Batteries as Storage in the Electricity Network
25 Jan 2021
Due to the energy transition and the higher proportion of renewable energy, batteries are becoming increasingly important as storage in the electricity network. They smooth over fluctuations resulting from weather-dependent energy sources such as solar and wind power, and ensure stable operation of the electricity network.
Germany has set itself ambitious climate policy targets: by 2050, annual greenhouse gas emissions must be reduced by 80 – 95 per cent (compared with 1990). They must be reduced by at least 55 per cent by 2030. The energy transition is key to achieving these goals. In fact, the proportion of renewable energy in the electricity we use is increasing: from around 6 per cent in 2000 to around 42 per cent in 2019.
Renewable sources of energy such as solar or wind power also have a disadvantage: they don’t produce electricity reliably. If the sun doesn’t shine or the wind doesn’t blow much, the relevant facilities will generate little or no electricity. The network operator must compensate for these variations to ensure a stable electricity supply. This is achieved with the help of controllable power stations or electricity storage. The more electricity that comes from decentralised renewable energy facilities, the more important the latter becomes. Electricity storage decouples electricity generation and electricity consumption, smoothing over the varying availability of renewable energies. It can also deliver electricity at peak times.
Batteries and other storage technologies
There are numerous storage technologies available, with different properties and applications. These range from compressed air storage, pumped storage power stations and superconductors, through to batteries and accumulators (accumulators: from the Latin ‘accumulare’, to collect or to store). This blog article considers the latter. First, the electricity is converted into a different form of energy for storage. This process is already associated with losses. Depending on the type of storage, more energy is lost while it is being stored. There are additional conversion losses during the last step, when the energy is converted back into electricity.
One example is battery storage fed by wind power: the rotating rotor generates electricity via a generator. This is stored chemically in a battery, while attempting to keep losses to a minimum. If the electricity is needed, the chemically-stored energy is reconverted to electrical energy. Here the aim is to minimise the losses and improve the efficiency. Battery storage can take up and release high-power electricity at short notice. According to the German Federal Association for Energy Storage (BVES), modern lithium-ion batteries can, on average, undergo 10,000 load cycles before needing to be replaced.
Energy density is extremely important for the performance of a rechargeable battery. This is specified in watt hours per kilogram (Wh/kg). Lead batteries – which, for example, are still used today as starter batteries for vehicles with combustion engines – achieve 40 to 70 Wh/kg. Lithium-ion batteries – for example, those used in smartphones or electric vehicles – achieve more than 160 Wh/kg.
The size or capacity of electricity storage is specified in kWh (kilowatt hours). This value shows how much energy the battery or accumulator can take up or release in a certain period. The dimension used is the hour, i.e. the amount of energy which can be made available in one hour. Example: a storage system with a capacity of 8 kWh for use in private houses can release or store one kilowatt of power for a period of eight hours.
Large batteries with capacities of multiple megawatt hours are usually used as a reserve for neighbouring fossil fuel power plants and provide the power for black starts, i.e. starting up a power station from shutdown without assistance from the public electricity network. Medium-sized batteries offers capacities upwards of 100 KWh and, for example, help companies smooth peak loads. They can also use the battery as a UPS (uninterruptible power supply), should the network power fail briefly.
Increasingly, small storage solutions for domestic use are coming on line, with single-figure or low-double-figure kWh ratings. These store solar power from the rooftop photovoltaic system, either for personal use, or for feeding into the electricity network. Electric cars, small electric transporters and electric buses also offer potential for storage. Modern electric buses have a capacity of around 300 to 400 kWh.
Lithium-ion batteries predominate
Currently, the most important batteries are lead acid batteries and lithium-ion batteries – whereby the latter have become significantly cheaper and now dominate the market. Redox flow batteries and sodium-sulphur batteries still play a minor role at the moment.
Lithium-ion batteries have a higher lifetime than lead batteries, and can be used for storing solar electricity for up to 20 years. Here, the number of cycles and the depth of discharge are also significantly higher, which offsets the higher cost. In addition, they offer a higher energy density. In contrast, lead batteries are much heavier and take up more space than lithium-ion batteries. This also makes them unsuitable for electric vehicles.
The problem: supplies of lithium on our planet are limited, and could be used up in just a few years. Redox flow batteries using vanadium are seen as the technology of the future, because vanadium as a raw material is widely available on the Earth. These batteries store energy in fluids and can be scaled as required – making their use as large-scale storage for electricity networks a conceivable prospect. However, redox flow batteries are relatively large, heavy and – due to their complex design – currently still too expensive to manufacture.