ANALYSIS AND EXPERIMENTAL INVESTIGATION OF THE NTCH BUILDING ON ENERGY CONSERVATION DESIGNS

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1 , Volume 2, Number 1, p.7-14, 2001 ANALYSIS AND EXPERIMENTAL INVESTIGATION OF THE NTCH BUILDING ON ENERGY CONSERVATION DESIGNS K.H. Yang and J.N. Lee Mechanical Engineering Department, National Sun Yat-Sen University, Kaohsiung, Taiwan 80424, R.O.C. (Received 8 March 2001; Accepted 18 April 2001) ABSTRACT Being located in the subtropical area at 24 degrees north latitude, the NTCH building s cooling load is tremendous, necessitating extra consideration on energy saving designs. External and internal shading devices were designed followed by sensitivity analysis to select an optimal solution to the building façade where glass block, or curtain walls will be installed. A full-scale experimental investigation was conducted successfully to validate the design effectiveness and is discussed in this paper. 1. INTRODUCTION The new Taichung City Hall, or the NTCH building is located in the central part of Taiwan, with 35 o C ambient temperature and 85% RH during the summer. The tremendous cooling demand urged a thorough study of the building facade, not only to save energy but to comply with the local building energy code as well. The index adapted in Taiwan s building code mandates an annual energy consumption of less than 110 kwh/m 2 -fl-area yr, applicable to office buildings of over 2,000 m 2 floor area since Unlike the OTTV (Overall Thermal Transfer Value) index adapted in Singapore which is climatically similar to Taiwan, the regulates an overall thermal load of the building perimeter zone per floor area, including internal heat gain, thermal insulation performance, and solar heat gain, as shown in equation, instead of a specific building material thermal resistance value alone as adapted in other countries in cold area. In 1987, Yang [1] conducted a series of full-scale experiment to validate the shading effectiveness vs. thermal mass effect of buildings in Taiwan, leading to the heavy weighting imposed to the solar incidence term on the index. In this way, the architect is allowed more flexibility in building envelope designs. For example, when a curtain wall or glass block wall (presumably the energy killer in this area) is designed, it can still be justified if it can be balanced out by other effective energy-saving designs such as external shading devices. This is exactly our case that the NTCH, a curtain-walled office building, trying to maintain a high-tech image yet has to comply with local building energy code by utilizing external and internal shading designs. = a 0 + a 1 G + a 2 L DH + a 3 ( Mk IHk) Where, : Building envelope annual energy consumption index (kwh/m 2 -fl-area yr) L : Envelop heat gain (W/m 2 -fl-area K) Mk: Coefficient of solar incidence in direction k (-) G: Internal heat dissipation rate (Wh/m 2 -fl-area yr) DH: Cooling degree hour (K h/yr) IHk: Solar incidence in k-direction (Wh/m 2 yr) a 0, a 1, a 2, a 3 : Regression coefficients These coefficients of equation can be obtained from tables listed in the Building Energy Conservation Code of Taiwan [2], which resulted from earlier work by Lin [3], and Yang et al. [1]. 2. BUILDING DESCRIPTION AND TRIAL ENVELOPE DESIGNS The NTCH is an office building with eleven floors and 46,000 m 2 total floor area, where Fig. 1 shows a perceptive view. Two design concepts were proposed for the external shading device, including case 1, a meshed structure as shown in Fig. 2 and with case 2, a simple flat plate as shown in Fig. 3. The overhang length under consideration ranges from 1.6 m to 2.2 m, while in case 1 an additional vertical shade with 1.6 m in length was used. The selection of glass is another key issue, where the SC (shading coefficient) value was emphasized. In defining the SC value, full-scale experimental investigation has been carried out on curtain walls with 10 mm clear glass as a comparison basis. The lower SC value apparently indicated its higher performance in absorbing heat. The internal shading presents another option. The corresponding values calculated in each case results in Table 1. 7

2 Fig. 1: A perspective view of the NTCH Building cleaning balcony fixed louvers shading glass coefficient : specification : 0.48 insulating glass alternative : mm float glass with hard coating section Fig. 2: Design Case 1 of the external shading of NTCH Building 8

3 cleaning balcony shading glass coefficient : specification : fixed louvers 0.48 insulating glass alternative : mm float glass with hard coating 0.24 insulating glass with silksreened dot or line pattern alternative: 0.21 laminated glass with silkscreened dot or line pattern section Fig. 3: Design Case 2 of the external shading of NTCH Building Table 1: calculation results of the NTCH building trial designs External shading type Glass type Internal shading Case 1 Clear Glass 10 mm, (SC = 0.48) None Fail Case 1 Clear Glass 10 mm, (SC = 0.48) Light-color Fail Case 1 Heat Absorbing Glass, (SC = 0.48) Light-color Pass Case 1 Heat Absorbing Glass, (SC = 0.48) Medium-color Fail Case 1 Heat Absorbing Glass, (SC = 0.42) Medium-color Pass Case 2 Heat Absorbing Glass, (SC = 0.42) Medium-color Pass The successful trial designs, which complied with the building energy code, should be considered further through sensitivity analysis to identify the most cost-effective design options as shown in the following. 3. SENSITIVITY ANALYSIS In case 1, sensitivity analysis is conducted by: 1-1 changing the horizontal shading, or overhang length 1-2 changing the vertical shading length 1-3 changing both lengths simultaneously While in case 2, only the horizontal overhang length will be changed. Table 2 showed the sensitivity calculation result of case 1. Actually, only the 120 cm 160 cm option failed to comply with the code. The overhang length reduction was justified by the other cases. Table 3 further indicated that the design options are all successful while the reduction of vertical shading length has an insignificant impact on the 9

4 value, implying that the vertical length reduction is also a feasible solution without causing adverse effect on value. Finally, when both lengths were reduced, such as from 200 cm 160 cm to 160 cm 120 cm, successful result can be obtained as shown in Table 4. On the other hand in case 2 design, only the horizontal overhang length was altered which resulted in Table 5. It indicated that the 200 cm overhang is a better solution, although the value of is marginal. Therefore, another thought is to utilize a lower SC-value glass simultaneously which results in Table 6, with the curtain wall SC-value reduced from 0.48 to A deep shading is obviously contributing to energy savings, but its impact to the outlook and structure of building façade, especially under strong wind conditions, is a major concern to the architect, not to mention the excessive cost incurred. In considering all these factors, the optimal design ends up with 200 cm external shading, with 0.42 SC-value glass block curtain wall and is adapted in this project. Table 2: Sensitivity analysis of Case 1-1 (cm cm) Pass Pass Pass Pass Pass Pass Pass Pass Fail Table 3: Sensitivity analysis of Case 1-2 (cm cm) Pass Pass Pass Pass Pass Pass Pass Pass Pass Table 4: Sensitivity analysis of Case 1-3 (cm cm) Pass Pass Pass Pass Pass Fail Table 5: Sensitivity analysis of Case 2-1 (cm x cm) Pass Pass Pass Pass Pass Pass Pass Pass Table 6: Sensitivity analysis of Case 2-2 (cm x cm) Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass Pass 10

5 4. EXPERIMENTAL INVESTIGATION In order to validate the design effectiveness, the final design was realized in the full-scale test building located in the National Sun Yat-Sen University in the southern part of Taiwan. The test building is 17 m 12 m 10 m in size, and was equipped with two identical rooms sized 5 m 5 m 3 m each. Each room was installed with independent air-conditioning system and the façade can be replaced individually to facilitate a realistic comparative study under local weather conditions. The two identical rooms are facing true west. Before test, each room was calibrated by recording the energy consumption under the same cooling load condition and HVAC system operation schedule in maintaining the same indoor environment. The comparison of power consumption in each room shows a deviation of 1%. Normally, sensitivity analysis was performed by researchers using computer simulation only. However, due to our unique experimental design, it can be validated on this test building in full-scale and under actual local weather conditions. The experimental data adapted in the building energy code in Taiwan were all generated this way from this test building. In room A, 10 mm clear glass wall without any shading was installed as comparison basis. In room B, the 200 cm external shading in case 2 with 0.42 SC glass was installed. Power consumption of each room was recorded automatically with a data acquisition system where the recording interval can be set in each 10 minutes, or an hour. During the experiment, internal shading is added as another option so that the comparative study can be performed on a more profound basis. Fig. 4 shows a close up of the experiment. The experimental result can be classified into three cases. In experimental case 1, comparison was made with or without installing the 200 cm external shading device, no internal shading was installed in either room. As shown in Fig. 5a, that in a typical summer day, the outdoor temperature rises from 28 o C at 08:00 to as high as 35 o C from mid-day till 15:30. While on the other hand, in the afternoon, both rooms begin to show significant differences in the indoor temperature swing and the power consumption needed to keep them within thermal comfort. In other words, when external shading was installed in room B, around 25% of energy savings was experienced as shown in Fig. 5b. In experimental case 2, only internal shading was installed in room B. The indoor temperature swing is more significant as shown in Fig. 6a. The comparison of Figs. 5a and 6a indicated that the external shading cut down the solar incidence to rooms more effectively. Fig. 6b showed around 15.7% energy savings when internal shading was installed which presents another option when external shading is difficult to apply in some cases. The comparison of Figs. 5b and 6b showed a difference of around 10% energy savings, indicating that external shading should indeed be the first priority for energy efficient building envelope designs under local weather conditions. In experimental case 3, an extreme case comparison was made. That is, in room A, internal shading was installed, while in room B, both internal and external shading were installed. In this case, both rooms kept the indoor environment in a thermal comfort condition almost perfectly as shown in Fig. 7a. The power consumption in achieving this is even more encouraging. As shown in Fig. 7b, the incremental energy savings of external shading alone is 25%. When one further compares room A in Fig. 5b with room B in Fig. 7b, that is, a case where only curtain wall was installed vs. one with both external and internal shadings installed, the energy savings of the latter could be up to 42% on a clear typical summer day in Kaohsiung, Taiwan. This is exactly the case of NTCH building where both external shading with 200 mm depth and internal shading with venetian blinds will be installed. 5. CONCLUSIONS Fig. 4: A close-up of the experiment In subtropical weather, the solar incidence imposes heavy cooling load and should be taken care of with efficient shading designs. The most effective design comes to external shading where 25% energy savings can be expected on a clear summer 11

6 Outdoor temperature Room A without external shading Room B with external shading (3) Temperature ( o C) (3) Fig. 5a: Experimental result of temperature with or without external shading, and no internal shading on a clear summer day in Kaohsiung, Taiwan Power consumption (kwh) Room A without external shading Room B with external shading Fig. 5b: Experimental result of power consumption with or without external shading, and no internal shading Outdoor temperature Room A without internal shading Room B with internal shading (3) Temperature ( o C) (3) Fig. 6a: Experimental result of temperature with or without internal shading, on a clear summer day in Kaohsiung, Taiwan 12

7 Power consumption (kwh) Room A without internal shading Room B with internal shading Fig. 6b: Experimental result of power consumption with or without internal shading, on a clear summer day in Kaohsiung, Taiwan Outdoor temperature Room A with internal shading Room B with internal & external shading (3) Temperature ( o C) (3) Fig. 7a: Experimental result of temperature in Room A with internal and Room B with internal & external shading, on a clear summer day in Kaohsiung, Taiwan Power consumption (kwh) Room A with internal shading Room B with internal & external shading Fig. 7b: Experimental result of power consumption in Room A with internal and Room B with internal & external shading on a clear summer day in Kaohsiung, Taiwan 13

8 day in Kaohsiung, Taiwan as experimented in this study. In some cases, where building façade does not allow external shading, internal shading can become another option with 15% energy saving potential. In extreme cases, such as a building with large opening area and glass block curtain walls, the architect would have to consider installing external and internal shading devices. In so doing, the energy savings could be up to 42%, which would hopefully offset the large building envelop cooling load and falling into the threshold value to comply with the code which is actually the case in NTCH building. On the other hand, the work done in this paper only shows the experimental result under peak cooling load conditions. On a seasonal or annual basis, the energy savings effect is expected to become moderate due to varying weather conditions, although the tendency stays the same. The analytical work and the sensitivity analysis through full-scale experiment performed in this study warrant the NTCH building as an energy efficient building, even with curtain wall installed. ACKNOWLEDGEMENT The authors would like to express their appreciation to the Taichung City Government of Taiwan for sponsoring this research project. REFERENCES 1. K.H. Yang and R.L. Hwang, The analysis of design strategies on building energy conservation in Taiwan, Building and Environment, Vol. 28, No. 4, pp (1993). 2. Building Energy Conservation Code of Taiwan, Ministry of Interior (1995) - In Chinese. 3. H.T. Lin, The design guide of building envelope designs in Taiwan (1994) - In Chinese. 14