WE-NET Home Page/Summary of Annual Reports 1997

5. Subtask 5 : Development of hydrogen transportation and storage technologies

5.1 Development of large-capacity hydrogen liquefaction facilities

5.1.1 R&D Goals

Goals of studies in the Phase I are that to make a conceptual design of large-scale liquefaction facilities, and to select technical items to be developed necessary for constructing it.

With only one year of Phase I remaining, it is now necessary to finalize the equipment specifications for the commencement of Phase II (Elemental Development), which in turn requires a prompt decision on the process to be employed in the WE-NET Project. In the fiscal year 1996, a trade-off study was conducted on several processes among those examined so far, namely the Hydrogen Claude Cycle, the Helium Brayton Cycle, the Neon Brayton Cycle, the Mixed Refrigerant Cycle and the Nelium (mixture of neon and helium) Brayton Cycle, and two processes were selected for the more detailed trade-off study.

5.1.2 Results in fiscal year 1997

In the fiscal year 1997, the final trade-off study was conducted including the optimization work of these two processes and one process was finally selected.

In the trade-off study in this fiscal year, comparison of the process efficiency is the most important issue. As the operation cost is mostly attributable to the process efficiency, it is particularly important for the WE-NET Project. In the last fiscal year, the conditions to study the processes were not sufficiently unified and, therefore, the evaluation was made from the viewpoint of determining whether or not the target process efficiency of 40% will be achieved in the trade-off study. In the case of the process evaluation conducted in the present fiscal year, the study conditions were unified as much as possible in order to compare the relative advantages of the processes in the trade-off study. The process study conditions used are given below. Process Study Conditions

Hydrogen liquefaction capacity :300 t/d

Feed hydrogen :Pressure: 1.05 atm (0.106 MPa)

Temperature :300K(refining not taken into account)

Liquid hydrogen tanks :Storage capacity: 50,000m³/tank, one tank installed for each liquefaction plant
Pressure: 1.05 atm (0.106 MPa)
Evaporation rate: 41.1 g/s (0.1%/day)
Para-hydrogen concentration: 99.8%
(equilibrium para-hydrogen concentration at 20.4K;
specification value is not less than 95%)

Boil-off gas (BOG) : Cold recovery carried out
Tank outlet temperature: 20.4 K (1.05 atm saturation temperature)
Para-hydrogen concentration: same as that in tank
Compressor suction pressure: 1.0 atm (0.1 MPa)
Compressor suction temperature: 300 K

Liquefying line high pressure : Maximum 50 atm (5.07 MPa)

BOG compressor : Installed

Liquid nitrogen (LN₂) : Inlet pressure: 1.2 atm (0.122 MPa)
Outlet pressure: 1.2 atm (0.122 MPa)
Unit power consumption: 0.5 kWh/Nm³ (1.44 kW/g/s)

Gas nitrogen (GN₂) : Inlet temperature: 78.9 K (1.2 atm saturation)
Outlet temperature: 300 K
Unit power consumption: 0.14 kWh/Nm³ (0.394 kW/g/s)

Compressor : Adiabatic efficiency: 85%
(gas leakage, etc. not taken into account)

Expansion turbine : Adiabatic efficiency: 82%
(81% for super-critical expansion turbine)
Power recovery efficiency: 90%
(gas leakage and bearing gas, etc. not
taken into account)

O-P conversion : Continuous conversion (combination with other conversion methods possible)

Pressure drops and heat-in-leak : Neglected
(except 0.05 atm or 5.07 kPa in BOG line)

Temperature difference at heat exchanger : Following minimum values at each
temperature level
around 300 K: 10 K, around 240 K : 3 K
around 80 K: 2 K, around 50 K: 1 K
around 30 K: 1 K, around 20K: 0.5 K

Conditions not unified : Layout of expansion turbine
Layout of O-P converter
Use/non-use of auxiliary refrigerator
(ammonia refrigerator)

Optimization based on the above study conditions led to the selection of the process shown in Figure 1 for the Hydrogen Claude Cycle and the process shown in Figure 2 for the Helium Brayton Cycle. In the case of the Hydrogen Claude Cycle, the optimum high pressures for the feed line and the recycling line were determined to be 30 atm (3.04 MPa) and 40 atm (4.05 MPa) respectively. The corresponding optimum high pressures for the Helium Brayton Cycle were 35 atm (3.55 MPa) for the feed line and 30 atm (3.04 MPa) for the recycling line. A trade-off study was conducted on the following evaluation items for these selected processes:

Evaluation Items

(1) Process Efficiency

Evaluation is conducted on the basis of the estimated process efficiency of each process. With a difference of less than 1%, both systems are evaluated very similar.

(2) Plant Cost (excluding rotating machine)

Equipment Cost
If the process requires the use of special equipment, it is evaluated to result in a higher cost. Accordingly, evaluation is made on the basis of whether or not each process requires the use of special equipment. Examples of special equipment are the hydrocarbon separation system for the Mixed Refrigerant Cycle and the cryogenic exhaust pump for the Neon Brayton Cycle, both of which were appeared in last year's trade-off study.
Refrigerant Cost
The prices of refrigerant gases used for the recycling line are evaluated, taking their availability into consideration.
Maintenance Cost
Even though the recycling line of the liquefaction plant is a closed cycle, gas refilling is still necessary to compensate for the open inspection of the plant and gas leaks. The cost of gas is, therefore, evaluated in the same manner as the refrigerant cost. The maintainability, i.e. the availability of gas at the time of planned regular open inspection and emergency open inspection, is also evaluated.

(3) Operation and Management of Refrigerant

Evaluation of the liability of each refrigerant to leakage is evaluated.

(4) Compressor

Liquefaction Line Compressor
As both of the cycles use hydrogen gas, comparative evaluation of the cost is conducted based on the compression volume and required number of compression stages. The boil-off gas compressor is neglected because of the extremely low compression ratio and compression volume.
Recycling Line Compressor
Comparative evaluation of the cost is conducted based on the design and manufacturing difficulties caused by different refrigerant gases, and on compression volumes and required number of compression stages of the compressor.

(5) Expansion Turbine

Liquefaction Line Turbine
Comparative evaluation of the cost is conducted based on the design and manufacturing difficulties of the turbine, processing volume and required number of turbines.
Recycling Line Turbine
Comparative evaluation of the cost is conducted based on the design and manufacturing difficulties of the turbine, processing volume and required number of turbines.

(6) Heat Exchanger

When considering the design and manufacturing difficulties of the heat exchanger, it is preferable that the volumetric flow of each fluid in the heat exchanger to be similar to each other. Consequently, comparative evaluation of the balance of volumetric flow in the heat exchanger used by each liquefaction process is conducted.

(7) Safety

Safety evaluation is conducted based on whether or not the refrigerant gas used is combustible. As combustible hydrogen gas is used in the liquefaction line, the use of combustible gas in the recycling line is judged to increase the risk.

The evaluation is conducted by comparing the both processes by each evaluation item above. A better process obtains "A", while the other gets "B". If the both liquefaction processes are deemed to be equal, "A " is given to the both. "A" carries a score of 10 while "B" carries a score of 5. The points for the most important three items, i.e. process efficiency, equipment cost and refrigerant cost, are doubled. The liquefaction process with the highest score is then selected as the process to be employed in the WE-NET Project. The results are shown in Table 5-1-1.

As shown in Table 5-1-1, the Hydrogen Claude Cycle is selected as the liquefaction process to be employed in the WE-NET Project. Based on this decision, it is now possible to determine the specifications of the two main components, i.e., the hydrogen compressor and expansion turbine.

5.1.3 Research plan for fiscal year 1998

We will elaborate the detailed plan for the Phase II (Elemental Development) based on the specifications which will be determined based on the decision mentioned above. As the planned period of five years for the Phase II is not enough to conduct every elemental development, we are planning to start preliminary study of a large scale hydrogen compressor, which is estimated one of the most important equipment, in the next fiscal year. It is also planned to study the layout of the hydrogen liquefaction plant through a general study on the required equipment and to prepare a drawing which will present an visual image of the pant.