The Future of the Hydrogen Economy - Part 2
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January 7, 2003 View for printing



To Part One

Packaging of Hydrogen Energy Needed to Compress Hydrogen


Energy is needed to compress gases. The compression work depends on the thermodynamic compression process. The ideal isothermal compression cannot be realized.
The adiabatic compression equation [2] is more closely describing the thermodynamic process for ideal gases.

By Baldur Eliasson and Ulf Bossel EV World

W = [n/(n -1)] po Vo [(p1/po) (n-1)/n - 1] (1)

with W specific compression work J/kg po initial pressure Pa = N / m 2 p1 final pressure Pa = N / m 2 Vo initial specific volume m 3 /kg n adiabatic coefficient, ratio of specific
heats

The compression work depends on the nature of the gas. This is illustrated by comparing hydrogen with helium and methane

H2 n = 1.41 Vo = 11.11 m 3 /kg He n = 1.66 Vo = 5.56 m 3 /kg CH4 n = 1.31 Vo = 1.39 m 3 /kg


Figure 2. Adiabatic compression work for hydrogen, helium and methane.

(Please visit http://evworld.com/databases/storybu ... subcookie=1 to view charts)

The energy invested in the adiabatic compression of ideal monatomic Helium, diatomic hydrogen and five-atomic methane from atmospheric conditions (1 bar = 100,000 Pa) to the final pressure is shown in Figure 2. Clearly, much more energy is required to compress hydrogen than methane.


Figure 3 Energy required for adiabatic and isothermal ideal-gas compression of hydrogen compared to its higher heating value HHV. The isothermal compression energy is determined by the simple equation: W = po Vo ln(p1/po)

Figure 3 illustrates the difference between adiabatic and isothermal ideal-gas compression of hydrogen. Multi-stage compressors with intercoolers operate between the two limits. Also, hydrogen readily passes compression heat to cooler walls, thereby approaching isothermal conditions. Numbers provided by a leading manufacturer [5a] of hydrogen compressors show that the energy invested in the compression of hydrogen is about 7.2% of its higher heating value (HHV). This number relates to a 5-stage compression of 1,000 kg of hydrogen per hour from 1 to 200 bar. For a final pressure of 800 bar 10% would perhaps be a realistic value. The analysis does not include
losses in the electric motor.

Energy Needed to Liquefy Hydrogen

Even more energy is needed to compact hydrogen by liquefaction. Theoretically only about 4MJ/kg have to be removed to cool hydrogen down to 20K (-253'C) and to condense the gas at atmospheric pressure. But the cooling process is extremely energy intensive with a Carnot efficiency of 7%. A theoretical analysis of the complicated, multi-stage liquefaction processes is difficult. We therefore use the actual energy requirements of existing hydrogen liquefaction plants as compiled by Linde Kryotechnik AG [5b]. The company is a well-known supplier of cryogenic equipment and cryogenic liquids.

The results of the compilation of the energy consumption of existing hydrogen liquefaction plants have been adapted to make them compatible with the current study. The liquefaction energy requirement depends on the process itself, the process optimization, the plant size, and on other parameters. Figure 4 shows the actual liquefaction energy consumption for plants having a capacity between 1 to 10,000 kg of liquid hydrogen per hour.


Figure 4 Typical energy requirements for the liquefaction of 1 kg hydrogen as a function of plant size and process optimization.

The energy requirement for liquefaction is substantial. For a plant capacity of 100 kg liquid hydrogen per hour about 60 MJ of electrical energy is consumed to liquefy 1 kg of hydrogen. The specific energy input decreases with plant size, but a theoretical minimum of about 40 MJ per kg H2 remains.

In Figure 5 the required energy input is compared to the higher heating value HHV of hydrogen.


Figure 5 Typical energy requirements for the liquefaction of 1 kg hydrogen compared to HHV of Hydrogen.

For small liquefaction plants the energy needed to liquefy hydrogen may exceed the HHV of the gas. But even with the largest plants (10,000 kg/h) about 30% of the HHV energy is needed for the liquefaction process.

Energy Needed to Store Hydrogen in Hydrides At this time only a generalized assessment can be presented for the physical (e.g. adsorption on metal hydrides) or chemical (e.g. formation of alkali metal hydrides) storage of hydrogen. There are many options for both types of hydrogen storage. This makes it difficult to present numbers. But a few cautious statements may be allowed.

The laws of nature certainly apply to this type of hydrogen storage as well. In the chemical case, a substantial amount of energy is needed to combine hydrogen with alkali metals. This energy is released when the hydrogen is liberated from the compound. The generated heat has to be removed by cooling and is normally lost.

For the physical hydride storage, the hydrogen gas must be pressurized. The energy required for compression has been assessed before. The compression energy is released as heat during the charging process. Also, external heating is needed to liberate the hydrogen from the hydride storage material. According to Ref. [11], p. 264 metal hydrides store only around 55-60 kg(H2)/m 3 compared to 71 kg(H2)/m 3 for liquid hydrogen. But 100 kg of hydrogen are contained in one cubic meter of methanol.

Hydride storage of hydrogen is by no means a low-energy process, but it may be compared to the compression of hydrogen. Generalized numbers cannot be presented today.