Transcript Slide 1

© Leigh & Orange Limited
| Calculation and Application of OTTV and U-value|
CONTENTS
Lesson 1
Calculation and Application of OTTV and U-value
• What is U-value?
• What is OTTV?
• Building Design and OTTV
• OTTV Requirements in Hong Kong
Summary and Further reading
| Calculation and Application of OTTV and U-value|
What is U-value?
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
U-value
is often called the overall heat transfer coefficient. It is a factor
for consideration in the design of buildings and choose of
building materials.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
U-value
is a measure of the rate of heat loss through a material over 1
square meter of area for every temperature degree difference,
under *a standardized condition.
*The usual standard is at a
temperature gradient of 24
°C, at 50% humidity with
no wind.
is the heat loss in Watt W
is the area calculated in square metre m²
is the degree difference in Kelvin scale K
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
The lower the U-value, the slower the rate of heat transfer per
unit square and the higher its insulting value, which is
desirable in building construction.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
Reinforced Concrete
Brick
Hardwood
Softwood
MATERIAL
U-VALUE
Softwood
0.13
Hardwood
0.18
Brick
0.77
Light-weight Concrete
(1800kg/m³)
1.13
Normal Concrete
(2400kg/m³)
1.93
Reinforced Concrete
2.3
Low-E Double Glazing
2.8
Single Glazing
5.7
Source: www.puravent.co.uk
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
Double glazing
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
[Example 1: Single Glazing]
U-value of a standard single glazed window = 5.7 W/m²K
For each degree difference (K) and per 1 m² of the surface area
of glass, 5.7W of thermal energy is transferred through from its
either side.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
[Example 2: Low-E Double Glazing]
U-value of a standard double glazed window = 2.8 W/m²K
For each degree difference (K) and per 1 m² of the surface area
of glass, 2.8 W of thermal energy is transferred through from its
either side. Therefore low-E double glazing has better
insulation performance.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
Low-E Glass
• Low-E glass (E = thermal emissivity)
• Metal oxide to reduce solar radiation entering indoor by
reflecting the radiant heat
• In summer, heat radiation from the sun is reflected away
• In winter, heat originated from indoors can be reflected back
inside, resulting in better heat insulation.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
Veterinary Laboratory at Tai Lung, Sheung Shui
has used low-e glass on its external façade to
reduce solar heat gain.
© Architectural Services Department
| Calculation and Application of OTTV and U-value| Lesson 1 - What is U-value?
What is OTTV?
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
OTTV
is the Overall Thermal Transfer Value. It is a value which
indicates the average heat gain into a building through the
building envelope.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
[2 major components in OTTV]
 Heat conduction through opaque
wall Qwc
 Solar radiation through window
glass Qgs
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
Basic principles of OTTV Calculation
The amount heat transfer through a building envelope can be
expressed as:
where Q = total heat transfer through envelope (W)
A = gross area of building envelope (m2)
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
1 Peking Road uses a triple-glazed active
wall system, combining 3 layers of low-E
clear glass with a ventilated cavity that
results in high light transmission and a low
OTTV.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
Calculations of heat transfer through
external walls in Hong Kong:
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
The OTTV equation for external walls in Hong Kong:
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
External shading of IFC 2 is not
considered as part of the roof
in OTTV calculation.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
[Extended Knowledge]
The general OTTV equation is used in the United States
and other countries.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
[Extended Knowledge]
Three major difference between the general OTTV
equation and the equation in Hong Kong:
•
•
•
The glass conduction term Qf is introduced.
A solar absorptivity term α is omitted.
External shading multiplier EMS is not calculated.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
How does the design of building affect
the OTTV value in Hong Kong?
The OTTV value of a building is affected by the following
factors:
• Building Orientation (Temperature Difference)
• Material of wall and roof (U-value)
• External finish and colour of walls (Solar Absorptivity)
• Type of Glass (Shading Coefficient)
• Shading of windows (External Shading Multiplier)
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
1. Comparing different dimensions of fenestration
Study the above wall sections and the following information:
a. All 3 walls are south-facing
b. Assume the width of the walls are 1 m
c. All 3 windows use tinted glass with a shading coefficient (SC) of 0.7
d. Solar Factor = 191
e. External shading multiplier (ESM) = 1
f. All walls and concrete beans have the same external surface and colour
g. Solar absorptivity of all walls and beams = 0.58
h. Equivalent temperature difference (K) =1.4 K
Given that the U-value of a 600mm concrete beam = 1.51 the U-value of a 100mm concrete panel = 2.32
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
Calculation of OTTV of the 3 walls per storey:
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
Calculation of OTTV of the 3 walls per storey:
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
Calculation of OTTV of the 3 walls per storey:
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
[Discussion]
By observing the drawings and the calculations, what
conclusion can you draw regarding the relationship between
the OTTV and the areas of the wall and the fenestration?
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
[Result]
OTTV1 > OTTV2 > OTTV3
The above calculations show that the OTTV is mainly governed
by the ratio of opaque wall area to fenestration area.
OTTV will increase the area of fenestration in the wall,
meaning the extensive use of glass will lead to high OTTV
(poor insulting performance).
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
2. Comparing different fenestration materials
Referring to Fig. 1, in wall section 2, if the tinted glass (SC = 0.7) is substituted by
reflective glass (SC = 0.4), and other conditions remaining the same, how will the result
change?
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
[Result]
OTTV2 in case 1 = 63.55W/m² > OTTV2 in case 2 = 36.57W/m²
From the above calculation, with the same proportion of wall
and fenestration area, the OTTV of wall section 2 is reduced
when reflective glass is used instead of tinted glass.
Compared to the previous case where tinted glass (SC=0.7) is
employed, reflective glass (SC=0.4) achieves a lower OTTV and
hence provides a better insulting performance. Therefore, the
type of glass for the fenestration used is an important factor
for OTTV.
| Calculation and Application of OTTV and U-value| Lesson 1 - What is OTTV?
OTTV requirement in Hong Kong
The Building (Energy Efficiency) Regulation (B(EE)R) aims at
• reducing heat transfer through the building envelope and
• minimising electricity consumption for air-conditioning
• by requiring the external walls and roofs of commercial buildings
to be designed and constructed to have a suitable Overall
Thermal Transfer Value (OTTV)
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
OTTV requirement in Hong Kong
• The OTTV code was subsequently amended (for all buildings
except residential buildings) in 1995 as follows:
• In the case of a building tower, the OTTV should not exceed
24W/m²
• In the case of a podium, the OTTV should not exceed
56W/m²
• open-front shops or the like on ground level may be
exempted from the OTTV calculations upon applications
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Exercise 1
Study the drawing and figures below and calculate the OTTV of this building. Assume that the
four elevations of the building are identical. Calculate the OTTV of the building.
Determine if this building follows the OTTV code in Hong Kong (24W/m²).
U-value of a wall Uw = 1.9 W/m2K
Outdoor temperature = 28.4oC
Indoor temperature = 27oC
Solar absorptivity of wall α = 0.58
External shading multiplier ESM= 1
Shading coefficient of window glass SC= 0.4
Solar factor SF = 191W/m2
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Reference answer
Considering the tower part of the building:
Total area of all windows on one façade Af = 2m x 2m x 18 = 72m2
Area of the wall Aw = (32m x 10m)-72m2= 248m2
Equivalent temperature difference TDeq = 28.4°C-27°C = 1.4°C
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Exercise 2
Please study the article “Limitations of the OTTV Standard in Hong Kong ”,
and information about OTTV Code
[Discussion]
Q1 What are the limitations of OTTV standard?
Q2 What are the difficulties of implementing OTTV controls in Hong Kong?
Q3 How can the government improve the standard of OTTV controls on buildings?
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Q1 What are the limitations of OTTV standard?
In controlling the energy efficiency of a building, OTTV has the following limitations:
• Cannot fully reflect how other parts of the building affect the heat gain
• Not precise and cannot ensure the efficient use of energy
• Limits the freedom of building design
• Shading from surrounding buildings affects the value measured
• OTTV might not be a valid standard in locations with extreme climate
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Q2 What are the difficulties of implementing OTTV controls in Hong Kong?
• Manual checking and calculation of OTTV value are not reliable.
• Lack of professional engineer responsible for OTTV submission.
• No control measures for the design and operation of existing buildings.
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Q3 How can the government improve the standard of OTTV controls on buildings?
1. Adopt a performance-based approach.
2. Incorporate both mandatory and voluntary requirements for OTTV controls to
accommodate greater flexibility in design.
3. Promote the use of compliance software to calculate and check the OTTV.
4. Research support on building energy standards. Study of energy data and design
practices will help setting up criteria which are tailored according to local conditions.
5. Provide education and training for designers and professional engineers on the
calculation and evaluation process of OTTV.
6. Regularly review the standard and set up an information network for information
exchange with other countries.
| Calculation and Application of OTTV and U-value| Lesson 1 – Application of OTTV?
Summary
1. U-value is called the overall heat transfer coefficient.
2. The lower the U-value, the higher the insulating value of the material.
3. OTTV stands for the ‘Overall Thermal Transfer Value’ .
4. The lower the OTTV, the higher the insulating performance of the building envelop.
5. Size of the fenestration and the type of glass used are important factor for OTTV.
6. OTTV has limitations to address the energy efficiency of a building at a certain
context.
7. A combined testing and tailored building energy standard can improve the
reliability of OTTV.
| Calculation and Application of OTTV and U-value|
Further Reading
1. Sam C. M. Hui. Introduction to OTTV and Building Energy Simulation.
http://www.arch.hku.hk/~cmhui/teach/65256-X.htm
2. Wong Wah-sang and Chan Hon-wah Edwin, eds. Building Hong Kong:
Environmental Considerations. Hong Kong University Press, 2000.
3. Building (Energy Efficiency) Regulation Hong Kong Government, 1995.
Building (Energy Efficiency) Regulation, Cap. 123 sub. Leg. M (L.N. 144 of
1995).
http://www.legislation.gov.hk/blis_pdf.nsf/4f0db701c6c25d4a4825755c00352e
35/25C2868DA2669A12482575EE003F079B/$FILE/CAP_123F_e_b5.pdf
4. Hong Kong Green Building Technology Net. EMSD.
http://gbtech.emsd.gov.hk/tc_chi/minimize/green_windows.html
| Calculation and Application of OTTV and U-value|