World’s Largest Batteries – (Pumped Storage)

Electricity faces a fundamental problem that
comes with pretty much any product that’s provided on-demand: our ability to generate
large amounts of it doesn’t match up that closely with when we need it. Wind and solar power are becoming more cost
effective, but they’ll always be unreliable and intermittent sources of energy. Retailers use warehouses to store goods between
manufacturer and sale. Water utilities use tanks and reservoirs. But the storage of electricity for later use,
especially on a large scale, is quite a bit more challenging. That’s why power grids are mostly real time
systems with generation ramped up or down to meet fluctuating demands instantaneously. That’s not to say that we don’t store
energy at grid scale though, and there’s one type of storage that makes up the vast
majority of our current capacity. Hey I’m Grady and this is Practical Engineering.

On today’s episode, we’re talking about
pumped hydroelectric storage. Although it’s a very convenient form of
energy to produce, transmit, and use, electricity has some disadvantages. We’ve talked a little bit about variability
in demand and generation capacity in previous videos of this series, but I’ll summarize
again here. The fundamental problem is that we use electricity
like this, with peaks in the morning and evening.

But, we generate electricity differently. Fossil fuel and nuclear plants generally have
a single capacity at which they run most efficiently with occasional need to go offline for maintenance. Solar, of course, follows the amount of sunlight
with some variability due to clouds. And wind follows weather patterns with potential
for lots of variability. You may have heard of the duck curve, which
is the name given to our electricity demand minus the contribution from solar.

You end up with this funky curve representing
the need for other sources of electricity. This creates a challenge because not only
does solar power start to die away right when we need it most during peak demands in the
evening, it also creates a much steeper demand curve, requiring grid operators to spin up
other types of generation more quickly. So, solar power is meeting some of our electricity
needs, but it’s not necessarily eliminating the need for other sources of electricity. And in some cases, it may actually be making
the grid less efficient by contributing to instability and requiring the use of peaking
plants that are generally heavier polluters. In fact, peaking plants are the go-to solution
for load following on the grid. These are smaller, more expensive sources
of electricity that only run for a few hours per day to make up the difference between
the base power load and the evening peaks. Another interesting solution to this problem
is called demand management, which is influencing the demand for electricity to reduce or shift
peaks and match generation capacity better. This can be as simple as marketing campaigns
encouraging you to set your thermostat a few degrees higher to sophisticated systems that
can tell your electric car when to start charging.

But, the holy grail in grid-scale power delivery
is simply to let the demand and generation curves be what they’ll be, storing energy
when generation exceeds demand and using that stored energy during demand peaks. There are a wide variety of fascinating ideas
for storing large amounts of energy, from molten salt to pressurizing the air in old
mines, but most of the current grid-scale storage relies on gravitational potential. That is: use excess energy to lift something
up, then use that thing to generate electricity as it falls back down, essentially treating
earth’s gravity as a spring. And the vast majority of current grid-scale
storage does this using water, in a scheme called pumped storage hydroelectricity. And I’ve built a little mini-scale version
of this as a demonstration.

In most cases, the way this works is to have
two reservoirs nearby but separated by a large difference in elevation, in this case two
buckets separated by a ladder. At night, when electricity prices are low,
you use that cheap power and pumps to fill the upper reservoir. During the day, when energy prices are high,
you use the water in the upper reservoir to spin turbines and generate hydropower. It’s essentially a giant water battery,
and storing energy in this way has a lot of benefits, besides just shaving off the peaks
of the demand curve. Hydropower is one of the most responsive ways
to generate electricity, so pumped storage allows grid operators to handle fluctuation
in demands quickly. Pumped storage is also valuable in an emergency,
providing quick access to power when other sources may be out of commission. Finally, these systems can provide a lot of
benefit on small, insular power grids (like on islands) where you don’t have as much
diversification in the generation portfolio. But, pumped storage has several major challenges
as well, and I’ll use this demo to illustrate the big ones. First is energy density, which is the term
to describe how much energy can fit into a unit volume.

And this is not a pumped storage facility’s
finest feature. Just for some reference here is the energy
density of gasoline, a lithium ion battery, and the water in a typical pumped storage
reservoir. I say typical because but the total energy
storage is both a function of height and volume. The greater the head above the turbines, the
more the generating capacity for a given volume of water. I’m using a little aquarium pump to fill
up my upper reservoir on top of this ladder. It’s pretty easy to see the difference in
energy density between a battery and the stored water. The water in this bucket has about the same
gravitational potential energy as the battery in your car’s key fob.

In fact, to reach the same density as a typical
lithium-ion battery, you’d have to have the water stored at a height of approximately
outside earth’s atmosphere, which wouldn’t be very convenient for an electric vehicle. In fact, this is one of the main disadvantages
of pumped storage facilities is that they require a very specific type of site where
you can locate two pools near each other while also separating them by as much vertical distance
as possible. And even then, because of the low energy density,
these are often massive reservoirs that are major civil engineering projects as compared
to something like a battery that can be manufactured in a factory. The other major challenge of pumped storage
is getting the energy back out once you’ve stored it.

Efficiency is the ratio of how much energy
you put in versus how much of it you can get out. You never get it all. That’s the second law of thermodynamics. But you hope to get most of it, otherwise
you’ve built a very big and very expensive battery that doesn’t work. As I mentioned, my model reservoir is holding
about a tenth of a watt-hour, but that’s not how much energy it took to get it there. I kept an eye on the power supply while the
bucket filled, and it took about 0.7 watt-hours of electricity. That means my pump’s efficiency was about
15%. So, the most energy I can even hope to recover
is a lot less than I’ve put in. Some pumped storage facilities can use reversible
pumps that act as turbines, but in my case I’m using a dedicated unit. I’ve got a power resistor as dummy loads,
and I’m measuring the voltage and current produced by the turbine to estimate the total
recovery of energy.

And… the numbers don’t look good. In fact, with the small amount of pressure,
my little mini hydro turbine could barely even spin at all. My best estimate is that I was able to generate
2 milliwatt-hours from the full bucket. That’s a whopping 0.3% efficiency and this
is the other reason we’re not hooking up tanks of water to our portable electronic
devices. At a small scale, this just isn’t a feasible
way to store power. Little pumps and turbines just aren’t very

But things look a little better on a larger
scale. Even considering all the potential losses
of energy from evaporation or leakage of water to friction and turbulence within the machinery,
many pumped storage facilities achieve efficiencies of 70 percent and higher. Of course that means they are net energy consumers,
since (as we mentioned) you can’t recover all the power used to pump the water to the
top, but if the cost of the energy consumed is lower than the price they can get out of
that energy (minus inefficiencies) during peak demand, they can still turn a profit. In fact, you might be surprised how many pumped
storage facilities already exist. In the U.S. the Energy Information Administration
has a nice online map where you can look around and see if there’s one near that you can
go visit. Of course, I’ve only had time to go into
the basics of pumped storage, and there are a lot of interesting advancements on the horizon,
like using abundantly available seawater instead of sometimes limited sources of freshwater.

Like demand management, storage is just one
part of improving the efficiency and stability of the power grid as we work to implement
more renewable and sustainable sources of electricity..

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