Flexibility (Power System)

Flexibility: Concept Definition

According to the International Energy Agency, the flexibility of a power system refers to "the extent to which a power system can modify electricity production or consumption in response to variability, expected or otherwise". Another source described it as "the modification of generation injection and/or consumption patterns in reaction to an external signal (price signal or activation) in order to provide a service within the energy system" .

Flexibility can therefore refer to the capability to change power supply/demand of the system as a whole or a particular unit (eg. a power plant or a factory).

Flexibility: Why It Matters

Load balancing is not the only service a power system must perform flexibly.There are many other services the operators of a power system must consider to make the grid stable. The three main services for the stability of a power system are: load balancing, frequency response, and voltage response.

Renewable sources, together with some battery storage, can already perform some of these services better than a conventional power system. For example, renewable sources can regulate reactive power (and therefore voltage response) even when they are not giving real power, and by curtailing or storing power output renewables can also perform frequency control. Batteries alone can already perform frequency response better than conventional sources, and they will contribute more to the fast frequency reaponse service required when fewer and fewer conventional power plants are online. Currently a renewable power source integrated with a battery storage system is already cost competitive to some conventional sources, and such implementation will boost the reliability of the grid.

However, current capacity of dispatchable renewables or batteriers are not enough to cover variations of the residual load curve. So the residaul load flexibility of a power system must still be performed mainly by conventional dispatchable sources, with the aid of some demand response.

For example, below is a simulated power output profile of the power system in Taiwan by summer 2025. Since most renewable capacity by then will be solar, there exists an obvious "duck curve" in this profile. The total dispatchable renewable capacity (hydro, pumped storage, bioenergy and geothermal) is about 5.5GW, but variations of the residual load can ramp up to 18GW in six hours, so in this specific redispatchment scenario fossil gas power plants perform most of the residual load flexibility. For more details, see Renewables are blooming; is the power system ready? A Prognosis on Residual Load Flexibility in Taiwan by 2025.

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Flexibility: Supply Side

Conventional Power Plants

Conventional flexible power plants, mainly the gas power plants, play an important role of varying power output flexibly. This is already the case in the traditional power systems, which is known as "load following". They will continue to play the same role when more variable renewables are fed into the grid, only to a greater extent.

Power plants which are conventionally considered inflexible, such as hard coal power plants or even lignite power plants, can be retrofitted or redesigned to become more flexible. This is what has happened in countries like Germany and Denmark.

The role of nuclear power plants in providing residual load flexibility is more unclear. Ramping the power output of nuclear power plants regularly tend to damage the revenues and lifespan of the plants. In countries where a clear nuclear phase out plan has been set, this is less of a problem; the nuclear fleet in Germany performed some 40% of power reduction during storms in autumn 2017. In countries where nuclear is likely to remain online for a while, this kind of flexible behavior is seldom observed; for example in Ontario, with only 6% of variable renewable electricity share in 2017, the operators already have to curtail 26% of the VRE generations in order to avoid shutting or ramping down nuclear power plants (in Germany this statistic is around 1%). 

Storage

Pumped storage systems have long been used, and certainly will play a bigger part in the new power system for providing flexibility. Apart from that, other storage options are currently still not economically feasible to provide residual load flexibility from a system perspective.

At a more local scale, variable renewables with small scale battery storage have already become competitve with other electricity sources . This kind of implementation can smooth the power output of VRE with less energy loss, and will play a bigger role in residual load flexibility in the future.

Interconnection

Better interconnections between grids can also ease some of the flexibility demand to neighboring grids. The more dispatchable power plants that are well connected in the system, the less ramping any single power plant will need to perform. Interconnections can also bringdown the wholesale electricity prices of the neighboring grid because electricity production with higher marginal costs can be reduced.

However, conventional electricity sources may often choose to export electricity rather than ramping or shutting down when interconnection is better. This causes a difficulty to reduce conventional electricity generation to full potential. For example, people have been suggesting that brown coal electricity generation in Germany could have dropped by 37%, should there be no electricity export and all the additional residual load flexibility was performed by brown coal power plants.

Flexibility: Demand Side

Flexibility on the demand side is typically known as demand response. Like conventional power plants, industrial owners can retrofit their factories or redesign their control system under the new mindset to meet the growing demand of residual load flexibility.

An example of this occurred during a solar eclipse event in Germany in 2015. Aluminium factories were asked to lower power demand during the few minutes, and the grid went through the event with no major incident.

Ways to Improve Flexibility

Technical Measures: Retrofit and New Design Mindset

There are three important factors that indicate the flexibility of a conventional power plant: the start-up time, the ramp rate, and the minimum load.

For existing conventional plants, retrofitting them may be able to increase their flexibility. One example is to add a pulverized coal (PC) storage facility and thereby decouples the direct supply chain between mills and burners. By doing so, the coal power plant can run at a lower minimum load because of a faster response to fire instabilities. It can also provide a higher ramp rate because of a reduced time lag between mills and burners.

For newly built conventional power plants, flexibility must be in the center role when designing. For instance, in the past when conventional power plants were designed to run around the clock with little variation of power output, the main consideration was to ensure that the inner components could withstand the high pressure and high temperature within the power plant; now engineers must also consider the thermal stress of the materials during a major ramp event.

Market Measures: Incentives for Flexibility

Sources have suggested that even in transition advanced nations such as Germany, the full potential of residaul load flexibility has still not been performed. The reason for this is mainly not the technical limits but the fear of losing revenue for conventional operators. Therefore, apart from retrofitting existing conventional power plants or build new ones under a new design mindset, the electricity market can also boost the flexibility of the overall system, if designed properly.

One example is the introduction of negative wholesale prices, which give operators of conventional power plants a stronger price signal to ramp down more or shut down their plants when variable renewable energy shows strong power output. In some of the extreme negative price events on new year day in 2018, Germany's lignite power plants reduced their power output to a minimum of around 4.5GW. Traditionally people expect the minimum of lignite power output to be 7GW in Germany

Another example is a well-constructed capacity market. Rather than receiving direct revenue from the total electricity they produced, operators in a capacity market are rewarded for providing the right amount of capacity in the right time. This increases the incentives for them to operate more flexibly.