Hydrogen – Electrolysis

In this process, hydrogen was produced by electrolysis in the Linde factory in Alenquer (ca 300 km south of Porto), compressed to 200 bar for transport to Porto and finally has a compression to 400 bar for filling the bus tanks.

Two options were considered, in the first the electricity used is obtained directly from the Electricity grid, and represents an average of the total electricity production plants in the country. In the second option, the electricity for the electrolysis process is of renewable source, all other processes (compression, transport) are similar.

The processes for electricity production were modeled according to the database of the GABI software. The efficiencies considered reflect the technology used. Renewable energy sources (wind, hydro, solar) are considered to have an efficiency of 100%, as they convert renewable energy directly into electricity, without a thermal cycle; solid waste use is also considered to have an efficiency of 100% due to the fact that it uses waste and not primary resources.

The sources of energy for electricity production were obtained from [5]. Table 4 provides an overview of the electricity production in Portugal.

Table 4. Sources of energy for electricity production in Portugal.

Power plant

Contribution

Efficiency

Coal

34,28%

30%

Hydro

25,55%

100%

Natural gas (simple and combined cycle)

14,07%

43%

Fuel oil + Diesel oil

11,71%

29%

Co-generation

10,90%

58%

Imports

2,00%

33%

Solid waste

1,17%

100%

Wind

0,31%

100%

According to [5], losses in the transport grid amount to 1,7% of total electricity production, the losses in the distribution grid are 8,4%.

Global efficiency of production, transport and distribution is 42,7%. When only renewable electricity is used, the overall efficiency is 90%.

Table 5. Energy consumption, losses and GHG emission for hydrogen production-grid electricity.

Energy supplied

Energy losses

GHG emission

(MJ/MJm)

(MJ/MJm)

%

(g/MJH2)

%

Production of H2

3,839

2,750

96,9

295,36

94,8

Primary compression

1,089

0,079

2,8

14,66

4,7

Transport of H2

1,010

0,003

0,1

0,20

0,1

Compression and filling

1,007

0,007

0,2

1,36

0,4

Total

3,839

2,839

too

311,58

100

According to [4], the efficiency of electrolysis plant is 65%, with an energy consumption of 0,43 kWh/MJ of hydrogen. In the case of the equipment used in the Linde Factory, the final pressure of the hydrogen is 10 bar.

This hydrogen suffers a primary compression up to 250 bar for transport (the tanker pressure is 200 bar). It is assumed that the efficiency of the compressor is the same than the one used for natural gas: 68%.

The compressed hydrogen is transported by road using diesel tankers, the total distance covered is 300 km, the return trip also considered. The Secondary compression raises the pressure from 200 to 400 bar to fill the vehicles. The results for the production of hydrogen are presented in Table 5 for grid electricity and in Table 6 for renewable electricity.

Table 6. Energy consumption, losses and GHG emission for hydrogen production-renewable electricity.

Energy supplied

Energy losses

GHG emission

(MJ/MJm)

(MJ/MJH2)

%

(g/MJm)

%

Production of H2

0

0

0

0

0

Primary compression

0,089

0,079

88,7

14,66

90,4

Transport of H2

0,010

0,003

3,4

0,20

1,2

Compression and filling

0,007

0,007

7,9

1,36

8,4

Total

0,089

0,089

too

16,22

too

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