Energy 155 (2018) 242e251
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Potential energy directly conversion and utilization methods used for heavy duty lifting machinery Yunxiao Hao, Long Quan*, Hang Cheng, Lianpeng Xia, Lei Ge, Bin Zhao Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan, 030024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 August 2017 Received in revised form 26 April 2018 Accepted 2 May 2018 Available online 3 May 2018
Hydraulic driven heavy duty lifting machinery is widely applied in mobile machinery. In traditional systems, the gravitational potential energy (GPE) is usually dissipated as heat through the throttling effect of the control valve, resulting in huge energy waste. To address the above issue, this paper proposes two direct GPE recovery (GPER) solutions based on hydraulic-pneumatic energy storage (HPES) principle. For system driven by double hydraulic cylinders, an independent HPES hydraulic cylinder is added to the system. For system driven by single hydraulic cylinder, the HPES is integrated into the original single rod hydraulic cylinder, functioning as a storage chamber. In both schemes, the HPES hydraulic cylinder or chamber is directly connected to an accumulator. With the self-weight of the lifting machinery is balanced by the precharge pressure of the accumulator, the GPE and hydraulic energy can be directly converted mutually. Both schemes have been analyzed in detail. Experimental prototypes have been constructed based on one 76-ton and one 6-ton hydraulic excavator. Experimental results indicate that as compared to the original system, 49.1% GPE recovery rate and 26.2% energy consumption reduction per operation cycle can be achieved for the 76-ton excavator. For the 6-ton excavator, the GPE recovery rate reaches 70.9% and 44.4% energy consumption reduction rate can be achieved for each operation cycle. Besides hydraulic excavator, the proposed solutions can also bring significant energy saving for all other lifting machinery. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Hydraulic excavator Hydraulic-pneumatic energy storage Three-chamber cylinder Gravitational potential energy recovery
1. Introduction Hydraulic excavator, wheel loader, various forklift and other mobile machinery use heavy duty lifting machinery controlled by hydraulic cylinder to drive the load. During working process, the lifting machinery lifts and lowers with high frequency. A large of the accumulated GPE is converted into heat and dissipated through the throttling effect of the control valve. That not only causes huge energy loss but also increases the hydraulic oil temperature rapidly, the cooling device should be added to lower the temperature which further aggravates the energy loss. Taking hydraulic excavator as an example, the wasted GPE accounts for 15% of the main hydraulic pump output energy during one standard work cycle [1]. Therefore, recovery and reutilization of this part of the wasted energy will significantly improve the energy efficiency of mobile machinery. There are hydraulic and electric two methods to recover and
* Corresponding author. E-mail address:
[email protected] (L. Quan). https://doi.org/10.1016/j.energy.2018.05.015 0360-5442/© 2018 Elsevier Ltd. All rights reserved.
reutilize the GPE of the heavy load lifting machinery. Regarding electric recovery method, it is mainly applied in oil-electric hybrid [2,3] or pure electric driving mobile machinery [4,5]. Its fundamental principle is that the hydraulic oil of the rodless chamber of the hydraulic cylinder is discharged to drive a hydraulic motor during lowering process, then the hydraulic motor drives an electric generator to convert the GPE into electric energy stored in supercapacitor or storage battery. To make up the shortcomings of slow torque building speed of the electric motor/generator, Ahn et al. developed a scheme that a bypass throttle valve is set parallel to the hydraulic motor to improve the lifting machinery control performance [6]. In Lin's study, a proportional throttle valve is in parallel and proportional directional valve is in series with the hydraulic motor to control the boom lowering velocity and improve the operation stability. The GPE recovery efficiency is 39% [7]. To eliminate the bypass throttling loss, Wang et al. studied a scheme that a throttle valve and a hydraulic motor are set in series. The valve pressure difference control strategy is used to improve the boom operation performance, and the GPE recovery efficiency is enhanced to 40%e50% [8]. Lin et al. used a hydraulic accumulator to
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separate the energy recovery and the conversion process. By prolonging the conversion time, the installed power of the hydraulic motor and electric motor/generator can be decreased by 60% and 39% of the potential energy is stored [9]. The closed pump controlled system can eliminate the throttling loss and improve the energy efficiency of the electric recovery system [10]. Yoon et al. studied the characteristics of a pump controlled system that a servomotor drives a pump to control the hydraulic excavator boom and the electrical battery is used to recover and store GPE. The energy consumption of the boom system is reduced by 47.8% compared with the load sensing system [11]. Zhang et al. studied a speed variable pump controlled hydraulic excavator scheme that each hydraulic cylinder is controlled by two pumps driven by a variable speed electric motor and GPE is stored by supercapacitor [12]. In the hydraulic recover system, to control the lifting machinery velocity, the simple way is to use a throttle valve to introduce the high-pressure oil of the hydraulic cylinder into the hydraulic accumulator to recover GPE. For energy regeneration, the stored energy can be used to drive the cooling system and other auxiliary equipment [13], or be used to drive the hydraulic pump by discharging the high-pressure oil of the accumulator into the suction port of the main hydraulic pump [14]. However, there is large throttling loss during the energy recovery process, and the regeneration of stored energy also causes secondary throttling loss. The hydraulic transformer [15] or closed pump controlled hydraulic cylinder system [16] can be used to solve the above issues. Zhang studied the scheme that hydraulic transformer is used to recover the GPE of a hydraulic excavator. The test results show that the GPE recovery efficiency reaches to 50% [17]. Daniel and Chen respectively studied the schemes that the hydraulic excavator boom is driven by two variable displacement pumps or two variable speed pumps, and the accumulator is used to recover GPE. The energy consumption of the boom system is reduced by 39% in Daniel's study [18] and 33.1% in Chen’ study [19]. Kim studied a GPER scheme that the high-pressure oil in the hydraulic cylinder is introduced into a variable displacement hydraulic motor to assist the engine to drive the main hydraulic pump when the hydraulic excavator boom lowers. The fuel consumption of hydraulic excavator can be reduced by 7% in the 90 truck loading tasks [20]. It is known from the above analysis that considering the regeneration of the stored energy, there are many energy conversion links and long energy transfer chain in electric recovery system and the overall energy efficiency is low. Although the hydraulic transformer has good GPER effect, no available commercial components can be used as yet, and the system with hydraulic transformer is complicated and costly [21]. In terms of the closed pump controlled system, the asymmetric area of the hydraulic cylinder should be compensated, and the system is complicated. Moreover, each hydraulic pump needs to be configured according to the peak flow rate of the driven hydraulic cylinder, which increases the installed power and cost of the system. To overcome the shortage, Liang proposed the principle of independent HPES hydraulic cylinder balancing the weight of lifting machinery in his doctoral thesis. The working performance and the energy saving effect of lifting machinery driven by two HPES hydraulic cylinders and one working hydraulic cylinder were analyzed and tested by him [22]. After that, the Germany Liebherr GmbH [23,24], the China Changlin Company [25] and Sunward Equipment Group [26] applied for the patents that the method is used to hydraulic excavator. Quan et al. proposed the scheme that the lifting machinery is driven by three-chamber cylinder constructed by integrating the HPES hydraulic cylinder into the original driving cylinder, and applied for and be authorized the patents that the three-chamber cylinder is used in the hydraulic excavator,
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wheel loader and other mobile machinery [27,28]. Zhao et al. also simulated the characteristics of the scheme that three-chamber cylinders are applied to hydraulic excavator boom and whole machine [29]. The HPES method has less energy conversion links and shorter energy transfer chain. However, so far only Liang has done fundamental research about the scheme with HPES hydraulic cylinder in his doctoral dissertation. The study of the three-chamber cylinder applied to the lifting machinery is only limited to simulation research. There are no research reports about the energy saving effect of the above two schemes applied to the real machine in the actual operation. To provide guidance and evidence for the further popularization and application of the HPES principle in heavy duty lifting machinery, the actual energy saving effect, the corresponding application situations and energy efficiency of the two GPER schemes based on HPES principle are all should be analyzed and compared through the experiment in the real machines. Therefore, in this paper, the prototypes of the boom driven by the HPES hydraulic cylinder of a 76-ton large hydraulic excavator and the boom driven by the three-chamber cylinder of a 6-ton small hydraulic excavator [27] are built, the characteristics of the two schemes are tested and researched. The remainder of this paper is organized as follows. In Section 2, the working principles are described and analyzed. In Section 3, the energy efficiency calculation model is presented and analyzed. The energy efficiency characteristics of the hydraulic excavator boom driven by HPES hydraulic cylinder are studied in section 4. In section 5, the energy efficiency characteristics of the hydraulic excavator boom driven by three-chamber cylinder are studied. In section 6, the energy saving effect of the proposed systems is compared and discussed. Finally, the conclusions are presented in Section 7. 2. Working principles 2.1. GPER system with HPES hydraulic cylinder The working principle of the GPER system with HPES hydraulic cylinder is illustrated in Fig. 1. The negative flow system is used as the hydraulic control circuit. Compared with the traditional driving system, one hydraulic cylinder used as an HPES hydraulic cylinder is set parallel to the original boom working hydraulic cylinders in the GPER system. The energy storage chamber C of the HPES hydraulic cylinder is connected to an accumulator to balance the weight of the working device by setting appropriate pressure of the accumulator. The rodless chamber A and rod chamber B of the working hydraulic cylinders are connected with the valve to control the boom. When the boom lowers, the high-pressure oil in the energy storage chamber is charged into the accumulator under the effect of the working device gravity. The GPE can be directly converted into hydraulic energy and stored in the accumulator. When the boom lifts, the accumulator discharges high-pressure oil into the energy storage chamber, the HPES hydraulic cylinder extends along with the working hydraulic cylinders to drive the boom. During this process, the stored hydraulic energy is directly converted into the GPE of the working device. 2.2. GPER system with three-chamber cylinder It is known that all the existing HPES schemes are added HPES hydraulic cylinder to balance the weight of the working device. These schemes are not suitable for the small lifting machinery driven by single hydraulic cylinder because of the restriction of the compact installation space. Therefore, a GPER system with threechamber cylinder is presented to address this issue. The working principle of the GPER system with three-chamber cylinder is shown
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A-rodless chamber, B-rod chamber, C-energy storage chamber Fig. 1. Working principle of GPER system with HPES hydraulic cylinder.
in Fig. 2. As shown in Fig. 2, the external shape of the three-chamber cylinder is the same as that of the single rod hydraulic cylinder, which makes the three-chamber cylinder can directly replace the original working hydraulic cylinder in the small hydraulic excavator. The three-chamber cylinder is formed by integrating the
HPES hydraulic cylinder in the single rod hydraulic cylinder. The energy storage chamber C is connected to an accumulator with appropriate pressure to balance the working device weight. The rodless chamber A and rod chamber B are connected with the control valves to drive the boom. The GPE recovery and reutilization principle of the GPER system with three-chamber cylinder is the
A-rodless chamber, B- rod chamber, C-energy storage chamber Fig. 2. Working principle of the GPER system with three-chamber cylinder.
Y. Hao et al. / Energy 155 (2018) 242e251
same as that of the GPER system with HPES hydraulic cylinder. The separate meter-in and meter-out system is adopted as the hydraulic control circuit. An electric joystick and a controller are used to control the electric proportional valves and an electric proportional variable displacement pump. According to the control signal of the electric joystick, the controller calculates the real-time required flow rate of the system to control the displacement of the pump. The pump outputs the required flow to meet the flow rate and pressure requirements of the system.
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pressures of the rodless chamber, rod chamber, and energy storage chamber of the hydraulic cylinders in the system with GPER device; m is the working device mass, B is the damping coefficient, x is boom displacement, FL is load force. In the system without GPER device, the piston cylinder A is connected to the pump, and the piston extends to lift the boom. Meanwhile, the piston cylinder B is connected to the tank, and the piston retracts. The energy consumption of the system during the boom lifting process can be calculated as follows:
Z
3. Energy efficiency calculation model
Eup ¼
To analyze the energy characteristics of the proposed systems, the rodless chamber A and rod chamber B in the system without GPER device are equivalent to two piston cylinders A and B. The hydraulic cylinders in the system with GPER device are equivalent to three piston cylinders A, B, and C which respectively represent the rodless chamber A, rod chamber B and energy storage chamber C. Fig. 3 shows the operation principle of the systems with and without GPER device. As shown in Fig. 3, the force equilibrium equations of the systems with or without GPER device are expressed respectively as follows:
pA AA pB AB ¼ mx€ þ Bx_ þ FL
(1)
PA AA PB AB þ PC AC ¼ mx€ þ Bx_ þ FL
(2)
where AA, pA, pB, and AB are the areas and pressures of the rodless chamber and rod chamber of hydraulic cylinders in the system without GPER device; A0 A, p0 A, p0 B, A0 B, p0 C, and A0 C are the areas and
pp qA dt
(3)
where pp and qA are the pump pressure and rodless chamber flow rate in the system without GPER device. In the system with GPER device, the accumulator discharges high-pressure oil into piston cylinder C; the piston extends along with the piston cylinder A. Neglecting the pressure loss between the accumulator and the piston cylinder C, the pressure of the piston C is the same as that of the accumulator. Because of the short operation cycle time of the boom, the gas state change in the accumulator is an adiabatic process, and the gas state change equation is described as follows:
p0C V n ¼ p0 V0n ¼ Const
(4)
where V is the gas volume of the accumulator at any time, p0 is the initial pressure of the accumulator and V0 is the corresponding gas volume, n is the gas index, n ¼ 1.4. During the boom lifting process, the gas volume of the accumulator is gradually increasing as shown in Fig. 3. The discharged energy of the accumulator is calculated as follows:
Z Edis ¼
p1 V1n AC v dt ðV1 þ vtÞn
(5)
where p1 is the highest pressure of the accumulator and V1 is the corresponding gas volume, v is the velocity of the cylinders. The energy consumption of the system with GPER device is calculated as follows: 0 Eup ¼
Z
p0p q0A dt
(6)
where p0 p and q0 A are the pump pressure and rodless chamber flow rate in the system with GPER device. During the boom lowering, the piston cylinder A in the system without GPER device is connected to the tank; the GPE is wasted through the throttling effect of the control valve. The system energy consumption and the wasted GPE are respectively shown as follows:
Z Edown ¼
pp qB dt
(7)
Z Epe ¼
Fig. 3. Equivalent operation principle of systems with and without GPER device.
ðpA AA pB AB Þvdt
(8)
In the system with GPER device, the GPE is mostly converted into hydraulic energy to be stored in the accumulator. Also, the accumulator stores energy from the piston cylinder B. The charged energy and recovered GPE of the accumulator are respectively calculated as follows:
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Y. Hao et al. / Energy 155 (2018) 242e251
Z Echa ¼
p2 V2n AC v dt ðV2 vtÞn Z
Er ¼ Echa
(9)
p0B q0B dt
(10)
where p2 is the lowest pressure of the accumulator and V2 is the corresponding gas volume. The energy consumption of the system with GPER device is calculated as follows: 0 Edown ¼
Z
p0p q0B dt
(11)
According to the above equations, the energy recovery efficiency of the systems with GPER device is calculated as follows:
hr ¼ Er Epe
(12)
During the boom lifting and lowering one cycle, the energy saving rate of the system with GPER device is calculated as follows:
hc ¼ Eup þ Edown
. 0 0 Eup þ Edown
(13)
4. Characteristics of GPER system with HPES cylinder In the GPER system, the structure parameters of the working hydraulic cylinders and HPES hydraulic cylinder are the same. The piston diameter of the hydraulic cylinders is 190 mm, and the piston rod diameter is 130 mm. 4.1. Parameters matching of accumulator As a key element of the proposed system, the accumulator has significant influences on the system's energy efficiency characteristics. During the boom lifting, the higher the pressure of the accumulator is, the larger the force of the energy storage chamber acting on the boom becomes. Moreover, the rod-less chamber's pressure of the working hydraulic cylinders is lower, and the energy consumption also becomes lower. However, if the initial pressure of the accumulator is too high, the rod chamber's pressure of the working hydraulic cylinders should be increased to drive the boom move down, which increases the system energy consumption. If the
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Lowering
Lifting
Pressure p/MPa
28 21 14 7 0 0
4
8
Time t/s
12
16
20
Fig. 4. Rod-less chamber pressure of the working hydraulic cylinders in the original system.
pressure of the accumulator is too low, the GPE of the working device cannot be fully recovered. Fig. 4 shows the rod-less chamber testing pressure of the boom working hydraulic cylinders in a 76 t hydraulic excavator without HPES hydraulic cylinder. The rod-less chamber pressure of the working hydraulic cylinders is about 13 MPae16 MPa during the lifting process and about 12 MPa during the lowering process. To recover the GPE as fully as possible and also not cause additional energy consumption, the maximum working pressure of the accumulator is selected 21 MPa, and the minimum working pressure of the accumulator is selected 13 MPa. According to 0.25p2
