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Potential Energy Storage
February 15, 2012
Last year, because of adverse
weather, there were quite a few
electrical power outages at my house. Several of my
neighbors have
gasoline-powered generators, generally about 5,000
watt capacity, that seem to work well. Several of them have had
transfer switch panels installed alongside their
circuit breaker boxes to make powering things in their home more convenient than having many
extension cords all over the place. A transfer switch panel is certainly the safer option, although their installation runs about double the cost of the generator.
During one of these
blackouts, I started to
brainstorm possible
energy storage devices that would charge during times when electricity was present, and would give about a 10
kilowatt-hour (kW-hr) reserve. My desire was for a low-cost, low maintenance system, so I immediately rejected
storage batteries.
Other storage options, such as
flywheel energy storage that I reviewed in a
previous article (Flywheel Energy Storage, July 21, 2011), are likewise too expensive for a single home. The best option I could think of using was
gravitational potential energy in the form of
pumping water up a height, when power was available; and then using the downwards flow to run a generator when power was needed.
At that point, it was time for a
back-of-the-envelope calculation.
Gravitational potential energy
U is given simply as
U = mgh
where
m is the
mass in
kilograms,
g is the
gravitational acceleration (9.8 m/sec/sec), and
h is the difference in height between the initial and final states in
meters. Ten kilowatt hours is 3.6 x 10
7 joules (1 kWh = 3,600,000 joule).
If we assume a height differential of twenty
feet, which is about residential roof height, how many
gallons of water does this energy represent? If we crank through the numbers, we get 160,000 gallons, which would occupy a
cube about 28 feet on a side.
This is a huge quantity of water, and the structure needed to hold it would be massive. If I were lucky enough to live on the side of a
hill, the
engineering would be a lot easier, with a
pond at the top and bottom of the hill. Of course, this analysis glosses over the
inefficiency of the generator and the fact that these water reservoirs would
freeze during a
New Jersey winter.
Water, which has a density of just 1 g/cc, is not that
dense. Most
metals, such as
iron and
zinc, have a density of about seven, although much more expensive
tungsten has a density of 19.25, and
depleted uranium has a density of 19.1. Common
rock has a density of almost three (the density of granite is 2.7). A
weight and pulley system, like that used on
grandfather clocks, but on a much grander scale, can store potential energy.
Detail from figure one of US Patent No. 383,539, "Pendulum for Clocks," by Albert L. Parcelle, May 29, 1888.
The mass at A' stores the potential energy for driving the clock mechanism.
(Via Google Patents). [1)]
In a recent
arXiv preprint,
mathematical physicist,
Nadja Kutz, has proposed some methods of weight lifting to store potential energy.[2] She argues that potential energy storage could be a helpful supplement to smart grids, even if the lifted mass is as simple as excavated material. She proposes several example applications in her article. Examples for storing
photovoltaically generated energy are explained in somewhat more in detail, but she suggests that similar methods can be used for energy storage from other intermittent sources, such as
wind energy.
In one example given in the paper, masses are lifted mechanically in shafts affixed to the outside of high buildings (see figure).[2] The potential energy stored in this fashion may be converted mechanically, by gears and rods, or by a system for
air compression. As the weight is lowered, the air in the shaft may be compressed to activate
wind turbines or
ventilation devices. Thus the shaft may either serve as weather protection or for air compression. It's a simple method that's nearly as easy as the hydraulic pumping of my example.
An imagined potential energy shaft affixed to a building exterior.
In this proposed implementation, photovoltaic energy is used for weight-lifting.
(Image courtesy of Nadja Kutz).[2)]
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References:
- Albert L. Parcelle, "Pendulum for Clocks," US Patent No. 383,539, May 29, 1888
- Nadja Kutz, "Storing unsteady energy, like photovoltaically generated electric energy, as potential energy," arXiv Preprint Server, February 13, 2012.
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