Win-win for home and grid

An increasing number of Swiss roofs have solar panels on them. This poses challenges for grid operators. After all, on a sunny day vast amounts of electricity can suddenly flow into the grid. If the grid is not built to withstand this surge, in the worst-case scenario it can even collapse. One possibility to prevent this kind of blackout would be to expand the grid infrastructure to withstand far greater maximum loads. However, this leads to considerably higher costs.

One alternative would be to prevent the grid from being inundated with large amounts of electricity. This would mean temporarily storing “surplus electricity” that had been produced locally. But is it worth the effort for the operators of the roof systems? What are the storage options? And can the power grid actually be stabilized in this way?

Empa researcher Philipp Heer set about exploring these questions. For his project, he used real measurement data from the local power and heat supplier Glattwerk in Dübendorf and studied two battery types: lithium ion batteries and salt water batteries of the sodium nickel chloride type, also known as ZEBRA battery (see box). In computer simulations, Heer calculated 160 different scenarios, varying both battery sizes as well as the entire system, which may be based on one central or several decentralized battery storage devices.

There are two parties with different interests. On the one hand, the grid operators: These companies run power grids in the mid and low-voltage range in order to distribute electricity to the consumers. In Switzerland, there are around 650 grid operators, which, taken together, maintain a grid covering around 200,000 kilometers. Their goal is to minimize the risk of a blackout on the grid, without having to expand the grid to accommodate a maximum load that can only be expected from time to time.

On the other hand, there are the consumers who simultaneously produce electricity themselves – referred to as prosumers. Their objective is to minimize their energy costs. This means that the self-produced electricity is supposed to be consumed when energy costs are high. As the feed-in tariffs are currently very low compared to the reference tariffs, it is hardly worth it for the prosumer to feed the generated electricity into the grid.

How can stationary batteries now be used in such a way that both sides stand to benefit? Imagine a sunny day: The photovoltaic systems provide power during daytime, i.e. when many residents are not at home. Feeding the electricity into the grid has drawbacks – for both sides: Consumers need to purchase the electricity in the evening again at a higher price, and grid operators have to expand their network to be able to accommodate the surge of electricity. If the self-produced electricity is stored temporarily in local batteries, on the other hand, prosumers can consume it in the evening "for free" – thereby relieving the pressure on the grid.

Of course, batteries are not without disadvantages. Their efficiency is not 100%. Overall, the average energy consumption on the entire grid, therefore, increases if battery storage devices are being used. In order to increase the battery’s usefulness for all parties, it would thus make sense to optimize the battery controls to accommodate the different interests of the stakeholders, instead of merely maximizing cost savings for the individual prosumer. In the worst-case scenario, all prosumers would fill their batteries with surplus PV electricity until they are fully charged at midday, for instance – and then suddenly all feed the electricity into the grid at the same time. The grid operators would then experience another peak in fed-in energy.

Optimized battery controls would charge the battery precisely when more power is added to the grid than is drawn from it. And this pays off: “Our simulations reveal that batteries that are optimized to accommodate the combined control objectives achieve a yield that is 15% higher than those, which are only optimized in favor of a single stakeholder,” explains Heer. Small, decentralized batteries can already be worth it for both parties – but larger, shared storage systems could carry even greater advantages.

In order to see whether the results of the simulation also prove to be true in reality, Heer and his team are now planning on testing battery controls that have been optimized in this way in a real system. To this end, they use Empa’s energy demonstrator, the Energy Hub, or ehub for short. The different NEST units act as prosumers, which produce and consume differing quantities of energy. Both a salt water and a lithium ion battery are on hand for the tests. “If the simulation results prove themselves in reality, the local grid in Dübendorf could also serve as a pilot project,” says Heer.

The salt water NaNiCl₂ battery, also dubbed ZEBRA battery, was developed in Pretoria, South Africa, in 1985. ZEBRA is an acronym of the project’s name, “Zero Emission Batteries Research Activity”. The batteries are based on raw materials, which are available in abundance compared to other battery technologies, such as lithium ion batteries. They are operated at temperatures of around 300 degrees Celsius.