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A hybrid energy storage system using pump compressed air and micro-hydro turbine
Renewable Energy 65 (2014) 117e122
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
A hybrid energy storage system using pump compressed air and micro-hydro turbine Jun lian Yin a, De zhong Wang a, Yu-Taek Kim b, Young-Ho Lee c, * a
School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Division of Marine Systems Engineering, Korea Maritime University, Busan 606-791, Republic of Korea c Division of Mechanical and Energy System Engineering, KMU, Busan 606-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2013 Accepted 22 July 2013 Available online 17 August 2013
In this paper, a micro-hybrid energy storage system, for a small power grid, which combines the concepts of pump storage plant (PSP) and compressed air energy storage (CAES), is proposed. There are two tanks, one open to the air and one subjected to compressed air, as well as a micro-pump turbine (MPT) in the hybrid system. The basic principle is that the MPT utilizes excess power from the grid to pump the water, which in turn compresses the air, and in this way, the energy is changed into internal energy of air. The energy in the air will be released to drive water passing through the MPT to generate power when the supply of power from the grid is insufficient. To validate the above proposal, such a micro-system was designed considering geometrical and operational conditions. Due to the large head variation for MPT, a variable speed machine [1] was designed by means of an inverse design method. After geometrical modeling and mesh generation for the complete configuration of the MPT, which consists of spiral casing, tandem, runner and draft tube, CFD simulations of typical operating points during pump mode and turbine mode were implemented. Special treatments of boundary conditions induced by the air compression or decompression were applied in the simulation. This energy storage system shows promising potential for application as the results indicated that the performance of the system and MPT was comparable. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Energy storage Micro-pump turbine CFD
1. Introduction Among existing energy storage system (ESS) technologies [1], only two technologiesdCAES (compressed air energy storage) and PHS (pumped hydroelectric storage [2])dare cost effective and successfully applied in industrial engineering. Conventionally, the working principle of CAES, which is shown in Fig. 1, can be illustrated as follows [3,4]: off-peak or excess power is taken from the grid at low cost and used to compress and store air within an underground storage cavern. When needed, this highpressure compressed air is then released, pre-heated and expanded in a gas turbine to produce electricity during peak demand hours. For additional efficiency, the compressed air can be mixed with natural gas, then burned (as is often done in conventional power generation). Thus, greenhouse gas (GHG) emissions from conventional CAES are not zero [5]. The basic principle of
* Corresponding author. Division of Mechanical & Energy System Engineering, Korea Maritime University, Busan 606-791, Republic of Korea. E-mail address: [email protected] (Y.-H. Lee). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.07.039
pumped hydro storage is also well established. Water is pumped to an upper reservoir at times of surplus supply and discharged through a pump turbine at times of high demand. However, a difference in geodetic height, which compromises applications, and high capital costs are limitations of PHS. Compared to CAES, a PHS system is simpler, as the pump turbine works bio-directionally, whereas compressors and gas turbines must be employed in CAES. Moreover, additional fuels are supplied to heat the air for high efficiency utilization. Referring to the two technologies, a hybrid energy storage system, shown in Fig. 2, is proposed to overcome the difficulties of energy storage for places where the geological structure for PHS and CAES is not suitable. It can be applied mainly for small scale energy storage. The point is the use of the pressure vessel to replace the reservoirs in PSP, where the head of water is provided by the compressed air trapped in the vessel. A micro-pump turbine (MPT) is used to achieve the charging/discharging process, as in PSP. Due to the high cost and construction difficulties of large pressure vessels, the hybrid system is more suited to small scale energy systems, for instance, energy storage for wave energy converters [6] or wind energy [7], where the capacity is no more than 100 kW.
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Vw ¼ Vt Vca ¼ 1674:5m3
(4)
The task for the MPT is to deliver a given volume of water with increasing head in the given time. In order to utilize the volume of the pressurized vessel adequately, the water will be released completely during the discharge process. This assumption means that the head variation ration is greater than 16, which is not out of the range of a pump turbine operated under a constant rotation speed. Thus, a variable speed scheme [9] is selected. Based on the similarity laws for variable speed, the relationship between the flow rate and head is:
Qt ¼
nt Q0 n0
(5) 0
Fig. 1. Schematic map of the conventional compressed air energy storage system.
To validate the proposed system, the design rules for the system were developed and then the MPT was designed. Subsequently, CFD simulations were carried out to validate the functions of the MPT. Finally, conclusions were drawn.
Zt B C Qt dt B C C p0 B Vt B C 0 2 1C þ Ht ¼ H0 þ kðQt Q0 Þ þ B C rg B A Zt B C @Vt Qt dt A ¼
nt n0
0
2 H0
(6)
2. System design The hybrid energy storage system includes two parts, the vessels that function as the reservoirs in PSP and the micro-pump turbine. The system was first designed so that the feasibility of the proposed system could be validated. Given the output power P ¼ 20 kW under the maximum head h ¼ 100 m and the working time T ¼ 6 h, the energy storage capacity is:
E ¼ PT ¼ 432MJ
(1)
Assuming the air is compressed by MPT from the ambient pressure p0 ¼ 101,325 Pa at t0 ¼ 298 K to the pressure p1 ¼ 1,075,844 Pa isothermally, the volume of compressed air is:
Vca ¼ E=ðp0 blnðbÞÞ ¼ 167:46m3
(2)
Here, b is the compression factor, b ¼ p1/p0. As identified by Hartmann [8], a huge efficiency difference exists between different adiabatic CAES plant configurations. For simplicity, an isothermal process is assumed in the present study. Consequently, the volume of the whole vessel is:
Vt ¼ Vca b ¼ 1842m3 and the maximum volume of water is:
Fig. 2. Schematic map of the hybrid energy storage system.
1
(3)
in which, H0, Q0, n0, p0 are the head, volume flow rate, rotation speed and pressure of the vessel, respectively, at the initial state; Ht, Qt, nt are the head, volume flow rate and rotation speed, respectively, at arbitrary time; r is the fluid density, A is the basal area and g is the gravity acceleration. The integral of the flow rate is:
Zt Qt dt ¼ Vw
(7)
0
Given the time needed to pump the water into the vessel as 6 h, combing Eqs. (5)e(7), the design specification for the MPT can be determined. Considering the optimal specific speed for pump design, the final design specification is that the MPT will operate from H0 ¼ 7 m, Q0 ¼ 171 m3/h, n0 ¼ 715.5 rpm to Ht ¼ 115 m, Q0 ¼ 693.18 m3/h, n0 ¼ 2900 rpm, i.e. the head variation ratio is 16.4. To validate this, the time variation of the above parameters is illustrated in Fig. 3. The next step is to design an MPT that is capable of operating under the complete system requirements. 3. MPT design and CFD validation 3.1. Hydraulic design Referring to the general design guidelines for large scale pump turbines, the MPT also contains spiral casing, stay vanes and guide vanes, runner and draft tube. The complete specification is listed in Table 1. As a preliminary design, the spiral type casing was adapted and its design was mainly conducted by the equivalent circulation method. The profiles of stay vanes and guide vanes are referred to existing ones. The number of stay vanes and guide vanes is 20, which is generally selected in large scale pump turbines and also allows the potential parameters to be optimized in future work. For uniform inflow in pump mode and good energy recovery in turbine mode, an elbow-type draft tube was selected. The runner is a key component of the MPT, so its design is very important. As the rotation speed is variable, the design of the runner was mainly based on the pump mode, and the shape of blades is calculated by
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119
Fig. 3. The time variation of concerned parameters during charging process.
Fig. 4. The 3D runner blades and the whole configuration of MPT.
an in-housing software which employs the quasi-3D inverse design method [10]. Fig. 4 displays the shape of the runner and the complete assembled MPT. It can be seen that the backwards blade is typical. Overall, the configuration of MPT includes a spiral casing with 345 wrapped angle, 20 guide vanes and 20 stay vanes, a runner with 9 backwards blades and an elbow type draft tube. 3.2. CFD validation To validate the hydrodynamic design, the CFD method [11,12] was applied to predict whether the performance of the MPT would match the system requirements. The flow field of the MPT was calculated by solving the three-dimensional steady incompressible Reynolds Averaged Navier Stokes (RANS) equations using the commercial software Ansys CFX 12.1. The turbulent term was modeled by RNG keε model [13], which is preferable to turbomachine with the curvature correction for turbulent viscosity. The equations were solved by the coupled solver, in which the advection term and turbulence term were discretized by the high resolution scheme. The computational domain embracing the whole flow passage, which consists of the spiral casing, all the passages of the stay vanes, guide vanes and runner vanes, and the draft tube, is shown in Fig. 5. The total pressure and designed mass flow rate were set at
Fig. 5. Grid generation of runner domain and the whole flow passage.
the inlet and outlet of the computational domain, respectively. Water was considered to be the working fluid, and the solid surfaces in the computational domain were considered to be hydraulically smooth with no-slip and adiabatic conditions. For the connection between the rotating runner and the adjacent vanes, as well as the guide vanes and draft tube, the stage method [14] was used. A hybrid grid system was constructed in the computational domain, with
Table 1 Configuration of the MPT. Component Spiral casing wrapped angle Stay vane number Guide vane number Runner vane number Draft tube
345 20 20 9 Elbow type
Fig. 6. Comparison of systematical requirement and predicted performance of MPT indicating a perfect coincidence between each other, the solid line and dotted line are the systematically required head and flow rate, respectively. The circle and triangle are the calculated results.
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J.lian Yin et al. / Renewable Energy 65 (2014) 117e122
Fig. 7. Predicted efficiency and power characteristics under different rotation speeds.
(1) hexahedral elements filling the runner domain generated by the automatic topology and meshing (ATM) feature and, (2) tetrahedral elements for the draft tube domain and other domains generated in the platform of ICEM. Eight layers of prism elements were generated to refine the grid density near the walls of the guide and stay vanes. A grid-dependency test was carried out with various numbers of grids, and the optimum number of grids was selected as approximately 4,000,000 grid points. Fig. 5 shows a typical example of the grid structure system used for the numerical analysis of the micro-pump turbine. In the computation, root mean square (RMS) residual values of the momentum and mass were set to fall below 1.0E-04 and the imbalances of mass and energy were kept below 1.0E-02 as part of the convergence criteria. The physical time scale was set to 0.1/u, where u is the angular velocity of the runner. The converged solutions were obtained after approximately 400 iterations. The computations were performed by a PC with an Intel Xeon CPU with a clock speed of 2.6 GHz. The computational time for single simulation was about 5e6 h. 4. Performance check 4.1. Pump mode With the design methodology focusing mainly on the pump mode [15], the first validation was to check whether the
performance of the pump could satisfy the requirements of the system. According to Eqs. (5) and (6), the head and flow variation of the whole system were calculated. The duty of MPT is to drive the water with the required flow rate and head under different rotation speeds. To check this, the flow of MPT under several rotation speeds with specified flow rates calculated by Eq. (5) were simulated by the CFD methodology introduced in Section 3.2. Fig. 6 shows the profiles of flow rate and head vs. rotation speed, where the solid line and dotted line represent the system requirements for flow rate and head, respectively, and the circles and triangles represent the calculated flow rate and head by CFD, respectively. It was found that the performance of MPT agrees well with the system requirements, i.e. the MPT can be capable of storing the energy and the system configuration is well posed in this sense. And also, the similarity laws applied for Eqs. (5) and (6) are effective for the MPT. Another aspect we focused is the efficiency level and power consumption characteristics, which are shown in Fig. 7. It can be seen that, under the different rotation speeds, the efficiency was maintained at a level higher than 86%, which is acceptable for utilization of energy. However, as mentioned in the Hydraulic design section, the whole design was referred to the large scale pump turbine, the capacity of which is usually larger than 100 MW. Thus, the efficiency can be improved further by optimization of the numbers for the runner blades and tandem vanes. With regards to the power consumption, it can be seen that, the power increases at a small rate during the first 5 h and increases rapidly in the last hour. This feature may bring some challenges for the design of the motor. 4.2. Turbine mode To obtain the hill chart for turbine mode, a series of operating points under six guide vane openings from 10 to 30 , shown in Fig. 8, were calculated. It can be seen that, as to the micro-pump turbine, the optimal efficiency is more than 93%, which is also superior than other energy discharging devices like battery. To attain an optimal operating efficiency for the whole system, the MPT should also change its rotation speed to adopt the variation of the head, which is depends on the air expanding process significantly and calculated by Eq. (8). Based on the optimal operating point (n11 ¼ 35 rpm, q11 ¼ 0.563 m3/s), the variations of the head, flow rate and the rotation speed can be computed according to Eqs. (8)e(10). The results are illustrated in Fig. 9.
Fig. 8. The calculated operating points and the corresponding efficiency level.
J.lian Yin et al. / Renewable Energy 65 (2014) 117e122
121
Fig. 9. Time variations of the flow rate, head and rotation speed during the discharging process.
5. Conclusion
Fig. 10. The power generation characteristic curve during the discharging process.
1
0
HðtÞ
Zt C B Vw QðtÞ dt C B C 1 B pm Vca C B 0 0 2 p0 C k QðtÞ þ ¼ B C rg B Zt A C B A @Vca þ QðtÞ dt
Acknowledgments This research was supported by China Postdoctoral Science Foundation 2013M531173. References
0
(8)
QðtÞ ¼ Q11 D2
nðtÞ ¼
n11
qffiffiffiffiffiffiffiffi HðtÞ
qffiffiffiffiffiffiffiffi HðtÞ D
In this paper, a hybrid energy storage system using compressing air and a micro-pump turbine is proposed. Compared with conventional pump storage plants and compressed air energy storage systems, the proposed system is characterized by no dependence on geological conditions and simple systematical configuration. The general rules of thumb for the whole system were concluded based on the energy storage capacity, thermodynamic process of compressing the air and performance of the variable speed MPT. To satisfy the requirements of the hybrid energy storage system, a design methodology combined with modern computational fluid dynamics was adopted to determine the MPT. From the numerical results, it is verified that the variable speed MPT is capable of compressing the air and generating power with a high efficiency. However, the hydraulic efficiency of the pump was not satisfactory and more effort for the hydrodynamic design is still needed. Also, the MPT presented herein is just a preliminary design and more details, such as the cavitation performance characteristics, and the interaction between water and air during the compression process, should be investigated.
(9)
(10)
Following the similarity laws, namely the efficiency is constant under the same n11 and q11, the power generation variation during the discharging process can be calculated and shown in Fig. 10. As same as the charging process, the generated power is decreasing rapidly under the high head conditions and slowly under lower head conditions. The case studied can be taken as an idealized one in that the MPT is always operating at the optimal condition.
[1] Ferreira HL, Garde R, Fulli G, Kling W, Lopes JP. Characterisation of electrical energy storage technologies. Energy 2013;53:288e98. [2] Kanakasabapathy P. Economic impact of pumped storage power plant on social welfare of electricity market. Int J Electr Power 2013;45(1):187e93. [3] Rutqvist J, Kim HM, Ryu DW, Synn JH, Song WK. Modeling of coupled thermodynamic and geomechanical performance of underground compressed air energy storage in lined rock caverns. Int J Rock Mech Mineral 2012;52:71e81. [4] Kim HM, Rutqvist J, Ryu DW, Choi BH, Sunwoo C, Song WK. Exploring the concept of compressed air energy storage (CAES) in lined rock caverns at shallow depth: a modeling study of air tightness and energy balance. Appl Energy 2012;92:653e67. [5] Jubeh NM, Najjar Ysh. Green solution for power generation by adoption of adiabatic CAES system. Appl Thermal Eng 2012;44:85e9. [6] Teillant B, Costello R, Weber J, Ringwood J. Productivity and economic assessment of wave energy projects through operational simulations. Renew Energy 2012;48:220e30. [7] Kapsali M, Anagnostopoulos JS, Kaldellis JK. Wind powered pumped-hydro storage systems for remote islands: a complete sensitivity analysis based on economic perspectives. Appl Energy 2012;99:430e44. [8] Hartmann N, Vohringer O, Kruck C, Eltrop L. Simulation and analysis of different adiabatic compressed air energy storage plant configurations. Appl Energy 2012;93(SI):541e8.
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[9] Miyagawa K, Fukuda N, Tsuji K, Suzuki K, Saotome J. Development of a Deriaz type pump-turbine with high head, large capacity and variable speed. In: Proceedings of the Xix Iahr symposium on hydraulic machinery and cavitation 1998. p. 394e403. [10] Jinling Lu, Guang Xi, Datong Qi. Blade optimization of mixed-flow pump by using inverse design method and neural network. J Xi’an Jiaotong Univ 2004;38(3):308e12. [11] Wang LQ, Yin JL, Jiao L, Wu DZ, Qin DQ. Numerical investigation on the “S” characteristics of a reduced pump turbine model. Sci China Technol Sci 2011;54(5):1259e66.
[12] Bjarne JB, Sverre KD. Numerical computation of the pump turbine characteristics. Lausanne; 2002. [13] Orszag SA, Yakhot V, Flannery WS, Boysan F, Choudhury D, Maruzewski J, et al. Renormalization group modeling and turbulence simulations. Tempe, Arizona; 1993. [14] Yin JL, Liu JT, Wang LQ, Jiao L, Wu DZ, Qin DQ. Performance prediction and flow analysis in the vaned distributor of a pump turbine under low flow rate in pump mode. Sci China Technol Sci 2010;53(12):3302e9. [15] Yin JL, Wang DZ, Wei XZ, Wang LQ. Hydraulic improvement to eliminate Sshaped curve in pump turbine. J Fluid Eng-T ASME 2013;135(7):71101e5.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
A hybrid energy storage system using pump compressed air and micro-hydro turbine Jun lian Yin a, De zhong Wang a, Yu-Taek Kim b, Young-Ho Lee c, * a
School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Division of Marine Systems Engineering, Korea Maritime University, Busan 606-791, Republic of Korea c Division of Mechanical and Energy System Engineering, KMU, Busan 606-791, Republic of Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 February 2013 Accepted 22 July 2013 Available online 17 August 2013
In this paper, a micro-hybrid energy storage system, for a small power grid, which combines the concepts of pump storage plant (PSP) and compressed air energy storage (CAES), is proposed. There are two tanks, one open to the air and one subjected to compressed air, as well as a micro-pump turbine (MPT) in the hybrid system. The basic principle is that the MPT utilizes excess power from the grid to pump the water, which in turn compresses the air, and in this way, the energy is changed into internal energy of air. The energy in the air will be released to drive water passing through the MPT to generate power when the supply of power from the grid is insufficient. To validate the above proposal, such a micro-system was designed considering geometrical and operational conditions. Due to the large head variation for MPT, a variable speed machine [1] was designed by means of an inverse design method. After geometrical modeling and mesh generation for the complete configuration of the MPT, which consists of spiral casing, tandem, runner and draft tube, CFD simulations of typical operating points during pump mode and turbine mode were implemented. Special treatments of boundary conditions induced by the air compression or decompression were applied in the simulation. This energy storage system shows promising potential for application as the results indicated that the performance of the system and MPT was comparable. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Energy storage Micro-pump turbine CFD
1. Introduction Among existing energy storage system (ESS) technologies [1], only two technologiesdCAES (compressed air energy storage) and PHS (pumped hydroelectric storage [2])dare cost effective and successfully applied in industrial engineering. Conventionally, the working principle of CAES, which is shown in Fig. 1, can be illustrated as follows [3,4]: off-peak or excess power is taken from the grid at low cost and used to compress and store air within an underground storage cavern. When needed, this highpressure compressed air is then released, pre-heated and expanded in a gas turbine to produce electricity during peak demand hours. For additional efficiency, the compressed air can be mixed with natural gas, then burned (as is often done in conventional power generation). Thus, greenhouse gas (GHG) emissions from conventional CAES are not zero [5]. The basic principle of
* Corresponding author. Division of Mechanical & Energy System Engineering, Korea Maritime University, Busan 606-791, Republic of Korea. E-mail address: [email protected] (Y.-H. Lee). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.07.039
pumped hydro storage is also well established. Water is pumped to an upper reservoir at times of surplus supply and discharged through a pump turbine at times of high demand. However, a difference in geodetic height, which compromises applications, and high capital costs are limitations of PHS. Compared to CAES, a PHS system is simpler, as the pump turbine works bio-directionally, whereas compressors and gas turbines must be employed in CAES. Moreover, additional fuels are supplied to heat the air for high efficiency utilization. Referring to the two technologies, a hybrid energy storage system, shown in Fig. 2, is proposed to overcome the difficulties of energy storage for places where the geological structure for PHS and CAES is not suitable. It can be applied mainly for small scale energy storage. The point is the use of the pressure vessel to replace the reservoirs in PSP, where the head of water is provided by the compressed air trapped in the vessel. A micro-pump turbine (MPT) is used to achieve the charging/discharging process, as in PSP. Due to the high cost and construction difficulties of large pressure vessels, the hybrid system is more suited to small scale energy systems, for instance, energy storage for wave energy converters [6] or wind energy [7], where the capacity is no more than 100 kW.
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J.lian Yin et al. / Renewable Energy 65 (2014) 117e122
Vw ¼ Vt Vca ¼ 1674:5m3
(4)
The task for the MPT is to deliver a given volume of water with increasing head in the given time. In order to utilize the volume of the pressurized vessel adequately, the water will be released completely during the discharge process. This assumption means that the head variation ration is greater than 16, which is not out of the range of a pump turbine operated under a constant rotation speed. Thus, a variable speed scheme [9] is selected. Based on the similarity laws for variable speed, the relationship between the flow rate and head is:
Qt ¼
nt Q0 n0
(5) 0
Fig. 1. Schematic map of the conventional compressed air energy storage system.
To validate the proposed system, the design rules for the system were developed and then the MPT was designed. Subsequently, CFD simulations were carried out to validate the functions of the MPT. Finally, conclusions were drawn.
Zt B C Qt dt B C C p0 B Vt B C 0 2 1C þ Ht ¼ H0 þ kðQt Q0 Þ þ B C rg B A Zt B C @Vt Qt dt A ¼
nt n0
0
2 H0
(6)
2. System design The hybrid energy storage system includes two parts, the vessels that function as the reservoirs in PSP and the micro-pump turbine. The system was first designed so that the feasibility of the proposed system could be validated. Given the output power P ¼ 20 kW under the maximum head h ¼ 100 m and the working time T ¼ 6 h, the energy storage capacity is:
E ¼ PT ¼ 432MJ
(1)
Assuming the air is compressed by MPT from the ambient pressure p0 ¼ 101,325 Pa at t0 ¼ 298 K to the pressure p1 ¼ 1,075,844 Pa isothermally, the volume of compressed air is:
Vca ¼ E=ðp0 blnðbÞÞ ¼ 167:46m3
(2)
Here, b is the compression factor, b ¼ p1/p0. As identified by Hartmann [8], a huge efficiency difference exists between different adiabatic CAES plant configurations. For simplicity, an isothermal process is assumed in the present study. Consequently, the volume of the whole vessel is:
Vt ¼ Vca b ¼ 1842m3 and the maximum volume of water is:
Fig. 2. Schematic map of the hybrid energy storage system.
1
(3)
in which, H0, Q0, n0, p0 are the head, volume flow rate, rotation speed and pressure of the vessel, respectively, at the initial state; Ht, Qt, nt are the head, volume flow rate and rotation speed, respectively, at arbitrary time; r is the fluid density, A is the basal area and g is the gravity acceleration. The integral of the flow rate is:
Zt Qt dt ¼ Vw
(7)
0
Given the time needed to pump the water into the vessel as 6 h, combing Eqs. (5)e(7), the design specification for the MPT can be determined. Considering the optimal specific speed for pump design, the final design specification is that the MPT will operate from H0 ¼ 7 m, Q0 ¼ 171 m3/h, n0 ¼ 715.5 rpm to Ht ¼ 115 m, Q0 ¼ 693.18 m3/h, n0 ¼ 2900 rpm, i.e. the head variation ratio is 16.4. To validate this, the time variation of the above parameters is illustrated in Fig. 3. The next step is to design an MPT that is capable of operating under the complete system requirements. 3. MPT design and CFD validation 3.1. Hydraulic design Referring to the general design guidelines for large scale pump turbines, the MPT also contains spiral casing, stay vanes and guide vanes, runner and draft tube. The complete specification is listed in Table 1. As a preliminary design, the spiral type casing was adapted and its design was mainly conducted by the equivalent circulation method. The profiles of stay vanes and guide vanes are referred to existing ones. The number of stay vanes and guide vanes is 20, which is generally selected in large scale pump turbines and also allows the potential parameters to be optimized in future work. For uniform inflow in pump mode and good energy recovery in turbine mode, an elbow-type draft tube was selected. The runner is a key component of the MPT, so its design is very important. As the rotation speed is variable, the design of the runner was mainly based on the pump mode, and the shape of blades is calculated by
J.lian Yin et al. / Renewable Energy 65 (2014) 117e122
119
Fig. 3. The time variation of concerned parameters during charging process.
Fig. 4. The 3D runner blades and the whole configuration of MPT.
an in-housing software which employs the quasi-3D inverse design method [10]. Fig. 4 displays the shape of the runner and the complete assembled MPT. It can be seen that the backwards blade is typical. Overall, the configuration of MPT includes a spiral casing with 345 wrapped angle, 20 guide vanes and 20 stay vanes, a runner with 9 backwards blades and an elbow type draft tube. 3.2. CFD validation To validate the hydrodynamic design, the CFD method [11,12] was applied to predict whether the performance of the MPT would match the system requirements. The flow field of the MPT was calculated by solving the three-dimensional steady incompressible Reynolds Averaged Navier Stokes (RANS) equations using the commercial software Ansys CFX 12.1. The turbulent term was modeled by RNG keε model [13], which is preferable to turbomachine with the curvature correction for turbulent viscosity. The equations were solved by the coupled solver, in which the advection term and turbulence term were discretized by the high resolution scheme. The computational domain embracing the whole flow passage, which consists of the spiral casing, all the passages of the stay vanes, guide vanes and runner vanes, and the draft tube, is shown in Fig. 5. The total pressure and designed mass flow rate were set at
Fig. 5. Grid generation of runner domain and the whole flow passage.
the inlet and outlet of the computational domain, respectively. Water was considered to be the working fluid, and the solid surfaces in the computational domain were considered to be hydraulically smooth with no-slip and adiabatic conditions. For the connection between the rotating runner and the adjacent vanes, as well as the guide vanes and draft tube, the stage method [14] was used. A hybrid grid system was constructed in the computational domain, with
Table 1 Configuration of the MPT. Component Spiral casing wrapped angle Stay vane number Guide vane number Runner vane number Draft tube
345 20 20 9 Elbow type
Fig. 6. Comparison of systematical requirement and predicted performance of MPT indicating a perfect coincidence between each other, the solid line and dotted line are the systematically required head and flow rate, respectively. The circle and triangle are the calculated results.
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J.lian Yin et al. / Renewable Energy 65 (2014) 117e122
Fig. 7. Predicted efficiency and power characteristics under different rotation speeds.
(1) hexahedral elements filling the runner domain generated by the automatic topology and meshing (ATM) feature and, (2) tetrahedral elements for the draft tube domain and other domains generated in the platform of ICEM. Eight layers of prism elements were generated to refine the grid density near the walls of the guide and stay vanes. A grid-dependency test was carried out with various numbers of grids, and the optimum number of grids was selected as approximately 4,000,000 grid points. Fig. 5 shows a typical example of the grid structure system used for the numerical analysis of the micro-pump turbine. In the computation, root mean square (RMS) residual values of the momentum and mass were set to fall below 1.0E-04 and the imbalances of mass and energy were kept below 1.0E-02 as part of the convergence criteria. The physical time scale was set to 0.1/u, where u is the angular velocity of the runner. The converged solutions were obtained after approximately 400 iterations. The computations were performed by a PC with an Intel Xeon CPU with a clock speed of 2.6 GHz. The computational time for single simulation was about 5e6 h. 4. Performance check 4.1. Pump mode With the design methodology focusing mainly on the pump mode [15], the first validation was to check whether the
performance of the pump could satisfy the requirements of the system. According to Eqs. (5) and (6), the head and flow variation of the whole system were calculated. The duty of MPT is to drive the water with the required flow rate and head under different rotation speeds. To check this, the flow of MPT under several rotation speeds with specified flow rates calculated by Eq. (5) were simulated by the CFD methodology introduced in Section 3.2. Fig. 6 shows the profiles of flow rate and head vs. rotation speed, where the solid line and dotted line represent the system requirements for flow rate and head, respectively, and the circles and triangles represent the calculated flow rate and head by CFD, respectively. It was found that the performance of MPT agrees well with the system requirements, i.e. the MPT can be capable of storing the energy and the system configuration is well posed in this sense. And also, the similarity laws applied for Eqs. (5) and (6) are effective for the MPT. Another aspect we focused is the efficiency level and power consumption characteristics, which are shown in Fig. 7. It can be seen that, under the different rotation speeds, the efficiency was maintained at a level higher than 86%, which is acceptable for utilization of energy. However, as mentioned in the Hydraulic design section, the whole design was referred to the large scale pump turbine, the capacity of which is usually larger than 100 MW. Thus, the efficiency can be improved further by optimization of the numbers for the runner blades and tandem vanes. With regards to the power consumption, it can be seen that, the power increases at a small rate during the first 5 h and increases rapidly in the last hour. This feature may bring some challenges for the design of the motor. 4.2. Turbine mode To obtain the hill chart for turbine mode, a series of operating points under six guide vane openings from 10 to 30 , shown in Fig. 8, were calculated. It can be seen that, as to the micro-pump turbine, the optimal efficiency is more than 93%, which is also superior than other energy discharging devices like battery. To attain an optimal operating efficiency for the whole system, the MPT should also change its rotation speed to adopt the variation of the head, which is depends on the air expanding process significantly and calculated by Eq. (8). Based on the optimal operating point (n11 ¼ 35 rpm, q11 ¼ 0.563 m3/s), the variations of the head, flow rate and the rotation speed can be computed according to Eqs. (8)e(10). The results are illustrated in Fig. 9.
Fig. 8. The calculated operating points and the corresponding efficiency level.
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Fig. 9. Time variations of the flow rate, head and rotation speed during the discharging process.
5. Conclusion
Fig. 10. The power generation characteristic curve during the discharging process.
1
0
HðtÞ
Zt C B Vw QðtÞ dt C B C 1 B pm Vca C B 0 0 2 p0 C k QðtÞ þ ¼ B C rg B Zt A C B A @Vca þ QðtÞ dt
Acknowledgments This research was supported by China Postdoctoral Science Foundation 2013M531173. References
0
(8)
QðtÞ ¼ Q11 D2
nðtÞ ¼
n11
qffiffiffiffiffiffiffiffi HðtÞ
qffiffiffiffiffiffiffiffi HðtÞ D
In this paper, a hybrid energy storage system using compressing air and a micro-pump turbine is proposed. Compared with conventional pump storage plants and compressed air energy storage systems, the proposed system is characterized by no dependence on geological conditions and simple systematical configuration. The general rules of thumb for the whole system were concluded based on the energy storage capacity, thermodynamic process of compressing the air and performance of the variable speed MPT. To satisfy the requirements of the hybrid energy storage system, a design methodology combined with modern computational fluid dynamics was adopted to determine the MPT. From the numerical results, it is verified that the variable speed MPT is capable of compressing the air and generating power with a high efficiency. However, the hydraulic efficiency of the pump was not satisfactory and more effort for the hydrodynamic design is still needed. Also, the MPT presented herein is just a preliminary design and more details, such as the cavitation performance characteristics, and the interaction between water and air during the compression process, should be investigated.
(9)
(10)
Following the similarity laws, namely the efficiency is constant under the same n11 and q11, the power generation variation during the discharging process can be calculated and shown in Fig. 10. As same as the charging process, the generated power is decreasing rapidly under the high head conditions and slowly under lower head conditions. The case studied can be taken as an idealized one in that the MPT is always operating at the optimal condition.
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