The wind and the sun are not interested in how much of their valuable energy is being used. The amount of energy they produce can vary strongly from one moment to the next. As wind and solar power begin to play a more important role in the electricity mix, the availability of high-volume storage capacity will become an increasingly critical factor.
The dishwasher is running, the washing machine is churning away and dinner is simmering on the stove. Time for a quick cup of tea, and boom – out go the lights! The water boiler overloaded the circuit, which was no longer able to meet demand and collapsed. The fuses blew and interrupted the power supply, just like they are designed to do.
While a situation like this is generally easily remedied with a quick trip to the circuit breakers, it can cause chaos throughout entire cities and regions when it happens on a bigger scale. Because the huge transmission and distribution networks also operate according to the same principles: they shut down when they are overloaded.
Bearing this in mind, grid operators and power producers always have to balance supply and demand for electricity. In the morning, when millions of people turn on the lights and the coffee machine, or huge industrial plants start operations, power stations everywhere have to ramp up production. In the old days, this was generally solved by just throwing some more coal in the furnace.
Nowadays, it’s more complex: green energy has priority in the grid. Consequently, network operators have to keep an eye on the weather forecasts to predict how much wind and solar capacity will be available. Conventional power stations are used to make up for the rest. So far, there have always been enough reserves to make this possible, but signs of bottlenecks are beginning to appear.
Another challenge is that the output of solar and wind farms can vary from second to second: all it takes is a cloud blocking the sun, and suddenly the power drops.
As a result of this, network frequency also declines all of a sudden. In Europe, the network frequency is 50 Hertz. Tiny variations of 0.01 Hertz are normal and are compensated by the inertia of electricity, various technical buffers and sheer size of the transmission networks. Deviations of just 0.2 Hertz, on the other hand, require active intervention.
This ‘primary control power’ is so important for the stability of the power grid that it is traded on its own independent market at prices set weekly. This system has a long tradition in Germany. And its significance is likely to increase as more and more renewables participate in generation.
Second-to-second imbalances in the system are nothing new, as they also theoretically occur every time any electric device is switched on. But these imbalances are growing with the rising share of renewables in the energy mix.
Consequently, the industry has been working for years to develop new solutions such as flywheel energy storage systems or capacitors. Modern battery storage systems are also very popular. In just a fraction of a second, they can store a relatively large amount of electricity or feed it into the grid. This allows them to provide larger amounts of primary balancing power and smooth out major network fluctuations for up to five minutes.
Using more intelligent power grids, known as smart grids, it may even be possible in the future to use the batteries of electric vehicles which are connected to the grid as a source of primary balancing power. Owners could then be paid for this contribution to maintaining the network frequency.
If a critical deviation in frequency lasts more than 30 seconds, transmission system operators request suppliers to provide what is known as a secondary reserve. To this end, at least five megawatts of power must be made available within five minutes.
So far, this job has mainly been handled by gas turbines, biogas plants or pumped-storage power stations. Similar to large battery storage units, the latter are well suited for storing the surplus production from renewables.
With the exit from nuclear power in 2022 and the gradual reduction of coal-based electricity generation, long-term storage options will become an increasingly decisive factor for security of supply.
Who will supply the electricity when the wind is still and the skies are cloudy for a few days, as was the case in January 2017? These periods of ‘dark doldrums’ lasting 48 hours or more are not very frequent. According to calculations by the German weather service, on average these conditions occurred twice annually between 1995 and 2015. Nevertheless, a way of maintaining the supply of electricity during these periods must be found whenever no conventional generation capacity is available.
In an ideal situation, long-term storage facilities would already be available, topped up with energy generated when the wind and the sun produced more power than was needed. This would also lead to an immediate, significant reduction in CO2 emissions.
However, the capacity of pumped-storage power plants in Germany alone amounted to around 40 gigawatt hours (GWh) in 2016. So, pumped storage is not a viable option for bridging extended periods of overcast, windless weather. And no other significant, long-term storage capacities are available. Even so, the technical potential of these facilities is immense.
Power-to-gas technology is also believed to possess great potential. The underlying principle is relatively simple: electricity from renewables is used to break down water into its components, oxygen and hydrogen, using electrolysis. The pure hydrogen can then be stored directly in gas storage facilities and used later to oxidise back to water and release electricity, going through the opposite process.
Furthermore, the hydrogen can also be combined with carbon dioxide, captured from the flue gases of coal-fired power plants for example, and processed into methane. Methane is the main component of natural gas and is used as a fuel in automobiles, home heating systems and fuel cells, as well as in gas-fired power plants to generate electricity.
Compared to other storage technologies, power-to-gas has one major advantage: the infrastructure and storage facilities already exist.While electricity storage units require massive investments to expand the transmission networks, gas can be transported using the existing gas grid.
Furthermore, Germany has the largest gas storage capacities in Europe. According to the Federal Association of Natural Gas, Crude Oil and Geothermal Energy (BVEG), additional investments are already planned. BVEG estimates the currently available volume of gas storage at around 24 billion cubic metres, which is equivalent to roughly one quarter of Germany’s annual gas consumption. In other words, the roughly 235 terawatt hours of stored energy is equivalent to more than 45 per cent of the electricity that was used in Germany in 2016 as a whole.
To date, gas made from wind and solar power has only really played an experimental role, as naturally occurring gas is far less expensive. Nevertheless, as a storage medium, synthetic gas may ultimately play a strategic role, as its use would also result in less dependence on imported natural gas.