In the current trend of mankind to switch to renewable energy, the ability to store intermittently generated energy plays a key role, but if wind energy is only available when it is windy and solar energy is only available during the day, and we would need it more in the evening and morning, it is important to be able to store that energy as efficiently and cheaply as possible, preferably with solutions that are sustainable over time and do not require replacement after a few years. A team of engineers from MIT University and the Wyss Institute in the US have just announced an unexpected and curious solution to storing electricity.
Their solution is a super capacitor based on cement and carbon. The notion of capacitor is used in Romanian more recently, when it has, in fact, a term known to physicists for much longer – condenser. The essence of a capacitor is the storage of electricity in the form of a magnetic field between two conductive fittings, both with the same electrical charge but with different polarity.
Capacitors can usually store small amounts of energy compared to their volume, and the figures also depend on their type. Their advantage was in the fact that they could discharge this energy very quickly and capture it just as quickly. In recent years, there has been a lot of development of super capacitors, or super capacitors, which can store much more electricity while retaining the advantage of being able to discharge it very quickly. With this development, supercapacitors have found widespread application in motorsport, on hybrid cars.
Le Mans cars in the LMP1 class have been using super capacitors for many years now, ever since the Toyote came on the scene and Porsche’s entries several years ago. What’s more, even in Formula 1, the KERS system, where drivers can get a short-lived boost of electrical power, is also based on super capacitors. And in all these motorsport disciplines, their essential advantage lies in their ability to pick up huge amounts of electricity very quickly on charging (when braking before cornering, for example), and rapidly discharging the capacitor, sending energy to the electro-motor. So you can, for example, provide a short-term boost of 60 kW, or about 82 hp, from a supercapacitor storing just less than 1 kWh of electricity. A lithium-ion battery so small in storage capacity would not be able to discharge with 60 kW of power, but the supercapacitor can.
So in motorsport these super capacitors are used extensively, they also have the advantage of keeping the weight of the car small due to their small size, but also being resilient to many, many charge and discharge cycles. So let’s take a look at what the cement and carbon-based supercapacitors, now announced by the US engineering team, are all about and how they can make a crucial difference to energy efficiency where they are applied.
The engineering team, led by Franz-Josef Ulm, Admir Masic and Yang-Shao Horn, has now developed relatively small demonstration supercapacitors, which can be seen in the image above. The essence of these is the material from which they are produced, to demonstrate its viability.
Basically, supercapacitors are composed of a mixture of cement and carbon, carbon black (a product consisting mainly of carbon) and petrified carbon, e.g. coal or an equivalent. And this means that carbon extracted from the air, in the form of carbon dioxide that becomes petrified in order to be buried deep underground and thus reduce the concentration of CO2 in the air, could theoretically also be used in the composition of this condenser if it were separated from oxygen. What is certain is that the engineers at the two universities have succeeded in creating a slightly porous mixture, the pores being formed naturally when mixed correctly. The presence of carbon between the porous cement network allows the formation of an inner fractal network, which, being interconnected, creates the structure necessary to provide the condenser effect.
The proper mixing and drying process now takes 28 days, with drying taking place slowly, insulated with temporary plastic to keep the internal network intact. Then the resulting material, reminiscent of building concrete, is cut to the desired shape and dipped in an electrolyte solution, for example potassium chloride, and two such cut blocks become electrodes, and, being placed side by side, with an insulating membrane between them. The two blocks connected to the load – one to the positive load and one to the negative load, and the whole construction thus becomes a super capacitor.
And here come the main curiosities. What we talked about above about the construction of the new super condenser means that it is effectively produced from just a normal mixture of cement, sand and water, like ordinary concrete, to which is added carbon powder of different variations, which, once mixed, forms a network within the cement and effectively the concrete, the key being in the slow drying of the final concrete to keep the network of carbon intact. Engineers Franz-Josef Ulm, Admir Masic and Yang-Shao Horn say that this mixture, and the process above, makes it possible to build an entire house foundation, for example, with such super-capacitor concrete. You can do thick concrete layers, you can do large surfaces, so the possible scaling is huge. And as a result, the foundation of your house effectively becomes an electricity storage battery, while also serving its primary role as a structural element in the construction.
Engineers also announce figures. As I said super capacitors don’t store electricity as densely as a lithium-ion battery, so a house foundation made of such concrete would need 45 cubic meters to store 10 kWh of electricity. If we think of a house with a footprint of 10 m x 10 m, this would mean a 100 square meter foundation area, and at a height of 45 cm of such a concrete element in the foundation, it would have 10 kWh storage capacity. If you decide to double the thickness, or have a larger surface area, or use such concrete in other structural elements, for example vertical partition walls, but electrically insulated afterwards, then the storage capacity of a house can increase to 20-30 kWh. Engineers say their material allows electrodes from 1 mm thick (so about 3 mm capacitor thickness) up to 1 meter thick electrode, so over 2 meters capacitor thickness.
One drawback of all super capacitors is that they cannot store energy for very long, so again the logic is that you can use the electricity stored in the foundation of your house for 24-48 hours, for example. You can store the 10-20 kWh or 30 kWh from the PV panels during the day and use them in the evening and morning, and have a home that stores its energy locally, without the need for an extra battery. What is the advantage over a battery? A huge one, mentioned above – resistance to a huge number of loads and discharges, which means longevity over time. Engineers say their capacitor concrete can withstand over 1 million load cycles without losing any of its properties! A lithium-ion battery has 3-5 thousand cycles expected, some a little more. But it’s on the order of thousands, whereas here it’s 1 million cycles. If we put up to 3-5 charge and discharge cycles in 24 hours, it would mean that this concrete capacitor would have a lifespan of 200,000-330,000 days, or 547-900 years! Even if it were used 5 times more intensively than the above average, it would still last over a century.
And because super capacitors can provide a discharge even at high power, this concrete in the building foundation can be easily adjusted to give the desired voltage and power to the grid, with very good resilience to load variation – for example when you connect another appliance to the grid, which suddenly consumes 3-5 kW more or even more. In other words, the home network could provide all the variations of real life.
And the engineers who created this super capacitor based on cement and carbon now see it being used in the future not only in houses and foundations, but also in larger blocks or even in road and motorway construction, where a 10 km or 100 km long motorway becomes a network of such super capacitors. Their purpose would be to be used where large quantities of concrete are already being placed anyway, transforming that concrete from an amorphous structure into an energy-producing one.