Subtitle: Storing the Wind-Energy Makes It Reliable
First, a quote (or paraphrase) from Confucius: “I show a
dull man one corner of a room, and he sits in the corner grinning. I show a wise man one corner of a room, and
he shows me the other corners, plus the entire house.” There
are quite a number of people grinning in the corner, based on some of the
comments made at WUWT, which started this entire post.
Background
Some low-information commenters on Anthony Watts excellent blog made light (jeered, even) at my suggestion that the recently-announced MIT storage spheres will solve the intermittency problem of wind-energy. A few, however, asked polite and intelligent questions. This is my response to those polite folks.
Pumped Storage Hydroelectric Basics
Now, as to the basics of how PSH (pumped storage
hydroelectric) works, and then how the MIT spheres work. There are numerous PSH sites in the US (more
than 22,000 MW at last count by EIA – Energy Information Agency). One of
the largest sites is on the eastern shore of Lake Michigan, the Ludington
Plant. Like all PSH plants, there is an
upper reservoir and a lower reservoir.
The lower reservoir is Lake Michigan.
The upper reservoir is man-made, about 360 feet above the lake, and on a
sandy cliff-like edge of the lake. At
night, six turbine/pumps run in pump mode by drawing power from the grid to
pump water from Lake Michigan into the upper lake. The next day, the flow is reversed so that
water flows from the upper lake through the turbine/pumps into Lake Michigan,
this time generating power as needed.
The upper reservoir and land occupy 1,000 acres. The generators produce 1,872 MW of power at
maximum flow. The penstocks (six of
them) or pipes, are 1,300 feet long and 28 feet diameter. These connect the upper and lower
reservoirs. The plant was built between
1969 and 1973. As PSH plants go, it is
large but has a low elevation change.
In contrast, the Castaic PSH in Southern California, near
Los Angeles, has an elevation change of 1,060 feet and produces 1,247 MW for up
to 10 hours in generating mode. Castaic
PSH also draws power from the grid at night to pump water uphill from Castaic
Lake into Pyramid Lake, the upper reservoir.
The tunnel connecting the two lakes is 7.2 miles long and is 30 feet in
diameter. Castaic PSH has six pump/turbines and one
standard turbine generator. Because the
elevation difference is greater, Castaic has much lower water flow than does
Ludington.
These two examples show a low-head and a high-head PSH plant
(Ludington is low-head, and Castaic is high-head). In this context, head is the elevation
difference in feet between the upper and lower reservoirs.
The MIT spheres on the ocean floor will do exactly the same function: draw
power from the grid at night to pump water out of the spheres. The spheres are not closed as one commenter assumes. Instead, they are vented
by a pipe to the atmosphere. The sphere
acts exactly like the lower reservoir.
It is at atmospheric pressure at all times. A turbine/pump connected to a
motor/generator draws power at night (or whenever the wind blows) and runs in
pumping mode to send water out of the sphere into the surrounding ocean. Air flows from the atmosphere through the vent
pipe into the sphere. The ocean, at that
depth, has considerable pressure. One
can estimate the water pressure by dividing the water depth by two. Thus, 1,000 feet of water will exert
approximately 500 pounds of pressure. (Engineers will know that the exact relationship
is 32.2 divided by 14.696, but for estimating purposes, two will suffice.)
During the day, when peak power is required, seawater is
allowed to flow by natural pressure from the ocean through the turbine/pump into
the sphere, turning the generator and producing power to the grid. Water flowing in forces air out of the sphere
through the vent line into the atmosphere. With proper design, about 80 MW will be
produced into the grid for each 100 MW consumed from the grid.
As to the servicing and maintenance issues someone asked
about, this is trivial. Proper design
will have the entire turbine/pump and generator/motor equipment in the atmospheric
pressure zone above the sphere. Simply
put, that building will also be vented to the atmosphere. Likely, an elevator will convey workers and
materials to the submerged sphere, much like in a mine shaft on land. There is no need to contemplate
high-pressure underwater activities.
Purists will say, at this point, yes but what about screens to keep fish
and other marine life out of the turbines?
Those screens or similar devices may require periodic cleaning, but that
can be done remotely with ROVs. (remote
operated vehicles, think unmanned submarines).
As to the MIT paper indicating 6 hours of storage, and the
naysayers objecting that this is far too little. It should be pointed out that Castaic PSH has
only 10 hours of generating capacity, and about 11 hours for Ludington. However, these spheres would be storing
offshore wind-energy and could require operation for several days. There are three salient points about PSH
generating time: one need only change the generating time by 1) increasing the
diameter, 2) adding more spheres, or 3) increasing the head. Put simply, if the sphere volume is the same
and only one sphere is used, one can obtain double the generating time by
setting the sphere twice as deep into the water – this increases the head. Similarly, if one maintains the head constant,
one can obtain 8 times the generating time by doubling the sphere’s
radius. Or, one could maintain the head
constant and add more spheres of constant radius to obtain the increased
generating time. Note that it is not
required to have a turbine/pump with motor/generator on each sphere. The spheres can be connected one to another
by suitable high-pressure pipes. Very
likely, the most economic choice for increased generating time is simply to
increase the spheres’ size. Spheres
have a nice property for that, as materials required go up with the square of
the radius, but volume increases with the cube of the radius. One may also excavate out a hollow in the
ocean floor and set the larger sphere in place, if water depth is an issue with
a larger sphere.
Now, as to the testing and prototyping as asked about: yes,
the MIT publications state the system has been built, has been tested, and
measurements taken on an actual sphere.
The economics are much criticized in the comments
on WUWT. It was overlooked, apparently, by
the naysayers that MIT stated the cost per sphere will decrease as more are
deployed. This is the economy of mass
production. Henry Ford recognized this
with automobiles; it still applies today.
Another cost-reduction will occur as spheres are made larger, this is
the economy of scale for unit production.
Yet another cost reduction will occur as spheres are installed along
trunk power lines laid on the ocean floor.
It will not be necessary to build the electrical infrastructure again
for each sphere.
Another word about economics: with a suitable number of
spheres in place, there will be no need for land-based fossil-fuel power plants
to be built in excessive numbers. Instead
of the 1,000 GW currently installed, the US could have only 600 GW installed,
and let the spheres do the peak load work.
The savings from not installing 400 GW of on-shore fossil-fuel power is
indeed large. That will offset much of
the cost of installing the spheres.
As to the land-locked cities, spheres can be installed in the
larger Great Lakes, with a power grid designed to send power from wind-farms in
the Great Plains to those storage systems, then back out the next day. Even shallow Lake Erie can have storage
spheres, they would simply be buried in a suitable hole in the lake bottom.
Conclusion
This wraps up the MIT sphere grid-scale storage
technology. It works. It has zero energy cost. It has very low environmental impact. It can be constructed now, without waiting
for offshore wind-turbines. It reduces
the cost of on-shore generating plants – fewer plants will be required. Power from the spheres is almost instantaneous
and can be at full power in less than 30 minutes. It quite easily follows the load. Economy of scale and mass production will
decrease the costs. There is a huge
coastline with shallow continental shelf along most of the Eastern seaboard and
Gulf of Mexico, so placing numerous spheres is quite possible. It makes intermittent wind energy very
reliable, available on demand.
Roger E. Sowell, Esq.
Marina del Rey, California
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