Off-shore wind-turbines generate electricity, as we all know. Now I’ll explain how off-shore solar and hydrogen can power our electricity too.
Solar at sea is easy. Simply mount photovoltaic panels on platforms isolated on their own or in the wide-open spaces between the off-shore wind turbines. Mount PV-panels high and dry but be sure to mount them below the height of the rotors of the wind turbines so as not to interfere with the wind flow.
Deep Sea Hydrogen Storage
The diagram shows how hydrogen gas can be used to store energy from renewable-energy platforms floating at sea by sending any surplus wind and solar electrical power down a sub-sea cable to power underwater high-pressure electrolysis to make compressed hydrogen to store in underwater inflatable gas-bags.
Later, when there is a lull in the wind or when it is dark, the hydrogen can be piped from the gas-bag up to the platform on the surface to fuel gas-fired turbine generators or hydrogen fuel cells to generate electricity on-demand in all weather conditions.
Air lifting bags for use in diving and salvage work, are available up to a volume of 50 metres-cubed.
It should be possible to make much bigger gas-bags and anyway it is possible to rig multiple gas-bags together – for example, as shown in this diagram to rig 3 gas-bags together.
Density of hydrogen gas with sea depth
Deeper seas are better because the water pressure is proportional to the depth allowing the hydrogen to be compressed more densely, so that more hydrogen and more energy can be stored in an inflatable gas-bag.
Consider how many 50 m3 gas-bags we’d need to store the energy required to provide 1 MW of electrical power for 1 day – a useful amount of back-up energy to store to serve one floating platform.
1 MW for 1 day = 1 MJ/s x 60 x 60 x 24 = 86.4 GJ of electrical energy which can be generated from 86.4/e GJ of hydrogen energy of combustion where “e” is the efficiency of the hydrogen-to-power generator and can vary from 30% to 60% depending on the complexity and expense of the generator.
The combustion energy from 1 gram of hydrogen is 143 kJ.
So the mass of hydrogen with 86.4/e GJ of energy is
mass = 86.4 x 109 J / (143 x 103 J/gram x e)
mass = 604/e Kg of hydrogen to provide 1 MW of power for 1 day
Consider three scenarios – 50 m3 gas-bags floating on the surface, at 200 metres depth and at 2000 metres depth.
Surface density of hydrogen 0.1g/L
Volume = 604,000g / (0.1g/L x e) = 6,040,000/e L = 6040/e m3
= 121/e x 50 m3 gas-bags
for efficiency of 30% that’s 121/0.3 = 403 x 50m3 gas-bags – far too many gas-bags!
200m density of hydrogen 1.8g/L
Volume = 604,000g / (1.8g/L x e) – 335/e m3 = 6.7/e x 50 m3 gas-bags
for efficiency of 30% that’s 6.7/0.3 = 23 x 50m3 gas-bags – inconveniently many gas-bags.
2000m density of hydrogen 16 g/L
V = 604,000g / (16 g/L x e ) = 37.75/e m3 = 0.755/e x 50 m3 gas-bags
for efficiency of 30% that’s only 0.755/0.3 = 3 x 50 m3 gas-bags – a practical number of gas-bags.
So the advantage of depth in reducing the volume and therefore the number of gas-bags required to store a given mass and energy content of hydrogen is clear.
I’m not sure if it is worth collecting the oxygen from the undersea electrolysis situation. I had in mind the option of just letting the oxygen gas bubble away.
One reason to store the oxygen would be to increase the efficiency and reduce the nitrogen oxide combustion by-products of hydrogen-fired generators. Whether that advantage is worth the cost of collecting the oxygen, I’m not sure.
Be aware that for undersea electrolysis in order to produce oxygen as the anode gas, a custom electrolyte solution will have to be used. If you try electrolysing sea water directly you get chlorine gas off at the anode, which is not so easy to dispose of and can be poisonous.
So the technique will be to separate the custom more-concentrated electrolyte solution from the sea water by a semi-permeable membrane and allow pure water to pass through it by osmosis from the relatively dilute sea water.
It’s worth pointing out that whereas we might describe this process as undersea “high-pressure” electrolysis, it is only so, “high-pressure”, because of the ambient high-pressure resulting from being under water at depth.
So there’s no high-pressure-vessel containment required for the electrolyte solution – as is required for high-pressure electrolysis which operates on the surface – and so undersea, a semi-permeable membrane is all that is required to keep the electrolyte solution contained.
Where is best for off-shore solar and hydrogen?
This “Atlas of Solar Power From Photo-Voltaic Panels” shows where on land and sea the most solar energy can be generated from a PV panel. (Reference – Fig 3. Global potential map of PV energy generation (Ypy) by c-Si PV module. Note – annual energy generation potential.)
So for example, the area of sea off the west coast of north Africa, between the Canary Islands and the Cape Verde Islands, coloured orange in the Atlas of Solar Power and scoring 1,600 – 1,800 looks like the highest scoring area for off-shore solar power which is not too far from western Europe.
Even closer to Western Europe, there are plenty of areas of sea, coloured yellow in the atlas of solar power, around Spain and in the Mediterranean and scoring not quite so high at 1,400 – 1,600, but which are closer to Western Europe and so would mean shorter and cheaper connection cables.
Deeper seas, which are better for storing hydrogen in, can be found from an atlas of the oceans, such as this one.
Looking at a close-up of the map for the area of sea closest to Scotland, Britain and Western Europe –
– this shows that deep sea water most suitable for hydrogen storage is not to be found around the coast of the British Isles but depths greater than 4,000 metres can be found in vast areas of the Atlantic beginning a few hundred miles to the south-west in the Bay of Biscay.
So one area of sea which looks suitable for both solar and hydrogen powered electricity generation appears to be just to the west and south-west of the Canary Islands and to the north of the Cape Verde Islands. Whether this area is near enough to western Europe to be the best choice to supply western Europe considering the additional costs of longer interconnection cables remains to be estimated.