6 Improved details on future of SPP fuel mix.
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HereWhile it's clear from this chart that while the trends for natural gas and coal roughly matchis the marginal fuel, coal dominates the mix by volumeinteresting thing is how it's cycled to accommodate the morning bump in wind generation. HoweverIn their most recent 10-year plan (from 2017), sinceSPP predicts retirement of at least 1 GW of coal capacity (of 26 GW currently online), and addition of as much as 10 GW of natural gas capacity (added to the existing 36 GW). Considering how coal is more variablecurrently being cycled, as coal plants closealong with the expected change in future generation mix, indicates that by 2025 natural gas will take pole position asalmost certainly be the marginal fuel in SPP.

Here it's clear that while the trends for natural gas and coal roughly match, coal dominates the mix by volume. However, since natural gas is more variable, as coal plants close, natural gas will take pole position as the marginal fuel.

While it's clear from this chart that coal is the marginal fuel, the interesting thing is how it's cycled to accommodate the morning bump in wind generation. In their most recent 10-year plan (from 2017), SPP predicts retirement of at least 1 GW of coal capacity (of 26 GW currently online), and addition of as much as 10 GW of natural gas capacity (added to the existing 36 GW). Considering how coal is currently being cycled, along with the expected change in future generation mix, indicates that by 2025 natural gas will almost certainly be the marginal fuel in SPP.

5 Fixed broken links. Found better data for MISO, PJM, and SPP
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MISO provides real time fuel mix data (herehere), but unfortunately doesn't provide a time series for this dataas well as historical real-time fuel on the margin data. So I pulled thisdownloaded the data up from their most recent Monthly Market Operations reportand did a quick analysis:

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"CC" stands for combined cycle and refers to power plants which burn natural gas and use multiple heat engines to improve plant efficiency. These plants can ramp faster than coalOn average, but not as fast as traditional natural gas "peaker" plantsgenerators are marginal every day from 7am to 11pm.

CombiningLooking at the percentages for CC and gastotals, natural gas was the fuel on the margin 64.7%is marginal 51% of the time, compared to 50.4% of time time forwhile coal, the next-most-marginal fuel (The numbers can sum to more than 100% because multiple fuels can be on the margin at is marginal 39% of the same time -- like two gas stations selling gas for the same price).

PJM provides marginal fuel data on a monthly basis, with a two month delay. I downloaded data from December and did some quickThe most recent State of the Market report also includes a historical analysis of real-time marginal units in Figure 3-5:

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PJM operates a 5-minute market, meaning that the fuel on the margin can switch as often as every five minutes. From the tableIn 2018, we see that natural gas was marginal within 744 hoursabout 60% of the monthtime, andwhile coal was the marginal fuel 67%30% of the time, more than twice as often as coal, the next most marginal fuel.

Central corridor -- coal and natural gas

SPP provides real-time generation mix data, but the graphic seems to be broken currently. I downloaded the data and attempted to analyze it a bit. Here's seven days of fuel mix data from February 2019did some analysis:

enter image description hereenter image description here

Here it's clear that while the trends for natural gas and coal roughly match, coal dominates the mix by volume. Overall thoughHowever, since natural gas is more variable, meaning that as as coal plants close, natural gas will take pole position as the marginal fuel.

MISO provides real time fuel mix data (here), but unfortunately doesn't provide a time series for this data. So I pulled this data up from their most recent Monthly Market Operations report:

enter image description here

"CC" stands for combined cycle and refers to power plants which burn natural gas and use multiple heat engines to improve plant efficiency. These plants can ramp faster than coal, but not as fast as traditional natural gas "peaker" plants.

Combining the percentages for CC and gas, natural gas was the fuel on the margin 64.7% of the time, compared to 50.4% of time time for coal, the next-most-marginal fuel (The numbers can sum to more than 100% because multiple fuels can be on the margin at the same time -- like two gas stations selling gas for the same price).

PJM provides marginal fuel data on a monthly basis, with a two month delay. I downloaded data from December and did some quick analysis:

enter image description here

PJM operates a 5-minute market, meaning that the fuel on the margin can switch as often as every five minutes. From the table, we see that natural gas was marginal within 744 hours of the month, and was the marginal fuel 67% of the time, more than twice as often as coal, the next most marginal fuel.

Central corridor -- coal and natural gas

SPP provides real-time generation mix data, but the graphic seems to be broken currently. I downloaded the data and attempted to analyze it a bit. Here's seven days of fuel mix data from February 2019:

enter image description here

Here it's clear that while the trends for natural gas and coal roughly match, coal dominates the mix by volume. Overall though, natural gas is more variable, meaning that as coal plants close, natural gas will take pole position as the marginal fuel.

MISO provides real time fuel mix data (here), as well as historical real-time fuel on the margin data. I downloaded the data and did a quick analysis:

enter image description here

On average, natural gas generators are marginal every day from 7am to 11pm. Looking at the totals, natural gas is marginal 51% of the time, while coal is marginal 39% of the time.

PJM provides marginal fuel data on a monthly basis, with a two month delay. The most recent State of the Market report also includes a historical analysis of real-time marginal units in Figure 3-5:

enter image description here

In 2018, natural gas was marginal about 60% of the time, while coal was marginal 30% of the time.

Central corridor -- coal

SPP provides real-time generation mix data, but the graphic seems to be broken currently. I downloaded the data and did some analysis:

enter image description here

Here it's clear that while the trends for natural gas and coal roughly match, coal dominates the mix by volume. However, since natural gas is more variable, as coal plants close, natural gas will take pole position as the marginal fuel.

4 added 186 characters in body
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The U.S. is divided into Regional Transmission Organizations and Independent System Operators, which are non-profit organizations that manage the grid in different regions of the country. I've taken a look at data from some of the largest to see which fuel is on the margin for each. These regions cover most of the U.S. population (the southeast and most of the west are excluded as these markets are still vertically integrated, meaning that each utility is self-regulated and there's no central source of data).

The Southwest Power Pool (SPP) covers a column of states between Texas and and the Canadian border, and operates 85 GW of capacity.

For Texas, I wasn't able to find information directly on fuel on the margin, or hourly output by fuel. However unlike most other ISOs/RTOs which operate energy andand capacity markets, Texas has an energy-only only market. This means thatAs a result, plants compete almost exclusively on the price of power they deliver to the grid. This means that the monthly breakdown of total generation (available here) gives strong clues about which is the de facto fuel on the margin:

The U.S. is divided into Regional Transmission Organizations and Independent System Operators, which are non-profit organizations that manage the grid in different regions of the country. I've taken a look at data from some of the largest to see which fuel is on the margin for each. These regions cover most of the U.S. population.

The Southwest Power Pool (SPP) covers a column of states between Texas and and the Canadian border, and operates 85 GW of capacity

For Texas, I wasn't able to find information directly on fuel on the margin, or hourly output by fuel. However unlike most other ISOs/RTOs which operate energy and capacity markets, Texas has an energy-only market. This means that plants compete almost exclusively on the price of power they deliver to the grid. This means that the monthly breakdown of total generation (available here) gives strong clues which is the de facto fuel on the margin:

The U.S. is divided into Regional Transmission Organizations and Independent System Operators, which are non-profit organizations that manage the grid in different regions of the country. I've taken a look at data from some of the largest to see which fuel is on the margin for each. These regions cover most of the U.S. population (the southeast and most of the west are excluded as these markets are still vertically integrated, meaning that each utility is self-regulated and there's no central source of data).

The Southwest Power Pool (SPP) covers a column of states between Texas and the Canadian border, and operates 85 GW of capacity.

For Texas, I wasn't able to find information directly on fuel on the margin, or hourly output by fuel. However unlike most other ISOs/RTOs which operate energy and capacity markets, Texas has an energy only market. As a result, plants compete almost exclusively on the price of power they deliver to the grid. This means that the monthly breakdown of total generation (available here) gives strong clues about which is the de facto fuel on the margin:

3 Added SPP.
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2 Found better data for MISO. Added Texas.
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