Storing energy

1. Production of hydrogen gas

The chapters on “Wind”, “Sun” and “Waves” explain in more detail how these energy sources are utilised on board of the ship to produce current electricity. This electricity, if not consumed directly, is fed to an electrolyser. This device, well known for a long time, is used to break water down into its component substances, oxygen and hydrogen, with the aid of electricity.

    The following draws on product information are provided by the Swiss firm AccaGen. This process is rated at 71 % efficiency, i.e. 100 kWh of electricity is enough to generate hydrogen gas with a calorific value of 71 kWh, or 2.5 kg (5.5 lbs). While the oxygen is not further required, some of it may be filled into pressure bottles and used in the ship’s workshop for welding.

2. Storing hydrogen gas

Upon leaving the electrolyser, the hydrogen is already under a pressure of up to 200 bar. It is then further compressed to 700 bar using a conventional compressor and subsequently stored in pressure vessels that, out of practical considerations, are found in the floats of the trimaran. Utilizing the changes of temperature which occur during the compression, the efficiency rate of this process may be sett­led to 90% (refer to Pelte 2002) . Of the 100 kWh supplied originally, 71 kWh is outputted by the electrolyser, of which 71 * 0.9 = 64 kWh reaches the storage device.

3. Data for the 72-ft-trimaran

As has already been explained in the section on “Energy”, only energy surpluses accumulated while mooring are stored in a long-term storage medium. Energy produ­ced during travel is  		completely used up by on-board energy requirements and the ship’s motors. When it is assumed that the vessel is moored for 273 days a year, the amounts of energy determined in the chapters on “Wind”, “Sun” and “Waves” result in the values shown in the following table:
hydrogen figures
The “Stand. north” and “Stand. south” columns refer to the standard operating sce­nar­ios presented in detail in the “Wind”, “Sun” and “Wave” chapters. For the sake of simplifying calculations, mooring time of 273 days and 42 days of travel were assumed. These operating periods can, of course, be further divided up. If, for example, they are divided in half, the required storage capacity is also reduced to half of the value given above in each case. Remaining with the volumes given in the table, concrete storage 
specifications for the ship are obtained as follows. A conventionally shaped pressure vessel (i.e. a cylinder with a hemispherical covering on both sides) is installed in each float. The total volume of 15,77 cu m (557 cu ft) determined for the standard northern scenario could be accommodated in pressure vessels, mounted in each of the floats, with a diameter of 1 m (3.28 ft) and a length of 6.4 m (21 ft; including cover).

4. Comparison with other storage media

In order to store 37.28 MWh of energy, the amount determined for the standard northern scenario, in rechargeable batte­ries, they would have a weight of 845 t, which is of course absurd for a 72-ft trima­ran.  		With Compressed air as storage medium the volume is the limiting factor. The volume of the floats is far beyond the demands. If diesel fuel were used, almost 4 cu m (141 cu ft) at a weight of 3.17 t would have to be kept on board.
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