How much are emissions reduced by renewables?

A new study has got some attention this week, claiming that shale gas had reduced emissions less than people had thought, and that renewables played a bigger role than perceived. The claims in the study can be summarized in the graph below.

Essentially natural gas has reduced emissions only 22%. However, their numbers, and assumptions, don’t stack up very well. Let’s consider why.

Here is their basic argument:

Coal’s market share declined from 50% in 2008 to a multi-decade low of 33% in April of last year. Natural gas was the biggest winner, growing from 20% of power generation in 2008 to more than 30% during 2012. But renewables played an important role as well. In 2005, renewables accounted for 7.9% of US power generation. During the first ten months of 2012, they accounted for 11.4%. And while one kWh of natural gas-fired generation emits roughly half as much CO2 as a kWh of coal-fired power, renewables emit no CO2 so each kWh of additional generation delivers a bigger emission reduction punch.

So, we have a bunch of things changing in the last few years. Their baseline year however is 2005, so comparing coal and gas in 2008 with coal and gas in 2012, and then suddenly compare renewables in 2005 with renewables in 2012 does not make a great deal of sense. Let’s make a meaningful comparison.

The actual percentages of electricity from each source can be found here.

• Coal goes from 49.6% to 37.4% of total production between 2005 and 2012. A 12.2% decline
• Gas goes from 18.8% to 30.3%. A 11.6% increase.
• Non-hydro renewables go from 2.2% to 5.4%. A 3.2% increase.

So, how much CO2 did gas and wind and solar save? This is a counter factual, so we need to ask what would have been burned instead of gas and renewables. Let’s assume, as they do, that it would have been coal. As the authors of the report say gas emits roughly half as much CO2 as coal, while renewables emit close to zero. The percentage of electricity from gas has gone up 11.6%, while non-hydro renewables have gone up by 3.2%. Some basic arithmetic would indicate that gas has thus saved 80% more CO2 than wind and solar, and not the 22% that the study claims.

So, they appear to be a reasonable bit out, and possibly even more. As Alex Trembeth pointed out in a Twitter conversation assuming that wind or solar are replacing nothing coal is a very questionable assumption, suggesting that wind and solar are mostly just displacing gas.

Let’s consider why this is the case. A simplified explanation of how an electricity grid works is this. Electricity demand varies throughout the day and year. There are basically enough power plants lying around to meet the maximum annual demand, which in most countries is around 6 or 7 pm on a cold winter night. The actual electricity mix at any particular time is driven by how cheap each source of electricity is. Take yesterday in Germany. Wind power was close to zero all day, so in essence it was almost an entirely non-renewable system.

This is total demand:

Running costs of nuclear power plants are very low and Germany, as most countries do, just runs them at 100% all of the time:

Lignite is very similar to nuclear, though coal output is a bit lower at night:

Gas however increases massively during the day.

In essence, when demand is high you turn on the gas plants. And when demand goes down you turn the gas plants off. Fuel costs are far more important in gas plants, so they prefer the price of electricity demand to be higher before firing them up, which happens when demand rises.

An alternative way of looking at wind and solar are not as electricity providers, but as demand reducers for non-renewable power sources. So, we can view a particularly windy day as one with lower demand. In Germany, gas will be displaced first, and then coal. However, if there is enough wind or solar on the grid it will displace both. Let’s consider the 25th May 2012, when German solar power hit a record high. This is solar that day:

As expected this results in gas not being ramped up during the day, as it was in earlier days that week:

22 GW of solar however is more power than Germany’s entire gas fleet can supply, so as expected the solar is also eating in to coal power:

Lignite and uranium plants, however, aren’t really impacted by solar:

So, I hope I have convinced you that saying that wind power is replacing coal 1-1 is rather simplistic, and probably deeply misleading.

What should we assume wind is displacing in America? At this stage we should move from considering Germany to considering the UK. The UK’s coal and gas mix is much more similar to the US’s, and it’s renewable energy is also more similar. Does wind displace coal or gas in the UK? It appears that it’s more or less entirely a 1 for 1 replacement of wind for gas. Chris Goodall and Mark Lynas ran the numbers here, concluding that:

“the exchange rate is about one for one: a megawatt hour of wind typically meant the UK grid used one less megawatt hour of gas-derived electricity.”

So, in the UK each additional kWh of gas and wind currently reduce carbon emissions by a very similar amount.

Back to this post’s question: how much CO2 is saved per kWh of wind and solar? At this point I will disappoint the reader by saying I don’t have a simple answer to the question. The key thing to recognise here is that this is not a simple equation. It is dependent on a wide range of factors: how much renewable energy is on the grid, levels of coal and gas, relative costs of coal and gas, and so on. And as a recent paper shows this equation is not the same in each region of the United States (h/t Costas Samaras). But one thing is clear: an assumption that wind and solar only displace coal is divorced from the basic realities of electricity generation.

Based on what I know, I suspect wind and solar are largely only displacing gas in America, but the question deserves a little more attention. The question could be answer quite easily with hourly electricity generation data by fuel type for all of America, which I cannot seem to find. If someone knows if this exists, please let me know and I can try to put a harder number to the emissions saved by renewables.