| Energy storage


Grid-scale renewable energy storage – the impossible dream?


How much grid energy storage do we need to smooth out the fluctuations inherent in weather dependent renewables and how much will it cost? Back of envelope calculations suggest some very large numbers


Euan Mearns senior researcher, Chair of Entrepreneurial Risks, ETH Zurich


The Energy Transition, which should actually be called an “energy replacement”, is a first in human history, and, to a large extent is a west European venture. It is underpinned by wind and solar PV power. The intermittent supply from these sources is widely recognised as the main barrier to achieving high levels of zero carbon renewables


penetration. Mitigating strategies often include: ● greater grid connectivity; ● combinations of different kinds of intermittent source;


● energy storage.


The only known, self-contained and secure means of converting variable renewables to dispatchable, load following or base load supply is via the energy storage route. Current commercially available “grid scale” storage options include pumped hydro storage and batteries. Among future technologies, green hydrogen is currently seen as the front-runner. Only pumped hydro storage (PHS) is deployed at scale today, with numerous schemes allowing specifications, performance and costs to be meaningfully assessed.


To analyse the feasibility of storage options, it is necessary to have a good understanding of the


following variables: ● the energy efficiency of storage media; ● the capital cost of storage media;


The energy efficiency of storage options


There is no such thing as a free lunch in the energy world and if you want to store electrical energy, and to then return it to the grid, energy is lost in the process (round trip efficiency). The following are round trip efficiency estimates for the three storage technologies mentioned above:


Pumped hydro storage Li-Ion battery


H2 O electrolysis – H2 storage - combined cycle turbine


82.0% (source: Swiss authorities)


89.5% (source: Tesla) 38% (source: various)


In short, both PHS and Li-ion batteries are reasonably energy efficient. Green hydrogen is the black sheep with very low round trip efficiency,


● the stochastic structure of the variable renewable energy sources on the grid;


● the time-cost of storage; ● the scalability of storage media; and ● the environmental impact of storage media.


This article will provide summary figures on efficiencies and costs but will then focus on pan-European wind in order to illustrate the scale and capital cost of the storage challenge using existing and planned PHS schemes to provide some framework numbers.


where heat is lost in the electrolysers and in the combined cycle gas turbine (CCGT) plant. We assume 75% efficiency for electrolysis and 50% efficiency for the CCGT resulting in 38% combined round trip. It is possible to find higher efficiency figures, but these tend to overlook the fact that this equipment will run periodically, resulting in sub-optimum operational conditions.


The capital cost of storage media Costs can be estimated for PHS and Li-ion batteries, both of which are in commercial operation. The absence of commercial green hydrogen projects makes reliable cost estimates for that technology impossible.


Pumped hydro storage


PHS is by far the most widely deployed grid-scale energy storage technology in the world today. Global generation capacity is estimated to be 181 GW with a storage capacity of 1.6 TWh. If the global installed PHS were switched on at capacity it would drain all reservoirs in 8.8 hours. Most PHS built to date has been used in tandem with base load nuclear power where night-time surplus is stored and released into daytime peak demand and price. A natural diurnal price arbitrage finances the whole operation where storage is filled and emptied on a daily basis.


Figure 1, top: stack of wind production from Sweden, Denmark, UK, France, Germany and Spain during September and October 2015. This period was cherry picked as having several pan-European wind lulls. Renewable energy systems need to be able to ride through the most adverse of conditions, such as these. The mean output across the two-month period was 17895 MW.


Figure 1, bottom: simulation of energy storage required to smooth out fluctuations in generation. Total generation above 17895 MW, energy is sent to storage (pumping), total generation below 17895 MW, energy is recovered from storage (generating). The opening storage value was adjusted to ensure that reservoirs did not run dry when the low point is reached at the beginning of week 8. The storage capacity required is 2.1 TWh (as indicated in the middle of week 3)


www.modernpowersystems.com | April 2022 | 23


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