~70 W) being self-consumed to maintain its temperature and operating system.

State of Charge

The graph below shows both my estimate of the state of charge of the battery (i.e. how full it is ) through each day. Also shown as red dots is the self-reported state of charge of the battery.

Click Image for a larger version. The estimated state of charge (SOC) of the battery (kWh) during the 23rd and 24th November. The green line is my estimate and the red dots show the battery’s self-reported estimate. Also shown is the solar power (kW) during the day which was substantial on the 23rd – enough to meet household demand and partially re-charge the battery – but not very substantial on the 24th. The green dotted line shows how the battery would discharge if there were no ‘solar boost’

The overall uncertainty in my estimate of the state of charge is quite large – perhaps about 0.5 kWh – as shown by the fact that on the 23rd the battery apparently ‘overfills’ and the 24th it ‘underfills’. However my estimates do coincide reasonably well with the Powerwall’s own estimates.

  • On the 23rd, the battery was initially not quite empty and then charged at approximately 3.6 kW using off-peak electricity available from 0:30 to 04:30. It discharged during the day at around 1 kW, and then after being boosted by 7.4 kWh of solar PV, ran out of charge between 23:00 and 24:00. That evening I was obliged to purchase only ~ 0.5 kWh of full-price electricity.
  • On the 24th, the battery was initially empty and then charged at approximately 3.6 kW using off-peak electricity available from 0:30 to 04:30. Discharging at ~1 kW as on the 23rd, but with just 0.94 kWh of ‘solar boost’, it ran out of charge between 18:00 and 19:00. That evening I was obliged to purchase ~5.4 kWh of full-price electricity!

Because the battery had been fully charged and emptied on the 24th, I could evaluate its round trip efficiency using data reported by the battery itself. It reported that it had received 13.4 kWh of charge and discharged 12.1 kWh – just over 90% of the charge, which is in line with specification. But I do not think this figure includes ‘self-consumption’ which I believe appears as a ‘phantom’ domestic load.

Click Image for a larger version. Grid use during the 23rd was almost exclusively during the cheap rate when the battery was charged while using cheap-rate electricity operate the domestic load, including a dishwasher cycle. On the 24th the battery ran out in the early evening and around 5.5 kWh of full price grid electricity was used.

This behaviour makes sense in general terms. If the battery is full (13.5 kWh) at 4:30 a.m., and demand is around 1 kW, then we would expect the battery to run out 13.5 h later i.e. around 6:00 p.m.. Each kWh of solar generation delays that time by an hour.

Costs & Carbon Emissions.

In the Table below I have summarised as best I can the costs and carbon emissions arising from the house on these two days and compared them with two alternative situations:

  • The first imagines that we had the same solar PV, and heated the house with a heat pump but with no battery.
  • The second imagines that we had the same solar PV, but heated the house with a gas boiler.

Click the image for a larger version of the table. Entries associated with burning gas are coloured in blue. Calaulations are bsed on Electricity prices of 5p/kWh (off-peak) and 16.3 p/kWh (peak) and gas prices of 3.83 p/kWh.

The analysis and evaluation of the alternative scenarios is tedious beyond measure, so allow me to simply summarise.

In terms of money:

  • The battery saves lots of money every day, with or without the ‘solar boost’.
  • It makes the use of an ASHP not only low-carbon, but also very cheap.

In terms of carbon dioxide emissions:

  • The battery reduces ‘my’ carbon dioxide emissions by optimally capturing solar energy.
  • On dull winter days the carbon dioxide emissions would be similar with or without a battery.
  • Using an ASHP – with or without a battery – drastically reduces carbon dioxide emissions compared to using a gas boiler.

Summary.

As I have commented before, the battery is primarily a financial investment.

In summer:

  • The battery it allows me to fully utilise the solar PV, taking the house essentially off-grid for around 6 months.
  • This saves me hundreds of pounds a year.

In winter:

  • The battery it allows me to fully utilise the small amount of solar PV available, and to time-shift the use of off-peak electricity.
  • This also saves me hundreds of pounds a year.

The apparent savings in carbon dioxide emissions associated with using a battery are illusory and in fact the battery is really an additional electrical item using power and causing further emissions.

The solar PV provides low-carbon electricity, but without a battery, any mismatch between production and our domestic use of electricity is expensive.

  • Overproduction of electricity is exported to the grid and I get 3p/kWh.
  • Underproduction of electricity requires me to import from the grid and I must pay 16.3p/kWh.

So without a battery, the low carbon dioxide electricity is still used and still helps the planet, displacing generation from gas-fired power stations. But I don’t get much benefit for it!

A weekend away…

November 15, 2021

Click image for a larger version. The view from Winchelsea village is splendid, and in my opinion, all the better for the views of Camber Sands wind farm and Dungeness nuclear power station.

Friends, I was just chatting with my heat pump (HP – he/him) the other day, when HP made an odd request:

HP: “Michael, would you and Stephanie mind going away for a few days?

I was honestly a bit alarmed. This is our home and although we have taken HP in and hope to provide him with a loving home, we really don’t want him to take over! But then HP explained.

HP: “I’ve got this issue with my Weather Compensation. And when you and Steph are inside the house, switching things on and off, cooking, and just generally heating the place with your fleshy metabolisms, I just can’t see if it’s working or not!”

It still seemed a bit previous, but after discussing it together as a family, Steph and I agreed to go away for a long weekend in Winchelsea and to leave HP to look after the house.

When we came back…

On returning, we were worried we might find the house trashed – you know what youngsters are like.

But in fact the house was pristine and HP had kept the temperature stable all weekend – all by himself!

And to cap it all, HP had left us with a whole pile of beautiful data. I was so proud that I told him I was going to share it on my blog.

Weather Compensation

Looking at 72 hours of data without us messing things up, it was clear that HP really did have good weather compensation.

As the external temperature went down, HP responded by increasing the temperature of the water flowing in the radiators.

The effect of HP’s efforts was to keep the internal temperature within ± 0.3 °C of 20.5 °C over most of this period.

And it is also worth noticing how low the flow temperatures were, peaking at just under 34 °C when the external temperature fell to 7°C.

Click image for a larger version. Graph showing the external temperature (°C) and the flow temperature (°C) of water in the radiators over the 72 hours starting at midnight on 10/11 November 2021. The grey dots show the data every 2 minutes and the green curve shows the data averaged over 1 hour. Notice how when the external temperature falls, the flow temperature rises. This is called ‘Weather Compensation’.

One can see the ‘Weather Compensation’ effect more clearly if one plots the hourly-averaged flow temperature versus the external temperature.

Click image for a larger version. Graph showing the average flow temperature (°C) of water in the radiators versus the external temperature (°C) over the 72 hours starting at midnight on 10/11 November 2021. The blue dashed line shows the nominal ‘Weather compensation’ curve that HP was trying to achieve.

The blue dashed line on the graph above shows the nominal ‘Weather Compensation Curve’ that Vaillant had suggested that HP should aim for – a curve which Vaillant helpfully label with the meaningless number ‘0.7’.

It suggests that even when the outside temperature reaches 0 °C, we should be able to stay cosy with a flow temperature in the radiators of around 40 °C.

In different houses, different compensation curves can be chosen to match the heat pump output to the heat losses from the house.

And over the period of 3 days, HP used just 19.8 kWh of electricity (275 W) but delivered a whopping 83.7 kWh of heat (1.16 kW) – an average coefficient of performance of 4.2.

Conclusion

HP really enjoyed his time at home alone – he seemed so proud that he had mastered the Weather Compensation which can be quite tricky even for clever heat pumps like HP.

It really is great having HP in our family, and Stephanie and I enjoyed our time away too.

I really wish everyone could have a heat pump as considerate as HP.

P.S. In case you haven’t been following, HP is a 5 kW Vaillant Arotherm plus heat pump.

Powerwall: Assessment of degradation of storage capacity after 8 months

November 9, 2021

Friends, it is now 8 months (235 days) since we installed the Tesla Powerwall domestic battery (link).

And one of the questions I am most commonly asked concerns its likely lifetime.

All batteries degrade over time, even Tesla’s, but the practical question is “How much degradation occurs and over what period?

By chance last night (8th/9th November) I had the opportunity to assess this degradation and, in case you are short of time and need to do something more important than read this blog, my estimate is that the battery degradation so far is immeasurably small.

For those of you still with me, allow me explain how I made the measurement and what the results suggest regarding the battery lifetime.

Powerwall Control

The Powerwall battery is apparently controlled by an ‘App’ on my phone.

The ‘Tesla App’ has pleasing – almost compulsively enticing – graphics showing power flowing to and fro from the grid; the battery; our house; and the solar panels. It’s a really engaging interface. It also allows detailed data to be downloaded for analysis.

Click image for a larger version. Screenshots from the Tesla ‘App’ showing the charging and discharging of the battery, its state of charge, and the overall operation of the battery system.

However, I say the battery is ‘apparently’ controlled by the App because in reality the battery is controlled and monitored 24/7 over the internet by Tesla. And Tesla give me only limited control via the App.

Currently the battery is set to ‘Time-Based Control‘ which charges the battery from solar PV when available or cheap rate electricity if required.

And Tesla make the choice about how much charge to take overnight based on it’s estimate for how much solar power will be available the following day.

I don’t have the algorithm it uses, but it seems to do a fair job.

The reason I consent to this egregious interference with my liberty is that in return for ceding control, Tesla promised that the battery would retain 80% of its specified 13.5 kWh capacity (i.e. 10.8 kWh) in 10 years time i.e. after a nominal 3650 partial charge cycles.

I think this is a guarantee worth having and so I submit to the Tesla-Brain.

In fact since I bought the Powerwall I believe this guarantee has been degraded to 70% after 10 years – which suggests it really is quite a tough specification.

Expected Battery Degradation

Various reports on the web, and Tesla’s 2020 environmental impact report, indicate that Tesla car batteries seem to retain around 90% of their range after 200,000 miles (320,000 km).

Click image to see a larger version. Excerpt from Tesla’s 2020 Environmental Impact Report showing roughly 10% reduction in EV range after 200,000 miles.

It’s hard to know how that colossal range would translate into the 3650 partial charge and discharge cycles of a domestic battery over 10 years.

In part it depends strongly on the range of the charge-discharge cycles. Charging and discharging over the middle of a battery’s range – between say 10% and 90% – is relatively benign. But rapidly charging and discharging from 0% to 100% degrades battery capacity. This is why Tesla want to have control over the battery.

My thought when I bought the battery, was that domestic service would be generally less stressful than service in a motor car. Why?

  • The Powerwall has it’s own re-circulating fluid temperature control and does not need to operate in the climate extremes of a car battery.
  • A Tesla car battery is about 4 times larger than a Powerwall’s 13.5 kWh, but the maximum EV discharging rates – which can affect battery life – are up to 25 times higher than the Powerwall’s transient maximum of 7 kW.

Other reports (link) suggest that Powerwall degradation might be considerably faster than for EV’s.

However, the general pattern of battery degradation (reflected in the figure above) is that the greatest rate of degradation is at the start of the service life of the battery.

If my Powerwall were to degrade linearly to 80% capacity over 10 years (120 months) then after 8 months I might expect to see 0.18 kWh decrease in capacity. Small, but possibly detectable.

What did I measure?

By chance last night the battery ran out just before midnight, so I knew it had zero ‘state of charge’.

Additionally the Tesla-Brain decided to fully charge the battery over the four hours of cheap rate (5p/kWh) electricity starting at 00:30. So I was able to observe a full charge from empty.

The graphs below (using data downloaded via ‘the App’) show what happened. Note the data only have a time-resolution of 5 minutes.

Click image for a larger version. Graph showing the charging of the battery from 00:30 at approximately 3.6 kW and the discharging of the battery after 04:30 at approximately 300 W to meet domestic demand.

Using the charging rate and the time I can work out the ‘state of charge’ of the battery and compare this with the specified capacity.

Pleasingly, the maximum state of charge appeared to correspond closely with the initially specified capacity.

Click image for a larger version. Graph showing the calculate ‘state of charge’ of the battery during charging from 00:30 to 04:30. Within the (considerable) uncertainties of this measurement, the maximum state of charge is closely in line with its specified original capacity.

Interestingly, while the battery was charging, the Tesla control circuitry also used cheap-rate electricity to run the dishwasher and top up the domestic hot water using the heat pump.

Click image for a larger version. Graph showing the household demand from midnight to 06:00. The battery was charging in the background during these high power events.

Conclusions

First of all, some caveats:

  • All these measurements are self-reported by the Powerwall, and so should rightly be subject to sceptical interpretation.
  • The data have limited resolution both in power and time.

However, when I have been able to check the reported values against independent measurements – e.g. for estimates of the energy reaped from the solar panels each day – I have found them in close agreement at the level of 0.1 kWh.

So taking these measurements at face value, I find no detectable degradation in battery capacity after 8 months or 235 days.

  • If the battery capacity were degrading linearly over time to 80% of initial capacity after 8 years I would have expected to see 0.18 kWh decline in capacity.
  • If the battery capacity were degrading faster than linearly – as it plausibly might – then I would have expected to see perhaps 0.3 kWh or 0.4 kWh degradation.

Obviously I will re-visit this issue at some point in the future, but the fact that there is no detectable degradation so far suggests that the retained capacity after 10 years may indeed exceed 80%.

Which would be nice.

A Year of Solar Energy

November 8, 2021

Friends, it’s just coming up to the anniversary of the installation of solar photo-voltaic (PV) panels at Podesta Towers in Teddington. So I thought it might be interesting to see what a year of generation has brought.

First I’ll describe the installation; then explain what I expected (or at least hoped for); and then outline what has actually happened together with a discussion of the role of our domestic battery.

And finally I’ll remind you – and myself – of how solar panels fit into to my efforts to reduce carbon dioxide.

But in case you’re short of time, here are the salient points.

  • 12 solar panels generated just over 3500 kWh of electricity which is close to last year’s total domestic consumption ~3700 kWh.
  • This will have avoided emission of ~0.8 tonnes of carbon dioxide this year.
  • On the sunniest days the system generated ~25 kWh/day and in mid-winter average generation was ~ 2kWh/day which compares with around 10 kWh/day of non-heating electrical use.
  • When used with a battery, we were substantially off-grid for around 4 months.

1. The Installation

I described the installation in some detail here, but briefly it consists of 12 panels from Q-Cells, each 1.0 m x 1.7 m in size with a nominal generating power of around 340 W. I think this year’s versions are already more powerful!

Click for a larger image. Google Maps view of my home showing the shape and orientation of the available roofs. And photographs before and after the installation.

The installation cost £4,230 pounds which did not include the cost of scaffolding which was already up on the house at that point.

I chose Q-cells panels because I liked their completely black appearance; they seemed adequately specified; they were readily available; and they were cheap – around £130/panel if I recall correctly.

I later found out that they have pleasingly low embodied carbon dioxide, less than 1.6 tonnes for this installation, and so the embodied carbon dioxide payback time will be around 2 years. Q-cells also score highly for the not producing toxic waste.

2. What did I hope for?

The quotation from local installer GreenCap Energy included an estimate of the expected output – 3780 kWh/year.

But rather than trust the installer I downloaded data from an EU project that cleverly allows one to estimate how a particular solar installation with an arbitrary location and orientation would have performed hour-by-hour over the entire period from 2005 to 2016. This data suggested I might reasonably expect 3847 ± 173 kWh/year.

Click for a larger image. Estimates from an EU re-analysis project of what my solar panels WOULD have generated over the years 2005 to 2016. Year-to-Year variability appears to be about 5%.

I then averaged these 11 years of hour-by-hour data to yield my estimate for the expected monthly performance. They are shown as yellow dots on the graph below.

Click for a larger image. Expected generation in kWh per day. The yellow dots are the monthly averages of the estimated generation from 2005 to 2016. The green dotted line is a crude guess based on a ‘sine-squared’ function. The red dotted line shows our typical average daily electricity consumption ~10.5 kWh/day.

One feature of the simulated data is that the peak generation is expected to occur from April to July – a range which is not centrally arranged around the longest day (June 21st).

3. What happened?

Solar generation was broadly in line – but a little lower – than expectations. But the day-to-day and week-to-week variability was much greater than I had appreciated.

This variability makes it hard to plot readable graphs because they look chaotic! So let me introduce the data one stage at a time.

Looking at the monthly averages (below) we see that most months were close to expectations, but April was especially sunny, and August was a bit disappointing. So far, November has been a brighter than normal. Please note, the December data is from December 2020, because the data from December 2021 is not yet available.

Click for a larger image. Comparison of monthly averages of actual solar generation (green dots) with expected generation in kWh per day. (yellow dots). The black error bars show the standard deviation of that months daily data.

Now let’s additionally plot the daily data.

Click for a larger image. Similar to the graph above but now additionally showing the actual daily solar generation (kWh/day)

A few features of the daily data are really quite remarkable.

  • Firstly, the day-to-day variability is large. This means that the solar generation on a given day is almost no indication of the likely generation on the next day.
  • Secondly, the peak generation of around 25 kWh/day can occur in either April or July despite the substantial differences in day length and solar path.
  • Thirdly, even in mid-summer there can be runs of several utterly miserable days with very little solar generation.

If we average the daily data over a week, (see below) then we still see variability – deviations from the nominally-expected generation – which deviate from the expected generation consistently over periods of up to 3 weeks

Click for a larger image. Similar to the graph above but now additionally showing the ±3 day average of the generation as pink or purple lines.

Another way to look at the data which emphasises the trend more strongly than the variability, is to show cumulative generation through the year.

Click for a larger image. Cumulative generation (kWh) shown as a blue line against nominal expected generation.

Actual generation (just over 3500 kWh) is about 8% lower than I expected, but I am not especially surprised.

It could be that 2021 was a ‘a bit dull’ over the summer months when most generation takes place, or because I had incorrectly allowed for losses at the inverter – which converts DC electricity into AC mains electricity.

Battery

The PV panels were installed in November last year and to our surprise they immediately made a measurable difference – reducing our use of electricity from the grid by around 2 units per day.

Click image for a larger version. Daily electricity usage (from a smart meter) before and after solar panel installation.

However, it was not till our battery was installed in March 2021 that the transformational power of solar became apparent. Within a couple of days, the household went ‘off-grid’ and remained off-grid for around 80 days.

Click image for a larger version. Daily electricity usage (from a smart meter) since solar panel installation. Install the panels led to a small reduction in grid use. Consumption rose over Christmas. Consumption began to fall as we entered spring, and then fell to zero once we installed the battery. As we enter Autumn and Winter grid consumption is rising because since July we are using electricity to power a heat pump which heats the house and provides hot water. The bold green line shows the daily consumption averaged over ± 1 week.

As we enter autumn and winter, the solar cells still contribute significantly – 48% of our electricity in October was solar. But generation will be just around 2 kWh/day through November, December and January.

Since July, the ASHP installation has been using ~1.5 kWh of electricity a day to provide ~4 kWh of domestic hot water.

Now (November) the ASHP is providing space heating within the house, and in the coldest weather (~0°C) I expect  this will require an additional 15 kWh/day of electricity to provide ~50 kWh/day of heating. Most of this electrical energy will be downloaded at cheap rates overnight.

Summary

The PV and battery system has been installed to reduce carbon dioxide emissions from the house. They are the part of a suite of measures to reduce heating demand, eliminate gas use, electrify heating and increase the use of renewable energy.

Together they have dramatically reduced the running costs of the house.

The graph below shows expected household carbon dioxide emissions (not including consumption or travel) over the period up to 2040.

Click image for a larger version. Anticipated household carbon dioxide emissions (not including consumption or travel) over the period up to 2040. The red line shows what would have happened if I had made no changes. The green line shows the expected outcome.

In the short term, all the actions I have taken have made things worse!

The embodied carbon dioxide in the solar panels, insulation, batteries, and heat pump amounts to ~11.5 tonnes, and this ‘debt’ will not be re-paid until the end of 2023.

I would love to add more solar panels, but I am resolved to hold off until my existing carbon dioxide debt is re-paid.

Overall, I hope you can see that the solar panels are central to the plan to reduce anticipated emissions by 60 tonnes by 2040

 

Re-visiting the “washing-up dilemma”

November 6, 2021

Friends, I can remember a time when the main question about washing up was: “Who is going to do it?”

But dishwashers changed all that. At first there were questions about whether dishwashers or regular washing-up was technically superior.

However nowadays, dishwashers do a very creditable job of cleaning crockery and cutlery – and in what was a devastating blow for the British Tea Towel industry – they also do a fine job of drying.

[Aside: If you would like to see how dishwashers work – check out these Technology Connections videos:

But this then raised the thorny question of whether hand-washing or using a dishwasher was cheaper.

And now of course, the BIG question is which is greener i.e. has the lowest associated carbon dioxide emissions. This article is an attempt to answer this question.

Energy and Carbon Emissions

In her bookEnergy and Carbon Emissions: the way we live today“, Nicola Terry does a fine job of answering such questions.

The book is a gold mine of information on all aspects of our current ‘carbon problem’.

And she looks at this problem in Chapter 7: Should I get a more efficient X?

She concludes that:

“Washing dishes by hand probably generates less carbon [dioxide] emissions if you heat water by gas – even less if you have a solar panel for hot water

And in characteristic and admirable detail, she explains the assumptions underlying her conclusion.

But as His Bobness might have commented… things have changed.

2011 versus 2021

The energy landscape has changed significantly since 2011 when Nicola Terry published her book.

  • Carbon dioxide emissions from electricity are now 235 gCO2/kWh rather than 545 gCO2/kWh in 2011.
  • Carbon dioxide emissions from gas are still roughly 200 gCO2/kWh assuming ~90% boiler efficiency: the same as 2011.
  • In our house,
    • Hot water comes from a heat pump which produces hot water around 3 times more efficiently than a gas boiler.
    • In summer, practically all our electricity comes from solar panels with no proximate emissions: 0 gCO2/kWh
    • In winter, we set the dishwasher to run on cheap-rate electricity which typically has 10% less CO2 emissions than daytime electricity.
  • In other houses,
    • Hot water might come from solar thermal panels.

So given this complexity, can we definitively compare the environmental impact of using a dishwasher and washing up by hand?

Well I have made an attempt – and below I will run through some example calculations and then I’ll collate a more extensive set of calculations at the end.

Dishwasher

Taking our recent Bosch model as typical, a dishwashing cycle seems to typically consume around 12 litres of water and around 1 kWh of electricity.

Comparing the consumption for different modes of operation, it is clear that nearly all this energy is used to heat the water. At 70 °C – way more than your hands could stand – fats within food residues soften considerably making them easier to remove in the water spray.

In the winter,

  • Carbon dioxide emissions associated with running a cycle in 2021 on average would have been 235 gCO2 using electricity direct from the grid.
  • At peak demand (4 pm to 7 pm), the emissions might be higher – but if run in the early hours of the morning – emissions might be ~10% lower.

In the summer,

  • Carbon dioxide emissions associated with running the dishwasher are around zero because the electricity is derived from the solar panels.

So assuming a year is 50% summer and 50% winter, carbon dioxide emissions associated with running a single cycle in 2021 on average would be 0.5 x 235 = 118 gCO2 per cycle.

So running the dishwasher 3 times per week would result in emission of around 354 gCO2 per week.

Dishwashers do offer ‘eco’ modes using cooler water which could reduce this.

Washing-up by hand

In general, hand washing probably consumes a bit more water than a dishwasher, but let us assume in the first instance that the same 12 litres of water will suffice, and that the dishes are rinsed in cold water.

Heating 12 litres of water by 40 °C from (say) 10 °C to 50 °C – roughly the hottest water one might reasonably use – requires:

12 litres x 10 °C  x 4200 J/°C/litre = 2,016,000 joules

Or in more familiar units, 0.56 kWh. If this water were heated with a heat pump with a COP of 2.5, this would require 0.224 kWh of electrical energy.

In the winter,

  • Carbon dioxide emissions associated with 0.224 kWh of electricity drawn from the grid would have been 0.224 x 235 gCO2/kWh ~ 53 gCO2

In the summer,

  • Carbon dioxide emissions associated with 0.224 kWh of electricity from the solar PV panels would have been zero.

So assuming a year is 50% summer and 50% winter, then washing up by hand 3 times a week results in weekly emissions of 3 x 27 = 81 g CO2/week

If the water were heated by gas, the equivalent emissions would have very similar in winter, but would not be reduced in summer, resulting in emissions of about 150 gCO2/week.

And if the water were heated by direct electrical heating without a heat pump, three washes per week would result in emissions of about 158 gCO2/week.

Comparison

I have carried out these calculations for a number of situations using this spreadsheet (Dishwasher Calculations). I compared:

  • Hand washing with water from a heat pump powered by solar energy and a battery for half of the year.
  • Hand washing with water from a heat pump powered by grid electricity.
  • Hand washing with water from a gas boiler.
  • Hand washing with water heated by an immersion heater powered by solar energy and a battery for half of the year.
  • Hand washing with water heated by an immersion heater using grid electricity.
  • A dishwasher powered by solar energy and a battery for half of the year
  • A dishwasher powered by grid electricity.

The figure below summarises my calculations assuming that a family washes up 3 times per week either by hand or with a dishwasher.

Click on the image for a larger version. Summary of calculations showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details.

It is clear that wash-for-wash, washing-up by hand results in lower CO2 emissions. This is especially striking when using a heat pump to generate the hot water.

The lower emissions arise primarily because the water used for washing-up by hand is cooler than the water used in a dishwasher.

However different households behave in different ways, and wash-for-wash comparisons may be unrealistic.

For example, in a small household, a dishwasher can store soiled dishes out of sight for a day or two, and so operating a dishwasher (say) 3 times a week is practical while not leaving out unsightly piles of unwashed dishes. However, if a similar household were relying on hand-washing, they might wash up once each day. Let’s call this Option#2.

Or for larger households, running a dishwasher once a day is more typically part of a family routine, whereas hand-washing for a larger family would be more likely – in my experience – to use rather more water – perhaps as much as twice as much. Let’s call this Option#3.

The results for Options 2 & 3 are shown below. Note that the vertical scales are different on each of the graphs shown.

Click on the image for a larger version. Summary of calculations for Option#2 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details. Note that the vertical is different from the other figures.

Click on the image for a larger version. Summary of calculations for Option#3 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details. Note that the vertical is different from the other figures.

The different options have a range of CO2 emissions that vary from ‘a few kilograms per year’ to ‘a few tens of kilograms per year’.

But looking across all the options, hand-washing generally results in reduced emissions, just as Nicola Terry had concluded back in 2011. But now the minimum emissions come with water heated by a heat pump.

Click on the image for a larger version. Summary of calculations for Options #1, #2, and #3 showing estimated annual CO2 emissions for a variety of ways to do washing up. See text for details.

Conclusions

My first conclusion is this: How can anyone be expected to wade through such a calculation to determine how to do the washing up!

My second conclusion is not that “Dishwashers are bad”. Indeed my wife and I own a dishwasher and use it regularly.

Personally, I am convinced of the overwhelming need for society as a whole to reduce carbon dioxide emissions, and I also feel this as an intense personal responsibility too.

Nonetheless, no decision can be entirely one-dimensional. As with many similar decisions, there are other criteria that can also be significant – criteria that have nothing to do with carbon dioxide emissions.

Off the top of my head, it could be that some people just don’t like washing up! Or it could be that some people are busy and using a dishwasher is helpful. Or perhaps that someone’s partner might just want a dishwasher for some reason, and so a dishwasher could be part of a declaration of love – an important human dimension.

But… although any decision about how to live one’s life is multi-dimensional, one of those dimensions is very likely to be the associated carbon dioxide emissions, almost no matter what the actual decision concerns.

In this case, my calculations tell me that:

  • For washing up, the overwhelming contribution to the carbon emissions is caused by heating water – so using less hot water will reduce emissions no matter how the washing up is done.
  • If using a dishwasher, it is a good idea to try to ensure that it is more-or-less full, and to think about using ‘eco’ modes.
  • And when washing up is done by hand, it is a good idea to be mindful of the amount of hot water used – and possibly use cold water to rinse.

Finally, in future years, it is possible many of these emissions may well change again.

It is planned that the carbon intensity of UK electricity should fall to 100 gCO2/kWh by 2030 – and this will reduce the emissions of any appliances which use that electricity. Additionally, it could be that dishwashers will be built which can use hot water heated by a heat pump instead of electrically heating their own water.

Finally this calculation does not include the embodied carbon dioxide emissions from either a dishwasher, a heat pump, a battery or solar panels. My guess – for what it is worth – is that assuming reasonably long lifetimes for these items, the embodied carbon dioxide emissions will not significantly alter these conclusions.

 

Heat Pump Operation: What it costs. And what it costs me.

October 26, 2021

Friends, yesterday I wrote about the way the new Vaillant 5 kW Arotherm plus Heat Pump performed during 24 hours of a mildly-cold autumn day.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

This was really important data for me. I have spent the best part of £60,000 insulating the house; installing solar PV and a battery; and plumbing in the air source heat pump (ASHP).

But this was the first real-world indication that the system worked more-or-less as I had anticipated.

Understanding how the heat pump works is complex. And surprisingly, explaining what it costs to operate is more complicated than I expected, and slightly personally embarrassing.

Let me explain

Basics

Recently I switched to Octopus Energy and they charge me a daily standing charge plus a rate dependent on how much gas or electricity I use.

Standing Charges 

  • Gas: 23.85p per day.
  • Electricity: 25p per day.

Rates

  • 3.83p per kWh of gas.
  • 5.0p per kWh of electricity between 00:30 and 04:30.
  • 16.26p per kWh of electricity between 04:30 and 00:30.

All these rates are likely to change – increase – in the future.

Domestic Hot Water

As the graph shows, on 24th October, the heat pump used 1.6 kWh of electricity to create 4.9 kWh of heat in the hot water cylinder.

If I had used gas to do this using a boiler which was 90% efficient I would have needed to consume 4.9/0.9 = 5.4 kWh of gas which would have cost 5.4 kWh x 3.83p/kWh = 20.7p.

Using the heat pump I consumed 1.6 kWh of electricity at 5 p/kWh (because it was heated between 00:30 and 04:30) which cost 1.6 kWh x 5.0p/kWh = 8p.

So for heating domestic hot water, the heat pump is 62% cheaper than using gas.

If we had considered heating hot water during the summer, then the electricity would have been drawn from the battery which would had been charged by solar electricity, and so would be free.

Space Heating

As the graph shows, on 24th October, I used 2.3 kWh of electricity to create 10.0 kWh of heat in the house.

If I had used gas to do this using a boiler which was 90% efficient I would have needed to consume 10/0.9 = 11.1 kWh of gas which would have cost 11.1 kWh x 3.83p/kWh = 42.5 p.

Using the heat pump I consumed 2.3 kWh of electricity at 16.26 p/kWh which cost 2.3 kWh x 16.26p/kWh = 37.4p.

So for space-heating in this mild climate, the heat pump is 12% cheaper than using gas.

Actually its a bit better than this. Because we have a battery, we fill up with cheap rate electricity at 5 p/kWh so the actual cost of using the heat pump during the day is closer to 2.3 kWh x 5p/kWh = 11.5p i.e. 73% cheaper than using gas.

And actually, its even a little better still. Even at the end of October typically 50% of our electricity is derived from the solar PV system and stored in the battery. So the actual cost per unit is closer to 2.5p/kWh, so the actual cost of using the heat pump during the day at this time of year is closer to 2.3 kWh x 2.5p/kWh = 6p i.e. 85% cheaper than using gas.

The solar contribution will go down as we head into winter, but even in midwinter we still get ~ 2 kWh/day of generation, all of which is now captured in the battery. See the graphs below for more details.

Click image for a larger version. This complicated graph shows solar generation since the solar panels were installed in November 2020. The thin green lines with circles are daily generation. The thick pink line is 7 day running average. The big green circles are monthly averages, and the yellow circles are monthly averages for this location from 2005 to 2016.

Click image for a larger version. This graph shows household electricity consumption (averaged over ±1 week) during 2021. After the battery was installed, we used very little grid electricity for about 3 months. The blue line shows actual household consumption As we head into winter, our use of grid electricity is increasing. The dotted green lines show the range into which I expect grid electricity to fall depending on how cold the winter is, and how well the heat pump works.

Preliminary summary

For heating hot water, using a heat pump costs less than half the cost of using gas when using cheap-rate electricity.

For space-heating, using a heat pump costs 12% less than using gas even when using full-price electricity. And accounting for the solar generation and use of cheap-rate electricity stored in the battery, space-heating will cost a small fraction of what it would have cost using gas.

When I take account of all the variables, including the standing charges, the heat pump, the solar PV and the battery, I estimate that my annual bills from electricity and gas will be reduced from roughly £1600 to about £500 including £178/year of standing charges.

So financially, I am benefiting from my £60,000 of investment to the tune of around £1,100 per year.

But there are some complications…

Complication#1

I feel obliged to disclose that when I installed the heat pump, in addition to a £5000 government grant, the kind staff at Enhabit pointed out that I could I apply for funding under the Renewable Heat Initiative (RHI).

This scheme funds people who switch to using renewable technologies to heat their homes, compensating for the fact that electrical heating is generally more expensive than gas.

It didn’t seem to matter that – as I showed above – in my circumstances the heat pump would be cheaper than using gas.

So in addition to saving around £1,100 per year, the RHI scheme will additionally pay me £129.83/quarter or £519.32 per year for the next seven (7) years. Yes, I really did write that.

So broadly speaking, I don’t expect to pay anything at all for electricity or heating for the next 7 years.

Complication#2

I also feel obliged to disclose that when I installed the heat pump, the kind staff at Enhabit also pointed out that I could I apply for funding to monitor the heat pump performance using the so-called Metering and Monitoring Service Package (MMSP).

Under this arrangement, I needed to pay £1,156 to have a system installed which monitored the performance of the heat pump, and reported the data back live to OFGEM every 2 minutes.

This helps OFGEM establish the real world performance of heat pumps rather than relying on manufacturer’s specifications.

In return, I get:

  • Access to the data – that’s the data I used for the previous article.
  • A one off payment of £805
  • Annual payments of £115 (paid quarterly) for the next 7 years.

Overall this amounts to £1,610. This easily covers my outlay; provides important data to OFGEM; allows me to write endless blog articles; and reduces even further the cost of heating my home for the next seven (7) years.

Final Summary 

Friends, even without any Government deals, the heat pump would already be saving me money over a gas boiler.

And the use of solar PV and a battery makes the advantage even larger.

But taking advantage of the Government’s subsidies I find myself in receipt of a financial windfall – and it seems unlikely I will need to pay for electricity or heating for the next seven years.

Frankly, I am a little embarrassed.

However, even without these deals, switching to a heat pump would already be saving me money.

Keep warm!

 

 

Heat Pump: First Space-Heating Results

October 25, 2021

Friends, the unseasonably warm autumn has meant that I have had to wait until 23rd October for our 5 kW Vaillant Arotherm plus system to switch itself on.

I only have a couple of day’s data, but the results are interesting and promising. First I will show the data for each day and then discuss what it means at the end.

23rd October 2021

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 23rd October 2021.

From midnight until 6 a.m., there is no space heating, but the heat pump operates for 1 hour to heat the domestic hot water (DHW) tank. Performance for hot water heating is described in these articles (1, 2)

From 6 a.m. until midnight the system is responding to the thermostat set at 19.5 °C and heated water is circulated around the radiators.

The heat pump operates 12 times up until 6 p.m. after which no additional heating was required.

As shown on the graph, 12.1 kWh of heat was delivered using only 2.9 kWh of electrical power. which over 12 hours amounts to an average heating power of around 1 kW using only 250 W of electrical power.

The ratio of thermal to electrical power is known as the coefficient of performance (COP) and this is summarised in the graph below

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 23rd October 2021.

The COP seems to increase slowly through the day peaking for a few minutes in each cycle at values as high as 6, but the space heating average is 4.2.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 23rd October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.

The initial flow temperature is ~34 °C which falls through the day to ~28 °C. This reduction in flow temperature is in response to the increase in external temperature from ~12 °C to ~15 °C.

This so-called ‘weather compensation’ allows the use of lower flow temperatures, which enables the system  to operate with the highest possible COP.

I was surprised that even with my unaltered radiators, flow temperatures of 35 °C were sufficient to warm the house.

24th October 2021

Here is the equivalent data to that for the 23rd – the graphs are similar and I show them only to show that the system seems to be behaving reproducibly.

Click image for a larger version. The graph shows electrical power consumed by the heat pump and the thermal power delivered by the pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows Coefficient of Performance (COP) of the heat pump during the 24 hours of 24th October 2021.

Click image for a larger version. The graph shows various temperatures relevant to heat pump operation during the 24 hours of 24th October 2021. The external temperature; the internal temperature; the flow temperature in the radiators; the temperature of water in the DHW cylinder.

Conclusions

My first conclusion is that the system works. This is quite a relief!

My second conclusion is that the system works slightly better than I had been hoping for.

Being able to heat the house with these low radiator temperatures means that over winter the average COP could be higher than my prior estimate of 3. This means I will use less energy, emit less CO2, and spend less than I had estimated.

My third conclusion is that average heating power was about 1 kW during both days. But the system has plenty of margin to deliver more power either by changing the duty cycle – the pump could stay on for longer – or by increasing the flow temperature.

As soon as the weather settles down to being reliably cold I will begin to carry out experiments to optimise the weather compensation.

My fourth conclusion will be the basis of another article – but I can see already that as far as heating is concerned – I am going to have a very cheap winter.

Keep warm.


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