Energy storage: the necessary supplement to sustainable energy sources

Of course, a sustainable energy supply will depend primarily on the use of sustainable energy sources. But their intermittent availability requires major changes in the electrical grid. At all levels in the grid, from the individual household to the national grid, energy storage will be a necessary supplement to solar and wind power.

Peter Notten
Peter Notten

Long transition

‘Energy storage is the key to the sustainable energy system,’ says Peter Notten, professor in Energy Materials and Devices at Eindhoven Technical University; ‘and this should be installed both at a small and a large scale. Smart grids, with intelligent switching on and off of devices and storage systems, will play a decisive role. In a grid that is fed by intermittent sources, it is impossible to store electricity merely at a large scale. Therefore we will have to introduce energy storage at all levels; we need all available technologies. Both batteries and fuel cells at a small scale, and pumped hydropower at a large scale.’

Electric car
Around 1900, many cars were electric (on lead batteries).

‘Energy storage in electric vehicles has an enormous potential,’ says Notten, ‘but we will need to re-engineer energy management systems in order to take them into account, and the growth of the fleet of electric vehicles will have to stay in tune with the supply of solar and wind power. Driving electrically on coal-based electricity is a waste of energy. Even though the energy efficiency of electric cars over the entire chain is double that of cars with internal combustion engines (34% vs. 17%), this efficiency doubles (to 69%) as we move to sustainably sourced electricity. Therefore it is advisable only to drive electric on the basis of sustainable energy. The implication of that is that we will face a long transition period, because we need to restructure the entire energy system. The interlocking of these processes implies a transition period of some 50 years. Driving electric is rather inefficient now, but that is not what we should be heading for.’

High-capacity energy storage

Peter Notten and his group research four areas. Firstly new materials, like silicon as a storage material for lithium in an Li-ion battery as a substitution for graphite. Secondly new technologies, like batteries integrated in chips; or micro storage combined with a solar cell. And modelling, in order to gain a better understanding of processes within batteries, particularly to prevent energy losses in charging and discharging. Finally application of those models in ‘battery management’, of major importance for safety and longevity.

Energy storage in Li-ion battery
Li-ion batteries are the most commonly used nowadays.

‘We have already witnessed major improvements in battery technology,’ says Peter Notten. ‘An Li-ion battery can store five times as much energy per kilogram compared to a lead battery. 1 kWh in an Li-ion battery now weighs 5 to 10 kilograms. At first sight, that number would seem to be high, but 1 kWh is the energy required to pump 360.000 litres of water 1 meter in height. That single kWh can now drive an electric car for 7 kms. For an operating range of 140 kms the battery should be able to store 20 kWh; it will weigh 100-200 kilograms and costs… € 10,000. Therefore, technologists all over the world look for cheaper batteries with larger storage capacities. This capacity problem in batteries also offers new opportunities for fuel cells. The fuel cell stores its energy externally and therefore potentially has a much larger capacity. But in batteries as well, there is much room for improvement. The lithium-air concept is promising. Here, the air forms one of the electrodes; it is freely available and does not have to be transported. The other electrode could be pure lithium, potentially leading to very high densities.’

Safe energy storage

‘In the long run, lithium has its disadvantages as well. Resources of lithium are abundant, but they are almost exclusively situated in South America. Whereas lead recycling is perfectly well-organised, there is no lithium recycling at present, because it is not cost-effective. One of the applications of used lithium is in concrete, it prevents carbonation. It is vital to do good LCAs if we increase the level of energy storage. And because of possible lithium shortages, we now also investigate sodium and magnesium as electrode materials.’

High energy densities also carry risks. Therefore battery management is one of the focal points of the research in Peter Notten’s group. ‘For instance, temperature gradients resulting from charging and discharging batteries are a threat to both longevity and safety. We develop methods for measuring and managing such temperature gradients. In order not to have to be confronted any more with news on laptops catching fire. And to ensure that both at a large and a small scale, safe energy storage will be an indispensable element of a sustainable energy system.’

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