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Research on frequency conversion technology of metro station's ventilation and air-conditioning system
Applied Thermal Engineering 69 (2014) 123e129
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research on frequency conversion technology of metro station’s ventilation and air-conditioning system Zhao Yang*, Zhuangzhuang Yu, Longqing Yu, Feng Ma School of Mechanical Engineering, Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, MOE, Tianjin University, 92 Weijin Road, Tianjin 30072, PR China
h i g h l i g h t s Use the FCT to reduce energy consumption of metro VAC is necessary and possible. Analyze the influence of running the chilled-water pumps with FCT. Results show that variable air volume of station public area is feasible. Calculations indicate that energy-saving effect of using the FCT is considerable.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2013 Accepted 9 April 2014 Available online 24 April 2014
Ventilation and air-conditioning system (VAC) is the most energy-saving potential system in the metro. This paper analyzes the passenger traffic, air-conditioning load and station air supply on the initial, recent and long-term phase of metro station. And it proposes that it is necessary to run chilled-water pumps, air handing unit (AHU) fans and back/exhaust fans with frequency conversion technology (FCT). Then it uses the thermodynamic method to analyze the impact of running chilled-water pumps with FCT. The results show that running chilled-water pumps with FCT can reduce the total power consumption of system, although increases chiller energy consumption. Then the temperature and velocity fields of the platform and station hall are simulated by CFD software according to the variable air volume. And the results show that under the condition of running the VAC system with FCT, temperature and velocity fields distribution are both in the comfortable range. Finally, by taking a typical summer day for example, this paper analyzes the energy savings of chilled-water pumps, AHU fans and back/exhaust fans on the initial, recent and long-term phase, and the calculation results show that the respective total energy savings are 1103.4 kWh, 1064.3 kWh and 926.2 kWh, and the respective total power saving ratio is 73.4%, 71.2% and 59.5%. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Metro station Energy saving Frequency conversion technology Thermodynamic method CFD
1. Introduction With the development of economy, urbanization and urban expansion, it increases pressure on urban transport and makes environmental degradation in China. In order to ease the traffic pressure and reduce environmental pollution, major cities begin to build metro which is efficient, green and has large passenger capacity. However, its high energy consumption cannot be ignored, only 20 km to the opening of the Shenzhen Metro, for example, its annual electricity consumption accounts for 1/3000 of the total electricity consumption in Shenzhen [1], of which the metro air-
* Corresponding author. Tel./fax: þ86 022 2789 0627. E-mail address: [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.applthermaleng.2014.04.016 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.
conditioning energy consumption accounts for about 35%. According to the code for design of metro, the maximum long-term load determines the capacity of the VAC equipments of metro station, which has 10e20% of the wealthy [2]. In the process of operation, full load operation time of the VAC will be less than 5% of the total runtime, especially in the initial stage. Meanwhile, metro’s characteristics of morning peak and evening peak make the airconditioning load of metro extremely volatile in a working day. Therefore, it is necessary to carry out energy-saving research on the VAC of metro [3]. Because of the climate difference and different metro design concept between southern and northern in China, the VAC of the metro station is different. According to the actual situation, the different energy-saving control program need to be constituted for the different place. In this paper, a metro station with designing and
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Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
Nomenclature
Ebf
Qe Qe,0 Q0 Wp Wwc Ke Ke,0 Fe Vw,e0 te tei COPI Ep
E1
Ef
chiller refrigeration capacity, kW chiller rated refrigerating capacity, kW refrigerating capacity, kW pump power, kW chiller power, kW evaporator heat transfer coefficient, kW/(m2 C) evaporator rated heat transfer coefficient, kW/(m2 C) evaporator heat transfer area, m2 chillers rated water flow, m/s evaporation temperature, C return chilled-water temperature, C integrated chiller COP overall energy consumption amount of chilled-water pumps, kWh overall energy consumption amount of AHU fans, kWh
installation of platform screen doors in the south China, for example, its corresponding energy-saving program is proposed and studied. The VAC of the metro station principle diagram is shown in Fig. 1. At present, one of energy-saving measures of the ground building is to use FCT to control the fluid machinery (fans and pumps) of VAC, which this article will introduce. Through empirical analysis, this paper intends to study the feasibility of FCT used in the equipments of chilled-water system and ventilation system of the station public area.
E2 E3 E4
xt
overall energy consumption amount of back/exhaust fans, kWh quantity of energy saving by chilled-water pumps frequency conversion, kWh quantity of energy saving by AHU fans frequency conversion, kWh quantity of energy saving by back/exhaust fans frequency conversion, kWh increased energy consumption amount of chiller, kWh total power saving ratio
Abbreviations VAC ventilation and air-conditioning system AHU air handing unit FCT frequency conversion technology VPF variable primary flow system VAV variable air volume technology
According to the station load, air supplies of the initial, recent and long-term stage are calculated, and the result is shown in Fig. 4. Air supply of the station public area also changes dramatically throughout a day. Especially in the initial and recent phase, the air supply is less than 60% of the maximum long-term phase. The prediction of hourly passenger traffic, air-conditioning load and air supply show that change of metro passenger traffic leads
2. Station passenger traffic and air-conditioning load analysis Hourly forecast passenger traffic is shown in Fig. 2, which shows that the passenger traffic difference is very large at different times a day, and the morning and evening peak passenger flow are very clear, with a variation hump-shaped. According to passenger forecasts and other information, and basing on the typical year weather data provided by special meteorological data set of analysis of china building thermal environment [4] and according to code for design of metro, this paper takes a typical summer day for example, hourly station airconditioning loads of the initial, recent and long-term stage are calculated and shown in Fig. 3. Most of the time, the air-conditioning load is 60e80% of the maximum long-term air-conditioning load. In a day, the chilled-water flow changes dramatically, because air-conditioning load reflects the needed of the chilled-water flow directly.
Fig. 2. Hourly distribution of station passenger (the morning rush is unit volume 1).
Fig. 1. The metro station VAC principle diagram.
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Fig. 5. Variable primary flow system schematic diagrams.
Fig. 3. Hourly station air-conditioning loads (maximum long-term air-conditioning load is unit volume 1).
the dramatic changes of air-conditioning load and air supply in the initial, recent and long-term stage, whose trend is an obvious hump. Meanwhile, the equipment selection of metro station VAC is according to the maximum long-term passenger traffic and airconditioning load, so it is necessary and possible to use the FCT to reduce energy consumption. 3. Chilled-water system variable flow feasibility analyses 3.1. Chilled-water system operation mode To reduce the energy consumption of chilled-water pump, the speed of pump can be adjusted according to air-conditioning load, but it also changes the chiller evaporator water flow, which is not allowed under normal circumstances [5]. In recent years, with the development of chiller control technology, electronic control
technology has replaced the mechanical and pneumatic control technology. The control system can make a timely response when there is little change in load. The chiller can work normally and stably even if evaporator water flow is variable. Therefore, this article uses variable primary flow system (VPF) [6] in the chilledwater system. Its principle is shown in Fig. 5, and its design temperature difference for the sending and return water is 5 C (7 C/ 12 C). Based on the air-conditioning load, VPF regulates the flow of chilled-water pump directly. When the water flow is reduced to the minimum flow decided by chiller and evaporator, the chilled-water pumps run in accordance with the minimum flow and the bypass valve opens now. The bypass valve in VPF is designed to ensure the normal operation of the chiller and evaporator. There are two frequency conversion control methods to control chilled-water pump of VPF: temperature difference control and pressure control. The pressure control is divided into trunk pipeline pressure control and the most unfavorable pressure control. As shown in Fig. 5, VPF has a few end devices (only two AHU). According to design requirements, the operating conditions of terminal equipments are the same. Meanwhile, the chilled-water system is simple. Therefore, from the system and energy-saving point of view, temperature difference control method is chosen to control VPF. Temperature difference control only changes water flow of the system, without changing the characteristics of the pipeline, so the water pump frequency conversion operation meets the similar law of fluid machinery, which makes the power and frequency into three power relations. Compared with the constant flow control, controlling the pump flow with the temperature difference control can achieve the biggest energy-saving rate [7]. 3.2. Running performance analysis of chilled-water variable flow At present, the reason of VPF hard to promote is that variable flow of chilled water makes the COP of chillers decrease, and the energy-saving effect of whole system is not obvious, even without energy saving. Therefore, the analysis should be based on specific circumstances with specific engineering applications. According to literature [8], for example, it provides a theoretical model with a typical case study to establish viability of providing variable frequency drive for cooling water pumps in power plants with sea water based once through condenser cooling water system. This article uses thermodynamic method to analyze the feasibility of chilled-water’s variable flow. Evaporation temperature is an important factor to determine the COP of chiller. Heat transfer and energy conservation equation of evaporator is
Fig. 4. Hourly air station air supplies (long-term maximum air supply is unit volume 1).
Qe ¼ Ke Fe
ðtei te Þ ðte0 te Þ ¼ Cw Vw;e rw ðtei te0 Þ e In tte0ei t te
(1)
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Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
By equation (1), derive the relationship between evaporation temperature and chilled-water flow rate
Q e i te ¼ teo h Ke Fe exp Cw Vw;e r 1 Cw Vw;e rw
(2)
w
When the chilled-water flow rate Vw,e changes, Qe, Ke and te change. When flooded evaporator’s water-side flow changes, the heat transfer coefficient Ke only has linear relationship with the chilledwater flow rate Vw,e sK;e
K e ¼ lK;e V w;e
(3)
where, K e for the relative heat transfer coefficient, is equal to the ratio of variable flow heat transfer coefficient divided by the design flow heat transfer coefficient, V w;e for the relative flow, is equal to the ratio of variable flow divided by the design flow; lK,e and sK,e are constants related with the evaporator, and this two number are positive numbers and not greater than 1 [9]. According to literature [10], analyze the relationship between relative load Q e and relative flow rate V w;e sQ
Q e ¼ V w;e
(4)
where, Q e for the relative load, is equal to the ratio of variable load divided by the design load, and sQ is a constant related with refrigeration coil, a positive number and not greater than 1. The equation (3), equation (4) generation into equation (2) sQ
V w;e Qe;0 !# lK;e V w;e Ke;0 Fe exp Cw V w;e Vw;e0 rw Cw V w;e Vw;e0 rw
te ¼ teo
sk;e
¼ teo ð1sQ Þ
1
("
V w;e
13
Qe;0 9 = Cw Vw;e0 rw
V w;e
Assume
Chilled-water variable flow operation leads not only to a decline in the chiller COP, but also reduces the energy consumption of chilled-water pump greatly. In order to analyze the impact on the whole system clearly, there is a definition of integrated COP of chiller, which is written as COPI, and the formula is
COPI ¼
Qe;0 ¼ Ae ; Cw Vw;e0 rw
Ke;0 Fe ¼ Be Cw Vw;e0 rw
Then the above equation simplifies to
te ¼ teo
variable flow operation, evaporation temperature will also rise when the water flow reduces, but less than the augmenter of design flow runtime. In 60% of design flow, evaporating temperature of variable flow is about 0.6 C lower than the design flow. Evaporation pressure is a positive relationship with the evaporation temperature, and chiller compressor energy consumption is an antirelationship with evaporation pressure. Thus, with the evaporation temperature decreasing, the chiller energy consumption increases, and the COP decreases, as shown in Fig. 7. The figure shows that the chiller COP decreases, as the chilled-water flow decreases. However, the COP changes little, even if the chilled-water flow rate is 60% of design flow, the COP of variable flow is only reduced by 6% relative to design flow. 3.3. Analysis of overall system running performance when chilled water uses variable flow operation
lK;e Ke;0 Fe A5 1 ð1sk;e Þ Cw Vw;e0 rw ;
exp
Fig. 6. Evaporation temperature contrast between chilled-water’s variable flow operation and design value operation.
ð1sQ Þ
V w;e
(
Ae "
exp
# lK Be
ð1s Þ V w;e k
)
(5)
Qe Wp þ Wwc
(6)
The integrated COPI of design flow and variable flow is shown in Fig. 8. The integrated chiller COPI of variable flow operation is always higher than the design flow operation. Thus, the chilled-water variable flow operation in accordance with the air-conditioning load variation is feasible and can reduce energy consumption of the whole system.
1
Because for a specific device, Qe,0, Ke,0, Vw,e0 values are known constants, Ae, Be are constants associated with equipments. Assume 1) Cooling water flow operation follows designed flow value and designed in and out water temperature (30 C/35 C); 2) When chilled water run by design flow value, its supply temperature is 7 C; 3) When chilled water run by flow change, refrigerating capacity with chilled-water flow relationships is equation (4). Under these assumptions, analyzing the evaporation temperature under the conditions of chilled-water’s variable flow and design flow operation, and the results and comparison are shown in Fig. 6. According to the results, if the chilled-water pump uses
Fig. 7. Relative COP of chilled-water variable flow contrasting with design flow operation.
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Fig. 10. Temperature field of platform.
Fig. 8. Integrated COPI of chilled-water design flow operation and variable flow operation.
4. Feasibility analysis of station public area variable flow operation A dual-fan total air system is used in VAC of this metro station, and the engine room is set in control rooms which are at both ends of the station. The AHU fans, back/exhaust fans, smoke exhaust fans and fresh air fans are arranged at the both ends of station symmetrically. Each end is responsible for half load of the station public area. Generally, fixed air flow design is used in the VAC of the metro station. Variable air volume technology (VAV) which is widely used in ground construction is adopted in this paper. Fluent software is used to simulate the effect of VAV operation of the station public area [11,12], and the model is shown in Fig. 9. By analyzing the temperature and velocity field, it determines whether VAV is suitable for metro station. Simulation condition is as follows: ambient temperature is 15 C, passenger traffic is 10% of maximum hourly flow rate in long-term, and air supply is 80% of the AHU rated air flow. Operation mode is that mechanical ventilation mode is used, and chillers are shut down. The simulation results are shown in Fig. 10e13 (the cross section of the platform and station is 1.65 m). 4.1. Analysis of VAV temperature field According to literature [13], it is very difficult to define acceptable thermal comfort criteria for underground railway environment. In this paper, based on the literature [14] to calculate comfortable temperature range, and the result is 18.7e26.3 C in accord with comfort requirements. As is shown in Fig. 10, most temperatures in platform are between 20 C and 26 C. But there is a small part whose temperature is higher than 26 C, such as yellow, orange and red areas. The yellow area (temperature range: 26e 28 C) is mainly between the shielded door and the elevator, for the region is narrow and the air flows slowly, so heat is concentrated
Fig. 11. Temperature field of station hall.
relatively, which is in accord with the actual subway and occupies a relatively small area. Orange and red areas whose temperatures are high are mainly concentrated in the area that near the elevator and the wall. This is mainly because fluid speed near the boundary wall is set equal to 0. This range is very small, and passengers are generally not close to the heating surface, so high temperature does not affect the comfort of passengers’ waiting environment. As shown in Fig. 11, majority of temperature area in station hall is mainly between 21 C and 26 C (green, light green, light yellow area). Although the temperature is higher than that in platform, it is still in comfortable range by the calculation which is between 18.7 C and 26.3 C. Temperatures field near the wall is similar with that in platform for the same reasons. Area of yellow, deep yellow and orange which means high-temperature region increased, because piping layout in station hall is different with platform. Return air pipes of station hall are set on both sides and the station hall is wider than platform, which results in that heat in regions far away from the return air pipes cannot be quickly exhausted and the temperature is relatively higher. But the high-temperature area is located near the wall and it does not affect the overall comfort of the station. 4.2. Analysis of VAV velocity fields Wind velocity in platform is between 0 m/s and 1 m/s, and it is zero near the walls, which is shown in Fig. 12. Wind velocity
Fig. 9. Model of station public area.
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Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
Fig. 12. Velocity fields of platform.
throughout the platform is mainly distributed in 0e0.3 m/s. It is greater than 0.9 m/s just below the outlet and then gradually reduces to 0.3 m/s. Velocity field of the transition is slow and the wind velocity is relatively low, so passengers will not feel the wind significantly. There is no area with wind velocity of 0 m/s, and the air distribution does not affect the platform. As shown in Fig. 13, wind velocity in station hall is between 0 m/ s and 0.8 m/s, and the maximum speed is smaller than platform. This is mainly because station hall is higher than platform. Wind velocity in the whole station hall is mainly between 0 m/s and 0.3 m/s and changes little without dead area and draught sensation.
Fig. 14. Quantity of energy saving by chilled pumps frequency conversion.
long-term stage is 125.6 kWh, 116.9 kWh and 153.3 kWh. In a word, energy-saving effect is very considerable.
4.3. Summary The above results show that when the system is operated by VAV technology, temperature field and velocity field of public areas of station are all in reasonable ranges to meet the comfort requirements of passenger, and AHU fan and back/exhaust fan can run with FCT according to load. 5. Analysis of energy-saving effect by FCT After analysis and demonstration, according to the airconditioning load, chilled-water pumps, AHU fans and back/ exhaust fans can run feasibly with FCT, and this paper takes a typical summer day for example, analyzes the energy-saving effect of frequency conversion operation. 5.1. Variable flow of chilled-water system According to the air-conditioning load, VPF adjusts the flow rate and running number of chilled-water pumps. Quantity of energy saving of the initial, recent and long-term stage obtained by chilledwater pump frequency conversion is shown in Fig. 14. The variable flow of chilled-water pumps leads to the energy consumption of chiller rise, which offsets part quantity of energy saving with pumps frequency conversion, and makes lower energy-saving rate of chilled-water pumps. But at most time the saving energy rate reaches more than 40% and the minimum value is 26.73%. In a typical summer day, the respective quantity of energy saving by chilled-water pumps frequency conversion of the initial, recent and
Fig. 13. Velocity fields of station hall.
5.2. Variable air volume of station public area As AHU fan and back/exhaust fan are used by FCT, station public area system can realize operation with VAV. Different from the chilled-water pump, all AHU fans and back/exhaust fans must run at the same working condition at any time. According to energysaving condition in the way of total volume air running (reference Figs. 15 and 16), the effects of energy saving by FCT of AHU funs and back-exhaust fans are much better than chilled-water pumps. And at most time the effect is very obvious, especially in the initial and recent stage. In a typical summer day, the respective quantity of energy saving by AHU fans frequency conversion of the initial, recent and long-term stage is 659.1 kWh, 640.3 kWh, 533.4 kWh. Meanwhile, the respective quantity of energy saving by back/ exhaust fans frequency conversion of the initial, recent and longterm stage is 318.7 kWh, 307.1 kWh, 239.5 kWh.
Fig. 15. Quantity of energy saving by AHU fans frequency conversion.
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3) The temperature and velocity fields of the platform and station hall are simulated by CFD software when the air volume is variable. The results indicate that the station can still maintain a comfortable temperature range when the air exchange volume is variable, and also do not affect the air flow layout. That is to say, all the fans (including AHU fans, back/exhaust fans) can be operated by FCT. 4) Detailed analysis is made on the energy-saving potential of the whole system when the water pumps and fans are operated by FCT. Taking a typical summer day as an example, the respective total energy-saving quantity is 1103.4 kWh, 1064.3 kWh and 926.2 kWh for the initial, recent and the long-term stage, and the respective total power saving ratio is 73.4%, 71.2% and 59.5%.
Acknowledgements
Fig. 16. Quantity of energy saving by back/exhaust fans frequency conversion.
Supported by the National Natural Science Foundation of China (Grant No. 51276124), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130032130006), Science and Technology Project of Tianjin City (Grant No. 12ZCDGGX49400).
5.3. The total power saving ratio
References
In a typical summer day, overall energy consumption of chilledwater pumps, AHU fans and back/exhaust fans are respectively Ep, Ef and Ebf without the use of frequency conversion technology, and the respective quantity of energy saving by using of frequency conversion technology is E1, E2 and E3. Meanwhile, the increased energy consumption amount of chiller is E4 because of variable flow. Then the total power saving ratio is
[1] Shigenobu Horita, Tsutomu Kayama, Toshifumi Nakamura, Ventilation and air-conditioning system design for the new Astram metro line in Hiroshima, Mitsubishi Electr. Adv. 74 (1996). [2] Ministry of Construction, GB50157e2003, Code for Design of Metro, China Planning Press, Beijing, 2003 (in Chinese). [3] Wang Feng, Study on Energy-saving of Ventilation and Air-conditioning System by Frequency Conversion in Subway, Southwest Jiaotong University, Shanxi, 2007. [4] Meteorological Data Room of Chinese Meteorological Information Center, Special Meteorological Data Set of Analysis of China Building Thermal Environment, China Architecture & Building Industry Press, Beijing, 2005 (in Chinese). [5] S.C. Sekhar, A critical evaluation of variable air volume system in hot and humid climates, Energy Build. 26 (1997). [6] F.W. Yu, K.T. Chan, Optimization of water-cooled chiller system with loadbased speed control, Appl. Energy 85 (2008). [7] Shen Shi-ping, Ning Shao-long, Chen Ming, Energy saving of the control strategy of main pipe differential pressure and differential temperature on chilled water in central air-condition systems, Build. Energy Environ. (Apr. 2012) 49e52. [8] R. Harish, E.E. Subhramanyan, R. Madhavan, S. Vidyanand, Theoretical model for evaluation of variable frequency drive for cooling water pumps in sea water based once through condenser cooling water systems, Appl. Therm. Eng. 30 (2010) 2051e2057. [9] J.M. Gordon, K.C. Ng, H.T. Chua, C.K. Lim, How varying condenser coolant flow rate affects chiller performance: thermodynamic modeling and experimental confirmation, Appl. Therm. Eng. 20 (2000). [10] Alfred Greenberg, Hydronic systems: analysis and evaluation, ASHRAE 10 (1969). [11] Anastasios I. Stamou, Ioannis Katsiris, Alois Schaelin, Evaluation of thermal comfort in Galatsi Arena of the Olympics “Athens 2004” using a CFD model, Appl. Therm. Eng. 28 (2008) 1206e1215. [12] Wei Qiaoli, VAV Application in the Subway Environmental Control System, Tianjin University, Tianjin, 2005. [13] F. Ampofo, G. Maidment, J. Missenden, Underground railway environment in the UK Part 1: Review of thermal comfort, Appl. Therm. Eng. 24 (2004) 611e 631. [14] United States Department of Transportation, Subway Environmental Design Handbook, vol. 1, 1976.
xt ¼
E1 þ E2 þ E3 E4 100% Ep þ Ef þ Ebf
(7)
Based on the above analysis and calculation, in a typical summer day, the respective total power saving ratio by frequency conversion technology of the initial, recent and long-term stage is 73.4%, 71.2% and 59.5%. 6. Conclusions Taking a southern subway station as a real instance, this paper analyzes the application feasibility of the FCT in the VAC of a metro station. The results can be concluded as follows: 1) This paper analyzes the hourly passenger traffic and air-conditioning load as well as the air supply of the metro from the aspect of the initial, recent and long-term stage respectively and the results show that it is necessary to carry out the FCT. 2) This paper states the influence of frequency conversion of the chilled-water pumps by the thermodynamic method theoretically, which presented that applying the FCT to the chilled-water pumps will be benefit for energy saving.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research on frequency conversion technology of metro station’s ventilation and air-conditioning system Zhao Yang*, Zhuangzhuang Yu, Longqing Yu, Feng Ma School of Mechanical Engineering, Key Laboratory of Efficient Utilization of Low and Medium Grade Energy, MOE, Tianjin University, 92 Weijin Road, Tianjin 30072, PR China
h i g h l i g h t s Use the FCT to reduce energy consumption of metro VAC is necessary and possible. Analyze the influence of running the chilled-water pumps with FCT. Results show that variable air volume of station public area is feasible. Calculations indicate that energy-saving effect of using the FCT is considerable.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2013 Accepted 9 April 2014 Available online 24 April 2014
Ventilation and air-conditioning system (VAC) is the most energy-saving potential system in the metro. This paper analyzes the passenger traffic, air-conditioning load and station air supply on the initial, recent and long-term phase of metro station. And it proposes that it is necessary to run chilled-water pumps, air handing unit (AHU) fans and back/exhaust fans with frequency conversion technology (FCT). Then it uses the thermodynamic method to analyze the impact of running chilled-water pumps with FCT. The results show that running chilled-water pumps with FCT can reduce the total power consumption of system, although increases chiller energy consumption. Then the temperature and velocity fields of the platform and station hall are simulated by CFD software according to the variable air volume. And the results show that under the condition of running the VAC system with FCT, temperature and velocity fields distribution are both in the comfortable range. Finally, by taking a typical summer day for example, this paper analyzes the energy savings of chilled-water pumps, AHU fans and back/exhaust fans on the initial, recent and long-term phase, and the calculation results show that the respective total energy savings are 1103.4 kWh, 1064.3 kWh and 926.2 kWh, and the respective total power saving ratio is 73.4%, 71.2% and 59.5%. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Metro station Energy saving Frequency conversion technology Thermodynamic method CFD
1. Introduction With the development of economy, urbanization and urban expansion, it increases pressure on urban transport and makes environmental degradation in China. In order to ease the traffic pressure and reduce environmental pollution, major cities begin to build metro which is efficient, green and has large passenger capacity. However, its high energy consumption cannot be ignored, only 20 km to the opening of the Shenzhen Metro, for example, its annual electricity consumption accounts for 1/3000 of the total electricity consumption in Shenzhen [1], of which the metro air-
* Corresponding author. Tel./fax: þ86 022 2789 0627. E-mail address: [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.applthermaleng.2014.04.016 1359-4311/Ó 2014 Elsevier Ltd. All rights reserved.
conditioning energy consumption accounts for about 35%. According to the code for design of metro, the maximum long-term load determines the capacity of the VAC equipments of metro station, which has 10e20% of the wealthy [2]. In the process of operation, full load operation time of the VAC will be less than 5% of the total runtime, especially in the initial stage. Meanwhile, metro’s characteristics of morning peak and evening peak make the airconditioning load of metro extremely volatile in a working day. Therefore, it is necessary to carry out energy-saving research on the VAC of metro [3]. Because of the climate difference and different metro design concept between southern and northern in China, the VAC of the metro station is different. According to the actual situation, the different energy-saving control program need to be constituted for the different place. In this paper, a metro station with designing and
124
Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
Nomenclature
Ebf
Qe Qe,0 Q0 Wp Wwc Ke Ke,0 Fe Vw,e0 te tei COPI Ep
E1
Ef
chiller refrigeration capacity, kW chiller rated refrigerating capacity, kW refrigerating capacity, kW pump power, kW chiller power, kW evaporator heat transfer coefficient, kW/(m2 C) evaporator rated heat transfer coefficient, kW/(m2 C) evaporator heat transfer area, m2 chillers rated water flow, m/s evaporation temperature, C return chilled-water temperature, C integrated chiller COP overall energy consumption amount of chilled-water pumps, kWh overall energy consumption amount of AHU fans, kWh
installation of platform screen doors in the south China, for example, its corresponding energy-saving program is proposed and studied. The VAC of the metro station principle diagram is shown in Fig. 1. At present, one of energy-saving measures of the ground building is to use FCT to control the fluid machinery (fans and pumps) of VAC, which this article will introduce. Through empirical analysis, this paper intends to study the feasibility of FCT used in the equipments of chilled-water system and ventilation system of the station public area.
E2 E3 E4
xt
overall energy consumption amount of back/exhaust fans, kWh quantity of energy saving by chilled-water pumps frequency conversion, kWh quantity of energy saving by AHU fans frequency conversion, kWh quantity of energy saving by back/exhaust fans frequency conversion, kWh increased energy consumption amount of chiller, kWh total power saving ratio
Abbreviations VAC ventilation and air-conditioning system AHU air handing unit FCT frequency conversion technology VPF variable primary flow system VAV variable air volume technology
According to the station load, air supplies of the initial, recent and long-term stage are calculated, and the result is shown in Fig. 4. Air supply of the station public area also changes dramatically throughout a day. Especially in the initial and recent phase, the air supply is less than 60% of the maximum long-term phase. The prediction of hourly passenger traffic, air-conditioning load and air supply show that change of metro passenger traffic leads
2. Station passenger traffic and air-conditioning load analysis Hourly forecast passenger traffic is shown in Fig. 2, which shows that the passenger traffic difference is very large at different times a day, and the morning and evening peak passenger flow are very clear, with a variation hump-shaped. According to passenger forecasts and other information, and basing on the typical year weather data provided by special meteorological data set of analysis of china building thermal environment [4] and according to code for design of metro, this paper takes a typical summer day for example, hourly station airconditioning loads of the initial, recent and long-term stage are calculated and shown in Fig. 3. Most of the time, the air-conditioning load is 60e80% of the maximum long-term air-conditioning load. In a day, the chilled-water flow changes dramatically, because air-conditioning load reflects the needed of the chilled-water flow directly.
Fig. 2. Hourly distribution of station passenger (the morning rush is unit volume 1).
Fig. 1. The metro station VAC principle diagram.
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Fig. 5. Variable primary flow system schematic diagrams.
Fig. 3. Hourly station air-conditioning loads (maximum long-term air-conditioning load is unit volume 1).
the dramatic changes of air-conditioning load and air supply in the initial, recent and long-term stage, whose trend is an obvious hump. Meanwhile, the equipment selection of metro station VAC is according to the maximum long-term passenger traffic and airconditioning load, so it is necessary and possible to use the FCT to reduce energy consumption. 3. Chilled-water system variable flow feasibility analyses 3.1. Chilled-water system operation mode To reduce the energy consumption of chilled-water pump, the speed of pump can be adjusted according to air-conditioning load, but it also changes the chiller evaporator water flow, which is not allowed under normal circumstances [5]. In recent years, with the development of chiller control technology, electronic control
technology has replaced the mechanical and pneumatic control technology. The control system can make a timely response when there is little change in load. The chiller can work normally and stably even if evaporator water flow is variable. Therefore, this article uses variable primary flow system (VPF) [6] in the chilledwater system. Its principle is shown in Fig. 5, and its design temperature difference for the sending and return water is 5 C (7 C/ 12 C). Based on the air-conditioning load, VPF regulates the flow of chilled-water pump directly. When the water flow is reduced to the minimum flow decided by chiller and evaporator, the chilled-water pumps run in accordance with the minimum flow and the bypass valve opens now. The bypass valve in VPF is designed to ensure the normal operation of the chiller and evaporator. There are two frequency conversion control methods to control chilled-water pump of VPF: temperature difference control and pressure control. The pressure control is divided into trunk pipeline pressure control and the most unfavorable pressure control. As shown in Fig. 5, VPF has a few end devices (only two AHU). According to design requirements, the operating conditions of terminal equipments are the same. Meanwhile, the chilled-water system is simple. Therefore, from the system and energy-saving point of view, temperature difference control method is chosen to control VPF. Temperature difference control only changes water flow of the system, without changing the characteristics of the pipeline, so the water pump frequency conversion operation meets the similar law of fluid machinery, which makes the power and frequency into three power relations. Compared with the constant flow control, controlling the pump flow with the temperature difference control can achieve the biggest energy-saving rate [7]. 3.2. Running performance analysis of chilled-water variable flow At present, the reason of VPF hard to promote is that variable flow of chilled water makes the COP of chillers decrease, and the energy-saving effect of whole system is not obvious, even without energy saving. Therefore, the analysis should be based on specific circumstances with specific engineering applications. According to literature [8], for example, it provides a theoretical model with a typical case study to establish viability of providing variable frequency drive for cooling water pumps in power plants with sea water based once through condenser cooling water system. This article uses thermodynamic method to analyze the feasibility of chilled-water’s variable flow. Evaporation temperature is an important factor to determine the COP of chiller. Heat transfer and energy conservation equation of evaporator is
Fig. 4. Hourly air station air supplies (long-term maximum air supply is unit volume 1).
Qe ¼ Ke Fe
ðtei te Þ ðte0 te Þ ¼ Cw Vw;e rw ðtei te0 Þ e In tte0ei t te
(1)
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By equation (1), derive the relationship between evaporation temperature and chilled-water flow rate
Q e i te ¼ teo h Ke Fe exp Cw Vw;e r 1 Cw Vw;e rw
(2)
w
When the chilled-water flow rate Vw,e changes, Qe, Ke and te change. When flooded evaporator’s water-side flow changes, the heat transfer coefficient Ke only has linear relationship with the chilledwater flow rate Vw,e sK;e
K e ¼ lK;e V w;e
(3)
where, K e for the relative heat transfer coefficient, is equal to the ratio of variable flow heat transfer coefficient divided by the design flow heat transfer coefficient, V w;e for the relative flow, is equal to the ratio of variable flow divided by the design flow; lK,e and sK,e are constants related with the evaporator, and this two number are positive numbers and not greater than 1 [9]. According to literature [10], analyze the relationship between relative load Q e and relative flow rate V w;e sQ
Q e ¼ V w;e
(4)
where, Q e for the relative load, is equal to the ratio of variable load divided by the design load, and sQ is a constant related with refrigeration coil, a positive number and not greater than 1. The equation (3), equation (4) generation into equation (2) sQ
V w;e Qe;0 !# lK;e V w;e Ke;0 Fe exp Cw V w;e Vw;e0 rw Cw V w;e Vw;e0 rw
te ¼ teo
sk;e
¼ teo ð1sQ Þ
1
("
V w;e
13
Qe;0 9 = Cw Vw;e0 rw
V w;e
Assume
Chilled-water variable flow operation leads not only to a decline in the chiller COP, but also reduces the energy consumption of chilled-water pump greatly. In order to analyze the impact on the whole system clearly, there is a definition of integrated COP of chiller, which is written as COPI, and the formula is
COPI ¼
Qe;0 ¼ Ae ; Cw Vw;e0 rw
Ke;0 Fe ¼ Be Cw Vw;e0 rw
Then the above equation simplifies to
te ¼ teo
variable flow operation, evaporation temperature will also rise when the water flow reduces, but less than the augmenter of design flow runtime. In 60% of design flow, evaporating temperature of variable flow is about 0.6 C lower than the design flow. Evaporation pressure is a positive relationship with the evaporation temperature, and chiller compressor energy consumption is an antirelationship with evaporation pressure. Thus, with the evaporation temperature decreasing, the chiller energy consumption increases, and the COP decreases, as shown in Fig. 7. The figure shows that the chiller COP decreases, as the chilled-water flow decreases. However, the COP changes little, even if the chilled-water flow rate is 60% of design flow, the COP of variable flow is only reduced by 6% relative to design flow. 3.3. Analysis of overall system running performance when chilled water uses variable flow operation
lK;e Ke;0 Fe A5 1 ð1sk;e Þ Cw Vw;e0 rw ;
exp
Fig. 6. Evaporation temperature contrast between chilled-water’s variable flow operation and design value operation.
ð1sQ Þ
V w;e
(
Ae "
exp
# lK Be
ð1s Þ V w;e k
)
(5)
Qe Wp þ Wwc
(6)
The integrated COPI of design flow and variable flow is shown in Fig. 8. The integrated chiller COPI of variable flow operation is always higher than the design flow operation. Thus, the chilled-water variable flow operation in accordance with the air-conditioning load variation is feasible and can reduce energy consumption of the whole system.
1
Because for a specific device, Qe,0, Ke,0, Vw,e0 values are known constants, Ae, Be are constants associated with equipments. Assume 1) Cooling water flow operation follows designed flow value and designed in and out water temperature (30 C/35 C); 2) When chilled water run by design flow value, its supply temperature is 7 C; 3) When chilled water run by flow change, refrigerating capacity with chilled-water flow relationships is equation (4). Under these assumptions, analyzing the evaporation temperature under the conditions of chilled-water’s variable flow and design flow operation, and the results and comparison are shown in Fig. 6. According to the results, if the chilled-water pump uses
Fig. 7. Relative COP of chilled-water variable flow contrasting with design flow operation.
Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
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Fig. 10. Temperature field of platform.
Fig. 8. Integrated COPI of chilled-water design flow operation and variable flow operation.
4. Feasibility analysis of station public area variable flow operation A dual-fan total air system is used in VAC of this metro station, and the engine room is set in control rooms which are at both ends of the station. The AHU fans, back/exhaust fans, smoke exhaust fans and fresh air fans are arranged at the both ends of station symmetrically. Each end is responsible for half load of the station public area. Generally, fixed air flow design is used in the VAC of the metro station. Variable air volume technology (VAV) which is widely used in ground construction is adopted in this paper. Fluent software is used to simulate the effect of VAV operation of the station public area [11,12], and the model is shown in Fig. 9. By analyzing the temperature and velocity field, it determines whether VAV is suitable for metro station. Simulation condition is as follows: ambient temperature is 15 C, passenger traffic is 10% of maximum hourly flow rate in long-term, and air supply is 80% of the AHU rated air flow. Operation mode is that mechanical ventilation mode is used, and chillers are shut down. The simulation results are shown in Fig. 10e13 (the cross section of the platform and station is 1.65 m). 4.1. Analysis of VAV temperature field According to literature [13], it is very difficult to define acceptable thermal comfort criteria for underground railway environment. In this paper, based on the literature [14] to calculate comfortable temperature range, and the result is 18.7e26.3 C in accord with comfort requirements. As is shown in Fig. 10, most temperatures in platform are between 20 C and 26 C. But there is a small part whose temperature is higher than 26 C, such as yellow, orange and red areas. The yellow area (temperature range: 26e 28 C) is mainly between the shielded door and the elevator, for the region is narrow and the air flows slowly, so heat is concentrated
Fig. 11. Temperature field of station hall.
relatively, which is in accord with the actual subway and occupies a relatively small area. Orange and red areas whose temperatures are high are mainly concentrated in the area that near the elevator and the wall. This is mainly because fluid speed near the boundary wall is set equal to 0. This range is very small, and passengers are generally not close to the heating surface, so high temperature does not affect the comfort of passengers’ waiting environment. As shown in Fig. 11, majority of temperature area in station hall is mainly between 21 C and 26 C (green, light green, light yellow area). Although the temperature is higher than that in platform, it is still in comfortable range by the calculation which is between 18.7 C and 26.3 C. Temperatures field near the wall is similar with that in platform for the same reasons. Area of yellow, deep yellow and orange which means high-temperature region increased, because piping layout in station hall is different with platform. Return air pipes of station hall are set on both sides and the station hall is wider than platform, which results in that heat in regions far away from the return air pipes cannot be quickly exhausted and the temperature is relatively higher. But the high-temperature area is located near the wall and it does not affect the overall comfort of the station. 4.2. Analysis of VAV velocity fields Wind velocity in platform is between 0 m/s and 1 m/s, and it is zero near the walls, which is shown in Fig. 12. Wind velocity
Fig. 9. Model of station public area.
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Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
Fig. 12. Velocity fields of platform.
throughout the platform is mainly distributed in 0e0.3 m/s. It is greater than 0.9 m/s just below the outlet and then gradually reduces to 0.3 m/s. Velocity field of the transition is slow and the wind velocity is relatively low, so passengers will not feel the wind significantly. There is no area with wind velocity of 0 m/s, and the air distribution does not affect the platform. As shown in Fig. 13, wind velocity in station hall is between 0 m/ s and 0.8 m/s, and the maximum speed is smaller than platform. This is mainly because station hall is higher than platform. Wind velocity in the whole station hall is mainly between 0 m/s and 0.3 m/s and changes little without dead area and draught sensation.
Fig. 14. Quantity of energy saving by chilled pumps frequency conversion.
long-term stage is 125.6 kWh, 116.9 kWh and 153.3 kWh. In a word, energy-saving effect is very considerable.
4.3. Summary The above results show that when the system is operated by VAV technology, temperature field and velocity field of public areas of station are all in reasonable ranges to meet the comfort requirements of passenger, and AHU fan and back/exhaust fan can run with FCT according to load. 5. Analysis of energy-saving effect by FCT After analysis and demonstration, according to the airconditioning load, chilled-water pumps, AHU fans and back/ exhaust fans can run feasibly with FCT, and this paper takes a typical summer day for example, analyzes the energy-saving effect of frequency conversion operation. 5.1. Variable flow of chilled-water system According to the air-conditioning load, VPF adjusts the flow rate and running number of chilled-water pumps. Quantity of energy saving of the initial, recent and long-term stage obtained by chilledwater pump frequency conversion is shown in Fig. 14. The variable flow of chilled-water pumps leads to the energy consumption of chiller rise, which offsets part quantity of energy saving with pumps frequency conversion, and makes lower energy-saving rate of chilled-water pumps. But at most time the saving energy rate reaches more than 40% and the minimum value is 26.73%. In a typical summer day, the respective quantity of energy saving by chilled-water pumps frequency conversion of the initial, recent and
Fig. 13. Velocity fields of station hall.
5.2. Variable air volume of station public area As AHU fan and back/exhaust fan are used by FCT, station public area system can realize operation with VAV. Different from the chilled-water pump, all AHU fans and back/exhaust fans must run at the same working condition at any time. According to energysaving condition in the way of total volume air running (reference Figs. 15 and 16), the effects of energy saving by FCT of AHU funs and back-exhaust fans are much better than chilled-water pumps. And at most time the effect is very obvious, especially in the initial and recent stage. In a typical summer day, the respective quantity of energy saving by AHU fans frequency conversion of the initial, recent and long-term stage is 659.1 kWh, 640.3 kWh, 533.4 kWh. Meanwhile, the respective quantity of energy saving by back/ exhaust fans frequency conversion of the initial, recent and longterm stage is 318.7 kWh, 307.1 kWh, 239.5 kWh.
Fig. 15. Quantity of energy saving by AHU fans frequency conversion.
Z. Yang et al. / Applied Thermal Engineering 69 (2014) 123e129
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3) The temperature and velocity fields of the platform and station hall are simulated by CFD software when the air volume is variable. The results indicate that the station can still maintain a comfortable temperature range when the air exchange volume is variable, and also do not affect the air flow layout. That is to say, all the fans (including AHU fans, back/exhaust fans) can be operated by FCT. 4) Detailed analysis is made on the energy-saving potential of the whole system when the water pumps and fans are operated by FCT. Taking a typical summer day as an example, the respective total energy-saving quantity is 1103.4 kWh, 1064.3 kWh and 926.2 kWh for the initial, recent and the long-term stage, and the respective total power saving ratio is 73.4%, 71.2% and 59.5%.
Acknowledgements
Fig. 16. Quantity of energy saving by back/exhaust fans frequency conversion.
Supported by the National Natural Science Foundation of China (Grant No. 51276124), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130032130006), Science and Technology Project of Tianjin City (Grant No. 12ZCDGGX49400).
5.3. The total power saving ratio
References
In a typical summer day, overall energy consumption of chilledwater pumps, AHU fans and back/exhaust fans are respectively Ep, Ef and Ebf without the use of frequency conversion technology, and the respective quantity of energy saving by using of frequency conversion technology is E1, E2 and E3. Meanwhile, the increased energy consumption amount of chiller is E4 because of variable flow. Then the total power saving ratio is
[1] Shigenobu Horita, Tsutomu Kayama, Toshifumi Nakamura, Ventilation and air-conditioning system design for the new Astram metro line in Hiroshima, Mitsubishi Electr. Adv. 74 (1996). [2] Ministry of Construction, GB50157e2003, Code for Design of Metro, China Planning Press, Beijing, 2003 (in Chinese). [3] Wang Feng, Study on Energy-saving of Ventilation and Air-conditioning System by Frequency Conversion in Subway, Southwest Jiaotong University, Shanxi, 2007. [4] Meteorological Data Room of Chinese Meteorological Information Center, Special Meteorological Data Set of Analysis of China Building Thermal Environment, China Architecture & Building Industry Press, Beijing, 2005 (in Chinese). [5] S.C. Sekhar, A critical evaluation of variable air volume system in hot and humid climates, Energy Build. 26 (1997). [6] F.W. Yu, K.T. Chan, Optimization of water-cooled chiller system with loadbased speed control, Appl. Energy 85 (2008). [7] Shen Shi-ping, Ning Shao-long, Chen Ming, Energy saving of the control strategy of main pipe differential pressure and differential temperature on chilled water in central air-condition systems, Build. Energy Environ. (Apr. 2012) 49e52. [8] R. Harish, E.E. Subhramanyan, R. Madhavan, S. Vidyanand, Theoretical model for evaluation of variable frequency drive for cooling water pumps in sea water based once through condenser cooling water systems, Appl. Therm. Eng. 30 (2010) 2051e2057. [9] J.M. Gordon, K.C. Ng, H.T. Chua, C.K. Lim, How varying condenser coolant flow rate affects chiller performance: thermodynamic modeling and experimental confirmation, Appl. Therm. Eng. 20 (2000). [10] Alfred Greenberg, Hydronic systems: analysis and evaluation, ASHRAE 10 (1969). [11] Anastasios I. Stamou, Ioannis Katsiris, Alois Schaelin, Evaluation of thermal comfort in Galatsi Arena of the Olympics “Athens 2004” using a CFD model, Appl. Therm. Eng. 28 (2008) 1206e1215. [12] Wei Qiaoli, VAV Application in the Subway Environmental Control System, Tianjin University, Tianjin, 2005. [13] F. Ampofo, G. Maidment, J. Missenden, Underground railway environment in the UK Part 1: Review of thermal comfort, Appl. Therm. Eng. 24 (2004) 611e 631. [14] United States Department of Transportation, Subway Environmental Design Handbook, vol. 1, 1976.
xt ¼
E1 þ E2 þ E3 E4 100% Ep þ Ef þ Ebf
(7)
Based on the above analysis and calculation, in a typical summer day, the respective total power saving ratio by frequency conversion technology of the initial, recent and long-term stage is 73.4%, 71.2% and 59.5%. 6. Conclusions Taking a southern subway station as a real instance, this paper analyzes the application feasibility of the FCT in the VAC of a metro station. The results can be concluded as follows: 1) This paper analyzes the hourly passenger traffic and air-conditioning load as well as the air supply of the metro from the aspect of the initial, recent and long-term stage respectively and the results show that it is necessary to carry out the FCT. 2) This paper states the influence of frequency conversion of the chilled-water pumps by the thermodynamic method theoretically, which presented that applying the FCT to the chilled-water pumps will be benefit for energy saving.