Future Hourly Marginal Emissions

It is vital to recognize that the true GHGI of a building is affected not only by the magnitude of its energy use but also the periods during which it draws electricity from the grid.

This is because the grid’s energy generation mix varies throughout the day. While the traditionally employed annual average factors represent grid emissions from the corresponding generation mix over a year, actual emissions associated with generation respond to variation in demand for every hour. It is, therefore, imperative to consider marginal hourly profiles 4 instead of an annual average factor when assessing the true GHG avoidance impact of design strategies. Figure 1 illustrates this difference for a reference project, where the annual average emission factor for grid electricity is published at 0.13 MTCO 2 e/MWh, and the published marginal emissions factor can be 6 times higher at 0.78 MTCO 2 e/MWh for peak hours during a summer week.

Figure 1:

The difference between annual average and marginal hourly emission factors for 2020 and the marginal hourly emission factors for 2020 and 2030. The ability to reduce building electricity demand during such high emissions intensity hours, therefore, offers the largest GHGI avoidance.

Second, in addition to accounting for current variations in grid emissions on an hourly basis, it is important to consider how the rapidly changing grid is expected to evolve over the next few years. For example, the dotted lines in Figure 1 compares 2030 grid GHG factor projections against 2020 factors. As the future grid gets cleaner and daytime factors approach zero due to high solar energy penetration, it will in fact be possible for a building to achieve zero-carbon operations if it can shift its load to only draw from the grid during these daytime hours; and carbon-positive operations if it can then feed surplus energy back into the grid during high intensity late afternoon

hours.