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Some facts I have learnt

The Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of different gases. Specifically, it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 [US] ton of carbon dioxide (CO2).

[...]

Nitrous Oxide (N2O) has a GWP 265–298 times that of CO2 for a 100-year timescale.
— United States Environment Protection Agency

According to the UK Government's Department for Environment, Food & Rural Affairs (DEFRA, 2021):

Table of greenhouse gasses released into the atmosphere per 1 kilowatt-hour of fuel used

With the average UK annual household gas usage being 12,000 kWh (UK Power, n.d.)

A heat pump that has a Coefficient of Performance (CoP) of three can create three kWh of heat from every one kWh of electricity. Therefore, compared to average gas usage, a heat pump with a CoP of three would use 4,000 kWh of electricity annually (Viessmann, n.d.)

UK average figures table for household greenhouse gas emissions

The UK Government is looking for everyone to install Heat Pumps for home heating and will be banning the installation of gas combi boilers.

But, from my calculations, heat pumps could provide little benefit to the environment in comparison to gas central heating. Where is the major benefit I am missing?

References

DEFRA (2021) UK Government GHG Conversion Factors for Company Reporting https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2021

UK Power (n.d.) Average gas and electric usage for UK households https://www.ukpower.co.uk/home_energy/average-household-gas-and-electricity-usage

Veissmann (n.d.) Do heat pumps use a lot of electricity? https://www.viessmann.co.uk/heating-advice/Do-heat-pumps-use-a-lot-of-electricity

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  • $\begingroup$ If the Government is promoting heat pumps as a form of heating for the future, it can be regarded as an admission that global warming is real. Heat pumps simply move heat from one side of a divide to the other side. Basically they transfer heat from outside a building to inside. This only works if there is heat to transfer and it is why heat pumps will not be used in cold environments such as Antarctica. If heat pumps are being recommended for the UK then it is an admission winters in the UK are getting warm enough for heat pumps to provide heat, instead of burning gas, coal or wood. $\endgroup$
    – Fred
    Oct 20, 2021 at 17:52
  • $\begingroup$ Interesting thought @Fred $\endgroup$
    – Chris Rogers
    Oct 20, 2021 at 17:57
  • $\begingroup$ @Fred heat pumps are also getting better: nrel.gov/news/features/2021/… $\endgroup$
    – LShaver
    Oct 20, 2021 at 20:48
  • $\begingroup$ @LShaver: Interesting! Thanks $\endgroup$
    – Fred
    Oct 20, 2021 at 20:50
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    $\begingroup$ While heat pumps with current energy generation mixes (e.g. mostly burning fossil fuels, with some nuclear and some green) already have an advantage, this will increase when energy generation is fossil fuel free. The UK government has pledged to phase out fossil fuel electricity generation on the same timescales as the introduction of heat pumps. $\endgroup$
    – Andy M
    Oct 21, 2021 at 8:52

1 Answer 1

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The values on the DEFRA table with the column heading $ kg\ \ce{N2O} $ already include the global warming potential (GWP) factor of 298, as indicated by a note on the column heading (looking at electricity):

Note on "kg N2O" reads "kg CO2e of N2O per unit

This is pretty confusing and took me a while to track down, but I figured it out when I noticed that the value in the first column is simply the sum of the values in the next three columns, which only makes sense once you understand that those three values are already converted to equivalent $\ce{CO2}$ emissions (or $\ce{CO2e}$).

There are a couple other minor issues with your calculation:

  • GWP is a factor per unit mass, so there's no need to convert to US tons.
  • The 12,000 kWh figure is for gas usage, or what's recorded at the meter. Since no furnace is 100% efficient, this means that the heating load (how much heat is actually added to the home) will be less. For years, natural gas furnaces had an efficiency of 80%, but newer models can reach closer to 98%. Let's assume it's 90% on average. This means the actual heating load is $12\,000 \times 0.9 = 10\,800\ \text{kWh}$, which makes the electricity needed to provide the same amount of heat $10\,800 \div 3 = 3\,600\ \text{kWh}$.

Here's how the final table should look:

Heating system Heating fuel (kWh) $ kg\ \ce{CO2e}$ per kWh Total $ kg\ \ce{CO2e}$
Natural gas furnace (90%) $12\,000$ $0.18316$ $1\,978$
Heat pump (COP 3) $3\,600$ $0.21233$ $764$

Finally, while the equivalent emissions factor for electricity is currently higher than that for natural gas, it's trending down, as indicated by this note in the spreadsheet:

In the 2019 GHG Conversion Factors, there was a 10% decrease in the UK Electricity CO2e factor compared to the previous year. In the 2020 update, the CO2e factor decreased (compared with 2019) again by 9%. In the 2021 update, the CO2e factor has again decreased by 9% (in comparison to the 2020 update). The above decreases are all due to a decrease in coal use in electricity generation and an increase in renewable generation.

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  • $\begingroup$ "The 12,000 kWh figure is for gas usage, so you need to convert to the actual heating load before calculating the electric usage." - So Viessmann have given bad information then? $\endgroup$
    – Chris Rogers
    Oct 20, 2021 at 7:21
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    $\begingroup$ @ChrisRogers it's an estimate and I didn't look into the accuracy, but they described it correctly as "gas usage." But 12,000 kWh of gas used (or gas recorded at the meter) doesn't translate to 12,000 kWh of heat energy, because no furnace is 100% efficient. $\endgroup$
    – LShaver
    Oct 20, 2021 at 13:58
  • $\begingroup$ As the proportion of renewable energy grows the emissions will go down when using a heat pump. With zero emissions electricity any electric heating can go all the way to zero with it but less clean energy needed with heat pump. With gas burning heating must always make emissions. $\endgroup$
    – Ken Fabian
    Oct 21, 2021 at 10:41

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