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H2 refueling assessment of composite storage tank for fuel cell vehicle
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Available online at www.sciencedirect.com
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H2 refueling assessment of composite storage tank for fuel cell vehicle Shitanshu Sapre a, Kapil Pareek a,*, Rupesh Rohan b, Pawan Kumar Singh c a
Centre for Energy and Environment, Malaviya National Institute of Technology, Jaipur, Rajasthan, 302017, India Indian Rubber Manufacturers Research Association, Thane, Maharashtra, 400604, India c Department of Mechanical Engineering, Indian Institute of Technology, Dhanbad, Jharkhand 826004, India b
highlights CFD simulation for refuelling process of Type IV tank is performed. Investigation of impact of refuelling conditions on storage density of Type IV tank. Analysis of heat transfer through tank walls based on heat capacity model. Validation of simulation and experimental results for end temperature and state of charge.
article info
abstract
Article history:
Hydrogen as compressed gas is a promising option for zero-emission fuel cell vehicle. The
Received 4 April 2019
fast and efficient refueling of high pressure hydrogen can provide a convenient platform
Received in revised form
for fuel cell vehicles to compete with conventional gasoline vehicles. This paper reports the
12 June 2019
finding of adiabatic simulation of the refueling process for Type IV tank at nominal working
Accepted 8 July 2019
pressure of 70 MPa with considering the station refueling conditions. The overall heat
Available online 7 August 2019
transfer involved in refueling process was investigated by heat capacity model based on MC method defined by SAE J2601. The simulation results are validated against experi-
Keywords:
mental data of European Commission’s Gas Tank Testing Facility at Joint Research Centre
Hydrogen storage
(GasTef JRC), Netherlands. The results confirmed that end temperature and state of charge
Hydrogen refueling
significantly depends on refueling parameters mainly supply hydrogen temperature and
Type IV tank
filling rate.
Fuel cell vehicle
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Development of clean transportation system is possible with revolutionary automotive technologies such as battery operated electric vehicle and hydrogen fuel cell vehicle (H-FCV) [1]. Particularly, hydrogen (H2) fuel cell vehicle has many benefits such as high energy conversion, efficient drivetrain, and zero
carbon dioxide emission than conventional gasoline vehicles [2e5]. One of the main challenges for commercial acceptance of H-FCV is the development of an efficient storage system which can store 5e7 kg of H2 for minimum drive range of 300 miles, set by United States Department of Energy (US DOE) [6,7]. To meet this criterion, technology of H2 storage in composite tanks at high pressure seems to be promising due to
* Correponding author. E-mail address: [email protected] (K. Pareek). https://doi.org/10.1016/j.ijhydene.2019.07.044 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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their lightweightness, and compact designs of the tanks, as well as maturity of the technology [8]. Type IV and Type III tanks have been used by many automobiles manufactures for H-FCV in recent years. However, still, research is required to reduce H2 filling time, improve gravimetric/volumetric H2 density, enhance onboard efficiency, and reduce the overall system cost [9]. The fast refueling of the storage tank can shorter the filling time but raises the critical issue of heat accumulation inside the tank, mainly caused by local thermodynamic and kinetic conditions during refueling, which depends upon factors including H2 filling rate (g/s), initial H2 temperature, tank volume, and tank materials. The low thermal conductivity of polymer liners and carbon fibers restrict the heat transfer through the tank’s surfaces and lead to a large amount of heat trapped inside the tank causing a high tank temperature, less state of charge (SOC), and degradation of tank materials. Numerous modeling and simulation studies have been carried out on filling process for investigation of filling parameters [10,11], thermodynamic and kinetics of refueling [12,13] in association with experimental studies on refueling process [14]. The experimental studies on Type III and IV tank have been conducted at the European Commission’s Gas Tank Testing Facility at Joint Research Centre (GasTef JRC), Netherlands to investigate the SOC and cooling demand of the tank and the findings are eventually validated by SAE J2601 look-up table approach [15]. Since, most of the simulation studies were based on pressure and temperature boundary conditions for predicting the end temperature and SOC [16,17], due to consideration of adiabatic filling, the reported errors are found up to 6% for end temperature and 4.5e6% for SOC of the tank [15]. The extent of error causes serious issues like low storage density, drive range and degradation of tank materials. One of the possible way to minimize the error is to consider the heat transfer during the filling process. Melideo et al. have considered conjugate heat tranfer from the tank and reported the results with the error is reduced up to 2e3% [18]. Consideration of overall heat transfer with refueling conditions mimimize the error which leads to higher amount of usable H2 inside the tank. In the present work, to understand the impact of refueling conditions on the evolution of vehicle tank temperature, pressure, and density at slow to high filling rates, the refueling process of Type IV 29L tank for filling time of 200 s is investigated, followed by implementation of heat capacity model (HCM) to simulation results to reduce the error. Finally, the simulation results are validated using experimental results of GasTef at JRC. The results confirm that the end temperature and SOC lie in the desired range of SAE J2601 for 40 C and 20 C H2 supply temperature. The end temperature is reduced to 1.3e2.4 C by implementing HCM in simulation results, leading to improvement in SOC of the tank. The end temeprature goes beyond the desired range for H2 supply temprature of 15 C and high fillling rates which also lowers the SOC less than 90%. The results provide insight for better understanding of the impact of refuling conditions to end temparature and SOC of the tank.
Methodology Structural model of the tank A 29 L Type IV tank was modeled using similar physical parameters used in the experimental studies conducted at GasTef JRC (Table 1) [19]. The dimensions taken are unpressurized at a nominal pressure of 70 MPa. The curvatures of both ends of tanks (end bosses) are assumed to be elliptical. The diameter of the injector is considered to be 6 mm, and the injector is either extended 40 mm inside the tank or inserted in gas space. The role of injector’s dimensions including diameter was not considered in the simulation. The physical properties of the tank-materials for simulation are reported elsewhere [14]. The tank contains solid domain for liner and CFRP layer and fluid domain for compressed H2. Both the domains have a hybrid mesh type as shown in Fig. 1. For good convergence and accurate results, finer mesh size was selected. The solid domain was composed of hybrid hexahedral and fluid domain with tetrahedral cells.
Simulation approach The initial pressure and temperature of the tank were considered up to 2 MPa and 20 C, respectively. While the ambient temperature and pressure were assumed up to 20 C and 0.1 MPa, respectively. The station reservoir pressure was 87.5 MPa which is 1.25 times of nominal working pressure of the tank. Constant and adiabatic mass filling rates ranging from 2 g/s to 10 g/s were considered for the filling time of 200 s. The filling direction was assumed to be uniform along the central axis. The mass flow inlet boundary condition was applied at the inlet of the tank. The Redlich Kwong equation of state was used for the evaluation of compressibility effect due to its good agreement with NIST data [20]. The turbulence was predicted by the realizable k-ε model. This k-ε model provides a better understanding of swirling filling, inlet jets, mixing layers and shear filling [21]. Suryan et al. in their study have suggested realizable k- ε model to be computationally economical, robust and more accurate than other models of turbulence [21]. The heat transfer through the out shell of the tank was set to be 6 W/ (m.K) [22]. No slip boundary conditions were specified at inner
Table 1 e Physical parameters of the tank model. S.No.
Parameters
1
Materials Liner Composite shell End bosses
2
H2 capacity (kg) (with fill density 40.22 kg/m3) External length (mm) Internal diameter (mm) External diameter (mm) Vessel mass (kg)
3 4 5 6
Type IV tank (29L) High Density Poly Ethylene Carbon Fibre Reinforced Polymer Stainless Steel 1.16 827 230 279 32.9
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Fig. 1 e Three dimensional model of the Type IV tank.
walls. The effect of buoyancy was considered negligible during the filling. The H2 supply temperature from the station reservoir to vehicle tank is based on the SAE J2601 protocol [23]. The simulation was conducted for transient filling conditions using density based solver of commercially available CFD package, ANSYS Fluent 16.0 [24].
Heat capacity model (HCM) For prediction of end temperature of the compressed gas inside the tank, various studies on the behavior of gas have been conducted [25e27]. By considering actual thermodynamics conditions at the station and vehicle tank, accurate tank filling results can be achieved. For this, Honda R&D Inc. has developed the new refueling method applicable to both communication and non-communication filling stations called MC method. Where MC stands for mathematical construct or total heat capacity of the system which includes combined specific heat and mass capacity of the tank [27,28]. Fig. 2 represents a schematic of a H2 tank with terminology involves in the development of the total heat capacity model. The heat capacity of the tank is a function of energy delivered to the tank in the form of heat and heat transfer from the tank walls. The heat capacity will be the deciding factor for the storage density of compressed H2 tank. Therefore, heat capacity is defined in terms of initial temperature, pressure, and volume of station and vehicle tank [29].
The required parameter for the development of heat capacity model and estimation of final temperature from station and vehicle are tabulated in Table 2. Based on the station and vehicle refueling parameter, total heat transfers, internal energy and amount of hydrogen transported from the refueling station to vehicle tank were calculated. In this study, we have taken an initial pressure of tank 2 MPa, and the initial temperature of tank 20 C which is similar to ambient temperature. We assumed that the vehicle is in a hot soak condition where the environment is hotter than ambient. The hot soak margin of safety to overheat has taken to be þ7.5 C [27]. So the initial temperature (Tinitial ) is the sum of ambient (Tambient ) and hot soak temperature ðDThot Þ as given by Equation (1). Tinitial ¼ Tambient þ DThot
(1)
Before delivering the hydrogen, the station needs initial information about the vehicle tank based on initial temperature and pressure. The initial mass (minitial Þ, density ðrinitial Þ and internal energy (uinital Þ of tank is given by the Equations (2)e(5). minitial ¼ Vvt rinitial ðTinitial ; PSinitial Þ
(2)
mCV ¼ Vvt rtarget
(3)
madd ¼ mcv minital
(4)
Fig. 2 e Schematic representation of the H2 tank during refueling (a) energy transferred in control volume (b) temperature distribution on a section of wall.
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specific heat (Cv Þ, adiabatic temperature (Tadiabatic Þ, mass (mcv ), and heat capacity (MC) for filling time of 200 s given by Equation (8).
Table 2 e Station and vehicle tank parameters. Station
Vehicle
Ambient temperature Hot soak temperature Storage pressure, temperature Tank initial pressure, temperature and density Volume of tank Enthalpy of H2 supplied at storage pressure and temperature Station type (A,B,C,D) Initial internal energy
uinitial ¼ uinital ðTinitial ; Pinitial Þ
(5)
After estimation of initial conditions of the tank, total heat transferred to the control volume (CV) is calculated at supply temperature and pressure of the H2 stream. The average enthalpy delivered to the vehicle tank is calculated using Runga-Kutta approximation method as given by Equation (6). Where.
Tend ¼
mcv Cv Tadiabatic þ MCTinitial ðMC þ mcv Cv Þ
(8)
The HCM provides significant advantage by including station and vehicle tank conditions during filling event by which accurate estimation of refueling results is possible. It follows the all field conditions of SAE J2601 protocol for refueling process. It dynamically measures the refueling parameters and provides faster vehicle refueling for better refueling experience of hydrogen fuel cell vehicles.
Results and discussion Simulation results Fig. 3 shows the temperature rise of H2 inside the tank during the filling process at filling rates ranging from 2 g/s to 10 g/s. The H2 supply temperature is as per SAE J2601 at 40 C, 20 C, 0 C and 15 C, respectively. Notably, a sharp tem-
Tprecooling Expected precooling temperature PstationInit Initial pressure at station PstationFinal Final pressure at station
3 2 PstationFinal PstationInit h Tprecooling ; PStationInit þ h Tprecooling ; PStationInit þ 7 16 4 7þ h¼ 6 5 2 44 3 2 PstationFinal PstationInit PstationFinal PstationInit þ h Tprecooling ; PStationInit þ 2 h Tprecooling ; PStationInit þ 6 7 4 4 6 7þ 4 5 2 3 2 PstationFinal PstationInit PstationFinal PstationInit þ h Tprecooling ; PStationInit þ 3 h Tprecooling ; PStationInit þ 6 7 4 4 6 7þ 4 5 2
(6)
3 2 PstationFinal PstationInit PstationFinal PstationInit þ h Tprecooling ; PStationInit þ 4 h Tprecooling ; PStationInit þ 3 6 7 4 4 6 7 4 5 2
Based on average enthalpy delivered (h) to tank, adiabatic internal energy (Uadaibatic Þ , and adiabatic temperature (Tadiabatic Þ can be obtained for the adiabatic filling condition where no heat is transferred from the system. The combined mass and specific heat capacity of characteristic volume (kJ/K) are calculated using the Equation (7) which represents the total heat absorbed by the wall. MC ¼ C þ A
j Uadaibatic þ g 1 ekDt Uinitial
(7)
The empirical coefficients (C, A, g, e, k, j) used in Equation (7) for calculation of MC are obtained from the confirmation test performed on Type IV tank for fill time longer 3 min [31]. Finally, the end temperature (Tend ) of the gas is determined using tank parameters initial temperature,
perature rise before 50 s followed by a knee point were observed in all four H2 supply temperature. The sharp temperature rise occurs mainly due to the real behavior of compressed H2 as it suddenly expands from the injector, in addition to negative Joule Thomason Coefficient of H2. Beyond the knee point, the temperature rise is moderate. The temperature of gas is inversely proportional to mass content inside the tank. Therefore, in the later stage of filling, the compressed mass of gas increases and temperature rise rate decreases, which is similar to the observation reported by Melideo et al. [16]. It is also observed that the temperature rise is directly proportional to the filing rates. The slow filling rates result less irreversibility and low entropy generation than higher filling rates. Hence, at higher filling rates temperature attained at the
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Fig. 3 e Temperature evolution inside the tank at different H2 supply temperatures of a) ¡40 C, b) ¡20 C, c) 0 C and d) 15 C.
As a consequence of temperature rise, a decrease in the density of compressed gas inside the tank is observed. The SOC is described as a function of density at end temperature (T) and pressure (P) attained during filling to nominal working pressure (70 MPa) and temperature (15 CÞ as given by the Equation (9). SOC ¼
Fig. 4 e SOC at a different filling rate and supply temperatures.
end of filling is higher. The end H2 temperature is above 85 C during filling rates of 8 and 10 g/s (Fig. 3d), which is not recommended by SAE J2601. This phenomenon is more significant with high H2 supply temperature. The end temperature of filling is less in 40 C and 20 C (Fig. 3a and b) than in 0 C and 15 C (Fig. 3c and d).
rðP; TÞ rð70MPa; 15 CÞ
(9)
Therefore, SOC decreases as the end temperature increases. The SOC of the tank at the end of the filling is depicted in Fig. 4. It can be also be seen that at high filling rates SOC decreases, but at low filling rates, it attains an acceptable peak. However, the dependency of SOC on supply temperature can not be ignored because at higher supply temperature it goes below the desired mark of 90%. Cebolla et al. in their experimental studies have also reported that cooling demand for supply H2 is highly required for improving the SOC of the tank [14]. The SOC attained at supply temperature of 0 C is in the desired range of more than 90% but, at 15 C and higher filling rates SOC goes below 90% which directly affects storage density and driving range of the fuel cell vehicle. The density obtained by filling of precooled H2 is very near to target density (rtarget Þ of 40.2 kg/m3, and the SOC varies from 95% to 98%. Although, the SOC could not be achieved the desired mark of 100%, but it is improved by an average of 3e4% at all filling rates for 40 C and 20 C.
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Fig. 5 e End temperature using heat capacity model (MC) at different supply temperature and filling rate (a) ¡40 C (b) ¡20 C (c) 0 C and (d) 15 C.
Heat capacity model results The energy transfers to vehicle tank from refueling station are in the form of average enthalpy supplied to the vehicle tank. Fig. 5 represents the end temperature as a function of different H2 supply temperature and filling rates. The higher end temperature is obtained with higher filling rates due to the fact that compressed gas has lesser time to transfer the generated
heat during refueling. The experimental studies by Liu et al. have also observed the fact that the heat transfer takes more time than the heat generation at higher filling rates [30]. Similarly, at low supply temperature less amount of heat is transferred to the vehicle tank which restricts the end temperature to prescribe limit of 85 C. Around 1.3e2.4 C decrease in temperature is observed at different filling rates and supply temperature compared to
Fig. 6 e SOC achieved using heat capacity model of Type IV tank at different H2 supply temperature and filling rates.
Fig. 7 e Maximum temperature inside the tank at filling rate of 10 g/s and different supply temperature.
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Fig. 8 e Comparison of state of charge obtained by simulation, heat capacity model and experimental (JRC) at different H2 supply temperature of (a) 0 C (b)15 C (c) ¡20 C and (d) ¡40 C.
adiabatic simulation. However, at higher filling rates and ambient H2 supply temperature, end temperature still exceeding the limiting value of 85 C. Fig. 6 represents the influence of filling rates and supply temperature on the SOC using the heat capacity model. The heat transfer from the characteristic volume of tank lowers the end temperature inside the tank which improves the state of charge.
Comparison with experimental results The end temperature reached its peak value at filling rate of 10 g/s. Fig. 7 shows the comparison of maximum temperature attained at the end of refueling for simulation, HCM and JRC experimental data for different supply temperature. It can be noted that the end temperatures of JRC experimental data are the lowest for all four H2 supply temperatures. Due to overall heat transfer consideration in HCM, the end temperatures are falls in the middle of JRC and simulation data. The simulation data due to adiabatic filling are the highest for all for H2 supply temperature. Fig. 8 shows SOC of the tank and calculated errors in SOC estimation. The SOC has shown a strong dependency on end temperature of filling, supply temperature and filling rates. The major contributor in lowering of SOC in the simulation are adiabatic conditions and target of short filling time. The adiabatic conditions do not permit any heat transfer from the
system. For a shorter filling time, fast filling rates accumulate a large amount of heat inside the tank. In both circumstances, the end temperature of refueling increases and eventually lowers the SOC. The HCM considered overall heat transfer through the walls of tanks or heat absorbed by the tank walls, which lead to higher SOC of the tank than SOC obtained from simulation data. On the other hand, in JRC experimental studies, due to transient heat transfer during filling, the estimated SOC is the highest. As per SAE J2601, SOC of 90% and above are in acceptable range. The simulation results for H2 supply temperature of 40 C and 20 C are in the desired range of more than 90% SOC (Fig. 8c and d). While the SOC drops to less than 90% for most of the times for H2 supply temperature of 15 C and very close to the desired range for 0 C at higher flow rates. In order to, better evaluate the accuracy of simulation and heat capacity model standard error and root mean square error (RMSE) are computed. The smaller the standard error is better the model performs. The standard error (x, y) in terms of percentage is presented in Fig. 8 which shows that maximum error lies between 1 and 2% in simulation and it is reduced to less than 1% in heat capacity model which can generally satisfy the precision requirement in engineering calculations [31]. The initial and boundary conditions applied in numerical computation are also partially responsible for such a small deviation in results.
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For more sensible comparisons among the results, RMSE is calculated. The RMSE of simulation found to be in range of 0.857e1.3153 for different supply temperature which is further reduced to less than 1% by implementing heat capcity model for all supply temperature. However, critical thermodynamics behavior of H2 during filling is responsible for the low state of charge. The heat capacity model improves the level of estimation of the state of charge and provides good agreement with the experimental results.
Jaipur for providing the facilities for execution of simulation studies.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.044.
references
Conclusions A Type IV tank of 29L at operating pressure of 70 MPa and filling time of 200 s has been investigated for adiabatic filling at different flow rates. Furthermore, heat capacity model based on MC method of SAE J2601 is utilized. The results of simulation and analytical heat capacity model are compared with the experimental results of GasTef at JRC. Results obtained by refueling simulations show that the temperature inside the tank gets affected by the boundary conditions such as filling rates and supply temperature. The maximum temperature in adiabatic simulation experienced in H2 supply temperature of 15 C at all filling rates which exceeds the limit of safety of 85 C. Although, the end temperature of filling significantly lowers by precooled supply temperature. The higher end temperature also affects the storage density attained at end filling. From this, it may also be concluded that Type IV tank needs precooled H2 supply for higher storage density for automotive applications. The thermodynamic involved in the refueling process was investigated by heat capacity model based on the MC method. The results obtained after considering the in and outflow of heat through the tank leads to lower the final temperature by 1.3e2.4 C at different filling rates. By this maximum temperature stay within limits defined by international standards and SOC has also improved. The outcomes have been compared to experimental results of GasTef at JRC which shows satisfactory agreement. The difference in simulation and experimental results may be due to the selection of boundary conditions, physical models and real behavior of H2. Finally, the simulation approach proved to be suitable for predicting the refueling parameters of compressed H2 system. The deviation in results can be overcome by transient heat transfer during the refueling process which provides good insight of heat losses during the refueling. The transient filling based vehicle tank parameters during refueling will provide better understanding of end temperature of refueling as well as exact estimation of storage density of storage system.
Acknowledgment The authors gratefully acknowledge the support of Early Carrier Research grant (ECR/2016/1001039) form SERB, Department of Science and Technology, Government of India The authors also acknowledge the Centre for Energy and Environment, Malaviya National Institute of Technology,
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
H2 refueling assessment of composite storage tank for fuel cell vehicle Shitanshu Sapre a, Kapil Pareek a,*, Rupesh Rohan b, Pawan Kumar Singh c a
Centre for Energy and Environment, Malaviya National Institute of Technology, Jaipur, Rajasthan, 302017, India Indian Rubber Manufacturers Research Association, Thane, Maharashtra, 400604, India c Department of Mechanical Engineering, Indian Institute of Technology, Dhanbad, Jharkhand 826004, India b
highlights CFD simulation for refuelling process of Type IV tank is performed. Investigation of impact of refuelling conditions on storage density of Type IV tank. Analysis of heat transfer through tank walls based on heat capacity model. Validation of simulation and experimental results for end temperature and state of charge.
article info
abstract
Article history:
Hydrogen as compressed gas is a promising option for zero-emission fuel cell vehicle. The
Received 4 April 2019
fast and efficient refueling of high pressure hydrogen can provide a convenient platform
Received in revised form
for fuel cell vehicles to compete with conventional gasoline vehicles. This paper reports the
12 June 2019
finding of adiabatic simulation of the refueling process for Type IV tank at nominal working
Accepted 8 July 2019
pressure of 70 MPa with considering the station refueling conditions. The overall heat
Available online 7 August 2019
transfer involved in refueling process was investigated by heat capacity model based on MC method defined by SAE J2601. The simulation results are validated against experi-
Keywords:
mental data of European Commission’s Gas Tank Testing Facility at Joint Research Centre
Hydrogen storage
(GasTef JRC), Netherlands. The results confirmed that end temperature and state of charge
Hydrogen refueling
significantly depends on refueling parameters mainly supply hydrogen temperature and
Type IV tank
filling rate.
Fuel cell vehicle
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Development of clean transportation system is possible with revolutionary automotive technologies such as battery operated electric vehicle and hydrogen fuel cell vehicle (H-FCV) [1]. Particularly, hydrogen (H2) fuel cell vehicle has many benefits such as high energy conversion, efficient drivetrain, and zero
carbon dioxide emission than conventional gasoline vehicles [2e5]. One of the main challenges for commercial acceptance of H-FCV is the development of an efficient storage system which can store 5e7 kg of H2 for minimum drive range of 300 miles, set by United States Department of Energy (US DOE) [6,7]. To meet this criterion, technology of H2 storage in composite tanks at high pressure seems to be promising due to
* Correponding author. E-mail address: [email protected] (K. Pareek). https://doi.org/10.1016/j.ijhydene.2019.07.044 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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their lightweightness, and compact designs of the tanks, as well as maturity of the technology [8]. Type IV and Type III tanks have been used by many automobiles manufactures for H-FCV in recent years. However, still, research is required to reduce H2 filling time, improve gravimetric/volumetric H2 density, enhance onboard efficiency, and reduce the overall system cost [9]. The fast refueling of the storage tank can shorter the filling time but raises the critical issue of heat accumulation inside the tank, mainly caused by local thermodynamic and kinetic conditions during refueling, which depends upon factors including H2 filling rate (g/s), initial H2 temperature, tank volume, and tank materials. The low thermal conductivity of polymer liners and carbon fibers restrict the heat transfer through the tank’s surfaces and lead to a large amount of heat trapped inside the tank causing a high tank temperature, less state of charge (SOC), and degradation of tank materials. Numerous modeling and simulation studies have been carried out on filling process for investigation of filling parameters [10,11], thermodynamic and kinetics of refueling [12,13] in association with experimental studies on refueling process [14]. The experimental studies on Type III and IV tank have been conducted at the European Commission’s Gas Tank Testing Facility at Joint Research Centre (GasTef JRC), Netherlands to investigate the SOC and cooling demand of the tank and the findings are eventually validated by SAE J2601 look-up table approach [15]. Since, most of the simulation studies were based on pressure and temperature boundary conditions for predicting the end temperature and SOC [16,17], due to consideration of adiabatic filling, the reported errors are found up to 6% for end temperature and 4.5e6% for SOC of the tank [15]. The extent of error causes serious issues like low storage density, drive range and degradation of tank materials. One of the possible way to minimize the error is to consider the heat transfer during the filling process. Melideo et al. have considered conjugate heat tranfer from the tank and reported the results with the error is reduced up to 2e3% [18]. Consideration of overall heat transfer with refueling conditions mimimize the error which leads to higher amount of usable H2 inside the tank. In the present work, to understand the impact of refueling conditions on the evolution of vehicle tank temperature, pressure, and density at slow to high filling rates, the refueling process of Type IV 29L tank for filling time of 200 s is investigated, followed by implementation of heat capacity model (HCM) to simulation results to reduce the error. Finally, the simulation results are validated using experimental results of GasTef at JRC. The results confirm that the end temperature and SOC lie in the desired range of SAE J2601 for 40 C and 20 C H2 supply temperature. The end temperature is reduced to 1.3e2.4 C by implementing HCM in simulation results, leading to improvement in SOC of the tank. The end temeprature goes beyond the desired range for H2 supply temprature of 15 C and high fillling rates which also lowers the SOC less than 90%. The results provide insight for better understanding of the impact of refuling conditions to end temparature and SOC of the tank.
Methodology Structural model of the tank A 29 L Type IV tank was modeled using similar physical parameters used in the experimental studies conducted at GasTef JRC (Table 1) [19]. The dimensions taken are unpressurized at a nominal pressure of 70 MPa. The curvatures of both ends of tanks (end bosses) are assumed to be elliptical. The diameter of the injector is considered to be 6 mm, and the injector is either extended 40 mm inside the tank or inserted in gas space. The role of injector’s dimensions including diameter was not considered in the simulation. The physical properties of the tank-materials for simulation are reported elsewhere [14]. The tank contains solid domain for liner and CFRP layer and fluid domain for compressed H2. Both the domains have a hybrid mesh type as shown in Fig. 1. For good convergence and accurate results, finer mesh size was selected. The solid domain was composed of hybrid hexahedral and fluid domain with tetrahedral cells.
Simulation approach The initial pressure and temperature of the tank were considered up to 2 MPa and 20 C, respectively. While the ambient temperature and pressure were assumed up to 20 C and 0.1 MPa, respectively. The station reservoir pressure was 87.5 MPa which is 1.25 times of nominal working pressure of the tank. Constant and adiabatic mass filling rates ranging from 2 g/s to 10 g/s were considered for the filling time of 200 s. The filling direction was assumed to be uniform along the central axis. The mass flow inlet boundary condition was applied at the inlet of the tank. The Redlich Kwong equation of state was used for the evaluation of compressibility effect due to its good agreement with NIST data [20]. The turbulence was predicted by the realizable k-ε model. This k-ε model provides a better understanding of swirling filling, inlet jets, mixing layers and shear filling [21]. Suryan et al. in their study have suggested realizable k- ε model to be computationally economical, robust and more accurate than other models of turbulence [21]. The heat transfer through the out shell of the tank was set to be 6 W/ (m.K) [22]. No slip boundary conditions were specified at inner
Table 1 e Physical parameters of the tank model. S.No.
Parameters
1
Materials Liner Composite shell End bosses
2
H2 capacity (kg) (with fill density 40.22 kg/m3) External length (mm) Internal diameter (mm) External diameter (mm) Vessel mass (kg)
3 4 5 6
Type IV tank (29L) High Density Poly Ethylene Carbon Fibre Reinforced Polymer Stainless Steel 1.16 827 230 279 32.9
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Fig. 1 e Three dimensional model of the Type IV tank.
walls. The effect of buoyancy was considered negligible during the filling. The H2 supply temperature from the station reservoir to vehicle tank is based on the SAE J2601 protocol [23]. The simulation was conducted for transient filling conditions using density based solver of commercially available CFD package, ANSYS Fluent 16.0 [24].
Heat capacity model (HCM) For prediction of end temperature of the compressed gas inside the tank, various studies on the behavior of gas have been conducted [25e27]. By considering actual thermodynamics conditions at the station and vehicle tank, accurate tank filling results can be achieved. For this, Honda R&D Inc. has developed the new refueling method applicable to both communication and non-communication filling stations called MC method. Where MC stands for mathematical construct or total heat capacity of the system which includes combined specific heat and mass capacity of the tank [27,28]. Fig. 2 represents a schematic of a H2 tank with terminology involves in the development of the total heat capacity model. The heat capacity of the tank is a function of energy delivered to the tank in the form of heat and heat transfer from the tank walls. The heat capacity will be the deciding factor for the storage density of compressed H2 tank. Therefore, heat capacity is defined in terms of initial temperature, pressure, and volume of station and vehicle tank [29].
The required parameter for the development of heat capacity model and estimation of final temperature from station and vehicle are tabulated in Table 2. Based on the station and vehicle refueling parameter, total heat transfers, internal energy and amount of hydrogen transported from the refueling station to vehicle tank were calculated. In this study, we have taken an initial pressure of tank 2 MPa, and the initial temperature of tank 20 C which is similar to ambient temperature. We assumed that the vehicle is in a hot soak condition where the environment is hotter than ambient. The hot soak margin of safety to overheat has taken to be þ7.5 C [27]. So the initial temperature (Tinitial ) is the sum of ambient (Tambient ) and hot soak temperature ðDThot Þ as given by Equation (1). Tinitial ¼ Tambient þ DThot
(1)
Before delivering the hydrogen, the station needs initial information about the vehicle tank based on initial temperature and pressure. The initial mass (minitial Þ, density ðrinitial Þ and internal energy (uinital Þ of tank is given by the Equations (2)e(5). minitial ¼ Vvt rinitial ðTinitial ; PSinitial Þ
(2)
mCV ¼ Vvt rtarget
(3)
madd ¼ mcv minital
(4)
Fig. 2 e Schematic representation of the H2 tank during refueling (a) energy transferred in control volume (b) temperature distribution on a section of wall.
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specific heat (Cv Þ, adiabatic temperature (Tadiabatic Þ, mass (mcv ), and heat capacity (MC) for filling time of 200 s given by Equation (8).
Table 2 e Station and vehicle tank parameters. Station
Vehicle
Ambient temperature Hot soak temperature Storage pressure, temperature Tank initial pressure, temperature and density Volume of tank Enthalpy of H2 supplied at storage pressure and temperature Station type (A,B,C,D) Initial internal energy
uinitial ¼ uinital ðTinitial ; Pinitial Þ
(5)
After estimation of initial conditions of the tank, total heat transferred to the control volume (CV) is calculated at supply temperature and pressure of the H2 stream. The average enthalpy delivered to the vehicle tank is calculated using Runga-Kutta approximation method as given by Equation (6). Where.
Tend ¼
mcv Cv Tadiabatic þ MCTinitial ðMC þ mcv Cv Þ
(8)
The HCM provides significant advantage by including station and vehicle tank conditions during filling event by which accurate estimation of refueling results is possible. It follows the all field conditions of SAE J2601 protocol for refueling process. It dynamically measures the refueling parameters and provides faster vehicle refueling for better refueling experience of hydrogen fuel cell vehicles.
Results and discussion Simulation results Fig. 3 shows the temperature rise of H2 inside the tank during the filling process at filling rates ranging from 2 g/s to 10 g/s. The H2 supply temperature is as per SAE J2601 at 40 C, 20 C, 0 C and 15 C, respectively. Notably, a sharp tem-
Tprecooling Expected precooling temperature PstationInit Initial pressure at station PstationFinal Final pressure at station
3 2 PstationFinal PstationInit h Tprecooling ; PStationInit þ h Tprecooling ; PStationInit þ 7 16 4 7þ h¼ 6 5 2 44 3 2 PstationFinal PstationInit PstationFinal PstationInit þ h Tprecooling ; PStationInit þ 2 h Tprecooling ; PStationInit þ 6 7 4 4 6 7þ 4 5 2 3 2 PstationFinal PstationInit PstationFinal PstationInit þ h Tprecooling ; PStationInit þ 3 h Tprecooling ; PStationInit þ 6 7 4 4 6 7þ 4 5 2
(6)
3 2 PstationFinal PstationInit PstationFinal PstationInit þ h Tprecooling ; PStationInit þ 4 h Tprecooling ; PStationInit þ 3 6 7 4 4 6 7 4 5 2
Based on average enthalpy delivered (h) to tank, adiabatic internal energy (Uadaibatic Þ , and adiabatic temperature (Tadiabatic Þ can be obtained for the adiabatic filling condition where no heat is transferred from the system. The combined mass and specific heat capacity of characteristic volume (kJ/K) are calculated using the Equation (7) which represents the total heat absorbed by the wall. MC ¼ C þ A
j Uadaibatic þ g 1 ekDt Uinitial
(7)
The empirical coefficients (C, A, g, e, k, j) used in Equation (7) for calculation of MC are obtained from the confirmation test performed on Type IV tank for fill time longer 3 min [31]. Finally, the end temperature (Tend ) of the gas is determined using tank parameters initial temperature,
perature rise before 50 s followed by a knee point were observed in all four H2 supply temperature. The sharp temperature rise occurs mainly due to the real behavior of compressed H2 as it suddenly expands from the injector, in addition to negative Joule Thomason Coefficient of H2. Beyond the knee point, the temperature rise is moderate. The temperature of gas is inversely proportional to mass content inside the tank. Therefore, in the later stage of filling, the compressed mass of gas increases and temperature rise rate decreases, which is similar to the observation reported by Melideo et al. [16]. It is also observed that the temperature rise is directly proportional to the filing rates. The slow filling rates result less irreversibility and low entropy generation than higher filling rates. Hence, at higher filling rates temperature attained at the
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Fig. 3 e Temperature evolution inside the tank at different H2 supply temperatures of a) ¡40 C, b) ¡20 C, c) 0 C and d) 15 C.
As a consequence of temperature rise, a decrease in the density of compressed gas inside the tank is observed. The SOC is described as a function of density at end temperature (T) and pressure (P) attained during filling to nominal working pressure (70 MPa) and temperature (15 CÞ as given by the Equation (9). SOC ¼
Fig. 4 e SOC at a different filling rate and supply temperatures.
end of filling is higher. The end H2 temperature is above 85 C during filling rates of 8 and 10 g/s (Fig. 3d), which is not recommended by SAE J2601. This phenomenon is more significant with high H2 supply temperature. The end temperature of filling is less in 40 C and 20 C (Fig. 3a and b) than in 0 C and 15 C (Fig. 3c and d).
rðP; TÞ rð70MPa; 15 CÞ
(9)
Therefore, SOC decreases as the end temperature increases. The SOC of the tank at the end of the filling is depicted in Fig. 4. It can be also be seen that at high filling rates SOC decreases, but at low filling rates, it attains an acceptable peak. However, the dependency of SOC on supply temperature can not be ignored because at higher supply temperature it goes below the desired mark of 90%. Cebolla et al. in their experimental studies have also reported that cooling demand for supply H2 is highly required for improving the SOC of the tank [14]. The SOC attained at supply temperature of 0 C is in the desired range of more than 90% but, at 15 C and higher filling rates SOC goes below 90% which directly affects storage density and driving range of the fuel cell vehicle. The density obtained by filling of precooled H2 is very near to target density (rtarget Þ of 40.2 kg/m3, and the SOC varies from 95% to 98%. Although, the SOC could not be achieved the desired mark of 100%, but it is improved by an average of 3e4% at all filling rates for 40 C and 20 C.
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Fig. 5 e End temperature using heat capacity model (MC) at different supply temperature and filling rate (a) ¡40 C (b) ¡20 C (c) 0 C and (d) 15 C.
Heat capacity model results The energy transfers to vehicle tank from refueling station are in the form of average enthalpy supplied to the vehicle tank. Fig. 5 represents the end temperature as a function of different H2 supply temperature and filling rates. The higher end temperature is obtained with higher filling rates due to the fact that compressed gas has lesser time to transfer the generated
heat during refueling. The experimental studies by Liu et al. have also observed the fact that the heat transfer takes more time than the heat generation at higher filling rates [30]. Similarly, at low supply temperature less amount of heat is transferred to the vehicle tank which restricts the end temperature to prescribe limit of 85 C. Around 1.3e2.4 C decrease in temperature is observed at different filling rates and supply temperature compared to
Fig. 6 e SOC achieved using heat capacity model of Type IV tank at different H2 supply temperature and filling rates.
Fig. 7 e Maximum temperature inside the tank at filling rate of 10 g/s and different supply temperature.
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Fig. 8 e Comparison of state of charge obtained by simulation, heat capacity model and experimental (JRC) at different H2 supply temperature of (a) 0 C (b)15 C (c) ¡20 C and (d) ¡40 C.
adiabatic simulation. However, at higher filling rates and ambient H2 supply temperature, end temperature still exceeding the limiting value of 85 C. Fig. 6 represents the influence of filling rates and supply temperature on the SOC using the heat capacity model. The heat transfer from the characteristic volume of tank lowers the end temperature inside the tank which improves the state of charge.
Comparison with experimental results The end temperature reached its peak value at filling rate of 10 g/s. Fig. 7 shows the comparison of maximum temperature attained at the end of refueling for simulation, HCM and JRC experimental data for different supply temperature. It can be noted that the end temperatures of JRC experimental data are the lowest for all four H2 supply temperatures. Due to overall heat transfer consideration in HCM, the end temperatures are falls in the middle of JRC and simulation data. The simulation data due to adiabatic filling are the highest for all for H2 supply temperature. Fig. 8 shows SOC of the tank and calculated errors in SOC estimation. The SOC has shown a strong dependency on end temperature of filling, supply temperature and filling rates. The major contributor in lowering of SOC in the simulation are adiabatic conditions and target of short filling time. The adiabatic conditions do not permit any heat transfer from the
system. For a shorter filling time, fast filling rates accumulate a large amount of heat inside the tank. In both circumstances, the end temperature of refueling increases and eventually lowers the SOC. The HCM considered overall heat transfer through the walls of tanks or heat absorbed by the tank walls, which lead to higher SOC of the tank than SOC obtained from simulation data. On the other hand, in JRC experimental studies, due to transient heat transfer during filling, the estimated SOC is the highest. As per SAE J2601, SOC of 90% and above are in acceptable range. The simulation results for H2 supply temperature of 40 C and 20 C are in the desired range of more than 90% SOC (Fig. 8c and d). While the SOC drops to less than 90% for most of the times for H2 supply temperature of 15 C and very close to the desired range for 0 C at higher flow rates. In order to, better evaluate the accuracy of simulation and heat capacity model standard error and root mean square error (RMSE) are computed. The smaller the standard error is better the model performs. The standard error (x, y) in terms of percentage is presented in Fig. 8 which shows that maximum error lies between 1 and 2% in simulation and it is reduced to less than 1% in heat capacity model which can generally satisfy the precision requirement in engineering calculations [31]. The initial and boundary conditions applied in numerical computation are also partially responsible for such a small deviation in results.
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For more sensible comparisons among the results, RMSE is calculated. The RMSE of simulation found to be in range of 0.857e1.3153 for different supply temperature which is further reduced to less than 1% by implementing heat capcity model for all supply temperature. However, critical thermodynamics behavior of H2 during filling is responsible for the low state of charge. The heat capacity model improves the level of estimation of the state of charge and provides good agreement with the experimental results.
Jaipur for providing the facilities for execution of simulation studies.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.07.044.
references
Conclusions A Type IV tank of 29L at operating pressure of 70 MPa and filling time of 200 s has been investigated for adiabatic filling at different flow rates. Furthermore, heat capacity model based on MC method of SAE J2601 is utilized. The results of simulation and analytical heat capacity model are compared with the experimental results of GasTef at JRC. Results obtained by refueling simulations show that the temperature inside the tank gets affected by the boundary conditions such as filling rates and supply temperature. The maximum temperature in adiabatic simulation experienced in H2 supply temperature of 15 C at all filling rates which exceeds the limit of safety of 85 C. Although, the end temperature of filling significantly lowers by precooled supply temperature. The higher end temperature also affects the storage density attained at end filling. From this, it may also be concluded that Type IV tank needs precooled H2 supply for higher storage density for automotive applications. The thermodynamic involved in the refueling process was investigated by heat capacity model based on the MC method. The results obtained after considering the in and outflow of heat through the tank leads to lower the final temperature by 1.3e2.4 C at different filling rates. By this maximum temperature stay within limits defined by international standards and SOC has also improved. The outcomes have been compared to experimental results of GasTef at JRC which shows satisfactory agreement. The difference in simulation and experimental results may be due to the selection of boundary conditions, physical models and real behavior of H2. Finally, the simulation approach proved to be suitable for predicting the refueling parameters of compressed H2 system. The deviation in results can be overcome by transient heat transfer during the refueling process which provides good insight of heat losses during the refueling. The transient filling based vehicle tank parameters during refueling will provide better understanding of end temperature of refueling as well as exact estimation of storage density of storage system.
Acknowledgment The authors gratefully acknowledge the support of Early Carrier Research grant (ECR/2016/1001039) form SERB, Department of Science and Technology, Government of India The authors also acknowledge the Centre for Energy and Environment, Malaviya National Institute of Technology,
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