The way we store our energy will be changing radically over the coming decades. Sustainable sources such as wind and solar will replace coal and gas. An important bottleneck of sustainable energy is its fluctuating supply, which rarely runs parallel to demand. Storing energy is a solution, but it’s not an easy one. How do you store power for a street, a neighborhood or even a whole city?
Article from Objective 21, 2014
Our current energy system works with large power stations that constantly generate power. Depending on the demand, the supplier can supply less or more power. It doesn’t work like that with sustainable energy, where the supply is dependent on climatological conditions and not on demand. The good thing about sustainable energy is that its sources are free and inexhaustible. The sun rises for nothing and solar panels are becoming cheaper. But at night time, when the demand for power peaks, the sun is gone. And wind turbines don’t move without wind. Sometimes it’s literally all or nothing. At the moment, large power stations can step in when the solar or wind supply is not sufficient. But as we begin to use more sustainable energy, this will become increasingly difficult. This means that we need forms of energy storage that can absorb sustainable energy during periods of low demand and make it available again during peak moments. Which technology is best suited depends on the specific application.
More than just batteries
Energy can be stored in many different ways. The common response currently is still the ‘battery reflex’: “Why don’t we store the surplus in the batteries of electric cars?” But what if those cars are parked at work or outside shops? Batteries can certainly play a role. But there are many more possibilities for energy storage, each of them with pros and cons. Small-scale, local storage at home might use batteries or can be done by converting energy into the heat or cold of the house or the refrigerators or heaters. Batteries can also be used on a larger scale, to provide storage for a whole neighborhood. Experiments with this are already underway, as can be seen from the article in Objective 18 about Smart Storage, which is already storing electricity for a residential area. One battery unit the size of an electricity substation can supply electricity to approximately 200 homes for more than 2 hours. This is a good scale for an experiment. But a lot of work still needs to be done before something like this can be used as a standard form of storage; both in the technical periphery and at the core of the batteries.
Chemical and electrical storage
Batteries are a chemical form of storage, where electricity is stored by means of a chemical reaction. Energy is lost in the process: the battery becomes warm. In addition, batteries have reaction time, both when they are being charged and when they are discharged. This is also the case with electric cars: you can’t simply ‘fill up’ your battery within minutes. Conversely, batteries can’t supply large amounts of power in a short period of time; they have to be discharged in a controlled manner. The chemical conversion takes time. Quick storage and discharge is possible with capacitors, the purest form of electrical storage. Here electricity is stored as charge, without chemical conversion. This makes it possible to store very high peak loads. It’s very energy efficient, therefore, because there is no conversion loss. However, the internal discharge resistance is not infinite; because the charge slowly ebbs away there is a small loss. Capacitors have become technologically very advanced over the past few years and have become cheaper by a factor of 100 over the last 10 years. The current ‘supercapacitors’ are the size of a coffee mug, have a capacity of no less than 5000 Farad, and are able to store the same charge as an AA battery. Thanks to their low internal resistance, capacitors can be charged and discharged very rapidly, without much generation of heat. This feature makes them particularly suitable for the supply and storage of peak loads.
You can also store and generate peak loads with a flywheel and dynamo. This is a technology that is used for instance in power stations for peak shaving, the absorbing of fluctuations in mains voltage. Flywheels are highly energy efficient, up to 99%, but the equipment is too large and requires too much maintenance to make them suitable for large-scale energy storage. Water power is another form of mechanical storage used in the Netherlands. We send our surplus power to Norway through a thick cable, where it is then stored as hydropower. A technology that we can use at home is compressed air energy storage. Just as you can inflate and deflate a balloon, you can also fill an underground space with compressed air, for instance an old mine or an existing geological structure. As soon as the demand for power increases, the compressed air is allowed to operate turbines. Compressed air installations are pretty expensive, but they are good value for money: they can supply energy for hours or even days, with peak capacities of several hundreds of megawatts.
There are many different versions of energy storage in the form of cold or heat. Large refrigerated warehouses are already being used as buffers. When there is a surplus of electricity, the refrigerator in these warehouses works a bit harder, and when there is a shortage, the temperature is turned up a few degrees. Real energy storage, where the cold or heat is subsequently converted back into energy again, is not the preferred option. Too much energy is lost in converting it into heat and then back into electricity again. The traditional way in which we produce energy also includes a thermal intermediate step, in which steam operates a turbine. The energy efficiency of this production is only 50 to 60%. Nevertheless, thermal storage has seen some use. There is an installation in Spain that stores solar energy in molten salt. When the sun does not shine, the heat is converted into electricity. The power station has a capacity of 20 MW and can keep up production for 15 hours without sunshine. The station can generate power for 75% of the year.
Converting energy into fuel
Low energy efficiency does not have to be an impediment for the storage technology in question. If you have an abundance of free energy, you may well be prepared to lose some of it in storing it. Certainly if you can solve this problem by continuity, as the Spanish solar power station does. This logic also applies to conversion into (liquid) fuel, a practical medium that has been used for energy storage for centuries. If only we would be able to do that: make free fuel from solar and wind power. It would be the alchemy of the 21st century.
Fuel from air or manure
And yet this conversion is as old as life on our planet: plants turn solar energy into fuel, absorbing CO2 from the air and emitting oxygen. All today’s fossil fuels originated in this way. It would be great, therefore, to be able to use these processes to store surplus energy as fuel. In the ideal case, you would make fuel out of the surplus energy and CO2 from the air. Technically, this is possible, and it already happens in laboratories. But there is still a long way to go. However, you don’t have to begin with air. Manure and other biological waste are also perfectly suited to be turned into liquid fuel. Manure is currently serving already as raw material for fermentation plants that use it to make methane. A few biochemical steps further and you can turn it into liquid fuel.
Fuel is compact
The big advantage of liquid fuel is that fits the existing situation: you can easily store it, transport and distribute it through the existing channels. You don’t have to build new networks or motors for it. In addition, fuel is the most compact way of storing energy. No other storage medium contains more energy (kWh) per liter.
Keeping it local
Storage helps to keep power local. It’s a resource that allows you to use as much as possible of the energy you generated yourself. After all, that is the most efficient way. It also means lower energy tax, because you pay that when purchasing energy, but you don’t currently receive it when you sell your surplus. In addition, the network benefits from keeping power local: it prevents overloading the network and transport across long distances. Local storage is not a solution if you generate more energy than you need yourself on a structural basis. Then you are better off selling the surplus immediately to the neighbors. An important question in that case is: who will match demand and supply, in what form and using which infrastructure?
In addition to the return, price and technical feasibility are important in judging storage methods. Many energy storage and conversion technologies are still very expensive. Batteries are becoming better and cheaper. But in order to serve a residential area, a storage system needs to be able to supply power in megawatts. That means you need a lot of batteries and that still costs a lot of money. Research into energy storage is continuing, with the focus primarily on materials science and chemistry. Thus graphene is currently being widely studied as a material that might give batteries a serious boost.
Keeping power local and storing it for periods of low demand also requires measuring and control engineering. You have to know when power needs to be stored and when the reserves need to be used. Some work still has to be done in this regard, such as equipping the network with the measuring and control engineering required for good use of storage that is integrated into the grid.
Technolution is already contributing to making the technology that is required for this: measuring and control engineering to match demand and supply, communication, price calculation models and payment systems. Where can we source the power and where does it need to be delivered? Who is going to pay for that, and how, and how much? All of this can be automated.
Energy storage is essential
The energy grid will only really become smart if we are able to store and control energy. That requires technical adjustments to the infrastructure, but also political and legal changes. A number of experiments are currently being conducted involving energy storage, but scaling up to the wider context is still a number of steps away. The transition to sustainable energy supply is inevitable. We can’t continue in the old fashion, with fossil fuels that are exhaustible and pollute the environment. We have to start working on the transition now, and energy storage is an essential aspect of that.