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Cities run on energy. Since the industrial revolution, urban environments have dominated energy consumption patterns in countries around the world. Today, over 50% of the world''s population lives in urban areas, collectively generating over 75% of the global gross domestic product (GDP). Attracted by this wealth, urban dwellers are expected to double by 2050. At that point, the urban built-up area is projected to more than triple1, accounting for over 70% of global carbon emissions2.
We collaborated with representatives from eight cities and municipalities around the world—Braga (Portugal), Cairo (Egypt), Dublin (Ireland), Florianopolis (Brazil), Kiel (Germany), Middlebury (Vermont, United States), Montreal (Canada), and Singapore. The cities were selected based on public calls for participation on building/urban science mailing lists and via direct contacts in our networks. A requirement for participation was that teams had some expertise in building energy modeling as well as existing relationships with local city representatives. We also aimed for a diverse set of cities with different climates, socioeconomic demographics, cultures, governing structures, and sizes.
Our educational goal for the collaboration was to train city representatives to conduct an urban building energy analysis for parts/segments of their building stock that they could later independently expand to the whole jurisdiction. For each city, we followed a study framework that consisted of individual pre-workshop meetings with city representatives, a joint three-day remote workshop including goal-setting and technical working sessions, and another set of individual debriefs. The workshop took place in January 2021. During the opening session, city representatives were invited to share their carbon reduction objectives for existing buildings as well as what retrofitting measures they were considering for those buildings.
We then built eight seed urban building energy models (UBEM) ranging from 38 to 399 buildings in neighborhoods for which building footprints, heights, program (usage type), and year of construction were available (Fig. 1). An UBEM is a physics-based model of buildings that estimates hourly energy use for heating, cooling, hot water, lighting, and equipment for "as is" conditions and any combination of possible retrofit upgrades. Non-geometric building properties such as construction characteristics, building age, heating, ventilation, and air-conditioning system properties were compiled for each city before the workshop (see Methods).
The colour of each building indicates the relative energy use intensity compared to other buildings in the model. For example, in Singapore with two archetypes defined (viz. residential and commercial), we observe that the commercial building (in red) has a relatively higher energy use intensity than the residential buildings, which performed rather similarly, reflecting how all residential buildings in the region of interest are similar-type public housing apartment units.
The concept of a "seed" UBEM was introduced for this project. A seed UBEM is a scaled-down version of a full UBEM that covers a limited part of a jurisdiction. Working with seed UBEMs (and fewer buildings) in the workshop is useful for staying nimble and supporting on-the-spot analysis. A seed model should ideally represent the city''s overall building stock—i.e., covers building typologies that represent a significant fraction of all buildings—and extend over an area that will soon undergo substantial renovation efforts. If well chosen, the seed model simulation and analysis results are indicative of the entire stock model since—with more buildings—the difference introduced stems mainly from building geometry.
The regions for the seed models were selected in consultation with participating city representatives. Florianopolis and Montreal selected typical mixed-use neighborhoods, including residential, retail, and larger commercial buildings. Braga, Dublin, Kiel, and Middlebury selected aging residential neighborhoods that are representatives of many similar neighborhoods surrounding the city core and are slated to undergo energy retrofits soon. Cairo and Singapore focused on multi-story public housing complexes that comprise most of those cities'' construction. For example, over 80% of Singaporeans live in public housing.
For each study area, we developed a baseline as well as shallow and deep retrofit scenarios based on input from the city representatives (see Results). Our research goals were to gauge the value city representatives would retain from using an UBEM-based model of their building stock and what specific building retrofit upgrades they were considering at the time from a list of possible options (see Methods). Although we provided some informal feedback at the end of the workshop to guide future development, we refrained from assuming the role of a "consultant" that provides custom-tailored, optimal solutions to each city.
In addition to building retrofits, we also predicted the maximum onsite electricity generation potential from PV assuming full rooftop utilization to provide an upper physical limit for onsite carbon emission reductions. To separate the emissions reduction contributions from building upgrades and grid decarbonization, future carbon emissions are shown as a range in the Results section, assuming current and projected future grid emissions, respectively.
This work contributes to urban-scale energy research and policy in multiple ways. Previous work on urban building energy modeling mainly focused on developing and validating simulation tools and identifying potential use cases for this new technology11,12. In select cases, the energy-saving potential from retrofitting existing buildings—for example, in San Francisco, CA13, and Venice, Italy14—was calculated. However, those studies do not report if and how the authors engaged with local governments. The LA10015 and Carbon Free Boston16 studies are notable exceptions where experts from a U.S. National Lab or university collaborated with the municipalities in Los Angeles and Boston to develop carbon reduction pathways using custom-built, fully integrated cross-sector models.
This paper presents the first study in which a scalable UBEM approach has been tested with multiple, diverse city representatives to understand whether local teams can learn how to use and independently apply the method and provide lasting value for participating jurisdictions. Our findings offer insight into what type of building retrofit packages energy policymakers are currently considering for their existing building stock and how resulting carbon emission reductions compare to politically motivated targets. This study also dovetails with parallel efforts to decarbonize the transportation, industrial, and electricity sectors.
The eight cities represent diverse cultures and climate zones where many urbanites live. In the following, we summarize the building retrofit upgrade scenarios that the city representatives formulated. The scenarios consist of combinations of building upgrades picked from a list of common technologies provided to city representatives before the workshop (see Methods). For each city, we defined two retrofit upgrade scenarios that mostly correspond to shallow (lower cost and/or easier to implement) and deep (more expensive and/or harder to implement) retrofits (Table 1). Details on how the baseline and upgrade scenarios were modeled are provided in the Methods section.
The selection process for the scenarios varied depending on the role of the city representatives, which ranged from city planners and sustainability directors to local NGOs. For some cities, a single representative "dictated" the scenarios, while other city teams were more collaborative, reflecting differences in how cities are generally governed.
Table 1 also lists carbon emissions reduction targets for buildings in each city. While some cities have separate near- and long-term goals for 2030 and 2050, others only have a 2050 goal or no goal at all. This information was provided by the city representatives and validated via official policy documents where possible. If no building-specific targets were available, we used economy-wide emission reduction targets. Braga, Florianopolis, and Montreal aimed for net-zero operational carbon emissions, meaning any remaining fossil fuel use will need to be balanced by carbon offsets or excess renewable energy generation.
Cairo has a subtropical desert climate with mild winters and hot summers. The city is cooling-dominated, with increasing risks of severe heat waves due to climate change21. The participating UN Habitat Egypt Office presented the twin goal of keeping the population healthy without excessive reliance on air-conditioning and a focus on the widespread deployment of rooftop photovoltaics (PV). The team did not provide a carbon reduction target. Based on previous experience working in the region22, we tested load reductions for shallow retrofits and enhanced air-conditioning and mechanical ventilation (ACMV) equipment for deep retrofits.
Florianopolis has a warm, humid subtropical climate with warm summers and mild winters. At the time of the workshop, the city did not have any carbon reduction goals for buildings but referred to Brazil-wide targets of 43% and 100% emissions reductions by 2030 and 2050, respectively. Technologies of interest to the local team, consisting of municipal representatives and a nearby university, initially ranged from architectural interventions such as added exterior shading (fixed individual window overhangs), reduced equipment loads, and enhancements to ACMV equipment. As the workshop progressed, the team increasingly concentrated on the latter two technology upgrades as they discovered the high cost and somewhat limited impact of exterior shading in retrofit projects.
Kiel is one of Germany''s major maritime centers, with a sub-oceanic climate influenced by currents from the Atlantic and the North Sea. Most homes in Kiel have no active cooling, and traditionally the emphasis has been placed on adding insulation to reduce heating energy use. The Director of the Kiel Authority for Environmental Protection expressed interest in adding insulation and weatherization to reduce heating loads and energy use. These measures are employed for the shallow retrofit scenario. For the deep retrofit scenario, electric heat pumps are added to electrify the remaining heating loads, a measure in line with ongoing German efforts to reduce the country''s reliance on natural gas from Russia26.
Dublin has a mild climate, mainly heating-dominated, with relatively comfortable summers and cold, humid winters. Due to the age and state of many buildings, thermal envelope upgrades represent one of the most effective retrofits27,28. Dublin contains a significant proportion of row houses, which is why the city targets coordinated group retrofits in the form of weatherization upgrades for the shallow retrofit scenario and added wall insulation and window replacements for the deep retrofits. During the workshop, the city representative did not express interest in heat pumps as the city was assessing the feasibility of expanding the existing district heating system.
Middlebury is situated in the U.S. state of Vermont, with warm, wet summers and frigid winters. The state has a largely decarbonized electric grid with high penetrations of renewables29. A key concern of participating representatives from the local energy committee and Middlebury College was understanding and quantifying the potential of heat electrification and grid resilience. Technologically, these goals translate into air-source heat pump adoption for space heating (shallow retrofit) and envelope upgrades (deep retrofit) to reduce the building''s winter peak demand caused by this newly-electrified heating.
Finally, Montreal enjoys a stable, decarbonized grid from hydropower. The city mainly seeks to reduce reliance on widely used electric resistance heating due to its inefficiency. The shallow retrofit replaces electric resistance heating with natural gas furnaces, which are cheaper to operate but more carbon-intensive. Ultimately, the city hopes to convince residents to invest in ground-source heat pumps for heating (deep retrofit) powered by their clean electricity supply. Given that natural gas furnaces, once installed in place, have a lifetime of 15+ years, the former approach is not readily compatible with the city''s 2050 target.
Predicted onsite baseline energy use intensities (EUI) range from under 89 kWh/m2 for Braga to 329 kWh/m2 for Middlebury (Fig. 2). EUIs are mainly influenced by program type, climate, construction standards, mechanical systems, and urban typology. EUI subcategories for heating, cooling, lighting, domestic hot water, and equipment reflect these relationships—i.e., Cairo, Florianopolis, and Singapore are cooling-demand dominated with no heating loads. In contrast, Dublin, Kiel, Middlebury, and Montreal are heating-dominated.
A baseline and two additional scenarios are defined for each city, with the colors representing energy use intensities for various end uses. Shallow retrofits naturally lead to smaller reductions in EUI, while deeper retrofits require more capital but often lead to significantly higher EUI reductions. In Braga, the "shallow retrofit" scenario accounts for a warmer climate in 2080; thus, the cooling demand greatly increases, increasing energy use.
The time stamp over each column marks the hour in the year when this peak occurs. We observe that the deployment of rooftop PV does not reduce the peak in many municipalities, as the peak usually occurs at the beginning or end of the day. The introduction of heat pumps for space heating significantly increases the peak in Kiel, and in Middlebury, the peak further shifts from summer afternoon to winter morning.
In Cairo, Florianopolis, and Singapore, shallow retrofits reduce the annual peak from 9% to 29%, while deep retrofits reduce the annual peak from 39% to 55%. In Dublin, the heating is provided by natural gas in all scenarios, so the electric peak demand from buildings is driven purely by winter lighting and equipment loads and is only slightly reduced in the shallow scenario. In Cairo, Braga, and Kiel, the peaks remain around the same time of year and occur in the evening/morning for cooling/heating-dominated climates. Given the limited availability of sunlight during those times, the deployment of PV does not affect the peak loads much except in Florianopolis, where the January 23rd 5pm mid-summer peak is delayed to March 23rd at 6pm and reduced by 20%.
Overall, our results show that the widespread use of rooftop PV will not significantly help utilities manage their building-related electricity peaks due to a temporal mismatch in production and demand. However, renewable energies will play a key role in reducing overall building-related carbon emissions as they provide a zero-carbon source of electricity to power electrified buildings.
Where applicable, carbon emission targets are shown in red lines. While many cities can meet their near-term carbon emissions reduction targets, meeting their long-term targets will require grid decarbonization and end-use electrification.
Singapore''s building energy use is modeled as all-electric, and the projected grid emission reductions would help it surpass its 2050 target for both shallow and deep retrofits. Much of the grid decarbonization will need to come from off-site sources as rooftop solar can only contribute to emissions reductions for a small part of the investigated residential high-rise buildings that make up much of Singapore''s building stock.
Cairo has no publicly available grid emissions reduction plans (nor emissions goals), and the rooftop solar production potential can reduce current emissions by 21% from baseline.
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