The infrastructure itself and the grid connections are more expensive offshore. Savings are due to the price of land , wind speeds and regulatory factors meaning larger turbines can be used. This potential to go offshore provides an opportunity for dealing with increased demands as the human population and consumption peaks this century, without losing the remaining biodiversity.
With increased size, and hence efficiency of wind turbines, offshore levelised cost of wind energy has been predicted to beat onshore wind this decade. Solar can already be cheaper in some cases, with cooling provided by the water increasing PV efficiency. What would really turn things around is if concentrated solar power could be done at scale offshore. This gets around the material constraints of PVs and so has more potential to produce the scale of power required to meet global energy needs. Using such energy to produce ammonia for fertiliser, and possibly for liquid fuel, could have the potential to act as energy storage in the system. Ammonia production currently accounts for 2% of global energy use and 3% of GHG emissions.
What about the relative biodiversity costs? Currently, siting of onshore energy farms are not optimised for biodiversity, and often displace other land uses which can have a knock-on biodiversity impact elsewhere. Land pressure is so tight that the best we can do is reduce impact with good planning, but even that isn’t happening. By comparison, the oceans have very large areas with relatively little biodiversity. It would be useful to investigate whether these locations are useful for energy collection. There is some speculation that platforms for wind farms might provide marine habitat, with sightings of marine life higher around the shelter provided by such infrastructure.
Siting solar farms offshore also provides an advantage by limiting reduction in the Earth's albido because oceans are darker than desert regions which are typically favoured for solar farms. About 6% of the reduction in global warming is lost due to the heating effect of reduced albido when solar farms are sited in deserts.
Currently, most biodiversity loss is caused by agricultural expansion. What if it were possible to site farms offshore? So far there are some small-scale, near-shore examples, including cattle and, of course, marine products such as fish and algae. Imagine fields of cereals with automated harvesting on the oceans. This is currently a feat of the imagination, but may be necessary to feed the expected human population.
Might growing fertiliser at sea have the potential to replace petrochemical fertilisers without the land cost? And could this contribute to atmospheric normalisation? It’s happened before, nearly 50 million years ago in The Azolla Event, this aquatic fern covered the Arctic Ocean, which was then landlocked and so developed a freshwater layer which supported the azolla, a nitrogen fixer. It sucked CO2 out of the atmosphere, changing the climate from palm trees, crocodiles and hippos at the poles, into an ice age, over a period of a million years.
The possible biodiversity advantages of substituting ocean use for land use need rigorous study. Effects on marine life reported so far are local effects, and such claims have been wrong for terrestrial biodiversity, where increased local abundance of common species masks large-scale reduction in rarer species.