SURGE CONTROL OF AIR COMPRESSOR UNDER VARIABLE OPERATING PRESSURE OF AUTOMOTIVE FUEL CELLS

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1 HEFAT04 0 th International Conference on Heat Transfer, Fluid Mechanics and Theodynamics 4 6 July 04 Orlando, Florida SURGE CONTROL OF AIR COMPRESSOR UNDER VARIABLE OPERATING PRESSURE OF AUTOMOTIVE FUEL CELLS Jaeyoung Han and Sangseo Yu* Deartment of Mechanil Engineering, National University of Chungnam, Daejon, , Korea, sangseo@cnu.ac.r ABSTRACT The comressor of a roton exchange membrane fuel cell for automotive alition requires severe dynamic erfoance under noal oerating conditions. Since air flow rate with a comressor should cover a wide range of oerating conditions, it is necessary to understand oerating trajectory of comressor. In this study, a simulation model of an automotive fuel cell system with a dynamic comressor is develoed to exlore roer trajectory of air flow rate on erfoance chart of an air comressor. A dynamic simulation model of a comressor is comosed of manifold dynamics of a suly and return line, static comressor model, and dynamic motor model. From comressor to fuel cell stac is considered as a lenum and an orifice between m is also assumed. An active control valve is also considered at exit of fuel cell stac so that n be actively rejected. The control strategy of a variable ressure comressor is concentrated on rejection of over various oerating conditions under load demands of driving mode. INTRODUCTION The centrifugal comressor is widely used from internal combustion engine to energy industry, to ressurize air flows. In se of automotive fuel cell, centrifugal comressor is resonsible for sulying ressurized air to fuel cell stac. High ressure oeration of a fuel cell stac imroves erfoance and comact acaging sace for system. When fuel cells are equied in automobiles, SUVs are tyil alition. However, since sedan is a target of automotive fuel cell system, variable ressure oeration is required to imrove erfoance with comact stacing []. A Surge is an unstable oerating mode of comressor systems that occurs at low mass flow where ressure delivered by comressor is less than lenum ressure []. As a comressor is oerated under a dynamic environment, comressor n occur. The n damage comressor, and durability of whole system is difficult ensure. Since automotive fuel cells must be oerated at a wide range of ressures, comressor of fuel cell must avoid s henomenon. NOMENCLATURE ψ [-] Head arameter U [m/s] Blade ti seed Φ [-] The noalized comressor flow rate ρ [g/m 3 ] density W [g/s] Air mass flow rate R [J/molK] Gas constant T [K] Temerature [a] Pressure M [-] Mach number d [m] diameter A [m ] Nozzle area V [V] Stac voltage P [W] Stac ower Secial characters α [-] Reference coefficient at suly manifold λ [-] Water content of H O/SO -3 B [-] Reference coefficient at thode manifold η [V] Cathode over-otential γ [-] Ratio of secific heat Subscrits c,in Comressor in c,out Comressor out c comressor a Air cr Air mass flow of comressor max Maximum 0 Comressor inlet ressure Comressor outlet ressure,out Suly manifold out Suly manifold Nern Nernst Fuel cell Cathode manifold Ca,out Cathode manifold out Cathode manifold 9

2 Surge control technology has been widely studied. Secifilly, Greitzer et al. reorted dynamic characteristics of entire axial load comression system of rotating stalll and by oretil and exerimental research [,3]. Moore & Greitzer roosed a model for a multistage axial load comression system [4]. Then Hansen et al. reorted a model in 98 based on one centrifugal comressor [5]. Shehata et al. reorted an imroved version of Moore & Greitzer model for a centrifugal comressor [6]. They suggested several control techniques to avoid s and consequently imrove efficiency of system [6]. Also, Galindo et al. reorted exerimental study of a centrifugal comressor of a turbocharger in all internal combustion engines [7]. Tirnovan, Miraoui and Giurgea resent a study of fuel cell system erfoance oerating at high ressure and different temerature levels [8]. In this study, a dynamic comressor model is develoed that n simulate comressor under a dynamic load follow-u of an automotive fuel cell system. The model is used to develo a rejection strategy of automotive fuel cells with two valves for suly manifold outlet and thode out manifold. The efficiency and erfoance of system is n discussed in tes of combination of inlet and outlet valves of fuel cell. A DYNAMIC MODEL OF THE COMPRESSOR Connection of odynamics with actual comressor ma In resent study, an Allied Signal comressor is used in modeling of comressorr to deteinee active control of. The model includes not only odynamic variable changes but also ractil oerating variables. The comressor model is divided into two arts: ( a comressor air flow rate in actual comressor (this is deteined by ressure ratio and ( rotational seed of comressor. Accordingly, erfoance curve indites variation of air flow rates in tes of ressure ratio and rotational seed of comressor. As inlet temerature of ressure affects exit condition of comressor, erfoance curve of comressor reflects changes of temerature and ressure such as modified flow rate, seed, and temerature. To noalize erfoance curve, this study emloys noalized charge roosed by Jensen & Kristensen [0]. The non-dimension head arameter is as follows: C T c, in U c c, out c, in Where, U is comressor blade ti seeds. c ( There, noalized comressor flow rate ( is n correlated with head arameter ( by following equation. max max ex ( Where, max, and max are olynomial functions of Mach number ( M. Therefore, air masss flow is n lculated using Equation (3 W cr a d 4 Nozzle Equation A nozzle is a device characteristic of a fluid flow as it exits an enclosed chamber or ie to increase velocity. To develo dynamic manifolds (inlet and outlet, this study utilizes nozzle flow equation. The nozzle flow equation is alied to internal combustion engine fundamentals [9]. The air flow rate is a function of ustream ressure and temerature, and downstream ressure of nozzle. If air flow rate ( W through a nozzle is zero, critil ressure ratio is lculated from following. 0 Figure Comarison of nozzle flow rate /( cu c / C D AR o W RT / o 0 ( ( / / (3 (4 (5 When ressure dro is less than ratio, aroriate equation is critil ressure 9

3 ( CD AR 0 / W / (6 RT o The lot of mass flow rate is shown in Fig.. If ressure difference between and is relatively all, mass flow rate is equal for various manifold ressures due to Equation (4 ~ (6. Therefore, a all mass flow rate region n be lculated by a linearized fo of Equation (4: W ( (7 Where, is valve oening coefficient. As shown in Equation (7, is increased from to and mass flow rate is increased as well. Model of a suly manifold A suly manifold sulies air from comressor outlet to thode of fuel cell stac. The Inlet air flow rate to thode side is deteined by ressure difference between comressor exit and thode inlet of fuel cell stac. Since ressure difference between suly manifold and thode is relatively all (7, nozzle equation of suly manifold is deteined by a linearized fo. W ( (8, out, out Where, is reference coefficient, The mass flow rate in suly manifold is lculated by conservation of mass equation and ideal gas law. dm d W W (9 ( c,out Ra ( WcTc, out W, outt (0 V Model of thode air flow rate A flow-assing thode of a fuel cell stac enters return manifold. The air flow rate n be considered as a thode air flow rate and air flow rate is sulied by ressure difference between thode downstream and return manifold. Since ressure difference between suly manifold and thode is relatively all, nozzle equation of thode outlet flow rate is deteined by a linearized fo. W ( (, out, out Where, is reference coefficient, The temerature of air leaving stac is relatively low when comared to air leaving comressor. Therefore, mass conservation rincile is used to lculate mass flow rate in return manifold. 6 6 dm d W W ( (, out, out Ra ( W, out W, out (3 V Model of return manifold As shown in Fig., if ressure difference is large, a nonlinear fo is used to lculate mass flow rate. Therefore, Beuse of ressure dro between return manifold and atmosheric is relatively large, and return manifold out flow rate is designed by nonlinear nozzle equations. ( C A / ( / /( D, T, atm atm atm W, / (4 out RT / /( C ( D, AT, / W atm, out / RT (5 The outlet mass flow rate is a function of manifold ressure,, and ressure downstream from manifold. The valve oening area,, is constant or variable, but this study set is constant beuse,, are variables to control. System integration with an automotive fuel cell stac In this study, a fuel cell system is comosed of a comressor, manifolds (suly and return, a humidifier, a hydrogen suly model, a fuel cell stac and a cooling system. Referenced from Han et al. ( and Yu et al (, a fuel cell mode was constructed to model secies mass transfer and an energy conservation equation. Therefore, fuel cell voltage and ower is lculated from V V J R( (6 P cv Nern mem V J n (7 Dynamic simulation A tyil vehicular fuel cell system was used in this study beuse transient resonse is very imortant in vehicle and control is very imortant during various oerating Table Comressor nozzle oening area with ses Case Nozzle oening area ercent(k Nozzle oening area ercent(k

conditions and to extend comressor s stable

Therefore CCV and TCV have advantages of

coefficient RESULTS AND DISCUSSION Transient

comressorr ma Figure 3 shows air flow versus

At this condition, valve of thode exit is

4 Conditions. The inut load rofiles were selected to createe henomenonn and to control. The detailed arameter is shown in Table below. Load rofiles The load rofile and nozzle oening coefficient are shown in Figure. and are CCV and TCV, resectively. The use of CCV and TCV is necessary to stabilize comressor under various oerating conditions and to extend comressor s stable range. Therefore CCV and TCV have advantages of ability to control. so, n be actively controlled. Furore, in this study, and weree changed from to %, resectively. Figure Load attern and nozzle oening coefficient RESULTS AND DISCUSSION Transient resonse of comressor on trajectory of comressorr ma Figure 3 shows air flow versus ressure ratio trajectory under a given load attern. The valve of suly manifold regulates flow such that nozzle coefficient is a fixed constant. At this condition, valve of thode exit is varied such that nozzle coefficient at exit manifold changes. The result shows that n be detected over some load conditions. The air flow rate is loted outside of line whichh imlies thatt n be detected. This is beuse comressor nnot rovide roer ressuree rise to overcome ressure loss through system. As a result, low mass flow rate of air is not roerly delivered to thode side of fuel cell stac. As exit valve of thode is fully oened, it is observed that system oerates outside margin. Figure 3 Comressor erfoance curve (K = 94

You n seee that erfoance curve is foed outside line at all regions.

rate does not ass stac. Therefore, ressure ration oscillation occurs.

erfoance curve is foed outside sure line at and in figure 6 beuse air flow rate

In contrast, when is and is and, henomenon does not occur beuse ressure loss does

Control strategy to avoid Figure7, 8, and 9 shows how much valve oening area is

Figure 7 shows changed suly manifold nozzle coefficient to from.

As n be seen in Fig.7 and Fig.8, when system is oerated around 80%, occurs.

9 maes it clear that and are dramatilly increased at a high nozzle coefficient.

As shown in Table 3, when current is 5%, and set u se 6 with high efficiency.

When load is between and, henomenon does not occur.

Therefore, when system is oerated from 50 to 65%, control oint is set as se.

every se. Also, henomenon section has much lower comressor efficiency.

Table Comressor nozzle oening area with ses Nozzle oening area Nozzle oening area

5 Figure 4 Comressor erfoance curve (K =. Figure 4 showss comressor erfoance ma that when suly manifold nozzle is oening. You n seee that erfoance curve is foed outside line at all regions. When suly manifold nozzle is oen, air flow rate does not ass stac at all regions. So, ressure ratio oscillation occurs. Therefore, nozzle oening area from 80% to, it is necessary to oen thode manifold control valve. When suly manifold nozzle coefficient is oen, you n see that erfoance curve is foed outside line at in figure5. When thode nozzle coefficient is lower or equal to suly nozzle coefficient, air flow rate does not ass stac. Therefore, ressure ration oscillation occurs. Therefore, nozzle oening area from 80% to, it is necessary to oen thode manifold control valve Next, when suly manifold nozzle coefficient oens at %, you n see that erfoance curve is foed outside sure line at and in figure 6 beuse air flow rate through suly manifold does not asss thode manifold due to a alll oening area. In contrast, when is and is and, henomenon does not occur beuse ressure loss does not exist at thode manifold outlet. Therefore, nozzle oening area from 80% to, it is necessary to oen thode manifold control valve. Figure 6 Comressor erfoance curve (K =. Control strategy to avoid Figure7, 8, and 9 shows how much valve oening area is required to avoid area from comressor system. Figure 7 shows changed suly manifold nozzle coefficient to from. Figure 8 shows changed return manifold nozzle coefficient to from. As n be seen in Fig.7 and Fig.8, when system is oerated around 80%, occurs. Thus, comressor efficiency is very low. Also, Figure.9 maes it clear that and are dramatilly increased at a high nozzle coefficient. Thus, when system is oerated at around 80%, and nozzle coefficients have to oen over. As shown in Table 3, when current is 5%, and set u se 6 with high efficiency. Therefore, when system is oerated from 5% to 30% %, control oint is set as se 6. When load is between and, henomenon does not occur. So, and set u se with good comressor efficiency. Therefore, when system is oerated from 50 to 65%, control oint is set as se. Also, when system oerates at 80%, we n see henomenon which that occurs in almost every se. Also, henomenon section has much lower comressor efficiency. Thus, as shown in Figure 9, and is set as se 6. Table Comressor nozzle oening area with ses Nozzle oening area Nozzle oening area Case ercent( ercent( Table 3 Comressor efficiency for current Case I=5% 75.7% 76.% 76.4% 76.% 75.9% 76.3% 76.7% 77.0% 76.0% 76.5% I=30% 77.4% 77.8% 78.% 78.3% 77.6% 78.% 78.4% 78.5% 77.8% 78.3% I=5 78.9% 78.9% 78.9% 78.9% 78.9% 78.9% 78.9% 78.9% 78.9% 78.9% I=65% 78.5% 78.4% 78.5% I=80% 9.% 30.0% 38.4% 4.6% 30.5% 35.4% 35% 4.0% 3.% 38.8% I= 76.% 76.% 76.0% 75.9% 76.% 76.0% 75.7% 75.5% 76.% 75.8% 95

3 4 5 6 76.9% 78.5% 78.9% 77.% 78.5% 78.9% 76.

9% 77.4% 78.9% 78.4% 39.6% 75.5% 78.3% 75.7% 75.

henomenon n be avoided by oening area of CCV to

In contrast, even if TCV is fully oen, control n

henomenonn does not occur in an oening area of

CCV is, and henomenonn n be controlled only if

Comarison of energy saving The ercentage of net

( m= (8 The gain of net energy of comressor

Figure9 Contour lot of efficiency (current=80%

6 % 78.5% 78.9% 77.% 78.5% 78.9% 76.% 77.9% 78.9% 76.7% 78.4% 78.9% 77.% 78.5% 78.9% 77.4% 78.9% 78.4% 39.6% 75.5% 78.3% 75.7% 75.3% 33.6% 76.% 78.5% 40.3% 75.7% 78.3% 7.0% 75.4% 78.% 76.7% 75.0% Therefore, simulations show that this henomenon n be avoided by oening area of CCV to over %. In contrast, even if TCV is fully oen, control n be limited at oening areas of CCV below Also, henomenonn does not occur in an oening area of only if TCV is fully oen and if oening area of CCV is, and henomenonn n be controlled only if TCV oens over. Comarison of energy saving The ercentage of net energy savings of comressor system is deteined by equation blow. The net energy of comressor system is: Figure8 Efficiency over return manifold nozzle coefficient. ( m= (8 The gain of net energy of comressor system is as follows: %,,,, 00, (9 The lculation shows thatt, and gained 0.8% net energy savings for comressor system. Figure9 Contour lot of efficiency (current=80% Control strategy to oerate system at otimal efficiency without Fig. 0 shows comressor erfoance curve trajectories during load attern under control logic. Figure7 Efficiency over suly manifold nozzle coefficient. ( = Figure 0 Comressor transient resonse on comressor ma under control logic. 96

7 The erfoance curve is in good agreement with that of comressor ma. If control valve is installed downstream of thode manifold good trajectory is achieved. Therefore, combination of CCV and TCV is needed to control. Also, when load is 80%, henomenon n be controlled only if CCV oens over and TCV oens at least over. CONCLUSION In this study, a dynamic simulation of an automotive fuel cell system was develoed to understand comressor over load changes.. The effect n be effectively reduced by increasing nozzle coefficient, but nozzle of comressor downstream does not affect avoidance of at 80%.. When system oerates at 5% and 30%, se 6 shows highest efficiencies of 77.4% and, resectively. As system oerate at and 65%, se results in high efficiencies 78.9% and, resectively. And at 80% and, se 6 results in high efficiencies 76.7% and 75.0%, resectively. 3. The lculation shows that, and gained 0.8% net energy savings for comressor system. 4. Accordingly, active control of a combination of CCV and TCV is necessary to achieve active reduction as well as efficient oeration. [6] Shelhata, R.S., Abdullah, H.A. and Areed, F.F.G., 009, "Variable structure control for constant seed centrifugal comressors," Control Engineering Practice, Vol 7,. 85~833. [7] Galindo, J., Serrano, J.R., Climent, H. and Tiseira, A., 008, " Exeriments and modelling of in all centrifugal comressor for automotive engines," Exerimental Theal and Fluid Science, Vol. 3,. 88~86. [8] Tirnovan, R., Miraoui, A. and Giurgea, S., 007, Modeling and Analysis of a High Pressure Oerating Fuel Cell Hydrogen/air system, Proceedings of IEEE. [9] Heywood, J.B., 998, Internal Combustion Engine Fundamentals. McGraw-Hill, New Yor,. 0~6 [0] Purushan, J., Stefanooulou, A. G. and Peng, H., 004, Control of Fuel Cell Power Systems, Sringer, London, First Edition,. 5~0. [] Han, J.Y., Lee, K.H. and Yu, S.S., 0, Dynamic Modeling of Cooling System Theal Management for Automotive PEM Alition, Trans of KSME(B, Vol. 36(,. 85~9. [] Yu, S.S., Kim, H.S., Lee, S.M., Lee, Y.D. and Ahn, K.Y., 007, Theal Management of Proton Exchange Membrane Fuel Cell, Trans. of Korean Hydrogen and New Energy Society, Vol. 8, No. 3,. 9~300. ACKNOWLEDGMENTS This research was suorted under a Basic Science Research Program (grant number: and Human Resource Training Project for Regional Innovation (Project No. 006A through National Research Foundation of Korea(NRF, funded by Ministry of Edution. REFERENCES [] htt:// [] Greitzer, E.M., 976, "Surge and Rotating Stall in Axial Flow Comressors, Part Ⅰ : Theoretil Comression System Model," ASME Journal of Engineering for Power, Vol. 98,. 90~98. [3] Greitzer, E.M., 976, "Surge and Rotating Stall in Axial Flow Comressors, Part Ⅱ: Exerimental Results and Comarison With Theory," ASME Journal of Engineering for Power, Vol. 98,. 99~. [4] Moore, G. K. and Greitzer, E.M., 986, "A ory of ost-stall transient in axial comression system: Part Ⅰ - develoment of equations," ASME Journal of Engineering for Gas Turbines and Power, Vol. 08,. 68~76. [5] Hansen, K.E., 98, "Exerimental and Theoretil Study of Surge in a Small Centrifugal Comressor," ASME Journal of Fluids Engineering, Vol. 03,. 39~

The Numerical Simulation of Gas Turbine Inlet-Volute Flow Field

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