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The case for marginal emission rates

The best opportunity for NYC buildings to reduce CO2e is by shifting or reducing hourly peak electric use each week so that NYISO can drop the marginal Queens/Brooklyn baseload generating plant from security-constrained unit commitment (SCUC) in the day-ahead market.

NYISO Security-Constrained Unit Commitment

In ISO day-ahead energy markets, efficiency programs and technologies affect how many and which baseload generating plants are selected for operation the next day – through a process called security-constrained unit commitment (“SCUC”).  By contrast, in ISO real-time energy markets, efficiency programs and technologies can only affect the incremental dispatch of such already selected and committed baseload generating plants.

As a result, day-ahead hourly marginal emission rates can be large and variable in contrast both to real-time hourly marginal emission rates and to the annual average emission rates calculated by the U.S. Environmental Protection Agency and broadly used in the energy industry.  Stated otherwise – day-ahead marginal hourly emission rates reveal a significant opportunity to reduce carbon emissions.

Energy efficiency, storage, and local distributed PV reduce SCUC

EMeister MPC is one of many examples of an efficiency measure that affects SCUC in the day-ahead energy market and therefore can make good use of marginal hourly CO2e emission rates that reflect SCUC.  (See examples on this website.) NYC buildings can reduce SCUC every week of the year through capital and operating improvements focused on:

  • reducing weekly peak hours electric use (energy efficiency).

  • shifting use out of weekly peak hours (storage).

  • providing operating reserves during weekly peak hours (storage or demand response).

  • taking advantage of local distributed PV and its coincidence with weekly peak hours.​

​EMeister MPC does all four as a byproduct of reducing building energy expense.

“Using eGrid for Environmental Footprinting of Electricity Purchases”*

This U.S. EPA publication excellently documents a method for using average regional emission rates for footprinting.  For example, for NYC for 2020, EPA recommends use of an average CO2e emission rate of 636.0 lbs/MWh or 0.32 tons/MWh for footprinting.  For evaluating the effect of efficiency measures at the margin, this same method recommends use of an average fossil fuel CO2e emission rate of 971.4 lbs/MWh or 0.49 tons/MWh.  This rate is “marginal” in that it properly reflects that efficiency measures reduce fossil plant operation, that is, do not reduce nuclear or renewable plant operation.  However, this rate implicitly reflects the real-time market, and not the opportunity provided by SCUC in the day-ahead market.  This EPA rate also does not reflect time-of-use.  The EPA does not have the local expert ISO knowledge needed to capture time-of-use or SCUC for marginal emission rates.

References to CO2e footprint throughout this website are based on this document.

*Huetteman, Justine, Travis Johnson, and Jeremy Schreifels. “Using eGRID for Environmental Footprinting of Electricity Purchases.” U.S. EPA. 2020. 

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