BCA SEMINAR ON
ENERGY EFFICIENCY IN BUILDING DESIGN
18 APRIL 2001
ENHANCING THE ENERGY STANDARDS
Thomas Goh
Building Management Department
Building And Construction Authority
Singapore
SYNOPSIS
The Building and Construction Authority or BCA has been actively involved in energy conservation in buildings. In September 1979, a set of prescriptive energy standards was incorporated in the Building Control Regulations. The most prominent standard is the maximum Overall Thermal Transfer Value (OTTV) of 45 W/m2 prescribed for air-conditioned building envelope. To keep abreast with advances in technology and global trend these standards were regularly reviewed. As a result of the review, it was found that OTTV does not reflect closely the thermal performance of a building. A more accurate formulation was obtained. The new formulation is known as the Envelope Thermal Transfer Value (ETTV). To improve the thermal performance of building envelope, it is also proposed to enhance the OTTV of 45 W/m2 to 35 W/m2. Feedback on ETTV and the enhancement from 45 W/m2 to 35 W/m2 was sought from the various parties in the building industry. Public comment was also invited through news release and the BCA website.
INTRODUCTION
BCA has been actively involved in energy conservation in buildings since the energy crisis in the 1970s. The former Building Control Division chaired a committee on energy conservation to spearhead a 2-year research project with the participation of various professional bodies, universities, statutory boards and some other government departments. The recommendations of the committee were incorporated in the Building Control (Space, Light and Ventilation) Regulations gazetted in August 1979. Since then the energy standards have been reviewed to take into consideration advances in building technology and improvement in energy efficiency of building services.
CURRENT ENERGY STANDARDS
Compliance with the energy standards are required during the building plans submission stage. Building designs that do not meet the required standards would not have their building plans approved. This is to ensure that buildings are designed to an acceptable level of energy efficiency. The Certificate of Statutory Completion for the building is issued after the works have been completed in accordance with the approved plans.
Before 1st July 2000
Building plans that were submitted to BCA for approval before 1st July 2000 were required to comply with the following energy standards:
The OTTV for the gross area of exterior walls shall not exceed 45 W/m2.
Where the roof is provided with skylights or any other material, which allows the passage of light through, the OTTV for the gross area of the roof shall not exceed 45 W/m2.
For any other type of roof not mentioned in (b) above, the average thermal transmittance for the gross area of the roof shall not exceed the prescribed limits shown in the table 1 below.
Table 1
|
Weight Group |
Weight Range (kg/m2) |
Max. Thermal Transmittance (W.m2 ° K) |
|
Light |
Under 50 |
0.5 |
|
Medium |
50 to 230 |
0.8 |
|
Heavy |
Over 230 |
1.2 |
The lighting load density shall comply with the limit specified in the Handbook on Energy Conservation in Buildings and Building Services.
(1) At least one thermostat for the regulation of space temperature shall be provided for each separate air handling system and zone.
(2) Each air handling system shall be equipped with a readily accessible means of shutting off or reducing the energy used for the air-conditioning system during periods of non-use or alternative uses of building spaces or zones served by the system.
(3) For the purpose of meeting the requirements of paragraph (2), the following devices shall be regarded as satisfactory:
(a) manually adjustable automatic timing devices;
(b) manual devices for use by operating personnel; or
(c) automatic control systems.
The design and installation of —
(a) any air-conditioning or mechanical ventilation system; or
(b) the alteration or extension of any existing air-conditioning or mechanical ventilation system,
shall be in accordance with the provisions of the regulations and with the Code of Practice for Mechanical Ventilation and Air-Conditioning in Buildings - SS CP 13.
All buildings used or intended to be used as offices, a hotel or shop or a combination thereof shall be provided with data logging facilities for the collection of data for energy auditing.
From 1st July 2000 Onwards
Several code of practices were recently revised by the Singapore Productivity and Standards Board and launched in Feb 2000. The revision affected some of the energy standards under BCA's purview. Since the industry would need sufficient time to be familiar with the revised codes only building plans submitted to BCA for approval from 1 July 2000 onwards are required to comply with the revised energy standards. Those standards that are not affected by the revision would continue to be applicable. The revised energy standards are:
a. SS CP 13:1999
CP 13:1999 was extensively revised. The revision includes indoor air quality guidelines from the Ministry of the Environment, energy conservation guidelines from BCA's 'Handbook on Energy Conservation in Buildings and Building Services' and fire safety requirements from FSB's 'Fire Precaution in Buildings.' However, only specific sections of CP 13 are under the purview of the BCA. They are enumerated below and supersede the technical requirements in Division 9 of the Building Control Regulations.
i Section 5 - Ventilation rates (cl 5.1 to 5.3)
ii Section 6 - Mechanical ventilation systems (cl 6.1 to 6.3 excluding fire safety related sub-clauses)
iii Section 7 - Air-conditioning design (cl 7.1 to 7.9 excluding fire safety related sub-clauses)
b. SS CP 24:1999
The revised CP 24:1999 sets the minimum energy efficiency standards for four major energy consuming equipment and system, viz. air-conditioning equipment, water heaters, electric motors and artificial lighting system. However, only the lighting power budget section of CP 24 is implemented.
c. SS CP 38:1999
For artificial lighting, the design illuminance shall be in accordance with the Code of Practice for Artificial Lighting in Buildings - SS CP 38:1999.
ENHANCEMENT OF ENERGY STANDARDS
In collaboration with the two local universities, BCA has reviewed the energy efficiency regulations. The energy standards to be enhanced includes the following:
a The ETTV Formulation
OTTV is a measure of the average heat gain into a building through its envelope. It is measured in W/m2. An air-conditioned building with a higher OTTV will impose a greater load on the air-conditioning system, which would have to expend more electrical energy in removing it. The Building Control Regulations stipulate that all air-conditioned buildings must be designed to have an OTTV of not more than 45 W/m2. This is aimed at achieving adequately designed building envelopes to cut down external heat gains and hence reduce the cooling load of air-conditioning systems.
. The OTTV concept takes into consideration three basic elements of heat gain through the external walls and windows of a building. These are:
These three elements of heat gain constitute the OTTV formulation as follows:
OTTV = TDeq´ (1-WWR)´ Uw + D T ´ WWR´ Uf + SF´ CF´ WWR´ SC where
WWR : window-to-wall ratio
Uw : thermal transmittance of opaque wall (W/Km2)
Uf : thermal transmittance of fenestration (W/Km2)
TDeq : equivalent temperature difference (K)
D T : temperature difference (K)
SF : solar factor (W/m2)
CF : correction factor for solar heat gain through fenestration
SC : shading coefficients of fenestration
The various weather-dependent factors in the OTTV formulation, such as SF, TDeq and D T, were derived from isolated solar and temperature measurements carried out in the late 70’s at the National University of Singapore and the meteorological stations. As the weather data were incomplete, the factors derived from them also inherited certain degree of inaccuracy.
Research done since 1985 using computer simulation and a complete set of weather data revealed that the present OTTV does not correlate well with the cooling energy required to remove the heat load. Furthermore, it was found that the existing OTTV formulation does not properly account for the relative contributions of the three elements of heat gain through the envelope. Since then, much research had been performed at the National University of Singapore to obtain a more accurate formulation for OTTV that correlates closely with the cooling energy imposed on air-conditioning systems. To differentiate the new formulation from the original OTTV formulation, the term ‘Envelope Thermal Transfer Value’ (ETTV) has been given to the new formulation.
The new ETTV formulation is basically a modified version of the original OTTV formulation. Details of the formulation are given in Appendix A. The various weather-dependent factors in the new formulation are determined through vigorous computer simulations, which provide an accurate account of the three elements of heat gain through the envelope. Thus, the ETTV formulation is better able to reflect the total heat gain through a building envelope under local weather conditions.
Computer simulations have been performed to compare the correlation of OTTV and ETTV with the total heat gain through the building envelope. The results are as follows:
Table 2: OTTV and Heat Gain Correlation
|
Cases |
Annual total gain through envelope (x 103 kwh) |
% increase in total gain through envelope |
|
Reference case |
273.1 |
- |
|
10 % increase in OTTV |
314.5 |
15.2 |
|
15 % increase in OTTV |
328.7 |
20.4 |
|
25 % increase in OTTV |
366.1 |
34.1 |
|
35 % increase in OTTV |
408.4 |
49.5 |
Table 3: ETTV and Heat Gain Correlation
|
Cases |
Annual total gain through envelope (x 103 kwh) |
% increase in total gain through envelope |
|
Reference case |
273.1 |
- |
|
10 % increase in ETTV |
298.9 |
9.4 |
|
15 % increase in ETTV |
313.5 |
14.8 |
|
25 % increase in ETTV |
339.2 |
24.2 |
|
35 % increase in ETTV |
366.9 |
34.3 |
Table 2 shows that an increase in OTTV results in a much higher increase in heat gain in terms of percentage. On the other hand, Table 3 shows that the percentage increase in the total heat gain through the building envelope is almost equal to the percentage increase in ETTV. Hence, ETTV correlates much better than OTTV with the total heat gain through the envelope.
b The ETTV Equivalent of 45W/m2 in OTTV
The key difference between the ETTV formulation and the OTTV formulation lies in the magnitude of the Solar Factor (SF) which determines the amount of radiation gain through the windows. The SF used in the OTTV formulation is 130W/m2 while the SF for ETTV is given as 210 W/m2. Hence, the ETTV of a building will be higher than its OTTV. For a reference building model that meets the current OTTV requirement of 45 W/m2, its ETTV works out to be approximately 65 W/m2.
c Roof Thermal Transfer Value (RTTV)
Based on the same approach used in formulating ETTV, a new Roof Thermal Transfer Value (RTTV) formulation has also been developed. Please refer to Appendix B for the new RTTV formulation.
d Enhancing Thermal Performance of Building Envelope
In the ETTV formulation, the two most critical factors are the Window-to-Wall Ratio (WWR) and the Shading Coefficient of glass (SC). WWR represents the relative size of the windows and usually varies within a narrow range of 0.3 to 0.6 for practical design consideration. On the other hand, SC is a function of glass technology and can vary over a wide range depending on the type of glass used.
The current standard of 65W/m2 for ETTV (45W/m2 for OTTV) was derived from a model building with a WWR of 0.33, which was glazed with glass having a SC of 0.6. (A SC of 0.6 was considered to be optimum for the range of ‘non-reflective glass’ available in the late 70’s.) Therefore, for consistency and ease of comparison, a review of the ETTV standard would have to be based on the same model building using the latest technical information available from the glass industry as reference.
A parametric study was carried out for a range of WWR from 0.3 to 0.7 and SC from 0.2 to 0.6 to determine the corresponding ETTV of the model building. The result of this study is presented in Appendix C.
For the same reference WWR of 0.33 as used in the 1979 standard, Appendix C shows that the ETTV could vary from a low of 35W/m2 to a high of 65W/m2 corresponding to the range of SC from 0.2 to 0.6. The new ETTV standard is thus confined to this range of value. A low value for the new ETTV standard would mean fewer choices of glass types for use in building design. On the other hand, a high value would have marginal improvement on energy efficiency.
Architects and developers are naturally concerned about the impact of an enhanced ETTV on design choices and construction costs. Hence, the new ETTV standard would be set at the mid-point (50W/m2) between the lower bound (35W/m2) and the upper bound (65W/m2) of the range. Appendix C shows that an ETTV of 50W/m2 corresponds to a SC of 0.4. (For the same model building, an ETTV of 50W/m2 is equivalent to an OTTV of 35W/m2.)
A literature survey based on some glass catalogues has indicated that glass with SC of 0.4 or better is available readily and at reasonable cost. A random survey done on several existing and recently completed government and private buildings has shown that 18 out of 25 buildings complied with the proposed enhanced OTTV standard. Recently completed buildings like the SIA Building, PWC Building, The URA Centre and Tuas Checkpoint achieved an OTTV standard below 30 W/m2. Please see Appendix D.
e. Introduction of the system analysis approach
The existing prescriptive standard is inflexible in that it requires all the systems to comply with the minimum requirements. The proposed system analysis approach offers much greater flexibility. In this method, the designed building is compared to a prototype building, which is modelled using the existing prescriptive standards. If the total energy consumption of the designed building is lower than that of the prototype, then compliance is achieved. It allows certain system to exceed the minimum requirements on condition that energy performance for the other systems in the building is improved to compensate for the non-compliance of the specific systems. The proposed system analysis approach is based on the energy budget method of ASHRAE/IES Standard 90.1 on energy efficient design of new buildings.
A computer programme, Building Energy STandard (BEST), has been developed by NUS in collaboration with BCA to facilitate the implementation of the energy budget method. BEST is one of the recognised software for demonstrating compliance under the system analysis approach.
f. Efficiency of Air-Conditioning Equipment
With energy efficient equipment readily available, the coefficient of performance of air-conditioning equipment shall comply with the minimum performance rating in accordance with the relevant provisions of the Code of Practice for Energy Efficiency Standard for Building Services and Equipment - SS CP 24:1999.
IMPLEMENTATION OF THE ENHANCED STANDARDS
Feedback on ETTV and the enhancement from 45 W/m2 to 35 W/m2 was sought from the Singapore Institute of Architects, Institution of Engineers Singapore, Association of Consulting Engineers Singapore and Real Estate Developer's Association of Singapore. Through news release and the BCA website the public was also invited to comment on the enhancement. No substantive comment was received.
BCA is planning for a trial implementation of the revised standards. After the trial implementation, the results would be reviewed before the new standards are legislated.
CONCLUSION
ETTV gives a more accurate correlation with the total heat gain through a building envelope and, hence, is a more reliable indicator for energy efficiency. It would replace the current OTTV formulation for envelope. Similarly, the OTTV formulation for roof would also be replaced with the new RTTV formulation. To enhance the thermal performance of the building envelope, a thermal transfer value of 50 W/m2 would be the new ETTV and RTTV standard. The system analysis approach allows for flexibility in design without compromising energy efficiency while the requirement on energy efficient equipment is introduced to ensure that only energy efficient equipment are used.
When compared to the old standards, it is estimated that the new standards on OTTV, maximum lighting power budget and the efficiency of air-conditioning equipment could reduce electricity consumption by as much as 24%.
REFERENCES
1. Turiel, I., Curtis, R. B., and Levine, M. D., Energy 10,95 (1985)
2. Turiel, I., Curtis, R. B., and Levine, M. D. (1988). 'Analysis of overall thermal transfer value (OTTV) energy conservation standards for Singapore office buildings', ASHRAE Trans., 90 (2B), 647-661.
3. Chou, S.K and Lee, Y.K. (1988). ‘A simplified overall thermal transfer value equation for building envelope’, Energy, the International Journal, 13(8), pp. 657-670.
4. Chou, S.K. and Chang, W.L. (1996). ‘A generalised methodology for determining the total heat gain through building envelopes,’ International Journal of Energy Research, Vol. 20, pp 887-901.
5. SS CP 13: 1999 Code of practice for mechanical ventilation and air-conditioning in buildings, PSB, 1999.
6. SS CP 24: 1999 Code of practice for energy efficiency standard for building services and equipment, PSB, 1999.
7. SS CP 38: 1999 Code of practice for artificial lighting in buildings, PSB, 1999.
8. Building Control Regulations (Cap 29, Rg 1)
Appendix A
Envelope Thermal Transfer Value (ETTV) Formula
The Envelope Thermal Transfer Value (ETTV) formula is given as:
where
ETTV : envelope thermal transfer value (W/m2)
WWR : window-to-wall ratio
Uw : thermal transmittance of opaque wall (W/Km2)
Uf : thermal transmittance of fenestration (W/Km2)
CF : solar correction factor for fenestration*
SC : shading coefficients of fenestration
*Table: Solar Correction Factors (CF) for Walls
|
Pitch |
Orientation |
|||||||
|
Angle |
N |
NE |
E |
SE |
S |
SW |
W |
NW |
|
70o |
1.17 |
1.33 |
1.47 |
1.35 |
1.21 |
1.41 |
1.56 |
1.38 |
|
75o |
1.07 |
1.23 |
1.37 |
1.25 |
1.11 |
1.32 |
1.47 |
1.28 |
|
80o |
0.98 |
1.14 |
1.30 |
1.16 |
1.01 |
1.23 |
1.39 |
1.20 |
|
85o |
0.89 |
1.05 |
1.21 |
1.07 |
0.92 |
1.14 |
1.31 |
1.11 |
|
90o |
0.80 |
0.97 |
1.13 |
0.98 |
0.83 |
1.06 |
1.23 |
1.03 |
|
95o |
0.73 |
0.90 |
1.05 |
0.91 |
0.76 |
0.99 |
1.15 |
0.96 |
|
100o |
0.67 |
0.83 |
0.97 |
0.84 |
0.70 |
0.92 |
1.08 |
0.89 |
|
105o |
0.62 |
0.77 |
0.90 |
0.78 |
0.65 |
0.86 |
1.01 |
0.83 |
|
110o |
0.59 |
0.72 |
0.83 |
0.72 |
0.61 |
0.80 |
0.94 |
0.78 |
|
115o |
0.57 |
0.67 |
0.77 |
0.67 |
0.58 |
0.75 |
0.87 |
0.73 |
|
120o |
0.55 |
0.63 |
0.72 |
0.63 |
0.56 |
0.71 |
0.81 |
0.69 |
Appendix B
Roof Thermal Transfer Value (RTTV) Formula
The Roof Thermal Transfer Value (RTTV) formula is given as:
where
RTTV : roof thermal transfer value (W/m2)
SKR : skylight ratio of roof, (skylight area/gross area of roof)
Ur : thermal transmittance of opaque roof (W/Km2)
Us : thermal transmittance of skylight area (W/Km2)
CF : solar correction factor for roof *
SC : shading coefficient of skylight portion of the roof.
*Table: Solar Correction Factors (CF) for Roof
|
Pitch |
Orientation |
|||||||
|
Angle |
N |
NE |
E |
SE |
S |
SW |
W |
NW |
|
0o |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
|
5o |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
1.00 |
|
10o |
0.99 |
0.99 |
1.00 |
1.00 |
1.00 |
0.99 |
0.99 |
0.99 |
|
15o |
0.98 |
0.98 |
0.99 |
0.99 |
0.99 |
0.98 |
0.98 |
0.98 |
|
20o |
0.96 |
0.97 |
0.98 |
0.98 |
0.97 |
0.97 |
0.97 |
0.96 |
|
25o |
0.93 |
0.95 |
0.96 |
0.96 |
0.95 |
0.95 |
0.95 |
0.94 |
|
30o |
0.91 |
0.92 |
0.94 |
0.94 |
0.93 |
0.93 |
0.93 |
0.91 |
|
35o |
0.88 |
0.90 |
0.92 |
0.91 |
0.90 |
0.90 |
0.90 |
0.89 |
|
40o |
0.84 |
0.87 |
0.89 |
0.88 |
0.87 |
0.87 |
0.87 |
0.85 |
|
45o |
0.8 |
0.83 |
0.86 |
0.85 |
0.83 |
0.84 |
0.84 |
0.82 |
|
50o |
0.76 |
0.80 |
0.83 |
0.82 |
0.79 |
0.80 |
0.81 |
0.78 |
|
55o |
0.72 |
0.76 |
0.80 |
0.78 |
0.75 |
0.76 |
0.78 |
0.75 |
|
60o |
0.67 |
0.72 |
0.76 |
0.74 |
0.70 |
0.73 |
0.74 |
0.71 |
|
65o |
0.63 |
0.68 |
0.73 |
0.70 |
0.66 |
0.69 |
0.71 |
0.67 |
Appendix C
ETTV for Different WWR and SC
|
SC |
0.20 |
0.25 |
0.30 |
0.35 |
0.40 |
0.45 |
0.50 |
0.55 |
0.60 |
|
WWR |
|||||||||
|
0.30 |
35.38 |
38.54 |
41.71 |
44.87 |
48.03 |
51.20 |
54.36 |
57.52 |
60.69 |
|
0.33 |
36.54 |
40.02 |
43.50 |
46.98 |
50.46 |
53.94 |
57.42 |
60.90 |
64.38 |
|
0.35 |
37.31 |
41.00 |
44.69 |
48.38 |
52.07 |
55.76 |
59.45 |
63.15 |
66.84 |
|
0.40 |
39.24 |
43.46 |
47.68 |
51.89 |
56.11 |
60.33 |
64.55 |
68.77 |
72.98 |
|
0.45 |
41.17 |
45.92 |
50.66 |
55.41 |
60.15 |
64.90 |
69.64 |
74.39 |
79.13 |
|
0.50 |
43.10 |
48.37 |
53.65 |
58.92 |
64.19 |
69.46 |
74.74 |
80.01 |
85.28 |
|
0.55 |
45.03 |
50.83 |
56.63 |
62.43 |
68.23 |
74.03 |
79.83 |
85.63 |
91.43 |
|
0.60 |
46.96 |
53.29 |
59.61 |
65.94 |
72.27 |
78.60 |
84.92 |
91.25 |
97.58 |
|
0.65 |
48.89 |
55.74 |
62.60 |
69.45 |
76.31 |
83.16 |
90.02 |
96.87 |
103.72 |
|
0.70 |
50.82 |
58.20 |
65.58 |
72.96 |
80.35 |
87.73 |
95.11 |
102.49 |
109.87 |
Appendix D
OTTV of Major Government and Private Buildings
|
S/No |
Building Project |
OTTV(W/m2) |
|
1 |
Changi Airport Terminal II Expansion (North Finger) |
20.4 |
|
2 |
Changi Airport Terminal II Expansion (South Finger) |
22.1 |
|
3 |
The Treasury |
26.6 |
|
4 |
SIA Building at Robinson Rd |
26.6 |
|
5 |
Tuas Checkpoint |
28.2 |
|
6 |
Causeway Point |
28.4 |
|
7 |
PWC Building |
29.2 |
|
8 |
URA Building |
29.3 |
|
9 |
Ngee Ann City |
30.7 |
|
10 |
NTUC Income, Tampines Point |
32.2 |
|
11 |
Immigration Building |
32.8 |
|
12 |
Singapore Art Museum |
33.8 |
|
13 |
Institute of Health Building |
34.0 |
|
14 |
Compac Centre, Tampines Plaza |
34.0 |
|
15 |
Changi Airport Terminal II |
34.1 |
|
16 |
Revenue House |
34.5 |
|
17 |
West Mall |
34.7 |
|
18 |
MOE HQ |
34.9 |
|
19 |
National Dental Centre Building |
35.4 |
|
20 |
Keppel Tat Lee Bank Building at Market St |
36.3 |
|
21 |
Civil Service College Building |
36.4 |
|
22 |
Republic Plaza |
38.8 |
|
23 |
ISEAS Building |
37.9 |
|
24 |
National Development Building |
39.9 |
|
25 |
KK Hospital |
41.4 |