Wind Energy FAQs: System-average GHG emissions

In this post we look at the GHG emissions of the entire US power generation system.

Following a 13-year period which has seen the rapid decline of coal and the equally rapid uptake of ‘natural’ gas, wind and solar; system average emissions of US power generation have sharply declined. At the end of 2019, GHG emissions reached a watershed moment which is likely to have a profound impact on public perceptions about the environmental impact of gas in an increasingly carbon-constrained world.

In December 2019, system average emissions reached an all-time low of 417.6 kilograms per megawatt hour of generation (kg/MWh). This level was, for the first time ever, lower than the emissions of the US average gas-fired power generation fleet (421.9 kg/MWh).

For the detail, read on..

Navajo Generating Station. Photo: Myrabella / Wikimedia Commons

GHG Calculation Methodology

US electricity is generated from three main fuels: Hydrocarbons, Nuclear and Renewables. GHG emissions during operations only arise from the combustion of hydrocarbons and this is what is of interest in calculating GHGs/unit of generation.

Power generation using hydrocarbons is undertaken in three sectors:

  • Electric Power Sector – which consists of those entities whose primary business is the generation of electricity
  • Industrial Sector – accounts for 30%+ of US energy consumption and includes heavy industry such as refineries, plane & car manufacturing, mining, paper mills, construction, food processing, LNG plants etc. Many entities within this sector have their own on-site generation facilities.
  • Commercial Sector – includes offices, hospitals, schools, warehouses, shopping malls, hotels etc. Entities within this sector often have their own on-site generation facilities which may be run to avoid peak demand charges or because of faults on the distribution system.

The Energy Information Administration (EIA) reports annual carbon emissions – by fuel type – from the entire electricity generation industry but usually with a 12-18 month lag. However, the EIA also reports GHG emissions by fuel type, for the Electric Power Sector only, on a monthly basis with a 2-3 month lag. Using EIA data on electricity generation by fuel type across the Electric Power, Industrial and Commercial sectors; we estimated the GHG emissions from the Industrial and Commercial sectors using actual EIA data from the Electric Power Sector.

The accuracy of this estimate is relatively easy to check by comparing our estimates of system-wide emissions with the lagged EIA data for the electric power system. That comparison, over the last 18 years, shows an error which is consistently less than 0.5%. For this reason, we have assumed that our model is sufficiently accurate for the purposes of the conclusions we discuss on this page.

System Average GHGs

Once we have the total GHG emissions it is simply a matter of dividing that number by the total electricity generation to arrive at the emissions per unit of power generation.

Shown to the right is a graph of the system-average power sector emissions (the black line), and average gas-fired power station emissions (orange line), since 2002. It shows that around 2007, which is when gas-fired power generation started to massively displace coal and when wind & solar growth started to accelerate, system-average emissions also fell sharply from around 613 kg/MWh to 418 kg/MWh at the end of 2019.

The majority of that fall was due to gas displacing coal but renewables also played a significant part. This trend has been a double-edged sword for gas: system-average emissions have declined so fast that, at the end of 2019, they fell below those of the average gas-fired power station (422 kg/MWh).

What this means from a practical perspective is that – conceptually at least – new gas additions actually increase system average emissions. This is not actually true in practice while gas is able to displace higher emitting coal-fired generation in specific locations. However it is, at least directionally, true. Gas is no longer an unequivocal environmental ‘good’. Indeed there are various regions – and the east coast of the US is one of them – where every single state has lower average emissions than that of gas plants. In those regions, gas can no longer justifiably argue that its use represents an environmental benefit by materially contributing to GHG reductions.

GHG Reductions by Fuel Type

Gas proponents tend to argue that , while gas displaces coal, it represents a major reduction in GHG emissions. However, increasingly gas is not displacing coal so much as competing head-to-head with wind and solar. It is therefore more accurate to estimate GHG savings by comparing gas with system average emissions.

The chart to the right shows the GHG benefits achieved from each generator type assuming that their operation displaces a unit with a system-average GHG emissions profile. The consequence of this is that hydro, wind and solar – which emit zero GHGs in operation – make a substantially larger contribution to GHG reductions than do gas-fired units.

There are several notable points from this,

  1. The GHG contribution of ‘natural’ gas turned negative in December: relative to system average GHG emissions, gas actually increased GHG emissions by 5.7 million tonnes through 2019.
  2. In 2019, wind energy surpassed hydro to become the renewable source with the single largest contribution to GHG reductions. Through 2019, wind use avoided the release of 124.3 million tonnes of GHGs.
  3. Although gas gets lots of credit for displacing coal, it never surpassed hydro in terms of it’s marginal contribution to GHG reductions and only briefly matched hydro in April 2008 when both were achieving annualised GHG reductions of 144 million tonnes.

GHG State Rankings

The following charts shows the GHG emissions of the power generation sector for individual states: the one on the left (for 2019) is the most recent while the one on the right shows the same information but for 2008. The dramatic shift over the 11 years is immediately obvious.

Of note,

  1. In 2008, only 11 states had GHG profiles that were lower than those of gas plants (which, at the time, were 451 kg/MWh). This number had almost tripled by 2019, following the significant expansion of gas and renewables (wind & solar) in power generation and the closure of multiple coal plants, when 31 states had lower emissions than gas plants.
  2. If the gas industry cares about climate change, as it frequently claims, then it makes little sense for it to be seeking to increase market share in states that already have low emissions and minimal coal use. This is particularly true for all states on the Eastern Seaboard where, as it happens, gas is engaged in an aggressive bid for increased market share. It is also true in California where gas is also seeking to expand its footprint.
  3. Gas industry GHG reduction efforts could most usefully be concentrated on coal-heavy states with high GHG footprints. In other words those on the right hand end of the 2019 chart i.e. Montana where, in 2019, coal generated 86% of load, Arkansas (43%), New Mexico (56%), Nebraska (64%), Colorado (42%), Wisconsin (35%), Ohio (30%), North Dakota (113%), Indiana (57%), Utah (76%), Missouri (67%), Kentucky (66%), Wyoming (193%) and West Virginia (160%).
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