This document discusses the challenges with using simple carbon intensity estimates of electricity and the need for more realistic numbers. It notes that emissions can vary significantly based on generation technology, operational characteristics like part-load operation and starting/stopping, and changing electricity systems with more renewables. Estimating past carbon intensity is more straightforward while future estimates are more difficult due to uncertainty over fuel mix, technologies, and plant operation. Better understanding the impacts of intermittent renewables on thermal plant operation is needed to develop improved carbon intensity estimates.

1. Carbon intensity of electricity:
towards more realistic numbers
Professor Gareth Harrison
Institute for Energy Systems
University of Edinburgh

2. Institute for Energy Systems
Our research spans and maps to the renewable energy supply chain
Resources Capture Conversion Delivery
Energy and Marine Machines and Power
Climate Energy Electronics Systems
Innovation Policy and Regulation Standards Environment

3. What are we trying to do?
• Gain an understanding of the carbon chain now and
in the future
• Estimate the carbon impact of intervention strategies
• Reward technologies that reduce carbon
• All of these require credible estimates of the carbon
intensity of electricity

4. How do we calculate carbon
intensity?
• The carbon intensity of electricity produced by a
given generation technology is given by
Emissions factor = fuel use / generation
• The DECC emissions factors are based on total fuel
use and total energy production per fuel type
• simple cross-industry measure
• reflect differences between fuels
• widely exploited for estimating carbon footprints, impacts
and payback

5. How good are these simple
estimates?
• Several areas where these values fall down
• Consumption patterns and production are not flat profiles
• Emissions vary within same fuel type
• System and generator operation ultimately dictates intensity

6. Time variability?
60000
50000
wind
Generation (MW)
40000 ps
30000 ocgt
coal
20000
ccgt
10000 intfr
0
nuclear
1 20 39 58 77 96 115 134 153 172 191 210 229 248 267 286 305 324
-10000
Period
Simple analysis: First week in Nov 2008, BMreports aggregated production.

7. Time variability?
700
Carbon Intensity (gCO 2/kWh)
600
2008:
500 Coal 903
400
Oil 730
Gas 404
300
This week:
200
Ave 551
100
0
1 20 39 58 77 96 115 134 153 172 191 210 229 248 267 286 305 324
Period
Simple analysis: First week in Nov 2008, BMreports aggregated production and 2008
DUKES emission factors; www.realtimecarbon.com, Ecotricity and GridCarbon do similar

8. Technology Considerations
• The generator technology has a very large impact on
emissions
• Gas peaking plant (OCGT) much less efficient
• Older power stations tend to be less efficient
Coal 903 Gas 404 Oil 730

9. Technology Considerations
• Generator operation has a big impact on its emissions
• Traditional steam and CCGT stations were designed
to operate at highest efficiency at full output
• Operation at part-load, starting and stopping all
result in additional emissions
• Part-loading may raise emissions by as much as 10%
Relative station efficiency

10. Technology Considerations
• Starting and stopping requires extra fuel to be used
to maintain at or return generator to operating
temperatures

11. Technology Considerations
• Starting and stopping requires
extra fuel to be used to
maintain at or return generator
to operating temperatures
• The frequency of cycle and Operation
duration of downtime are both
important
• Intensity during warm up:
• CCGT ~800gCO2/kWh
• Coal ~1200-1400gCO2/kWh
Emissions

12. Towards better estimates?
• Highly complex problem
• Emissions are largely market-driven and increasingly
reflect non-dispatchable renewable generation
• Simple emissions factors not sufficient to inform important
choices
• Estimating historic carbon intensity conceptually simpler
• We can measure production and fuel burn
• The small extent of changes (wind etc) means simple marginal
assumptions (dCO2/dP) can be credible
• We still need to reflect operational characteristics in estimates

13. Towards better estimates?
• Estimating future intensity is much harder…
• Substantial uncertainty over fuel types, technology
and operational characteristics
• Large injections of wind will see more flexible use of
thermal plant: more cycling and greater reserve
holding
• Lower emissions due to energy substitution but more
emissions per kWh of thermal generation
• We need to fully understand this…