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Green construction hoist with customized energy regeneration system
Automation in Construction 45 (2014) 66–71
Contents lists available at ScienceDirect
Automation in Construction journal homepage: www.elsevier.com/locate/autcon
Green construction hoist with customized energy regeneration system Myungdo Lee a,1, Taehoon Kim b,⁎, Hong-Ku Jung c,2, Ung-Kyun Lee d,3, Hunhee Cho e,1, Kyung-In Kang f,1 a
School of Civil, Environmental and Architectural Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea Research Institute of Engineering and Technology, College of Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea c Liftec Corp., 248-8, Siheung-Dong, Sujeong-Gu, Seongnam-Si, Gyeonggi-Do 461-370, Republic of Korea d School of Architecture, Kwandong University, 522 Naegok-Dong, Gangneung-Si, Gangwon-Do 210-701, Republic of Korea e School of Civil, Environmental and Architectural Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea f School of Civil, Environmental and Architectural Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 18 November 2013 Revised 15 April 2014 Accepted 2 May 2014 Available online xxxx Keywords: Construction hoist Green technology Energy regeneration system Regenerative braking
a b s t r a c t Construction hoists are large energy consumers in building construction sites. The application of green technology to hoists can be a useful means to reduce energy consumption during construction. We propose a green construction hoist that uses an energy regeneration system (ERS) to reduce its operating energy requirements. The energy regeneration system was customized to improve its energy-saving efficiency and to cope with exposed weather conditions as well as power supply problems. The prototype was applied to an actual building construction project to verify its operating performance, which showed a cost-effective, stable performance for an energy recovery of 55.5% on average. It is expected that the proposed system can be employed as a practical means to save energy usage in construction sites. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Construction hoists are essential equipment for vertical transportation of construction resources, such as workers and materials [1]. There have been many studies and developments to improve the performance or productivity of construction hoists by increasing their capacity, size, speed, and reliability, or by adapting intelligent control with group control to solve site logistic problems at large construction sites, fault diagnostics, and so on [2–4]. However, there have been few efforts to reduce the energy consumption of the hoist itself in response to the era of the low energy society. Construction hoists have large energy consumption. The need for high-performance construction hoists with low energy consumption is increasing because of the trend to high-rise buildings and the global energy crisis. Therefore, it is necessary to develop green technology to reduce the operating energy of hoists in construction sites. Energy regeneration systems (ERSs), which are a green technology, have been widely used in various types of machinery to improve their energy efficiency [5,6]. ERSs can recover electrical energy by regenerative
⁎ Corresponding author. Tel.: +82 2 921 5920; fax: +82 2 923 4229. E-mail addresses: [email protected] (M. Lee), [email protected] (T. Kim), [email protected] (H.-K. Jung), [email protected] (U.-K. Lee), [email protected] (H. Cho), [email protected] (K.-I. Kang). 1 Tel.: +82 2 921 5920; fax: +82 2 923 4229. 2 Tel.: +82 31 322 3000; fax: +82 31 322 3010. 3 Tel.: +82 33 649 7548; fax: +82 33 645 8120.
http://dx.doi.org/10.1016/j.autcon.2014.05.001 0926-5805/© 2014 Elsevier B.V. All rights reserved.
braking to decelerate or stop movement. Therefore, the downward half of the hoist's moving path can offer energy savings in each operation cycle in which the ERS is applied. Moreover, an ERS can be applied to construction hoists without significant alteration of the hoist's original configuration. Thus, an ERS could be a useful alternative to reduce the operating energy of construction hoists. To apply an ERS for construction hoists, its design should consider their unique application conditions and environmental obstacles. The construction industry has a natural reluctance to the application of new technologies because of their inherent risks [7]. To overcome these perceptions, an ERS for hoists needs to establish simple structures with minimized modifications and also improve its energy-saving performance. In addition, construction hoists are exposed to the weather and frequently face unexpected power supply because they are installed on the external faces of buildings. The aim of this study is to propose a green construction hoist by adopting a customized ERS to reduce energy consumption. In this study, the ERS applied to construction hoists contributed to decreased total energy expenditure of the construction site by transmitting the generated power back to the power distribution network of the site. To enhance the efficiency of energy recovery and deal with the possible problems resulting from their adverse working conditions, the ERS is modified for construction sites after full consideration of the various properties of construction hoists. Then, a prototype with the customized ERS is developed based on the most commonly used type of hoist. Through a field application, the energy-saving performance and economic feasibility are analyzed.
M. Lee et al. / Automation in Construction 45 (2014) 66–71
2. ERS for construction hoists 2.1. Overview of ERS An ERS recovers electrical energy by regenerative braking [8,9]. Regenerative braking can be used with hybrid or hybrid hydraulic drives powered by an electric motor. During the braking, the motor generates electrical energy to decelerate or stop movement. In conventional
67
braking systems, the energy is dissipated as heat in a braking resistor because it has a random frequency that can cause trouble in the power system. An ERS recovers this generated energy as electricity that can be either used immediately or stored. The basic components of an ERS are the electric motor, an inverter unit, and a power regenerator. The electrical energy generated during braking flows into the inverter unit. The inverter unit consists of a direct current (DC) to alternating current (AC) converter, an electrolytic
Fig. 1. Configuration of the customized ERS.
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M. Lee et al. / Automation in Construction 45 (2014) 66–71
capacitor, and a rectifier. The inverter unit controls the voltage and frequency of the electrical energy to match the available electric power. The refined electric energy in the inverter unit is DC. The power regenerator converts DC into AC to transmit the electric power back to the power distribution network of the construction site. In construction machinery, ERSs use two main recoverable forms, kinetic energy and potential energy [10]. Regeneration from kinetic energy has often been applied in wheel-type construction machines based on a hydraulic accumulator [11], a battery, possibly in combination with a supercapacitor [12], or a flywheel system [13]. Potential energy regeneration has been implemented in machines used for lifting loads, for instance, excavators [5,6,14], elevators [15,16], and forklifts [17]. When the machines use regenerative brakes, their kinetic or potential energy is converted to electric energy. Both energy forms provide an opportunity to reduce the demand on the original energy source by adopting ERSs [10]. However, the existing ERS applications for construction machines require significant modification of the original configuration and mechanisms [18]. Because the existing machines originally use a combustion engine for their operation, a complex hybrid or hybrid hydraulic mechanism should be established with an electric motor for the ERS. In contrast, ERSs can be applied to construction hoists without significant alteration of the original hoist configuration because generally hoists operate using an electric motor instead of a combustion engine. Thus, an ERS for a hoist can be established with a relatively simple driving mechanism compared with existing hybrid mechanisms. Moreover, energy management on construction sites has become increasingly important, and the power consumption of hoists is predicted to increase significantly with high-rise building trends. Therefore, adoption of ERSs for construction hoists can be a useful approach to saving the operating energy if the cost effectiveness can be made satisfactory.
A customized ERS configuration for construction hoists is presented in Fig. 1. As mentioned above, the general ERS consists of the electric motor, an inverter unit, and a power regenerator. In this study, the proposed ERS was customized to reflect their unique working conditions. The newly added or modified components of the proposed system are: (1) clutch-type motor system; (2) inverter controller; (3) subcontactor; (4) power-monitoring system; and (5) main contactor. The clutch-type motor system is specifically designed to improve energy recovery as well as ensure simple structures. The energy-saving ratio Γs was calculated from Eq. (1):
2.2. Design considerations
Γs ¼ X
Table 1 Main parameters of the targeted hoist type. Parameter
Value
Allowable load capacity (Kg) Cage weight (Kg) Drive unit & electric parts weight (Kg) Cage internal width × length × height (m) Max. speed (m/min) Max. lifting height (m)
2000 2100 1200 1.5 × 4.0 × 2.6 70 400
Finally, devices for protecting the main components of the ERS from external environmental conditions should be considered. Construction hoists are installed on the external faces of buildings and the hoist operating systems are installed on the top of the cage. Thus, the components of the ERS can be affected by environment conditions such as temperature, rainwater, and relative humidity. 3. Prototype of the green construction hoist 3.1. Configuration of customized ERS
X
Before designing an ERS for construction hoists, the properties of hoists and possible problems were firstly examined through site surveys and consultations with experts from hoist manufacturers and suppliers and construction managers. From these examinations, the design considerations for green construction hoists were derived as follows. First, to counter the resistance to the application of new systems on construction sites, an ERS for a hoist should minimize the modification of the existing system while ensuring improved performance. When compared with other industries, the employment of new technologies in the construction industry has been relatively slow and difficult because of contractors' reluctance in increasing the initial investment and performance uncertainties [7]. Even though energy management has become increasingly important in construction sites, construction managers may have antipathy toward applying a new system if it requires considerable changes from the traditional system in terms of configurations and costs. Thus, to ensure the feasibility of the ERS for hoists, it is necessary to minimize the modification of the original hoist configurations and cost increases while enhancing energy recovery performance. Second, a power-monitoring device is required to protect the ERS from power supply problems. The hoist's power is supplied by power cables connected to the cage. The power cable is affected by winds because it hangs from the cage and is exposed to the weather. If the cable kinks and the tensional force exceed permissible limits because of winds, the cable may cause problems such as momentary disconnections, shorts, and phase unbalances. Such situations often occur while hoists are operating and increase as the building height increases. They can lead to breakdown of the main control system, including the ERS, and can cause safety problems. A power-monitoring device can prevent these problems.
electricity regeneration electricity consumption
¼
Eout ; Ein
ð1Þ
where Eout and Ein are the total output and input energies, respectively, during the total operating time. According to Eq. (1), the energy-saving ratio through the ERS is theoretically estimated at about 33% considering the power losses from each electric drive, including the motor, reducer, inverter, and regenerator [19,20]. A method for improving the energy-saving ratio is to use electric drives with higher efficiency. However, this leads to a considerable increase in the initial investment. Although the application of motors with larger capacity can also increase the total amount of energy recovery, it not only results in simultaneous unnecessary energy consumption, but also requires significant alteration of the structure. Therefore, in this study, the clutch-type motor system is introduced to achieve improved energy recovery with
Fig. 2. Clutch-type motor system on mock-up test.
M. Lee et al. / Automation in Construction 45 (2014) 66–71
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Fig. 3. The prototype and main components installed on the hoist cage.
the minimized modification of hoist configurations and cost increases. The motor system has two components: the main motor and submotor. The main motor consists of two general-purpose electric motors and the submotor consists of an electric motor integrated with an electric clutch. When the hoist travels upwards, only the main motor operates and the submotor is excluded from the operating mechanism because the main motor is designed to have sufficient power for hoist's operation; additional operation of the submotor would cause unnecessary energy consumption. On the other hand, when the hoist travels downwards, the submotor is connected to the hoist's operating mechanism by the clutch, so all three motors operate for energy regeneration. As a result, the submotor improves energy recovery through complementing the power losses in the electric drives during energy regeneration. The inverter controller is adopted to check the direction of motion of the hoist and to send a signal to the subcontactor controlling the clutch operation of the submotor. The power-monitoring system aims to cope with unexpected power supply problems such as disconnection, shorts, and phase unbalances. The power-monitoring system checks for unstable power conditions and sends a signal to the main contactor to stop the hoist's operation when stability limits are exceeded. When this situation occurs, the main contactor shuts out the power to protect the entire system from inflow of incompatible voltage levels and the inverter unit forces the motor system to stop the cage slowly to protect workers and loads in the cage from the shock of a sudden stop. External panels are installed to protect the main components of the proposed system from comparatively severe external environmental conditions such as temperature, rainwater, and relative humidity. The external panels have a small fan and an electric heater to maintain temperatures of less than 50 °C and to sustain relative humidity of less than 95%. The panels also have a waterproofing function. Those functions were set to prevent malfunction and to maximize their life. 3.2. Development of the prototype Construction hoists are of several types classified by speed and capacity. In this study, the prototype green construction hoist has a 2ton allowable load and a speed of 70 m/min because this is one of the typical types used in building construction. According to a survey result from 444 hoists in 293 construction projects from 2005 to 2010 in Korea, this hoist type accounted for about 37% of all hoists, which
was the highest proportion. The main parameters of the green hoist are presented in Table 1. In the clutch-type motor system, the motor capacity was determined as 36 kW according to the calculation of motor capacity introduced by Ryff [21] and consultations with experts. The inverter unit and the power regenerator were designed to match the capacity of the motor system. A minimum inverter capacity of 108 kW is required because the three motors are operated during downward travel. Therefore, the capacity of the inverter and power regenerator was set as 110 kW. In developing the prototype, we performed mock-up tests through a test tower of 50 m height. Several clutch locations on the submotor were examined to improve the efficiency of energy regeneration. Fig. 2 shows the chosen location of the clutch and the clutch-type motor system after the tests. The structural safety was also checked. Compared with general hoists, the hoist with the proposed ERS has additional weight of about 232 kg for the submotor and power regenerator. However, the braking resistor used in the general hoist is replaced by the power regenerator. Thus, the total additional weight of the proposed system becomes about 150 kg. Our tests showed that the hoist had sufficient capacity to ensure structural safety. 4. Field application 4.1. Description of the field application A field application was performed to verify the energy-saving performance of the prototype and to examine its economic feasibility. We selected a building construction site using construction hoists with 2-ton allowable load and 70 m/min speed. The prototype was applied in a 38-story apartment building that had the total floor area Table 2 Results from data-gathering system during total duration. Parameters
Output
Total application period (days) Total operating time (h) Average daily service time (h) Average daily operating time (h) Total input electricity energy (kWh) Total output electricity energy (kWh)
281 660.4 10.2 2.4 37,023.0 20,572.4
70
M. Lee et al. / Automation in Construction 45 (2014) 66–71
Fig. 4. Electricity consumption and recovery, and energy-saving ratio.
of 17,827 m2. Twin hoists, one for transport of workers and one for materials, were attached to the building. The prototype green hoist replaced the hoist used for worker transport. The total period of the application was 281 days excluding holidays (from June 12, 2012 to April 30, 2013). Fig. 3 shows the main components of the ERS installed on the upper structure of the cage. To measure the energy-saving ratio accurately, a data-gathering system was installed in the electric part of the main components consisting of two electric meters and an operation recorder. One meter was connected to the input power line to check the electricity use, and the other was connected to the output power line to check the electricity recovery, which was transmitted back to the power line of the construction site. 4.2. Results and discussion The results acquired from the data-gathering system during the experiment's duration are shown in Table 2. The total operating time except for the queuing time, including loading and unloading time, was about 660 h over 281 days. The total amount of electricity consumed was 37,023.0 kWh and the amount recovered was 20,572.4 kWh. Thus, the energy-saving ratio of the prototype during total duration was calculated as 55.5% from Eq. (1). As shown in Fig. 4, the energysaving ratio showed a monthly minimum of 54.1% and a maximum of 58.0%. The results show that the prototype has stable performance for energy regeneration. Fig. 5 presents the daily average electricity recovery based on the hoist's operating duration in the corresponding month. As shown in Fig. 5, the daily electricity recovery generally increases until January 2013, then decreases after February 2013. According to the survey on the site, the traveling distance of the hoist was continuously extended by construction progress until January 2013. Thus, the hoist was more likely to travel longer distances when it moved downwards. From February 2013, however, manpower on the test site gradually decreased
because most of the work had been completed. These results indicate that the amount of electricity recovered by the hoist may be affected by the loads and the continuous traveling distance; this was identified in previous studies [22,23]. To ensure the economic feasibility of the proposed system, we analyzed the break-even point (BEP) of the system. BEP was calculated based on the additional costs for applying the ERS and the reduced cost from the energy savings of the prototype. In this study, we assumed that the general contractors purchased the customized ERS with the hoist because they are the main beneficiary of energy savings. The additional costs of the customized ERS over the original hoist configuration were estimated at US$5100, including the cost of the clutch housing ($1200), clutch ($800), regenerator ($1800), and external panel ($1300). The reduction in costs from the energy savings was calculated from the daily energy recovery of the prototype. The average amount of energy recovery per day was 73.2 kWh and the minimum and maximum were 67.0 kWh and 79.6 kWh. We assumed that the hoist would be operated for 25 days per month based on the minimum value during the field application. Then, the reduced costs per month were estimated based on average power rate of OECD unit cost of electric power, US$0.123 per kilowatt-hour. The results showed that BEP would be reached between a minimum of 18 and a maximum of 25 months, as shown in Fig. 6. We found that the proposed system had satisfactory cost-effectiveness. These results show that the system proposed in this study can be employed as a practical means to save energy in construction sites. However, through the case study, we found several aspects that should be improved. The power-monitoring system should first be redesigned to avoid unnecessary stopping of the hoists. During the field application, the hoist stopped frequently, which led to decreased energy recovery and inconvenience to workers. These situations often occur when an unstable power supply is shut off because of power cable problems such as momentary disconnections, shorts, and phase imbalances. However, there were many negligible situations such as momentary power failures. We found that the hoists could be operated stably by using
Fig. 5. Daily average electricity recovery.
M. Lee et al. / Automation in Construction 45 (2014) 66–71
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Fig. 6. BEP considering the range of energy saving performance.
the internal electric energy in the inverter system during the first minute of an outage. Therefore, the power-monitoring system should be redesigned to send a signal to the main contactor to stop the hoist only when the unstable power conditions continued for more than one minute. Furthermore, additional experiments are required to reduce the total amount of energy consumption. In this study, the prototype's self-weight was increased by about 150 kg with ERS compared with the weight of a conventional hoist. Although this may affect both the amount of energy consumption and recovery, a lower weight can reduce total energy consumption. Therefore, we need to find ways to reduce total self-weight, such as adopting a submotor with lower capacity and sourcing ERS components with lower weight, while sustaining their energy saving performance.
5. Conclusions We have proposed a green construction hoist using an ERS to reduce energy consumption during construction. To verify its energy-saving performance and feasibility, it was applied to an actual building construction project. The proposed system showed stable performance for energy recovery as well as a high level of satisfaction with its cost effectiveness. Moreover, its energy-saving performance was enhanced as the building height and scale increased. In this study, the system was developed targeted at only one type of hoist with a 2-ton allowable load and a speed of 70 m/min. Although it is one of the most common hoist types, experiments targeting other hoist types with higher allowable loads and speeds are necessary given the trend toward construction of high-rise buildings. For commercialization, it will be necessary to perform additional experiments with hoists used for lifting materials. In addition, further research is required to assess green hoist operation strategies, such as analysis of the optimal stopping position during waiting to improve energy-saving performance.
Acknowledgments This work was supported by a grant (No. 00043975-1) from the Business for Cooperative R&D between Industry, Academy, and Research Institute funded by the Korea Small and Medium Business Administration in 2012.
References [1] S. Hwang, Planning temporary hoists for building construction, Construction Research Congress, ASCE, Seattle, WA, 2009, pp. 1300–1307. [2] Alimak Hek, Alimak Hek Rack and Pinion construction and industrial elevators, available online at http://alimakhek.com/Products (accessed October 8, 2013). [3] J.H. Bae, J.Y. Soh, J. Lee, C.H. Han, A study on efficient management by analysis of highspeed lift A/S case, Conference Proceedings of KICEM, Daegu, 2012, pp. 428–430. [4] C.Y. Cho, S. Kwon, T.H. Shin, S. Chin, Y.S. Kim, A development of next generation intelligent construction liftcar toolkit for vertical material movement management, Automation in Construction 20 (1) (2011) 14–27. [5] D. Wang, C. Guan, S. Pan, M. Zhang, X. Lin, Performance analysis of hydraulic excavator powertrain hybridization, Automation in Construction 18 (3) (2009) 249–257. [6] T. Lin, Q. Wang, B. Hu, W. Gong, Research on the energy regeneration system for hybrid hydraulic excavators, Automation in Construction 19 (8) (2010) 1016–1026. [7] M.J. Skibniewski, L.C. Chao, Evaluation of advanced construction technology with AHP method, Journal of Construction Engineering and Management 118 (3) (1992) 577–593. [8] W.J. Lee, R. Kenarangui, Energy management for motors, systems, and electrical equipment, IEEE Transactions on Industry Applications 38 (2) (2002) 602–607. [9] K.E. Rydberg, Energy efficient hydraulic hybrid drives, The 11th Scandinavian International Conference on Fluid Power, SICFP'09, 2009, Linköping, Sweden, 2009. [10] M. Ochiai, S. Ryu, Hybrid in construction machinery, Proceedings of the 7th JFPS International Symposium on Fluid Power, TOYAMA, 2008. [11] T.H. Ho, K.K. Ahn, Design and control of a closed-loop hydraulic energy-regenerative system, Automation in Construction 22 (2012) 444–458. [12] Y. Bian, L. Zhu, X. Xu, Regenerative brake system in electric hoist with ultracapacitor, International Conference on Electrical and Control Engineering, Wuhan, 2010, pp. 4571–4575. [13] S. Hui, J. Junqing, Research on the system configuration and energy control strategy for parallel hydraulic hybrid loader, Automation in Construction 19 (2) (2010) 213–220. [14] T. Wang, Q. Wang, T. Lin, Improvement of boom control performance for hybrid hydraulic excavator with potential energy recovery, Automation in Construction 30 (2013) 161–169. [15] S. Tominaga, I. Suga, H. Araki, H. Ikejima, M. Kusuma, K. Kobayashi, Development of energy-saving elevator using regenerated power storage system, PCC-Osaka, 2, 2002, pp. 890–895. [16] H. Yang, W. Sun, B. Xu, New investigation in energy regeneration of hydraulic elevators, IEEE/ASME Transactions on Mechatronics 12 (5) (2007) 519–526. [17] T.A. Minav, A. Virtanen, L. Laurila, J. Pyrhönen, Storage of energy recovered from an industrial forklift, Automation in Construction 22 (2012) 506–515. [18] T. Lin, Q. Wang, B. Hu, W. Gong, Development of hybrid powered hydraulic construction machinery, Automation in Construction 19 (1) (2010) 11–19. [19] Gaius Co., Ltd., Power regenerative unit, available online at http://www.gaius.co.kr/ mproduct/product_cont.asp?p_idx=877 (accessed October 11, 2013). [20] Seoho Electric Co., Ltd., Energy saving and conversion: power regenerative unit, available from http://www.seoho.co.kr accessed October 15, 2013. [21] P.F. Ryff, Electric machinery, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1994 [22] R.D. Peters, P. Mehta, Green lift control strategies, International Journal of Elevator Engineers 2 (1998). [23] T.A. Minav, L.I.E. Laurila, J.J. Pyrhönen, Analysis of electro-hydraulic lifting system's energy efficiency with direct electric drive pump control, Automation in Construction 30 (2013) 144–150.
Contents lists available at ScienceDirect
Automation in Construction journal homepage: www.elsevier.com/locate/autcon
Green construction hoist with customized energy regeneration system Myungdo Lee a,1, Taehoon Kim b,⁎, Hong-Ku Jung c,2, Ung-Kyun Lee d,3, Hunhee Cho e,1, Kyung-In Kang f,1 a
School of Civil, Environmental and Architectural Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea Research Institute of Engineering and Technology, College of Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea c Liftec Corp., 248-8, Siheung-Dong, Sujeong-Gu, Seongnam-Si, Gyeonggi-Do 461-370, Republic of Korea d School of Architecture, Kwandong University, 522 Naegok-Dong, Gangneung-Si, Gangwon-Do 210-701, Republic of Korea e School of Civil, Environmental and Architectural Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea f School of Civil, Environmental and Architectural Engineering, Korea University, Anam-Dong, Seongbuk-gu, Seoul 136-713, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 18 November 2013 Revised 15 April 2014 Accepted 2 May 2014 Available online xxxx Keywords: Construction hoist Green technology Energy regeneration system Regenerative braking
a b s t r a c t Construction hoists are large energy consumers in building construction sites. The application of green technology to hoists can be a useful means to reduce energy consumption during construction. We propose a green construction hoist that uses an energy regeneration system (ERS) to reduce its operating energy requirements. The energy regeneration system was customized to improve its energy-saving efficiency and to cope with exposed weather conditions as well as power supply problems. The prototype was applied to an actual building construction project to verify its operating performance, which showed a cost-effective, stable performance for an energy recovery of 55.5% on average. It is expected that the proposed system can be employed as a practical means to save energy usage in construction sites. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Construction hoists are essential equipment for vertical transportation of construction resources, such as workers and materials [1]. There have been many studies and developments to improve the performance or productivity of construction hoists by increasing their capacity, size, speed, and reliability, or by adapting intelligent control with group control to solve site logistic problems at large construction sites, fault diagnostics, and so on [2–4]. However, there have been few efforts to reduce the energy consumption of the hoist itself in response to the era of the low energy society. Construction hoists have large energy consumption. The need for high-performance construction hoists with low energy consumption is increasing because of the trend to high-rise buildings and the global energy crisis. Therefore, it is necessary to develop green technology to reduce the operating energy of hoists in construction sites. Energy regeneration systems (ERSs), which are a green technology, have been widely used in various types of machinery to improve their energy efficiency [5,6]. ERSs can recover electrical energy by regenerative
⁎ Corresponding author. Tel.: +82 2 921 5920; fax: +82 2 923 4229. E-mail addresses: [email protected] (M. Lee), [email protected] (T. Kim), [email protected] (H.-K. Jung), [email protected] (U.-K. Lee), [email protected] (H. Cho), [email protected] (K.-I. Kang). 1 Tel.: +82 2 921 5920; fax: +82 2 923 4229. 2 Tel.: +82 31 322 3000; fax: +82 31 322 3010. 3 Tel.: +82 33 649 7548; fax: +82 33 645 8120.
http://dx.doi.org/10.1016/j.autcon.2014.05.001 0926-5805/© 2014 Elsevier B.V. All rights reserved.
braking to decelerate or stop movement. Therefore, the downward half of the hoist's moving path can offer energy savings in each operation cycle in which the ERS is applied. Moreover, an ERS can be applied to construction hoists without significant alteration of the hoist's original configuration. Thus, an ERS could be a useful alternative to reduce the operating energy of construction hoists. To apply an ERS for construction hoists, its design should consider their unique application conditions and environmental obstacles. The construction industry has a natural reluctance to the application of new technologies because of their inherent risks [7]. To overcome these perceptions, an ERS for hoists needs to establish simple structures with minimized modifications and also improve its energy-saving performance. In addition, construction hoists are exposed to the weather and frequently face unexpected power supply because they are installed on the external faces of buildings. The aim of this study is to propose a green construction hoist by adopting a customized ERS to reduce energy consumption. In this study, the ERS applied to construction hoists contributed to decreased total energy expenditure of the construction site by transmitting the generated power back to the power distribution network of the site. To enhance the efficiency of energy recovery and deal with the possible problems resulting from their adverse working conditions, the ERS is modified for construction sites after full consideration of the various properties of construction hoists. Then, a prototype with the customized ERS is developed based on the most commonly used type of hoist. Through a field application, the energy-saving performance and economic feasibility are analyzed.
M. Lee et al. / Automation in Construction 45 (2014) 66–71
2. ERS for construction hoists 2.1. Overview of ERS An ERS recovers electrical energy by regenerative braking [8,9]. Regenerative braking can be used with hybrid or hybrid hydraulic drives powered by an electric motor. During the braking, the motor generates electrical energy to decelerate or stop movement. In conventional
67
braking systems, the energy is dissipated as heat in a braking resistor because it has a random frequency that can cause trouble in the power system. An ERS recovers this generated energy as electricity that can be either used immediately or stored. The basic components of an ERS are the electric motor, an inverter unit, and a power regenerator. The electrical energy generated during braking flows into the inverter unit. The inverter unit consists of a direct current (DC) to alternating current (AC) converter, an electrolytic
Fig. 1. Configuration of the customized ERS.
68
M. Lee et al. / Automation in Construction 45 (2014) 66–71
capacitor, and a rectifier. The inverter unit controls the voltage and frequency of the electrical energy to match the available electric power. The refined electric energy in the inverter unit is DC. The power regenerator converts DC into AC to transmit the electric power back to the power distribution network of the construction site. In construction machinery, ERSs use two main recoverable forms, kinetic energy and potential energy [10]. Regeneration from kinetic energy has often been applied in wheel-type construction machines based on a hydraulic accumulator [11], a battery, possibly in combination with a supercapacitor [12], or a flywheel system [13]. Potential energy regeneration has been implemented in machines used for lifting loads, for instance, excavators [5,6,14], elevators [15,16], and forklifts [17]. When the machines use regenerative brakes, their kinetic or potential energy is converted to electric energy. Both energy forms provide an opportunity to reduce the demand on the original energy source by adopting ERSs [10]. However, the existing ERS applications for construction machines require significant modification of the original configuration and mechanisms [18]. Because the existing machines originally use a combustion engine for their operation, a complex hybrid or hybrid hydraulic mechanism should be established with an electric motor for the ERS. In contrast, ERSs can be applied to construction hoists without significant alteration of the original hoist configuration because generally hoists operate using an electric motor instead of a combustion engine. Thus, an ERS for a hoist can be established with a relatively simple driving mechanism compared with existing hybrid mechanisms. Moreover, energy management on construction sites has become increasingly important, and the power consumption of hoists is predicted to increase significantly with high-rise building trends. Therefore, adoption of ERSs for construction hoists can be a useful approach to saving the operating energy if the cost effectiveness can be made satisfactory.
A customized ERS configuration for construction hoists is presented in Fig. 1. As mentioned above, the general ERS consists of the electric motor, an inverter unit, and a power regenerator. In this study, the proposed ERS was customized to reflect their unique working conditions. The newly added or modified components of the proposed system are: (1) clutch-type motor system; (2) inverter controller; (3) subcontactor; (4) power-monitoring system; and (5) main contactor. The clutch-type motor system is specifically designed to improve energy recovery as well as ensure simple structures. The energy-saving ratio Γs was calculated from Eq. (1):
2.2. Design considerations
Γs ¼ X
Table 1 Main parameters of the targeted hoist type. Parameter
Value
Allowable load capacity (Kg) Cage weight (Kg) Drive unit & electric parts weight (Kg) Cage internal width × length × height (m) Max. speed (m/min) Max. lifting height (m)
2000 2100 1200 1.5 × 4.0 × 2.6 70 400
Finally, devices for protecting the main components of the ERS from external environmental conditions should be considered. Construction hoists are installed on the external faces of buildings and the hoist operating systems are installed on the top of the cage. Thus, the components of the ERS can be affected by environment conditions such as temperature, rainwater, and relative humidity. 3. Prototype of the green construction hoist 3.1. Configuration of customized ERS
X
Before designing an ERS for construction hoists, the properties of hoists and possible problems were firstly examined through site surveys and consultations with experts from hoist manufacturers and suppliers and construction managers. From these examinations, the design considerations for green construction hoists were derived as follows. First, to counter the resistance to the application of new systems on construction sites, an ERS for a hoist should minimize the modification of the existing system while ensuring improved performance. When compared with other industries, the employment of new technologies in the construction industry has been relatively slow and difficult because of contractors' reluctance in increasing the initial investment and performance uncertainties [7]. Even though energy management has become increasingly important in construction sites, construction managers may have antipathy toward applying a new system if it requires considerable changes from the traditional system in terms of configurations and costs. Thus, to ensure the feasibility of the ERS for hoists, it is necessary to minimize the modification of the original hoist configurations and cost increases while enhancing energy recovery performance. Second, a power-monitoring device is required to protect the ERS from power supply problems. The hoist's power is supplied by power cables connected to the cage. The power cable is affected by winds because it hangs from the cage and is exposed to the weather. If the cable kinks and the tensional force exceed permissible limits because of winds, the cable may cause problems such as momentary disconnections, shorts, and phase unbalances. Such situations often occur while hoists are operating and increase as the building height increases. They can lead to breakdown of the main control system, including the ERS, and can cause safety problems. A power-monitoring device can prevent these problems.
electricity regeneration electricity consumption
¼
Eout ; Ein
ð1Þ
where Eout and Ein are the total output and input energies, respectively, during the total operating time. According to Eq. (1), the energy-saving ratio through the ERS is theoretically estimated at about 33% considering the power losses from each electric drive, including the motor, reducer, inverter, and regenerator [19,20]. A method for improving the energy-saving ratio is to use electric drives with higher efficiency. However, this leads to a considerable increase in the initial investment. Although the application of motors with larger capacity can also increase the total amount of energy recovery, it not only results in simultaneous unnecessary energy consumption, but also requires significant alteration of the structure. Therefore, in this study, the clutch-type motor system is introduced to achieve improved energy recovery with
Fig. 2. Clutch-type motor system on mock-up test.
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Fig. 3. The prototype and main components installed on the hoist cage.
the minimized modification of hoist configurations and cost increases. The motor system has two components: the main motor and submotor. The main motor consists of two general-purpose electric motors and the submotor consists of an electric motor integrated with an electric clutch. When the hoist travels upwards, only the main motor operates and the submotor is excluded from the operating mechanism because the main motor is designed to have sufficient power for hoist's operation; additional operation of the submotor would cause unnecessary energy consumption. On the other hand, when the hoist travels downwards, the submotor is connected to the hoist's operating mechanism by the clutch, so all three motors operate for energy regeneration. As a result, the submotor improves energy recovery through complementing the power losses in the electric drives during energy regeneration. The inverter controller is adopted to check the direction of motion of the hoist and to send a signal to the subcontactor controlling the clutch operation of the submotor. The power-monitoring system aims to cope with unexpected power supply problems such as disconnection, shorts, and phase unbalances. The power-monitoring system checks for unstable power conditions and sends a signal to the main contactor to stop the hoist's operation when stability limits are exceeded. When this situation occurs, the main contactor shuts out the power to protect the entire system from inflow of incompatible voltage levels and the inverter unit forces the motor system to stop the cage slowly to protect workers and loads in the cage from the shock of a sudden stop. External panels are installed to protect the main components of the proposed system from comparatively severe external environmental conditions such as temperature, rainwater, and relative humidity. The external panels have a small fan and an electric heater to maintain temperatures of less than 50 °C and to sustain relative humidity of less than 95%. The panels also have a waterproofing function. Those functions were set to prevent malfunction and to maximize their life. 3.2. Development of the prototype Construction hoists are of several types classified by speed and capacity. In this study, the prototype green construction hoist has a 2ton allowable load and a speed of 70 m/min because this is one of the typical types used in building construction. According to a survey result from 444 hoists in 293 construction projects from 2005 to 2010 in Korea, this hoist type accounted for about 37% of all hoists, which
was the highest proportion. The main parameters of the green hoist are presented in Table 1. In the clutch-type motor system, the motor capacity was determined as 36 kW according to the calculation of motor capacity introduced by Ryff [21] and consultations with experts. The inverter unit and the power regenerator were designed to match the capacity of the motor system. A minimum inverter capacity of 108 kW is required because the three motors are operated during downward travel. Therefore, the capacity of the inverter and power regenerator was set as 110 kW. In developing the prototype, we performed mock-up tests through a test tower of 50 m height. Several clutch locations on the submotor were examined to improve the efficiency of energy regeneration. Fig. 2 shows the chosen location of the clutch and the clutch-type motor system after the tests. The structural safety was also checked. Compared with general hoists, the hoist with the proposed ERS has additional weight of about 232 kg for the submotor and power regenerator. However, the braking resistor used in the general hoist is replaced by the power regenerator. Thus, the total additional weight of the proposed system becomes about 150 kg. Our tests showed that the hoist had sufficient capacity to ensure structural safety. 4. Field application 4.1. Description of the field application A field application was performed to verify the energy-saving performance of the prototype and to examine its economic feasibility. We selected a building construction site using construction hoists with 2-ton allowable load and 70 m/min speed. The prototype was applied in a 38-story apartment building that had the total floor area Table 2 Results from data-gathering system during total duration. Parameters
Output
Total application period (days) Total operating time (h) Average daily service time (h) Average daily operating time (h) Total input electricity energy (kWh) Total output electricity energy (kWh)
281 660.4 10.2 2.4 37,023.0 20,572.4
70
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Fig. 4. Electricity consumption and recovery, and energy-saving ratio.
of 17,827 m2. Twin hoists, one for transport of workers and one for materials, were attached to the building. The prototype green hoist replaced the hoist used for worker transport. The total period of the application was 281 days excluding holidays (from June 12, 2012 to April 30, 2013). Fig. 3 shows the main components of the ERS installed on the upper structure of the cage. To measure the energy-saving ratio accurately, a data-gathering system was installed in the electric part of the main components consisting of two electric meters and an operation recorder. One meter was connected to the input power line to check the electricity use, and the other was connected to the output power line to check the electricity recovery, which was transmitted back to the power line of the construction site. 4.2. Results and discussion The results acquired from the data-gathering system during the experiment's duration are shown in Table 2. The total operating time except for the queuing time, including loading and unloading time, was about 660 h over 281 days. The total amount of electricity consumed was 37,023.0 kWh and the amount recovered was 20,572.4 kWh. Thus, the energy-saving ratio of the prototype during total duration was calculated as 55.5% from Eq. (1). As shown in Fig. 4, the energysaving ratio showed a monthly minimum of 54.1% and a maximum of 58.0%. The results show that the prototype has stable performance for energy regeneration. Fig. 5 presents the daily average electricity recovery based on the hoist's operating duration in the corresponding month. As shown in Fig. 5, the daily electricity recovery generally increases until January 2013, then decreases after February 2013. According to the survey on the site, the traveling distance of the hoist was continuously extended by construction progress until January 2013. Thus, the hoist was more likely to travel longer distances when it moved downwards. From February 2013, however, manpower on the test site gradually decreased
because most of the work had been completed. These results indicate that the amount of electricity recovered by the hoist may be affected by the loads and the continuous traveling distance; this was identified in previous studies [22,23]. To ensure the economic feasibility of the proposed system, we analyzed the break-even point (BEP) of the system. BEP was calculated based on the additional costs for applying the ERS and the reduced cost from the energy savings of the prototype. In this study, we assumed that the general contractors purchased the customized ERS with the hoist because they are the main beneficiary of energy savings. The additional costs of the customized ERS over the original hoist configuration were estimated at US$5100, including the cost of the clutch housing ($1200), clutch ($800), regenerator ($1800), and external panel ($1300). The reduction in costs from the energy savings was calculated from the daily energy recovery of the prototype. The average amount of energy recovery per day was 73.2 kWh and the minimum and maximum were 67.0 kWh and 79.6 kWh. We assumed that the hoist would be operated for 25 days per month based on the minimum value during the field application. Then, the reduced costs per month were estimated based on average power rate of OECD unit cost of electric power, US$0.123 per kilowatt-hour. The results showed that BEP would be reached between a minimum of 18 and a maximum of 25 months, as shown in Fig. 6. We found that the proposed system had satisfactory cost-effectiveness. These results show that the system proposed in this study can be employed as a practical means to save energy in construction sites. However, through the case study, we found several aspects that should be improved. The power-monitoring system should first be redesigned to avoid unnecessary stopping of the hoists. During the field application, the hoist stopped frequently, which led to decreased energy recovery and inconvenience to workers. These situations often occur when an unstable power supply is shut off because of power cable problems such as momentary disconnections, shorts, and phase imbalances. However, there were many negligible situations such as momentary power failures. We found that the hoists could be operated stably by using
Fig. 5. Daily average electricity recovery.
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Fig. 6. BEP considering the range of energy saving performance.
the internal electric energy in the inverter system during the first minute of an outage. Therefore, the power-monitoring system should be redesigned to send a signal to the main contactor to stop the hoist only when the unstable power conditions continued for more than one minute. Furthermore, additional experiments are required to reduce the total amount of energy consumption. In this study, the prototype's self-weight was increased by about 150 kg with ERS compared with the weight of a conventional hoist. Although this may affect both the amount of energy consumption and recovery, a lower weight can reduce total energy consumption. Therefore, we need to find ways to reduce total self-weight, such as adopting a submotor with lower capacity and sourcing ERS components with lower weight, while sustaining their energy saving performance.
5. Conclusions We have proposed a green construction hoist using an ERS to reduce energy consumption during construction. To verify its energy-saving performance and feasibility, it was applied to an actual building construction project. The proposed system showed stable performance for energy recovery as well as a high level of satisfaction with its cost effectiveness. Moreover, its energy-saving performance was enhanced as the building height and scale increased. In this study, the system was developed targeted at only one type of hoist with a 2-ton allowable load and a speed of 70 m/min. Although it is one of the most common hoist types, experiments targeting other hoist types with higher allowable loads and speeds are necessary given the trend toward construction of high-rise buildings. For commercialization, it will be necessary to perform additional experiments with hoists used for lifting materials. In addition, further research is required to assess green hoist operation strategies, such as analysis of the optimal stopping position during waiting to improve energy-saving performance.
Acknowledgments This work was supported by a grant (No. 00043975-1) from the Business for Cooperative R&D between Industry, Academy, and Research Institute funded by the Korea Small and Medium Business Administration in 2012.
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