Pure hydrogen can be stored on-board in tanks under pressure. The mass of hydrogen stored in a container of volume γ and pressure p can be calculated using the ideal gas equation:
In the formula: p and P are the pressure and volume in the container, respectively; R is the gas constant (8.31J/(mol·K)); T is the absolute temperature; WH is the molecular weight of hydrogen (2.016g/mol).
Thus, the energy stored in hydrogen is
EH = mHHV
Where: HV is the calorific value of hydrogen, depending on whether the condensation energy of the generated water is recovered, the calorific value is either high calorific value (HHVH=144MJ/kg) or low calorific value (LHVH=120MI/kg). In the case of internal combustion engines, the most commonly used is the low calorific value.
Figure 1 shows the mass and energy of 1 L of hydrogen at room temperature (25°C), corresponding to different pressures, and the equivalent gasoline liter. Equivalent gasoline liters are defined as liters of gasoline that contain the same energy as 1 liter of hydrogen. Under the pressure of 350bar (1bar=105Pa), the energy per liter of hydrogen is less than 1kW·h, which is equivalent to about 0.1L of gasoline. Even if the pressure is increased to 700bar, which is considered to be the maximum pressure that can be achieved, the energy per liter of hydrogen is still less than 2kW·h, which is about 0.2L equivalent gasoline. Furthermore, a certain amount of energy is required to compress the hydrogen gas from low pressure to high pressure. In the hydrogen compression process, it can be assumed to be an adiabatic process, that is, no heat exchange occurs in the process, so the energy consumed can be expressed as
In the formula: m is the mass of hydrogen; H is the molecular weight of hydrogen; γ is the specific heat coefficient (γ=1.4); p is the pressure of hydrogen; p0 is the atmospheric pressure. This energy consumption is also shown in FIG. 1 .
As can be seen from Figure 1, about 20% of the hydrogen energy must be consumed in the process of compressing the hydrogen to high pressure. After accounting for compressor and motor inefficiencies, it is estimated that about 25% of the hydrogen energy is consumed.
Storing gas at a pressure of several hundred standard atmospheres requires a very strong gas tank. In order to make the weight of the tank as light as possible and its volume reasonable, at present, the manufacture of hydrogen storage tanks for automobiles uses composite materials, such as carbon fiber materials. Thus, the cost of compressed hydrogen storage tanks may be higher. The flammability of onboard compressed hydrogen must be considered. In addition to the risk of hydrogen leakage due to cracks in the tank walls, seals, etc., there is also the problem of hydrogen permeation through the tank wall material. This is because molecules containing two hydrogen atoms are so small that they can diffuse through certain materials.
Also, in the event of a crash, the compressed hydrogen tank is a potential bomb. In the case of hydrogen, it’s even more dangerous. In air, hydrogen has a wide explosion range from 4% to 77% and can mix with air very rapidly. Compared with gasoline, the explosion range of gasoline is only 1%~6%, and it is liquid. It should be noted. Hydrogen has a high auto-ignition temperature (571°C), and although gasoline’s auto-ignition is around 220°C, it must first be gasified. So far, the storage technology of on-board compressed hydrogen is still a very complex problem applied to automobiles.
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