What is the best size and location for Energy Storage?
Energy storage technologies are available over a range of scales. Different scales are suitable for different applications, with very large scale technology options more suitable for transmission connected centralized storage facilities providing energy management, or for the storage of large amounts of renewable energy. Smaller scale facilities can be used in distribution networks, providing support for sections of the distribution network or even for individual houses. The technologies on the different scales can often be effectively used for the same purpose provided the storage efficiency and self-discharge characteristics are similar, i.e. a large scale centralized storage option could be used for energy management or the coordinated action of a large number of domestic scale storage devices could provide energy management for many individual properties.
Figure: Illustrating where storage on different scales typically connects to the electricity network.
The scale on which energy storage is most suitable depends not only on the technology, but also on the network in question, and what the task of the energy storage is. For example, in order to peak shave and lower the risks of blackouts due to transmission constraints at peak times the storage would be best sited on the demand side of the transmission. Hence a domestic or distributed energy storage product may be the most attractive option. This might suggest a storage size of 2-5 kW and 4-10 kWh that could cover 2-3 hours of domestic peak load or district scale energy storage up to 1MW and up to around 2 MWh. These systems would then reduce the size of the peak demand as seen by the distribution and transmission networks. Alternatively, to provide reserve services in the event of an unexpected output loss from a large coal or nuclear plant, a centralised transmission connected energy storage facility would be preferable. This would probably require hundreds of MW’s to GW’s of output, and need to be able to maintain this output over several hours.
In the UK there has been some discussion about using energy storage to store large amounts of wind energy , both to try and “firm up” wind energy to displace fossil generation and in the nearer-term avoid the rare situations in which wind energy is available but it cannot be used. In order to achieve this using large-scale energy storage (with either centralised storage on the transmission network or storage on the wind-farm side of the meter) would probably require upwards from 50 MW power and hundreds of MWh’s capacity. Alternatively the co-ordinated action of a large number of small scale storage facilities may be able to perform this task, provided there exists the transmission capability to effectively transfer the power. Obviously if the windfarm output was constrained due to a transmission constraint, then meter-side storage would be the preferable option.
The optimal location and capacity of energy storage depends on the desired application and the surrounding network. In terms of alleviating any network constraints the best placed storage would be in close proximity to the source of the constraint. However energy storage at many scales can provide similar services provided that it has similar characteristics, with numerous smaller energy storage devices being more flexible than a single large-scale device. Scale is also intrinsic to most storage technologies – i.e. pumped hydro only makes sense on a large-scale, say for transmission connected centralized energy storage, whereas electrochemical batteries seem much better suited to distributed or even domestic storage options.