© TU Berlin

We all know that wind power, photovoltaics and other renewables do not provide a steady supply of energy. The generation of renewable energy greatly fluctuates, and sometimes even comes to a halt if there is no sun or wind. So there are times when there is little energy available and times when there is plenty. In order to ensure security of supply in Germany at all times as the share of renewables grows, you can either install a huge amount of capacity creating a situation where there is always more energy available than needed – but resulting in high costs – or you can store excess energy so that it can be fed into the grid when it is actually needed.

However, energy storage units are expensive, too. Before setting up a storage unit, you therefore have to take a very close look at what makes sense economically – what is the right size of the storage unit, where is it to be installed, for what purpose, and at what cost.

Apart from installing stationary units for storing energy for a certain amount of time, we are currently also discussing using the batteries of electric and hybrid vehicles connected to the grid. As it is very likely that the number of these types of vehicles will increase in future, I believe that this is something we need to do. As most cars are not being used most of the time, they could – if plugged in – absorb excess electricity from the grid or feed it back in when it is needed. The extra stress on the battery is quite low. Batteries also age if they are not being used. Sometimes, due to the car being used only very little, the battery even fails before having reached its maximum cycle count. So this could actually result in better use of the battery.

The additional costs for using car batteries as mobile storage units are considerably lower than the costs for installing stationary storage units. In order to ensure that energy remains affordable, even as the share of renewables increases, we simply have to harness this opportunity.

Prof Julia Kowal is a professor of Electrical Energy Storage Technology at the TU Berlin.

© RWTH Aachen/JARA Energy

No, we do not need car batteries as mobile storage units in order to ensure a reliable and stable electricity supply and make the energy transition a success. However, we do need the energy transition to make electric mobility a success, because using low-carbon electricity is a prerequisite for zero-emissions mobility. As of today, we do not need electric vehicles as storage units in order to further expand renewables. It is getting increasingly difficult to obtain primary balancing energy from conventional plants as very few of these plants are connected to the grid for a whole consecutive week. In order to account for this development, we are currently installing many battery storage units with a capacity of 5 to 10 MW. At the moment, we do not need any more storage technology than this. There are also quite a few alternatives to storing electricity, for example demand side management and PV storage solutions for private households.

But as the share of renewables grows, the need for storing electricity will rise as well. I am convinced that by then, we will have a high number of electric vehicles on our roads. The batteries of these vehicles are not used for 22 to 23 hours every day and from an economic perspective, we have to do everything that we can to tap this potential. This could go as far as making it mandatory to ‘plug in’. A battery has enough charging cycles to be used as a mobile storage unit and feed back energy into the grid, and the charging process does not put any extra stress on the battery. If we leave this potential untapped, we would have to find other ways of storing electricity and making our system more flexible, and consumers would have to foot the bill. The costs for additional storage units would be passed on automatically to all consumers of electricity. Vehicle batteries are already there and the additional costs for using them are low. Car manufacturers, energy companies, IT service providers are preparing to access this market.

Prof Dirk Uwe Sauer is a professor of ‘Electrochemical Energy Conversion and Storage Systems’ at the ‘Institute for Power Electronics and Electrical Drives’ (ISEA) at RWTH Aachen University.