Power System Operation - The challenge of the 100% Renewable Target

  ⁽²⁾Senior Researcher and Assistant Professor, INESC TEC and Faculty of Engineering of the University of Porto

1. Introduction

Increasing requirements to reduce GreenHouse Gas (GHG) emissions, together with declining Renewable Energy Sources (RES) costs, triggered the consensual commitment to decarbonising the economy. In order to achieve this objective, it’s crucial to overcome technical and economic challenges in the framework of the smart-grid concept.

To achieve a reduction of 80-95% in GHG emissions by 2050, the European Union expects that RES should hold a 97% share of the European power generation mix considering the high-RES scenario envisioned for the decarbonisation process⁽¹⁾. A deep transformation of the power system is currently being implemented, through the massive integration of non-dispatchable RES, widely deploying high-energy efficient technologies, a stronger commitment towards electrifying the economy, and the elimination of coal as a source of energy. Decarbonising the energy sector requires electricity to be the key vector in this paradigm change. Since the potential sites for hydropower are already being exploited in Europe, renewable generation is expected to develop through biomass, geothermal and increased shares of Variable Renewable Energy (VRE) that are not dispatchable due to their fluctuating nature, such as wind and solar power. Most likely, the total cost of a 100% renewable power system would be higher than an equivalent power system with low carbon technologies – nuclear and carbon capture and storage, due to seasonal mismatches, among other aspects.

The traditional generators are typically synchronous rotating machines that are manageable and dispatchable in terms of power output, enabling a suite of services to the power grid, e.g., inertia and capacity to provide regulating power, therefore allowing a successful, secure, and stable operation of the power system. Increased variability resulting from the paradigmatic changes in the generation and demand side calls for more capacity to permanently assure the balance between generation and consumption. Hence, flexibility⁽²⁾ is the key word: it allows the power system to manage changes in multiple technical dimensions, time scales, and geographies (local services versus systemic ones).

2. How to reach the target of near 100% renewables?

The path towards near 100% renewable power system will require the gradual implementation of certain elements, namely:

• More RES generation and more transmission capacity.
• A diversity of RES and energy carriers (hydrogen, for instance), while fostering sector coupling (transportation, industrial heat).
• Well-thought integration of heat pumps and electric vehicles (EV) to reduce demand peaks and biogas needs.
• Energy efficiency and conservation measures (use less energy with a negligible loss of comfort) to smooth the load curve.
• Extensive use of “Power-to-X”, “Power-to-H2” and “Power-to-H2O” technologies (desalination and water purification) to enable RES to convert surplus energy into other energy carriers. These energy carriers can be stored for later use.
• Enhanced cross-system coordination between transmission system operators (TSO) and distribution system operators (DSO) under a smart grid paradigm.
• Develop the interconnection capacity between countries, therefore achieving power system flexibility from the Pan-European market, based on effective collaboration among system operators.
• Provide innovative ancillary services (a variety of operations beyond generation and transmission that are required to maintain grid stability and security) from both conventional VRE sources and eventually innovative solutions using energy storage and variable speed hydro power.
• Evolution of market arrangements to reflect real system needs: markets operation closer to real-time, involving the full separation of energy and capacity-related services.
• Evolution of grid codes, identifying new requirements in advance, hence facilitating the transition to green power.
• Integration of distributed energy resources into the market, for instance, EV with Vehicle-to-Grid (V2G) capability.
• New operational procedures to enhance flexibility, for instance, advanced weather forecasting for accurate VRE generation forecasts.
• Dynamic line rating, i.e., operating the power lines closer to their thermal limits.
• Virtual power lines, i.e., utility-scale storage systems connected to the grid at the supply side, storing surplus that could not be transmitted due to grid congestion, and on the demand side, charged whenever grid capacity allows and discharged when needed.

Figure 1 shows the main drivers of the power system operation change.

3. The challenges of the new paradigm of operating the power system

The progressive implementation of a near 100% renewable power system will require visible changes regarding its operation. Perhaps the most significant ones are related to the wide deployment of storage devices, implementation of demand-side programs, the transition to an inverter-based power system, and the emergence of microgrids, as discussed in the next paragraphs.

The time-variable nature (but increasingly predictable in its variability) of many RES makes it more challenging to match demand and generation, therefore the need for storage technologies that will change the mindset of the operation of the power system. A massive integration of storage devices (batteries, thermal, mechanical, hydrogen, and pumped hydropower) will need to be incorporated into the power grid.

In the past, demand for energy has been met by ‘firm’, easy-to-control power, where supply followed energy demand, and there was no effort taken to manage that demand. With increasing VRE in the grid, the situation changes, and demand-side management (DSM) is required to achieve the balance between not-so-easy to control variable generation and variable demand. Demand-side flexibility is expected to play an important role in a 100% renewable power system, with tomorrow’s smart homes and digital technologies managing the assets. Smart charging EV, and time-of-use tariffs are also part of the solution. To fully exploit the potential flexibility from the distribution side (batteries and DSM), TSO should ensure that they do not cause technical problems to areas managed by DSO and vice-versa.

The 100% renewable grid will feature many distributed inverter-based generators (DC/AC converters), therefore composing an inverter-dominated grid. To overcome the shortcoming of a low inertia power system, grid-forming inverters (with the capacity to emulate virtual inertia) will play a crucial role. Advanced control functions, which are currently being heavily researched, must be designed to assure the proper operation of the power system steady state, but mainly in transient state, to prevent load shedding and ultimately blackouts.

The decentralisation of the power system will promote the advent of renewable-based microgrids that can be operated isolated from the main grid. The transient behaviour of these microgrids needs to be addressed to ensure their secure operation, making sure that they do not disturb the existing grid. Smart Transformers are expected to play a decisive role in this matter, by providing voltage and frequency control, black start, fault-ride-through provision, and reconfiguration of the distribution grid.

4. Conclusion

The 100% renewable power system is not just feasible and needed: it is already taking place. Countries like Iceland, Paraguay, Costa Rica, Norway, Austria, Brazil, and Denmark run 100% renewable power systems or with high incorporation of RES. In Portugal, data from 2021 show that renewables accounted for 59% of the electricity demand. The path towards a near 100% renewable power system is underway, and the operation of the power system must adapt to the new paradigm. This paper outlined the main changes that need to be implemented to achieve said goal. Flexibility is the keyword. Furthermore, we have identified the deployment of storage devices, the demand side programs, the relevance of inverter-based generators, and the arrival of microgrids as the main challenges the green power system must face. In short, the 100% renewable power system requires flexibility to maintain high stability and reliability standards.