In the electric power system, electricity supply needs to be balanced with electricity demand and network losses at all times to maintain safe, dependable and stable system operation. Fluctuating demand from hour to hour, day to day, season to season is a fundamental characteristic of all power systems. The ongoing changes in electricity production and consumption create uncertainty into the system and make difficult to meet the moment-by-moment challenge of balancing supply and demand for electricity across a power system. If system balance between supply and demand is not maintained, the operational system frequency will deviate from its reference, potentially resulting to system instability and outages. Power flexibility is therefore required to counterbalance supply and demand of electricity to maintain equilibrium at every second on the every location of grid. Flexible power system has to maintain continuous service in the face of rapid and large swings in supply or demand. Future electricity system is expected to steer away from the conventional means of production. Nevertheless, driven by international and national climate and energy plans, the introduction of very high penetration levels of variable renewable energy sources (VRES) like wind and solar energy shall displace the part of the conventional generation capacity energy generation and VRES is subject to weather conditions and suchlike makes production unpredictable. The move towards more renewable energy paradigm shift shall increase the requirements for more power system flexibility as on the supply side. This development will make planning and balancing of supply and demand more challenging to meet the transition. The “power system flexibility” gap has to be covered by new flexibility options.
Electricity consumers are supplied with electricity through sizable technical infrastructure combined with a manoeuvered operating system that ensures balancing of supply and demand. Power system to ensure spatial and temporal balance of generation and consumption at all times consist inherent feature of flexibility in its design but their ability to alter their output differs considerably. Dispatchable power generators can be turned on or off and can adjust power to match generation and demand. Peaking plants like gas or diesel turbines and reservoir hydropower can respond on the minute-to-minute time scale to start up and shut down. Mid-merit plants including coal, biomass and solar plants respond within the hour to ramp down to a minimum operating scale and thus provide significant flexibility. Base load plants mostly geothermal have slower response times. Other resources that may potentially be used for balancing are storage, demand side management (DSM) or response, and interconnection to adjacent power systems. In practice balancing reseed through predicting the electricity demand within the time-frame and scheduling the operation of generation units accordingly.
Flexibility, in power system thus refer to sources of electricity that can be used on demand and dispatched at the request of power grid operators, according to the need within certain available parameters to increase or decrease the output over a defined period. “Up regulation” provides additional power needed to maintain system balance and “down regulation” reduces the power generation in system. Both up and down regulations can be controlled by reducing and increasing the load. Ramping capability in its common parlance represent how fast flexible resources can change demand or supply of power.
International Energy Agency (IEA) defines the “electrical flexibility” as the ability of a power system to reliably and cost-effectively manage the variability and uncertainty of demand and supply across all relevant time scales for ensuring instantaneous stability of the power system to support long-term security of supply.
The Flexibility Assessment Method
The IEA has developed the flexibility assessment (FAST) method to guide decision-makers through a holistic assessment of system side measures for balancing variability. The technical flexible resource is assessed over the balancing time scale. The time scales used are 15 minutes, 1 hour, 6 hours and 36 hours ahead of the moment resources are actually required to provide their flexibility. Resources are quantified in megawatts and summed up, resulting in technically ramp electricity supply or demand; up or down; to balance variability and uncertainty in the net load. The availability of flexible resource is captured to know whether the technical flexible resource will be available to operate in the desired way or not. The extent of the existing need is identified to find out the maximum expected value and rate of variability and uncertainty. When flexibility needs and resources are both known, they are compared to assess the extent to which the existing flexible resource might provide for the additional needs resulting from variable power plants. This step results in a megawatt number that expresses how much variable renewable energy capacity can be reliably balanced by the system.
Conventional sources of power generation such as fossil fuel power plants or nuclear plants are able to adjust to changes in power demand. However rapid growth in variable generation is driving the need for a more flexible power system. The transition to a more flexible power system requires a new portfolio of technologies and methodologies.
In prevailing coal-fired power plants to minimise frequent
shutdowns, augment aggressive ramp rates and lower minimum sustainable load to enable flexibility, fossil generators need holistic layup techniques to reduce impacts on boiler tubes and addressing cycling repercussions on heat rate which deteriorates significantly at lower and transient loads with better understanding of damage mechanisms, considering defense against water side corrosion, methodologies for reducing minimum loads, advanced monitoring capabilities during transients, and methods to minimise emissions during cyclic operations. For ameliorating flexibility of natural gas fired generation fast-start technology, remote starting and integration of equipment into a unit that can minimises the effects of thermal cycling is essential. Converting nuclear plants to flexible operation the overarching need is to implement new operating practices, increased staff training and awareness. Safe operating of nuclear power plants has to be clearly defined. Advanced sensor and monitoring methods needs to be developed to detect possible impacts of high-cycle fatigue caused by changes in flow-induced vibrations during more frequent plant heat and cool down cycles; and changes in flow rates, pump speeds, and valve repositions. The acceptable ramp rates, depth of power reductions, duration of reduced power operation, and frequency of power level changes deserve consideration. Hydroelectric generation for flexible operation poses unique challenges. In a course for converting hydropower to flexible operation appropriate path is to develop improved turbine runner materials/coatings and for improving operational performance, maintenance strategies are obligatory to keep availability, reliability of major components through improved inspection, along with ancillary supporting.
Flexible Power System a Key to Renewables Push
India has positioned itself to leapfrog into a cleaner electricity system in its development pathway. Increasing the flexibility of the power system is necessary and urgent. Failure to do so will constrain the growth of renewables. To tap the potential of green energy, power plants need to balance their demand-supply equation better. Solar output is zero at night, rapidly rising to near 100 per cent at mid-day, and falling again to zero in the evening. This cyclicality, while relatively predictable, imposes large stresses on the power system. Another group of renewables including wind, wave and tidal energy are also based on resources that fluctuate over the course of the day and from season to season. These fluctuations are likely to mean that, in order to maintain the balance between demand and supply, other parts of the power system will have to change their output or consumption more rapidly and more frequently than already required. Variable renewable electricity and uncertainty in the system is an additional, rather than a new. There is inability to perfectly forecast the output from renewable energy resources which are not uniform across all geographical locations and operating with non-synchronous technologies. The transitions towards an energy system where the majority of energy provided by VRES shall require sufficient ability of generators to effectively balance the variations that occur in demand and generation.
The Energy and Resources Institute (TERI) has been leading a flagship project called the Energy Transitions Commission India, along with several international partners. The project has developed a roadmap inter alia for increasing the flexibility of the Indian grid, in order to enable large-scale integration of cheap renewables.
Storage as an Option for Providing Flexibility
Electric energy storage offer promising solutions in improving flexibility in power system. The development of storage technologies that release energy on an as-needed basis can address several of the challenges presented by the use of increased renewables. Energy storage provides an inventory of electricity to the power system, adding a buffer to the current “just-in-time” system. It has the potential to enhance grid flexibility in peak shaving, load leveling, frequency regulation, voltage control, and renewables integration. It can follow power system ramps and relieve Transmission and Distribution (T&D) congestion. Storage can make the overall grid more flexible by accommodating more variable, renewable generation resources.
Battery energy storage shall provide substantial benefits not only to grid operators but also to customers. For customers, storage technologies ensure availability of power during non-generation hours, uninterruptible power supply, and possibly opportunities to trade power with local utilities. Pumped hydro is the dominant technology where water is pumped uphill to a reservoir and released to generate electricity when it is required. Thermal energy storage (TES) stocks, thermal energy by heating or cooling a storage medium so that the stored energy can be used at a later time. Flywheel stores energy is in the form of rotational kinetic energy. Energy from off-peak electricity is stored underground as compressed air. Grid scale energy storage can respond quickly to second-to-minute changes in electricity demand and supply changes resulting from variable generation. As a temporary “shock absorber” it can dampen transient electric conditions on local generation, transmission, and distribution network equipment.
Flexibility Options Disjunctive to Storage
As one of the flexibility options to manage reliable and cost-effective, variability and uncertainty of demand in power system, demand-side management response can be used as an alternative to energy storage. The implementation of DSM programs can range from improving energy efficiency with better insulation materials to fully autonomous energy systems that automatically respond to shifts in supply and demand. It is an important and integral strategy for addressing the challenges of chronic peak and energy shortages, improving access and affordability of power. The utilities are being mandated to draw up cost-effective demand side management action plans and programs to prioritise them as per their specific needs. The benefits from DSM are potentially twofold; first, consumers can reduce their electricity bills by adjusting the timing and amount of electricity use. Second, the energy system can benefit from the shifting of energy consumption from peak to non-peak hours.
Transmission plays a critical role in facilitating flexibility derived from resources with changing demand and renewables production. Ensuring system flexibility presents a new challenge to transmission system planning and operation. As more variable generation connects to a network, the direction of power flow and transmission networks may experience a reversal of flows.
Transmission system design and operation is evolving to consider more uncertain and variable power flows. Adopting power electronics devices, known as Flexible AC Transmission System (Facts) technologies, aide balancing of operations. Resources are used to overcome certain limitations in the static and dynamic transmission capacity of electrical networks to enhance controllability and increase power transfer capability.
Increasing the flexibility of the electricity system is an urgent, complex and substantial challenge. Variable generation output, stronger transmission and distribution systems, increasing storage capacity and demand-side management all help to boost system flexibility, as do renewable-based heat and hydrogen production. Increased flexibility should also contribute to long-term decarbonisation, which is essential to ensure a sustainable energy future.
*Harsha Rajwanshi is Assistant Professor of Law, Dean, External Relations and Centre Director, Gujarat National Law University & Faculty Advisor to GUVNL-GNLU Research Fellowship on Energy Law and Policy.