Long-term carbon intensity reduction potential of K-12 school buildings in the United States

Abstract

School buildings have a great potential for carbon emission reduction since their annual emission is about 72 million metric tons. Currently, more than 30 % of school buildings were built before 1960 and are underperforming. To effectively reduce carbon emissions via school building retrofits, it is critical for policymakers to understand the carbon intensity reduction potential of retrofitting school buildings in different regions. Hence, this study develops a method to comprehensively assess the long-term carbon intensity reduction potential of aggregated commercial buildings on a county-by-county basis in the continental U.S. We apply this method to the K-12 school buildings including primary and secondary school buildings. This paper predicts the carbon intensity reduction potential of K-12 school buildings with eight building retrofit measures from 2022 to 2050 in the continental U.S. The results reveal several interesting findings: (1) In the approximately 3,000 counties of the U.S. from 2022 to 2050, the carbon intensity reduction potential of retrofitting K-12 school buildings in each county ranges from 0.41 kg/m² to 40.00 kg/m². (2) Even in the same climate zone, the trends of carbon intensity reduction potential from 2022 to 2050 are different depending on their electricity sources. For example, in a hot and humid climate zone, the carbon intensity reduction potential in Florida will decrease from 2044 to 2048. However, in Mississippi, the carbon intensity reduction potential from 2044 to 2046 will increase due to the termination of the nuclear energy usage. (3) Generally, reducing lighting power density leads to more carbon intensity reduction in most states, but it might not be applicable for states with high clean energy penetration, such as Washington.

Introduction

To mitigate climate change caused by greenhouse gas (GHG), the United States (U.S.) has outlined a pathway to reduce carbon emissions 50 % by 2030 [1] and 80 % by 2050 [2]. Being one of the significant infrastructures in the U.S., K-12 school buildings, including primary and secondary schools, have a great potential to reduce carbon emissions. According to the climate advocacy organization Generation180, school facilities annually emit about 72 million metric tons of carbon dioxide, which is equivalent to the emissions from about 8.6 million homes [3]. Currently, more than 30 % of school buildings are constructed before 1960 and are underperforming [[4], [5]]. The underperforming buildings are buildings with low energy efficiency and high carbon emission intensity. Carbon intensity, which is the kilograms of carbon dioxide equivalent emitted per square meter, is one of the emerging metrics to measure the carbon emissions of buildings [[6], [7], [8]]. Hence, it is critical for the policymaker to know the reduction potential of carbon intensity for underperformed buildings by adopting building retrofit measures. The high operating cost of the K-12 school buildings also forces public administrations to make strategic decisions concerning the refurbishment of the school building stock. According to the 2012 Commercial Buildings Energy Consumption Survey (CBECS) conducted by the U.S. Energy Information Administration (EIA), schools annually spend around $8 billion on utility bills [9]. Moreover, reducing the carbon intensity of K-12 school buildings is a good practice for promoting sustainable development for the pupils and their families [10].

The existing study has proved the effectiveness of building retrofits to reduce carbon intensity in specific cities [[11], [12]]. There are several building retrofit measures that have the potential to reduce carbon intensity, such as increasing the insulation of the building envelope, improving the efficiency of the HVAC system, and adjusting the lighting power density of buildings [[13], [14], [15], [16], [17], [18], [19], [20], [21]]. Moreover, building retrofits have a positive economic impact on reducing the operational costs of buildings, which can be an incentive for stakeholders to adopt the building retrofit measures, reducing carbon intensity as a result [22]. Gamarra et al. indicated that schools could reduce the fossil energy demand of the building in the operation and maintenance phase via building retrofits in a hot climate zone [23]. Many researchers realize that carbon intensity reduction on the county level is very important to achieve low-carbon development goals [[24], [25], [26], [27]].

However, challenges remain regarding presenting comprehensive carbon intensity inventories at the county level. For the U.S., the situation becomes particularly difficult because of the significant geographical and social-economic diversity across the country. Furthermore, generating comprehensive carbon intensity inventories requires very detailed carbon accounting for each county as well as a comprehensive understanding of the local climate. Compared with states, counties have various definitions regarding their boundaries and non-centralized statistics, which produce uncertainties in carbon intensity calculation. Facing this challenge, a method emphasized on a county-by-county basis is needed to predict the carbon intensity reduction potential of school buildings. Carbon assessment on a county-by-county basis is a key research direction in the field of carbon neutrality. Understanding carbon intensity at the county and regional levels have been highlighted in the carbon management literature [[26], [28], [29]].

The above literature review reveals that there is no existing method for predicting the large-scale carbon intensity reduction potential of K-12 school buildings on a county-by-county basis. To fill the gap, this study investigates the large-scale carbon intensity reduction potential of K-12 school buildings on a county-by-county basis, which is not fully investigated yet. Furthermore, the new methods proposed in this paper can also be applied to any aggregated commercial buildings in the continental U.S. Based on the result of building energy modeling, the dynamic carbon emission factor of electricity is used to explore the carbon intensity reduction potential of commercial buildings on a county-by-county basis in the continental U.S. Then, to calculate the aggregated carbon intensity reduction of commercial buildings, the weighting values of floor area calculated from the existing construction database are considered. According to the research objective, this study adopts this method to comprehensively assess the carbon intensity reduction potential of K-12 school buildings in the continental U.S. on a county-by-county basis.

The rest of this paper is organized as follows: Section 2 introduces the methodology of predicting county-by-county based carbon intensity reduction potential of aggregated commercial buildings; Section 3 describes the application of this method in K-12 school buildings including building energy model preparation, information on building locations, building retrofit measures and weighting values of floor area between primary and secondary school buildings; Section 4 presents the results of carbon intensity reduction of K-12 school buildings; Section 5 discusses the spatiotemporal variability of the long-term carbon intensity reduction of K-12 school buildings. Finally, Section 6 concludes and discusses future work.