Enhancing Energy Efficiency of Built Environment through Daylighting
Ir. Yiu-chung WU
Senior Building Services Engineer
The Government of the Hong Kong SAR
Dr. Danny H.W. LI
City University of Hong Kong
Enhancing Energy Efficiency of Built Environment through Daylighting
Daylight through Light Transportation System Daylight through Window
Light Transportation Systems
Sun Pipe System Hybrid Solar Lighting System
Sun Pipe System
Design Concept Tubular skylight with reflective
pipe Collects daylight on rooftop Guides daylight down Diffuses daylight into the building
interior
Sun Pipe System
Components
Domeconsists of clear polycarbonate
curved reflector to capture daylight and light intercepting transfer device to redirect daylight
increases the daylight collection and harvest for low sun angles
Sun Pipe System
Components
Pipeconsists of a very high reflective
internal finish with 98% to 99.7% reflectance
straight run or with elbow to convey daylight
Sun Pipe System
Components
Ceiling diffuser
Sun Pipe System
Performance
Depends on location climate weather
Sun Pipe SystemPerformanceLimitationin vertical length
• 530mm – 13m• 350mm – 10m• 250mm – 6.5m
horizontal run• not recommended
one dome supplies one diffuser point only
Sun Pipe System
Case Study
Electrical and Mechanical Services Department Headquarters 10 nos. 250mm sun pipe at a corridor on the top floor artificial lights (CFT) to back up
Sun Pipe System
Case Study
Diffusers
Sun Pipe System
Case Study
Domes
Sun Pipe System
Case Study
Diffuser of
sun pipe
(right)
Artificial
light (left)
Sun Pipe System
Case Study
Results of half months’ measurements in December 2005
Time Indoor Lux Outdoor Lux Weather Condition
09:00-10:00 80-126 8936-16467 Foggy to sunny
10:00-11:00 270-285 >100000 Foggy to sunny
11:00-13:00 294-321 >100000 Foggy to sunny
13:00-15:00 101-243 16000-32048 Very foggy to sunny
15:00-17:00 22-85 2313-12801 Foggy to sunny
Sun Pipe System
Observation and Discussion To be installed with artificial lighting with photo sensor Can direct daylight to more interior areas; and can
provide more stable and uniform artificial daylight than skylight
Recommended for obvious energy and environmental benefits
Initial cost is not very attractive and competitive at present; but expected to be lower after further development and advancement of material and equipment
Hybrid Solar Lighting System
Design Concept and Case Study
Sacramento Municipal Utility District
1200mm diameter roof-mounted solar collector to concentrate daylight into a bundle of optical fibres
Hybrid Solar Lighting System
Design Concept and Case Study
Infra-red and ultra-violet filter to filter out unwanted heat to the interior which can further save energy for cooling
Hybrid Solar Lighting System
Design Concept and Case Study Automatic sunlight tracking
system to rotate the collector to collect maximum sunlight
Hybrid Solar Lighting System
Design Concept and Case Study The optical fibres are very flexi
ble and configurable, hence
• require a small penetration through the roof
• allow light transmitted around and through complex building environment with bents of any angles
Hybrid Solar Lighting System
Design Concept and Case Study
A bundle of 127 nos. optical fibres connecting the solar collector to the hybrid lighting fittings with a running length of 20m
Hybrid Solar Lighting System
Design Concept and Case StudyHybrid lighting fittings connected 3 nos. fluorescent fitting each
with 3 nos. 1200mm T8 tube and 2 nos. emitting rods each connected with 15 nos. of optical fibres
3 nos. 20W halogen spot light and 3 nos. incandescent lamp bulb each connected to 4 nos. of optical fibres
Hybrid Solar Lighting SystemDesign Concept and Case Study On a sunny day, the system can deliver 50,000
lumens (equivalent to 55 nos. of 60W incandescent lamps)
Daylight through Light Transportation System
Conclusion Various light transportation systems are available
in the market which can transmit daylight effectively and efficiently
Two most mature systems are discussed, both of them have their pros and cons
Designers are encouraged to adopt the kind of system that suits their application to best use of daylight to achieve energy saving
Daylight through Window
Daylight is the best light source in the form of renewable energy
Lighting control by photo sensors with dimmers becomes mature
Proper lighting control integrated with daylight can reduce artificial lighting energy effectively
Prediction of indoor daylight illuminance is critical in the design of lighting control to achieve most optimal energy saving
Simplified Daylight Illuminance Prediction Method Prediction of daylight illuminance is very complica
ted Simulation computer software tools available are to
o sophisticated and time-consuming to run A simple software tool would be very useful especi
ally during preliminary design stage with different conceptual design schemes are being considered
Importance of Daylighting
Daylighting is an effective approach to have a more flexible building façade design strategy
Enhance a more energy-efficient building design (always an energy saver)
Provide visual comfort Greener building development People desire good natural lighting in their livin
g and working environments
Estimation of daylight illuminance
A key step in evaluating daylighting performance
Once the daylight illuminance obtained, it is quite straightforward to compute the lighting energy savings
The presentation mainly for the indoor daylight illuminance estimation
Prediction method
Traditional daylight factor approachBased on CIE standard overcast sky onlySimpleNot flexible :Cannot cater for the dynamic
variation of daylight illuminance for different solar positions under non-overcast sky conditions
The CIE Standard Skies
In 2003, the International Commission on Illumination (CIE) has adopted 15 standard skies
Containing 5 clear, 5 partly cloudy and 5 overcast skies
Covering the whole spectrum of usual skies found in nature
The CIE standard skies
The relative luminance distribution, lv, for a standard sky combining gradation function (Z) and indicatrix function f()
0Zf
Zf
L
Ll
szv
The CIE standard skies gradation function (Z)
indicatrix function f()
bexpa1
Zcos/bexpa1
0
Z
s
2s
2
s Zcose)2/dexp(dZexpc1
cose)2/dexp(dexpc1
Zf
f
Classify the CIE standard skies for Hong Kong
Measured sky luminance were compared with modeled standard sky models to determine the root-mean-square error (RMSE):
The standard skies classified with the lowest RMSE
The CIE Standard Skies
21
2
mea
meapred
L
LL
N
1RMSE
The CIE standard skies
Standard sky numbers 1 (overcast) and 13 (clear) are of the highest frequency of occurrence
Methodology
For daylight prediction, three components are involvedSky component (SC)Externally reflected component (ERC)Internally reflected component (IRC)
Only SC and IRC are considered in this program : For buildings facing small sky obstructions such as top floors
in high-rise blocks or buildings located in low density business areas
Methodology
Internal daylight illuminance depends on the outdoor illuminance and exact sky luminance distribution at that time
The approach:
relates the luminance distribution of the sky to the illuminance at a point in a room
Sky Component
is the sum of the sine function of the elevation for each the sky element that is seen by the reference point times solid angle times angle between the sky element and the line normal to the window facade
For those sky element which can be “seen” by reference point, it can expressed as:
n
1iiii SLSinE
Internally reflected component
Lighting coming directly from the sky C1
Reflected lighting coming from below the horizon C2
A = the total area of all the interior surfaces R = the average reflectance of all the interior surfaces Rcw = the average reflectance of the ceiling and upper walls above
the mid-height of the window Rfw = the average reflectance of the floor and lower walls below the
mid-height of the window τ = the overall transmittance of the window W = the window area (i.e. they are building parameters)
Internally reflected component
R1A
RCRCWIRC cw2fw1
Internally Reflected component
C1 is the ratio of illuminance received by vertical window to the horizontal diffuse illuminance, Evd :
C2 :
For overcast sky:For non-overcast sky:
(For 0≤θ<π/2, -π/2<Φ-Φn<π/2; =0 otherwise)
vd
gt2 E2
REC
g2 R5.0C
vd
ii
145
1ini
2i
1 E
coscosLC
Calculation Sequence
Devise a set of factors for sky and
Internally Reflected components
Calculate a sky luminance
distribution pattern
Calculate hourly daylight level of a typical day of
each month
Predict lighting power
consumption
Input and output of software
BuildingGeometry
SurfaceReflectance
LightingProperties
IlluminanceDatabase
Software Tool
IndoorIlluminance
level
LightingPower
Consumption
LightingEnergySaving
User Interface
Lighting Inputs
Building Geometry
Surface reflectance
A Case Study
External Conditions
Sky number 1 (overcast) and 13 (cloudless polluted)
Building height 3m
Building width along the window façade infinite
Ground Reflectance 20%
Obstruction None
Room Parameters
Room Length 6m
Room Depth 6m”””
Room Height 3m
Sill Height 0.75m
Window dimensions 1.8m x 4.8m
Ceiling Reflectance 70%
Wall Reflectance 50%
Floor Reflectance 20%
Glazing Type Clear, 85% transmittance
External conditions and room parameters for validation:
Validation For sky type 1, the
estimated value by the software is 4.89% while RADIANCE is 4.32, the discrepancy is less than 0.6%.
The prediction by the software is slightly smaller than the prediction by RADIANCE.
The peak different between the software prediction method and RADIANCE is only 2.7% (for clear sky 13).
Using traditional CIE overcast sky pattern only would considerably underestimate the interior daylight illuminance.
0
5
10
15
20
25
30
0 5 10 15 20 25 30
DF computed by Simple software (%)
DF
com
pute
d by
RA
DIA
NC
E (
%)
sun lit window facade
sun shaded window facade
Conclusions
The software can be used to estimate the indoor daylight illuminance and hence the likely electric lighting energy savings under CIE standard skies 1 and 13.
The software is convenient for architect and building engineers during initial design stage when different schemes and concepts are being considered
Enhancing Energy Efficiency of Built Environment through Daylighting
Thank You
