What an interesting and intriguing subject! I understand that when gas powered turbines are used to generate electricity, the efficiency is ~40% which has always made me shudder! I’ll follow with great interest!  Regards, Toodles.

When a gas fired generating unit is operating in open cycle, its efficiency will probably be in the order of 35% to 36%. This is because only the generator being driven by the gas turbine is producing electricity. The exhaust gases from the gas turbine are at a temperature of 600C or more, but are expelled to the atmosphere via the chimney.

When operating in combined cycle, the exhaust gasses from the gas turbine are diverted to a Heat Recovery Steam Generator (HRSG) and used to produce high temperature, high pressure steam, with the exhaust gases from the HRSG going to the chimney at a much lower temperature.

The steam produce by the HRSG is then used to drive a steam turbine which is connected to a second generator, thereby producing an additional supply of electricity. The overall efficiency would now be in the order of 50%.

A typical Combined Cycle Gas Turbine (CCGT) unit may consist of a 300MW gas turbine driven generator and a 150MW steam turbine driven generator.

When starting from cold it does not take too long before the gas turbine driven generator is producing electricity, but it would be several hours before there would be sufficient steam for the steam driven generator to become operational.

Posted by: @transparent

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I have been asked a question by another community energy group who wish to evaluate the overall efficiency of

• delivering gas to a home which is heated using a (condensing) gas boiler
• using gas at a power-station to generate electricity. The electricity is then used in the home by an ASHP

 

That's a brilliant question, to which we really should know the answer

It is indeed, but it’s not quite comparing like with like.

In order to make a real comparison it would be necessary to look at the efficiency of all the sources of the ASHP’s consumed electricity. As I write this, the grid energy where I live is only 36% fossil fuels (all gas) and 14% renewables. Confusingly, it’s 48.6% imports so that would also have to be broken down to give an accurate picture. What is clear, though, is that if, for example, gas only supplies half the grid’s leccy and is only 50% efficient then that means the fossil fuel energy wastage equates to only a quarter of the energy supplied.

If you heat your home and water with gas, that’s a simple real world calculation. As far as I know there is no significant loss of gas between where it is pumped in and where the boiler takes it out so you're just calculating how well the boiler can convert energy in to heat energy out.

If you run a heat pump the electricity doesn’t just come from gas, it comes from a wide range of sources, each with its own efficiency signature for creating useable electricity. Solar PV is very inefficient in technical terms but far more so in environmental terms. Hydro-electric power is hugely efficient in all senses. Nonetheless, electricity from each of these sources still has to be transported to the home and there are signficant losses there. All this adds up to a much more complex picture than "is it better to burn gas at home or burn it to generate electricity that's then used at home".

I'm not suggesting we should avoid the discussion just because the initial question was a bit simplistic, though. I would love to know the answer. However, I'd probably prefer any discussions to centre around environmental damage rather than an ambiguous term like efficiency. After all, a wind turbine might only be 30% efficient but any wastage is irrelevant environmentally.

‘Natural’ gas energy content is usually given at the Lower Heating Value (LHV).  This value is not the total chemical energy, but assumes all water created when the gas burns is left as a vapour.  There is also a Higher Heating Value (HHV), which assumes all water created is cooled to be a liquid - the difference between these values is about 11%.  Gas boilers use the  LHV in their marketing unsurprisingly, and so can sometimes produce what they describe as efficiencies of over 100%!  Strictly speaking, the HHV only should be used for efficiency calculations.  I don’t know which value the electricity industry uses.

Agree with @majordennisbloodnok that considering 100% electricity from gas is a bit unfair.
And I recognize the gas power plant efficiencies from @derek.

If you're looking for efficiencies, I believe the grid losses are minimal both for gas and electricity. So I would ignore them probably.

If you're looking for the CO2 emissions, here would be my calculation:

Gas boiler:
Annual CO2e emissions = Gas consumption x CO2 emission intensity = 12,000 kWh x 183 gCO2e / kWh of gas = 2.2 tCO2e / year

ASHP:
Electricity consumption = Gas consumption x boiler efficiency / COP = 12,000 kWh x 83% / 300% = 3,320 kWhe
Annual CO2e emissions = electricity consumption x average emission intensity of the grid = 3,320 kWhe x 140 gCO2e / kWhe = 0.46 tCO2e / year

Note that I used the average emission intensity of the grid as opposed to electricity 100% from gas. 
 

Posted by: @hcas

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Agree with @majordennisbloodnok that considering 100% electricity from gas is a bit unfair.
And I recognize the gas power plant efficiencies from @derek.

If you're looking for efficiencies, I believe the grid losses are minimal both for gas and electricity. So I would ignore them probably.

I don't think we're going to assume the scenario of 100% electricity being derived from gas turbine generation.

I have the (hourly) figures for the energy-mix being supplied to different areas of Britain, so we can create a reasonable set of statistics per location and per time-period.
It depends what level of detail the original questioner wants.

There isn't going to be a single figure we can use now that electricity generation is so widespread and variable.

I'm going to be sent data on gas transmission losses next week.
The relevant engineer isn't around this week.

The losses on the electricity grid (10%) will be much higher than with gas.
Those losses are mostly at the local substations, and have doubled over the last 20-years.

However, on the plus side, it's something which we could address as end-users if we knew enough about it.
That alone is a good enough reason for pooling this information here on the forum.

@transparent Agree, depends how granular they want to go. The emission factor I used above was average UK for 2023.

Hmm interesting on the grid losses. I always heard the losses were lower to mid single digits. But perhaps that's the transmission part only?

What's the source of your 2023 CO2 emissions @hcas ?

Unfortunately the most recent edition of the Digest of UK Energy Statistics (DUKES) is still 2022.
It will be another 7 weeks before they'll publish the 2023 figures.

... and that's assuming they're on-time.
I believe that the Office of National Statistics has lost 20% of its workforce (958 people) during 2023.

I dunno how they work that out 🤔 

@transparent Here you go

Box 3.6 page 16

Posted by: @transparent

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Posted by: @hcas

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Agree with @majordennisbloodnok that considering 100% electricity from gas is a bit unfair.
And I recognize the gas power plant efficiencies from @derek.

If you're looking for efficiencies, I believe the grid losses are minimal both for gas and electricity. So I would ignore them probably.

I don't think we're going to assume the scenario of 100% electricity being derived from gas turbine generation.

I have the (hourly) figures for the energy-mix being supplied to different areas of Britain, so we can create a reasonable set of statistics per location and per time-period.
It depends what level of detail the original questioner wants.

There isn't going to be a single figure we can use now that electricity generation is so widespread and variable.

 

I'm going to be sent data on gas transmission losses next week.
The relevant engineer isn't around this week.

The losses on the electricity grid (10%) will be much higher than with gas.
Those losses are mostly at the local substations, and have doubled over the last 20-years.

However, on the plus side, it's something which we could address as end-users if we knew enough about it.
That alone is a good enough reason for pooling this information here on the forum.

 

Perfect. You’ve outlined exactly the approach I’d want to see. It might not be perfect but it’ll be a damned good approximation.

@hcas

What you also need to consider is that unless there is a vast increase in renewable energy production and storage capacity, that if most households switch from gas central heating to heat pumps, there would need to be many more gas or coal fired power stations operational to meet demand.

That is without considering the impact of switching to EV's.

The changeover from fossil fuels to renewable energy is going to be a long, expensive and disruptive process.

Reducing overall energy consumption should be one of the highest priorities that people should endeavour to achieve.