A landscape of hydride compounds for off-board refilling of transport vehicles

https://doi.org/10.1016/j.ijhydene.2019.09.173Get rights and content

Highlights

  • Hydrogen cars have the lowest primary energy consumption for long cruising ranges.

  • Off-board refilling is the only solution for solid state hydrogen storage materials.

  • Off-board refilling enables short refilling times and fulfills the DOE requirements.

Abstract

The authors compare the energy consumption of hydrogen cars (using fuel cells) with electric cars (using batteries) and conventional petrol cars finding that hydrogen cars are preferable to electric cars for long distances. They evaluate several types of hydrogen storage materials in terms of off-board refilling, in which hydrogen uptake takes place outside the vehicle. Literature values for enthalpy and entropy of formation etc. are used to calculate hydrogen densities, heat production and theoretical desorption temperature. Additionally, experimental literature values for temperature and pressure of (de)hydrogenation, kinetics and cycling stability are summarized. The results are discussed assuming that hydrogen refilling takes place in a replaceable tank outside the vehicle, which reduces the DOE requirements to high volumetric and gravimetric density, moderate release temperature, sufficiently fast release and high reversibility. They are fulfilled by materials like NaAlH4, while even better performance can be expected from compounds like LiBH4+MeHx or Mg-Ti composites.

Introduction

The world energy consumption is constantly rising because the population is still growing and a rising number of people can afford to buy cars, air condition, travels and various technical equipment. As most of the energy comes from burning fossil fuels, which produces the greenhouse gas carbon dioxide, the content of this gas in the atmosphere is constantly rising [1]. This is the main contribution to the climate change that the humanity experiences the last decades [2]. Progress in efficiency of various technical equipment and efforts to reduce consumption, e.g. by improved insulation, could only slow down the increase in CO2 output.

The only way out is to strengthen the efforts to generate electricity from renewable sources and to use it as efficiently as possible. (Using nuclear energy can help during a transition period, but is not a sustainable solution. The fusion technology might solve the energy problem one day, but it is still unreachable for the short term [3].) The renewable sources that have to be utilized in the future are hydropower, solar, wind and geothermal energy. The use of hydropower is increasing slowly in many parts of the world [4], the geothermal power technology is still being developed for worldwide use [5] so that the main additional contribution in the next years are from wind energy and solar energy.

The problem is that the generation of electricity from these two sources varies with time and cannot be adapted to the actual consumption. If their contribution to the total electricity production is not too large, e.g. less than about a third, the variation can be balanced by varying the electricity generation from other sources like gas power plants. Additionally, electricity may be transported according to lack or excess of generation in different countries or regions. But this is limited by the transport capacity and the loss of electrical energy of about 1% per 100 km [6].

The closer one gets to 100% electricity generation from renewable energies, which usually means a high percentage from wind and solar energy, the more it gets necessary to have an efficient large scale energy storage system [7,8]. This is especially true for regions above 40° latitude where solar energy is mainly available during summer, while the energy consumption is higher in winter when heating is needed, while tropical regions might come along with any kind of electrical or mechanical storage that is able to buffer day to night variations.

So far, only two systems for storage of large amounts of energy are known: pumped storage or storage in form of chemical energy. Pumped storage is a well-established system with a quite high efficiency of 70%–80% [9], but the resources are limited in many countries.

For chemical storage, hydrogen is one of the substances that has been suggested as energy carrier. Its advantages are (1) its high energy density of 120 MJ/kg (compared to 44 MJ/kg for petrol) [10], (2) the fact that only water is produced when it is used and (3) its flexibility - it can be produced from various sources and used for combustion or electricity generation.

When more electrical energy is generated than used, hydrogen could be produced. It can then be used at any time by industry, for transport or heating. For the stationary storage of hydrogen several solutions are suggested [11]. In densely populated areas, it seems to be the best solution to store it directly inside a pipeline network (at moderate pressures) [12]; alternatively, caverns or large tanks could be used. Liquefaction or high pressure gas storage need a lot of energy and are therefore unfavorable [13].

While the use of renewable energy in various areas is rising, mainly via a rising percentage of electricity generated from renewable sources, the transport is still dominated by cars driven by fossil fuels. Electric cars are available, but not yet widely used, though they are energy efficient. Apart from the high price, the long charging times and the short cruising range are the problems. The reason for the latter is the low energy density of the batteries, which requires large and heavy batteries for a proper driving range. Li-ion batteries have at present the highest energy density of 0.875 MJ/kg [6] which results in a battery mass of at least 100 kg per 100 km range. So it works fine for a city car, but it cannot well be used for long distances.

The energy density of a hydrogen tank can be much higher. Therefore, a hydrogen car is an alternative to the electrical car, especially for cars meant to be used for long distances. In addition, refilling can be much faster (see below). So a hydrogen car is probably easier accepted by customers as a car needed for long range driving.

Hydrogen cars usually use electric motors. (Combustion of hydrogen is also possible, but less efficient.) The electricity is generated by a fuel cell. If the hydrogen is generated from electricity, hydrogen cars have a lower overall efficiency than electric cars, because of the two transformations from electricity to hydrogen and back, which both have a limited efficiency (cf. Fig. 1a, Table 1a, Table 1ba and b, Table A.1 in Appendix 1). However, a wide use of electric cars with electricity generated from renewable sources, only works well if a high fraction of the electricity is generated in hydropower plants or a large storage capacity in pumped storage power stations is available; otherwise electricity consumption and generation cannot be matched.

Nowadays, there is not enough electricity generated from renewable sources for all possible applications. Additional applications require therefore the generation from hydrocarbons. This electricity can be used to run cars; or the hydrocarbons are used to produce hydrogen (to run cars) or it is directly used in a car. These options are illustrated in Fig. 1 and compared in Table 1a, Table 1ba and b.

The data in Table 1a show that an electric car using a Li battery works clearly most efficient for short distances. However, this calculation leaves out the fact that the production of a battery car causes at least twice the amount of CO2 emission than that of a normal car (or a hydrogen car) because of its large battery [14,15]. This means that a real gain is reached only after a long driving distance (130000 km or more depending on the size of battery and car, conditions of electricity generation and battery construction etc.). It also does not consider additional consumption for heating the interior of the car during winter, because waste heat from engine or fuel cell, which is used in other types of cars, is not available.

In addition, the mass of the battery becomes more and more of a problem with increasing cruising range. It requires a stronger engine and a more solid chassis, which both increase the weight and the consumption. This requires an even larger battery, which again results in a heavier car and so on. This effect was already described earlier and the mass of the car calculated as a function of driving range for different kinds of batteries in a battery electric vehicles (BEV) and for fuel cell electric vehicles (FCEV) [6].

This kind of calculation is repeated here to determine the total energy consumption using many of the parameters given there. We do this for a hypothetical, medium sized car equipped with different kinds of engines, which has 1200 kg net weight (with full tank) as a petrol car. The idea is to investigate principal properties, not to evaluate existing cars, especially differences between cars made for short and long distances. Therefore, we have chosen a very short cruising range of only 150 km (for a city car) and a long one of 500 km (for a long distance car).

We follow the idea from Ref. [6] to keep the ratio between power and mass fixed to enable constant acceleration and climbing ability. We also used several factors from that publication, but not all numbers needed to calculate the car masses were given there, so that we had to determine or estimate some of them. They might be wrong by some percent, but this will not significantly alter the result, because the main effect is the increasing battery weight. All number are listed in Appendix 1:

  • 1.

    The energy densities are taken from the literature (see Appendix 1)

  • 2.

    The efficiency ϵcar inside the car is the efficiency of the complete propulsion system times the efficiency of the fuel cell or discharge where applicable (see also Appendix 1)

  • 3.

    The total efficiency ϵtot includes additionally (where applicable): transport, conversion and processing (charging, compression, chemical reaction) ϵtot = ϵcar ϵtrans ϵconv•ϵproc

  • 4.

    For the city car, a cruising range of 150 km is assumed, for the long distance car, a range of 500 km. The large difference is chosen to bring out the principal differences between these two types of cars.

  • 5.

    It is assumed that the mass mbase of chassis plus vehicle body increases with total car weight. Assuming that the linear part is 15% of the total mass and a total mass of mtot = 1200 kg for the petrol car (s.a, Table 1b), we get the base mass mbase,0 mbase,0 = mtot - 0.15 mtot – mmotor – mfuel – mtank – mFC+batt = 735 kg and thus the equation for the mass of chassis plus superstructure: mbase = mbase,0 + 0.15 mtot. to reach the planned total mass of 1200 kg of the petrol car (see Table 1b).

  • 6.

    The mass mmotor of the propulsion system is assumed to be proportional to the power of the motor with the following factors: 4 kg/kW for a combustion engine and 2 kg/kW for an electric motor.

  • 7.

    The battery of a normal car weighs about 15 kg (0.72 kWh, measured), the one for a FCEV is supposed to be somewhat heavier, 20 kg (corresponding to 1 kWh for a lead battery and 17.5 kWh for Li-ion battery). The battery of a battery electric car is by far heavier. Its mass is determined by the energy Etot needed for the wanted cruising range, the energy density available (ρE,Li = 0.875 MJ/kg = 0.243 kWh/kg for L-ion batteries), and the fraction κ = 0.7 [6] of the battery charge that is available: mbatt,BC = EtotE,Li 1/κ

  • 8.

    The mass of the fuel cell is assumed to be proportional to the power; a recently produced cell had a factor of 0.694 kg/kW [16].

  • 9.

    The masses of the tanks (including additional devices like control and discharging systems) are also assumed to be proportional to the amount of fuel: mtank = K mfuel using KCH4 = 5.0, KCGH2 =18.3 or 22.5 [23], KNaAlH4 = 10.0+1/mass%. Only for petrol a fixed mass of 7 kg is assumed.

  • 10.

    The total mass mtot is the sum of the five masses described.

  • 11.

    The power Pmot is set proportional to the to car mass Pmot = 0.05 kW/kg mtot

  • 12.

    The energy consumption increases with the total mass of the car C/C0 = (mtot/mtot,0)0.6 [6]. The standard mass mtot,0 is set to 1200 kg and the standard energy consumption to C0 = 6 L/(100 km) corresponding to c0 = C0/100 ρpetrol ρE,petrol/3.6 kWh/MJ = 0.557 kWh/km giving the energy consumption at the wheels c0,wheel = c0 ϵcar,petrol = 0.143 kWh/km and c = c0 (mtot/mtot,0)0.6

  • 13.

    This allows calculating the primary energy consumption needed c0,tot = c0,wheeltot

The calculated values of the motor power Pmot is used to calculate the new motor and fuel cell mass. The calculated consumption is used to calculate the new fuel, tank and battery mass. The calculated car mass is used to calculate the new base mass. This gives a new total mass leading to a new motor power and a new consumption. This procedure is repeated until it has converged.

The result of this procedure is shown in Table 1a, Table 1ba and b for two cruising ranges of 150 km and 500 km: For up to 150 km, the masses of all cars are rather similar (s. Table 1a). The heavier tank of the cars driven by an electric motor is compensated by the lighter motor. As a consequence, the electric car has the lowest energy consumption due to the high efficiency of its propulsion system and its energy conversion.

However, for a cruising range of 500 km, the situation looks different: Starting from about 1200 kg, the mass of the BEV has increased to over 2000 kg caused by a battery of over 800 kg. As a consequence, the consumption at wheel has increased by 37.3%. If electricity from natural sources is available, then this is still the most efficient car, even if loss by energy storage, e.g. in a pumped storage power station is considered as in Table 1b, because an electric motor is so much more efficient than a combustion engine and the energy conversion from electricity to hydrogen and back reduces the efficiency of the hydrogen car. But if the electricity is generated from hydrocarbons, then the electric car shows the highest consumption of primary energy (see Table 1b). Under these conditions, the hydrogen car has the lowest energy consumption in spite of the twofold energy conversion from methane to hydrogen and from hydrogen to electricity, because the car is rather light and the motor works with high efficiency. A similar calculation using somewhat different numbers by Thomas [6], who compared cars with electric motors, using different batteries with those using hydrogen fuel cells, gave similar results. The advantage for fuel cell driven cars was even larger.

The resulting numbers depend on the factors assumed, but the overall result is always the same. If we assume the proportional part of mbase to be 10% (20%) of the total mass, the battery car has a total mass of 1990 (2159) kg and a consumption of 0.653 (0.686) kWh/km compared to 2066 kg and 0.668 kWh/kg calculated for 15%. All other results do not significantly change.

The driving range, on the other hand is crucial. While the electric car has about the same total mass as the other cars for the 150 km chosen, has only 10–51 kg higher mass than the other cars, the difference increases to 83–133 kg for 200 km.

Summarizing, an electric city car run on battery is unbeatable; but an electric car made for long distances becomes necessarily very heavy with existing battery technology. So a small car running on batteries for long distances cannot be built. In contrast, long distance cars running on fuel cells can have about the same mass as a conventional combustion cars and therefore retain a good energy efficiency.

The calculation also shows that reducing the weight of the tank by an improved hydrogen storage system still has a significant effect on the energy efficiency (cf. last two lines in Table 1b). Therefore, searching for better hydrogen storage materials still makes sense - for a higher safety and for a better energy efficiency.

Combined with the approach to produce hydrogen at times of excess of renewable energy, a hydrogen car is a good solution in addition to the electric car.

Another solution is a combination of electric and hydrogen car. As most hydrogen cars have anyway a battery to regain the kinetic energy that is lost during braking, a larger battery that allows short distance driving and a hydrogen tank for long distances would be the ideal combination, both in terms of fuel efficiency and practicability.

As electric motors supplied by batteries are best suited for vehicles used for small distances, it is clear that for vehicles like trucks, busses or ferryboats it is a much better choice to use hydrogen as energy source.

The three main ways to store hydrogen in a compact form are: hydrogen gas under high pressure, liquid hydrogen at −253 °C and hydrogen in chemical compounds in solid state form. Most of the hydrogen cars produced, both prototypes and production cars, use the first storage method: gas in high-pressure tanks (of up to 700 bar) inside the car. One possible explanation why this method is preferred by car producers is that it allows easy and fast refilling of the hydrogen container inside the vehicle. On-board refilling meets the customers’ expectations. The solid state hydrogen storage materials, although they offer better security, a higher volumetric density and, if the tank is included, even higher gravimetric density than compressed gas (see Table 2), are still not in favorable position for application purposes due to disadvantageous material properties like slow reaction kinetics or high temperature and/or pressure needed for the re-hydrogenation process (see Table 3). A solution for solid state hydrogen materials utilization may appear by implementation of a changeable container, where recharging of the tank can take place outside the vehicle. Løvvik discussed this way of applying solid state materials for hydrogen storage in cars [18]. The author elucidates the idea in terms of using the heat excess for high-temperature electrolysis, which appears to be a more efficient hydrogen production method than hydrolysis [19].

In the present article, important parameters like heat excess, volumetric and gravimetric density are calculated for various hydride systems in order to evaluate the possible use of the materials in off-board and on-board refilling.

Section snippets

Thermodynamics of different hydrogen storage systems

In order to determine whether the hydrogen storage properties of the materials are appropriate for refilling outside the vehicle, several parameters were determined. The basic properties are taken from literature in order to calculate the rest of the parameters. It is assumed that 5 kg of hydrogen are needed.

If one uses the known numbers for energy content of hydrogen and petrol, the density of petrol, efficiencies of 32% for a petrol engine and 99% for an electric motor, the same losses (of

General results

A striking result is that enormous amounts of nitrogen would be needed to cool the tank during hydrogen uptake for practically all solid state materials investigated (see Table 2). That makes on-board refilling an unpractical solution, unless a better cooling procedure will be found. Therefore, the solution of off-board refilling as suggested by Løvvik [18] is clearly preferable.

The concept of off-board hydrogen refilling has implications for the choice of the best storage material. The

Conclusion

While electric cars usually have the lowest primary energy consumption for short ranges, it is minimal for hydrogen cars for long cruising ranges, if losses due to storage of electricity and/or energy needed to produce the car are taken into account. Therefore, the development of all parts of hydrogen cars need further development. Due to the drawback of both hydrogen storage methods, compressed gas and liquid hydrogen, it is worth continuing to search for good solid state materials for

Acknowledgement

The authors would like to thank the Northern-Norway Research Council for financial support (pre-project 238664) in performing this study.

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