- Email: [email protected]
Urban Cooling Potential: System Losses from Microclimates
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 78 (2015) 3072 – 3077
6th International Building Physics Conference, IBPC 2015
Urban Cooling Potential: System Losses from Microclimates Forrest Meggersa,b*, Gideon Aschwandenc, Eric Teitelbauma, Hongshan Guoa, Marcel Bruelisauerd a Princeton University, School of Architecture, Princeton, NJ, 08544, USA Princeton University, Andlinger Center for Energy and the Environment, USA c The University of Melbourne, Melbourne School of Design, Melbourne, Austrailia d Future Cities Laboratory, Singapore-ETH Centre, Department of Architecture, ETH Zürich, Zürich, Switzerland b
Abstract Temperatures in the cities are amplified through the urban heat island effect by an additional 2-4 °C. The trapping of solar energy in urban canyons and infrastructural plays the most significant role. But use of air conditioning for cooing during summer months is very prevalent in large cities such as New York, and it converts even more energy (electricity) into heat that is rejected into the external environment. The temperature of the environment actually directly controls the efficiency of the common refrigeration cycle found in air conditioning systems by the second law of thermodynamics. A city, with its complex topography of urban canyons and skyscrapers, produces small microclimates with varying temperatures. This project investigates three urban settings that create microclimates that are detrimental for the efficiency of cooling in New York. First, the overall urban heat island effect, second the effect of roof temperature on rooftop package air conditioning units, and third, the impact of local heat emission from agglomerations of window air conditioners. The efficiency loss is investigated by considering the range of temperature changes that can be observed in the surrounding environment of air conditioning system, and determining the subsequent impact on the Coefficient of Performance (COP). Our COP analyses indicate a range of potential energy increases of around 10% to 70% due to increases in environmental temperature around air conditioners. An analysis of the building stock of New York City showed that the electrical energy demand is potentially increased by these effects by 500 TJ from UHI, 75 TJ from rooftop package units, and 370 TJ from window units. © 2015 2015Published The Authors. Published by Elsevier Ltd. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL. Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL Keywords: COP, energy efficiency, cooling, city
* Corresponding author. Tel.: +1-609-258-3178; fax: +1-609-258-4740. E-mail address: [email protected]
1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL doi:10.1016/j.egypro.2015.11.759
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1. Introduction This paper investigates the impact of high-density urban situations on the energy efficiency of air condition units. The focus lies on three factors that change the environment in which the heat exchanger of the air conditioner is working. The first is the UHI effect that raises the local temperature for all units. The second is the surface material in the immediate surrounding of the units raising the temperature locally. The last factor is the geometrical distribution of units where the dissipated heat raises the temperature of units in the vicinity or themselves (see Figure 1). Even though these three factors are complimentary this paper investigates the impact of them individually.
Figure 1 Image illustrating the stacking of heat exchange units in the urban canyons of New York City (left), and attached to buildings in Singapore (right)
2. Background 2.1. Urban Heat Island (UHI) The urban heat island effect has been identified as, “high surface and thermal structure heterogeneity, complex localized flow patterns inside street canyons radiative trapping, artificial materials, anthropogenic heat releases, as well as finite domains for heat conduction, instead of the semi-infinite sub-surface domain that can be assumed over natural terrain” [1] It has been identified to be caused by the excessive anthropogenic heat, storage of solar radiation in large thermal masses with insufficient circulation, reducing the ability for infrared radiation to escape atmosphere, causing nighttime temperatures kin urban areas to rise significantly above surrounding conditions [2]. Since the UHI effect is most prominent during the night hours heat exchange units are only affected 1/3 of the time. The absorbed radiation stored in the attached materials and dissipated through radiation [3]. Depending on the thermal storage capability of the surrounding materials the dissipation of the absorbed energy occurs with a delay over hours and leads to the known fact of increased temperature in cities in the night [2]. The adverse impact of urban heat island have been extensively studied and identified to increase energy consumption and peak electricity demand during cooling seasons as well as decreasing the Coefficient of Performance (COP) by as much as 25% [4], eventually deteriorating the outdoor thermal comfort [5]. Strategies on maintaining the urban thermal balance have been developed including increasing the albedo of the urban environment, expanding the urban green spaces and utilizing natural heat sinks within or near the vicinity of the urban area. Such mitigation techniques have been demonstrated to be effective and beneficial for large-scale applications [6]. Of the aforementioned mitigation techniques, some are not as usable as the others: The available free ground in cities, for example, isn’t always as abundant as the designers would like to believe and could render the implementation of large scale mitigation methods ineffective. Similarly, the availability of green spaces with the rapid urbanization process could also lead to problematic strategy implementation. The mitigation methods that are based on roofs, on the other hand, are highly preferable due to its flexibilities and ease of implementation due to the costs, thereby cultivating strategies that aimed at increasing the albedo effect of cool/reflective roofs [7].
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2.2. Surface Materials The local temperature is impacted by the materials in its immediate surroundings in particular their ration of absorption and storage capability. Prado argues that the reflection of the solar radiation depends on the material and can range from 10% to 73% [8]. Choosing appropriate surface materials could also lead to significant increase in energy efficiencies thus regaining the lost potentials of the split units and potentially achieving energy savings. To be more specific, we look at how the materials change the environmental temperature for the rooftop package units that led to a change of performance for them [9]. Our three major areas of interest in this direction are the influence of the reflectivity, thermal capacity and the capabilities of evaporative cooling. According to the research of [10], between the unpainted cement surface and different colored paints, the cool white paint could reduce the heat flux up to 37% with a solar reflectance of 0.78[10]. As with thermal capacity of significant thermal masses, a study by Jim in 2014 revealed that with a higher level of building thermal insulation in contrast to unshielded building blocks could not only cut heat ingress, but could also suppress the cooling load by 33.15% [11]. This coincides with the [12] research in 2010 that high-mass building leads to a low sum of cooling degree-days, but could be improved even further by installing indirect evaporative passive cooling system, with a comparable temperature depression (6.3˚C comparing to 6.1˚C, hence a 3% variation) with that of high-mass building envelope. A simplified estimation can therefore be made as to the impact of choosing appropriate envelope materials has on the performance of window air conditioning units and can be calculated by assuming the same cool white paint used on a building envelope. By using medium-mass materials with an indirect evaporative cooling system such as the one developed by Gonzalez et al. in 2000, the VBP-1, an overall reduction of reduced heat flux up to 55% for extreme cases compared to that of bare concrete with minimum thermal mass and no evaporative cooling could be achieved. These effects lead to the introduction of the term SUHI (Surface Urban Heat Island). Observations in Minneapolis quantified the difference between the surface temperature in the urban areas and the surrounding rural areas to 24 K [9]. Wong measured temperature above different materials with respect and found that the air 1m above hard surfaces is up to 30 K higher than above vegetation [13]. 2.3. Stack Effect A primary culprit in the increased energy demand due to UHI effect could also be the stacking of condensing units for window air conditioners leading to decreased COPs. As was identified by previous studies [14], the stacking of the condensing units could lead to localized urban heat island effect resulting in the increase of local temperature at 2-4 K at 40 stories, more recent researches have found that such setup could lead to decreased COP of the air conditioning units, could experience a 40% decline of COP at 25 floor level due to the ‘stuck and stack’ effect identified by Bruelisauer et al. [15]. They were able to show that the local microclimate generated by air conditioner heat rejection differs from the surrounding environment by up to 13 °C from the stacking of units, and in the case of a confined spaces designed to hide unsightly mechanical units, an additional 7 °C are is contributed by the effect of the unit being “stuck.” This work indicates that local microclimatic variances have an even higher effect than the overall urban heat island. The changes to the outside temperature depend both the amount of energy dissipated into the environment and the size of the heat sink that absorbs the energy. In most cases the assumption is that the heat sink is the surrounding air that constantly moves and can carry the emitted energy away. This is not the case in the urban environment where distances between buildings are limited, heat exchangers are hidden behind blinds, and the urban canyon creates eddies that accumulate the dissipated heat. In all of these cases there are impacts on efficiency. 3. Method This part introduces the calculation methods used to evaluate the energy efficiency for cooling in the urban setting by evaluating the ‘coefficient of performance’ (COP) for the air condition system and its dependency on the
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temperature at which heat is rejected from the system. The ambient temperature is impacted by the urban heat island and microclimatic effects discussed above. 3.1. COP Calculation The COP is calculated according to Equation 1. The formula indicates that performance decreases with an increase in the temperature difference of ܶ௧ െ ܶௗ . Since we are limiting our investigation to the use cooling for thermal comfort ܶௗ can be regarded as constant (selected for an indoor temperature 20 °C with an evaporator temperature of 10 °C). This leaves the major impact on the performance on the outside temperature ܶ௧ . We used Equation 1 to calculate and plot the impact of the three factors on the air conditioner efficiency. ܱܲܥ ൌ
் ் ି்
(1)
3.2. UHI Calculation Since the UHI effect is most prominent during the night hours, the air conditioner heat exchange units are estimated to be affected 1/3 of the time or 8 hours of the day [2]. Economic data from NYCEDC (New York City Economic Development Corp) show the overall energy used in NYC to be 1000 million MMBtu (1 EJ), of which cooling is responsible for 9%, for a total of 27 TWh (90 PJ) as the total cooling electricity demand for NYC [16]. Materials and delayed dissipation are a major contributor of night temperature rises as observed in Tokyo by 8 °C, in Seoul by 4.5 °C, Bangkok by 5 °C [17]. New York City is experiencing a UHI of around ~7.2˚F (~4˚C) [18]. Taking into account that the UHI increases the temperature between 2K and 8k the COP can be calculated and the increase in electricity respectively, and the New York City impact was estimated for the 4 °C level. 3.3. Roof top effect The calculation of rooftop packaged air conditioning units’ contribution to electricity consumption was based on the number of commercial buildings in the city, the percentage of commercial buildings that are cooled, the percent of cooled commercial buildings that use rooftop package air conditioning units, the average square footage of office buildings, and the energy consumption per square foot of package units. Data for office buildings in the northeast provided by CBECS were used to determine that 41.1% of office buildings in the northeast use packaged air conditioners, giving us a total of 2494 office buildings for calculation, and additionally, data for Mid-Atlantic office building square footage from CBECS were used to arrive at an average of 20,000 ft 2 per office building [19]. This was used as a lower bound, and data for Manhattan office space of 100,000 ft2 per office building were used as an upper bound in calculations. A value of 6 kWh/ft-2 was used as the electricity consumption of packaged units to calculate the overall electricity consumption of all packaged units in NYC. The COP impact was estimated from the work of Bourbia that showed surface temperature rise of up to 25 K [20]. 3.4. Stack effect To estimate the scale of stack effect influences to overall consumption, the individual energy consumption of window air conditioning units and total number of these units had to be determined. With these numbers, the annual electricity consumption of al window air conditioning units in NYC could be extrapolated. Data was extracted from Urban Green Council [21] and the Department of Energy, Energy Information Agency, Residential Energy Consumption Survey survey to provide an overall number of 4,000,000 window air conditioning units and 286 hours of operation for a standard 1 ton window unit in NYC, respectively [22].
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4. Results 4.1. COP Calculation Figure 1 shows the impact on COP from our three scenarios. The temperature is a range based on our literature review. For the NYC case, individual temperatures at +4 °C, 15°C and 25 °C were used for the three cases respectively.
Figure 2: Performance of a heat pump as a function of the outside temperature variance
4.2. UHI Resulting Impact Multiplying by a factor 1/3 for fraction of nighttime impact on the systems results in 2500 TJ to be impacted by UHI at night. The increase in the COP of the machine for the range of 2 to 8 K UHI results in an increase in electricity consumption of 5 – 29%. For the New York City case, the 4 °C UHI increases the electricity balance sheet by 500 TJ or almost 1% of the total energy budget of the city. 4.3. Rooftop / Materials: The local temperature increase through different materials is up to 25 K. Using the same calculation as above leads to an increase in electricity demand of 56%. The number of high-rise office buildings in NYC is 6061 out of which 41.14% have a packaged air condition on top of the building, giving us 2494 office buildings. With an average use of 600’000 kWh per building gives us a hidden total of 70 TJ annual saving potential. 4.4. Window / Stack Effect: An increase between a 5 K and 20 K rise in the ambient temperature leads to an increase of electricity consumption between +20% to +50%. Urban green estimates 4 million window units in NYC [21]. With an average of 3.5kW electricity consumption and 286 hours operating per year we get to ~1000 ݄ܹܭȀሺ ݎݕή ݐ݅݊ݑሻ. Not all units are affected by other units below. Assuming 15 K rise for NYC, and that 25% of the units are on the lowest level hence not affected, this leaves 3 million units affected. Taking a median value of 35% increase leads to an increase of electricity consumption of 370 TJ. 5. Conclusion This paper investigated the impact of the three factors, UHI / material / stack effect, on the energy consumption for cooling of the city. The impact of each individual factor on the ambient temperature was investigated first to calculate the impact on the COP. The next step was to estimate the number of units influenced by the effect and calculate the total increase in electricity demand from each factor.
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The influence of each factor is significant for the total energy budget of the city, between 60-500 TJ annually. The stack effect factor, guided by geometry, holds nearly the highest potential and also requires the least impact to change. While the adjustment of surface materials and the mitigation of the UHI requires a lot of financial investment the stack effect requires only the adjustment of the individual location of each heat exchanger in respect to the other heat exchangers. The next steps are to quantify further the effect of the stack effect by measuring different location in the city. This provides a solid foundation for further inquiry. To broaden the impact of this investigation a cost / benefit analysis to find the time until the break even is reached for individual apartments. A cost benefit analysis would help motivate residents to invest and change the location of their heat exchangers and mitigate the energy budget of the city. References [1] [2] [3] [4] [5]
[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Z. H. Wang, E. Bou-Zeid, and J. A. Smith, "A Spatially-Analytical Scheme for Surface Temperatures and Conductive Heat Fluxes in Urban Canopy Models," Boundary-Layer Meteorology, vol. 138, pp. 171-193, Feb 2011. T. R. Oke, "Canyon geometry and the nocturnal urban heat island: comparison of scale model and field observations," Journal of climatology, vol. 1, pp. 237-254, 1981. H. Takebayashi and M. Moriyama, "Surface heat budget on green roof and high reflection roof for mitigation of urban heat island," Building and Environment, vol. 42, pp. 2971-2979, 2007. M. Santamouris, N. Papanikolaou, I. Livada, I. Koronakis, C. Georgakis, A. Argiriou, et al., "On the impact of urban climate on the energy consumption of buildings," Solar energy, vol. 70, pp. 201-216, 2001. G. A. Meehl, T. Karl, D. R. Easterling, S. Changnon, R. Pielke Jr, D. Changnon, et al., "An Introduction to Trends in Extreme Weather and Climate Events: Observations, Socioeconomic Impacts, Terrestrial Ecological Impacts, and Model Projections*," Bulletin of the American Meteorological Society, vol. 81, pp. 413-416, 2000. M. Santamouris, A. Synnefa, and T. Karlessi, "Using advanced cool materials in the urban built environment to mitigate heat islands and improve thermal comfort conditions," Solar Energy, vol. 85, pp. 3085-3102, 2011. M. Zinzi, "Cool materials and cool roofs: Potentialities in Mediterranean buildings," Advances in Building Energy Research, vol. 4, pp. 201-266, 2010. R. T. A. Prado and F. L. Ferreira, "Measurement of albedo and analysis of its influence the surface temperature of building roof materials," Energy and Buildings, vol. 37, pp. 295-300, 2005. F. Yuan and M. E. Bauer, "Comparison of impervious surface area and normalized difference vegetation index as indicators of surface urban heat island effects in Landsat imagery," Remote Sensing of Environment, vol. 106, pp. 375-386, 2007. K. L. Uemoto, N. M. Sato, and V. M. John, "Estimating thermal performance of cool colored paints," Energy and Buildings, vol. 42, pp. 17-22, 2010. C. Jim, "Air-conditioning energy consumption due to green roofs with different building thermal insulation," Applied Energy, vol. 128, pp. 49-59, 2014. E. Krüger, D. Pearlmutter, and F. Rasia, "Evaluating the impact of canyon geometry and orientation on cooling loads in a high-mass building in a hot dry environment," Applied Energy, vol. 87, pp. 2068-2078, 2010. N. H. Wong, Y. Chen, C. L. Ong, and A. Sia, "Investigation of thermal benefits of rooftop garden in the tropical environment," Building and environment, vol. 38, pp. 261-270, 2003. S.-H. Choi, K.-S. Lee, and B.-S. Kim, "Effects of stacked condensers in a high-rise apartment building," Energy, vol. 30, pp. 968-981, 2005. M. Bruelisauer, F. Meggers, E. Saber, C. Li, and H. Leibundgut, "Stuck in a stack—Temperature measurements of the microclimate around split type condensing units in a high rise building in Singapore," Energy and Buildings, vol. 71, pp. 28-37, 2014. J. B. a. A. Moore, "Economic Snapshot: A Summary of New York City's Economy," NYCEDC Economic REsearch & Analysis Center for Economic TransformationJuly, 2013 2013. H. Tran, D. Uchihama, S. Ochi, and Y. Yasuoka, "Assessment with satellite data of the urban heat island effects in Asian mega cities," International Journal of Applied Earth Observation and Geoinformation, vol. 8, pp. 34-48, 2006. C. Rosenzweig, W. Solecki, and R. Slosberg, "Mitigating New York City’s heat island with urban forestry, living roofs, and light surfaces," A report to the New York State Energy Research and Development Authority, 2006. DOE EIA, "Commercial Buildings Energy Consumption Survey," United Sates Department of Energy, Ed., ed, 2003. F. Bourbia and H. Awbi, "Building cluster and shading in urban canyon for hot dry climate: Part 1: Air and surface temperature measurements," Renewable Energy, vol. 29, pp. 249-262, 2004. M. Zuluaga, S. Maxwell, J. Block, and L. Eisenberg, "There are holes in our the wall," Urban Green Council, New York Chapter of the U.S. Green Building Council, 2011. DOE EIA, "Residential Energy Consumption Survey," United States Department of Energy, Ed., ed, 2009.
ScienceDirect Energy Procedia 78 (2015) 3072 – 3077
6th International Building Physics Conference, IBPC 2015
Urban Cooling Potential: System Losses from Microclimates Forrest Meggersa,b*, Gideon Aschwandenc, Eric Teitelbauma, Hongshan Guoa, Marcel Bruelisauerd a Princeton University, School of Architecture, Princeton, NJ, 08544, USA Princeton University, Andlinger Center for Energy and the Environment, USA c The University of Melbourne, Melbourne School of Design, Melbourne, Austrailia d Future Cities Laboratory, Singapore-ETH Centre, Department of Architecture, ETH Zürich, Zürich, Switzerland b
Abstract Temperatures in the cities are amplified through the urban heat island effect by an additional 2-4 °C. The trapping of solar energy in urban canyons and infrastructural plays the most significant role. But use of air conditioning for cooing during summer months is very prevalent in large cities such as New York, and it converts even more energy (electricity) into heat that is rejected into the external environment. The temperature of the environment actually directly controls the efficiency of the common refrigeration cycle found in air conditioning systems by the second law of thermodynamics. A city, with its complex topography of urban canyons and skyscrapers, produces small microclimates with varying temperatures. This project investigates three urban settings that create microclimates that are detrimental for the efficiency of cooling in New York. First, the overall urban heat island effect, second the effect of roof temperature on rooftop package air conditioning units, and third, the impact of local heat emission from agglomerations of window air conditioners. The efficiency loss is investigated by considering the range of temperature changes that can be observed in the surrounding environment of air conditioning system, and determining the subsequent impact on the Coefficient of Performance (COP). Our COP analyses indicate a range of potential energy increases of around 10% to 70% due to increases in environmental temperature around air conditioners. An analysis of the building stock of New York City showed that the electrical energy demand is potentially increased by these effects by 500 TJ from UHI, 75 TJ from rooftop package units, and 370 TJ from window units. © 2015 2015Published The Authors. Published by Elsevier Ltd. © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL. Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL Keywords: COP, energy efficiency, cooling, city
* Corresponding author. Tel.: +1-609-258-3178; fax: +1-609-258-4740. E-mail address: [email protected]
1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL doi:10.1016/j.egypro.2015.11.759
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1. Introduction This paper investigates the impact of high-density urban situations on the energy efficiency of air condition units. The focus lies on three factors that change the environment in which the heat exchanger of the air conditioner is working. The first is the UHI effect that raises the local temperature for all units. The second is the surface material in the immediate surrounding of the units raising the temperature locally. The last factor is the geometrical distribution of units where the dissipated heat raises the temperature of units in the vicinity or themselves (see Figure 1). Even though these three factors are complimentary this paper investigates the impact of them individually.
Figure 1 Image illustrating the stacking of heat exchange units in the urban canyons of New York City (left), and attached to buildings in Singapore (right)
2. Background 2.1. Urban Heat Island (UHI) The urban heat island effect has been identified as, “high surface and thermal structure heterogeneity, complex localized flow patterns inside street canyons radiative trapping, artificial materials, anthropogenic heat releases, as well as finite domains for heat conduction, instead of the semi-infinite sub-surface domain that can be assumed over natural terrain” [1] It has been identified to be caused by the excessive anthropogenic heat, storage of solar radiation in large thermal masses with insufficient circulation, reducing the ability for infrared radiation to escape atmosphere, causing nighttime temperatures kin urban areas to rise significantly above surrounding conditions [2]. Since the UHI effect is most prominent during the night hours heat exchange units are only affected 1/3 of the time. The absorbed radiation stored in the attached materials and dissipated through radiation [3]. Depending on the thermal storage capability of the surrounding materials the dissipation of the absorbed energy occurs with a delay over hours and leads to the known fact of increased temperature in cities in the night [2]. The adverse impact of urban heat island have been extensively studied and identified to increase energy consumption and peak electricity demand during cooling seasons as well as decreasing the Coefficient of Performance (COP) by as much as 25% [4], eventually deteriorating the outdoor thermal comfort [5]. Strategies on maintaining the urban thermal balance have been developed including increasing the albedo of the urban environment, expanding the urban green spaces and utilizing natural heat sinks within or near the vicinity of the urban area. Such mitigation techniques have been demonstrated to be effective and beneficial for large-scale applications [6]. Of the aforementioned mitigation techniques, some are not as usable as the others: The available free ground in cities, for example, isn’t always as abundant as the designers would like to believe and could render the implementation of large scale mitigation methods ineffective. Similarly, the availability of green spaces with the rapid urbanization process could also lead to problematic strategy implementation. The mitigation methods that are based on roofs, on the other hand, are highly preferable due to its flexibilities and ease of implementation due to the costs, thereby cultivating strategies that aimed at increasing the albedo effect of cool/reflective roofs [7].
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2.2. Surface Materials The local temperature is impacted by the materials in its immediate surroundings in particular their ration of absorption and storage capability. Prado argues that the reflection of the solar radiation depends on the material and can range from 10% to 73% [8]. Choosing appropriate surface materials could also lead to significant increase in energy efficiencies thus regaining the lost potentials of the split units and potentially achieving energy savings. To be more specific, we look at how the materials change the environmental temperature for the rooftop package units that led to a change of performance for them [9]. Our three major areas of interest in this direction are the influence of the reflectivity, thermal capacity and the capabilities of evaporative cooling. According to the research of [10], between the unpainted cement surface and different colored paints, the cool white paint could reduce the heat flux up to 37% with a solar reflectance of 0.78[10]. As with thermal capacity of significant thermal masses, a study by Jim in 2014 revealed that with a higher level of building thermal insulation in contrast to unshielded building blocks could not only cut heat ingress, but could also suppress the cooling load by 33.15% [11]. This coincides with the [12] research in 2010 that high-mass building leads to a low sum of cooling degree-days, but could be improved even further by installing indirect evaporative passive cooling system, with a comparable temperature depression (6.3˚C comparing to 6.1˚C, hence a 3% variation) with that of high-mass building envelope. A simplified estimation can therefore be made as to the impact of choosing appropriate envelope materials has on the performance of window air conditioning units and can be calculated by assuming the same cool white paint used on a building envelope. By using medium-mass materials with an indirect evaporative cooling system such as the one developed by Gonzalez et al. in 2000, the VBP-1, an overall reduction of reduced heat flux up to 55% for extreme cases compared to that of bare concrete with minimum thermal mass and no evaporative cooling could be achieved. These effects lead to the introduction of the term SUHI (Surface Urban Heat Island). Observations in Minneapolis quantified the difference between the surface temperature in the urban areas and the surrounding rural areas to 24 K [9]. Wong measured temperature above different materials with respect and found that the air 1m above hard surfaces is up to 30 K higher than above vegetation [13]. 2.3. Stack Effect A primary culprit in the increased energy demand due to UHI effect could also be the stacking of condensing units for window air conditioners leading to decreased COPs. As was identified by previous studies [14], the stacking of the condensing units could lead to localized urban heat island effect resulting in the increase of local temperature at 2-4 K at 40 stories, more recent researches have found that such setup could lead to decreased COP of the air conditioning units, could experience a 40% decline of COP at 25 floor level due to the ‘stuck and stack’ effect identified by Bruelisauer et al. [15]. They were able to show that the local microclimate generated by air conditioner heat rejection differs from the surrounding environment by up to 13 °C from the stacking of units, and in the case of a confined spaces designed to hide unsightly mechanical units, an additional 7 °C are is contributed by the effect of the unit being “stuck.” This work indicates that local microclimatic variances have an even higher effect than the overall urban heat island. The changes to the outside temperature depend both the amount of energy dissipated into the environment and the size of the heat sink that absorbs the energy. In most cases the assumption is that the heat sink is the surrounding air that constantly moves and can carry the emitted energy away. This is not the case in the urban environment where distances between buildings are limited, heat exchangers are hidden behind blinds, and the urban canyon creates eddies that accumulate the dissipated heat. In all of these cases there are impacts on efficiency. 3. Method This part introduces the calculation methods used to evaluate the energy efficiency for cooling in the urban setting by evaluating the ‘coefficient of performance’ (COP) for the air condition system and its dependency on the
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temperature at which heat is rejected from the system. The ambient temperature is impacted by the urban heat island and microclimatic effects discussed above. 3.1. COP Calculation The COP is calculated according to Equation 1. The formula indicates that performance decreases with an increase in the temperature difference of ܶ௧ െ ܶௗ . Since we are limiting our investigation to the use cooling for thermal comfort ܶௗ can be regarded as constant (selected for an indoor temperature 20 °C with an evaporator temperature of 10 °C). This leaves the major impact on the performance on the outside temperature ܶ௧ . We used Equation 1 to calculate and plot the impact of the three factors on the air conditioner efficiency. ܱܲܥ ൌ
் ் ି்
(1)
3.2. UHI Calculation Since the UHI effect is most prominent during the night hours, the air conditioner heat exchange units are estimated to be affected 1/3 of the time or 8 hours of the day [2]. Economic data from NYCEDC (New York City Economic Development Corp) show the overall energy used in NYC to be 1000 million MMBtu (1 EJ), of which cooling is responsible for 9%, for a total of 27 TWh (90 PJ) as the total cooling electricity demand for NYC [16]. Materials and delayed dissipation are a major contributor of night temperature rises as observed in Tokyo by 8 °C, in Seoul by 4.5 °C, Bangkok by 5 °C [17]. New York City is experiencing a UHI of around ~7.2˚F (~4˚C) [18]. Taking into account that the UHI increases the temperature between 2K and 8k the COP can be calculated and the increase in electricity respectively, and the New York City impact was estimated for the 4 °C level. 3.3. Roof top effect The calculation of rooftop packaged air conditioning units’ contribution to electricity consumption was based on the number of commercial buildings in the city, the percentage of commercial buildings that are cooled, the percent of cooled commercial buildings that use rooftop package air conditioning units, the average square footage of office buildings, and the energy consumption per square foot of package units. Data for office buildings in the northeast provided by CBECS were used to determine that 41.1% of office buildings in the northeast use packaged air conditioners, giving us a total of 2494 office buildings for calculation, and additionally, data for Mid-Atlantic office building square footage from CBECS were used to arrive at an average of 20,000 ft 2 per office building [19]. This was used as a lower bound, and data for Manhattan office space of 100,000 ft2 per office building were used as an upper bound in calculations. A value of 6 kWh/ft-2 was used as the electricity consumption of packaged units to calculate the overall electricity consumption of all packaged units in NYC. The COP impact was estimated from the work of Bourbia that showed surface temperature rise of up to 25 K [20]. 3.4. Stack effect To estimate the scale of stack effect influences to overall consumption, the individual energy consumption of window air conditioning units and total number of these units had to be determined. With these numbers, the annual electricity consumption of al window air conditioning units in NYC could be extrapolated. Data was extracted from Urban Green Council [21] and the Department of Energy, Energy Information Agency, Residential Energy Consumption Survey survey to provide an overall number of 4,000,000 window air conditioning units and 286 hours of operation for a standard 1 ton window unit in NYC, respectively [22].
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4. Results 4.1. COP Calculation Figure 1 shows the impact on COP from our three scenarios. The temperature is a range based on our literature review. For the NYC case, individual temperatures at +4 °C, 15°C and 25 °C were used for the three cases respectively.
Figure 2: Performance of a heat pump as a function of the outside temperature variance
4.2. UHI Resulting Impact Multiplying by a factor 1/3 for fraction of nighttime impact on the systems results in 2500 TJ to be impacted by UHI at night. The increase in the COP of the machine for the range of 2 to 8 K UHI results in an increase in electricity consumption of 5 – 29%. For the New York City case, the 4 °C UHI increases the electricity balance sheet by 500 TJ or almost 1% of the total energy budget of the city. 4.3. Rooftop / Materials: The local temperature increase through different materials is up to 25 K. Using the same calculation as above leads to an increase in electricity demand of 56%. The number of high-rise office buildings in NYC is 6061 out of which 41.14% have a packaged air condition on top of the building, giving us 2494 office buildings. With an average use of 600’000 kWh per building gives us a hidden total of 70 TJ annual saving potential. 4.4. Window / Stack Effect: An increase between a 5 K and 20 K rise in the ambient temperature leads to an increase of electricity consumption between +20% to +50%. Urban green estimates 4 million window units in NYC [21]. With an average of 3.5kW electricity consumption and 286 hours operating per year we get to ~1000 ݄ܹܭȀሺ ݎݕή ݐ݅݊ݑሻ. Not all units are affected by other units below. Assuming 15 K rise for NYC, and that 25% of the units are on the lowest level hence not affected, this leaves 3 million units affected. Taking a median value of 35% increase leads to an increase of electricity consumption of 370 TJ. 5. Conclusion This paper investigated the impact of the three factors, UHI / material / stack effect, on the energy consumption for cooling of the city. The impact of each individual factor on the ambient temperature was investigated first to calculate the impact on the COP. The next step was to estimate the number of units influenced by the effect and calculate the total increase in electricity demand from each factor.
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The influence of each factor is significant for the total energy budget of the city, between 60-500 TJ annually. The stack effect factor, guided by geometry, holds nearly the highest potential and also requires the least impact to change. While the adjustment of surface materials and the mitigation of the UHI requires a lot of financial investment the stack effect requires only the adjustment of the individual location of each heat exchanger in respect to the other heat exchangers. The next steps are to quantify further the effect of the stack effect by measuring different location in the city. This provides a solid foundation for further inquiry. To broaden the impact of this investigation a cost / benefit analysis to find the time until the break even is reached for individual apartments. A cost benefit analysis would help motivate residents to invest and change the location of their heat exchangers and mitigate the energy budget of the city. References [1] [2] [3] [4] [5]
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