Global Journal of Researches in Engineering, J: General Engineering, Volume 1 9 Issue 2

Urban Heat Island Effect on Building Electricity use Menglin S. Jin ɲ & Rebecca Huff ʍ Abstract- The campus-wide electricity use in University of Maryland, College Park (UMCP) is highly correlated with the outdoor 2-meter surface air temperature, at hourly, daily, and monthly scales, with the correlation coefficients normally > 0.70 in 2014 and 2015. Nevertheless, 2-meter surface air temperature has evident spatial heterogeneity, determined by underlying surface types and surrounding vegetation fraction, with up-to 6 °F difference between a roof on campus and a vegetation-covered airport for the clear days on Julys 2014 and 2015. Such urban heat island effect (UHI) signal suggests that urban local surface air temperatures, instead of those in an nearby airport, may be needed in order to accurately forecast the electricity use for a given urban community. In addition to outdoor weather conditions, campus electricity use amount is also affected by other factors such as human behavioral pattern, for example, weekdays vs weekends. Therefore, interdisciplinary effort from weather system, society, and mechanical engineering is needed to fully understand and thus forecast electricity use. I. I NTRODUCTION lectricity is needed to power heating, ventilation and air conditioning (HVAC). An average of 41% of the consumed electricity in the U.S. is used by HVAC systems [Goetzler et al . 2014], which is widely implemented on buildings to maintain human comfortable level. In addition, lightings and lab equipment such as computers also need electricity. Accurately forecasting electricity need for a building, a community, or a city is critical for the facility management to plan the resources in advance for sustainable development and electricity savings. Various natural weather factors, in particular, the ambient air temperature and humidity, affect the amount of electricity used in buildings [Jin 2018]. In addition, the configuration of the building structure such as the materials of the roof and exterior walls, the shape of the building, the slope of the roof and the number and size of the windows affect building energy use [DOE 2015, Wei et al., 2016]. Various studies assess building contributions to the Urban heat island effect (UHI) and vice versa. For example, Shahmohamadi et al. [2011] showed that the lack of impervious surface materials in the city Tehran, Iran forced “an evaporation deficit in the city continues to build structures using “waterproof and low albedo” materials, the surface air temperature there would further rise. UHI is mainly caused by reduced surface albedo [Jin et al. 2005], less vegetation coverage in the city, less soil moisture, and reduced heat capacity in urban surfaces [Table 1]. Specifically, for vegetation surface, the heat capacity is 1300J/g/K while the asphalt parking lot and roof are only 1000 J/g/K and 837 J/g/K, respectively. Therefore, with the same amount of solar radiation absorbed, vegetative covered airport would have less ground temperature increase than the parking lot and roof since part of the solar radiation absorbed in the airport is redistributed as latent heat flux. Furthermore, parking lot and roof surface albedo differs from vegetation- covered airport, as shown in Table 1, and results in UHI (Jin et al. 2005). Via evapotranspiration, soil moisture affects atmospheric humidity, another parameter important for HVAC control on building environment. Urban regions have less soil moisture for evaporation, a natural physical process that cools down the surface [Zhao et al. 2013]. Dickinson [1992] concluded that “presence or absence of vegetation is significant”, which can be revealed through the diurnal temperature and humidity variations between urban and rural surfaces. Humidity affects electricity use similarly to how outdoor temperatures do. The specific heat capacity of water, as expressed by Perlman, is defined as “water has to absorb 4.184 Joules of heat for the temperature of one gram of water to increase 1 degree Celsius (°C).” Therefore, it takes electricity to make the air drier just as ground water needs absorption of solar radiation to evaporate. According to Byrd Heating and Air Conditioning, “air conditioners cool homes by removing heat and moisture from the air. When humidity levels are excessive, they need to work a lot harder.” As HVAC systems work through high humidity levels, more electricity is needed to power moisture off the room and cool a building. Nevertheless, due to the limited availability of humidity data, this study only studies the air temperature effect on building electricity use. This study compares 2-meter surface air temperatures measured from various urban surfaces with that in a local airport, College Park, MD. Temperature heterogeneity throughout a small city like E Author Į ı : Oceanic Science, University of Maryland – College Park, MD. e-mail: mjin1@umd.edu city which is caused intensity of urban heat island.” If the © 2019 Global Journals Global Journal of Researches in Engineering ( ) Volume XIxX Issue II Version I 1 Year 2019 J