Grid-scale battery storage to complement renewable power
Updated: Jul 10, 2020
As the quantity and proportion of renewable energy on many developed electricity grids increases, grid operators face a growing problem of integrating intermittent, uncertain, energy sources with conventional power to meet electricity demand on a minute-by-minute basis. To solve this issue, the operator has a number of potential tools in his box, from “spilling” excess renewable power in times of high supply to providing short term gas-powered backup generation in times of shortage to, in the much longer term, demand side management.
A major solution to this issue in the relatively short term is thought to be various forms of energy storage. If you have already read the earlier “insight” papers in this series, you will understand why we need large scale storage on many European and N American grids as the amount of renewable generation on these systems increases.
In figure 1 above, these storage methods are classified by their power ratings (expressed in MW) on the x-axis and their potential output (expressed in MWh or duration of delivery at their rated output) on the y-axis. They range in size from small scale supercapacitors and batteries at the kW end to the only two current technologies which have GW scale, namely pumped hydro and CAES. This paper focusses on the larger battery storage options.
The pros and cons of Battery Energy Storage (BES) as a generic type can be summarised in a SWOT analysis as below:
Large scale battery options
Leaving aside very small-scale batteries such as those for domestic appliances and conventional cars, and focussing on battery technology for grid-scale energy or services provision, there are two main types of large battery technology in development: flow batteries and sodium-sulphur batteries.
These two types are shown towards the top of figure 1 and their respective pros and cons tabulated below. Other types, such as lead-acid and Li-ion batteries are available but tend to be very much smaller scale and therefore best suited for very local applications.
Figure 2: Schematics of Na-S (upper) and flow batteries (lower) (Cheng Zhong, 2019)
Some of the pertinent technical and operational characteristics of these two types of battery storage are shown in the table below:
Integration of Renewable Energy Sources on the grid
In general, the battery technologies utilized in grid-scale storage are expected to meet the following demands:
(1) peak shaving and load levelling; this refers to processes during which the battery energy storage system stores electrical energy (charging process) under low electrical load and releases the stored electrical energy (discharging process) under high electrical load. To the extent that such peaks and troughs can be levelled out, battery storage removes the need for expensive spinning thermal reserve on the system. This is shown in figure 3 below.
(2) voltage and frequency regulation; sudden loads or demand reductions on the grid can push both the system voltage and frequency outside regulated limits. In the context of renewable energy integration, these variations can be caused by the sudden introduction or falling away of, for example, significant quantities of wind power on the grid. The fluctuations must be corrected quickly if the grid operator is to avoid such breaches of its service provisions. Batteries can provide one method of providing this service which is typically required many times per year; and
Figure 3: (A) Electrical energy storage in peak shaving; and (B) in load levelling (also called demand shifting). (Cheng Zhong, 2019)
(3) emergency energy storage is associated with the requirements of backup devices with a millisecond-level response in case of a grid shortfall. The supply must achieve full power discharge in any state in the event of a wide-scale active power shortage. Batteries can provide such a back-up until alternative sources can be brought online.
Figure 4: a standalone 250 kW, 500 kWh, flow battery (ZnBr) system (Energy Institute, 2019)
Current research on the exploration of battery technologies at the grid scale is focussed on the comprehensive development and overall performance of batteries, including: energy and power densities, response time, operating temperature, lifetime (cycle and shelf life), battery voltage, energy efficiency, memory effect, safety, environmental friendliness, and most importantly, cost.
Cheng Zhong et al, “Battery Technologies for Grid-Level Large-Scale Electrical Energy Storage”, Online pub., 2019
Dehghani-Sanij, A.R. et.al., "Study of energy storage systems and environmental challenges of batteries", Renewable and Sustainable Energy Reviews, vol 104, pp 192 - 208, April 2019
Energy Institute, “Battery Storage Guidance Note 1: Battery Storage Planning, first edition, London, 2019
Nikolaidis, P, Chatzis, S and Poullikkas, A, “Life cycle cost analysis of electricity storage facilities in ﬂexible power systems”, INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY, VOL. 38, NO. 8, 752–772, 2019
SNL 2013: Sandia National Laboratories, DOE/EPRI 2013 Electricity Storage Handbook in Collaboration withNRECA, Report SAND2013-5131, July 2013.