A major increase in the power production from renewable sources will emphasize the need for load balancing measures. Adding buffers of energy storage within the grid might allow for renewables to increase their share beyond what would otherwise be practical. A wide range of various storage methods is on the table, and thanks to recent progress in batteries and materials technology they are beginning to look more attractive.
A future where solar, wind and tidal constitute the lion’s share of the power production, would also be a future where the amount of available electricity varies over time and with the weather. To a great extent, electricity would probably also be generated by small, distributed devices, giving centralized power plants less opportunity to adapt production to consumption.
In a way, the picture painted is one of a dynamic future, where power supply is organic. But, it would need some kind of skeleton lest it would crumble – an internal supporting structure, capable of evening out power fluctuations and offer consumers a predictable supply.
A regulating role for the grid
Such a skeleton might be formed by new functionality in the grid. The capability of the smart grid to communicate with end users, in part controlling and scheduling their energy use, would be a first step. The construction of super grids, spanning wide areas and multiple weathers, would be another. (See also “The Smart Grid and the Super Grid”).
It is likely, however, that those steps will not suffice. To complement them, the grid will need ability to maintain a stockpile of energy, acting as a buffer; at times when production fall short of demand, the buffer is drained, but whenever windmills are spinning or sunlight hits the solar cells, it is replenished.
A smorgasbord of storage methods
There are plenty of conceivable storage methods. As kinetic energy in a flywheel, for example, or as potential energy in a water reservoir. The flywheel would be accelerated by an electric motor, while electric pumps would bring water upstream into the reservoir. (See also articles “Pump power stores energy” and “Energy storage with flywheel technology”). Pump power is the most established method of energy storage so far, with a 99 percent market share and a storage capacity of more than 100 000 MW throughout the world.
The problem with all forms of energy storage is the added cost. Often, in practice, just increasing production turns out to be more economical. There is also the issue of loss, since every energy conversion dissipates some of the energy. The challenge will be to find a spectrum of cheap and highly efficient alternatives, to be committed where they are suitable given the conditions in the grid.
Similar to how pump power uses water as the medium of storage, air can be compressed using electricity, and then stored in vessels. Thermal storage is another option, which might be especially convenient for balancing the output of solar power – for instance, the heat from focused sunlight can be used to melt salt. The molten salt stays hot in insulated tanks, at a later time providing steam to spin turbines. (See also “A blooming desert“).
A method currently being tried out in the United Kingdom uses electricity to cool air until it liquefies. The liquid air takes up much less volume, and can be stored in vacuum flasks. When reheated, a hefty expansion takes place, which can be used to drive a turbine. An interesting aspect of this method is that the reheating step can be done in conjunction with energy harvesting: by using otherwise lost low-grade waste heat, efficiency increases. (See also “Energy Harvesting – recycling waste energy ”).
Then there is electrochemical storage – i.e., batteries. Battery technology has been steadily progressing, increasing energy density while lowering cost. The success of electric vehicles like Tesla piggybacks on the evolution of lithium-ion-batteries, a technology which have also been used in pilot energy storage projects – albeit to a high relative cost.
Current battery performance is adequate only for balancing moderate amounts of power over short time spans, and for a limited number of charge-discharge cycles. But a wide array of batteries are in development, and if a new, reliable combination of electrodes and electrolytes is found that boosts efficiency and lifespan, large-scale batteries are likely to have a place in the future grid. In a future where the vehicle fleet is largely electric, the grid could also benefit from the combined storage capacity of their batteries; vehicles not being used would be connected to the grid for charging, but also allowing them to act as a temporary power supply if power production falls behind.
A lot of expectations are built on so-called flow batteries. Unlike ordinary batteries, they use two electrolytes, stored in tanks and circulated through the battery, past the electrodes, divided by a membrane. There are numerous varieties being researched, with applications for electric vehicles as well as energy storage. They can be easily scaled up by increasing the size of the tanks – a favourable condition for grid energy storage. Common batteries store their energy in the electrodes, while the energy content of flow batteries are found in the electrolytes. A vehicle powered by flow batteries could hence be recharged quickly by refueling electrolyte fluids, and pouring the old ones out.
A current example worth mentioning is the German company NanoFLOWCELL. Their electric car, The Quant, is powered by flow batteries containing unspecified salt solutions. A prototype, claimed by the company to have a performance-to-weight ratio five times higher than lithium-ion competitors, has been shown at the Geneva Motor Show.
Other ideas also look promising – but, likely, lie further into the future: the storied hydrogen fuel cell, for instance, or new kinds of energy dense capacitors. Such supercapacitors would charge extremely fast, and have almost indefinite lifespans. They might be based on the unique properties of graphene, a novel and Nobel prize winning material, consisting of a single layer of carbon atoms. (See also “Super material with environmental potential”).
Energy storage can be accomplished in a number of ways, and relevant technologies are progressing. Time and continued development will make them mature and cost effective, and when different methods find their respective niches in the grid, the opportunity to increase power production from renewable sources will grow – hopefully, to a level where they will make a genuine difference.
The article was published in November 2014