This aspect has a wider range of significance than might at first spring to mind. Obviously the intermittent nature of renewable sources is a problem during periods of undersupply. In fact during such periods energy demand is likely to be at its greatest for the very reason that caused the undersupply; for example, solar energy is at a premium during nights and during periods of cold weather.
However, during periods of local oversupply, where say, both wind and solar energy are flooding in simultaneously, and the grid is committed to accepting and paying for electricity from private generators, it happens on occasion that the network is locally overloaded by the excess. At such times any means of energy storage is doubly welcome if it can absorb the temporary excess for consumption during subsequent periods of shortage, thereby converting an embarrassment, even a threat to the functioning of the grid, to an asset.
- It would take the most dangerous unit out of reach and out of mind of most meddlers, saboteurs, and many other hazards
- It would protect the most vulnerable potential victims in the event of a rupture.
- Storage below ground level also would permit the tank to contain a far higher pressure, as long as the wall were suitably mated to the surrounding soil or rock.
- The increased working pressure for a given cost of tank construction could offset the increased cost of underground installation.
Obviously of course, there would be many a stage of design, redesign, installation and development of infrastructure before anything like the scale I propose here would even be contemplated, but there is no point my wasting time on the initial and intermediate stages; the first prototypes probably would be developed in baths or bathing pools, and none the worse for it.
The devil is always in the details. One possible concern is the question of how to prevent tackle from tangling, breaking, or sinking when sinking would be disastrous, or floating when floating would be no better. The scheme would hardly have been worth discussing say seven decades ago. Of course, it would be desirable to be able to make suitable members of the structure rigid, others flexible, all of them indifferent to salt or water, and pretty close to neutrally buoyant in sea water. For example, we want the membranes to remain in place inside their mesh caps whether they are empty or full; we want the ballast chains to remain sunk and we don't want cables or struts anywhere to tangle, knot, develop cracks, or snap on becoming fatigued after a few million cycles of charging and discharging.
Some of these attributes are tricky, especially when dealing with depth of kilometres under the sea. For example, deep-sea floats are dangerous toys. An inflated bag is fine near the surface, but a kilometre down...
But at our disposal we now have materials technology that even in my childhood would have been seen as miraculous. We have industrially available polymers ranging from densities well above that of salt water, to just below. Polypropylene and polybutylene PB-1 for example, remain slightly buoyant at all oceanic depths and pressures, and can be fabricated to resist enormous wear and tension.
The best material for the ballast weights might be lead slugs in dense plastic envelopes; solid, oriented polyester capsules might be especially suitable, strong enough for the tension, non-buoyant, and resistant to water leaching inwards or lead leaching out.
From the ecological point of view, it is interesting to compare such tent farms with oil platforms. Oil platforms originally met with hysterical resistance because of the ecological harm they would do. Eventually the hysteria died down because the oil companies promised to be good and to remove them once they had outlived their usefulness. And when the companies eventually did prepare to live up to their promises, there were hurried negotiations to prevent their doing so. Even during their active lives the platforms had caused little harm if any, and once they stood idle they were a pure ecological boon.
In this respect the tent farms should differ only in that they would involve no significant ecological harm from scratch and that their useful lives apart from maintenance, should be indefinite.
Conversely, there is the question of fouling organisms growing on the structures. Since they are static, this is not as serious a matter as it would be on the hulls of ocean-going vessels. However, if growths were to become troublesome, it might well be prudent to dedicate purpose-built robots to scavenge and scrape fouling organisms off each other as well as off ropes, pipes and the like on a fixed schedule.