following up to accommodate increasing load. For load following down, generation starts at ahigh 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 morefuel and results in increased air emissions relative to generation operated at its design outputlevel. Also, varying the output of generators (rather than operating at constant output) mayincrease fuel use and air emissions, and it increases the need for generator variable maintenance.Storage is well-suited to load following for several reasons. First, most types of storage canoperate at partial output levels with relatively modest performance penalties. Second, most typesof storage can respond very quickly (compared to most types of generation) when more or lessoutput is needed for load following. Consider also that storage can be used effectively for bothload following up (as load increases) and for load following down (as load decreases), either bydischarging or by charging. (See Appendix D for details.)When charging storage for load following, the energy stored must be purchased at the prevailingwholesale price. This is an important consideration – especially for storage with lower efficiencyand/or if the energy used for charging is relatively expensive – because the cost of energy used tocharge 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 determinedby the prevailing price for wholesale energy. Depending on circumstances (i.e., if the price forthe load following service does not include the value of the wholesale energy involved), whendischarging for load following, two benefits accrue – one for the load following service andanother for the energy.Storage competes with central and aggregated distributed generation and with aggregateddemand response/load management resources including curtailable/interruptible loads and directload control.3.3.1.2. Technical ConsiderationsStorage used for load following should be somewhat-to-very reliable or it cannot be used to meetcontractual obligations associated with bidding in the load following market. Storage used forload following will probably need access to automated generation control (AGC) from therespective independent system operator (ISO). Typically, an ISO requires output from an AGCresource to change every minute.For this application, storage could provide up to two service hours per hour of dischargeduration. (See Appendix D for details.)3.3.1.3. Application SynergiesLarge/central storage used for load following may be especially complementary to otherapplications if charging and discharging for the other applications can be coordinated withcharging and discharging to provide load following. For example, storage used to providegeneration capacity mid-day could be charged in the evening thus following diminished systemdemand down during evening hours.Load following could have good synergies with renewables capacity firming, electric energytime-shift, and possibly electric supply reserve capacity applications. If storage is distributed,