The effect of urban evaporation on building energy demand in an arid environment

1
The effect of urban evaporation on building
energy demand in an arid environment

• E.L. Krüger, D. Pearlmutter
• Presented by Kassandra Marshall and Jason
  OGrady
• 5 June 2009

2
Objective

• To demonstrate that in urban settings vegetated
  areas and pools can effectively reduce the
  outdoor temperature through evapotranspiration,
  thus reducing the cooling demand in adjacent
  buildings.

3
Outline of Research

• Previous research only considered dry conditions,
  with negligible latent heat
• Vegetated areas vary significantly within urban
  areas and are difficult to model
• Scaled model needed to be implemented to
  incorporate the effects of moisture on outdoor
  temperature changes, as well as vapor pressure
  and humidity within the urban canopy
• From the model, ambient parameters could be
  implemented by IDA Indoor Climate and Energy
  thermal simulation software to gauge the cooling
  demand of a reference building in the area

4
Scaled Model

• Sede-Boqer Campus of Ben-Gurion University in
  Negev Highlands region of Southern Israel
• Strong daily thermal fluctuations, dry air, clear
  skies, intense solar radiation
• Summer winds come from northwest and are
  consistently strong in late afternoon and
  evenings
• Scaled buildings were constructed from
  0.2 x 0.2 x 0.4 m hollow concrete
  masonry blocks.
• Blocks and soil had similar thermal and optical
  properties to local building materials
• Evaporation pans (2.0 x 0.1 x 0.3 m) were added
  to sides of buildings to simulate pools and
  irrigated vegetation.

5
Scaled Model

• Evaporation rates of pans correlated to Class-A
  pans used in external research by Brutsaeft. A
  pan coefficient is used to relate the pan
  evaporation to vegetative transpiration.
• Three water coverage ratios observed AW/AH 0
  (no pans), AW/AH 0.1 (one pan), and AW/AH 0.2
  (two pans).
• Two street canyon aspect ratios observed H/W
  .33 (one story) and H/W .66 (two stories).
• Temperature measurements were taken at head
  height (0.1 m) and above urban canopy (0.9 m) as
  a reference (expected to stay constant).

6
Reference Building

• Student dormitory close to site of scaled model
  used as a reference building
• Aspect ratio (H/W) approximately 0.6
• Air temperatures measured throughout and outside
  of building, in winter, spring and summer
• Daily activity of building, such as operation of
  windows, shutters, and doors, was monitored
• Building material properties were obtained to be
  used with software
• The window-to-wall ratio was measured for each
  side of the building
• IDA Indoor Climate and Energy thermal simulation
  software used to simulate occupation of the
  building
• A chilled beam-type cooling device was used to
  measure the cooling demand, and was controlled by
  the building tenants to keep the building at an
  average comfort temperature of about 25.5 deg C

7
Reference Building
8
Temperature Model

• Pearlmutter et al. (2007) used semi-empirical
  model to predict temperature within street
  canyons (Ta) based on temperature above the urban
  canopy (TA)
• The coefficients were found empirically to
  predict temperatures in the absence of surface
  moisture

9
Temperature Model

• Model is accurate at dry conditions, in the
  absence of evaporation pans (AW/AH 0)
• For wetted conditions, coverage ratio equal to
  0.1 or 0.2, the model becomes inaccurate by 1 5
  C
• New coefficients were determined for wetted
  conditions by similar means

10
Vapor Pressure and Humidity Models

• Water vapor concentration was measured with
  Li-Cor infrared gas analyzer and was compared to
  measurements from nearby meteorological station
• A model was developed for vapor pressure based on
  the vapor pressure measured at meteorological
  station (VPMET), the hourly global solar
  radiation (IMET), and four coefficients which
  were found empirically for each mode
• Relative humidity was computed using the
  predicted vapor pressure and air temperatures

11
Results

• The software, after inputting calculated ambient
  estimations, computed the cooling demand for each
  scenario (i.e. building height, coverage ratio,
  wind direction)
• Two orientations were considered the building
  face with the highest window-to-wall ratio facing
  the northwesterly wind (windward), and same
  side facing away from the wind (leeward)
• Windward orientation allowed for lower cooling
  loads largely due to ventilation

12
Results

• There were significant decreases in the cooling
  loads as evaporation pans were added
• A one-story building had a more dramatic decrease
  than a two-story when water pans were added
• The cooling demand fraction was plotted against
  the complete evaporating fraction, the area of
  water over the complete 3D area
• The benefit to increasing the complete
  evaporating fraction tends to eventually diminish

13
Conclusions

• Temperature prediction model developed for dry
  conditions is applicable to quantify street
  evaporation if new coefficients are implemented
• Outdoor cooling in urban area is a direct
  function of the availability of moisture in the
  environment, with respect to the urban surface
  area
• A correlation can be determined between wetted
  surfaces (evaporation pans) and actual vegetated
  areas to estimate cooling in an urban setting
• Results show the importance of accounting for
  urban density when establishing vegetative
  surfaces for cooling purposes, expressed by
  aspect ratio of street canyons
• Air temperature is reduced outdoors by
  evaporative/vegetative areas, which moderates the
  cooling load needed to meet indoor comfort
  temperatures