The underestimated factor in the energy transition: battery storage

Wind and solar energy are important pillars of the energy transition. And while their share of the electricity mix is steadily growing, another pillar is increasingly coming to the fore: battery storage. Because without intelligent storage, the energy transition cannot be 100 per cent successful. What solutions already exist and what research is being done?

The expansion of renewable energies in Germany is going according to plan. In fact, that's an understatement. In the case of solar energy, Germany already achieved the expansion target set by the Renewable Energy Sources Act for the end of 2024 in May. In the case of wind energy, on the other hand, Germany is lagging behind schedule, but this is offset overall by the increase in solar energy.

Wind and solar energy are available in almost unlimited quantities. So all we need is enough wind and solar installations to collect the amount of energy that our society needs to live and operate, right? Not quite. Both energy sources are highly dependent on the weather and the season.

Why we need more than wind and solar power plants

Since the sun does not shine around the clock and the wind cannot be controlled, the energy supply needs another building block. So far, this has been gas and coal-fired power plants. However, for a complete decarbonisation of the energy industry, these central power plants must also be shut down in the near future. They will serve as an emergency reserve for a transitional period when peak loads occur or the dreaded ‘dark doldrums’ should occur. As long as the expansion of renewable energies continues, these large central power plants will continue to play an important role in the supply and stability of the electricity grids.

But even today, there are days when renewable energy sources generate so much energy that a surplus occurs. These days can be identified by negative electricity prices from suppliers with dynamic tariffs. Customers who have signed up for such a tariff earn money on days with electricity surpluses when they consume electricity. The reason for this is that the electricity grids themselves cannot store electricity; they must release the fed-in electricity. In the case of a surplus, either consumption must be increased or generation reduced. Consequently, current prices on the electricity exchange drop into negative territory.

While this is beneficial for these customers, such days are detrimental to the energy transition. They are costly for both the federal budget and commercial electricity producers. Negative electricity prices put a strain on the state’s EEG account, as feed-in tariffs remain high as expenses, while revenues from negative prices disappear. On the other hand, the price pressure could slow down or even halt the necessary expansion of the EEG, as the high investments may no longer be worthwhile.

The increasing number of days with negative electricity prices clearly indicates that the energy transition still lacks a crucial component to be successful in the long term: electricity storage. These storage systems could stabilise prices in the short-term market by absorbing surplus electricity from the grids. At the same time, this would enhance the reliability of the electricity networks, as they could take in electricity on a short-term basis and also feed it back into the grid when needed.

Different energy storage solutions for different tasks

One way to store surplus energy is by converting electricity into hydrogen. Converting it into heat can also be a sensible option. Both methods help to regulate surpluses. However, they are not suitable for the opposite case of under-supply.

In pumped storage power plants, compressed air storage, and flywheel storage systems, energy is also converted but can be transformed back into electricity through turbines or generators.

A significant role in storage technology is played by various batteries of different sizes:

• Small storage: These small stationary battery storage systems are often installed in conjunction with photovoltaic systems in residential homes. They capture a portion of the solar power generated on the rooftop and store it for times when the PV system does not produce electricity.
• Large storage: These systems store several megawatts of electricity and are therefore suitable for balancing the natural fluctuations in the EEG grid. They are used in the commercial sector or directly by energy suppliers.
   

Both storage sizes are primarily based on lithium-ion batteries. Although this is not a completely new technology, the development of lithium-ion storage on a large scale has led to significant cost reductions. Since 2010, the prices for lithium-ion batteries have decreased by 90 percent. Furthermore, due to research and development, they have become more resilient and durable, giving them an advantage over alternative battery technologies.

However, it is expected that alternative chemical batteries will become increasingly competitive in the coming years. For example, sodium-ion batteries are about 20 to 30 percent cheaper to produce, but they have not yet reached the development stage for mass deployment.

Outlook: Supercapacitors as an alternative to batteries

As chemical batteries always exhibit a certain degree of inertia, they are not always suitable for applications that require speed. This is particularly true for balancing voltage fluctuations in the grid. In the future, so-called supercapacitors, also known as ultracapacitors or supercaps, could step in. They operate not with chemical processes but with physical ones, which occur significantly faster. This makes them interesting for all applications that require ultra-rapid energy discharge.

However, it is not just their speed that makes supercapacitors so appealing; they can also be integrated extremely flexibly. Researchers at the Massachusetts Institute of Technology (MIT) have developed a supercapacitor made from everyday materials: cement, carbon black, and water. This could allow, for example, the foundation of a wind turbine to serve simultaneously as an energy storage system, storing surplus electricity directly at the site.

These "cement storage" solutions would also be very useful for construction. The researchers have calculated that a foundation of 45 cubic meters is sufficient to meet the electricity demand of a day (approximately ten kilowatt-hours).

Thinking further, cement-based supercapacitors could connect our motorways into a gigantic storage network. In combination with induction technology, electric vehicles could be wirelessly charged while driving in the future. This would simultaneously solve one of the biggest problems of electric cars: expensive and heavy batteries with limited range.

Text: Falk Hedemann