Solid-state hydrogen storage system design

Solid-state hydrogen storage system design

4 Solid-state hydrogen storage system design D. E. D E D R I C K, Sandia National Laboratories, USA 4.1 Introduction Efficient hydrogen storage is ...

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4 Solid-state hydrogen storage system design D. E. D E D R I C K, Sandia National Laboratories, USA

4.1

Introduction

Efficient hydrogen storage is a significant challenge inhibiting the use of hydrogen as a primary energy carrier. In general, three main categories of energy storage applications require the use of energy-dense hydrogen storage materials; transportation, portable auxiliary power, and personal electronics. Each of these applications requires relatively high energy and power density capability. For transportation applications, the US Department of Energy (US DoE) has specified challenging volumetric and gravimetric energy density targets for technology acceptance (3 kW h/kg and 2.7 kW h/l) [1]. Portable power systems, including man-portable and auxiliary power systems, need to compete with the power densities of other technologies such as gas turbine and lithium-based battery technologies to be viable (0.100–1.5 kW h/kg and kW h/l) [2]. Personal electronics power and energy density requirements are continually increasing as portable computation demands increase. Lithium ion batteries are the current technology baseline and are characterized by energy densities of 0.2 kW h/kg and 0.3 kW h/l. This chapter will focus on automotive applications as these will likely have the most extensive impact on consumer-scale energy storage technologies. Automotive systems are arguably the most challenging energy storage application since the low cost and high performance of current fossil fuel technologies cause consumers to expect similarly plentiful energy and power from new transportation technologies. For automotive systems, current hydrogen storage technologies (liquid and compressed gas) fall short in terms of energy density and cost and do not provide a clear path for efficiency improvement and the resulting increase in energy density (Fig. 4.1). Hydrogen storage technologies that are efficient, low cost, and robust must be developed to enable the use of hydrogen fuel in transportation applications. Solid-state hydrogen storage solutions are theoretically able to store more hydrogen per unit volume than liquid or solid storage systems [3]. Given this potential for high energy density reversible 82

Solid-state hydrogen storage system design

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System volumetric capacity (g/l)

100

80

2015 US DoE targets

60 2010 US DoE targets Chemical hydride

40

Complex hydride

20

Liquid hydrogen

700 bar compressed

300 bar compressed

0 0

2

4 6 8 System gravimetric capacity (wt%)

10

4.1 The current status of automotive hydrogen storage technologies indicates that significant performance improvements must be realized prior to attaining 2010 and 2015 goals (adapted from [1]).

hydrogen storage, significant effort has been applied to develop storage solutions based on this technology. Some automotive-scale solid-state hydrogen storage systems have been developed and demonstrated. Sandia National Laboratories and General Motors Corporation developed automotive-scale hydrogen storage systems based on metal hydrides [4]. United Technologies Research Corporation was funded by the US DoE to demonstrate a large scale (~0.5 kg H2) hydrogen storage tank based on sodium alanates [5]. A few larger-scale demonstrations exist including a fuel-cell powered Class 212 German submarine [6, 7] and a Snowcat [8]. The submarine, built by Howaldtswerke–Deutsche Werft (HDW), utilizes Fe–Ti metal hydride tanks capable of storing a tonne of hydrogen. The storage of hydrogen in solid-solution materials is characterized by endothermic and exothermic phase change reactions. The thermodynamic nature of these reactions requires the full characterization of influential material properties to enable the optimization of heat and mass transfer within the system. Additionally, the thermodynamic nature of the materials will define the containment technologies required to withstand the operational pressures and temperatures. In addition to optimizing the hydrogen uptake and delivery performance of the system, safe operation must be ensured through hazard-minimizing design. Since some solid-solution hydrogen storage materials tend to be highly reactive in nature, extra care must be taken to ensure that the system user is not exposed to an unnecessary amount of risk. The system designer must fully understand the risks involved in order to design appropriately safe systems and controls.

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Solid-state hydrogen storage

4.2

The behavior of solid-state hydrogen storage materials in systems

Metrics for system performance are typically described in terms of gravimetric and volumetric energy density. These two quantities define the mass and volume required within the application to provide sufficient hydrogen storage and delivery. These two performance metrics are directly influenced by hydrogen storage material properties including thermodynamic, physical, and chemical/kinetic. Additional influences on energy densities include, but not limited to, properties of the containment material, available energy, operational environment, and required hydrogen delivery rates. Figure 4.2 describes the hierarchical nature of the material properties’ influence on system energy densities. The thermodynamic characteristic (∆H – enthalpy of reaction) of the phase change material has a two-fold influence on the system. First, this property defines the amount of required heat transfer during hydrogen uptake and release. Secondly, the containment structure must be constructed based on the thermodynamic nature of the material since the enthalpy of reaction is defined by the equilibrium pressures and temperatures. The physical properties of the storage material, such as thermal conductivity, are highly influential on the design of a solid-state hydrogen storage system as these define the temporally varying rate at which heat can be removed or added to the system to support the phase change reaction. The chemical/ kinetic properties of the hydrogen storage material also influence the system energy densities by defining the operational conditions, such as temperature, and quantities of reacting material required to supply the application-specific

Thermodynamic properties

Physical properties

Chemical/kinetic properties

∆H

Pressure

Temperature

Containment technology, e.g. metal, composite

Solid density

Geometry, defined by heat transfer

Thermal properties

Inefficiencies and losses

Uptake and release rates

Gravimetric and volumetric system energy density

4.2 The hierarchical influence of various hydrogen storage material properties on the system characteristics and efficiencies.

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hydrogen flow demands. Other metrics, such as cost, are also important and must be considered based on the target application. Although less quantifiable, the safety performance of the system can be equally or exceedingly important to the system design. For consumer applications, the hazards associated with the use and operation of systems must be minimized to meet levels of acceptable risk. The optimum system design for any hydrogen storage application will vary based on acceptable cost, operational environment, and the safety and performance demands.

4.3

Thermodynamic properties of hydrogen storage materials

Reversible hydrogen storage materials are characterized by endothermic decomposition and exothermic recombination. The enthalpy of the reversible reaction defines the quantity of heat that will be moved in and out of the system during one complete fueling cycle. The pressure and temperature operating regime of an on-board automotive hydrogen storage system limits the possible variation of the enthalpy of reaction regardless of the material chosen. In Fig. 4.3, equilibrium pressures are plotted as a function of temperature for various hydrogen storage materials. The boxed area represents the approximate regime appropriate for automotive applications based on

Pressure (atm)

100

10

1

0.1 1.5

2.0

2.5 3.0 1000/T (1/K) Thermodynamic data include: V LaNi4.6Mn0.4 TiFe MmNi4.5Mn0.5 LaNi5 TiMn1.5 LaNi4.7Al0.3 ZrFe1.5Cr0.5 MmNi4.5Al0.5 MmNi3.5Co0.7Al0.8 CaNi5 MmNi4.2Co0.2Mn0.3Al0.3 Ca0.7Mm0.3Ni5 LaNi4.8Sn0.2 TiFe0.8Ni0.2 TiV0.62Mn1.5 TiFe0.9Mn0.1 Zr0.8Ti0.2MnFe

3.5

4.0

ZrMn2 ZrCr2 TiCo Mg2Ni ZrNi NaAIH4 Na3AIH6

4.3 Van’t Hoff data of a variety of hydrogen storage materials. The shaded area describes approximate area appropriate for automotive applications.

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system temperature and delivery pressure. For on-board hydrogen storage, the boundaries lie approximately between 20 and 120 °C and 1 and 10 atm. The plateau pressure expression for any of these materials is described as follows:

ln( p H 2 ) = – 1 ( ∆H – T∆S ) = – ∆H + ∆S RT RT R

[4.1]

where p H 2 is the plateau pressure, R is the gas constant, T is the temperature, ∆H is the enthalpy of reaction, and ∆S is the change in entropy. The change in entropy is approximately equivalent for any hydride solid as the hydrogen gas has lost nearly all its translational degrees of freedom when bonding to the metal species. Since the intercept of the plateau pressure expression is ∆S/R, hydride solids that operate within a limited pressure and temperature regime have similar enthalpies of reaction (∆H). If higher temperature fuel cells or off-board refueling storage systems are utilized, this region could expand, but for our applications of interest, these approximate boundaries on thermodynamics are appropriate. Upon observation of the associated enthalpies of reaction within this regime, approximately 40 kJ per mol of hydrogen sorbed must be accommodated during each fueling and delivery cycle. The most the enthalpy of reaction can vary in this automotive-specific regime is approximately ±10 kJ/mol. Considering a 5 kg hydrogen system, approximately 100 MJ of heat must be moved into and out of the bed. Due to the refueling goals of 2 kg H2/min [1], absorption is the most demanding energy transfer process, requiring a cooling rate of nearly 0.7 MW on average. If you consider that the natural absorption of hydrogen into these materials is typically nonlinear, then you can assume that up to three times that heat rate must be removed from the bed. This amount of heat in a compact automotive system requires careful engineering and a quantitative understanding of all associated heat transfer properties. From an infrastructure point of view, each hydrogen refueling station that is capable of refueling hydride-containing storage systems must include the necessary industrial cooling hardware to remove this quantity of heat. For example, a station capable of refueling 100 automobiles per day must dissipate and/or utilize up to 10 000 MJ of low-quality heat per day at an average rate of ~300 kW over a 10 hour operational period. Significant engineering design is required to enable an efficient, automotive hydrogen refueling station that is capable of refueling hydride-containing storage systems in a cost-effective manner.

4.4

Thermal properties of hydrogen storage materials

So far in this chapter we have described the sorption of hydrogen in reversible hydrogen storage materials as being a significant challenge in heat transfer.

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In this section, we will explore the modes of heat transfer that the system designer must learn to accommodate to design an efficient hydrogen storage system. The thermal properties of hydrogen storage materials directly influence the efficiency, performance, and cost of hydrogen storage systems based on complex metal hydride materials. Thermal design is the most challenging aspect of hydride-based automotive hydrogen storage system design due to the thermodynamic characteristics of typical hydrogen storage materials operating in the pressure and temperature regime of conventional polymer electrolyte membrane (PEM) fuel cells. Hydrogen uptake is the most thermally challenging operational state as the reaction is exothermic and on-board automotive refueling scenarios require rapid refueling rates. As discussed in the introductory section, during rapid hydrogen uptake of an automotive scale system, nearly 0.7 MW of cooling may be required to maintain a constant system temperature. These intensive heat flux conditions demand a detailed understanding and optimization of the thermal transfer properties present within a hydrogen storage system. High thermal conductivity of the metal hydride and low thermal resistance at the wall mitigate the effects of thermally limited reactions, enable the use of larger containment vessel diameters, and reduce the number of parts required for efficient operation. Since thermal resistance at the vessel wall can generally be accommodated during exothermic hydrogen absorption with the temperature and flow characteristics of the coolant fluid, the effective thermal conductivity of the system is of specific interest as this can limit the rate of the absorption reaction most significantly.

4.4.1

Thermal conductivity of a hydrogen storage packed particle bed

Solid hydrogen storage materials generally take the form of packed particles or porous structures immersed in hydrogen gas. The effective thermal conductivity of a packed particle bed can be described in terms of three distinct gas pressure regimes: low, intermediate, and high. The low-pressure regime is characterized by molecular or rarified gas transport in the interparticle space and typically exists at pressures much less than 1 atm owing to the small characteristic distance between each solid particle. The thermal properties of the bed are relatively invariant with gas pressure in the lowpressure regime as the gas has a nearly insignificant role in heat transfer. The intermediate-pressure regime is characterized by transition between molecular and continuum transport within the void spaces. In this pressure regime, thermal properties are significantly affected by the gas pressure as the gas plays an important role in the bed heat transfer. The high-pressure regime is characterized by continuum gas transport within the void space and thus

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pressure has no further influence on the thermal properties of the bed. The inflection point between transition and continuum transport is called the critical pressure. The boundaries of each of these regimes depend on the characteristic void space or pore size of the bed. The intermediate- and highpressure regimes above 1 atm are of specific interest for automotive hydridebased hydrogen storage systems. There are a total of six modes of heat transfer within the bulk metal hydride packed particle structure as described in Oi et al. [9]: 1. 2. 3. 4. 5. 6.

Heat Heat Heat Heat Heat Heat

conduction at the particle contacts. conduction through a thin hydrogen ‘film’. radiation between the particles. conduction through the particles. conduction through hydrogen in larger void spaces. radiation between vacant spaces.

Models have been constructed describing each of these heat transfer mechanisms. Yagi and Kunii [10] developed generalized resistance models for packed beds which others adapted for application to metal hydride beds [9, 11–13]. For lower and moderate temperature applications of these models, radiation heat transfer can be neglected [9, 11, 12, 14, 15]. In general, the resistance model of effective thermal conductivity of a packed metal hydride bed can be described as:

1 – φv  K eff = K H 2 φv +  K   H 2 γ+ 2     3  K p   

[4.2]

where K H 2 is the thermal conductivity of the hydrogen gas, Kp is the thermal conductivity of the metal hydride solid, φ v is the void fraction of the bed. The term γ is a constant defined as the ratio of the effective length of the solid particle relating to conduction to the average hydride particle diameter. This ratio is difficult to quantify and is a function of particle contact angle, shape, and roughness. For metal hydrides, typical values fall between 0.01 and 0.1 Figure 5 in [16]. Using this resistance model, Oi et al. [9] predicted the thermal conductivity of a typical metal hydride bed as a function of metal hydride particle thermal conductivity for various porosities. In general, resistance models predict that metal hydride beds exhibit thermal conductivities between 0.5 and 2 W/m K for typical porosities and particle thermal conductivities. Resistance models are useful for making quick estimations of packed bed effective thermal conductivities, yet they tend to be highly subjective and, in practice, lack accuracy. Other types of models have been developed including numeric methods [17]. The most useful and accurate model was developed

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by Zehner, Bauer, and Schlünder and reported in Tsotsas and Martin [18] and adapted by Rodriguez Sanchez et al. [19] which consists of a unit cell of two particle halves of equivalent shape encased in a cylinder of fluid (hydrogen). The model presented here neglects radiation and assumes spherical particles. The effective thermal conductivity of the bed is calculated by:

K eff = K H 2

1  (1– 1 – Ψ ) ⋅ Ψ ⋅   Ψ–1+ 1 KH2      + 1 – Ψ ⋅ [ϕ ⋅ K p + K pgp ⋅ (1 – ϕ )]

[4.3]

In this expression, K H 2 is the thermal conductivity of hydrogen, Ψ is the void fraction, φ is a flattening coefficient which defines contact quality, Kp is the particle thermal conductivity, and Kpgp is an expression for the thermal conductivity at the particle–gas–particle interface and includes particle diameter Dp. The effective thermal conductivity is highly influenced by Kpgp as it describes the contribution of the fluid to the particle thermal contact quality. For a complete description of the model refer to Rodriguez Sanchez et al. [19]. Using the Rodriguez Sanchez model, effective thermal conductivities (Kth) can be calculated as a function of hydrogen pressure, particle thermal conductivity, particle diameter, and void fraction (Fig. 4.4). Ranges selected for these variables were chosen based on typical metal hydride packed bed characteristics. Other model inputs include the deformation factor and contact flattening coefficient of the particle – indicating the area of contact between the particles. These factors are difficult to calculate or predict and are usually estimated from experimental results. Overall, the effective conductivity of the bed is less sensitive to the deformation factor and contact flattening coefficient compared with the other variables. For these calculations, typical values of these coefficients were used as in Rodriguez Sanchez et al. [19]. Effective thermal conductivity as a function of hydrogen pressure and void fraction is described in Fig. 4.4(a). In general, higher pressures and lower void fractions lead to higher conductivities up to the critical pressure. The critical pressure for these calculated geometries is near 1 × 108 Pa as observed by a reduction in rate of effective thermal conductivity increase with pressure. The effective thermal conductivity as a function of particle thermal conductivity is shown in Fig. 4.4(b). In general, particle thermal conductivities above 50 W/mK do not significantly influence bed effective thermal conductivity due to dominance in this regime by porosity and the particle to particle thermal contact. The effective thermal conductivity as a function of particle diameter is presented in Fig. 4.4(c). Larger particle diameters lead to higher effective conductivities due to the reduction of particle to

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Solid-state hydrogen storage 4

Dp = 15 µm Kp = 100 W/m K 4

2

0 1×105 1×106 1×107 1×108 Hydrogen pressure (Pa) (a)

Effective K th (W/m K)

Effective K th (W/m K)

6

2

Dp = 15 µm pH2 = 10.3 MPa 0 100 200 Particle Kth (W/m K) (b)

Effective K th (W/m K)

6

pH2 = 10.3 MPa Kp = 100 W/m K 4 Void fraction = 0.3 Void fraction = 0.4 Void fraction = 0.5

2

0 1×10–6 1×10–5 1×10–4 Particle diameter (m) (c)

4.4 Calculated effective thermal conductivity of a packed particle bed as a function of void fraction, hydrogen pressure, and particle characteristics.

particle contacts and an increase in continuum-regime gas transport relative to transition-regime gas transport. The calculations presented here are consistent with many models and measurements described in the literature [11–14, 17, 20, 21]. Models and measurements indicate that the effective thermal conductivity of particles loaded in a packed bed is generally limited to values below ~5 W/m K, even with significant increases in the particle thermal conductivity (Fig. 4.4(b)). More clever methods must be employed to enhance thermal conductivity to levels above 5 W/m K. Additionally, the models discussed above have been developed for distinct particles typical of classic/interstitial hydride materials. These classic/interstitial beds are generally characterized as unsintered powders while complex hydrides, such as sodium alanates, can become porous sintered solids as seen in Fig. 4.5. Application of packed particle models have not been directly applied to sintered solid materials.

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10 µm 2000× (a)

(b)

4.5 (a) The sintered solid resulting from hydrogen cycling of sodium alanates. (b) A scanning electron microscope (SEM) image of the sintered solid.

Measurements at Sandia National Laboratories, California, of sodium alanate sintered solids have demonstrated that the effective conductivities are similar to packed particle beds [22]. The thermal conductivities of stoichiometric sodium alanates compacted at 40% of the single crystal density were found to vary between 0.5 and 1.0 W/m K depending on cycle, hydrogen content, and gas pressure. Effective thermal conductivities as a function of gas pressure for fully cycled stoichiometric sodium alanates are shown in Fig. 4.6. As with packed particle beds, low thermal conductivities of complex hydrides such as sodium alanates present an engineering challenge when integrated within a system. Although the physical appearance of a sintered sodium alanate particle is dissimilar to a bed of close packed spheres, the thermal transport behaviors of both cases are similar. Both cases contain a characteristic thermal path length that influences the thermal transport within the bed when compared to the mean free path of the gas. Additionally, the sintered sodium alanate bed can be modeled as spheres (or islands) of material with improved thermal contact between each sphere or ‘island’. Given the similarities in fundamental mechanisms of heat transport of each case, we can assume that many of the same thermal conductivity enhancement strategies will work for both a packed particle and a sintered solid bed. Using our understanding of heat transfer within metal hydride beds, the following section will explore heat transfer enhancement mechanisms that can be exploited to improve the overall performance of metal hydride systems.

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Solid-state hydrogen storage NaH + Al (H2)

1.00

NaH + AI 0.80

Kth (W/m K)

Na3AIH6 0.60

NaAIH4

0.40 Characteristic equations NaH + Al: Kth = 0.068 ln(PH2) + 0.71 Na3AIH6: Kth = 0.061 ln(PH2) + 0.50 NaAIH4: Kth = 0.037 ln(PH2) + 0.51

0.20

0.00 1

10 Pressure (atm)

100

4.6 Effective thermal conductivity (Keff) as a function of gas pressure for fully cycled stoichiometric sodium alanates [22].

4.4.2

Thermal properties enhancement methods for solid storage materials

Several authors have described the importance of thermal conductivity enhancement to enable improved performance of metal hydride-based hydrogen storage systems [23–25]. Lower thermal resistances enable the use of larger thermal length scales and an increased rate of hydrogen uptake. Gas flow Flow through the porous bed enhances the radial effective or apparent thermal conductivity of packed beds [10, 26]. Winterberg and Tsotsas [26] developed models and heat transfer coefficients for packed spherical particle reactors that are invariant with the bed-to-particle diameter ratio. The radial effective thermal conductivity is defined as the summation of the thermal transport of the packed bed and the thermal dispersion caused by fluid flow, or: Kbed + flow = Kbed + Kflow

= K bed + X1 ⋅ Pe 0 ⋅

uc ⋅ f ( r ) ⋅ K gas uave

[4.4]

The coefficient X1 is a correlation function that describes the rate of increase of the effective thermal conductivity with flow velocity, Pe0 is the Péclet number, which describes the contribution of forced convection relative to hydrogen heat conduction, uc is the velocity at the centerline of the bed, uave is the average velocity, f(r) describes the radial variation in dispersion, and

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Kgas is the thermal conductivity of the fluid (hydrogen). In many cases, uc and uave are assumed to be near equivalent due to the large vessel-to-particle diameter ratios. The radial dispersion variation f(r) is assumed to be unity for similar reasons. For a complete description of the model and correlations, refer to Winterberg and Tsotsas [26]. Although the model is promoted to be invariable to the vessel diameter to particle diameter ratio, the correlations were built using experimental data with ratios from 5.5 to 65. High conductivity coatings Researchers have experimented with enhancing heat transport by coating metal hydride pellets with high-conductivity, high-ductility metals. Copper has been frequently used for this purpose [27–30]. Kim et al. [28] reported conductivities as high as 9 W/m K for higher packing densities of 25% by weight copper-coated LaNi5 powders (thermal conductivity of uncoated powders typically less than 1 W/m K). This corresponds well with results published by Kurosaki et al. [27], which describes effective thermal conductivity of LaNi5 as a function of copper mass percent. This approach may not be the most favorable method of thermal properties enhancement as significant copper mass is required to attain moderate increases in thermal conductivity. Additionally, gas and solid mass transport may be adversely affected when this technology is applied to complex hydrides. High thermal conductivity composite alloying Various high conductivity materials have been alloyed with metal hydrides to form enhanced heat transport composite materials. Eaton et al. [31] experimented with various alloyed metal additives including copper, aluminum, lead, and lead–tin. The samples were alloyed at elevated temperature (200– 600 °C) and cycled. In many samples, cycling resulted in the separation and fracture of the alloy and thus a reduction in composite thermal conductivity. Sintered aluminum structures of 20% solid fraction have been integrated with LaNi5 hydride materials with success, resulting in effective thermal conductivities of 10–33 W/m K [32–34]. Temperatures required for this process and added mass and volume may exclude application to some complex hydrides. High thermal conductivity structures High thermal conductivity structures have been used to enhance thermal conductivity including copper wire matrices [35], periodic plates [25], nickel foams [30], and aluminum foams [36]. Nagel et al. [35] designed and integrated a 90% porous corrugated copper wire matrix with a MmMi4.46Al0.54 hydride

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bed and improved the overall conductivity by a modest 15%. Aluminum and nickel foams have been used with some success [30]. Practically, metal foams tend to be costly and are a significant challenge to integrate with metal hydrides without the creation of high void fractions. High thermal conductivity additives Expanded natural graphite fibers have been described as appropriate additives due to their characteristic high thermal conductivity, porosity, dispersibility, and low cost [19,38,39]. Expanded natural graphite (ENG) fibers are produced from natural graphite that is treated in sulfuric acid and heated to high temperatures, thereby expanding to very fine flakes. The thermal properties of ENG fibers are highly anisotropic; in-plane thermal conductivities in excess of 500 W/m K with through-plane conductivities of less than 5 W/m K. Effective thermal conductivities as high as 20 W/m K are theoretically attainable for classic metal hydrides combined with 10% volumetric fraction of ENG fibers assuming 100% fiber alignment with the direction of heat transfer. Experimentally, composite thermal conductivities as high as 10 W/ m K were measured with mass fractions as low as 5% although these results were obtained with very small sample sizes and presented with significant measurement scatter [19]. The measurement of larger composite samples may result in somewhat lower thermal conductivities (as much as half) due to randomization of the ENG fiber orientation. Many options exist for the enhancement of the thermal properties of reversible hydrogen storage materials. The optimum method must be chosen based on the optimization of the total system energy densities, hydrogen uptake and delivery performance, and cost targets.

4.5

System heat exchange design

The combined influence of the thermodynamic, chemical/kinetic, physical properties, and application environment define the heat exchange requirements for the entire system. With this information the system designer is able to evaluate various methods for heat transfer within the system to obtain an overall optimized design based on the end-use requirements.

4.5.1

Absorption heat exchange

The most significant challenge arises from removing the heat of absorption as discussed in earlier sections. Many standard heat exchanger designs are viable; however, the optimal solution depends on the application. Heat exchange systems include but are not limited to the designs listed below. In these examples, ‘vessel’ refers to the hydride-containing pressure vessel.

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Internally finned vessels: Fins enhance heat transfer out of the containment vessel and thus allow for more monolithic tank designs. Monolithic tank designs are generally less complex systems, which is attractive for automotive applications. Attaining adequate heat transfer without adding excessive weight and volume can be challenging with finned vessels. Vessels in cross or axial flow: Vessels distributed within a fluid flow field can improve system heat transfer. The distributed nature of the vessels can result in increased system complexity and a reduction in volumetric and gravimetric efficiency. Internal flow cooled vessel: Designs that incorporate internal flow provide similar benefits of finned vessels by allowing for monolithic tank designs. Some clever engineering may be required to allow for penetration of the containment vessel with the cooling plumbing. Fluidized beds: Fluidization of the bed can result in enhanced heat and mass transfer performance. Unfortunately, many fluidized bed designs tend to be voluminous, which reduces overall storage efficiency.

This is not a comprehensive list of examples, and the designer may determine that a hybrid, or combinatorial, solution is appropriate, depending on the application.

4.5.2

Desorption heat exchange

Although less thermally rigorous, delivery of hydrogen to the fuel cell requires the transfer of heat into the bed to support the endothermic reaction. If the quality of available waste heat in the system from a fuel cell stack or other energy conversion device is higher than the quality of heat required to support the endothermic reaction, this heat can be transferred to the hydride at low cost. On the other hand, if the required quality of heat is greater than that of the available waste stream, then auxiliary heating will be needed to support the endothermic reaction. Depending on the heat of reaction, this can contribute a significant loss to the overall system efficiency. For example a ~20% loss to efficiency can be expected for a 40 kJ/mol material with an 80% efficient auxiliary heater. If resistive heating is used in conjunction with a fuel cell, losses can begin to overwhelm the benefit of using phase-change materials in place of lower energy density solutions such as compressed gas. It should be noted that the quality of the waste heat must be significantly higher than that of the required heat to enable heat transfer. Heat flux is defined as HA∆T, where H is the heat transfer coefficient in W/m2 K, A is the active surface area in m2, ∆T is the temperature difference in K between the available and required temperatures. Thus, if ∆T is very small, the heat flux is similarly minimized and will not allow for sufficient hydrogen fuel delivery to the conversion device.

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Identification of the most efficient design for a combination of material properties, fuel demand requirements, and operating environment parameters, requires the consideration of all operational modes including hydrogen refueling and hydrogen delivery. No single storage system design can be recommended without a detailed understanding of the material properties and application requirements.

4.6

Safe systems design

Although hydrogen uptake and delivery properties of storage systems are critical to the success of solid-state hydrogen storage systems, operator and consumer safety is of highest importance of all engineering-related properties. Safety properties are defined as the potential health and safety hazards realized during both abnormal scenarios (such as accidents) and also normal operation over the entire life cycle of the system. These properties have direct implications on the health and well-being of the vehicle worker and operator. Considering the automotive platform, approximately 12 million new motor vehicles are produced every year within the United States alone. Owing to the technology usage density and diverse user demographic, robust and safe systems must be designed and demonstrated. By definition, high-efficiency energy storage systems are capable of releasing significant amounts of the stored energy and present a potential hazard. Although it is improbable that this hazard can be eliminated entirely, engineering controls and appropriate protocols can minimize the potential hazard to a tolerable level. In the case of metal hydrides, the reaction processes with contaminants, especially air and water vapor, must be understood to enable the implementation of engineering controls and the definition of appropriate protocols. Another safety-related property, volumetric expansion of the solid materials during normal hydrogen sorption, is also an important property that must be considered during the design of solid-state hydrogen storage systems.

4.6.1

Volume expansion and decrepitation

Volumetric expansion-induced pressure due to expansion of the storage material during hydrogen sorption is a classic metal hydride safety system issue. This pressure originates from expansion of the crystal structure during hydrogen sorption and can be exacerbated by a physical process called decrepitation. Decrepitation is the systematic physical breakdown of larger particles into smaller fines due to high strain induced by hydrogen cycling [39]. Following decrepitation, the fines tend to settle under the influence of gravity and/or vibration, thus causing a density gradient to form. Upon subsequent cycling, these high-density sections of the bed expand. Owing to a lack of void space in these regions, the expansion is directed outward, exerting large forces on

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the wall of the containment vessel. In extreme cases, structural failures of the containment vessel can occur, causing a release of hydrogen gas and exposure of the reactive storage materials to the surrounding environment. Every hydrogen sorption material will behave differently with respect to expansion forces. Some materials will not establish large density gradients due to sintering, while others will experience significant decrepitation events with just a few cycles. This property disparity requires the systems designer to characterize each material as a function of sorption cycle by monitoring the strain in the containment vessel wall. Nasako and colleagues have described methods for measuring stress and strain in a hydride-containing vessel wall [40]. The strain measurement of a hydride-containing vessel wall can be difficult for two reasons: (1) the temperature excursions intrinsic to the uptake and release of hydrogen can cause significant strain measurement errors, and (2) the strain induced by the gas pressure can complicate the measurement of the mechanical expansion-induced strain. In practice, these difficulties can be overcome with clever instrumentation, including temperature compensation and gas pressure-induced signal calibration.

4.6.2

Contamination of metal hydrides – oxygen and water reactivity

Another important set of safety properties can be described as the health, safety, and performance effects of the products, pathways, and rates of reactions occurring between hydrogen storage materials and their potential contaminants. The automotive platform is a dynamic hydrogen storage application subject to a highly variable operating environment, uncertain hydrogen refueling quality control, and accidents, such as collisions and externally fueled fires. The nature of this duty cycle forces the system designer to consider that the storage materials or containment structures may be somehow compromised during the lifetime of the hydrogen storage system. In an accident or contaminated refueling scenario, the hydride material may be exposed to air, water, and other contamination. Owing to the requirements for high-energy density hydrogen storage solutions, hydrides (complexes) comprising light metal elements are of specific interest – including, but not limited to, lithium (Li), sodium (Na), magnesium (Mg), calcium (Ca), boron (B), and aluminum (Al). Hydrides and complexes synthesized from these elements are typically found in the form of finely divided powders that can be pyrophoric and water reactive. Additionally, the oxidation reaction products may also present hazards. In the case of alkali metal-based materials, the oxidation products may include materials that present health hazards or form hydrates in a humid environment that can decompose rapidly as a result of friction or heat. The associated chemical pathways and reaction rates must be quantified to enable the design and implementation of safe hydride-based automotive

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hydrogen storage systems. In addition, chemical hazard mitigation methods must be developed to enable the safe handling of hydride materials during the entire life cycle of the storage system. In this section, we will explore the safety properties of a specific set of complex hydrides, sodium alanates. These materials represent the most advanced solid-state hydrogen system developed to date and allow us to understand the safety properties associated with this class of materials.

4.6.3

Sodium alanates: prototypical complex metal hydrides

Sodium aluminum tetrahydride (NaAlH4) is a complex hydride historically used as a reducing agent in chemical synthesis, which was found to reversibly release and accept hydrogen in a two-step phase change process in 1997. This discovery of destabilization by titanium doping made sodium alanates (NaAlH4, Na3AlH6) promising materials for hydrogen storage systems [41,42]. The thermochemistry characteristics of sodium alanates This complex hydride absorbs and releases hydrogen in a two-step decomposition and recombination reaction shown in the following reaction: NaAlH4 s 1/3Na3AlH6 + 2/3Al + H2 s NaH + Al + 3/2H2 Sodium alanate-based systems are currently under development for use as high energy density hydrogen storage solutions [4,5]. Reversible hydrogen storage materials are characterized by endothermic decomposition and exothermic recombination and the thermodynamics of these reactions are well characterized [43,44]. It should be noted that sodium alanates will not meet the current US DOE goals for hydrogen storage performance; however, similar higher energy density complexes are actively being investigated that may have structural and chemical reactivity characteristics similar to that of sodium alanates. Sodium alanates are similar to traditional metal hydrides in the fact that they are typically found as finely divided metal powders and undergo significant morphological changes during hydrogen sorption. Figure 4.7 illustrates this morphology change using SEM imaging of the highly crystalline sodium aluminum tetrahydride (hydrogen charged phase, A) compared to the highly porous form of the fully decomposed material (hydrogen released phase, B). The sodium alanates are high surface area materials that are pyrophoric and react readily with water. Intuitively, it would be expected that the system exothermically reacts to form NaOH and Al2O3 upon exposure to an oxidizing species. Despite the relatively benign oxidation products, some researchers have observed samples becoming friction sensitive after uncontrolled oxygen

Solid-state hydrogen storage system design

A

99

B 5 µm 5000×

5 µm 5000×

4.7 SEM images of the highly crystalline sodium aluminum tetrahydride (A) compared with the highly porous form of the fully decomposed material (B).

exposure. Researchers as early as Ashby in 1969 [45] reported sensitivities of oxidized alanates to friction. This behavior could indicate that other reactions could occur, perhaps resulting in oxides such as sodium superoxide as determined by Desreumaux [46]. The oxidation reactions of these materials are not fully understood – which causes difficulties when designing safe and effective controls for systems. Since large quantities of reactive sorption materials must be integrated into the automotive platform, the hazards associated with contamination and environmental exposure must be fully quantified and understood so appropriate codes and standards can be specified and engineering controls can be designed and integrated. A large number of oxidation reactions are possible between sodium alanates and the oxidizing gases present in the atmosphere (oxygen and water vapor). Many reactions are thermodynamically favorable as shown by free energy and enthalpy changes of a few selected reactions in Table 4.1. The thermodynamic property data were obtained from Barin et al. [47]. Reactions that yield sodium oxides are of particular interest as some of these compounds can exhibit structural instability. The oxidation of sodium alanates by oxygen to yield various Na–O compounds and either hydrogen or water vapor are reactions that are particularly interesting. Oxidation to sodium oxides such as peroxide, Na2O2, and superoxide, NaO2, are both favorable according to the free energy changes although less so than to sodium oxide, Na2O. The water vapor product is capable of further oxidizing either the alanate or some of the oxidation products of the initial reactions.

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Table 4.1 Thermochemistry and volumetric changes of a selection of oxidation reactions of sodium alanate by oxygen or water vapor. Free energy and enthalpy changes are given per mol of NaAlH4. Volumetric changes refer to the solid compounds only and are percentages based on NaAlH4 ∆G (kcal/mol)

∆H (kcal/mol)

–22.59

–25.70

–10.4

Oxidation by O2 to Na–O compounds – H2O product O2 + 2NaAlH4 → Na2O + 2AlH3 + H2O –49.98

–54.68

–10.4

Proposed processes (abbreviated list)

Oxidation by O2 to Na–O compounds – H2 product 1 /2 O2 + 2NaAlH4 → Na2O + 2AlH3 + H2

∆V (%)

Oxidation to Al2O3 as one product 4O2 + 2NaAlH4 → Na2O + Al2O3 + 4H2O 4H2O + 2NaAlH4 → Na2O + Al2O3 + 8H2

–332.14 –112.99

–338.85 –107.05

–19.0 –19.0

Oxidation to NaOH 4O2 + 2NaAlH4 → 2NaOH + Al2O3 + 3H2O H2O + NaAlH4 → NaOH + AlH3 + H2

–350.71 –13.76

–362.23 –20.10

–12.7 –8.3

Calculations of volumetric changes of the solid compounds associated with these reactions were also performed to determine potential effects on the bed, such as consolidation or unpacking, or expansion that might affect the container. These results were obtained from published values of the bulk density of the various solid oxidation products and data for the phases of sodium alanates [47]. The volumetric calculations are summarized in Table 4.1. The volumetric changes are percentages based on the presence of only NaAlH4 initially. In general, significant volumetric changes are not realized during bed oxidation, with the prevailing trend toward volumetric reduction. System implications of the oxidation reactions Since the reactions described in Table 4.1 are exothermic, the rate of the oxidation processes must be controlled to avoid rapid heating and, possibly, ignition of the metal hydride. In the case of the thermodynamically aggressive postulated process: 4O2 + 2NaAlH4 → 2NaOH + Al2O3 + 3H2O the heat of oxidation reaction is –1516 kJ/mol NaAlH4. For a sodium alanatebased system that reversibly stores 5 kg of hydrogen, the total quantity of heat removal required is approximately 5 GJ – the amount of energy released when burning 38 kg of hydrogen fuel. If the majority of this heat is not removed, it is plausible that ignition can occur. In a postulated event in which the oxidation process is active for 4 hours, the average heat rate is 333 kW. Since this is a significant amount of energy and power to manage during

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an automotive accident scenario, controls must be in place to limit the impact of such an oxidation event to avoid ignition. In addition to the ignition and fire hazard posed by the oxidation of the metal hydrides, other safety factors must also be considered. The oxidation reaction products may be unstable, toxic, or otherwise hazardous. For example, the reactions described in Table 4.1 that result in sodium superoxide could result in the production of friction-sensitive materials and may add risk to the disassembly of handling contaminated hydrogen storage materials.

4.7

Enabling safe systems based on hydrogen sorption materials

Although sodium alanates have been the focus of this discussion, other chemically similar materials will likely have very similar safety hazards. Given thermochemistry considerations of complex metal hydrides, some effort is required to address the contamination issues and demonstrate that hazards associated with the safe production and operation of the automotivescale complex hydride-based hydrogen storage systems can be effectively minimized. To enable the realization of safe systems based on reactive solids, the developers must identify and characterize chemical processes and hazards associated with hydride exposure to air, water vapor, and other relevant contaminants. These hazards could include human toxicity and environmental pollutants. From this information, system designers can predict system contamination scenarios and identify hazard mitigation strategies to allow for handling and disposal of contaminated materials. Eventually, full-scale testing using standardized methods will be required to obtain regulatory approval for consumer use of new hydrogen storage technologies.

4.8

Future trends

As novel hydrogen storage materials are developed and become better understood, engineers will be responsible with developing safe, efficient, and application-relevant systems. As energy densities of the materials increase, system design becomes more challenging as more heat must be transferred per unit volume. Fortunately, as we described earlier in this chapter, we already have a general understanding of the physical and thermodynamic characteristics of many potential solid-state hydrogen storage materials, which allows system designers to develop engineering techniques and methods to enable the utilization of this future technology. Although no single system design is appropriate for all hydrogen storage materials and applications, intelligent engineering and optimization efforts will enable safe, high energy density storage solutions based on solid sorption materials.

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4.9

Solid-state hydrogen storage

Sources of further information and advice

Currently the US DOE is funding efforts to develop hydrogen storage systems based on complex metal hydrides. Annual review proceedings that discuss US DOE-funded hydrogen storage engineering efforts can be accessed at http://www.eere.energy.gov/. Although references on hydrogen storage systems are very limited, hydrogen storage system design is discussed in brief in Fuel Cell Systems Explained by James Larminie and Andrew Dicks (Wiley).

4.10

References

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