Flexible Peaking Resource

Executive Summary

Many types of electricity storage are well-suited to provide what is often referred to as “peaking” electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. supply resources. Peaking resources are an important element of utilities’ portfolio of electric supplyA source of electric energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... and/or capacity, possibly including generationThe manner in which electricity is generated. The electricity that flows through California facilities and purchases. resources that are used to serve {peak demand} for electricity. The operational role for peaking resources (generation and demand-side) during a day with high peak demand1. The rate at which electric energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by... is depicted graphically in Figure 1 below. (Figure 1 depicts a typical electric resources “stack.”) 

As shown in Figure 1, peakers are the last powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. plants to be turned on and the last to be called upon (dispatched) to provide power during times when peak demandThe maximum power draw from end-user loads during specified times. For example, most utilities experience peak demand during hot summer afternoons. occurs. 

Typically, peak demand occurs during what is sometimes called “on-peak” hours, times when electricity use and price are highest. In most regions, annual peak demand occurs during summer afternoon hours for durations ranging from four to six hours (e.g., Noon to 6:00 pm). A key driver of peak demand that occurs during hot summer days is air conditioning (cooling). 

Perhaps not surprisingly, a generation plant that is used to serve peak demand is often referred to as a {peaker}. For most situations, peakers are simple cycleOne sequence of storage charging and discharging. Also known as charge-discharge cycle. combustion turbine (CT) generation. CTs are mature, familiar and responsive. However, CT peakerAn electric supply resource, typically a combustion turbine generator or reciprocating engine, whose primary purpose is to generate electricity when peak demand occurs. plants tend to have relatively low fuel efficiency, and they tend to have relatively high air emissions (per kWh of energy generated), especially when operated at part loadAn end-use device or an end-use customer receiving electric power and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or.... Other common generation-related means to address peak demand include: a) reciprocating engines, especially diesel generators (gen-sets), b) less responsive natural gasA mixture of hydrocarbon gases occurring with petroleum deposits, mainly combining methane with varying quantities of ethane, propane, butane and other gases. Used as a fuel and for manufacturing organic compounds. This type of gas generates about 38... fired steam generation plants, c) hydroelectric generation, d) photovoltaics (PV) and e) solar thermal generation. 

Three “demand-side” alternatives to peakers exist. They reduce electric demand during on-peakTimes when demand for electricity is highest (a.k.a peak demand). Typically on-peak times occur during weekdays during the hottest summer months, when normal demand is high and when air conditioning is operating. Similarly, in some areas on-peak times... times which reduces the need for peakers. First are electricity end-user energy efficiency measures that reduce peak demand coincidentally. Second is {demand response} (DR) which involves direct control of electricity using equipment – by the grid system operatorAn individual at a control center (Balancing Authority, TransmissionAn interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. Operator, Generator Operator, Reliability Coordinator) whose responsibility it is to monitor and control that electric system in real time., with the end-user’s consent – such that electric power draw is reduced. Third, utilities are using increasingly rich pricing mechanisms for energy and/or power that are used during peak demand periods. Pricing involves high or very price for electric energy and/or power during peak demand periods. 


Electricity storage is a compelling alternative to peakers and to demand-side1. Of or related to end-user electric demand, often said to be “on the customer side of the meter.” 2. Of or pertaining to electricity demand, sometimes referred to as “customer side of the meter.” approaches to manage peak demand. Indeed, there is a significant amount of utility-owned pumped hydroelectric storage capacityThe rate at which equipment can either generate, convert or transfer energy. installed that is used extensively as peaking resources. 

Although storage may have a higher up-front cost than peakers or demand-side alternatives,storage is a very responsive and flexible peaking resource that can provide more utility and benefits. So, despite storage’s relatively high up-front cost, additional benefits are likely to accrue – relative to use of peakers or demand-side approaches – such that the overall net benefit from the storage exceeds the net benefit from alternatives. 

Some important advantages of storage – used to serve peak demand – include the following: 

Storage can serve a broader array of applications than peakers or demand-side approaches.

Most storage types tend to startup much faster than peakers and their output can be varied – {ramping} – much more rapidly than peakers’ output, so storage is a significantly more flexible alternative. 

Because most storage peaker charging occurs at night, storage used as peaking resources increases utilization of cleaner, more efficient “{baseload}” generation and will increase utilization of transmission capacity and, depending on the location, it may also increase utilization of distributionThe practice of and infrastructure for distribution of electricity to end-users by utilities. Typical voltages range from 12 to 138 kiloVolts (kV) capacity (“increased asset utilization”). 

There are no direct emissions from storage and most storage types produce limited or noise so it may be easier to site than generation-based peakers. 

Some storage types are modular and thus can be deployed and operated in distributed mode, where they are most effective and/or most valuable. If it is also transportable, modular storage can be redeployed in other locations, as conditions and needs evolve and to so its benefits can be optimized. 

Depending on 1) the type of generation equipment and/or fuel used to generate energy that is stored and 2) the type of peaking resource displaced by storage peakers, fuel use and/or air emissions may be reduced (per kWh delivered to end-users). 

There are two versions of storage peakers: 1) bulk/central facilities comprised of one large plant at sites that tend to be somewhat or very remote from load centers and 2) modular/distributed storage located near or within load centers. Bulk/central peakers are most likely to involve pumped hydroelectric and compressed air energy storage (CAES), though many modular storage units could be co-located at a central site and operated as one resource. 

Modular/distributed storage peakers could be installed at or near loads if locational benefits or other advantages related to distributed deployment are significant.The primary benefit is related to the reduced need for generation equipment. An important incremental benefit is the value of {electric energy time-shift}. Several other circumstance-specific benefits could also accrue. Storage used as a peaker during peak demand periods could also provide ancillary services during times when demand is lower. 

Net fuel use and/or emissions associated with storage peakers may be comparable or perhaps even lower than those for CT peakers. That is because the stored energyThe energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh. is likely to be from relatively efficient generation with relatively low or no air emissions (e.g., high efficiency natural gas fueled “combined cycle” power plants or nuclear power plants) and/or from wind generation or baseload renewable generation such as geothermal and biomass. 

Distributed peakers could be comprised of a combination of storage, dispatchable distributed generationSmall, modular electric generation that is interconnected with the electricity grid at the subtransmissionPart of an electricity transmission and distribution system whose voltage is lower than that of the transmission system and higher than that of the distribution system. Subtransmission circuits are usually arranged in loops so that a single line... or distribution level. and demand responseReductionA chemical process that results in the acceptance of electrons by an electrode’s active material. The opposite of oxidation. of retail electricity end-users’ electric load (power draw) in response to control or price signals. DR resources are deployed and used in lieu of installing/operating peaking generation capacity.. They could be operated individually or they may be aggregated into composite “power blocks.” 

If the storage flexible peaker capacity is sited near electricity end-users – in distributed locations – it could provide potentially significant benefits related to: a) transmission congestion relief, b) T&D {upgrade deferral} or {life extension}, c) local {voltage support} and {power factor correction} and d) improved electric service {reliability} or power qualityA measure of the level of voltage and/or frequency disturbances.. If the distributed peaker is modular and/or transportable, then the value may be somewhat higher yet.

Conclusions and Observations

Peakers are a critical, but expensive, element of the electrical1. An adjective meaning “pertaining to electricity”. Electrical Engineer. 2. Related to, pertaining to or associated with electricity but not having its properties or characteristics. grid. Electricity storage is quite well-suited to service as a flexible peaking resource. Advantages are numerous. Generation is freed up to provide service for which it is designed: generation of electricity at the plants’ full rated output (power), at a constant rate. That optimizes production cost, fuel use, air emissions and maintenance cost. Storage is more responsive than generation-based peakers. The ability to both “absorb” and dischargeThe process of extracting stored energy from the storage system. energy adds to storage’s flexibilityRobust responses to changing needs and opportunities that are enabled by use of a device or technique that is adaptable, versatile and, in some cases, transportable.. Storage used for peaking could be used for ancillary services during thousands of hours per year.