Salient White Elephant

August 7, 2009

Energy Storage for Offshore Wind Turbines

Offshore Turbine with Energy Storage

When the wind turbine is producing power that isn’t being used, the energy is used to drive the reversible hydro turbine in the direction that evacuates the storage tank. Later, when the power is needed on shore, water is allowed to refill the evacuated tank. The process of refilling the tank drives the hyrdo turbine in the reverse direction to produce electricity.

Instead of having one tank per turbine, maybe a long underwater hallway passes by each wind turbine, and then goes to the shore. The wind turbines pump water out of the hallway, and water can refill the hallway by flowing through the hydro turbines near the shore.

One problem with this idea is that you want to put wind turbines in shallow water, but you want to put this energy storage device in deep water. The Hoover Dam is 220 meters high – ten times higher than the depth to which offshore turbines are currently built. But of course, the storage chamber doesn’t have to be directly beneath the turbines. Think of the incredibly high energy density you could achieve if the storage tank were very very deep! You’d be moving the water against such a large head that you wouldn’t have to move much water, and the size of the storage chamber could be reduced. What if you could find a place with a depth of 20 meters for the wind turbines, and a nearby drop-off that led to deep water where the storage chamber would be placed? Doesn’t seem so improbable does it? Aren’t there lots of “cliffs” on the ocean floor?

No expensive, intrusive reservoirs are required for this system. The water simply goes into the ocean when it exits the storage tank, and returns from the ocean when producing power. Also note that the water storage tank can be built beneath the ocean floor in order to use less concrete. Since it’s an offshore turbine, the turbine can be close to its associated energy storage mechanism (rather than having to transmit the power a long ways to the storage facility). The energy storage mechanism is harmless to ocean wildlife, and does not cause any visual pollution.

The Hoover Dam is just 220 meters high, and it produces a slapsnoggeling 2 GW. But there are places where the ocean is easily 20 times deeper than the Hoover Dam! (Even the Great Slave Lake reaches three times this depth!) Can you imagine the energy density that could be acheived with this form of storage! With such a favorable energy density, it may be worthwhile to go to a lot of extra expense to transmit power to and from these places, and to figure out how to actually build something at that depth. (Maybe much could be built on the surface, and then sunk to the bottom. Maybe some stuff could even be raised from the bottom to the surface for maintenance and repair!) If these problems could be solved, it may prove cost effective to store energy from many sources – not just wind turbines.

Deep Water Variation

The deepest part of Lake Superior is 400 meters (about twice as deep as the Hoover Dam is high). Imagine allowing a gigantic plastic chamber to fill with water, and then sink to this deepest spot. The chamber does not need to withstand any pressure differential because it is filled with water (and vented to the outside water as it sinks). Once on bottom, robotic machines dig up the material at the bottom of the lake and pile it on top of the plastic chamber. They keep piling stuff on top of the chamber until they have essentially created a “false bottom”. In other words, the chamber is nothing but a mold for the shape of an underground room at the bottom of Lake Superior. Finally, concrete tubes that have the reversable water turbines inside are sunk to the bottom, and the robotic machinery then manipulates these tubes so that they connect the underground room with the lake above. Voila! Deep water high density energy storage!!

Compressed Air Variation

As usual, I am running my mouth in this post about technology that I know little or nothing about. It occurs to me that a “reversable water turbine” may be a bigger deal than I think. Or perhaps the efficiency is high in one direction, but not in the other. Or perhaps the efficiency of an air compressor would be higher at those astronomical pressures. In any of these cases, the following design may be considered. Note that there are 4 possibilities with this design:

  1. air turbine compresses air to store energy, water flows through water turbine when refilling tank to harvest the stored energy
  2. water turbine removes water from the chamber to store energy, air turbine harvest energy of escaping air when water flows back in to the tank
  3. air turbine alone is used when storing or harvesting stored energy
  4. water turbine alone is used when storing or harvesting energy (as in original diagram above)

Lake-Bottom Energy Storage with Both Air and Water Turbines

Underground Tunnel Variation

Another twist similar to the one just described would bore a tunnel from the shore to some point beneath the water. Then, a large “underwater room” is built just above the end of the tunnel as described previously (by sinking a giant plastic bottle and then covering it up with mud). Now the tunnel is connected to the underwater room, the underwater room is vented to the lake water through a concrete tube that has a water turbine, and the air turbine is placed near the shore at the mouth of the tunnel that runs underneath the lake.

Mechanical Drive Variation

An offshore wind turbine has a direct mechanical drive that actuates a water pump. The water pump acts to evacuate the storage chamber at the base of the turbine. Electricity is involved only when the chamber is refilled, and the system is sending electrical power to the shore for consumption. A little thought will show that the turbine can be producing power and consumers can be using the power, both at the same time. All that is required is that one system doesn’t “outrun” the other, so that the storage tank either gets completely filled with water or completely evacuated. If the chamber is completely filled with water, then no electrical power may be produced. If the chamber is completely evacuated, then the turbine is no longer able to store power, and must be shut down.

Energy Density

Well, maybe I’m getting excited over nothing. It’s been so long since I crunched numbers, I don’t trust my numbers anymore. But I banged out a few quick equations to try to figure the energy density at the depth that a sperm whale can reach, which is 7,400 feet. The result, if correct, says that an evacuated chamber at a depth of 7,400 feet has an energy density of 23 kilojoules/liter. By contrast, according to Wikipedia, the energy density of a lead-acid battery is 360 kilojoules/liter. So even a stupid lead-acid battery has ten times the energy density of even that ridiculously deep chamber. Oh well… you just can’t win at this invention game, can you? Back to the drawing board.

In case you want to double check my numbers, here’s a rough idea of what I did. It’s 223 atmospheres at 7,400 feet of depth, which is 22.6 MegaNewtons/cubic meter. Because work is equal to force times distance, this is the same as saying that it’s 22.6 Megajoules/cubic meter, or just 23 kilojoules/liter. I got the energy density of other materials from the Wikipedia article on energy density.

Crash and burn folks – that’s my middle name. Ol’ Salient “Crash ‘n Burn” White Elephant, does it again. Why couldn’t I have been born an accountant?

*****

Well, this is later after I wrote the above, and I was reading about compressed air energy storage in Wikipedia. It said that first of all, it’s tricky to compress and expand the air, because it heats up and cools down in the process. And secondly, it said “if 1.0 m3 of ambient air is very slowly compressed into a 5-liter bottle at 200 bars (20 MPa), the potential energy stored is 530 kJ (or 0.15 kW·h)“. So this is saying they get 530 kilojoules for a cubic meter while I’m getting 22.6 Megajoules/cubic meter (40 times more), and I don’t have all the thermodynamic challenges that they do. I guess I just don’t have time to sort all this stuff out. But I think this idea will go the way of all the other Salient White Elephant inventions – straight to the wastebasket. That’s my home… the wastebasket. Just dig down beneath the wadded up paper and coffee grounds if you’re ever looking for me… that’s where I’ll be.

This Solution Doesn’t Feel Right

Another of my posts, How I Invent Wind Turbines, explains how I reject bad ideas to make room for good ideas. One of the points made in that post is that sometimes you just have to depend on how something feels, rather than on how it looks on paper. This post is a good example of a solution that just doesn’t feel right, not because the energy density is disappointing, but for another reason. What is that reason? Well… you know… you know… that by the time you could build a 7,400 foot deep energy storage reservoir (if you could ever build such a thing), someone would have invented the high voltage super capacitor, and you’d be left with a White Elephant at the bottom of the ocean… and he wouldn’t even be Salient anymore! (Actually that sounds like a good book title doesn’t it? White Elephant at the Bottom of the Ocean. I’ll have to remember that.)

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