Carbon Intensity of the UK Electricity Grid

The window over the kitchen sink faces due east and the top of the 199 m-high chimney at Ratcliffe-on-Soar power station is just visible over a hill, nearly 7 miles away. It makes for an interesting contrast to the wind turbines visible from windows facing north.

Ratcliffe-on-Soar Power Station Chimney from Kitchen Window

I quite like being able to see the power station and it only takes a glance to check whether it’s running or not. Over the summer it was rare to see it operating at all. Now autumn is here I’ve noticed it seems to be running most of the time – though not at full capacity, judging from the modest amount of steam from the cooling towers. Today’s news reports help to explain why: Polluting UK coal plants export power to France as cold weather bites. Personally I’d prefer a less sensationalist headline since the French Interconnector has a maximum capacity of 2 GW compared to a total UK consumption figure typically in the 30 – 50 GW range. The underlying data shows this interconnector has indeed been exporting at full capacity to France recently whereas normally it’s importing nuclear-generated French electricity to the UK.

Ratcliffe-on-Soar has quite a lot of pollution mitigation measures installed and hence isn’t too bad for SOx and NOx emissions, but coal is still fundamentally a high-carbon fuel – significantly worse than gas. Why is this important? Because Marsh Flatts Farm is all-electric for cooking and heating – and the addition of an electric car will further increase the reliance on electricity. It’s all very well having zero carbon emissions at the point of use but if those translate to high carbon emissions at the power station that’s clearly no good overall.

When specifying the heating system I initially had some misgivings about “wasting” electricity by using it for heating. Initially it seemed better to preserve electricity for things which absolutely demand it and to provide heat by burning wood or gas. In particular, why burn gas at a power station and suffer the unavoidable losses from turning that into electricity and transmitting it over a long distance when you can just burn gas directly? Well: a) there’s no mains gas available at Marsh Flatts Farm, and b) with a good heat pump it’s easy to get much lower carbon emissions than with a gas boiler – especially now that the carbon intensity of the UK electricity grid has reduced so much.

A good (91% efficient) gas boiler produces 240 g of CO2 per kWh (ref. this CIBSE Journal article) but that’s not far off the typical overnight figure for UK Grid Carbon Intensity (ref. the Carbon Intensity API website). A good Ground Source Heat Pump will deliver more than 4 times as much heat as it consumes in electricity, which equates to roughly 60 g of CO2 per kWh. (I used the overnight figure for the grid since a GSHP will typically do most of its heating overnight – when it’s colder outside – than in the daytime.)

As evidence of “..more than 4 times as much heat as it consumes in electricity”, in the past 24 hours my NIBE F1145 Ground Source Heat Pump consumed 8.53 kWh of electricity and generated 36 kWh of heating water at a nominal 30 degrees C. That’s a coefficient of performance of 4.22.

Electric Vehicle Charging

I predict an electric car in my future – I had a brief test drive in a BMW i3 the other day and was very impressed. Got a longer test drive scheduled in a couple of weeks and I’ll see how the numbers stack up after that.

Electric car = electric car charging which warrants some research. While it’s perfectly possible to connect a low-current charger (as supplied with the car) to one of the external 13A sockets and leave it at that there are some other / better options to consider:

  • A dedicated EVSE (Electric Vehicle Supply Equipment) “charger”
    • The actual “battery charger” is part of the car and the various alternative solutions are just different ways of supplying electricity to the car so it can decide how to proceed
  • Some means of dynamically scheduling the charging, to either take advantage of a cheaper electricity tariff or spare solar generation capacity

Initially I found the terminology around the many options for EV charging quite confusing (Type versus Level versus Mode etc.) but I think I’ve fought my way through that now:

  • Type refers to the style of socket on the car
    • The BMW i3 models made for the European market are fitted with an IEC 62196 “Type 2” (aka Mennekes) socket
  • Level refers to the nature of the electricity supply
    • Level 1 is 120V AC (mostly of relevance in the Americas)
    • Level 2 is 240V AV (standard in Europe)
    • Level 3 is 480V DC (not relevant for domestic charging)
  • Mode refers to the nature of the communications between the car and the EVSE – basically:
    • The simple EVSEs typically supplied with EVs and fitted with a standard 13A plug are Mode 2 and operate at a maximum of 10A
    • The hard-wired EVSEs are Mode 3 and communicate with the car using a protocol defined as part of the SAE J1772 standard
      • The most interesting part of this is that the car decides what current to draw but it does that based on information advertised by the EVSE
      • Mostly this is used to protect the weakest part of the electrical circuit from overload (typically it’s the cable) but it can also be used to “ask” the car to charge at a lower rate than it might otherwise try to achieve

There are various commercial options for EVSEs – e.g. BMW offer a Wallbox (in BMW colours) – but I’m currently favouring the Open Source OpenEV which is available from the OpenEnergyMonitor folks (and integrates with their other solutions – or other Open Source solutions).

To be able to charge at 7.4 kW (32A @ 230V AC) requires a dedicated circuit with some fairly chunky wiring direct from the main consumer unit (or in my case from a new consumer unit since the two primary consumer units are already “full”). Fortunately I predicted the need for some sort of external connection and there’s already some ducting installed to take it. It will also warrant an extra sub-meter to monitor how much electricity is being used to charge the car.