Science Teaching Kit for Senior Secondary Curriculum ...

Calculation and Application of OTTV and U-value

Science Teaching Kit for Senior Secondary Curriculum

[Teacher notes]

Energy and Use of Energy

Organizer Sponsor Research Team

ContentsPreamble

Teaching plan i

DisclaimerCreate 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

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?

1.2.1 Principles of OTTV

1.2.2 The OTTV Equation in Hong Kong

1.3 Building Design and OTTV

1.3.1 Comparing Different Wall Designs

1.3.2 Comparing Different Types of Glass

1.4 OTTV Requirements in Hong Kong

Exercise 1

Exercise 2

Summary, Key words and Further reading

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Cover Photo © Leigh & Orange Limited

Science | Calculation and Application of OTTV and U-value

Topic 06Calculation and Application of OTTV and U-value

Major teaching areasPhysics: Chapter VIII Energy and Use of Energy• Energy Efficiency in Building and Transportation

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 planLesson ContentsLesson 1

Calculation and Applica-tion of OTTV and U-value

• 1.1.1

• 1.1.2

• 1.2.1

• 1.2.2

• 1.3.1

• 1.3.2

• 1.4

• Exercise 1

• Exercise 2

The principles of U-value

The U-values of common construction materials

The principles of OTTV

The OTTV equation in Hong Kong

Discussion on the relationship between building designs and OTTV value

Discussion on the relationship between types of glass and OTTV value

OTTV requirements in Hong Kong

Calculation of OTTV for a building under Hong Kong OTTV requirements

Discussion on the limitations of OTTV in regulating the energy efficiency of a building

Interdisciplinary teaching areas Design and Applied Technology• Strand 3 Value and Impact

Liberal Studies• Module 2 Hong Kong Today• Module 6 Energy Technology and Environment

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Lesson 1Calculation and Application ofOTTV and U-Value

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Lesson 1Calculation and Application of OTTV and U-value

Material U-value (Wm-2K-1) Material U-value (Wm-2K-1)Softwood 0.13 Normal Concrete (2400 kgm-3) 1.93

Hardwood 0.18 Reinforced Concrete 2.3Brick 0.77 Low-E Double Glazing 2.8Light Weight Concrete (1800 kgm-3) 1.13 Single Glazing 5.7

Reinforced concrete Brick Hardwood Softwood Double glazing

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.

1.1.2 U-values of Common Construction Materials

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.

source: www.puravent.co.uk

*The usual standard is a temperature gradient of 24 °C, at 50% humidity with no wind.

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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 glazed window diagramSingle glazing section Double glazing section

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 03


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

where Q = total rate of heat transfer through envelope (W) A = gross area of building envelope (m2)

1.2.1 Principles of OTTVThe rate of heat transfer through a building envelope can be expressed as:

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.

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.

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1.2.2 The OTTV Equation in Hong Kong:

where Aw, Af = wall and window area (m2) Uw = U-values of wall (Wm-2K-1) α = solar absorptivity of wall TDeq = equivalent Temperature Difference (K) SC = Shading Coefficient of the glazing ESM = External Shading Multiplier SF = Solar Factor (Wm-2)

Solar absorptivity of wall α

Solar absorptivityα 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

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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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 should not project more than 1.5 m from the external wall.

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

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.

The OTTV equation for external walls in Hong Kong:

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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)

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.

[Extended Knowledge]

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1.3.1 Comparing Different Wall Designs

Study the above wall sections and the following information:

a. All three walls are south-facingb. Assume the width of the walls is 1 mc. All three windows use tinted glass with a shading coefficient (SC) of 0.7d. Solar Factor = 191 Wm-2

e. External Shading Multiplier (ESM) = 1f. All walls and concrete beams have the same external surface and colourg. Solar absorptivity of all walls and beams = 0.58h. 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

1.3 Building Design and OTTVThe 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)

Wall section 1 Wall section 2 Wall section 3

Fig. 1 3 wall sections with different window sizes

Concrete Beam

Concrete Panel

Concrete Beam

Concrete Panel

Concrete Panel

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OTTV of Wall section 1



Calculation of OTTV of the three walls:

Possible perspectivesOTTV1 > 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).

Various materials and different types of glass are used on the external façade of the Stanley Municipal Services Building

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?

[Discussion]

Note: The Wall Conduction of Concrete Beams and Concrete Panels are different - therefore they must be calculated separately because of the different U-values.

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1. Compare the result in case 1 and explain the difference.

1.3.2 Comparing Different Types of GlassReferring 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?

p Tinted glass (Left) and reflective glass (Right) in the Stanley Municipal Services Building

Possible perspectivesOTTV2 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.

[Discussion]

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1.4 OTTV Requirements in Hong KongThe 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 stations2. Hotels defined by the Hotel and Guesthouse Accommodation Ordinance

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:

• 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 application

Source: Building (Energy Efficiency) Regulation, Hong Kong Government, 1995.

Teaching TipsMore 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’.

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Possible perspectives

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 wall Aw = (32 m x 10 m)-72 m2= 248 m2

Equivalent temperature difference TDeq = 28.4 °C-27 °C = 1.4 °C

OTTV of the building

The building follows the OTTV code in Hong Kong.

U-value of a wall Uw = 1.9 Wm-2K-1

Outdoor temperature = 28.4 oC

Indoor temperature = 27 oC

Solar absorptivity of wall α= 0.58

External shading multiplier ESM= 1

Shading coefficient of window glass SC= 0.4

Solar factor SF = 191 Wm-2

[Exercise 1]

Glass

ConcreteTower

PodiumGlass

15m

4.5m1.5m

2m2m

32m

6m

10m

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).

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— Extract from Overall Thermal Transfer Value (OTTV): How to Improve Its Control in Hong Kong by Hui, Sam. C. M.,1997

[Exercise 2]

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.

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.

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.

Revision of the OTTV CodeThe 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)

Please study the article and information below:

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1. What are the limitations of the OTTV standard?

2 What are the difficulties of implementing OTTV requirements in Hong Kong?

3 How can the Government improve the standard of OTTV requirements on buildings?


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.


• 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.


• 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.

[Discussion]

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Key wordsOTTV (Overall Thermal Transfer Value)U-valueHeat transfer

Further reading1. 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/4f0db701c6c25d4a4825755c00352e35/25C2868DA2669A12482575EE003F079B/$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>.

Summary

1. U-value is the rate of heat transfer of a material. 2. The lower the U-value, the higher the resistance of heat flow through the material.3. OTTV stands for ‘Overall Thermal Transfer Value’.4. The lower the OTTV, the higher the resistance of heat flow through the building envelope. 5. The size of the windows and the type of glass used are important factors for OTTV.6. OTTV has limitations in addressing the energy efficiency of a building.7. A combined testing and tailored building energy standard can improve the reliability of

OTTV.

Organizer Sponsor Research Team

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http://gbtech.emsd.gov.hk/tc_chi/minimize/green_windows.html

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