The goal of this report is to provide a basic overview of electricity storage technologies and their potential applications, especially with regards to the transition to an electricity system with high shares of renewable energies.
Motivated by the threat of global warming and other environmental impacts of energy usage, security of energy supply and economic reasons, there is an international effort to increase the share of renewable power generation in the electricity supply. However, the two fastest growing renewable energy sources, wind and solar power, are naturally fluctuating due to weather conditions as well as diurnal and seasonal patterns. Furthermore, the best harvesting potential does often not coincide spatially with the centers of power consumption.
Therefore, the transition to a power system with high shares of renewable power generation requires a differently structured electricity grid with higher transmission capacity in order to bridge spatial distance between supply and demand. Additionally, the power system requires capabilities in order to bridge distance in time between supply and demand, which we may call flexibility. Flexibility is the ability of the power system to match fluctuating generation with - also fluctuating - demand. Sources of flexibility can be dispatchable generation (fossil, hydro or biomass), demand response, the curtailment of renewable generation and/or electricity storage.
In general, different sources or combinations thereof can provide flexibility. The choice will depend on economic factors, social acceptance, ecological considerations and other factors. This report focuses on storage technologies and specifically those that are able to absorb electricity from the system and reconvert it into electricity at a later time.
With increasing shares of renewable generation, the electricity system is in a transition from currently demand driven centralized and fossil-based generation, where flexibility is predominantly provided by dispatchable generation, towards more supply driven regenerative and distributed energy production where additional sources of flexibility will be required.
This also affects the operating principles and the system stability. Traditionally, the major conventional power plants supply energy and balancing power to the grid and the power flow is always directed from the higher voltage levels (location of power plants) to the lower voltage levels (location of consumers). Due to an increase in renewable electricity generation on the mid and low-voltage level (residential PV systems, onshore wind power) the power flow direction can be inversed. Furthermore, during times with high renewable energy feed-in the electricity demand can be covered completely by renewable generators. This leads to situations where new ways of guaranteeing system stability have to be found: either conventional power plants have to be run at minimal load with high specific emissions, or electricity storage systems, demand side response and the renewable energy generators themselves have to supply the necessary system services.
Besides the integration of renewable electricity generation the most active field of development for electricity storage systems is the electrification of the transport sector. The demand for especially lithium-ion battery systems rises rapidly due to the introduction of plug-in hybrid and full electric vehicles. This transition is also linked to the electricity sector for two reasons: Firstly, the increased demand fosters mass-production of batteries which causes decreasing battery prices which also helps to introduce storage systems in the electricity grid. Secondly, the battery storage systems of the traffic sector can also be used as grid storage during times when the vehicles are plugged-in for charging. The link between these two sectors causes major transitions in the traditional world of utilities as automobile manufacturers start to produce their one “green” electricity for their electric vehicle fleet. On the other hand utility companies start getting involved in the mobility sector as they deploy the charging infrastructure and develop business models for electricity supply for e-mobility. Both trends result in a closer link of these two important branches of industry.
In order to keep this report reasonably compact it was limited to the discussion of electricity storage systems. However, this is not to be mistaken as a statement of preference for electricity storage as a source of flexibility. In some cases electricity storage is unavoidable, however, for the time being all flexibility sources are to be considered as at the current state of development there is no single solution that serves all flexibility needs. One alternative to electricity storage is heat storage where heat is the final form of energy used. Especially low temperature heat in buildings and process heat in the industry have the potential to provide significant amounts flexibility. A short discussion of potentials and limitations is given in chapter 2.
All applications of electricity storage make it necessary to understand the technology options and their alternatives. As the cost of electricity storage strongly depends on the specifics of the application, this report cannot give general recommendations for the use of certain technologies for given applications. It is a first introduction into electricity storage systems and their possible applications. The report is structured as follows:
Chapter 2 gives an overview on selected applications for energy storage systems. This covers mainly electricity grid services but also other important systems like electromobility .
Chapter 3 gives an introduction into important parameters and terminologies. A systematic classification of storage technologies helps to better understand the different main characteristics.
Chapter 4 is the main part of this report and contains the description of important (technically and operationally proven) electricity storage technologies with their technical parameters and their deployment potential. Mechanical, electrical, chemical, thermal and electrochemical (batteries) storage systems are covered.
Chapter 0 discusses the potential role of the different storage technologies.
A comprehensive discussion of demand and cost of storage for the integration of renewable energy exceeds the scope of this report. The feasibility of different storage options, the amount of storage required at different shares of renewable energy and the related costs are being discussed among experts and in public. The main complications in the search for answers to these questions is the fact that demands for generation, transmission and flexibility are interlinked and can replace each other to some extent.
An estimate of the storage demand for the coming years therefore depends on the share of renewable energy, the fraction of the different renewable sources, the flexibility of the conventional generation portfolio and the transmission capacities, the cost structure of each component above and their development throughout Europe.