EME 812
Utility Solar Power and Concentration

10.1 Purpose and Function of Utility Scale Storage Systems


Purpose and Function of Utility Scale Storage Systems

Energy storage technologies are expected to enable of electric grid modernization, addressing the current limitations of electricity infrastructure and increasing grid stability and resiliency. We can identify a number of critical functions that we expect the energy storage systems to perform. 

  • Integration of renewable energy into the electricity grid

The variability of solar and wind power makes it hard for electricity providers to plug them into the electricity grid. Grids constantly balance the supply and demand of electricity and thus benefit most from dispatchable sources of energy (so far fossil fuels that could be burned on demand provided that sort of convenience). Energy storage makes the solar and wind energy more dispatchable (available on-demand of grid operators) and hence more competitive with traditional fuel options.

  • Addressing peak demand

Responding to peak demands requires the ability to generate power quickly. The traditional choice for peak power generation are natural gas turbines. The energy stored in the batteries is immediately available and can be used to meet peak demand. This helps use the renewable power for peak generation and avoid grid disruptions or blackouts. 

  • Time shifting

PV Solar panels generate power only during the daytime, with the peak at noon hours, while the peak energy demand is often located during evening hours, when the solar irradiation is low. So the solar power needs to be “time-shifted” to be available during the time of high demand, and this can be achieved by means of utility scale storage. 

  • Energy autonomy and independence

For communities living in areas without access to electricity grid, combined renewable energy plus storage systems may be the best option to provide for constant supply of electricity. This autonomous approach can be realized at both distributed (house / community) and utility (area / region) scales.

Probing Question

Can you think of other major purposes or functions provided by energy storage systems? What other services to industry and community can we expect from the storage technology development in the future?

Check out Table 10.1, which provides more detail on how storage systems serve the grid and help diversify the energy resources.

Table 10.1 Key roles of energy storage in the integration of renewable energy systems in the electric grid [IES, Electrical Energy Storage, White Paper, Switzerland, 2011]
Role Time scale(s) Description Benefits to RE integration Examples of EES technologies
Time shifting / Arbitrage / Load levelling Hours to days EES allows storage of off-peak energy and release during high-demand period A solution to diurnal generation cycles that do not match load cycles NaS batteries, CAES, PHS, RFB
Seasonal shifting Months EES stores energy for months at a tie, releasing it at times of the year when RE output is typically lower Allows use of renewably-generated energy year-round reducing reliance on traditional generation in seasons with, e.g., low sunlight Hydrogen, SNG
Load following / Ramping Minutes to hours EES follows hourly changes in demand throughout the day May mitigate partial unpredictability in RE output during critical load times Batteries, flywheels, PHS, CAES, RFB
Power quality and stability < 1 second Provision of reactive power to the grid to handle voltage spikes, sags and harmonics Mitigate voltage instability and harmonics caused or exacerbated by uncontrollable variability of capital RE generation LA batteries,NaS Batteries, flywheels, RFB
Operating reserves
Frequency regulation Seconds to minutes A fast-response Increase or decrease In energy output to stabilize frequency Mitigate uncontrollable moment-to-moment variability in RE generation output Li-ion Batteries, NaS Batteries, Flywheels, PHS (with advanced variable speed control)
Spinning Reserves ~10 Minutes A fast-response increase or decrease in energy output to cover a contingency, e.g. generator failure Mitigates partial unpredictability RE generation output, providing (or removing)  energy win the RE resource does not perform as expected PHS, flywheels, batteries
Supplemental reserves Minutes to hours A slower response resource that comes online to replace a spinning reserve Provide a firm power in the event of an especially severe and long-lasting drop in RE output. Use for RE integration is expected to be infrequent and low-value PHS
Efficient use of transmission network Minutes to hours EES can help grid operators defer transmission system upgrades through time-shifting and more efficient operating reserves Reduced transportation costs, mitigate locational dependency challenges of RE generation Li-ion
Isolated grid support Seconds to hours EES can assist in the integration of RE on small power grids, such as those in use on islands Time-shifting and power quality applications to mitigate variability and unpredictability of RE generation LA batteries
Emergency power supply / Black start Minutes to hours EES may be used to re-start the power system in the event of a catastrophic failure No specific benefit accrues to RE integration, but storage resources may nonetheless provide black start capability to gird LA batteries

These purposes and applications require storage systems of diverse scale. The concept of scale has two dimensions: space and time. Space scale is related to the size and capacity of the storage, while timescale indicates how long the energy can be stored. Take a look at the figure below, which presents rough classification of storage systems in terms of size and time.

storage scale diagram see text for description
Figure 10.1. Typical time and size scales associated with energy storage technologies
(Aneke and Wang, 2016)

In this diagram, we see that such devices as capacitors store small amount of energy on the scale of seconds and minutes. At the same time, the systems shown in the right upper corner - pumped hydro storage, chemical storage - can store amounts of energy worth of gigawatt-hours over long periods of times (months to years).  The same as with power generation technologies, storage system variety is important to satisfy various applications and demands and also to provide service storage in diverse natural and industrial settings. 

The selection of the energy storage depends on many technical characteristics (besides scale), which would help us to understand why some technologies are preferred over others, and what trade-offs are involved in this selection. Let us look at some key storage characteristics next.