Thinking more about batteries

Thanks to everyone who got in touch about the previous article on batteries.

Two communications in particular – from correspondents in Canada and Japan – gave me pause for thought, reminding me of my UK perspective.

And on reflection I realised again that I was really hoping to do two similar, but quite district, things with a battery, Both involved storing electricity and using it later: that’s what batteries do! But the first involved storing solar electricity and the second involved storing grid electricity.

Which of these tasks was relevant would change with the seasons, and the optimal storage of solar electricity depends quite a bit on the day-to-day variability of the weather.

Why use a domestic scale battery?

My aim is to reduce carbon emissions from my house. And just to be clear – my primary motive is moral, not financial. But nonetheless, money matters. I don’t have infinite resources and methods for reducing carbon emissions that don’t make financial sense are unlikely to catch on with less zealous carbon-noughts than I.

Defn: Carbonought/Carbonaut: a person seeking to live without net emission of carbon dioxide.

There are two distinct ways in which a battery can help.

Reason#1: Winter heating 

My gas boiler emits around about 0.2 kg for every kilowatt hour (kWh) of heat it provides. Over the winter of 2019/2020 it released 2.5 tonnes of CO2 just keeping the house at a modest 19 °C.

My recent improvements to the insulation will have reduced that by about half for the current winter – but that is still more than a tonne of CO2! A tonne! The gas boiler is more than 90% efficient at converting chemical energy to heat, so in order to reduce emissions. further I need to find another way to heat the house.

The answer is to “go electric”. As the UK electricity network has increased the contributions from renewable sources of electricity, the so-called carbon intensity (kg CO2 per kWh) has fallen steadily year-on-year. In 2020 the average was about 200 gCO2/kWh so using electric heating will not immediately result in lower CO2 emissions than burning gas directly. But the carbon intensity of UK electricity has fallen from about 450 gCO2/kWh in 2012 and is expected to be close to 100 gCO2/kWh in 2030 – and so switching to electric heating should result in a gradual year-on-year decline in emissions.

But a kWh of electricity costs around £0.24 whereas a kWh of gas costs only about £0.05 – so just going electric without doing anything else would cost more than 4 times as much!

My solution is to insulate the house reducing the overall heating load by about half, and then to meet the remaining requirement with a heat pump.

A heat pump is an electrical device that moves heat from outside to inside. Critically, one kWh of electricity can move two kWh of heat from outside resulting a net heating power of three kWh. This should reduce the annual bill for heating electrically to something similar to the previous gas bill.

But by using a battery, I can pay even less! I can buy the electricity to operate the heat pump at night – when it costs much less – and then use the battery to power the heat pump during the winter days. This arrangement should reduce the cost of heating the house to well below what I currently pay.

• So in this case I would be using the battery to shift the time at which I purchased the electricity from the grid.

Reason#2: Summer Solar Electricity 

In the summer my house requires very little heating – but it still uses electricity. During the summer months I expect the solar panels on my roof to produce enough electricity to power the house – but they do not produce it at the correct time.

During the summer, the role of battery is to store solar electricity generated during the day which I can then use to power the house at night.

• So in this case I would use the battery to shift the time at which I used locally-generated solar electricity.

If I can get this right I should not need to rely on grid electricity at all for many days at a time i.e. there would be practically zero carbon emissions during the summer – and additionally some electricity could be exported.

Examples

Let’s see the difference between summer and winter operation by using a few examples. 

The graphs below were generated using this spreadsheet – that simulates the interaction of solar panels and batteries.

Screen shot from a spreadsheet: Click for a larger view. The boxes in the top left with yellow backgrounds allow different parameters – such as battery size – to be changed. Various parameters are then calculated minute-by-minute through the day. Daily performance is summarised in the boxes with red backgrounds.

The graphs show two types of quantities versus hour-of-the-day.

• For conceptual and programming simplicity, the time-axis shows hours after 11:30 p.m. rather than the more normal midnight. This allows me to plot the ‘off-peak’ electricity period (11:30 p.m. to 6:30 a.m.) as a contiguous area on the graph – it is shown as dark grey.
• The battery capacity (- – -) and state of charge are shown in kW) against the left-hand axis.
• Electrical demand (in kW), exported power (in kW) and solar power (in kW) are shown against the right-hand axis.

Case#1. At the moment (January in the UK) the house uses about 12 kWh of electricity each day – equivalent to a 0.5 kW continuous demand – and we don’t have a battery. The graph shows 0.5 kW of ‘demand’ represented by a thick black line which should be read against the right-hand axis.

The nominal daily cost of this electricity is £2.30 and the CO2 emissions amount to 2.33 kgCO2/day.

Case#1: Click for a larger version

Case#2. During the winter the solar panels generate on average 2 kWh per day. This weak solar generation (2 kWh) is shown as a yellow line in the graph below. This reduces the cost of electricity to £1.81 (saving £0.49) and reduces the CO2 emissions to 1.94 kgCO2/day (saving 0.39 kgCO2).

Case#2: Click for a larger version

Case#3. But sometimes – on clear cold days – the solar panels can generate 5 kWh per day as shown in the graph below. Because the instantaneous generation (up to 2 kW) exceeds the household demand – the bulk of the solar generation is exported (blue line). The price paid for this is generally low – EDF offer 1.8 pence per unit. But somebody somewhere will benefit from ‘my’ low carbon electricity!

Comparing Case#3 to Case#2, even though the panels generated 2.5 times more electricity than Case#2, the daily cost of electricity would only be reduced by a further £0.04 to £1.77 and ‘my’ CO2 emissions by a further 0.04 kg to 1.90 kgCO2/day.

Case#3: Click for a larger version

Case#4. So what would a battery do? Using the same 5.0 kWh of solar generation as in Case#3, if I had a 13.5 kWh Tesla Powerwall 2 battery then the solar generation would be captured and re-used later in the day. Very roughly such a battery would cost £10,000. Yes. That much.

The state of charge of the battery is shown as a green line in the graph below and should be read against the left-hand axis. Looking closely, one can see that once the solar generation exceeds the local demand – the excess is stored. Then when solar generation falls below local demand, the battery begins to discharge.

Case#4: Click for a larger version

By displacing expensive ‘peak’ electricity the daily cost is slashed to just £1.06 and the CO2 emissions reduced to 1.34 kgCO2/day. But all the stored electricity is used up before the end of the day.

Case#5. Now let’s imagine a summer day with 15 kWh of solar generation. Now the daily cost is slashed to just £0.20 and the CO2 emissions reduced to 0.35 kgCO2/day. Remember without solar panels or batteries the cost and emissions were £2.30 and 2.33 kgCO2/day. So we see that there are potentially large savings of both money and CO2 to be made.

Case#5: Click for a larger version

Case#6. At the end of the day described in Case#5, the battery still held 6.6 kWh of charge . So the next day – if solar generation were similar – would look like the graph below. The battery would discharge further over night and then be re-charged during the day.

Case#6: Click for a larger version

The daily cost is now £0.00 and the CO2 emissions are 0.00 kg. This is the situation in which the house is off-grid and all the electricity is used locally. Halleluiah!

Case#7. But now, at the end of the day, the battery holds 9.6 kWh of charge. So the next day – if solar generation were similar – would look like this:

Case#7: Click for a larger version

Again the daily cost and CO2 emissions are zero. But now we have reached a stable situation where the battery holds the same charge at the beginning and end of the day.

During the day – the storage capability of the battery was exceeded – and 2.9 kWh of electricity was exported to the grid. Potentially this export might generate a few pennies of payment but in general it represents a loss to me personally – but a boon for the planet because someone somewhere gets to use it as it displaces CO2-producing electricity generation.

Case#8. But now suppose the weather on the next day was dull – with just 4.0 kWh of generated electricity.

Case#8: Click for a larger version

In this case the battery will discharge – again the cost and carbon emissions are both zero – but at the end of the day the battery charge is low (1.6 kWh). In this case the optimal strategy depends on knowledge of the next day’s weather.

Case#9. Let’s suppose that Case#8 was followed by another poor generation day but that one just left the battery to discharge overnight – this situation is illustrated below in Case#9. Now one needs to purchase grid electricity at both peak and off-peak rates – this would cost £1.22 and emit 1.25 kg/CO2.

Case#9: Click for a larger version

Case#10. However if after Case#8 one had charged the battery from the grid overnight (see below) then one could avoid purchasing grid electricity at peak rate – this option would cost only £0.35 (less than a third of Case#9 cost) but emissions would be similar 1.26 kg/CO2.

Case#10: Click for a larger version

Case#11. But if one had charged the battery in readiness for a poor generation day (Case#10) and then by chance the weather had turned out fine, then one might have the situation as shown below.

Case#11: Click for a larger version

In this situation the cost and CO2 emissions are the same as previously (£0.35 and 1.26 kg) but now some of the electricity (1.9 kWh) is exported.

Case#12. If one could have anticipated Case#11, then one might have chosen to charge the battery less overnight:

Case#12: Click for a larger version

By reducing the charging rate, the cost and CO2 emissions are both reduced (£0.25 and 0.88 kg) and no electricity is lost by export to the grid.

Summary so far

In the summer the optimal financial strategy is to:

• Have sufficient charge in the battery – through overnight charging – that no peak electricity need be purchased towards the end of the next day.
• Have sufficient spare capacity in the battery so that no solar electricity will be lost to the grid – for negligible recompense.

However a strategy should not be so complicated that one has to spend significant effort programming the battery. Obviously bigger batteries makes these choices less critical,  but I think I need some real-world data before I can recommend a strategy for a UK summer.

In terms of reducing carbon emissions globally, then what I want is a situation in which – in addition to meeting my own needs – I export as much electricity as possible. – like Case#7.

Winter with a heat pump 

After installation of the external wall insulation, the house can now be heated 20 °C above the external temperature using less than 3 kW of heating power. Using a heat pump, this 3 kW of heating power can be supplied using just 1 kW of electrical energy.

Case#13. So after a heat pump has been installed, I can expect an additional roughly constant demand of between 0.5 kW and 1 kW on top of normal household use. So total demand will be between 1 kW and 1.5 kW.

Case#13: Click for a larger version

This is illustrated above:

• For an additional 0.5 kW of demand i.e. 1 kW in total, the daily cost and CO2 emissions would be would be (£4.60 and 4.66 kg)
• For an additional 1.0 kW of demand i.e. 1.5 kW in total, the daily cost and CO2 emissions would be would be (£6.90 and 6.99 kg)

Case#14. On a typical winter generation day (2 kWh), the solar panels will reduce this slightly:

• For an additional 0.5 kW of demand i.e. 1 kW in total, the daily cost and CO2 emissions would be would be (£4.10 and 4.26 kg)
• For an additional 1.0 kW of demand i.e. 1.5 kW in total, the daily cost and CO2 emissions would be would be (£6.40 and 6.59 kg)

Case#14: Click for a larger version

Case#15. In Winter in general a battery would be used to purchase slightly greener but much cheaper electricity at night.

For example in the situation below with an additional 0.5 kW of demand due to the heat pump i.e. 1 kW in total, the daily cost and CO2 emissions would be would be £1.40  and 4.00 kg – saving £3.30 with respect to the situation with solar panels but no battery.

Notice though that battery runs out of charge before the end of the day requiring the purchase of roughly 1.5 kWh of peak electricity.

Case#15: Click for a larger version

Case#16. Below I consider what I think is a “Reasonable Worst Case” corresponding to very cold winter day with prolonged temperatures around 0 °C.  There would be an additional 1 kW of demand due to the heat pump i.e. 1.5 kW in total, and even with the solar panels and the heat pump, the daily cost and CO2 emissions would be £3.70  and 6.33 kg.

Case#16: Click for a larger version

This comprises 24 kWh of demand from the heat pump which would deliver 72 kWh of heat. If that 72 kWh of heat had been provided by gas – as it is now – the daily cost and CO2 emissions would be £3.60  and 14.4 kg.

So in this reasonable worst case, the cost is similar to using gas – but there are big reductions in carbon emissions.

Can I ever reach zero emissions?

I think that ‘zero emission’ is possible in a certain sense, but it won’t be easy. And I may need to invest beyond what I have planned already. Let me explain:

• In the summer it should be possible to operate ‘off grid’ for several days or weeks at a time, and indeed – with the battery fully-charged – to export a fraction of the solar-generated electricity. I think it would be fair to consider this exported electricity as “CO2 emissions avoided” because it is displacing the use of CO2-emitting generation for someone else. Let’s call the amount of CO2 production avoided X kg.
• But in the winter it will be necessary to draw upon grid resources – and so CO2 will be emitted to generate that electricity. Let’s call the amount of CO2 production Y kg.

Click for a larger version: Illustration of how my come could become carbon neutral. Excess solar electricity exported in the summer avoids the emission of X kg carbon dioxide somewhere else on the UK grid. In winter when I need to draw electricity from the grid I will cause Y kg carbon dioxide to be emitted. If I can make X equal to Y then I think I claim that my home is carbon neutral.

To make the operation of this household ‘carbon neutral’ when averaged over one year, I need X to be equal to Y. Unfortunately at the moment I can only guess at X and Y in the most general terms – I hope to get the data I require this year. I am modestly confident that with a battery and a heat pump I can get the difference down to hundreds of kilograms rather than tonnes. But I am not sure I can get it down to zero.

But as the carbon intensity of UK electricity falls, year-upon-year, both X and Y will reduce and hopefully their difference will also get smaller.

Additionally, there is still space on my roof for more solar panels so perhaps in a year or two  I could increase X further. And of course we could just switch stuff off, turn down the thermostat and wear a pullover!

Summary

In writing this article – and the previous one – I have become convinced of the utility of domestic scale batteries and I hope to order one as soon as the last of the quotations comes in.

I am very excited by the prospect – External Wall Insulation AND a domestic battery. Truly I am living the dream!