The future of renewables, energy storage.

The biggest challenge today for our electric power generation and distribution networks is to have a means for storing energy, so that generation and consumption are independent.

Storing electrical energy would allow any of the renewable energies to become independent from consumption and take advantage of their full generation potential when it occurs in nature, increasing their competitiveness and equating them to conventional energies, encouraging the entry of new generations of distribution networks called “Smart grids”.

Electrical energy storage systems would therefore be configured as reversible systems, in which the important thing to consider is the overall performance achieved in the transformation and storage of energy, always taking into account the time variable (to take into account aspects such as losses during storage, power, response time, etc.).

Using Gravity.

Within mechanical storage systems, perhaps the most intuitive and cheapest form that currently exists is that of storing energy in the form of gravitational potential energy, which basically consists of using the electrical energy generated in excess to lift something to a greater height, gaining potential energy, and letting it descend to generate energy again.

The most logical thing is to use water as a medium, due to its availability, and your thing is to convert the energy storage that is already available, water reservoirs, converting them into reversible hydraulic reservoirs, in which water is pumped to an upper reservoir when there is a surplus of electricity generation, and is dropped, when necessary, into the lower reservoir, turbining it to recover this energy (as if it were a closed circuit hydroelectric plant).

And from here, the proposals that emerge today are very diverse, typical of a mature technology that seeks to adapt to diverse locations with potential, and there are already experiences of pure pumping plants, storage using seawater or the existence of deep caverns, as lower storage systems, among others.

With an overall efficiency of close to 80% in energy conversion in new power plants, hydraulic pumping technology is a well-proven energy storage proposal with extensive implementation experience around the world, with nearly 5,300 MW installed in Spain (far from countries such as Japan, with 25 MW).

Compressing Energy.

Elastic potential energy could be the other means of storing energy by mechanical means, causing a material to pass from a state of compression to another of release, generating energy, something known to the common portals who have been able to see the potential of compressed air in pneumatic systems of all kinds (hammers, pumps, motors, tools, etc.).

Systems that use air for energy storage are called CAES (Compressed Air Energy Storage), and their function is to compress air up to about 70 bars of pressure and store it in caves to, when necessary, decompress it and take advantage of energy.

The main problem, in this case, is the gas laws (PxV=NxRxt), which insist on relating pressure and volume to temperature, forcing the system to assume energy losses due to heat dissipation, during compression, or heating of the gas, during decompression. Storage yields stay that way at 42%.

Obviously, some of the readers who follow us will have realized that if we mix the heat storage techniques we saw in a previous post, with this system, we could have a better storage cycle... Not to worry about the patent, because the idea is already taken, it is called Adiabatic CAES and represents the next generation of compressed air storage systems, with several demonstration projects in Germany (ADELE Project), the United States, etc.

In addition, other interesting alternatives emerge in this line, such as

• The storage of compressed air in underwater “balloons”, which could be a revelation for offshore wind power,
• The ICAES (Isothermal Compressed Air Energy Storage), which proposes a compression/decompression of air in multiple stages up to maximum working pressures of about 70 bar, or
• The compression of air to liquefaction, called LAES (Liquid Air Energy Storage), with which they propose to compress, purify and liquefy air, by cooling it, to store it at low pressures in insulated tanks that function as energy stores, allowing higher levels of compression and therefore lower storage volumes.

The Chemistry of Combustion.

When we talk about storing energy in chemical form, it is common for us to fall into the temptation to remember batteries or batteries, one of the systems that has perhaps reached the strongest today for the storage of electrical energy, due to their proliferation in our daily lives.

However, combustion is still one of the most used forms of energy today. In it, energy storage is found in the fuel itself, which stores an enormous amount of useful energy, being one of the sources of supply with the highest energy density.

A kilo of gasoline accumulates about 12.2 kWh/kg of energy inside, while pumping hydraulics have a density of 1.5 Wh/kg.

The main problem with using fuels is that many are not renewable (once burned there is no way to reuse them) and that their use is extremely poor, with efficiencies that at best reach 56% in their combustion.

Does this mean that we have to discard them? ... Obviously not, especially with that energy density. In fact, they should be the first line of battle.

If we talk about its origin, what is theirs is that we close the cycle and choose to generate fuel from the electricity we want to store, and much better if we can do it using some natural or waste flow that we can recycle into a useful fuel, such as hydrogen and its generation by electrolysis of water, a process that, even if it still has low yields (70% conversion), could have a great future.

The other interesting alternative that is beginning to be considered in terms of chemical energy storage is the recycling of CO2, through methanation or catalytic reduction processes, which can generate light hydrocarbons such as methanol, with an energy density much higher than that of hydrogen, although currently with lower storage yields.

“Power to Gas” (P2G) or “Power to Liquid” (P2L) systems, which is how these media are defined for the generation of fuels from electricity, are therefore an alternative for the future to be taken into account, and today there are already a multitude of experiences in the market implemented and in operation.

If we talk about use, their thing is that we replace combustion, which provides a very incomplete use, with controlled oxidation reactions that allow maximum conversion to electrical energy and, therefore, the highest yields.

This is where so-called batteries, cells or fuel cells arise, which are nothing more than batteries in which the reagents that produce electricity are consumed, and therefore require continuous replacement.

In a fuel cell, fuel (hydrogen, methanol, CO, etc.) is supplied to the anode, while the oxidant (oxygen, air, etc.) is supplied to the cathode, both consisting of electrodes that allow the reaction to be catalyzed. In the middle of both is the electrolyte, which acts as an electrical insulator, a medium for the diffusion and/or exchange of protons and a separator of chemical reactions.

Following this operating principle, there are currently several different fuel cells on the market, differing depending on the accepted fuel and the type of electrolyte used, mainly.

The future of these systems obviously lies in the generation of energy to replace current combustion engines, typical of a technology of the past, but perhaps fuel cells can offer us even more, and in the future we can see reversible fuel cells (also known as regenerative cells), systems with greater energy efficiency, lower manufacturing costs and even greater robustness and flexibility in the use of fuels.

Charging the batteries.

The other chemical alternative for energy storage is the use of oxidation and reduction reactions in which the chemicals are not oxygen and a fuel, but pairs of substances that interact (redox pairs) being reduced and oxidized in cells (anode, cathode and electrolyte systems) that connected in series together form electrochemical batteries that are capable of providing different powers and energy capacities.

These storage systems have been used for almost 200 years in a wide variety of systems, evolving considerably since Alessandro Volta invented them in 1800. But it was not until the last few decades that the proliferation of energy needs in portable systems (from mobile phones, to tablets, computers or even electric cars) has made them truly known, and their application to large scale energy storage is still a very young alternative with many limitations.

For the storage of energy from the grid in stationary regulation applications such as those we are analyzing, right now the only batteries that exist on a commercial scale are those developed by the Japanese at NGK Insulators, Ltd., based on the patent developed by Ford Motor Company in 1960, and then sold to the company, which developed it on a commercial scale by TEPCO (Tokyo Electronic Power Company), currently at 75% storage efficiency, including system losses.

Currently, the largest facilities developed by NGK have a storage capacity of 245 MWh and a power of 35 MW, located in Japan (for a 51 MW wind farm) and in Italy (within the framework of collaboration with TSO TERNA).

Alternatives to sodium technology are still scarce, and the more or less commercial-scale experiences carried out (for large scale energy storage) are reduced to a few plants of a few MW, installed with greater or lesser success, and fundamentally based on technologies already implemented on a smaller scale.

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