Predicting Building Emissions in the United States | MIT News



The United States is entering a construction boom. Between 2017 and 2050, it will build the equivalent of New York City 20 times. Yet to meet climate goals, the country must also significantly reduce greenhouse gas (GHG) emissions from its buildings, which account for 27% of the country’s total emissions.

A team of current and former MIT Concrete Sustainability Hub (CSHub) researchers are responding to these conflicting demands with the aim of giving policymakers the tools and information they need to take action. They detailed the results of their collaboration in a recent article in the journal Applied energy which projects emissions from all buildings in the United States under two GHG reduction scenarios.

Their article found that “embedded” emissions – those from materials production and construction – would account for around a quarter of emissions between 2016 and 2050 despite significant construction.

In addition, many regions would have different GHG reduction priorities; some, like the West, would benefit the most from intrinsic emission reductions, while others, like parts of the Midwest, would benefit the most from interventions on energy-related emissions. If these regional priorities were dealt with aggressively, emissions from the building sector could be reduced by around 30% between 2016 and 2050.

Quantify contradictions

Modern buildings are much more complex – and efficient – than their predecessors. Thanks to new technologies and stricter building codes, they can offer lower energy consumption and operational emissions. And yet, more efficient materials and improved construction standards can also generate higher intrinsic emissions.

Concrete, in many ways, embodies this compromise. While its durability can minimize energy-consuming repairs over the operational life of a building, the scale of its production means that it contributes a large part of the intrinsic impacts in the building industry.

As such, the team focused the GHG reductions from concrete in their analysis.

“We took a bottom-up approach, developing benchmark designs based on a set of residential and commercial building models,” says Ehsan Vahidi, assistant professor at the University of Nevada at Reno and former postdoctoral fellow at CSHub. “These designs differed in roof and slab insulation, HVAC efficiency, and construction materials, primarily concrete and wood.”

After measuring the operational and intrinsic GHG emissions for each benchmark design, the team extended their results to the county level and then to the national level based on the building stock forecast. This enabled them to estimate emissions from the entire building sector between 2016 and 2050.

Understand how various interventions could reduce GHG emissions, the researchers executed two different scenarios – a “projected” scenario and an “ambitious” scenario – within their framework.

The projected scenario corresponded to current trends. He assumed that the decarbonisation of the network would follow the forecasts of the Energy Information Administration; the widespread adoption of new energy codes; improving the efficiency of lighting and household appliances; and, for concrete, the implementation of 50% low carbon cements and binders in all new concrete construction and the adoption of full carbon capture, storage and utilization (CCUS) of all cement and concrete emissions.

“Our ambitious scenario was intended to reflect a future where more aggressive action is taken to reduce GHG emissions and meet targets,” explains Vahidi. “Therefore, the ambitious scenario took these same strategies [of the projected scenario] but included more aggressive targets for their implementation.

For example, he assumed a 33% reduction in grid emissions by 2050 and advanced the planned timelines for lighting and appliances and thermal insulation to five and 10 years, respectively. The decarbonization of concrete has also happened much faster.

Discounts and variations

The strong growth forecast for the US construction sector will inevitably generate a significant number of emissions. But how much can this figure be downplayed?

Without the implementation of GHG reduction strategies, the team found that the construction sector would emit 62 gigatonnes of CO.2 equivalent between 2016 and 2050. This is comparable to the emissions generated by the 156,000 billion kilometers traveled by passenger vehicles.

But the two GHG reduction scenarios could significantly reduce emissions from this unmitigated business-as-usual scenario.

In the projected scenario, emissions would drop to 45 gigatons of CO2 equivalent – a decrease of 27 percent over the period of analysis. The ambitious scenario would offer an additional reduction of 6% compared to the projected scenario, reaching 40 gigatons of CO2 equivalent – such as the removal of approximately 55 trillion kilometers of passenger vehicles from the road during the period.

“In both scenarios, the biggest contributor to the reductions was the greening of the energy grid,” notes Vahidi. “Other notable downsizing opportunities have been increasing the efficiency of lighting, HVAC and appliances. Together, these four attributes contributed 85% of emissions during the analysis period. Their improvements offered the greatest potential emission reductions.

The remaining attributes, such as thermal insulation and low carbon concrete, had a lower impact on emissions and, therefore, offered lower reduction opportunities. This is because these two attributes were only applied to new constructions in the analysis, which outnumbered existing structures throughout the period.

The disparities in impact between strategies targeting new and existing structures underscore a broader observation: despite significant construction during the period, intrinsic emissions would only represent 23% of cumulative emissions between 2016 and 2050, the rest coming mainly from of operation.

“This is because the existing structures far exceed the new ones,” says Jasmina Burek, postdoctoral fellow at CSHub and new assistant professor at the University of Massachusetts Lowell. “The operational emissions generated by all new and existing structures between 2016 and 2050 will still far exceed the intrinsic emissions of new structures at any given time, even as buildings become more efficient and the grid becomes greener. “

Yet the emission reductions from the two scenarios were not distributed evenly across the country. The team identified several regional variations that could have implications for how policymakers should act to reduce emissions from the building sector.

“We found that the western regions of the United States would see the greatest opportunities for reducing interventions to residential emissions, which would constitute 90 percent of the region’s total emissions during the analysis period,” explains Vahidi.

The predominance of residential emissions stems from the region’s continued population growth and subsequent growth in the housing stock. The proposed solutions would include CCUS and low-carbon binders for the production of concrete, and improvements to energy codes for residential buildings.

As with the West, ideal solutions for the South East would include CCUS, low carbon binders, and improved energy codes.

“In the case of the south-eastern regions, interventions should also target commercial and residential buildings, which we believe were more evenly distributed across the housing stock,” Burek explains. “Due to the strict energy codes in both regions, interventions on operational emissions had less impact than those on intrinsic emissions. “

Much of the Midwest has seen the opposite result. Its energy mix remains one of the most carbon-intensive in the country and improvements in energy efficiency and the network would be very profitable, particularly in Missouri, Kansas and Colorado.

New England and California would see the smallest cuts. As their already strict energy codes would limit further operational reductions, the opportunities to reduce intrinsic emissions would be the most powerful.

This huge regional variation discovered by the MIT team is in many ways a reflection of the great demographic and geographic diversity of the nation as a whole. And there are still other variables to consider.

In addition to GHG emissions, future research could consider other environmental impacts, such as water consumption and air quality. Other mitigation strategies to consider include longer lifespans of buildings, retrofits, rooftop solar power, as well as recycling and reuse.

In this sense, their conclusions represent the lower limits of what is possible in the building sector. And while further improvements are ultimately possible, they have shown that regional variation will invariably inform these environmental impact reductions.



About Author

Comments are closed.