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Renewable Energy 65 (2014) 117e122

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Renewable Energy

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A hybrid energy storage system using pump compressed air andmicro-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, ChinabDivision of Marine Systems Engineering, Korea Maritime University, Busan 606-791, Republic of KoreacDivision of Mechanical and Energy System Engineering, KMU, Busan 606-791, Republic of Korea

a r t i c l e i n f o

Article history:Received 21 February 2013Accepted 22 July 2013Available online 17 August 2013

Keywords:Energy storageMicro-pump turbineCFD

* Corresponding author. Division of Mechanical &Korea Maritime University, Busan 606-791, Republic o

E-mail address: lyh@hhu.ac.kr (Y.-H. Lee).

0960-1481/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.renene.2013.07.039

a b s t r a c t

In this paper, a micro-hybrid energy storage system, for a small power grid, which combines the conceptsof 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 thehybrid 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. Theenergy in the air will be released to drive water passing through the MPT to generate power when thesupply of power from the grid is insufficient. To validate the above proposal, such a micro-system wasdesigned considering geometrical and operational conditions. Due to the large head variation for MPT, avariable speed machine [1] was designed by means of an inverse design method. After geometricalmodeling 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 andturbine mode were implemented. Special treatments of boundary conditions induced by the aircompression or decompression were applied in the simulation. This energy storage system showspromising potential for application as the results indicated that the performance of the system and MPTwas comparable.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Among existing energy storage system (ESS) technologies [1],only two technologiesdCAES (compressed air energy storage) andPHS (pumped hydroelectric storage [2])dare cost effective andsuccessfully applied in industrial engineering.

Conventionally, theworking principle of CAES,which is shown inFig. 1, can be illustrated as follows [3,4]: off-peak or excess power istaken from the grid at low cost and used to compress and store airwithin an underground storage cavern. When needed, this high-pressure compressed air is then released, pre-heated andexpanded in a gas turbine to produce electricity during peak de-mand hours. For additional efficiency, the compressed air can bemixed with natural gas, then burned (as is often done in conven-tional power generation). Thus, greenhouse gas (GHG) emissionsfrom conventional CAES are not zero [5]. The basic principle of

Energy System Engineering,f Korea.

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pumped hydro storage is also well established. Water is pumped toan upper reservoir at times of surplus supply and dischargedthrough a pump turbine at times of high demand. However, a dif-ference in geodetic height, which compromises applications, andhigh capital costs are limitations of PHS. Compared to CAES, a PHSsystem 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 ef-ficiency utilization.

Referring to the two technologies, a hybrid energy storage sys-tem, shown in Fig. 2, is proposed to overcome the difficulties ofenergy storage for places where the geological structure for PHSand CAES is not suitable. It can be applied mainly for small scaleenergy storage. The point is the use of the pressure vessel to replacethe reservoirs in PSP, where the head of water is provided by thecompressed air trapped in the vessel. A micro-pump turbine (MPT)is used to achieve the charging/discharging process, as in PSP. Dueto the high cost and construction difficulties of large pressurevessels, the hybrid system is more suited to small scale energysystems, for instance, energy storage for wave energy converters [6]or wind energy [7], where the capacity is no more than 100 kW.

Fig. 1. Schematic map of the conventional compressed air energy storage system.

J.lian Yin et al. / Renewable Energy 65 (2014) 117e122118

To validate the proposed system, the design rules for the systemwere developed and then the MPT was designed. Subsequently,CFD simulations were carried out to validate the functions of theMPT. Finally, conclusions were drawn.

2. System design

The hybrid energy storage system includes two parts, the vesselsthat function as the reservoirs in PSP and the micro-pump turbine.The systemwas first designed so that the feasibility of the proposedsystem could be validated. Given the output power P ¼ 20 kWunder the maximum head h ¼ 100 m and the working time T¼ 6 h,the energy storage capacity is:

E ¼ PT ¼ 432MJ (1)

Assuming the air is compressed by MPT from the ambientpressure p0 ¼ 101,325 Pa at t0 ¼ 298 K to the pressurep1 ¼ 1,075,844 Pa isothermally, the volume of compressed air is:

Vca ¼ E=ðp0blnðbÞÞ ¼ 167:46m3 (2)

Here, b is the compression factor, b ¼ p1/p0. As identified byHartmann [8], a huge efficiency difference exists between differentadiabatic CAES plant configurations. For simplicity, an isothermalprocess is assumed in the present study. Consequently, the volumeof the whole vessel is:

Vt ¼ Vcab ¼ 1842m3 (3)

and the maximum volume of water is:

Fig. 2. Schematic map of the hybrid energy storage system.

Vw ¼ Vt � Vca ¼ 1674:5m3 (4)

The task for the MPT is to deliver a given volume of water withincreasing head in the given time. In order to utilize the volume ofthe pressurized vessel adequately, the water will be releasedcompletely during the discharge process. This assumption meansthat the head variation ration is greater than 16, which is not out ofthe range of a pump turbine operated under a constant rotationspeed. Thus, a variable speed scheme [9] is selected. Based on thesimilarity laws for variable speed, the relationship between theflow rate and head is:

Qt ¼�ntn0

�Q0 (5)

Ht ¼ H0 þ kðQt � Q0Þ2 þp0rg

0BBBBBBB@

Vt

Vt �Zt

0

Qtdt

� 1

1CCCCCCCA

þ

Zt

0

Qtdt

A

¼�ntn0

�2

H0

(6)

in which, H0, Q0, n0, p0 are the head, volume flow rate, rotationspeed and pressure of the vessel, respectively, at the initial state;Ht,Qt, nt are the head, volume flow rate and rotation speed, respec-tively, at arbitrary time; r is the fluid density, A is the basal area andg is the gravity acceleration.

The integral of the flow rate is:

Zt

0

Qtdt ¼ Vw (7)

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 bedetermined. Considering the optimal specific speed for pumpdesign, the final design specification is that the MPT will operatefrom 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 is16.4. To validate this, the time variation of the above parameters isillustrated in Fig. 3. The next step is to design anMPT that is capableof 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 pumpturbines, the MPT also contains spiral casing, stay vanes and guidevanes, runner and draft tube. The complete specification is listed inTable 1. As a preliminary design, the spiral type casing was adaptedand its design was mainly conducted by the equivalent circulationmethod. The profiles of stay vanes and guide vanes are referred toexisting ones. The number of stay vanes and guide vanes is 20,which is generally selected in large scale pump turbines and alsoallows the potential parameters to be optimized in future work. Foruniform inflow in pumpmode and good energy recovery in turbinemode, an elbow-type draft tube was selected. The runner is a keycomponent of the MPT, so its design is very important. As therotation speed is variable, the design of the runner was mainlybased on the pump mode, and the shape of blades is calculated by

Fig. 3. The time variation of concerned parameters during charging process.

Fig. 4. The 3D runner blades and the whole configuration of MPT.

Fig. 5. Grid generation of runner domain and the whole flow passage.

J.lian Yin et al. / Renewable Energy 65 (2014) 117e122 119

an in-housing software which employs the quasi-3D inverse designmethod [10]. Fig. 4 displays the shape of the runner and the com-plete assembled MPT. It can be seen that the backwards blade istypical. Overall, the configuration of MPT includes a spiral casingwith 345�wrapped angle, 20 guide vanes and 20 stay vanes, arunner 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 MPTwould match the system requirements. The flow field of the MPTwas calculated by solving the three-dimensional steady incom-pressible Reynolds Averaged Navier Stokes (RANS) equations usingthe commercial software Ansys CFX 12.1. The turbulent term wasmodeled by RNG keε model [13], which is preferable to turbo-machine with the curvature correction for turbulent viscosity. Theequations were solved by the coupled solver, in which the advec-tion term and turbulence term were discretized by the high reso-lution scheme.

The computational domain embracing the whole flow passage,which consists of the spiral casing, all the passages of the stayvanes, guide vanes and runner vanes, and the draft tube, is shownin Fig. 5. The total pressure and designed mass flow rate were set at

Table 1Configuration of the MPT.

Component

Spiral casing wrapped angle 345�

Stay vane number 20Guide vane number 20Runner vane number 9Draft tube Elbow type

the inlet and outlet of the computational domain, respectively.Water was considered to be the working fluid, and the solid sur-faces in the computational domain were considered to be hydrau-lically smooth with no-slip and adiabatic conditions. For theconnection between the rotating runner and the adjacent vanes, aswell as the guide vanes and draft tube, the stage method [14] wasused.

A hybrid grid system was constructed in the computationaldomain, with

Fig. 6. Comparison of systematical requirement and predicted performance of MPTindicating a perfect coincidence between each other, the solid line and dotted line arethe systematically required head and flow rate, respectively. The circle and triangle arethe calculated results.

Fig. 7. Predicted efficiency and power characteristics under different rotation speeds.

J.lian Yin et al. / Renewable Energy 65 (2014) 117e122120

(1) hexahedral elements filling the runner domain generated bythe automatic topology and meshing (ATM) feature and,

(2) tetrahedral elements for the draft tube domain and other do-mains generated in the platform of ICEM. Eight layers of prismelements were generated to refine the grid density near thewalls of the guide and stay vanes. A grid-dependency test wascarried out with various numbers of grids, and the optimumnumber of grids was selected as approximately 4,000,000 gridpoints. Fig. 5 shows a typical example of the grid structuresystem used for the numerical analysis of the micro-pumpturbine.

In the computation, root mean square (RMS) residual values ofthe momentum and mass were set to fall below 1.0E-04 and theimbalances of mass and energy were kept below 1.0E-02 as part ofthe convergence criteria. The physical time scale was set to 0.1/u,where u is the angular velocity of the runner. The converged so-lutions were obtained after approximately 400 iterations. Thecomputations were performed by a PC with an Intel Xeon CPU witha clock speed of 2.6 GHz. The computational time for single simu-lation was about 5e6 h.

4. Performance check

4.1. Pump mode

With the design methodology focusing mainly on the pumpmode [15], the first validation was to check whether the

Fig. 8. The calculated operating points an

performance of the pump could satisfy the requirements of thesystem. According to Eqs. (5) and (6), the head and flow variationof the whole system were calculated. The duty of MPT is to drivethe water with the required flow rate and head under differentrotation speeds. To check this, the flow of MPT under severalrotation speeds with specified flow rates calculated by Eq. (5) weresimulated 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 re-quirements for flow rate and head, respectively, and the circlesand triangles represent the calculated flow rate and head by CFD,respectively. It was found that the performance of MPT agrees wellwith the system requirements, i.e. the MPT can be capable ofstoring the energy and the system configuration is well posed inthis 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 powerconsumption characteristics, which are shown in Fig. 7. It can beseen that, under the different rotation speeds, the efficiency wasmaintained at a level higher than 86%, which is acceptable forutilization of energy. However, as mentioned in the Hydraulicdesign section, the whole design was referred to the large scalepump turbine, the capacity of which is usually larger than 100 MW.Thus, the efficiency can be improved further by optimization of thenumbers for the runner blades and tandem vanes. With regards tothe power consumption, it can be seen that, the power increases ata small rate during the first 5 h and increases rapidly in the lasthour. This feature may bring some challenges for the design of themotor.

4.2. Turbine mode

To obtain the hill chart for turbine mode, a series of oper-ating points under six guide vane openings from 10� to 30�,shown in Fig. 8, were calculated. It can be seen that, as to themicro-pump turbine, the optimal efficiency is more than 93%,which is also superior than other energy discharging deviceslike battery.

To attain an optimal operating efficiency for the whole sys-tem, the MPT should also change its rotation speed to adopt thevariation of the head, which is depends on the air expandingprocess significantly and calculated by Eq. (8). Based on theoptimal operating point (n11 ¼ 35 rpm, q11 ¼ 0.563 m3/s), thevariations of the head, flow rate and the rotation speed can becomputed according to Eqs. (8)e(10). The results are illustratedin Fig. 9.

d the corresponding efficiency level.

Fig. 9. Time variations of the flow rate, head and rotation speed during the discharging process.

Fig. 10. The power generation characteristic curve during the discharging process.

J.lian Yin et al. / Renewable Energy 65 (2014) 117e122 121

HðtÞ ¼ 1rg

0BBBBBBB@

pmVca

Vca þZt

0

QðtÞdt

� p0

1CCCCCCCA

� k0Q2ðtÞ þ

Vw �Zt

0

QðtÞdt

A

(8)

QðtÞ ¼ Q11D2

ffiffiffiffiffiffiffiffiHðtÞ

q(9)

nðtÞ ¼n11

ffiffiffiffiffiffiffiffiHðtÞ

qD

(10)

Following the similarity laws, namely the efficiency is constantunder the same n11 and q11, the power generation variation duringthe discharging process can be calculated and shown in Fig. 10. Assame as the charging process, the generated power is decreasingrapidly under the high head conditions and slowly under lowerhead conditions. The case studied can be taken as an idealized onein that the MPT is always operating at the optimal condition.

5. Conclusion

In this paper, a hybrid energy storage system using compressingair and a micro-pump turbine is proposed. Compared with con-ventional pump storage plants and compressed air energy storagesystems, the proposed system is characterized by no dependenceon geological conditions and simple systematical configuration.

The general rules of thumb for thewhole systemwere concludedbased on the energy storage capacity, thermodynamic process ofcompressing the air and performance of the variable speed MPT. Tosatisfy the requirements of the hybrid energy storage system, adesign methodology combined with modern computational fluiddynamics was adopted to determine the MPT. From the numericalresults, it is verified that the variable speed MPT is capable ofcompressing the air and generating power with a high efficiency.

However, the hydraulic efficiency of the pump was not satis-factory andmore effort for the hydrodynamic design is still needed.Also, the MPT presented herein is just a preliminary design andmore details, such as the cavitation performance characteristics,and the interaction between water and air during the compressionprocess, should be investigated.

Acknowledgments

This research was supported by China Postdoctoral ScienceFoundation 2013M531173.

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