⁽¹⁾PhD candidate, Faculty of Engineering of the University of Porto & Senior Advisor, REN

  ⁽³⁾Full Professor, Faculty of Engineering of the University of Porto & Associate Director, INESC TEC

1. The need for an energy transition

Climatic change does no longer represent future threats, since the changes are already happening all over the world. Heat waves and prolonged droughts in Europe, hurricanes and floods in Asia are hitting the planet. These effects determine the need of decarbonisation of the world economy and, consequently, of a growing electrification of the economic activity - together with the utilisation of renewable power sources to produce electricity.

However, the complete decarbonisation of the society and of the economy will not be possible without exploiting another energy vector – Hydrogen (H2) – a renewable gas, if produced from electrolysis of water and using electricity obtained from renewable sources, mainly wind power and solar photovoltaic (PV). The use of biogases is also expected to increase. In fact, the need for industrial high temperature heat, the need for new green fuels for long-range road transportation, maritime and air transportation justify the exploitation of H2 as a complementary vector to support a faster decarbonisation path. Apart from this, H2 can also be used to provide seasonal energy storage by using the excess of renewable generation under a power-to-power (P2P) solution and providing adequate security of supply levels to the power system.

The progressive electrification of the economy, together with an increased use of renewable gases, where sector coupling plays an important role, and the need to optimise the use of heat leads to the development of multi energy systems, requires an overall optimisation on the planning and operation of these infrastructures, with a common goal: Sustainability.

2. Challenges and opportunities – The role of renewable gases

Increased integration of renewable power sources brings the challenge of dealing with variable generation. The production of green H2 using renewable electricity can play a significant role in the context of the decarbonisation of the energy sector. In fact, the production of H2 can bring significant flexibility for the power system, if electrolysers are exploited as flexible loads able to address frequency changes, as well as seasonal storage of renewable electricity by developing a P2P solution, blending H2 into natural gas grids or by injecting directly to dedicated H2 infrastructures.

The variability of renewable generation will lead to surpluses of renewable energy in some periods of the year, and to scarcity of energy in others. As the excess of renewable electricity is seasonal, usually during spring (for wind) and in summer (for PV), there is a high interest in storing the surplus of electricity and electrolysers arise as a new load that may absorb this energy. H2 can then provide a seasonal storage solution if one can store this H2 in large reservoirs like caverns, which happens to be the case in Portugal - where large salt caverns are already used to store natural gas for use it in late autumn or winter. This solution can lead to the deployment of a sector coupling approach, where H2 is used to blend natural gas that is going to fuel combined cycle gas turbine plants or is directly used by stationary fuel cells (used also to produce heat that can be of interest for industrial cogeneration) or used by H2 turbines, leading to a P2P solution. The adoption of a P2P solution, where a H2 plant is used, provides a firm capacity to the power system, which allows keeping the security of supply levels within the desired limits.

The main technical challenges on the electricity sector relate to the efficiency increase in the P2P solution (electrolysers and H2 power plants) and the adaptation of the cavern reservoirs to store H2. Additionally, there are also regulatory challenges regarding the definition of an adequate framework for the P2P solution economic viability when dealing with the surplus of renewable generation, while assuring security of supply.

H2 is also used by the industry in several domains: oil-refining, ammonia for production of fertilisers, methanol production and steel production. In these industrial facilities, self-production of renewable electricity should be promoted via PV plants and cogeneration facilities, where natural gas should be blended with green H2.

The mobility sector is one of the main consumers of fossil fuel. Therefore, it is urgent to replace fossil fuels by green electricity and use biofuels and H2. H2, as a renewable gas, may play a central role in the decarbonisation of the mobility sector, namely for urban and regional buses, trains, long travel heavy transportation, and maritime and air transportation in the long-term. Green H2 can also be used nowadays as a sustainable aviation fuel (SAF) after duly conversion, contributing to decarbonise the aviation sector. The main technical challenge is how to store the amount of H2 required for an extended driving range within the vehicular constraints of weight, volume, efficiency, safety, and costs. Therefore, high-range and heavy-duty transportation should be the preferable mean of H2 deployment, as batteries should be more effective for light-duty.

At the buildings level, energy systems pose a unique challenge, which is the idiosyncrasy of the consumers, especially the particular renovation needs of old buildings. Energy poverty is also rising great concerns in the current geopolitical energy context and supply scarcity. Therefore, demand side response and energy efficiency, together with comprehensive techno-economical methodologies, are key to draft new decarbonisation pathways and increase its pace. Furthermore, in renovations where architectural challenges are massive, there are no silver bullets, so every technological approach should be considered in a multi-energy design.

3. Multi-energy systems

An approach based on multi-energy systems is the step forward that must be taken to ensure an efficient and flawless energy transition, while ensuring that all the benefits of renewable electricity and gases and flexible loads (e.g., electric vehicles) are properly utilised.

In technical terms, a multi-energy system considers different types of energy networks and sectors that interact at different levels in a building, city, or region, as shown in Figure 1. The optimised operation and planning of multi-energy systems is expected to bring several benefits to the global system, like delivering cost-effective energy services, increasing reliability and quality-of-service, enhancing overall security of supply, while reducing the impact over the environment.

Green H2 may play an important role to foster multi-energy systems implementation. In fact, the H2 economy has grown exponentially in the past few years and presents an opportunity to research and develop new technological solutions to accelerate the energy transition.

Despite the potential benefits, there are still several challenges that one must overcome to achieve the implementation of the concept:

• Isolated institutional and market structures for the different energy sectors;


• Complicated technical, economic and market-related interdependencies between energy systems that are not holistically addressed by network operators;


• High operational and management complexity due the integration of multiple energy carriers;


• Lack of tools ready to guide the design and planning of integrated and interdependent energy systems.

Several efforts have been made in the last years to overcome these challenges, together with significant technical advancements in terms of operational and planning tools, and at the level of markets coupling.

4. Next steps

To effectively achieve the desired energy transition, several gradual measures should be put into practice, in different phases:

A. Phase 1 – Electrification of the economy will increase and advance renewable energy technologies deployment to decarbonise the electric power system.

B. Phase 2 – With a decarbonised electricity system, the development of new management approaches and tools to supervise and keep power system reliability levels should be a priority. Efforts should be made to decarbonise heating and transportation sectors, together with an investment in the real-world implementation of multi-energy systems. Renewable gases, namely H2, will increasingly decarbonise the energy sectors where electricity cannot effectively solve the decarbonisation problem.

C. Phase 3 – Reform regulatory and market frameworks to facilitate and leverage general electrification and general implementation of the multi-energy systems concept. Renewable gases will achieve the main support for the decarbonisation of the economy, using H2 in the industry and mobility sectors, fostering endogenous resources, while optimising renewable energy harvesting.

D. Phase 4 – Enforce a general implementation of the multi-energy systems concept, coupling electricity, gas, and heating networks together with the flexibility of multi-vector resources. Joint operational planning and long-term investment planning should be mandatory.

Figure 2 shows the global roadmap for a low carbon economy.