Every time we talk about renewable energy we end up in the same conundrum: offices work during office hours, factories work according to their own schedules and, in our homes, we want light as soon as we hit the switch. This means a permanent and consistent energy supply. Even if Nature covers the first characteristic, it sure doesn’t cover the second as the below charts clearly demonstrate.


Solar and eolic energy production in Spain in the period 2014-16 (source: e-sios/ REE)

This variability makes it a necessity for all Transmission System Operators (TSO), i.e. the companies that manage the energy grids such as the REE in Spain, to always rely on a backup from regulatable energy sources (which right now means nuclear centrals or combustion engines).

Therefore, any sort of transition into a CO2-free energy economy, will require the introduction of large-scale storage mechanisms to work in conjunction with energy generation (especially if we consider the future impact of distributed generation, as we’ve discussed in the past here), in order to give us, not just the exact amount of energy we want, but as importantly: when and where we want it.

But what exactly are we storing?

As you well know, we use electricity to power almost everything. A good example is a hair dryer: once you switch it on you create a difference of potential (a sort of an “electric vacuum”) which turns the potential energy waiting in your house’s wiring into a current of moving electrons. The energy contained in these moving particles generates a magnetic field (that in turn generates the force) that moves the fan, but it also generates heat (by being moved through a highly resistive wire) and also the light that tells you the hair drier is on (photons from an LED). Noticed that word: “potential”? That word is one of the keys to solving the vast majority our energy problems in a CO2–free economy.

The subject of energy if profound. It’s part of myth, philosophy and underpins all the theories of natural sciences. From what we know, Energy is… everything. There’s no context-free definition for it, but so far we do know that natural laws prevent us from creating it. All we do is make it change “form” or simply transfer it from one particle/body to another. By that time-changing process we measure it – whether it’s through “work” or “heat flux” – and by it we also generate electricity.

But what happens when there’s nothing moving? If I just said that I cannot create energy, yet when I connect the hair drier things happen, that must mean that energy is actually waiting somewhere. That “energy-in-waiting” is called “potential energy” (because it has the potential to do things) and it can take many forms. It may reside in the chemical bonds of the substances contained in your typical car battery (lead and acid), in the weight of the water behind a dam’s wall or in the particles that compose matter itself (nuclear energy).

So, if we want to turn the volatile renewable energy supply into a constant electrical supply, all we have to do is change the kinetic energy of the moving wind into potential energy. We could use batteries, but even though the cost of a battery dropped a lot in recent years, they are expensive and pollutant so they remain in the realm of small things (such as electric cars and mobile phones).To ensure there are no misconceptions, we’ll call these alternatives by the more generic term of “accumulator.

How do they work?

Nowadays the best example of an “accumulator” is pumped-storage hydropower (PSH). This system uses two water reservoirs at different heights to store energy: a large dam and a smaller reservoir. Water flows into the lower reservoir to generate electricity (at a controlled rate) during peak (expensive) hours and is pumped up again to the upper reservoir at cheaper hours, usually the ones where most wind energy (unregulated) is present. A working example of this technology is Iberdrola’s “La Muela”: the largest PSH complex in Spain and Europe. La Muela I (seen in the image below) provided Spain with 630MW of hydropower and 555MW of PSH, but the complex was overhauled in 2013 with La Muela II, a completely underground system, that increased the central’s output to 1,77GW of hydropower and 1,28GW of PSH.


Aerial image of Iberdrola’s Cortes-La muela hydropower complex in Valencia, Spain. source: El Pais

But is this the only method? Definitely no! Apart from PSH (and excluding batteries) there are four other main methods which show promise (some are already in use).

Thermal Energy Storage (TES)

This term actually encompasses a very broad range of technologies and systems, designed for all sorts of applications. Nevertheless, they all share the same common background: they temporary store energy in the form of heat and release it at a time of our convenience.

A famous application is molten salt: in some thermal solar plants the excess energy produced during peak sunlight is used to melt salt, which is stored in reservoirs, so that the plant can produce steam later in the day (and even during the night) when (obviously) there’s no sunlight available. This can increase immensely a plant’s efficiency.

The main reference facility in the domain of thermal solar plants with storage is Soltar Reserve’s Crescent Dunes plant, in Nevada (US), which has an ouput of 110MW. Spain was a pioneer of this system since 2011, with Gemasolar operated by Terrasol Energy. This central in Andalusia has a power output of 19MW and can generate power for 15h without any direct sunlight.

Compressed Air Energy Storage (CAES)

These plants are much like PSH plants conceptually, but instead of relying on the weight of water, they rely on the pressure of air. Ambient air is compressed and stored under pressure in an underground reservoir (usually a cavern to keep projects affordable). When electricity is required, the pressurized air is heated and expanded in an expansion turbine driving a generator for power production. The heating is done using a natural gas turbine, a recuperator or by creating a hybrid system (by merging it with a thermal storage system).

Hydrostor, a Canadian company in partnership with AECOM, is leading the implementation of this technology, both on sea and land. Just some days ago it introduced their most recent system which (they claim) can compete with natural gas plants. It already has a pilot project running for the Utilities Company Toronto Hydro.

Flywheel Energy Storage Systems (FESS)

Flywheels store energy in the form of motion, in this case the motion of a spinning mass, called a rotor. The rotor (a massive rotating cylinder) spins in a vacuum to (almost) eliminate drag, and is usually mounted on magnetically levitated bearings to ensure the movement is almost frictionless. The inertia of its movement allows for it to continue spinning, even without power, which is to produce electricity.

Right now the company Temporal Power claims to create the most powerful flywheels in the world with 500kW of output per flywheel.

Gravitational Energy Storage (GES)

This system generates electricity by releasing a heavy cargo from a certain height when required. The cargo is moved upwards and downwards at will and uses regenerative braking (usual on electric vehicles, such as trams) to convert the energy of braking to electricity. These systems are very interesting because they have a very fast response (or discharge) time if required, but can be regulated as well. They also (in theory) are several times cheaper than the previously mentioned systems.

A leading company in this field is Ares, which claims it costs 40% less than PHS and is more efficient. The system is composed of a single uphill track with a central queue of shuttle-trains, loaded with concrete blocks, which travel up and down. It operates a 55MW facility in Nevada.


Storage technologies comparison. Source: ARES

Where do we go from here

In February 2017 the European Commission published a document outlining the role of energy storage in relation to electricity, presenting different technologies and discussing policy approaches.

Nevertheless, we cannot forget that some storage systems (particularly the PSH) take up a lot of electricity as well and therefore, in times of expensive electricity, the energy they deploy to the grid can be, not just more expensive than usual, but even more expensive than thermal coal and far more than natural gas.


Hours in which PSH and Thermal Coal market the daily price of electricity in the Iberian Market, distributed by price range. Source: OMIE

Even so, the price of a CO2-free energy economy is not just measured in money, but in the end result for us as a Civilization. I, for one would gladly pay more for storage generated power (in the existing marginalist market) if that is what’s required to boost renewable energy investment and sustainability.

Hugo Martins | Analyst

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