We have been exploring future technologies for energy storage on the grid a lot lately.
To me, it's the most important technology humans need to develop in our battle against
climate change. Solar and wind technologies have reached maturity and are capable of providing
more than enough energy for all of humankind for a fraction of the cost of older fossil
fuel power, and before I hear the same old argument in the comments, no they aren’t
cheaper because of subsidies. In 2017 the fossil fuel industry received
subsidies totalling 447 billion dollars worldwide, dwarving the 128 billion renewables received.[1]
Despite renewables providing a larger share of new capacity installations in 2017 at around
75% of total new electricity generation capacity. [2] Renewables are cheaper, cleaner and sustainable.
That’s why countries are investing in renewables, that’s the point of subsidies, it’s government
funding for economy sustaining infrastructure. The faster we can transition the better. Countries
will become less reliant on constant imports of fossil fuels from war ravaged regions,
will create new jobs in the renewable industry, and because we are looking down the barrel
of a climate crisis that is going to send the world into chaos if we don't address it
now. The primary factor holding renewables back today is this energy storage problem.
Lithium ion batteries and other novel battery technologies are rapidly becoming the forerunners
to form the brunt of our energy storage needs, while hydrogen looks poised to seize long
duration energy storage applications. However, in all of this analysis, we have glossed over
the oldest and most dominant form of energy storage. A simple method of energy storage
that converts electricity to potential energy by pumping water to an elevated reservoir,
where it can later be released to drive an impeller turbine when the electricity is needed.
Pumped hydro is one of the oldest technologies still in use in our modern day electricity
grid. It’s a rugged, long-lived and mature technology that provides an incredibly valuable
service to grids across the world for more than a century. There have been calls around
the world to increase our energy storage capacity in this tried and tested technology. However,
as we will find out, it’s not quite as simple as investing in the infrastructure.
To learn more about this essential technology, I visited Turlough Hill, Ireland’s first
and only pumped hydro station. Construction began on this landmark infrastructure project
in 1968 and after 6 years of laborious excavation the final components of the generators were
installed and became operational in 1974. Since then Turlough Hill has provided valuable
load shifting services for the Irish electricity grid, requiring just one overhaul of its facilities
in 2012, 38 years into its operation. That is an impressive life cycle.
Historically, Turlough Hill has taken thermal power generation from coal, oil and peat fueled
stations, which could not be turned off at night, and released that power during the
day when needed most.
However, today it has become a valuable resource as Ireland rapidly increases it’s wind power
generation. Helping the country to gradually decommission those heavily polluting fossil
fuel plants and replacing them with wind. Ireland, with it’s windy position on the
edge of the Atlantic, has prime real estate to begin growing it’s offshore and onshore
wind resources and with the help of energy storage, become a net exporter of energy.
To its neighbours in the UK and wider Europe.
However, the question I find myself asking, is how many Turlough Hills would Ireland need
to fully convert to renewables?
To understand the scale of the task at hand, let’s imagine a 1 cubic metre cube of water.
We can raise its energy by increasing its height. We can calculate the increase in energy
using the equation for gravitational potential energy, which is simply the mass of the object
multiplied by the acceleration due to gravity multiplied by its height. Here we are defining
height as the difference in height between our starting point and end point.
The mass of a 1 cubic metre of water is 1000 kilograms, so with every 1 metre gain in height,
we add 9810 joules of energy. We will convert to watt hours here as it's a more commonly
used unit. 9810 joules equals about 2.725 Watt Hours. That’s not a lot.
That could run a 100 watt lightbulb for just 98.1 seconds, but we can’t convert that
energy perfectly. Turlough Hill is about 80% efficient, so that would be closer to 78.5
seconds. If we raised it to 286 metres, the head of Turlough Hill, that 1 cubic metre
block of water could power that same light bulb for 22,451 seconds, or about 6.2 hours.
We of course couldn’t drip feed water like this through a generator over nearly 6 hours.
Let’s see how the power station buried deep within this mountain works. To get there we
drove down this tunnel. It felt like we were entering some villain’s lair from James
Bond. The tunnel is 600 metres long and the granite rock was blasted out of the mountain
using explosives until the desired location of the internal cavern was met.
Here they cleared a cavern 28 metres high. 23 metres wide and 82 metres long to install
the four 73 megawatt generators deep inside the mountain. [3]
Animation 2a The cavern cross section looks like this.
We enter the cavern here on the upper level where the pony motors are located. These are
electric motors which take electricity from the grid to spin impellers located on the
lower level, which pumps water uphill. When we need to generate electricity this valve,
also on the lower level, opens to allow water to flow through the impeller. This now rotates
impellers in the opposite direction and drives the generator, located just under the pony
motor. This is a single machine that is capable of both pumping water and generating electricity.
There is no control over the rotational speed here. It’s a fixed speed generator. Locked
to the grid 50 Hertz Frequency. However, there are wicket gates between this valve and the
impeller which limits the flow rate into the impeller and allows the generator to be throttled
down to 5 megawatts. This is an important feature that allows Turlough Hill to quickly
ramp it’s production up or down with grid demands. It also features a compressed air
evacuation system that allows the impeller chamber to be quickly emptied of water using
a blast of compressed air. This allows the impeller to quickly reverse direction without
the resistance of the water impeding it. Allowing Turlough Hill to quickly switch from generation
to pumping.
At maximum flow rate, water rushes through the machine at 28.3 cubic metres per second.
For a total of 111.3 cubic metres per second if all four 73 megawatt generators are used.
That is an ungodly rush of water. One hundred and eleven metric tonnes of water, every second.
At this flow rate the 2.3 million cubic metre upper reservoir would be drained in a little
over 5 and a half hours.
When this valve opens to the water pressing down on it with the force of 29 atmospheres
you can feel the ground beneath you shake. I was in the drive shaft access room during
generation and the power behind it was truly awe inspiring. This massive chunk of steel
rotates at 500 revolutions per minute, driving the rotors to rotate inside the magnetic stators
to generate 73 megawatts of power. That’s an enormous quantity of water and electricity.
It felt like the entire mountain was shaking around me.
Together these four generators can provide 292 MW of power.
Capable of providing about 4.8% of Ireland’s total electricity needs at its peak demand
of about 6000 MW, which occurs at about 5.30 pm every day.[4]
This is truly a massive battery that helps Ireland immensely in smoothing out it’s
erratic wind generation. Over the last month, this is what wind generation in Ireland looked
like. [5] Going to a maximum of 4249 megawatts over 80% of Ireland’s maximum demand, to
a minimum of 300 megawatts, just 5% of maximum demand. Thankfully, Ireland’s grid operator
has become incredibly skilled in forecasting wind generation using weather data. This graph
shows the actual wind generation and this is the forecasted generation.
This ability to predict what power will be available ahead of time allows Ireland’s
grid operators to predict when quick response generators, like Turlough Hill and natural
gas power plants, will need to kick in and take up the slack.
Going forward though, we will want to completely eliminate this natural gas generation. So
let’s look at the average natural gas generation in Ireland, and figure out how much pumped
storage would be needed to replace it.
This will be a little imprecise and won’t account for many scenarios, but we can get
a general idea of the challenge that awaits us.
I started by downloading the generation data from the Irish Grid dashboard website and
found the average generation for the past month, which came out to 4840 megawatts.[6]
Over that period roughly 50% of power generation came from fossil fuels. So, let’s say we
need 2420 megawatts of power generation.
That would require 8.3 more pumped hydro stations like Turlough hill to satisfy at any one moment,
but the problem is, Turlough Hill can only run for 5.3 hours at peak generation.
This power generation would need to be available 24 hours a day, so we are going to need 4
to 5 groups of 8 pumped hydro stations available to come online at different periods of the
day. That would require about 37 stations of equivalent size.
Now keep in mind that Ireland is a relatively sparsely populated country. New York City
has a larger population than the entire island of Ireland. 37 facilities like this for such
a small population is a massive undertaking. If we were to create one massive reservoir,
with the same head as Turlough Hill, that reservoir would need a volume of 85 million
cubic metres. That is roughly the same volume of Ireland’s 9th largest lake, and trying
to find space for that on top of large hills and mountains isn’t easy.
Finding suitable sites for pumped storage is difficult. Extremely difficult. We need
not one, but two reservoirs capable of holding massive volumes of water separated by a meaningful
height, at least 200 metres, but the horizontal distance between the two reservoirs needs
to be relatively short, as a long passage between the upper and lower reservoir will
result in greater energy losses due to friction and viscous fluid effects. The cost of boring
the tunnels between the reservoirs will also be higher.
Typically the cut off point is defined by a ratio of head height to horizontal distance.
Anything greater 1:10 is usually deemed uneconomical. [7] So a 200 metre head could have a maximum
horizontal distance of 2 kilometres between the reservoirs.
Turlough Hill has a head to height ratio of about 1:5.
Next, the site needs to be relatively close to population centres to avoid transmission
losses or expensive purpose built high voltage transmission lines to connect to a distant
grid.
We also need a supply of fresh water. Which is a much larger logistical issue than people
anticipate. Freshwater resources are valuable and interfering with them often comes with
environmental concerns. Even with a closed loop storage system, like Turlough Hill, where
water is just swapped from one reservoir to the other without an outflow. This water can
evaporate over time. Thankfully it’s rarely sunny and rains so often in Ireland that any
evaporation is more than replaced by ground water gathered by the lower reservoirs catchment
area.
Because finding suitable sites for pumped hydro storage is difficult, we have designed
algorithms [8] that scour over databases of map data looking for suitable sites, and there
are plenty of proposed sites currently vying for planning permission or already under construction,
but there isn’t enough to satisfy our total energy storage needs.
Ireland has just one active site seeking permission to begin construction. The estimated cost
of the facility will be 948 million dollars and will use a disused strip mine as it’s
lower reservoir. [9] It now has EU backing, but still hasn’t begun construction a decade
after first being proposed. The combination of high initial capital cost and environment
concerns often block the few suitable sites we can find. This progress is too slow.
To this end, some companies are looking to fix these problems. A pumped storage facility
that used salt water instead of fresh water would open some locations where freshwater
is scarce.
A seawater pumped storage facility was tested to limited success in Japan [10], but that
experimental facility eventually closed due to lack of demand for the electricity.
This could open many new possibilities, and would be particularly valuable for places
like California, where fresh water is a very valuable asset and solar energy is abundant.
One such facility was proposed in County Mayo in Ireland on this large flat topped mountain
directly next to the sea. A flat topped mountain of this size could house a massive upper reservoir
and simply use the Atlantic ocean as it’s lower reservoir.
Others, like Quidnet Energy, are looking to instead pump water into underground rock layers
under pressure, which would then be released to drive a generator when needed.
Pumped storage is a reliable and long lasting energy storage method. It’s not going anywhere.
It is coming under pressure from competition from cheaper batteries that don’t require
massive 1 billion dollar investments, but it’s ability to store energy for longer
durations will ensure its continued use.
One energy storage method isn’t going to fix this problem. We are going to see a world
where multiple methods are used.
This graph, taken from a fantastic paper on levelized cost of storage, shows the mix I
see be utilized by 2050. [11]
A combination for several forms of batteries for fast frequency response and short duration
load shifting, with pumped hydro being used for longer duration storage from 12 to 72
hours, while hydrogen being the only feasible option for longer duration storage.
There won’t be just one solution that solves this energy storage crisis. This problem is
going to need a holistic analysis of the grid and it’s needs.
Electricity generation is just one aspect of how humans need to adapt to a more sustainable
existence on this planet. We have a long way to go in our quest to avoid a climate disaster.
This video was created in partnership with Bill Gates, inspired by his new book “How
to Avoid a Climate Disaster”, which approaches this problem with the exact holistic analysis
I mentioned above. You can find out more about how we can all work together to avoid a climate
disaster with the link below.