Studies Say Wind+Gas reduces CO2

TL:DR; Probably in England 2014, not so much in the U.S. 2024

First off, I’ve come up with a great way to illustrate the issue of Wind and CO2 emissions. We have had something called Pumped Hydro for decades. Overnight when power demands are low, they pump water up to a lake on top of a mountain. During peak usage, they run that water down to the lower lake to generate electricity. It’s a closed system.

When the water is running down through the generators during the day, that’s zero CO2 generation. So can you call that clean energy? Well, it depends. Because if the water is pumped up at night via power from a coal fired plant then no. All that generation actually has the CO2 output from the coal plants as it’s source.

Same issue for the backup power for Wind. It needs to include the CO2 generated by the backup power for when the Wind is not blowing.

And with that, on to the two reports Will Toor touted as addressing the CO2 emissions of Wind’s backup. The easiest first.

Michigan Study

Wind Power And Air Quality: Reducing Air Pollution And Carbon Emissions In Michigan

This study is worthless. It cites 12 studies that find that wind emits no CO2 while generating electricity. Well no shit Sherlock. What none of them speak to¹ is the trade-off between Wind+SCGT vs CCGT.

The one place this study does speak to the backup issue they say:

Katzenstein and Apt noted that natural gas turbine fuel consumption (and NOx emissions) could increase, depending on the type of turbine and on how the turbines are ramped up and down, when the gas turbines are quickly ramped up and down to balance the fluctuations of wind energy.

They then go on to use the Environmental Protection Agency’s (EPA) Green Power Partnership Green Power Equivalency Calculator. What does this calculator do? It tells you for a given GWh/year, how many homes it will power and how many wind turbines or solar panels will generate that. Not a word about backup power.

And again, they discuss the CO2 savings when the wind blows, but not the backup

Their approach accounted for not only the marginal unit of generation displaced by wind, but also the marginal emissions from the displaced generation.

In my opinion this study is a puff piece where using wind (or solar) has no impact on the CO2 difference of its backup. The one paragraph where they do speak to the issue of the backup, they acknowledge that the wind+backup will increase CO2.

Great Britain Study

Marginal greenhouse gas emissions displacement of wind power in Great Britain

Ok, here we have someone who dove in and, in my opinion, did a good job of trying to model the CO2 savings of Wind+Backup vs. Coal or Gas.

The authors developed a novel methodology to isolate the marginal emissions displacement of wind power using historical empirical data from Great Britain between 2009 and 2014. This approach takes into account the impact on the operating efficiency of coal and combined cycle gas turbine (CCGT) plants, providing a more accurate assessment of wind power's effect on GHG emissions.

What I really like about this is truly modeling what power source is used when the wind drops is a Sisyphean task. There is no one to one relationship on this. A balancing authority sees demand exceed generation and spins up a generator somewhere. An hour later when it happens again, they could spin up a totally different type of generator. Kudos to them for trying² to model this.

Arguments in favor of this study

• Published in Energy Policy, a reputable journal in the field.

• The study analyzed data over a five-year period, providing a robust dataset for their conclusions.

Limitations

• It’s for Great Britain 2009 - 2014 when wind was generating ~ 10% of their electricity.³

• In Great Britain, at that time, their backup is coal and CCGT⁴. In the U.S. peaker plants are generally SCGT.

They do acknowledge the issue we have in the U.S.

A particular challenge in determining the MDF⁵ of wind power is that operating fossil-fueled generation at part-load has an efficiency penalty that increases the fuel consumption and GHG⁶ emissions per unit of energy generated; analysis of the MEF⁷ of demand fluctuations in the USA found efficiency penalties have a significant impact on emissions reductions (Siler-Evans et al., 2012).

They note an assumption about startup/shutdown. Based on what I’ve read, especially about coal plant startup/shutdown, this is something I hope they investigate as this can be significant⁸ ⁹.

Note that the analysis does not capture the GHG emissions of generator start-up and shut-down. Instead it assumes these can be approximated by extrapolating the efficiency curves below ‘normal’ minimum stable generation levels (shown by dotted line). Start-up and shut-down are complex processes that have different fuel consumption characteristics from part-load operation; however, it is expected that these emissions do not currently contribute significantly to the marginal displacement of wind power. Further work is required to confirm this assumption.

They conclude with the following. As I read this, and I may be wrong, they find wind has a larger impact when CCGT is primarily in use because the Wind replaces coal also being brought online. And when it’s primarily coal the wind is replacing CCGT and it has a smaller impact

When CCGT is being operated in preference to coal (the situation from 2009 to 2011), the average emissions of the system are low and wind will also be displacing the most carbon-intensive generation, reducing these emissions even further. In contrast, when coal is being operated in preference to CCGT (as in 2012–2014) average emissions tend to be higher, and wind is less effective at emissions reduction because it is displacing less carbon-intensive generation.

Unfortunately coal played such a large part in Great Britain, and in their study, it’s difficult to pull out how wind+CCGT vs. CCGT plays out. I think they struggle with that in their study. That’s the nature of the complexity of the grid. I wish they had been able to pull clearer conclusions out of this. More than “less effective at emissions reduction.”

I think that if we could spin up CCGTs quickly, to full efficiency, then Wind+CCGT would probably emit less CO2 than just CCGT. It would be more expensive, but it would reduce CO2.

Unfortunately for our grid, they have found that CCGT is not a good solution for peak use as they take time to get running at full efficiency. So for very good technical reasons, the backup here is SCGT.

So the above study is good news for Great Britain at 10% wind and way too much coal (at that time). But I think it’s of marginal relevance to the U.S. in a BA with 30% wind that uses SCGT for most peak needs.

I do think the paper is very well researched and they did a great job trying to measure an incredibly complicated system. Kudos to them.

Future Research

I think it would be great if someone could perform similar research for a balancing authority in the U.S. Preferably the PSCO (Colorado) but really any BA where SCGT is the primary backup power source.

If they do that, a couple of suggestions:

1. Measure the hit of starting/stopping the backup plants

2. Measure the hit of running the CCGT and SCGT plants at partial load.¹⁰

3. Measure this for multiple BAs where they have a spread of wind generation from 20% - 50%.

4. Give us specific numbers for savings. It can be a range. But there’s a big difference in paying for 1 GW of wind that saves 1.5, 15, or 150 tonnes/hour.

This is incredibly hard to measure. But it’s important to get a better measure to determine if wind+backup is better than a CCGT. And if so, how much better.

Wind Turbine with Gas Turbine backup

1. I did not read all 12 reports in full. But in skimming them, they are studies of the advantage when the wind is blowing.

2. I say “trying” because I don’t think this can be done perfectly. I think these researchers did a very good job.

3. Balancing a change in 10% of generation is a lot easier than doing so for 30% or in the future 70%.

4. CCGT instead of SCGT may work ok for 10% wind.

5. Marginal Displacement Factor

6. GreenHouse Gas

7. Marginal Emission Factor

8. Emissions Catalyst Issues for Fast-Start Combined Cycle Power Plants

9. Analysis of the impact of Combined Cycle in the energy transition

10. For some generators, when they run at less than 100%, while their total gas/coal consumption and CO2 emissions are less in total, they are less efficient and spew a higher percentage of CO2.