Science Teaching Kit for Senior Secondary Curriculum
Energy and Use of Energy
Calculation and
Application of
OTTV and U-value
[Teacher notes]
Organizer
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Research Team
Contents
Teaching plan
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Lesson 1 : Calculation and Application of OTTV and U-value
1.1 What is U-value?
1.1.1 Principles of U-value
1.1.2 U-values of Common Construction Materials
1.2 What is OTTV?
02
02
02
04
1.2.1 Principles of OTTV
04
1.2.2 The OTTV Equation in Hong Kong
05
1.3 Building Design and OTTV
08
1.3.1 Comparing Different Wall Designs
08
1.3.2 Comparing Different Types of Glass
10
1.4 OTTV Requirements in Hong Kong
11
Exercise 1
12
Exercise 2
13
Summary, Key words and Further reading
15
Cover Photo © Leigh & Orange Limited
Disclaimer
Create Hong Kong of the Government of the Hong Kong Special Administrative Region provides funding support to the project only, and does not otherwise
take part in the project. Any opinions, findings, conclusions or recommendations expressed in these materials/events (or by members of the project team) do not
reflect the views of the Government of the Hong Kong Special Administrative Region.
© 2012 Hong Kong Institute of Architects
Science | Calculation and Application of OTTV and U-value
Preamble
Topic 06
Calculation and Application of OTTV and U-value
Interdisciplinary teaching areas
Major teaching areas
Physics: Chapter VIII Energy and Use of Energy
•
Energy Efficiency in Building and Transportation
Design and Applied Technology
• Strand 3 Value and Impact
Liberal Studies
Module 2 Hong Kong Today
Module 6 Energy Technology and Environment
Learning objectives
•
To understand the meaning of U-value
•
To calculate the OTTV
•
To apply basic OTTV calculations in problem solving
•
To evaluate the thermal performance of different building designs via scientific measurements
Teaching plan
Lesson
Lesson 1
Contents
• 1.1.1
The principles of U-value
Calculation and Application of OTTV and U-value
• 1.1.2
The U-values of common construction materials
• 1.2.1
The principles of OTTV
• 1.2.2
The OTTV equation in Hong Kong
• 1.3.1
Discussion on the relationship between building designs
and OTTV value
• 1.3.2
Discussion on the relationship between types of glass
and OTTV value
• 1.4
OTTV requirements in Hong Kong
Science | Calculation and Application of OTTV and U-value
•
•
• Exercise 1 Calculation of OTTV for a building under Hong Kong OTTV
requirements
• Exercise 2 Discussion on the limitations of OTTV in regulating the
energy efficiency of a building
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Science | Calculation and Application of OTTV and U-value
Lesson 1
Calculation and Application of
OTTV and U-Value
01
Lesson 1
Calculation and Application of OTTV and U-value
1.1 What is U-value?
1.1.1 Principles of U-value
U-value is a measure of the rate of heat transfer through a one-square-metre area of a material for
every temperature degree difference under *a standardized condition. It is a factor for consideration in
the design of buildings, and the choice of building materials. The lower the U-value, the lower the rate
of heat transfer per unit area and the higher its resistance to heat flow, which is desirable in building
construction.
*The usual standard is a
temperature gradient of 24 °C,
at 50% humidity with no wind.
1.1.2 U-values of Common Construction Materials
Reinforced concrete
Brick
Hardwood
U-value (Wm-2K-1) Material
Material
Softwood
Hardwood
Brick
Light Weight Concrete (1800 kgm )
source: www.puravent.co.uk
Softwood
-3
0.13
Normal Concrete (2400 kgm-3)
0.77
Low-E Double Glazing
0.18
1.13
Reinforced Concrete
Single Glazing
Double glazing
U-value (Wm-2K-1)
1.93
2.3
2.8
5.7
Science | Calculation and Application of OTTV and U-value
U-value with SI units of Wm-2K-1
where Q stands for rate of heat transfer in Watt W;
A stands for the area of the material calculated in square metres m2;
TDeq stands for the equivalent temperature difference in Kelvin scale K.
02
U-value of a standard single glazed window = 5.7 Wm-2K-1
U-value of a double glazed window = 2.8 Wm-2K-1
This tells us there is less heat transfer through a double glazed window
than a single glazing, i.e., double glazing has a significantly better
insulation performance.
Double glazing section
Double glazed window diagram
Low-E glass - a building material with low U-value
Low-E glass (E = thermal emissivity) is a construction material that is coming into wide use. Low-E coatings such as
metal oxide is applied to the surface of the glass to reduce the amount of solar radiation entering the interior of the
building. With the reflective coating, radiant heat is kept out (lowering the U-value of the material) while visible light
can still pass through the glass. This means that in summer, heat radiation from the sun is reflected away while in
winter, heat originating indoors can be reflected back inside, resulting in better heat insulation.
p
The Veterinary Laboratory at Tai
Lung, Sheung Shui uses low-E
glass on its external façade to
reduce solar heat gain.
© Architectural Services Department
Science | Calculation and Application of OTTV and U-value
Single glazing section
03
1.2 What is OTTV?
OTTV stands for ‘Overall Thermal Transfer Value’. It is a value that indicates
the average rate of heat transfer into a building through the building
envelope.
Building envelope
The term building envelope refers to the outermost
layer of a building. It includes the roof, the walls and
windows of all sides.
Science | Calculation and Application of OTTV and U-value
Its two major components include:
1.
Rate of heat transfer through conduction through opaque walls, Qwc
2.
Rate of heat transfer through solar radiation through window glass, Qgs
1.2.1 Principles of OTTV
The rate of heat transfer through a building envelope can be expressed as:
where Q = total rate of heat transfer through envelope (W)
A = gross area of building envelope (m2)
1 Peking Road uses a triple-glazed active wall
system, combining three layers of low-E clear
glass with a ventilated cavity that results in
high light transmission and a low OTTV.
04
1.2.2 The OTTV Equation in Hong Kong:
where Aw, Af
Uw α
TDeq
SC
ESM
SF
= wall and window area (m2)
= U-values of wall (Wm-2K-1)
= solar absorptivity of wall
= equivalent Temperature Difference (K)
= Shading Coefficient of the glazing
= External Shading Multiplier
= Solar Factor (Wm-2)
Solar absorptivity α is a multiplication constant to the equivalent temperature
difference depending on the exterior surface and colour.
Different external surfaces and wall colours of the same material have different energy
absorptivity, and hence have a significant effect on chiller energy used. U-value is,
however, a standard measurement that does not consider the external surfaces and
colours of the material.
Calculation of U-value:
Therefore, the solar absorptivity of walls is taken into account in the heat gain
calculation. For example, the solar absorptivity of black concrete is 0.91 while that of
brown concrete is 0.85. The heat transfer is higher for concrete of darker colour.
Equivalent Temperature Difference TDeq
Science | Calculation and Application of OTTV and U-value
Solar absorptivity of wall α
In the following calculation, we simplify the equivalent temperature difference into
measured temperature difference between indoors and outdoors.
The equivalent temperature difference is a combined effect on the building envelope, of
incidental solar radiation, radiant energy and convective heat exchange with the sky and
outdoor air, which takes into account the type of building orientation, and wall mass and
density.
Shading Coefficient of the glazing SC
The shading coefficient is the ratio of the solar heat gain through a particular type of
glass under a specific set of conditions to the solar heat gain through that of double
strength sheet clear glass under the same conditions. SC has a maximum value of 1.
The higher the shading coefficient is, the lower the shading performance of the glass.
The value is always provided by glass manufactures.
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External Shading Multiplier ESM
The external shading multiplier is the shading coefficient to be multiplied with the
shading coefficient of the glazing SC. The projections over the windows, or at the sides
of the window, or a combination of both, provide shading effects on the fenestration
and can significantly reduce the heat transfer through the glazing. ESM has a maximum
value of 1.
Windows with no shading projection would have ESM = 1. Fenestration with both
overhanging and side projections would take the smallest value of either projection as
ESM in the calculation of OTTV.
Solar Factor SF
Additional
calculation of OTTV for roofs
As the OTTV for roofs is similar to that for walls, and the value for skylights is similar to that
of windows,
the roof can be taken as part of the walls in calculating OTTV while skylight
glazing
can be treated as part of the windows in the calculation.
However,
when external shading is considered as a shading projection over windows, the
heat transfer of the projections over windows should not be included in the OTTV of the
roof. According to the Building (Planning) Regulations (Cap. 123 F), an external shading
not project more than 1.5 m from the external wall.
should
The OTTV
equation for external walls in Hong Kong:
Science | Calculation and Application of OTTV and U-value
The solar factor is the hourly radiation per square metre for horizontal and vertical
surfaces (Wm-2). Energy simulation has calculated solar factors for the Hong Kong
climate at various orientations. Any sloped or angled wall can be resolved into vertical
and horizontal components. The vertical components are treated as wall elements at
respective orientations, and the horizontal ones are treated as roof components. The
horizontal solar factor for all orientations in Hong Kong is 264.
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[Extended Knowledge]
The OTTV requirement was first proposed in the American Society of Heating, Refrigerating and
Air-Conditioning Engineers (ASHRAE) Standard in 1975. The general OTTV equation is used in the
United States, Singapore and other parts of Southeast Asia.
There are three major differences between the general OTTV equation and the equation in Hong
Kong:
a) The glass conduction term Qgc is introduced.
b) A solar absorptivity term α is omitted.
c) The external shading multiplier EMS is not calculated.
Science | Calculation and Application of OTTV and U-value
The external shading of IFC 2 is not
considered part of the roof in OTTV
calculation.
The general OTTV equation for external walls:
where Aw , Af = wall and window area (m2); A = Aw + Af
Uw , Uf = U-values of walls and windows (Wm-2K-1)
TDeq = equivalent Temperature Difference (K)
SC = Shading Coefficient of window glass
SF = Solar Factor (Wm-2)
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1.3 Building Design and OTTV
The OTTV of a building is affected by the following factors:
•
•
•
•
•
Building orientation (Temperature Difference)
Material of walls and roof (U-value)
External finish and colour of walls (Solar Absorptivity)
Type of glass (Shading Coefficient)
Shading of windows (External Shading Multiplier)
1.3.1 Comparing Different Wall Designs
Wall section 1
Concrete
Beam
Concrete
Beam
Concrete
Panel
Concrete
Panel
Wall section 2
Wall section 3
Science | Calculation and Application of OTTV and U-value
Concrete
Panel
Fig. 1 3 wall sections with different window sizes
Study the above wall sections and the following information:
a.
b.
c.
d.
e.
f.
g.
h.
All three walls are south-facing
Assume the width of the walls is 1 m
All three windows use tinted glass with a shading coefficient (SC) of 0.7
Solar Factor = 191 Wm-2
External Shading Multiplier (ESM) = 1
All walls and concrete beams have the same external surface and colour
Solar absorptivity of all walls and beams = 0.58
Equivalent Temperature Difference (K) =1.4 K
Given that the U-value of a 600 mm concrete beam = 1.51 Wm-2K-1
the U-value of a 100 mm concrete panel = 2.32 Wm-2K-1
08
Various materials and different types of
glass are used on the external façade of
the Stanley Municipal Services Building
Note: The Wall Conduction of Concrete Beams and Concrete Panels are
different - therefore they must be calculated separately because of the
different U-values.
OTTV of Wall section 1
OTTV of Wall section 2
OTTV of Wall section 3
Science | Calculation and Application of OTTV and U-value
Calculation of OTTV of the three walls:
[Discussion]
1.
Based on the drawings and the OTTV calculations, what conclusion can you draw regarding the OTTV value in relation to the areas of the wall and the window?
Possible perspectives
OTTV1 > OTTV2 > OTTV3
The above calculations show that OTTV is mainly governed by the ratio of opaque wall area to
fenestration area. The higher the proportion of window area, the higher the OTTV, especially in cases
where glass with a high Shading Coefficient (SC) has a greater contribution to heat gain. To conclude,
OTTV will increase with the area of windows in the wall, meaning the extensive use of glass will lead
to high OTTV (poor insulating performance).
09
Tinted glass (Left) and reflective glass (Right) in the Stanley Municipal Services Building
1.3.2 Comparing Different Types of Glass
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 remain the same, how would the result change?
Science | Calculation and Application of OTTV and U-value
p
[Discussion]
1.
Compare the result in case 1 and explain the difference.
Possible perspectives
OTTV2 in case 1 = 63.55 Wm-2 > OTTV2 in case 2 = 36.57 Wm-2
From the above calculation, with the same proportion of wall and fenestration
area, the OTTV of wall section 2 is lowered from 63.55 Wm-2 to 36.57 Wm-2 when
reflective glass is used instead of tinted glass. Compared to the previous case where
tinted glass (SC=0.7) was employed, reflective glass (SC=0.4) achieves a lower OTTV
and hence provides a better insulating performance. Therefore, the type of glass
used for windows is an important factor of OTTV.
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1.4 OTTV Requirements in Hong Kong
The Building (Energy Efficiency) Regulation ‘B(EE)R’ came into effect in 1995. The following
buildings are covered under the B(EE)R:
1.
Commercial buildings, except for domestic, industrial, bulk storage, and utility buildings such as sub-stations and power stations
Hotels defined by the Hotel and Guesthouse Accommodation Ordinance
2.
The B(EE)R aims at reducing heat transfer through the building envelope thus minimizing electricity
consumption for air-conditioning by requiring the external walls and roofs of commercial buildings
to be designed and constructed for a suitable OTTV. The suitable level of OTTV and the methodology
of OTTV calculations are specified in the Code of Practice for Overall Thermal Transfer Value in
Buildings 1995 (the OTTV Code).
During the second review of the OTTV requirements by the Buildings Department, the OTTV Code
was subsequently revised as below:
application
Source: Building (Energy Efficiency) Regulation, Hong Kong Government, 1995.
Teaching Tips
More information on:
• Regulations and guidelines of green buildings under Liberal
Studies Topic 07: ‘Video: Environmentally Friendly Green
Buildings’;
• Energy-saving applications under Science Topic 07: ‘Video:
Energy-saving Approaches in Architecture’;
• Design for sustainability under Design and Applied Technology
Topic 05: ‘Video: Sustainability in Architecture’; and
• Aesthetics of materials and textures under Arts Topic 08: ‘Visit:
Tin Shui Wai Municipal Services Building’.
Science | Calculation and Application of OTTV and U-value
• In the case of a building tower, the OTTV should not exceed 24 Wm-2
• In the case of a podium, the OTTV should not exceed 56 Wm-2
• Open-front shops or the like on ground level may be exempted from the OTTV calculations upon
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[Exercise 1]
1.
Study the drawing and figures below and calculate the OTTV of the 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 (24 Wm-2).
2m
2m
Concrete
Tower
Outdoor temperature = 28.4 oC
Indoor temperature = 27 oC
Solar absorptivity of wall α = 0.58
External shading multiplier ESM= 1
10m
1.5m
Shading coefficient of window glass SC= 0.4
Solar factor SF = 191 Wm-2
Glass
Possible perspectives
4.5m
Considering the tower part of the building:
Total area of all windows on one façade Af = 2 m x 2 m x 18 = 72 m2
Area of the wall15m
Aw = (32 m x 10 m)-72 m2= 248 m2
Equivalent temperature difference TDeq = 28.4 °C-27 °C = 1.4 °C
6m
odium
U-value of a wall Uw = 1.9 Wm-2K-1
Science | Calculation and Application of OTTV and U-value
32m
Glass
OTTV of the building
The building follows the OTTV code in Hong Kong.
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[Exercise 2]
Please study the article and information below:
Limitations of the OTTV Standard in Hong Kong
The biggest limitation of the OTTV method is that it only deals with the building envelope and
does not consider other aspects of building design (such as lighting and air-conditioning) and the
coordination of building systems to optimise the combined performance. The use of OTTV as the only
control parameter is inadequate and cannot ensure energy is used efficiently in the building. Before
other energy codes are implemented, the effect of the OTTV standard on ‘real’ energy savings is
questionable, although it helps to increase concern and awareness of energy efficiency matters.
Evolution of the ASHRAE’s energy standards has indicated that more flexible approaches are
needed to encourage innovative energy efficient design in buildings. Many countries are now
moving towards energy performance criteria which give designers greater flexibility. To maintain its
economic competitiveness in the international market, Hong Kong should not lag behind the current
development.
Other problematic areas are related to the implementation issues. At present, the code of practice
for OTTV only provides a manual approach to the OTTV calculations. The compliance and calculation
procedure is often lengthy and time-consuming, so that checking of the results during submission
and enforcement is not easy. The lack of a competent professional engineer to be responsible for
the OTTV submission is also affecting the actual implementation. In addition, as there are no control
measures for the design and operation of existing buildings, which constitute a large portion of the
energy consumption, the energy savings arisen from the OTTV standard are limited.
Science | Calculation and Application of OTTV and U-value
The OTTV method has also been criticized for limiting design freedom in architecture and restricting
innovation. Although the OTTV approach has made code compliance simple for conventional building
designs, it has tended to restrict designers from innovation and more challenging work. If alternative
paths for code compliance are not provided, innovative designs that may exceed the OTTV limits
but can achieve a higher overall efficiency will be excluded and discouraged. For example, designs
employing daylight to reduce energy consumption by electric lights will be restricted.
— Extract from Overall Thermal Transfer Value (OTTV): How to Improve Its Control in Hong Kong by Hui, Sam. C. M.,1997
Revision of the OTTV Code
The OTTV Code has been revised throughout the years as below:
a) In the case of a tower, the OTTV
should not exceed 35 Wm-2 (first implemented in 1995)
should not exceed 30 Wm-2 (first revision)
should not exceed 24 Wm-2 (second revision)
b) in the case of a podium, the OTTV should not exceed 80 Wm-2 (first implemented in 1995)
should not exceed 70 Wm-2 (first revision)
should not exceed 56 Wm-2 (second revision)
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[Discussion]
1.
What are the limitations of the OTTV standard?
2
What are the difficulties of implementing OTTV requirements in Hong Kong?
Possible perspectives
• Manual checking and calculation of OTTV value is not reliable.
• There is a lack of professional engineers responsible for OTTV submissions.
• There are no control measures for the design and operation of existing buildings.
3
How can the Government improve the standard of OTTV requirements on buildings?
Science | Calculation and Application of OTTV and U-value
Possible perspectives
In regulating the energy efficiency of a building, the OTTV standard has the following limitations:
• As OTTV deals with the building envelope but not other aspects of the building design,
it cannot fully reflect how other parts affect heat gain. For example, lighting levels
inside a building and the heat generated are not examined under the OTTV standard.
• The control parameters of OTTV value are not precise and cannot ensure the efficient
use of energy.
• It limits the freedom of building design.
• The surrounding buildings may provide shading for the building and affect the value
calculated.
• Outdoor temperature is not uniform throughout the year. OTTV will be different at
different times of the year, especially in locations with extreme climates.
Possible perspectives
• Adopt a performance-based approach.
• Incorporate both mandatory and voluntary OTTV requirements to accommodate
greater flexibility in design.
• Promote the use of compliance software to calculate and check the OTTV.
• Support research on building energy standards. Study of energy data and design
practices will help in setting up criterias tailored to local conditions.
• Education and training for designers and professional engineers on the OTTV
calculation and evaluation process.
• Review the standard regularly and set up an information network for information
exchange with other countries.
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Summary
1.
2.
3.
4.
5.
6.
7.
U-value is the rate of heat transfer of a material.
The lower the U-value, the higher the resistance of heat flow through the material.
OTTV stands for ‘Overall Thermal Transfer Value’.
The lower the OTTV, the higher the resistance of heat flow through the building envelope.
The size of the windows and the type of glass used are important factors for OTTV.
OTTV has limitations in addressing the energy efficiency of a building.
A combined testing and tailored building energy standard can improve the reliability of
OTTV.
Key words
Heat transfer
Further reading
1. Hui, Sam C.M. Introduction to OTTV and Building Energy Simulation. Architecture Department of Hong
Kong University, 1997. <http://www.arch.hku.hk/~cmhui/teach/65256-X.htm>.
2. Wong, Wah Sang, and Hon Wah Chan, eds. Building Hong Kong: Environmental Considerations. Hong
Kong: Hong Kong University Press, 2000.
3. Legislative Council. Building (Planning) Regulations. February 2012.
<http://www.legislation.gov.hk/blis_pdf.nsf/4f0db701c6c25d4a4825755c00352e35/25C2868DA2669A12482575EE003F07
9B/$FILE/CAP_123F_e_b5.pdf>.
4. Electrical and Mechanical Services Department. HK Green Technology Net.
<http://gbtech.emsd.gov.hk/english/minimize/green_windows.html>.
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Sponsor
Science | Calculation and Application of OTTV and U-value
OTTV (Overall Thermal Transfer Value)
U-value
Research Team
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