Emissions from buildings tend to be associated with the direct consumption of fossil fuels, which are mainly intended for space and water heating. However, electricity consumption also produces considerable off-site emissions, especially if the local grid relies on fossil fuels as the main energy source.
When buildings use fuels like natural gas or propane directly, emissions per BTU of energy input are relatively constant. They can increase based on the size of the building and decrease when using high-efficiency heating systems. However, emissions per unit of fuel consumed tend to remain the same. On the other hand, emissions per kilowatt-hour can change significantly, based on the energy sources used for power generation.
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Depending on how electrical energy is produced locally, emissions may increase or decrease when a building switches from combustion heating to electric heating. For example, a heat pump running on solar energy is environmentally friendly, but not a resistance heater using coal energy!
How are emissions per kilowatt-hour calculated?
Electric grids use many types of power plants, and emissions per kilowatt-hour vary depending on generation technology and energy consumption. For example, natural gas produces fewer emissions than coal, and a combined cycle gas plant produces fewer emissions than a simple cycle plant. To estimate emissions per kWh from an electrical grid, it is necessary to consider the individual impact of all energy sources.
- An electricity grid using only coal would produce about one kilogram of CO2 per kilowatt-hour, or one metric ton per megawatt-hour.
- On the other hand, a network that runs solely on natural gas would fall somewhere between 400 and 600 grams of CO2 per kWh.
In New York City, the emissions intensity specified in Local Law 97 is 0.000288962 tCO-e per kWh. This equates to almost 289 g CO2e per kWh. New York State gets 90% of its electricity from natural gas, nuclear power and hydroelectric turbines. Since nuclear power and hydroelectricity have minimal emissions compared to natural gas, the average value per kWh is reduced.
Reduction of emissions from electricity consumption
When buildings use fossil fuels directly, the only way to reduce emissions is to reduce consumption. However, when it comes to electricity, there are three possible approaches:
- Reducing the amount of kilowatt-hours consumed.
- Generate electricity on site using solar panels or other clean technologies.
- Increase the percentage of clean energy sources at grid level, which reduces average emissions per kWh.
Building owners have no control over the energy mix used by the local grid, but they can reduce their electricity consumption or install their own generation systems. However, if local energy companies start replacing fossil fuels with clean sources, emissions from buildings will automatically be reduced. For example, if New York's emissions factor were reduced from 289 to 200 g CO2-e per kWh, buildings would reduce their emissions from grid electricity by 30%.
When using natural gas for on-site generation, building emissions can increase or decrease depending on the energy mix used by the grid. For example, a gas microturbine will reduce emissions if the local power grid relies on coal. However, this same measure will actually increase emissions if the grid uses a high percentage of renewable sources.
Conclusion
After energy consultants analyze a building, they typically recommend energy efficiency measures first and then on-site generation. The logic is simple: if a building first reduces its consumption with energy efficiency measures, it needs less production capacity to later cover its energy needs.
Building electrification can achieve continuous emissions reductions over time, even in cases where the initial benefit is small. If the grid starts using a higher percentage of renewable energy, the corresponding emissions from buildings will continue to decrease – without additional changes.
