Stand-alone grids distribute electricity within closed areas without external dependencies. Self-sufficiency and sustainability are drivers for their establishment, as is the development of areas where the expansion of interconnected grids is not worthwhile. The higher the share of renewable energies in electricity generation in an island grid, the more important the use of suitable storage technologies is for security of supply.

Relevance of the field of application

Stand-alone grids serve to secure the power supply in areas that are not or no longer connected to a supra-regional power grid - whether for reasons of self-sufficiency and sustainability or because grid expansion would be unprofitable. Such "off-grid supplies" play a comparatively minor role in Germany, although their relevance is also increasing here.

Globally, the importance of island grids is many times greater: around 770 million people still have no direct access to electricity, 75 % of them in sub-Saharan Africa away from central interconnected grids. Yet electrical energy is seen as a central basis for economic development. According to estimates, access to electricity is likely to have declined in 2020 for the first time since 2013 as a result of the Corona crisis.¹

The existing island grids have recently been increasingly converted from fossil (mainly diesel) to renewable energy sources (solar and wind, but also small hydropower plants and biogas plants), for both ecological and economic reasons. In any case, renewable energies play the main role in the realisation of new off-grid projects.

Battery storage systems are particularly important in regenerative island grids based on solar and wind energy: they ensure the balance between fluctuating power generation determined by environmental conditions on the one hand and consumption on the other. In addition, they also ensure the reliability of the island grid and provide system services, such as balancing power to compensate for short-term supply or demand surpluses.

Use of battery storage

The design of island grids is as individual as the regions in which they are used. There are considerable differences on both the consumer and the generator side. On the consumer side, the number and structure of the consumers with the associated load profiles are the determining factors. On the generation side, the mix of electricity sources used and the corresponding environmental conditions are decisive.

In the field of energy storage, batteries are proving to be particularly suitable due to their high energy efficiency and flexible applicability. Depending on the application, it is obvious to use only one battery system or a combination of two battery systems: a "power battery" to compensate for fluctuations in the power grid and to serve high loads, and an "energy battery" with lower power but higher capacity.² Together with the generation units, the consumers and an energy management system, the battery storage systems form the island grid.

Performance requirements

The establishment of new stand-alone grids or the conversion of existing ones is oriented towards the goal of a sustainable and at the same time secure energy supply. Harmful effects on the environment and resources should be minimized. This requirement is transferred to the battery storage systems used: their production and operation must be particularly energy- and resource-efficient in order to meet the sustainability requirements of stand-alone grids.

In addition, stand-alone grids place high demands on the service life of the battery storage systems: These should function reliably over a long period of time in order to ensure investment security. The benchmark for this is often the service life of the other system components, such as the generation units. The more battery storage systems function as "power batteries", the more important the requirement for cycle stability becomes, as frequent charging and discharging processes are to be expected here.