Again in World Conflict II, the American navy had an issue, the identical one the Russians are going through right this moment. It wanted absurd quantities of gasoline for its navy efforts, and aboveground tanks have been large, stationary, thin-skinned targets stuffed with flammable materials. So the US navy began digging underground caverns for its gasoline provides close to bases the place it may and utilizing gravity to run the gasoline right down to the bottom.

In 1940, the US authorities was wanting on the winds of warfare and appeared on the gasoline tanks at Pearl Harbor and mentioned to itself, that’s a strategic weak spot. In any case, loads of the Pacific Fleet was operated out of the Harbor, and in the event that they couldn’t put gasoline within the ships and planes, then that they had loads of costly metal, not a navy pressure. So that they surveyed the close by hills and settled on a web site 2.5 miles (4 km) from the harbor. It turned the Purple Hill Underground Gasoline Storage facility.

Web site of Purple Hill Underground Gasoline Storage in O’ahu courtesy Google Maps

They utilized some modern development methods together with vertical tanks and increasing shafts with rock conveyors on the backside that enabled them to construct the ability rather more rapidly than historic methods. It wasn’t in time for them to be full for the Japanese assault in 1941, nonetheless. The development space turned the grave web site of lots of the casualties.

Gasoline flowed by means of a number of pipes for several types of gasoline operating down a tunnel to the harbor. A pumping station on the harbor managed the stream. The fairly excessive elevation of the ability in comparison with the harbor meant gasoline could possibly be loaded in a short time.

Tanks superimposed to point out their location at Purple Hill facility Courtesy EPA

Initially supposed for less than a handful of underground caverns with room for growth, the positioning quickly grew to twenty caverns able to holding 250 million gallons, approaching a billion liters, of assorted forms of liquid fuels for warfare machines.

A number of many years later, the priority was not fueling an enormous warfare fleet and air craft, however poisoning the bottom water of O’ahu. This turned a problem in 2014 when operations employees managed to spill 27,000 gallons (102,000 liters) of jet gasoline whereas refilling the ability. That led to a bunch of soil and water desk testing, and unsurprisingly discovered that many years of fossil fuels had left elevated ranges of unhealthy hydrocarbons within the ecosystem. Ingesting water remained inside federal and state well being customary ranges.

The ability was left in operation and issues went again to regular till 2021. Two spills that 12 months. One impacted the consuming water of 93,000 navy personnel, and the opposite dumped 19,000 extra gallons of jet gasoline. A number of hew and cry. Visions of 250 million gallons of diesel and jet gasoline flowing into the in any other case tourist-friendly ocean. Sierra Membership and different teams chiming in saying it was time to close the ability.

And so, in 2022 Secretary of Protection Austin gave the order to the DOD to tug the plug. So lengthy, Purple Hill. A number of work ensued. Plans, feedback on plans, rejections of plans, alteration of plans, beginning of labor, and so forth. The navy expects to completely shut the ability down by 2027, however depart the tanks in place for some unspecified future use that doesn’t contain gasoline or different poisonous substances.

At present somebody reached out to me to ask about one potential use they’d thought up, which is to repurpose the caverns for pumped hydro storage of electrical energy. Up till the purpose the query was requested, I had no thought the ability existed. Twenty seconds of studying prompt that the positioning was extra more likely to be a brand new Superfund web site requiring many years of remediation fairly than one thing you needed to flush water by means of frequently. My opinion hasn’t essentially modified, however I stored going.

The primary query I requested myself was what the top peak was. That’s the peak from the floor of the water in a hydroelectric facility to the generators. It’s essential as a result of the formulation for vitality from pumped hydro is mass instances gravity instances peak. Kilograms instances 9.8 m/s^2 instances meters, as a result of I’d choose to finish up in helpful measurements, joules and MWh. The vitality in a pumped hydro facility scales linearly with peak, clearly, and whereas the strain offered for the gasoline appeared fairly affordable, that doesn’t imply that it was appropriate for pumped hydro.

Naturally none of that was in any public documentation I may discover on the ability. Maybe that’s one thing to do with navy secrecy, or maybe nobody thought anybody would care. Off to a useful instrument which provides the elevation at a particular latitude and longitude, with these offered by Google Maps. Elevation above sea stage turned out to be 104 meters.

However the excavated caverns themselves have been 30 meters underground. Onerous to bomb, don’t you understand. So the utmost head peak was solely about 74 meters. After all, with 20 unconnected tanks, the water stage doesn’t drop equally because it does in a standard reservoir, which might be one thing you’d wish to repair with cross tunnels if this have been to change into a factor. Nevertheless, the tanks are reportedly sufficiently big to accommodate a 20-story constructing, in different phrases about 280 ft or 86 meters.

Clearly there’s an issue, as a result of the engineers wouldn’t construct the underside of the tanks beneath the extent of the bottom and under sea stage. The Google Maps location is under the ridgeline, so let’s give it one other 20 meters of head peak so that every one the numbers a minimum of roughly make sense. 94 meters of head peak, then.

However again to the unconnected tanks, that head peak drops fairly rapidly. It’s a reasonably deep however slender reservoir and when 1 / 4 of the liquid is gone, so is 1 / 4 of the top peak. For whole vitality storage, the typical head peak of about 50 meters might be the fitting one to make use of. (The shortage of precision is because of the lack of precision within the knowledge.)

We’ve got the mass of the roughly billion liters of water which is a billion kilograms, as a result of within the truly helpful metric system main models of measurement are associated to 1 one other deliberately. Mass instances gravity instances peak offers us a really massive quantity in joules, and after we convert that to megawatt hours we get just below 140 MWh of electrical energy storage.

Hmmm… that’s not that a lot, but it surely’s actually not a tiny quantity. How does that evaluate to the electrical energy consumption of Hawaii? Effectively, the islands eat about 9,000 GWh yearly, or about 25 GWh a day. About one million of the 1.4 million Hawaiians stay on O’ahu, so many of the demand goes to be there as effectively. Because the map above reveals, Purple Hill is adjoining to Honolulu, the most important metropolis within the islands as effectively. Presumably round 18 GWh per day on common is O’ahu’s demand, and a Purple Hill pumped hydro facility would supply virtually 1% of that. That may be a helpful addition.

After all, there’s a wrinkle right here. This assumes that the ocean is used as the underside reservoir. Utilizing present our bodies of water as backside reservoirs is a reasonably customary observe in pumped hydro, with the Michigan facility utilizing the eponymous lake, and the Clever Land Funding Purple John web site in Scotland designed to make use of Loch Ness.

However sea water is far nastier to mechanical gear than contemporary water. Much more minerals, salt being an apparent one, that construct up on issues. And much more microscopic life that likes to cling to issues to create a slurry that barely larger issues can develop on and so forth till you’ve gotten a mass of barnacles and different shell fish with trailing seaweed by means of which you’ll vaguely see no matter was there within the first place. Probably not an incredible supply of mass for pumped hydro in comparison with contemporary water.

Leaving that apart, there’s the little matter of the fossil gasoline residues within the tanks and pipes. That stuff is leached into the rocks. I’m fairly certain that nearly each Hawaiian and loads of different individuals and the EPA would look unfavorably on frequently flushing the worst of many years of sludge into Pearl Harbor and the Pacific Ocean. There are most likely options for that, however they received’t be low cost, straightforward, or most likely as fast as constructing the ability within the first place.

Is popping Purple Hill into Purple John Pacific a good suggestion? In all probability not, but it surely’s an attention-grabbing thought. A number of work is being executed round deserted coal mines with the intent to repurpose fossil gasoline extraction websites into closed-loop, off-river pumped hydro, so there’s precedent for rather more benign repurposing of fossil gasoline infrastructures.

After all, there’s loads of topography in Hawaii, so it’s most likely higher simply to construct precise closed-loop, off-river pumped hydro with a lot greater head heights and contemporary water. However that doesn’t imply somebody shouldn’t look severely at Purple Hill and do greater than this serviette math. 140 MWh of high-efficiency storage subsequent to Honolulu that reuses present infrastructure isn’t one thing that ought to be dismissed out of hand.