Batteries, a continuous research

When it comes to renewables, the main obstacle they present stems from their intermittency and variability. Creating energy from the sun, wind, ocean or even agricultural waste means coming to terms with irregular or even unpredictable performance. Another problem is that the most useful form of energy that we have is, of course, electricity. We use it to get around, generate light and heat, cook and run all of the devices that surround us. Electricity, however, isn’t readily available in nature. We have to produce it using other forms of energy.

Additionally, once we’ve generated it, if we can’t find a way to use it immediately, we lose it. Forever. Given that we can’t sit around in the dark, we have to find a way to obtain energy exactly when we need it. To achieve this, all of the renewable energy plants in existence today are connected to a storage system or grid. When the sun is shining or the wind is blowing, isolated plants store the energy that goes unused at that time and maintain it for when it is needed. Plants connected to grids, instead, simply transfer the excess energy to the grid, from which it can be retrieved when needed. Grid operators do the same with their grids, which are often as large as countries or continents.

Usually, excess electricity is stored as hydraulic energy; leftover energy is used to obtain water from the sea or low-elevation lakes and pump it to high-elevation lakes. When energy is needed, the water is then released through a penstock, and the force of the water moves a turbine, which generates new electricity. If there is an uptick in the demand for energy and the grid doesn’t contain enough, it can be purchased from abroad, or, in the worst case, thermal power stations can be brought into operation.

The use of batteries

For this reason, the ability to accumulate and store electricity is mandatory in order to make the use of renewable resources and fossil fuels really efficient, to strike a balance between the intermittent nature of the sources and the variable demand for energy in industrial and civil contexts, and improve the stability, flexibility and reliability of distribution grids.

Batteries are therefore a key tool for energy access because they allow for resources with a lower environmental impact to be used more intelligently, and therefore reduce CO2 emissions. Batteries can come in a wide variety of sizes, and each is suited to a specific use. There’s everything from watch, mobile phone and notebook batteries, to energy storage batteries connected to a house’s solar panel system, to those used in large infrastructures and connected to power stations.

Battery types also vary according to use and the requested average storage time: from the batteries in our smartphones, to those present in large facilities, which have to compensate for the variations in the supply and demand for energy between day and night. The largest ones have to manage the fluctuations over entire seasons. If we think about it, the main limitations of all mobile devices, from smartphones to electric cars, stem from weight, production cost and disposal to the low capacity of the batteries that power them. The same problem applies for larger systems, from electric cars to large facilities: to fix the problem, you can’t just create larger batteries. All over the world, new solutions to this problem are being sought. State-of-the-art research is also happening in Italy; electrolysers, for example, make use of surplus electricity to produce hydrogen from water in an electrolytic cell, and then run them in reverse when the electricity is needed by consuming the stored hydrogen. A promising alternative Eni is also developing a redox flow battery, which is an electrochemical cell connected to two tanks containing different electrolytes dissolved in solution. The electrolytes are pumped into the cell, where a special semipermeable barrier prevents the fluids mixing. Here an oxidation-reduction (redox) reaction occurs that transforms the chemical energy stored in the two fluids into electricity, which can be removed from the cell and used.  In contrast, when it comes to a renewable source –for example, a solar panel on a bright and sunny day– the energy produced by the system would enter the electrochemical cell and be used to trigger the same redox reaction, but in the opposite direction. The two fluids would thus return to storing ready-to-use chemical energy when needed. Redox flow battery technology is one of the most promising, not only in terms of its current level of development (industrial installations already exist), but also in terms of its potential (the research is constantly evolving). It also makes it possible to easily separate the power component (the size of the cell) from the energy storage component (the volume of the tanks). This decoupling removes the possibility of the self-discharge phenomenon (you know when you take out a battery that you charged a while ago, and it’s completely empty even though you never used it?) and makes it possible to create a custom battery for whatever necessary power and storage needs. Lastly, at the end of their lives, redox flow batteries are much more easily recycled than other types of batteries; electrolytes, which make up the majority of the system, can be recovered and purified. Even the rest of the system, which consists of metal alloys, plastics and commercial electronics, can be sorted and recovered. Until studies on redox flow batteries can lead to the creation of industrial systems, the development of renewables cannot reach its full potential. The research just needs to carry on.

The author: Luca Longo

Industrial chemist specialized in theoretical chemistry. He was a researcher for 30 years before moving on to Eni's scientific communication.

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