In order for the lights to come on each time we flick the switch, it is necessary for an entire electrical network to be coordinated in a subtle balancing act, so that the energy produced at a given moment corresponds exactly to the consumption demanded at that moment. But what if this generation is not constant over time, for example in the case of clean energies such as photovoltaics or wind power? The ability to store large amounts of energy, at an affordable cost, is one of the great challenges for the sector, which would enable it to take full advantage of renewable sources. Here we analyse what needs to be done to reach this goal.
The wind does not always blow, nor is the weather always good, and, in addition, the energy needs of consumers do not always correspond with the weather. To achieve the smooth integration of renewables into the network, it is essential not to alter the functioning of the system. “A badly managed imbalance can lead to power cuts and even breakdowns of the generating equipment and that of the customers,” says Ricardo Luis Guerrero, a member of one of the panels of experts in renewables at the International Energy Agency.
Currently, this balance between supply and demand is usually obtained by combining renewable generation (wind and solar) with conventional generation, produced in hydroelectric or gas plants, whose sources normally guarantee more stable production cycles over time. Specialists such as those at the Electricity Control Centre (CECOEL) of Red Eléctrica de España —the only operator and transporter in the Spanish electricity system— are dedicated to this delicate task. There, algorithms and control technologies of the power plants allow the generation and distribution of electricity to be monitored, avoiding imbalances with respect to demand. “We’ve been doing this for many years,” says Juan Bola, head of the renewable energy operating unit at this centre. “The energy transition we are going to undergo from now on is an additional challenge.”
Energy storage of the future
The sector is looking for new formulas to integrate these clean sources, also with a view to the possibility of their accelerated growth causing production surpluses, according to the different experts consulted. One of the main lines followed, they agree, is the development of energy storage technologies.
“Some countries are now teaching us that they can rely 100% on renewables to obtain electricity for long periods of time,” says Kasper Moth-Poulsen of Chalmers University of Technology (Sweden). “Part of the challenge is to find storage solutions that are very high-performance (for air and ground transport) or very low-cost and scalable (heating applications).”
The real possibilities are multiple, adds Mark Z. Jacobson at Stanford University (California, USA). To store electricity, there are options such as pumping stations (hydroelectric plants connected to renewable energy facilities that feed off the excess production of the latter and, in exchange, intervene to compensate for moments of low generation), batteries, storage in the form of compressed air or solar thermal power plants, he explains.
There are also systems that allow heat and cold to be stored, such as water tanks (for both), ice storage (for cold), or underground thermal energy storage in wells and aquifers (used in some countries as a form of seasonal storage, for example in summer to be used later in winter). Finally, Jacobson notes, there are tanks that permit the storage of hydrogen, indicated by some —including the U.S. government— as one of the most promising technologies in this area.
How do you choose between so many alternatives? Moth-Poulsen believes that “it depends on the application and the geographical location.” There are countries, he explains, where differences in altitude and high water availability mean that solutions based on the combination with hydroelectric power “can work very efficiently.” In other cases, he contrasts, it may be better to apply thermal storage solutions. “In countries like Denmark, which has many challenges linked to heating, but also large areas that have district heating systems installed, large thermal heat storage pits are currently being implemented for seasonal storage.”
A question of economics
One of the main challenges to increase the presence of these technologies is the cost, admit the specialists consulted. Research groups such as the one led by Jessika Trancik at the Massachusetts Institute of Technology have among their main objectives to study how electricity produced from renewable sources can be economically competitive with that generated from conventional sources.
“Our results suggest that technologies whose energy storage costs fall below $20 per kilowatt-hour can be applied to renewables,” explain Trancik and Micah S. Ziegler, a member of her lab. Other factors limiting the production of energy that is price competitive and of good quality in terms of resources for the development of storage technologies, they add, include both the “availability of materials” and their “productive scalability at speeds fast enough to support broad deployment over a given period of time.”
Specialists are reluctant to give specific time frames to see electrical systems powered by 100% clean energy, but they are willing to suggest a possible path forward. “Electrifying all energy sectors (heating and cooling of buildings, industry, vehicles) means significantly reducing energy demand, because we will use heat pumps and electric vehicles, which need a quarter of the energy required by their corresponding fossil fuels,” Jacobson maintains. “That way, it will be easier to match the energy demand to the supply.”
Networks with artificial intelligence
There are also alternatives to energy storage, such as so-called “demand response,” say Trancik and Ziegler. This involves the creation of networks managed by algorithms in which users play an active and collective role in maintaining the balance between demand and supply, because they can reduce their consumption at a specific moment if the system so requires. The aggregation of many of them at the same time allows large amounts of energy to be saved.
Programmes based on this idea have been running for decades in some countries such as the United States and the United Kingdom, explains Manuel Alcázar of the Polytechnic University of Valencia. But in almost all the countries of the EU until a few years ago, the existing regulations hindered their development. Now the rules have changed, and demand response can take off in these areas as well. “These programmes would greatly facilitate the integration of renewables into the system,” says Alcázar.