Flexibility (Power System)

Flexibility: Concept Definition

As the world shift to more renewable energy, especially variable ones such as wind and solar, a paradigm shift in the power sector has gradually taken place to meet the transition. In particular is the growing trend of more focus on the so-called "Power System Flexibility" in the academic and industrial sectors in this field.

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.

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.

As an example of how much the flexibility of a conventional power plant can affect the electricity generation portfolio, consider the following "low carbon" scenario of the power output curve in Taiwan by summer 2025. In the scenario, the hard coal power plants performed their flexibility potentials to the most, and are only fed to the grid when the residual load is above a certain threshold. Compared to the scenario in the previous section, in this scenario coal use dropped from 26.8% to 2.6%. The coal power plants in the UK are already operating their entire coal power fleet in a similar manner.

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 had to curtail 26% of the VRE generations in order to avoid shutting or ramping down nuclear power plants (in Germany this statistic was less than 1% in 2013); in 2018, when VRE generation share rose to 7%, their curtailment rate actually decreased to 19% due to less nuclear output that year. With a regulated requirement of the minimum power output from nuclear power plants, VRE power plants in Japan also suffer from heavy curtailment rates.

Renewable Energy Power Plants

If a renewable technology is fully dispatchable, such as biogas power plants, hydroelectric power plants, and geothermal power plants, then they can also provide the flexible operations needed to balance the residual load variations. In fact, hydroelectric power plants and geothermal power plants are perhaps the most flexible among all the fully dispatchable energy sources.

For wind and solar which are variable in their nature, it is still possible to provide some flexibility services on the economic dispatch level. Of course, when there is no wind nor solar, other sources will still be needed, but flexible variable renewables still have some system, economic and environmental benefits in the near future. For instance, they can provide positive and negative balancing energy when they are available, and therefore reduce the need of conventional power plants for providing system must run. A detailed example of this "flexible VRE" can be seen in here.

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.

Since parking electric vechicles can also be a potential storage option, electricification of the transportation sector can also increase flexibility of the power system. Using EV as a source of performing residual load flexibility has become important especially in isolated grids such as Hawaii.

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 bring down the wholesale electricity prices of the neighboring grid because electricity production with higher marginal costs (known as the spark spread ) can be reduced. For example, during the winter of 2016 and 2017, large amount of nuclear capacity was unavailable to serve electricity in France. Imports from British, Germany, and other neighbors reduced France's need to turn on too much gas power plants (their marginal electricity producers) during the events. This means that interconnection is usually a win-win situation, where most nations are willing to further cooperate with each other.

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.

As more energy demand such as that from the heating or the transportation sector is electrified, it can also provide innovative ways of demand side management to contribute to power system flexibility. For example, Agora Verkehrswende has conducted a study that concluded that smart charging of electric vehicles could reduce the peak residual load and the variations in the residual load curves, thereby reduce need of distribution grid investment in the future.

Ways to Improve Flexibility

Retrofit and New Design Mindset of Power Plants

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.

Smart Grid

With the digitalization of the grid and smart meters implemented for consumers, the demand and supply of electricity can be adjusted with automated real time communication between the devices. In theory, this will increase the flexibility of the power system, increase the network capacity and reduce demand of additional storage.

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 not only the technical limits but also the fear of losing revenue for conventional operators. Therefore, 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, when 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.

A new study from Agora Verkehrswende suggested that distribution systems operators charge the customers a lower energy-related grid fee if they provide grid-friendly control services, which could in theory give incentives to more flexible demand side management or variable renewable operations.

Flexibility in Decentralized Systems

The discussions above mainly apply to a centralized power system. For decentralized power systems such as micro-grids or emergency backup systems, diesel generators are currently the main dispatchable sources. The question of how to integrate as much VRE as possible into these systems is sometimes more challenging, and while battery storage is usually a solution, there exist other optimized re-dispatchment models that allow the minimum conventional fuel consumption with no storage capacity .

Cost-Effectiveness of Different Flexibility Resources

In the above sections we had mentioned many sources for providing power system flexibility. It is important to note that some measures will be more cost-effective in the early stages of energy transition, and some will only make sense in the latter stages. For example, demand side management is usually considered as the most cost-effective resource in the current power system, following the retrofitting of conventional power plants and flexibility from renewable energy sources. Mass deployment of new energy storage technologies will only be economically reasonable and necessary in the late middle stages of the energy transition, and seasonal storage technologies such as synthetic gas will be important in the final push for 100% renewable-based energy systems. A detailed categorization of different stages of energy transition and the corresponding cost-effective flexibility resources can be found in reports such as the world energy outlook of IEA.

Further Reading

Residual Load

IEA Status of Power System Transformation 2018