Of course, with the advent of new and less predictable sources of electricity, everyone is looking for the most efficient way to store it for when it is really needed.
There are many ways, due to the state of technology and its commercialization, in the end, in the vast majority of cases, the most affordable electrochemical storage, for example in the form of lithium batteries, wins.
Not everyone has Long Hills in their garden
After all, the technical room in the house, armed with an army of lithium cells, is only slightly more accessible than, say, the Long Side itself. The idea of a private pumped-storage power plant is tempting, but who has a high enough hill in their garden, you see?
Be careful, the hill doesn’t have to be that high. They are already in development tiny transfer pumps for example for Australian farmers:
In the same way, you probably won’t succeed in front of the State Mining Administration and your neighbors with a request that you would like to dig a mine shaft behind the garage for concrete battery Gravitricity, which stores energy by pulling a weight up and converts it back into electricity by releasing the brake and spinning the generator.
Carnot battery
So will we be forever dependent on stock lithium and other conventional chemical electrolytes? Maybe not, we could also store excess energy in the form of heat. Simply by heating a suitable medium and keeping it at a constant temperature, and when it is needed, we spin a steam turbine with its help, or produce electricity completely silently with some highly efficient Peltier cells the future.
Basic diagram of a thermoelectric Carnot battery
One of the ways is the so-called Carnot battery named after the father of thermodynamics, Nicolas Carnot, and a thermal accumulator that will be cheap, available, heat it up relatively quickly, but at the same time have enough capacity to at least charge the damn cell phone when the heat is converted back into electricity.
Different energy storage techniques in terms of output power and storage time
We’ve had the basic theory for ages, but the practical technology is still relatively new and still evolving, because it didn’t make much economic sense until now. The advent of cheap electricity in times of surplus – for example on a windy Sunday morning, when the propeller blades are still spinning, but the factories stand with a bit of exaggeration – nevertheless gave the green light to all novel ways to store this energy.
Stone dust
So what would such a Carnot battery look like in practice? Many people might think of a pool filled with liquid, which we heat with excess electricity in the same way as when we prepare water for tea in the kettle in the morning. In practice, however, a solid substance is much more suitable – for example, crushed granite.
There is plenty of stone everywhere, and if it is ground to the right grain size to get rid of air bubbles, we could use it to fill, for example, a suitably insulated container under the garden with pansies. All this without worrying about how many charging and discharging cycles it can handle. It is, after all, stone – a stable substance that can last for millions of years on its own.
When charging, we will reheat its hot core, just like when we keep the stove running at the cottage, and when discharging, we will take heat from the peripheral parts so that the core does not burn out too much.
Can something like this really work? But of course it is, a prototype battery consisting of 132 tons of stone dust for example, heat pump expert Stanislav Mach from Moravské Krumlov built it. With the results he boasted on the Electro Dad YouTube channel.
132t dust battery for a family house in Moravské Krumlov:
Experiment from CTU
Mr. Mach’s experience was also used by researchers from University Center for Energy Efficient Buildings of CTU (UCEEB) who recently published message about their own concept of electricity storage in the Carnot battery and they collaborated on the project with Tepelná pombra Mach.
So let’s say we have some excess heat and electricity. Excess heat, for example from thermal solar collectors it preheats the ground dust to the basic temperature, well photovoltaic panels then they just electrically heat it up to the target. This is exactly how Stanislav Mach’s dust battery is charged.
Small experimental Carnot battery from CTU UCEEB. Note the carefully insulated and roughly 1.5 meter tall cylinder filled with rock dust
The concept from CTU works similarly. It works with a compressor at the input heat pumpwhich can utilize already existing industrial waste low-potential heat (60 °C) and heat the dust to the final temperature using electricity from renewable sources or cheap surpluses from the grid 125 °C. The laboratory prototype simulates the input waste industrial heat of the heating system of the CTU UCEEB building.
Control terminal from TC Mach
Universal granite dust
An experimental accumulator uses to store heat granite dust, which is easily available in the Czech Republic and is crushed by every quarry. What’s more, the dust bin is in contrast to the electrochemical cells much more scalable.
From the development of storage with stone dust interwoven conductors for heat supply and removal
While, for example, such 4.2 V lithium cells must be connected in series and parallel to complex battery structures for the target capacity and electrical output, while individual cells can degrade over time at different rates, we can increase the capacity of a dust battery simply by increasing its storage, which at the same time we will also achieve a higher efficiency of the heat accumulation itself.
In this respect, therefore, the Carnot battery is more like a pumping station, the capacity of which will also be inflated jen by simply raising the level in the upper tank.
And why only 125 °C?
As we said above, UCEEB heats the dust battery to 125 °C, and an astute reader could argue why such a low value, when the stone can withstand many times higher temperatures. After all, such Finns heat 100 tons of sand up to 500°C in a similar experiment.
Temperature stratification of a dust cylinder with an apparent hot core in the center
The limit is in this case heating method. While the Finns heat the sand using a stream of hot air that is heated by electrical resistance heat (parallel to our electric kettle), the Czechs are experimenting with more efficient heat pumps.
However, an ordinary compressor pump cannot be heated to hundreds of degrees Celsius, so even the best ones work with temperatures up to 180 °C. Lower temperatures are gentler for both the compressor pistons and the heat medium inside it.
Organic Rankine cycle
Okay, so we’ve stored the heat in the dust in the reservoir, but now the conversion back to electricity. Here, too, the Carnot battery has one great advantage, as it uses already widely used principles and existing technologies that we know from ordinary steam power plants using Rankine cycle.
General diagram of the simplest Rankine cycle
Simply put, water is heated in the boiler, it turns into steam, which under high pressure spins the blades of the steam turbine and the generator produces electricity. Then we cool the steam in the condenser again and turn it into water, after which the whole cycle is repeated.
However, the Carnot battery from CTU works at lower temperatures, so it uses organic Rankine cycle (ORC), which replaces water with a more suitable mineral oil R-1233zd. So instead of a gas, coal or nuclear boiler we have hot stone dust, instead of steam we have oil and instead of a steam turbine a much smaller lamella expander.
The vane expander is actually a small turbine with an electrical generator for the ORC
Theoretical efficiency of 10-20 percent
However, the efficiency of RTE’s conversion of electricity into heat and back is still much lower (Round-Trip Eefficiency), which currently cannot compete with the efficiency of electrochemical storage. However, these focus on shorter accumulation times and faster cycles on a smaller scale. Therefore, the competitor of the theoretical large-capacity dust heat battery is again a smaller water pumping power plant with an efficiency of around 70%.
Carnot battery of CTU in operation
The prototype from CTU, which at this stage aimed to verify the basic functions of all three components (heating, storage, conversion to electricity), could, according to model calculations, achieve an efficiency of approx. 10-20 %, although real values are still lower. But again, it should be remembered that high RTE was not the primary goal of the research.
Where could it store electricity in practice?
The experimental and relatively small dust battery from CTU has a thermal capacity of approx 44 MJ (12 kWh)and since it works on the principle of a heat pump with two inputs, the adept for real implementation is rather than a family house outside the city, an industrial enterprise with enough waste heat (60 to 80 °C), which is heated to the target temperature with cheap electricity from the network, or its own of the RES system (photovoltaics, wind power, etc.).
Carnot battery vs Li-ion batteries and economics
On the other hand, the prototype of a family house from Moravský Krumlov and similar experimental installations from, for example, the aforementioned Finland show that, under suitable conditions, dust thermal batteries could also work for ordinary housing.
Initially, perhaps only as heat accumulators for the winter, with the development of the most efficient ORC techniques, which will extract the maximum even from relatively lower temperatures, but also for the actual production of electricity.
2023-04-23 16:45:26
#CTU #produced #experimental #Carnot #battery #granite #dust #lithium #capacity #kWh
