Not all storage solutions are created equal

Contributed by Hans Maghon, Head of Energy Storage at Siemens Energy

As we head towards a decarbonized energy future, it’s becoming more and more clear that energy storage systems will form not just building blocks, but be part of the very foundation that future will rest on.

Batteries are making headway in power generation. For example, close to Antioch, California, a town not far from San Francisco, a 720MW gas-fired plant named Marsh Landing has so far relied on diesel engines to ensure it could perform a black start in case of a power outage. But soon, that’s going to be a thing of the past.

Currently, the plant is being equipped with a customized battery storage system by Siemens Energy. It supports up to three attempts to restart the power facility on an expedited basis, and it also reduces emissions over its traditional back-up systems.

Doing so doesn’t make Marsh Landing an exception: today, there are numerous plants around the world taking advantage of using batteries for carbon-free black-start capabilities.

This black-start capability is just one benefit among many. As the share of renewables keeps increasing, energy storage systems will be essential to balance fluctuating energy supply, grid stability, and 24/7 availability of renewable power.

However, not all energy storage solutions are created equal, as they will play different roles. For example, batteries serve a different purpose than rotating grid stabilizers, which in turn fulfill different functions than thermal energy storage or green hydrogen.

If we look at these different energy storage systems more closely, we don’t just get an impression of the variety of those systems: we gain insight into the shape of our future energy system.

Batteries

Let’s first focus on batteries. A rather mature technology, currently, they are not only well suited for black-start capabilities: they economically solve a variety of issues that arise when trying to decarbonize.

Batteries support carbon-free energy production, as they address the volatility of renewable energy sources by storing energy when it’s available in abundance and providing it when there’s a shortage.

They are also flexible, allowing fast supply of energy when needed; and they are plannable and reliable. Additionally, batteries help power producers avoid curtailment, a mostly involuntary reduction of energy output.

Likewise, they make energy arbitrage feasible – storing energy when it is cheap and selling it when prices are high. In combination with renewables such as wind farms, battery storage helps manage power depending on current needs.

And large offshore vessels and drilling platforms use it to minimize the use of diesel generators, as well as to reduce CO2 and NOx-emissions.

However, we have not yet unlocked the full potential of batteries. In the coming years, batteries will be able to help virtually expand grids (by providing and consuming electricity) and managing grid congestion.

That, in turn, allows companies to flexibly handle the increase in energy demand. For industrial assets, batteries will enable peak shaving by supplying electricity demand during peak hours.

That is not to say that there aren’t open questions. First, today’s batteries have limited capacity, meaning they can provide power for only a few hours, and, over time, they degrade.

Second, there is the continued usage of rare elements in the production of batteries, raising environmental issues as well as concerns over the dependency on countries supplying them.

But through continued research, rare elements should become less important. This goes along with the importance of finding ways to either reuse or recycle batteries – and developing new battery concepts such as metal-free flow batteries which may achieve longer discharge periods.

Overall, batteries are one option among many, and the potential shortcomings of one solution are easily matched by others, such as rotating grid stabilizers, thermal, or hydrogen storage.

Rotating grid stabilizers

As the need for power from renewable sources increases, fluctuating power is not the only concern for power generation.

Another important challenge is that with less conventional synchronous power generation, grid frequency is getting more sensitive due to the reduced number of rotating machines.

Grid operators are already faced with the challenge of providing sufficient system inertia of synchronous generators with high rotating masses to avoid black-outs due to fast frequency and voltage drops.

Rotating grid stabilizers (RGSs) solve this challenge, as they provide additional system inertia and short circuit power to the grid – and they actually do so at critical grid locations worldwide today.

A typical RGS system consists of a synchronous condenser and a flywheel. The flywheel stores energy as rotational energy. As soon as the grid frequency drops, the flywheel responds, resulting in balanced and more stable grid frequency.

The task of the synchronous condenser is to connect the flywheel to the electric grid and thus help stabilize the grid.

This way, RGSs also enable the grid to handle fluctuating renewable infeed. As they release no emissions, they are as environmentally sound as the energy that feeds them. And they are cost-efficient, as their lifetime ranges from about 30 to 40 years.

Additionally, by replacing the system inertia that is currently being supplied by fossil power plants, RGSs enable a share of renewables of up to 100%. Another benefit they provide is the repurposing of conventional power plants which otherwise might be phased out, re-using their infrastructure, and offering a second life for these assets.

Thermal storage

Thermal energy storage supports decarbonization by handling another important building block to a future energy system: heat. It makes use of heat produced by renewable energy or captured from waste heat or exhaust gas, ranging in discharge duration from mid-term to long-term storage.

Thermal energy storage improves a plant’s efficiency as well as its operational flexibility, and also increases the energy yield.

A great variety of heat storage media are available, such as liquids like molten salt and pressurized water, or solids like stone, steel, concrete, or sand. Thermal energy storage also feeds thermal energy across sectors back into various processes and makes them more flexible, in heating as well as cooling applications for buildings or industrial processes.

Also, renewable electricity can be fed into thermal storage via resistive heating, helping to decarbonize heat production and to balance availability and demand for thermal energy.

So, the potential is undoubtedly great. Heating and cooling, for example, are Europe’s largest energy consumers, using more energy than mobility or electricity sectors.

Green hydrogen

Now let’s turn to green hydrogen – produced via the electrolysis of water with electrical energy from renewable sources, meaning it´s completely free of CO2 emissions.

Green hydrogen will enable long-term storage that, in combination with other storage solutions, allows the efficient coupling of all sectors of the economy. It’s an excellent solution for long-term energy storage, particularly in hydrogen pipeline and cavern storage networks.

With the ongoing build-up of a pan- European pipeline network and with sufficient cavern storage to be built up by 2050, it will enable seasonal power-to-power storage on a large scale.

Re-electrification will be realized in H2capable gas turbines, engines, or fuel cells to provide security of electricity supply; in periods of low renewable energy supply, for example, when there is lack of wind.

Compared to the other storage solutions mentioned thus far, hydrogen also enables other applications. It can be used directly as fuel for mobility or as a feedstock for various industries. Via synthesis with carbon dioxide, it can be converted into synthetic, sustainable e-fuels such as e-methanol, e-methane, e-diesel, e-jet fuel, or other carbon-based chemicals.

That’s not to say there aren’t challenges. One is the present cost of producing hydrogen, another the current lack of infrastructure for producing, distributing, and storing hydrogen.

But these aren’t permanent roadblocks. Transportation costs can be significantly reduced by using existing gas infrastructure. Also, production cost can be cut by upscaling industrial processes.

So, in short, the potential is great. Estimates are that sector coupling via hydrogen has the potential to reduce primary fossil energy consumption by 50% even while power demand grows by 25%.

Long-duration solutions

While the picture drawn above gives a rough idea of the essential role energy storage systems will play in our energy future, today the idea of energy storage is mainly associated with batteries, which store energy only for short periods of time.

That’s why next to green hydrogen, other long-duration energy storage systems should be mentioned.

There is pumped hydro, globally the most widely deployed bulk energy storage solution, which has been used for millennia. It produces energy when stored water flows downhill and is capable of supplying reactive power when there is an imbalance in the grid.

Compressed air energy storage is a mechanical storage solution that offers a reliable, cost-effective, and long-duration energy underground storage solution at grid scale. It’s especially attractive in areas where geography does not support pumped hydro, but large caverns are available.

And finally, ETES (Electric Thermal Energy Storage) by Siemens Gamesa Renewable Energy enables long-duration storage by using electricity to heat volcanic stones to temperatures of 600°C and higher.

The stored heat can be converted back into electricity using a steam turbine.

All in all, it is these long-term solutions that will make a decarbonized energy system robust and sustainable. Solutions for longer duration storage – with the exception of pumped hydro – still have to become known more widely and accepted by the market. But as the share of renewables increases, the need for storage systems will become not just a building block for sure, but part of the very foundation our energy future rests on.