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Space temperature difference, cooling coil and fan—energy and indoor air quality issues revisited
Energy and Buildings 37 (2005) 49–54
Space temperature difference, cooling coil and fan—energy and indoor air quality issues revisited S.C. Sekhar∗ Department of Building National University of Singapore, 4 Architecture Drive, Singapore 117566, Singapore Received 10 March 2004; received in revised form 26 April 2004; accepted 6 May 2004
Abstract In designing an energy-efficient air-conditioning system that also simultaneously addresses the needs of adequate ventilation and acceptable indoor air quality, several factors begin to play an important role. Among several others, the cooling coil, the fan and the temperature difference between the space and the supply air (commonly known as the Space T) can be considered to be crucial. For a given space cooling load, the choice of a particular Space T has an implication on the amount of supply air required, which further has an impact on the performance of the cooling and dehumidifying coil as well as the fan. Inherent in these implications are issues related to energy, ventilation and indoor air quality. This paper investigates these implications and quantifies them by considering a hypothetical building in a tropical climate and subjecting the design to several parametric variations involving different Space Ts for a given space temperature and humidity condition. The total power requirements, comprising additional cooling, reheating and higher fan power, for a design involving a Space T of 5 ◦ C can be as high as a factor of 2.2 of the total power for a design with a Space T of 8 ◦ C. The implication of higher supply air flow rates on duct design is qualitatively discussed. For a given space cooling load and a given Space T, the implication of increased design ventilation rates to address part-load ventilation requirements can lead to an additional installed cooling capacity of 17.5%. Finally, emerging technologies that are aimed at addressing both energy efficiency and IAQ are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Cooling coil; Supply air temperature; HVAC system; Energy; Ventilation; Indoor air quality
1. Introduction The optimal design of an air-conditioning system and its successful implementation and operation in a building is obviously a great challenge, especially when both energy and indoor air quality (IAQ) issues are of equal importance. It is, indeed, the designer’s knowledge and understanding of the dynamically varying system characteristics as a dynamic response to the inevitably varying cooling load characteristics that will ultimately prove to be crucial in the design of an optimal and successful system. Often, some of the fundamental principles in psychrometrics are likely to be either overlooked at design or accepted as limiting constraints in current state-of-the-art technology that lead to designs, which may be considered as the best compromise. Among several factors concerning energy consumption and IAQ that are intertwined in conventional design practice, it is often economics, and particularly, the capital cost, that become the governing criterion in the selection of a design. ∗ Tel.: +65-874-3479; fax: +65-775-5502. E-mail address: [email protected] (S.C. Sekhar).
0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.05.003
In recent times, the need for a life cycle costing approach towards the selection of air-conditioning and air-distribution systems is strongly advocated, considering that the economic life span of such systems are reasonably long and are, typically, in the range of 10–15 years [1–3]. This paper revisits some of the fundamental thinking in the design considerations of two of the most important and critical components in an air-conditioning system: • Cooling and dehumidifying coil • Fan Having estimated the cooling loads in a building, the volume flow rate of air supplied to the various zones is then a function of the space dry bulb temperature (DBT) and the supply air DBT. The difference in these temperatures, called the Space T, is a critical design parameter as it has implications in terms of both the cooling coil capacity and its performance and the amount of supply air necessary for handling the space cooling loads. The implication includes both energy and IAQ and is prevalent throughout the entire operating range of the air-conditioning system. For example, a small Space T for a given space DBT would not
50
S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
only result in a high air flow requirement but would also make it impossible for the cooling and dehumidifying coil to maintain space humidity levels within the desired comfort range without resorting to overcooling and reheating. This is particularly relevant in tropical designs which experience warm and humid climates and would naturally lead to an energy penalty. In the absence of any permissible reheating provisions, a low Space T design will lead to the space DBT gradually getting colder than desired and eventually leading to thermal discomfort. Zaheer-Uddin and Zheng [4] have observed that an optimal supply air temperature exists in a climate with high relative humidity and a high recirculation rate. Ke and Mumma [5] investigated supply air temperature reset controls from ventilation and energy perspectives and concluded that optimization is the only solution. Ke et al. [6] studied eight ventilation control strategies in VAV systems of which three of the strategies considered were changes in supply air temperature, leading to a conclusion that supply air temperature and supply air flow rate were two key parameters for optimization on the air-side.
2. Problem definition A hypothetical building of about 1200 m2 with a total space cooling load of 100 kW and a Room Sensible Heat Ratio (RSHR) of 0.75 is considered. The air-conditioning system being considered is of the variable air volume (VAV) type. The implication of different temperature differences between the space condition and the supply air temperature is investigated both in terms of the cooling coil performance
and the associated fan performance. A qualitative discussion on the implication of higher air flow rates on duct design is also presented. A psychrometric overview of the various cases studied is presented in Fig. 1. If the VAV fan at part load is expected to provide minimum fresh air requirements to the critical zones, a higher percentage of fresh air needs to be designed for peak or design load. The implication of such designs in terms of increased energy consumption for cooling coil and fan is evident and an attempt is made in this paper to quantify the energy implication. If lower space temperature differences, which are quite typical in several tropical designs, are to be considered in conjunction with higher fresh air quantities at design load, the overall increase in energy consumption is phenomenal. A standard coil selection program is used in the selection of coils and in exploring the performance characteristics to satisfy the total space cooling load of 100 kW and RSHR of 0.75 [7].
3. Results and discussion For a given space condition, the energy consumption under scenarios of different supply air temperatures resulting in different Space Ts is investigated on the basis of both the cooling coil and the fan performance. The results of the coil selection are first presented to be followed by those of the fan selection. The basis of the coil selection is to ensure the leaving conditions of the air to be commensurate with the required dew point temperature that would result in the space condition to be held constant both in terms of the DBT and the relative
Fig. 1. A psychrometric overview of the cases studied.
S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
humidity. This may, in certain cases, necessitate overcooling to achieve a warmer supply air temperature that lies on the same load ratio line having a room sensible heat ratio of 0.75, as shown in Fig. 1. 3.1. Coil selections for various space temperature differences (Space ∆T) 3.1.1. Space ∆T=8 ◦ C: BASE case The outdoor design condition for Singapore is considered as 32 ◦ C dry bulb temperature (DBT) and 26.2 ◦ C wet bulb temperature (WBT). The space temperature is designed to be maintained at 23 ◦ C DBT and 60% relative humidity and the Space T in BASE case is 8 ◦ C, associated with a total supply air flow rate of 10.3 m3 /s which is determined for the given space sensible load of 75 kW. The fresh air quantity in the BASE case is 18% of the total air flow rate and the resulting entering condition of the coil is psychrometrically obtained as 25.3 ◦ C DBT and 19.8 ◦ C WBT. A chilled water flow rate of 8 l per second and chilled water supply temperature of 6.7 ◦ C is considered for the BASE case and all other variations of Space T. The desired supply air temperature (which may or may not be the same as the leaving condition of the coil) in the BASE case is 15 ◦ C DBT. A four-row deep coil of 32 tubes (1.3 m) high and 3.2 m long with a fin density of 11 fins per inch (fpi) is selected that has
51
a “full circuiting” arrangement for the chilled water flow. The performance characteristics of this coil are presented in Table 1. The face velocity of air through the coil is 2.48 m/s and is kept constant in all the subsequent cases that are studied. The total cooling coil capacity is 212 kW with an air pressure drop of 151 Pa and water pressure drop of 32 kPa. 3.1.2. Space ∆T=7 ◦ C: Case 1 The space conditions are maintained at 23 ◦ C DBT and 60% relative humidity, resulting in a supply air temperature of 16 ◦ C DBT. For handling the same space sensible load of 75 kW at a reduced Space T of 7 ◦ C, the total supply air flow rate increases to 11.74 m3 /s. This constitutes a fresh air quantity of 15.8% that results in a slightly lower entering air enthalpy. However, the higher air flow rate leads to a higher cooling coil capacity. It is also to be noted that a warmer supply air temperature that still needs to maintain the same space condition necessitates some overcooling to 15 ◦ C DBT, resulting in a cooling coil capacity of 223 kW. Raising the supply air temperature to the desired level of 16 ◦ C DBT requires an additional reheat capacity of 14.3 kW. A coil different from the BASE case (Space T=8 ◦ C) is selected, which is obviously larger in order to handle the increased cooling capacity. The performance characteristics of this coil are included in Table 1.
Table 1 Psychrometric and coil performance parameters for various Space Ts BASE case
Case 1
Case 2
Case 3
Increased ventilation at peak load Case 4
Space total cooling load (kW) Room sensible heat ratio (RSHR) Space DBT (◦ C) Space relative humidity (%) Space temperature difference: Space T (◦ C) Entering air DBT (◦ C) Entering air WBT (◦ C) Leaving air DBT (◦ C) Leaving air WBT (◦ C) Air volume (m3 /s) Outdoor air percentage Face velocity (m/s) Air pressure drop (Pa) Cooling coil capacity (kW) (inclusive of overcooling, if any) Reheat (kW) Chilled water supply temperature (◦ C) Chilled water return temperature (◦ C) Chilled water flow rate (lps) Water pressure drop (kPa) Coil geometry Tube diameter (mm) Tubes high Finned height (mm) Finned length (mm) Fin density (fins/in.) Circuiting Number of rows
D
100 0.75 23 60 8 25.3 19.8 15 14.1 10.3 18 2.48 151 212
C
100 0.75 23 60 7 25.1 19.6 15 14.3 11.74 15.8 2.48 151 223
B
100 0.75 23 60 6 25 19.4 16 14.7 13.7 13.5 2.48 129 231
A
Case 5
100 0.75 23 60 5 24.9 19.2 16.5 15 16.5 11.2 2.48 144 249
100 0.75 23 60 8 25.7 20.3 15 14.1 10.3 22 2.48 129 233
100 0.75 23 60 8 26 20.7 15 14.1 10.3 26 2.48 129 249
– 6.7 13 8 32
14.3 6.7 13.4 8 17.9
16.7 6.7 13.6 8 39
30 6.7 14.1 8 17.4
– 6.0 12.2 9 39.5
– 6.0 12.6 9 39.5
16 32 1299 3200 11 FULL 4
16 48 1947 2430 11 FULL 4
16 32 1299 4250 9 FULL 4
16 64 2594 2560 6 FULL 6
16 32 1299 3200 9 FULL 4
16 32 1299 3200 9 FULL 4
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S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
3.1.3. Space ∆T=6 ◦ C: Case 2 The space conditions are again maintained at 23 ◦ C DBT and 60% Relative Humidity, resulting in a supply air temperature of 17 ◦ C DBT. For the given space loads at a reduced Space T of 6 ◦ C, the total supply air flow rate increases to 13.7 m3 /s. This constitutes a fresh air quantity of 13.5%. The need to overcool to 16 ◦ C DBT results in a cooling coil capacity of 231 kW which is also associated with a reheat capacity of 16.7 kW. A coil different from BASE case and Case 1 is selected, which is obviously larger in order to handle the increased cooling capacity. The performance characteristics of this coil are included in Table 1.
ues are presented in Table 2 which also summarises the key energy-related parameters that are extracted from Table 1. A coefficient of performance (COP) of 4.5 is used in obtaining the electrical power requirements for the chiller from the cooling capacity in Table 2. The adverse energy implication of higher flow rate coupled with the difficulty in achieving the desired psychrometric conditions without resorting to overcooling and reheating is evident from Table 2. The fan power, chiller power, the reheater power and the total electrical power requirements are plotted in Fig. 2. Whilst the total power (energy) required for the BASE case is 55.5 kW, it is more than double in Case 3 where the Space T is 5 ◦ C.
3.1.4. Space ∆T=5 ◦ C: Case 3 There is no change in the space conditions and for the given space loads at a reduced Space T of 5 ◦ C, the total supply air flow rate increases to 16.5 m3 /s. This constitutes a fresh air quantity of 11.2%. The need to overcool to 16.5 ◦ C DBT results in a cooling coil capacity of 249 kW which is also associated with a reheat capacity of 30 kW to reach a supply air temperature of 18 ◦ C DBT. The performance characteristics of the coil selected are included in Table 1.
3.3. Duct sizing Whilst increased supply air flow requirements imply a larger fan and increased energy consumption, they also have an implication on the air distribution system. Higher air flow quantities would typically lead to larger duct sizes. For a given diameter of the duct, an increase in flow rate leads to an exponential increase in duct velocity, and correspondingly, the pressure loss. This would have an obvious adverse effect on the fan selection and performance. If the pressure loss is to be kept constant, an increase in flow rate would be accompanied by a significant increase in the duct diameter and a marginal increase in duct velocity.
3.1.5. Energy implications It is apparent from Table 1 and the ensuing discussion that the cooling capacity increases as the Space T decreases if the space condition is to be held constant for a given space cooling load and RSHR. It is also seen from Table 1 that sensible heat correction in the form of reheating to achieve the desired supply air temperature increases as the Space T decreases.
3.4. Coil performance for satisfying part load ventilation provision In a typical variable air volume (VAV) system, the outdoor air provision for ventilation at part loads could easily be compromised if sufficient consideration is not accorded in the design of peak load requirements. For the BASE case, the cooling coil energy implication in the consideration of 20% and 40% more outdoor air quantity at design is presented as Cases 4 and 5, respectively in Table 1. These assumptions of increased outdoor air quantities at design are representative of solutions that could likely be considered to address ventilation provision for critical zones during part load operation in VAV system. Fig. 3 represents the increase in cooling coil capacities due to increase in ventilation rates for a Space T of 8 ◦ C. The implication of additional installed capacity at design condition is the selection of an
3.2. Fan selection and energy implication Associated with decreasing Space T is the need to raise the total supply air flow rates to handle a given space cooling load. The increasing air flow quantities required are tabulated in Table 1. An attempt is made to quantify the extent of additional fan energy amongst the three cases being compared with the BASE case. A standard fan with forward-curved blades is selected for the base case that requires 11.2 hp [8]. Based on the fact that the fan power varies as the cube of the flow rate, the fan power requirements for Cases 1, 2 and 3 are computed as 16.5, 26.36 and 46, respectively. These valTable 2 Psychrometric and coil performance parameters for various Space Ts Space T (◦ C)
8 7 6 5
Supply air DBT (◦ C)
15 16 17 18
Total air flow (m3 /s)
10.3 11.74 13.7 16.5
Increase in total air flow (%)
Outdoor air (%)
– 14 33 60
18 15.8 13.5 11.2
Fan power
Cooling energy (COP = 4.5)
hp
kW
Cooling capacity (kW)
Electrical power (kW)
11.2 16.5 26.36 46
8.4 12.3 19.7 34.3
212 223 231 249
47.1 49.6 51.3 55.3
Reheat electrical power (kW)
Total electrical power (kW)
Additional energy/power required (%)
– 14.3 16.7 30
55.5 76.2 87.7 119.6
– 37.3 58 115.5
S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
53
Fig. 2. Electrical power requirements for fan, chiller and reheater.
oversized cooling coil whose part-load dehumidifying performance is significantly compromised and, often, leads to an inefficient operation. It is thus apparent from Figs. 2 and 3 that the cumulative impact of higher ventilation rates and lower Space Ts
could lead to significantly higher energy consumption in the overall operation of an air-conditioning system. It, therefore, pays to direct more attention to the efficiency of the cooling coil as well as the effectiveness with which ventilation can be provided to the occupied zones of a building from the
Fig. 3. Cooling coil capacities for increased ventilation rates.
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S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
perspective of human exposure. High driving potential (HDP) is a new method of achieving efficient cooling and dehumidification of the hot and humid outdoor air by treating it separately through a coil and thus maximizing the driving potential to dehumidify [9]. The concept of Dedicated outdoor air system (DOAS) for providing ventilation air directly to the occupied zone without mixing with the recirculated air has been proposed [10,11]. The non-ventilation related recirculated air that is necessary to handle the ambient cooling loads in the space in order to address total cooling capacity requirements could be provided by localized terminal devices, such as chilled ceilings [12]. In line with this new paradigm of separating the “ventilation air” and the “cooling air”, the Single coil twin fan (SCTF) air-conditioning and air distribution system has recently been developed which provides separate and independently controlled “demand ventilation” and “demand cooling” [13]. The SCTF system employs a dual duct concept at the air distribution system level and the conditioned outdoor air and the conditioned recirculated air do not mix until they reach the respective variable air volume (VAV) boxes in the occupied zones of the building. Additionally, the psychrometric performance of the cooling and dehumidifying coil is optimized and made energy efficient by the use of a newly developed compartmented coil that consists of two separate sections for the two air streams albeit connected by a common chilled water circuit [13].
4. Conclusions In conjunction with the choice of the Space T, cooling coil and fan are the most important factors that need to be considered in the optimal design of an air-conditioning system from both energy and IAQ perspectives. This paper presents the energy and IAQ implications for a typical design in the tropics by exploring different scenarios that involve the Space T as a primary design variable. The basis for comparison across all the cases studied is that the space condition is always maintained at a typical design value of 23 ◦ C DBT and 60% RH. The total power requirements, comprising overcooling, reheating and increased fan power increases significantly as the Space T decreases from 8 to 5 ◦ C. It has been observed in this study that the total power for a design involving a Space T of 5 ◦ C can be as high as a factor of 2.2 of the total power required for a design with a Space T of 8 ◦ C. The implication of higher supply air flow requirements on the sizes of the ducts in the air distribution system is also discussed. For a given space cooling load and a given Space T, the study has shown
that increased design ventilation rates to address part-load ventilation requirements can lead to an additional installed cooling capacity of 17.5%. This implies a larger than desired effective surface area of the cooling coil which would lead to inefficient dehumidifying performance at part-load operating conditions. The notion of separate tracks for ventilation/outdoor air and recirculation air is introduced as a new paradigm and emerging technologies based on such a paradigm that are aimed at addressing both energy efficiency and IAQ are identified.
References [1] Morteza M. Ardehali, Theodore F. Smith, Evaluation of variable volume and temperature HVAC system for commercial and residential buildings, Energy Conversion and Management 37 (9) (1996) 1469– 1479. [2] E.H. Mathews, C.P. Botha, D.C. Arndt, A. Malan, HVAC control strategies to enhance comfort and minimize energy usage, Energy and Buildings 33 (8) (2001) 853–863. [3] Arthur Williams, M. Akbar Hessami, Two case studies of cogeneration systems design and economic feasibility, Heat Recovery Systems and CHP 13 (2) (1993) 167–186. [4] M. Zaheer-Uddin, G.R. Zheng, A VAV system model for simulation of energy management control functions: of normal operation and duty cycling, Energy Conversion and Management 35 (11) (1994) 917–931. [5] Y.-P. Ke, S.A. Mumma, Optimised supply air temperature (SAT) in variable air volume (VAV) systems, Energy 22 (6) (1997) 601–614. [6] Y.-P. Ke, S.A. Mumma, D. Stanke, Simulation results and analysis of eight ventilation control strategies in VAV systems, ASHRAE Transactions 103 (Part 2) (1997) 381–392. [7] SPC 2000: Standard—Version 2.1, Coil selection software (self-selection software—free download from www.spcoils.co.uk), S&P Coil Products Limited, United Kingdom, 2003. [8] Steve Y.S. Chen, Stanley J. Demster, Variable Air Volume Systems for Environmental Quality, McGraw-Hill, 1996. [9] R.E. Luxton, P.G. Marshallsay, Optimal design of air-conditioning systems: towards an integrated approach, IIR Bulletin, 98.2 (1998) 2–13 (Part 1); 98.3 (1998) 2–13 (Part 2). [10] Stanley A. Mumma, Overview of integrating dedicated outdoor air systems with parallel terminal systems, ASHRAE Transactions 107 (1) (2001) 545–552. [11] S.A. Mumma, B.W. Lee, Extension of the multiple spaces concept of ASHRAE Standard 62 to include infiltration, exhaust/exfiltration, interzonal transfer, and additional short circuit paths, ASHRAE Transactions 104 (2) (1998) 1232–1241. [12] Jae-Weon Jeong, Stanley A. Mumma, William P. Bahnfleth, Energy conservation benefits of a dedicated outdoor air system with parallel sensible cooling by ceiling radiant panels, ASHRAE Transactions, 109 (Part 2) (2003). [13] S.C. Sekhar, Uma Maheswaran, K.W. Tham, K.W. Cheong, Development of energy efficient single coil twin fan air-conditioning system with zonal ventilation control, ASHRAE Transactions, 2004, in press.
Space temperature difference, cooling coil and fan—energy and indoor air quality issues revisited S.C. Sekhar∗ Department of Building National University of Singapore, 4 Architecture Drive, Singapore 117566, Singapore Received 10 March 2004; received in revised form 26 April 2004; accepted 6 May 2004
Abstract In designing an energy-efficient air-conditioning system that also simultaneously addresses the needs of adequate ventilation and acceptable indoor air quality, several factors begin to play an important role. Among several others, the cooling coil, the fan and the temperature difference between the space and the supply air (commonly known as the Space T) can be considered to be crucial. For a given space cooling load, the choice of a particular Space T has an implication on the amount of supply air required, which further has an impact on the performance of the cooling and dehumidifying coil as well as the fan. Inherent in these implications are issues related to energy, ventilation and indoor air quality. This paper investigates these implications and quantifies them by considering a hypothetical building in a tropical climate and subjecting the design to several parametric variations involving different Space Ts for a given space temperature and humidity condition. The total power requirements, comprising additional cooling, reheating and higher fan power, for a design involving a Space T of 5 ◦ C can be as high as a factor of 2.2 of the total power for a design with a Space T of 8 ◦ C. The implication of higher supply air flow rates on duct design is qualitatively discussed. For a given space cooling load and a given Space T, the implication of increased design ventilation rates to address part-load ventilation requirements can lead to an additional installed cooling capacity of 17.5%. Finally, emerging technologies that are aimed at addressing both energy efficiency and IAQ are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Cooling coil; Supply air temperature; HVAC system; Energy; Ventilation; Indoor air quality
1. Introduction The optimal design of an air-conditioning system and its successful implementation and operation in a building is obviously a great challenge, especially when both energy and indoor air quality (IAQ) issues are of equal importance. It is, indeed, the designer’s knowledge and understanding of the dynamically varying system characteristics as a dynamic response to the inevitably varying cooling load characteristics that will ultimately prove to be crucial in the design of an optimal and successful system. Often, some of the fundamental principles in psychrometrics are likely to be either overlooked at design or accepted as limiting constraints in current state-of-the-art technology that lead to designs, which may be considered as the best compromise. Among several factors concerning energy consumption and IAQ that are intertwined in conventional design practice, it is often economics, and particularly, the capital cost, that become the governing criterion in the selection of a design. ∗ Tel.: +65-874-3479; fax: +65-775-5502. E-mail address: [email protected] (S.C. Sekhar).
0378-7788/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2004.05.003
In recent times, the need for a life cycle costing approach towards the selection of air-conditioning and air-distribution systems is strongly advocated, considering that the economic life span of such systems are reasonably long and are, typically, in the range of 10–15 years [1–3]. This paper revisits some of the fundamental thinking in the design considerations of two of the most important and critical components in an air-conditioning system: • Cooling and dehumidifying coil • Fan Having estimated the cooling loads in a building, the volume flow rate of air supplied to the various zones is then a function of the space dry bulb temperature (DBT) and the supply air DBT. The difference in these temperatures, called the Space T, is a critical design parameter as it has implications in terms of both the cooling coil capacity and its performance and the amount of supply air necessary for handling the space cooling loads. The implication includes both energy and IAQ and is prevalent throughout the entire operating range of the air-conditioning system. For example, a small Space T for a given space DBT would not
50
S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
only result in a high air flow requirement but would also make it impossible for the cooling and dehumidifying coil to maintain space humidity levels within the desired comfort range without resorting to overcooling and reheating. This is particularly relevant in tropical designs which experience warm and humid climates and would naturally lead to an energy penalty. In the absence of any permissible reheating provisions, a low Space T design will lead to the space DBT gradually getting colder than desired and eventually leading to thermal discomfort. Zaheer-Uddin and Zheng [4] have observed that an optimal supply air temperature exists in a climate with high relative humidity and a high recirculation rate. Ke and Mumma [5] investigated supply air temperature reset controls from ventilation and energy perspectives and concluded that optimization is the only solution. Ke et al. [6] studied eight ventilation control strategies in VAV systems of which three of the strategies considered were changes in supply air temperature, leading to a conclusion that supply air temperature and supply air flow rate were two key parameters for optimization on the air-side.
2. Problem definition A hypothetical building of about 1200 m2 with a total space cooling load of 100 kW and a Room Sensible Heat Ratio (RSHR) of 0.75 is considered. The air-conditioning system being considered is of the variable air volume (VAV) type. The implication of different temperature differences between the space condition and the supply air temperature is investigated both in terms of the cooling coil performance
and the associated fan performance. A qualitative discussion on the implication of higher air flow rates on duct design is also presented. A psychrometric overview of the various cases studied is presented in Fig. 1. If the VAV fan at part load is expected to provide minimum fresh air requirements to the critical zones, a higher percentage of fresh air needs to be designed for peak or design load. The implication of such designs in terms of increased energy consumption for cooling coil and fan is evident and an attempt is made in this paper to quantify the energy implication. If lower space temperature differences, which are quite typical in several tropical designs, are to be considered in conjunction with higher fresh air quantities at design load, the overall increase in energy consumption is phenomenal. A standard coil selection program is used in the selection of coils and in exploring the performance characteristics to satisfy the total space cooling load of 100 kW and RSHR of 0.75 [7].
3. Results and discussion For a given space condition, the energy consumption under scenarios of different supply air temperatures resulting in different Space Ts is investigated on the basis of both the cooling coil and the fan performance. The results of the coil selection are first presented to be followed by those of the fan selection. The basis of the coil selection is to ensure the leaving conditions of the air to be commensurate with the required dew point temperature that would result in the space condition to be held constant both in terms of the DBT and the relative
Fig. 1. A psychrometric overview of the cases studied.
S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
humidity. This may, in certain cases, necessitate overcooling to achieve a warmer supply air temperature that lies on the same load ratio line having a room sensible heat ratio of 0.75, as shown in Fig. 1. 3.1. Coil selections for various space temperature differences (Space ∆T) 3.1.1. Space ∆T=8 ◦ C: BASE case The outdoor design condition for Singapore is considered as 32 ◦ C dry bulb temperature (DBT) and 26.2 ◦ C wet bulb temperature (WBT). The space temperature is designed to be maintained at 23 ◦ C DBT and 60% relative humidity and the Space T in BASE case is 8 ◦ C, associated with a total supply air flow rate of 10.3 m3 /s which is determined for the given space sensible load of 75 kW. The fresh air quantity in the BASE case is 18% of the total air flow rate and the resulting entering condition of the coil is psychrometrically obtained as 25.3 ◦ C DBT and 19.8 ◦ C WBT. A chilled water flow rate of 8 l per second and chilled water supply temperature of 6.7 ◦ C is considered for the BASE case and all other variations of Space T. The desired supply air temperature (which may or may not be the same as the leaving condition of the coil) in the BASE case is 15 ◦ C DBT. A four-row deep coil of 32 tubes (1.3 m) high and 3.2 m long with a fin density of 11 fins per inch (fpi) is selected that has
51
a “full circuiting” arrangement for the chilled water flow. The performance characteristics of this coil are presented in Table 1. The face velocity of air through the coil is 2.48 m/s and is kept constant in all the subsequent cases that are studied. The total cooling coil capacity is 212 kW with an air pressure drop of 151 Pa and water pressure drop of 32 kPa. 3.1.2. Space ∆T=7 ◦ C: Case 1 The space conditions are maintained at 23 ◦ C DBT and 60% relative humidity, resulting in a supply air temperature of 16 ◦ C DBT. For handling the same space sensible load of 75 kW at a reduced Space T of 7 ◦ C, the total supply air flow rate increases to 11.74 m3 /s. This constitutes a fresh air quantity of 15.8% that results in a slightly lower entering air enthalpy. However, the higher air flow rate leads to a higher cooling coil capacity. It is also to be noted that a warmer supply air temperature that still needs to maintain the same space condition necessitates some overcooling to 15 ◦ C DBT, resulting in a cooling coil capacity of 223 kW. Raising the supply air temperature to the desired level of 16 ◦ C DBT requires an additional reheat capacity of 14.3 kW. A coil different from the BASE case (Space T=8 ◦ C) is selected, which is obviously larger in order to handle the increased cooling capacity. The performance characteristics of this coil are included in Table 1.
Table 1 Psychrometric and coil performance parameters for various Space Ts BASE case
Case 1
Case 2
Case 3
Increased ventilation at peak load Case 4
Space total cooling load (kW) Room sensible heat ratio (RSHR) Space DBT (◦ C) Space relative humidity (%) Space temperature difference: Space T (◦ C) Entering air DBT (◦ C) Entering air WBT (◦ C) Leaving air DBT (◦ C) Leaving air WBT (◦ C) Air volume (m3 /s) Outdoor air percentage Face velocity (m/s) Air pressure drop (Pa) Cooling coil capacity (kW) (inclusive of overcooling, if any) Reheat (kW) Chilled water supply temperature (◦ C) Chilled water return temperature (◦ C) Chilled water flow rate (lps) Water pressure drop (kPa) Coil geometry Tube diameter (mm) Tubes high Finned height (mm) Finned length (mm) Fin density (fins/in.) Circuiting Number of rows
D
100 0.75 23 60 8 25.3 19.8 15 14.1 10.3 18 2.48 151 212
C
100 0.75 23 60 7 25.1 19.6 15 14.3 11.74 15.8 2.48 151 223
B
100 0.75 23 60 6 25 19.4 16 14.7 13.7 13.5 2.48 129 231
A
Case 5
100 0.75 23 60 5 24.9 19.2 16.5 15 16.5 11.2 2.48 144 249
100 0.75 23 60 8 25.7 20.3 15 14.1 10.3 22 2.48 129 233
100 0.75 23 60 8 26 20.7 15 14.1 10.3 26 2.48 129 249
– 6.7 13 8 32
14.3 6.7 13.4 8 17.9
16.7 6.7 13.6 8 39
30 6.7 14.1 8 17.4
– 6.0 12.2 9 39.5
– 6.0 12.6 9 39.5
16 32 1299 3200 11 FULL 4
16 48 1947 2430 11 FULL 4
16 32 1299 4250 9 FULL 4
16 64 2594 2560 6 FULL 6
16 32 1299 3200 9 FULL 4
16 32 1299 3200 9 FULL 4
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3.1.3. Space ∆T=6 ◦ C: Case 2 The space conditions are again maintained at 23 ◦ C DBT and 60% Relative Humidity, resulting in a supply air temperature of 17 ◦ C DBT. For the given space loads at a reduced Space T of 6 ◦ C, the total supply air flow rate increases to 13.7 m3 /s. This constitutes a fresh air quantity of 13.5%. The need to overcool to 16 ◦ C DBT results in a cooling coil capacity of 231 kW which is also associated with a reheat capacity of 16.7 kW. A coil different from BASE case and Case 1 is selected, which is obviously larger in order to handle the increased cooling capacity. The performance characteristics of this coil are included in Table 1.
ues are presented in Table 2 which also summarises the key energy-related parameters that are extracted from Table 1. A coefficient of performance (COP) of 4.5 is used in obtaining the electrical power requirements for the chiller from the cooling capacity in Table 2. The adverse energy implication of higher flow rate coupled with the difficulty in achieving the desired psychrometric conditions without resorting to overcooling and reheating is evident from Table 2. The fan power, chiller power, the reheater power and the total electrical power requirements are plotted in Fig. 2. Whilst the total power (energy) required for the BASE case is 55.5 kW, it is more than double in Case 3 where the Space T is 5 ◦ C.
3.1.4. Space ∆T=5 ◦ C: Case 3 There is no change in the space conditions and for the given space loads at a reduced Space T of 5 ◦ C, the total supply air flow rate increases to 16.5 m3 /s. This constitutes a fresh air quantity of 11.2%. The need to overcool to 16.5 ◦ C DBT results in a cooling coil capacity of 249 kW which is also associated with a reheat capacity of 30 kW to reach a supply air temperature of 18 ◦ C DBT. The performance characteristics of the coil selected are included in Table 1.
3.3. Duct sizing Whilst increased supply air flow requirements imply a larger fan and increased energy consumption, they also have an implication on the air distribution system. Higher air flow quantities would typically lead to larger duct sizes. For a given diameter of the duct, an increase in flow rate leads to an exponential increase in duct velocity, and correspondingly, the pressure loss. This would have an obvious adverse effect on the fan selection and performance. If the pressure loss is to be kept constant, an increase in flow rate would be accompanied by a significant increase in the duct diameter and a marginal increase in duct velocity.
3.1.5. Energy implications It is apparent from Table 1 and the ensuing discussion that the cooling capacity increases as the Space T decreases if the space condition is to be held constant for a given space cooling load and RSHR. It is also seen from Table 1 that sensible heat correction in the form of reheating to achieve the desired supply air temperature increases as the Space T decreases.
3.4. Coil performance for satisfying part load ventilation provision In a typical variable air volume (VAV) system, the outdoor air provision for ventilation at part loads could easily be compromised if sufficient consideration is not accorded in the design of peak load requirements. For the BASE case, the cooling coil energy implication in the consideration of 20% and 40% more outdoor air quantity at design is presented as Cases 4 and 5, respectively in Table 1. These assumptions of increased outdoor air quantities at design are representative of solutions that could likely be considered to address ventilation provision for critical zones during part load operation in VAV system. Fig. 3 represents the increase in cooling coil capacities due to increase in ventilation rates for a Space T of 8 ◦ C. The implication of additional installed capacity at design condition is the selection of an
3.2. Fan selection and energy implication Associated with decreasing Space T is the need to raise the total supply air flow rates to handle a given space cooling load. The increasing air flow quantities required are tabulated in Table 1. An attempt is made to quantify the extent of additional fan energy amongst the three cases being compared with the BASE case. A standard fan with forward-curved blades is selected for the base case that requires 11.2 hp [8]. Based on the fact that the fan power varies as the cube of the flow rate, the fan power requirements for Cases 1, 2 and 3 are computed as 16.5, 26.36 and 46, respectively. These valTable 2 Psychrometric and coil performance parameters for various Space Ts Space T (◦ C)
8 7 6 5
Supply air DBT (◦ C)
15 16 17 18
Total air flow (m3 /s)
10.3 11.74 13.7 16.5
Increase in total air flow (%)
Outdoor air (%)
– 14 33 60
18 15.8 13.5 11.2
Fan power
Cooling energy (COP = 4.5)
hp
kW
Cooling capacity (kW)
Electrical power (kW)
11.2 16.5 26.36 46
8.4 12.3 19.7 34.3
212 223 231 249
47.1 49.6 51.3 55.3
Reheat electrical power (kW)
Total electrical power (kW)
Additional energy/power required (%)
– 14.3 16.7 30
55.5 76.2 87.7 119.6
– 37.3 58 115.5
S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
53
Fig. 2. Electrical power requirements for fan, chiller and reheater.
oversized cooling coil whose part-load dehumidifying performance is significantly compromised and, often, leads to an inefficient operation. It is thus apparent from Figs. 2 and 3 that the cumulative impact of higher ventilation rates and lower Space Ts
could lead to significantly higher energy consumption in the overall operation of an air-conditioning system. It, therefore, pays to direct more attention to the efficiency of the cooling coil as well as the effectiveness with which ventilation can be provided to the occupied zones of a building from the
Fig. 3. Cooling coil capacities for increased ventilation rates.
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S.C. Sekhar / Energy and Buildings 37 (2005) 49–54
perspective of human exposure. High driving potential (HDP) is a new method of achieving efficient cooling and dehumidification of the hot and humid outdoor air by treating it separately through a coil and thus maximizing the driving potential to dehumidify [9]. The concept of Dedicated outdoor air system (DOAS) for providing ventilation air directly to the occupied zone without mixing with the recirculated air has been proposed [10,11]. The non-ventilation related recirculated air that is necessary to handle the ambient cooling loads in the space in order to address total cooling capacity requirements could be provided by localized terminal devices, such as chilled ceilings [12]. In line with this new paradigm of separating the “ventilation air” and the “cooling air”, the Single coil twin fan (SCTF) air-conditioning and air distribution system has recently been developed which provides separate and independently controlled “demand ventilation” and “demand cooling” [13]. The SCTF system employs a dual duct concept at the air distribution system level and the conditioned outdoor air and the conditioned recirculated air do not mix until they reach the respective variable air volume (VAV) boxes in the occupied zones of the building. Additionally, the psychrometric performance of the cooling and dehumidifying coil is optimized and made energy efficient by the use of a newly developed compartmented coil that consists of two separate sections for the two air streams albeit connected by a common chilled water circuit [13].
4. Conclusions In conjunction with the choice of the Space T, cooling coil and fan are the most important factors that need to be considered in the optimal design of an air-conditioning system from both energy and IAQ perspectives. This paper presents the energy and IAQ implications for a typical design in the tropics by exploring different scenarios that involve the Space T as a primary design variable. The basis for comparison across all the cases studied is that the space condition is always maintained at a typical design value of 23 ◦ C DBT and 60% RH. The total power requirements, comprising overcooling, reheating and increased fan power increases significantly as the Space T decreases from 8 to 5 ◦ C. It has been observed in this study that the total power for a design involving a Space T of 5 ◦ C can be as high as a factor of 2.2 of the total power required for a design with a Space T of 8 ◦ C. The implication of higher supply air flow requirements on the sizes of the ducts in the air distribution system is also discussed. For a given space cooling load and a given Space T, the study has shown
that increased design ventilation rates to address part-load ventilation requirements can lead to an additional installed cooling capacity of 17.5%. This implies a larger than desired effective surface area of the cooling coil which would lead to inefficient dehumidifying performance at part-load operating conditions. The notion of separate tracks for ventilation/outdoor air and recirculation air is introduced as a new paradigm and emerging technologies based on such a paradigm that are aimed at addressing both energy efficiency and IAQ are identified.
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