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SimulationofintermittenttransientcoolingloadcharacteristicinanacademicbuildingwithcentralizedHVACsystem

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Simulation of intermittent transient cooling load characteristic in an academic building with centralized HVAC system

Petrus Tri Bhaskoro

Mechanical Engineering Department University Teknologi PETRONAS

Bandar Seri Iskandar, Malaysia petrustribhaskoro@gmail.com

Syed Ihtsham Ul Haq Gilani

Mechanical Engineering Department University Teknologi PETRONAS

Bandar Seri Iskandar, Malaysia syedihtsham@petronas.com.my

Mohd Shiraz Bin Aris

Mechanical Engineering Department University Teknologi PETRONAS

Bandar Seri Iskandar, Malaysia mshiraz_aris@petronas.com.my

Abstract— This paper aims to estimate intermittent transient cooling load characteristics in an academic building with large glazing area using TRNSYS. Simulation and experimental studies have been done in the past to estimate cooling load characteristic and to find strategies to reduce the energy used by the centralized HVAC system in various ways. However, these studies were mostly conducted for a building with simultaneous occupancy pattern. Since occupancy pattern in a university is intermittent, the cooling load characteristic would be different with that with simultaneous occupancy pattern. The building thermal and simulation model was first built and validated by comparing the results with experimental results. The validated model was then used to estimate the cooling load characteristic on a year basis. In addition, the effect of the occupancy pattern were also analyzed and discussed. The results showed that the ratio of cooling loads at minimum and maximum were 0.52, 0.54, and 0.68 for the workshop, classroom and office due to the occupancy pattern. The results also implied that the HVAC system should be equipped with additional equipment to track the occupancy and make it work based on the occupancy pattern. This would reduce the energy wasted during unoccupied period.

Index Terms— Keywords: Building simulation, intermittent transient cooling load characteristic, centralized HVAC system, academic glazed building

I. INTRODUCTION Recently, efforts to reduce energy usage are doubled due

to global warming and energy source shortage. With the fact that HVAC system is the major energy consumption in office buildings in Malaysia (up to 64% of the total energy consumed) [1] and a trend of higher cooling energy usage due to climate change [2], strategies to reduce the cooling load is highly needed .

Davis III and Nutter found that occupancy pattern in a university differ from that in many office and residential buildings [3]. Since cooling load in those building is mostly driven by the occupancy pattern, the difference will result in

different cooling load characteristics. In office or residential buildings, the room is usually occupied during cooling period even though the number of occupants may be changing over the period [4,5]. In a university, there are occupied and unoccupied hours during cooling period [6]. Due to this, the cooling type is likely to be continuous load for office or residential buildings and intermittent load for a university. The types of rooms are also found to be more various in a university than that in office or residential building.

Simulation and experimental studies showed that external window glazing could have significant effect on indoor thermal comfort and energy consumption. Due to the glazing constructions and properties, more shortwave solar radiation will be transmitted into the zone while long wave solar radiation from internal heat sources will be trapped inside the zone. On the other hand, it will reduce heat gain from artificial lighting during daytime and release the heat buildup during daytime easily during nighttime [7,8,9,]. These results indicate that there is a trade-off between thermal comfort, lighting need and energy consumption.

Simulations and experimental studies to study cooling load characteristic have been done in the past by taking into account internal (occupant, lighting, etc) and external (wall heat gain, window heat gain and solar radiation heat gain) cooling load. The building thermal model was first constructed based on physical and operation characteristic of the building [4,9,10,11]. The model was then validated by comparing simulations results with experimental results before it is used for further analysis. From the simulations results, it was found that for office and residential buildings, heat gained from building envelope and lighting are the major cooling load [4,9]. The previous studies of cooling load characteristics, however, were mostly conducted for office and residential building.

Therefore, the objective of this work is to do simulation study of intermittent transient cooling load characteristic of an academic building with large glazing area using transient system simulation software TRNSYS. A building thermal and

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2011 International Conference on Environment Science and Engineering IPCBEE vol.8 (2011) © (2011) IACSIT Press, Singapore

simulation model was first constructed and validated by comparing the results with experimental results. The validated model was then used to simulate the hourly intermittent transient cooling load characteristic for a year. In addition, the effect of usage factor and the occupancy pattern on the energy consumption and the HVAC system performance were analyzed and discussed.

II. BUILDING THERMAL AND SIMULATION MODEL A transient building thermal and simulation model for the

academic blok was developed using energy building simulation software TRNSYS. The building thermal and simulation model was developed based on the following: • Building construction detail and occupancy schedules • List of indoor lights, equipments and machines • Centralized HVAC system in the building • Cooling load types in the building

A. Building thermal details and occupancy schedules An academic block located at Universiti Teknologi

PETRONAS (UTP) was chosen for the study. The blok is facing east with large window glazing area (nearly 100% of glazing area) and on 32m above sea level. It has three levels with 4m height on each floor. Floor lay out for the block was detailed on construction drawing [12].

As an academic block, it has offices, classrooms and workshop to support practical/research works by the students and staffs. Material used for walls on the workshop, conductivity and U-values of each material were described in Table 1 and 2. Student academic schedule was described in Table 3 while staff schedule was based on UTP working days

(Monday-Friday), working hours (08.00-17.00) and Malaysia national holidays.

TABLE 1 BLOCK 16 BUILDING MATERIAL SPECIFICATION [13]

Building Construction Details ( thickness-thermal conductivity)

External Wall

Steel (5mm-54 kJ/hmK), Air gap (0.047 hm2K/kJ), Steel (11mm-54 kJ/hmK)

Partition Wall

Plasterboard (25mm-0.576 kJ/hmK), Air Gap (92mm-0.047 hm2K/kJ), Plasterboard (25mm-0.576 kJ/hmK)

Flooring Concrete slab (100mm-4.07 kJ/hmK), Common concrete (550mm-7.56 kJ/hmK)

Window Optiwhite glass (8mm-3.24 kJ/hmK)

Roofing Alumunium (1mm-846 kJ/hmK), Rockwool (25mm-0.162 kJ/hmK), Alumunium foil (1-846 kJ/hmK), Common concrete (10mm-7.56 kJ/hmK)

Activity schedule for each workshops in block 16 are based on practical schedule of each subject. Numbers of equipment used and usage factors of the equipments in each type of rooms were based on observation data and discussion with the technician/staff. Activities in the workshop with no specific schedule were not considered in this simulation.

TABLE 2 U-VALUES OF BLOCK 16 WALLS

Type U-values (conduction) (W/m2K)

U-value (Overall) (W/m2K)

Floor 6.74 6.433 Roof 6.425 4.928 Partition Wall 7.474 5.522 Window 31.25 20.448 External Wall 21.143 10.58

TABLE 3 STUDENTS ACADEMIC SCHEDULE 2009

Month Jan Feb Mar Apr May Jun Week

I B S S S E B II B S S S E B III S S B S E B IV S S S S E B V - - S S - B

Month Jul Aug Sep Oct Nov Des Week I B S S S E B II B S S S E B III S S S S E B IV S S B S E B V - - S S - B

B : Mid semester or semester break E : Examination S : Study week

B. Centralized HVAC system in the building Centralized AC system with absorption chiller was used to

cover the cooling load from all the academic blocks. Each block has 3 levels and each level have two wings of group zones (left and right wing). The buildings are fully sealed and fresh air is supplied by the air handling unit (AHU) on each floor. There are two air handling unit (AHU) on each level. AHU 1 is used to provide air supply to the right wing while AHU 2 is for the left wing. A variable fan speed and a

Nomenclature Ts Surface temperature, oC Tstar Artificial temperature node, oC As Surface area, m2 Requiv Equivalent resistant between the walls with a

node, h m2 K kJ-1 qc,s Convection heat flux an the surface of the wall,

kJ h-1 qr,s Long wave radiation heat flux on the surface of

the wall, kJ h-1 hconv,s, Convective heat transfer coefficient on the

surface of the wall, kJ h-1 m-2 K-1 σ Stephan-Boltzmann constant εs, Long-wave emissivity of the surface Ta,s Ambient temperature, oC Tfsky Fictive sky temperature, oC qcomb combined convective and radiation heat

transfer, kJ h-1 RH Relative humidity, % Superscripts i inside o outside

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regulating valve were used in the AHU to give variable supply air and chilled water flow rate.

The HVAC system use variable air volume (VAV) system to control supply air flow rate enter the room according to the cooling load. A temperature sensor is placed in each room to monitor room temperature [14]. The AC system was operated from 7 a.m till 7 p.m to keep indoor temperature set point (24oC). Indoor relative humidity was kept floating depend on the indoor temperature.

C. Cooling Load in the Building Heat gain from equipments (QEQUIP), heat gain from

lighting (QLIGHT), heat gain from electronic equipment and heat gain from building envelope due to solar radiation (QENV) would contribute to sensible cooling load (QSEN). Heat gain from occupants (QPERSON), heat gain from ventilation (QVENT) and heat gain from infiltration (QINF) would contribute to both latent (QLAT) and sensible cooling load. These heat gains were considered in the simulation.

The heat balance method is used in TRNSYS as a base for all calculations. For conductive heat gain at the surface on each wall, TRNSYS use transfer function method (TFM) as a simplification of the arduous heat balance method [15]. Heat gain through radiation and convection within the zone were calculated using the star network given by Seem:

(1)

Heat gain through radiation and convection for external surface were calculated by:

(2)

(3)

(4)

Convection heat transfer coefficient for inside and outside walls were set 11 kJ/hm2K and 64 kJ/hm2K as recommended by the software [15]. Minimum ventilation rate required in each room is calculated based on ASHRAE standard [16]. Degree levels of activities were based on the types of activities in each room and it was inputted to get the portion of sensible and latent heat from ISO 7730 table. Heat gains from the equipments were calculated based on rated power, usage factor, load factor and its efficiency. Convective and irradiative fraction for these heat gains were 0.7 and 0.3 while for artificial lights, the values were 0.6 and 0.4 [17].

III. ASSUMPTIONS AND STANDARDS Building and occupant ventilation components were

considered based on ASHRAE standard [16]. Building ventilation component is at rate 0.3 L/sm2 and occupant ventilation rates per person are 2.5 L/s per person. Infiltration air exchange due to door openings and leakage air was assumed to be 0.05/h during AC operation and 0.15/h during non-AC operation.

For calculation simplification, influence of indoor furnishing to indoor RH can be neglected since indoor furnishing will affect on indoor RH level at less than 2% RH, especially during daytime operating mode condition [2]. Occupant activity level in classroom and office was set at level 4 with sensible and latent heat values 269kJ/hr. Level 5 of activity level was applied for workshop where practical work existed. The sensible heat value is 332.33kJ/hr and the latent heat value is 342.88kJ/hr. These values are based on ISO 7730 table.

IV. EXPERIMENTAL DATA MEASUREMENTS Indoor and outdoor experimental measurements were

done for 2 weeks for validation purposes only. Indoor temperature, indoor relative humidity, supply air flow rate and supply air temperature were measured to get indoor environmental and supply air conditions. Supply air flow rate and temperature measurements are performed under the maintenance department supervision. Scheduled calibrations were performed by the department to maintain the accuracy of the equipments.

Ambient temperature, ambient relative humidity, global solar radiation, wind speed and wind directions measurements were done to get outdoor environmental conditions. The first three outdoor parameters measurements were done every 5 minutes by Firmanda [18] while wind speed and wind direction were not measured on this locations due to technical difficulties. These measurements data were collected from nearby weather station at Ipoh.

V. COMPARISON OF EXPERIMENTAL AND SIMULATION RESULTS

The comparison of indoor temperature between experimental measurement and simulation model is shown in Fig. 1 and 2. 81.25% of the simulated temperature values were found to be within +1oC with the measured values. The difference occurs may be due to climatic difference between building area and outdoor experiment area.

Figure 1. Comparison of indoor temperature between measured values and

simulation model during a holiday.

os,r,os,c,os,comb, qqq +=

( )os,sa,os,conv,os,c, TThq −=

( )4fsky4os,os,os,r, TTσεq −=

)(1,

,,staris

isiequiv

TTAR

−=+= is,r,is,c,is,comb, qqq

293

Figure 2. Comparison of indoor temperature between measured values and

simulation model during a working day.

Indoor relative humidity comparison between experimental measurement and simulation model is shown in Fig. 3 and 4. 72.5% of the simulated relative humidity values were found to be within +5% compare to measured values. It may be due to influence of indoor temperature which will amplify the difference.

Figure 3. Comparison of indoor relative humidity between measured values

and simulation model during a holiday.

Figure 4. Comparison of indoor relative humidity between measured values

and simulation model during a working day.

The simulation model was then used to calculate intermittent transient cooling load characteristic of the building for a year. The result was then used to investigate the effect of usage factor and the HVAC system performance.

VI. INTERMITTENT TRANSIENT COOLING LOAD CHARACTERISTIC

Intermittent transient cooling load for a year was calculated by the software using hourly climatic weather data collected from Ipoh weather station. The characteristics for typical workshop, classroom and office in the building during peak and off-peak period were shown in Fig. 5 to Fig. 10

Figure 5. Intermittent transient cooling load characteristic of workshop in the

academic building during peak period.

In the workshop, during peak period, the sensible and

latent heat rise due to occupancy and practical activity. It can be seen that peak cooling load during peak period occur during occupied hours. However, the occupant exists only for several hours and become empty after that. The peak cooling load during peak period and off-peak period were counted up to 73 MJ and 39 MJ.

In the classroom, sensible and latent heat rise due to occupancy and more ventilation air needed. Similar to workshop, the peak cooling load during peak period occur during occupied hours. However, occupancy in the classroom was more frequent compare to that in the workshop. The peak cooling load during peak period and off-peak period were counted up to 51 MJ and 25 MJ.

Figure 6. Intermittent transient cooling load characteristic of workshop in the

academic building during off-peak period.

294

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additional equipment is needed by the current system to track the occupant and control the HVAC system based on the occupancy.

Strategies to solve the problems and handle the intermittent load by taking into account usage factor and occupancy pattern for each type of room will be discussed and presented in the near future.

REFERENCES [1] Energy efficiency. Pertubuhan arkitek Malaysia. CPD seminar. 2004 [2] K. K. W. Wan, D. H. W. Li, D. Liu, and J. C. Lam (2010). Future

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[4] Z. Li, W. Chen, S. Deng, and Z. Lin (2005). The characteristic of space cooling load and indoor humidity control for residences in the subtropics. Building and environment. Volume 41 (2006), PP 1137-1147. Available: http://www.elsevier.com/locate/enbuild

[5] K.W. Mui, and L.T. Wong (2006). Cooling load calculations in subtropical climate. Building and Environment. Volume 42 (2007), PP 2498–2504. Available: http://www.sciencedirect.com

[6] P. T. Bhaskoro, “Transient cooling load simulation of a mechanical workshop at UTP,” presented at the ICPER , Kuala Lumpur, Malaysia, June 15-17, 2010, Paper EF 12

[7] A. Stegou-Sagia, K. Antonopoulos, C. Angelopoulou, and G. Kotsiovelos (2007). The impact of glazing on energy consumption and comfort. Energy Conversion and Management. Volume 48 (2007), PP 2844–2852. Available: http://www.sciencedirect.com

[8] N. A. M. Al-Tamimi, S. F. S. Fadzil, and A. Abdullah, “The effect of orientation and glazed area to the indoor air temperature in unventilated building in hot-humid climate,” 3rd International Conference on Built Environtment in Developing Countries, Kuala Lumpur, 2009

[9] J. C. Lam, and D. H. W. Li (1998). An analysis of day lighting and solar heat for cooling dominated office buildings. Solar Energy. Volume 65 (1999), No. 4, PP 251–262

[10] M. Bessoudo, A. Tzempelikos, A.K. Athienitis, and R. Zmeureanu (2010). Indoor thermal environmental conditions near glazed facades with shading devices-Part I: experiments and building thermal model. Building and Environment. Volume 45 (2010), PP 2506 – 2516. Available: http://www.elsevier.com/locate/buildenv

[11] Z. Lin, and S. Deng (2004). A study on the characteristics of nighttime bedroom cooling load in tropics and subtropics. Building and Environment. Volume 39 (2004), PP 1101 – 1114. Available: http://www.sciencedirect.com

[12] Construction drawing, Universiti Teknologi PETRONAS [13] Material specification, University Teknologi PETRONAS [14] Air conditioning and ventilation drawing, University Teknologi

PETRONAS [15] TRNSYS 16 manual [16] Ventilation for acceptable indoor air quality, ASHRAE, Inc., 2005 [17] J. D. Spitler, D. E. Fisher, and C. O. Pederson, “The radiant time series

cooling load calculation procedure,” ASHRAE trans. Vol. 103, part 2, 1997

[18] D. Firmanda,”Energy and techno-analysis of solar power system for residential house lighting,” Symposium paper, unpublished

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