Hmm another thing that could be done is to split Rejected Energy Incurred into light emissions and heavy emissions.
The light emissions would be the portion of energy consumed with after netting out heavy emission.. The heavy emissions would be the portion that emits 200 million tonnes (0.2Gt) per Quad. This is two times the equivalent of simply burning coal at 220 pounds (100 kg) per mmBtu. It also make it east to know how many Gt of carbon the system is emitting just by multiplying Quads of Heavy Emissions by 5.
So for example natural gas emits 117 kg/mmBtu. So 1 Quad of NG is 0.266 Quads Heavy Emissions (=117/440) and 0.734 Quads Light Emissions. Now if you use that gas to generate power at say 40% efficiency, then let's net that out of the Zero emission part. Thus:
1.000 Quads of Natural Gas (40% efficiency)
=
0.400 Quads of Electricity Generated
+ 0.266 Quads of Rejected Energy Heavy Emissions
+ 0.334 Quads of Rejected Energy Light Emissions
+ 0.000 Quads of Rejected Energy Avoided
Compare that with coal assuming 35% efficiency.
1.000 Quads of Coal (35% efficiency)
=
0.350 Quads of Electricity Generated
+ 0.500 Quads of Rejected Energy Heavy Emissions
+ 0.150 Quads of Rejected Energy Light Emissions
+ 0.000 Quads of Rejected Energy Avoided
Note if we could generate coal at a higher efficiency, it would have the same heavy emissions but less light emission.
Now suppose we want to compare that with Solar with say 10% curtailment displacing 36% efficient fossil generation
1.000 Quads of Solar (36% efficiency, 10% curtailment)
=
0.360 Quads of Electricity Generated
+ 0.000 Quads of Rejected Energy Heavy Emissions
+ 0.040 Quads of Rejected Energy Light Emissions
+ 0.600 Quads of Rejected Energy Avoided
If we could reduce the curtailment, we'd have less light emissions and more Rejected Energy Avoided. Curtailment usually involves cutting renewable energy while must-run generation capacity continues to put surplus power into the grid. So the gird is still generating some carbon emissions even though there was surplus renewable power available. We need a lot more storage and non-grid offtake markets to avoid light emissions.
So if we aggregate a mix of energy in to such bucks, we get a clearer picture of how the system is doing along key attributes.
Electricity Generated, this is good, we want plenty of this.
Rejected Energy Heavy Emissions, this is very bad, we want to reduce this.
Rejected Energy Light Emissions, this is not ideal, but better than Heavy emissions, we want this to be small.
Rejected Energy Avoided, this is positive, we want more renewable energy displacing fossils.
Decarbonization consists in reducing Heavy Emissions to zero or net negative if negative emissions power generation becomes a thing.
So this is a rough idea of how to do analysis that maps both balance of energy and emissions. I don't like defining heavy emissions as 0.2 Gt/Quad. That constant is not so nice. At first I thought 0.1 Gt/Quad as this matches the combustion of coal. However, when you attempt to attribute emissions just to the rejected energy portion this leads to negative values for coal's rejected light emissions. I don't see a good way to interpret negative zero emissions or represent it in this sort of infographic. So I doubled it. But what do we do with metallurgical coal were coal is being used for industrial heat. That may well have high emissions that exceed the total amount of rejected energy. So the problem occurs because we want to think of the useful energy as clean and allocate all emissions to the rejected energy. Another possibility is to define heavy emissions at such a high concentration, e.g., 1Gt/Quad, that it avoid these netting out problems. But that send the odd message that 1 Quad coal = 0.35Quad Electricity + 0.10Quad Heavy Emissions + 0.55Quad Light Emissions. Yikes, that actually make coal look pretty clean! So I compromised and set heavy emissions at 0.2Gt/Quad. But maybe there is a more elegant why to frame this.
Of course you can make a flow chart for just the carbon, separate from the energy flow. LLNL has done that, but the result does not really give you a sense of efficiency, how it compares with the value that energy creates. It also does not illustrate how electrification can assist in decarbonization. It tells a very limited story.
Carbon Flow Charts
