Other Related Uses
Load Following/Ramping Support for Renewables
Electricity storage is eminently suitable for damping the variability of wind and PV systems and is being widely used in this application. Technically, the operating requirements for a storage system in this application are the same as those needed for a storage system to respond to a rapidly or randomly fluctuating load profile. Most renewable applications with a need for storage will specify a maximum expected up- and down-ramp rate in MW/minute and the time duration of the ramp. This design guidance for the storage system is applicable for load following and renewable ramp support; this Handbook therefore treats them as the same application.
Load following is characterized by power output that generally changes as frequently as every several minutes. The output changes in response to the changing balance between electric supply and load within a specific region or area. Output variation is a response to changes in system frequency, timeline loading, or the relation of these to each other that occurs as needed to maintain the scheduled system frequency and/or established interchange with other areas within predetermined limits.
Conventional generation-based load following resources’ output increases to follow demand up as system load increases. Conversely, load following resources’ output decreases to follow demand down as system load decreases. Typically, the amount of load following needed in the up direction (load following up) increases each day as load increases during the morning. In the evening, the amount of load following needed in the down direction (load following down) increases as aggregate load on the grid drops. A simple depiction of load following is shown in Figure 9.
Normally, generation is used for load following. For load following up, generation is operated such that its output is less than its design or rated output (also referred to as ‘part load operation’). Consequently, the plant heat rates, fuel cost, and emission are increased. This allows operators to increase the generator’s output, as needed, to provide load following up to accommodate increasing load. For load following down, generation starts at a high output level, perhaps even at design output, and the output is decreased as load decreases.
These operating scenarios are notable because operating generation at part load requires more fuel per megawatt hour (MWh) and results in increased air emissions per MWh relative to generation operated at its design output level. Varying the output of generators (rather than operating at constant output) will also increase fuel use and air emissions, as well as the need for generator maintenance and thus variable operations and maintenance (O&M) costs. In addition, if a fossil plant has to shut down during off-peak periods, there will be a significant increase in fuel use, O&M, and emissions. Plant reliability will also deteriorate, resulting in the need for significant purchases of replacement energy.
Storage is well-suited to load following for several reasons. First, most types of storage can operate at partial output levels with relatively modest performance penalties. Second, most types of storage can respond very quickly (compared to most types of generation) when more or less output is needed for load following. Consider also that storage can be used effectively for both load following up (as load increases) and for load following down (as load decreases), either by discharging or by charging.
In market areas, when charging storage for load following, the energy stored must be purchased at the prevailing wholesale price. This is an important consideration, especially for storage with lower efficiency and/or if the energy used for charging is relatively expensive, because the cost of energy used to charge storage (to provide load following) may exceed the value of the load following service.
Conversely, the value of energy discharged from storage to provide load following is determined by the prevailing price for wholesale energy. Depending on circumstances (i.e., if the price for the load following service does not include the value of the wholesale energy involved), when discharging for load following, two benefits accrue – one for the load following service and another for the energy.
Note that in this case, storage competes with central and aggregated distributed generation and with aggregated demand response/load management resources including uninterruptible loads and direct load control.
Technical Considerations
Storage System Size Range: 1 – 100 MW
Target Discharge Duration Range: 15 minutes – 1 hour
Minimum Cycles/Year: Not Applicable
Storage used for load following should be reliable or it cannot be used to meet contractual obligations associated with bidding in the load following market. Storage used for load following will probably need access to AGC from the respective independent system operator (ISO). Typically, an ISO requires output from an AGC resource to change every minute.
Other considerations include synergies with other services. Large/central storage used for load following may be especially complementary to other services if the charging and discharging for the other services can be coordinated. For example, storage used to provide generation capacity mid-day could be charged in the evening, thus following diminished system demand down during evening hours.
Load following could have good synergies with renewables capacity firming, electric energy time-shift, and possibly electric supply reserve capacity applications. If storage is distributed, then that same storage could also be used for most of the distributed applications and for voltage support.
Frequency Response
Frequency response is very similar to regulation, described above, except it reacts to system needs in even shorter time periods of seconds to less than a minute when there is a sudden loss of a generation unit or a transmission line. As shown in Figure 10, various generator response actions are needed to counteract this sudden imbalance between load and generation to maintain the system frequency and stability of the grid. The first response within the initial seconds is the primary frequency control response of the governor action on the generation units to increase their power output as shown in the lower portion of the figure. This is followed by the longer duration secondary frequency control response by the AGC that spans the half a minute to several minutes shown by the dotted line in the lower portion of Figure 10. It is important to note that the rate at which the frequency decays after the triggering event – loss of generator or transmission – is directly proportional to the aggregate inertia within the grid at that instant. The rotating mass of large generators and/or the aggregate mass of many smaller generators collectively determines this inertia.
The combined effect of inertia and the governor actions determines the rate of frequency decay and recovery shown in the arresting and rebound periods in the upper portion of Figure 10. This is also the window of time in which the fast-acting response of flywheel and battery storage systems excels in stabilizing the frequency. The presence of fast-acting storage assures a smoother transition from the upset period to normal operation if the grid frequency is within its normal range. The effectiveness of fast-acting storage in this application has been successfully utilized by utilities and also described in other reports and papers.
The size of storage systems to be used in frequency response mode is proportional to the grid or balancing area in which they are needed. Generally, storage systems in the 20 MW and greater size can provide effective frequency response due to their fast action; some studies have shown that the response is twice as effective as a conventional fossil-fueled generator, including combustion turbines (CTs) and coal units. However, location of the storage system within the grid with respect to other generation, transmission corridors, and loads plays a crucial role in the effectiveness as a frequency response resource.