URBAN HEAT ISLAND MITIGATION: LOOKING BEYOND POLICIES TO LOWER AIR TEMPERATURE

Transcription

1 Cities and Climate Change Conference March 5 7, 2018 Edmonton, Alberta, Canada URBAN HEAT ISLAND MITIGATION: LOOKING BEYOND POLICIES TO LOWER AIR TEMPERATURE Evyatar Erell Ben Gurion University of the Negev Israel erell@bgu.ac.il

2 Another look at some UHI mitigation strategies U.S. EPA, 2008 THE OBJECTIVE What works best for reducing the hazards of climate extremes such as heat waves in the cities we work in? Cool roofs High albedo green Cool paving High albedo Porous + water evaporation Vegetation Parks Localized trees Surface cover (grass, cover plants) Shading 2

3 Strategies assessed 3 1. Vegetation Shashua Bar L., Pearlmutter D. and Erell E. (2009). "The cooling efficiency of urban landscape strategies in a hot dry climate". Landscape and Urban Planning, 92(3 4): Shashua Bar L., Pearlmutter D. and Erell E. (2011). "The influence of trees and grass on outdoor thermal comfort in a hot arid environment". International Journal of Climatology, 31(10): Albedo Erell E., Boneh D., Bar (Kutiel) P. and Pearlmutter D. (2014) Effect of high albedo materials on pedestrian thermal stress in urban street canyons. Urban Climate, 10: Building height and canyon aspect ratio Erell E. and Kalman Y. (2015). "Impact of increasing the depth of urban street canyons on building heating and cooling loads in Tel Aviv, Israel". ICUC9 The 9th International Conference on Urban Climate, Toulouse, France, July

4 Vegetation and thermal stress (i) 4 A semi controlled experiment was carried out in two adjacent courtyard spaces at Sde Boqer, Israel. The courts are similar in their geometry and material attributes, except for the presence of trees in one of them. WEST EAST

5 Vegetation and thermal stress (ii) it s the cool surface and reduced IR flux 5 Exposed-Bare Exposed-Grass Index of Thermal Stress (ITS) is the equivalent latent heat of sweat evaporation required for the body to maintain thermal equilibrium under warm environmental conditions. (Givoni, 1963; Pearlmutter et al, 2007) Grass did NOT cool the air, but still reduced thermal stress

6 Vegetation and thermal stress (iii) it s the shade 6 Mesh-Bare Exposed-Bare Trees-Bare Exposed-Grass Mesh-Grass Trees and fabric shade had almost identical effect on thermal stress Trees-Grass

7 Albedo and thermal stress (i) 7 The Canyon Air Temperature (CAT) model: T_base = T_base Simulates urban effects on temperature, humidity and wind in a street canyon, accounting for: T_urb T_met Building geometry T_urb T_met Anthropogenic heat Advected moisture Generates climate data for a typical meteorological year (TMY) at an urban site, based on measured data from a reference station (rural, airport).

8 Albedo and thermal stress (ii) CAT model validation: Adelaide 8 CAT simulation results for May 2000

9 Albedo and thermal stress (iii) 9 Canyon environment in Adelaide simulated by CAT on hot summer day used as input for calculating ITS for surfaces with different albedos (0.2, 0.45, 0.7): open space (H/W=0.1) N S canyon (H/W=2) heat stress (W) heat stress (W) time of day time of day High albedo cooled the air but increased (simulated) thermal stress, rather than reducing it!

10 Albedo and thermal stress (iv) external fluxes (W/m2) SW reflected SW sun LW terrestrial LW sky convection *External (incoming) fluxes do not include LW radiation given off by the person or latent heat loss by sweat. surface albedo N S canyon, H/W=2, at noon on typical summer day, Adelaide Increased SW reflection offsets effect of high albedo on surface temperature and IR emission

11 Building height and canyon aspect ratio (i) 11 We know that the aspect ratio (H/W) of streets affects microclimate in cities, especially the urban heat island, but also wind speed. QUESTIONS: How will addition of several floors to existing buildings affect : a. building energy consumption b. thermal comfort in non conditioned apartments 2 story buildings on a 15m wide street 8 story buildings on a 15m wide street N. America Maximum intensity of the nocturnal UHI (Oke, 1987) o Europe + Australasia

12 Building height and canyon aspect ratio (ii) 12 T canyon (K) Simulated temperature difference: Tel Aviv Bet Dagan 8 stories 5 stories 2 stories time of day (hours) Intra-urban temperature variation due to geometry is largely a nocturnal phenomenon. met stn wind speed (m/s) Simulated wind speed at 1.5 m height in streets with different building heights met stn. 2 stories 5 stories 8 stories time of day canyon wind speed (m/s) Height increase from 2 to 5 floors reduces wind speed at street level substantially.

13 Building height and canyon aspect ratio (v) No AC? Night ventilation The climate cooling potential (Artmann et al., 2007) is the difference between the hourly indoor air temperature, approximated by a sine curve, and concurrent outdoor air temperature, summed over the cooling period. reference 13 T bh : 24.5 o C T crit : 2 o C by the sea 8 story buildings Adding floors increased air temperature and eliminated night cooling in non AC buildings

14 Building height and canyon aspect ratio (iii) 14 EnergyPlus building thermal simulation (using ENERGYui interface developed for Israel standard) Occupancy, internal loads and HVAC setpoints (20 o C winter, 24 o C summer) are fixed Shading by adjacent buildings of equal height modeled by adding nonconditioned extensions to the building Add floors, then generate new urbanized.epw input with CAT model, repeat To isolate urban effects, added floors are assumed to be identical to existing ones N S NW SW NE SE

15 Simulated Annual Energy Demand (kwh/sq.m) Building height and canyon aspect ratio (iv) Heating and cooling loads North South 1 story North South 3 North South 5 story stories Number of floors Heating Cooling North South 7 stories Average annual HVAC load in 7 stories building [kwh/m 2 ] 1 st floor 16.6 Middle floors 23.5 Top floor Adding floors increased air temperature but reduced average (simulated) HVAC loads

16 Takeaways 16 Air temperature is a good headline indicator of weather conditions but when used as the primary tool for assessing the impact of proposed interventions, it might lead to inaccurate or even erroneous conclusions. More useful benefits of UHI mitigation studies may include recommendations for strategies based on their contribution to practical benefits: a. Improving human thermal comfort (esp. in outdoor spaces) response to impact of climate change b. Conserving energy in buildings to reduce GHG emissions and combat climate change THANK YOU!

17 References 17 Artmann N, Manz H and Heiselberg P: (2007). "Climatic potential for passive cooling of buildings by night time ventilation in Europe", Applied Energy, 84: Erell E. and Kalman Y. (2015). "Impact of increasing the depth of urban street canyons on building heating and cooling loads in Tel Aviv, Israel". ICUC9 The 9th International Conference on Urban Climate, Toulouse, France, July Erell E. and Williamson T. (2006) Simulating air temperature in an urban street canyon in all weather conditions using measured data at a reference meteorological station, International Journal of Climatology, 26(12): Erell E., Boneh D., Bar (Kutiel) P. and Pearlmutter D. (2014) Effect of high albedo materials on pedestrian thermal stress in urban street canyons. Urban Climate, 10: Givoni B (1963). Estimation of the effect of climate on man development of a new thermal index. PhD thesis, Technion Israel Institute of Technology. Oke, T. R. (1987). Boundary Layer Climates. London & New York, Methuen. Pearlmutter D., Shaviv E. and Berliner P. (2007). "Integrated modeling of pedestrian energy exchange and thermal comfort in urban street canyons", Building & Environment, 42: Shashua Bar L., Pearlmutter D. and Erell E. (2009). "The cooling efficiency of urban landscape strategies in a hot dry climate". Landscape and Urban Planning, 92(3 4): Shashua Bar L., Pearlmutter D. and Erell E. (2011). "The influence of trees and grass on outdoor thermal comfort in a hot arid environment". International Journal of Climatology, 31(10):