It's not really fair to compare this, which takes heat at 2000°C, to a steam turbine that takes heat at say 550°C.
A turbine system would have much better efficiency than 40% if its heat was available at that temperature. For example a closed Brayton cycle gas turbine + steam turbine system. Certainly complex and expensive, but could get much better than 40% efficiency.
If a steam turbine could operate at those higher temperatures it would be more efficient. But it cannot do so under any reasonable condition. If you want the higher efficiency from storing high grade heat energy, it's not feasible to use a steam turbine.
Think of the steam turbine as a baseline. Like rating a vehicle in horse power.
Also I wonder if, using similar principles to a heat pump's operation, you could still get juice out of stored heat at lower temperatures. Surely you can have this hot graphite sitting at under 2000°C, heat some fluid/gas to say 1000°C, and then compress the gas to increase its temperature? Surely that would be the ideal solution anyway, since you don't want your hot graphite to become a chunk of useless heat simply for dropping below temp briefly.
On the topic of heat pumps, you could also use a TPV for geothermal power. Since there are no moving parts and presumably no huge steam engine installation, it would be more feasible to have one of these in your back yard. The grid powers a heat pump, you compress the fluid till it hits 2000°C, and your TPV extracts power. The heat pump itself is >100% efficient, so overall you can steal a fair bit of electricity from the ground. Right?
I just want to point out that compressing gas takes a non-zero amount of work. Compressing gas to increase temperature to use temperature to make energy would be better spent using the compressing work directly to make electricity.
The best theoretical efficiency of a heat pump is 1/CarnotEfficency. So the best you can do is to have a round trip efficiency of 1. See Carnot batteries. So using a heat pump to increase the temperature so that a thermal engine is more efficient can’t possibly increase the overall efficiency.
Heat pumps are different than heat engines... I don't think Carnot efficiency applies. A heat pump can move several units of heat energy per input unit of electrical energy (thanks to awesome compressor / refrigerant / phase-change technology). ...They're >100% efficient.
For instance, take a Brayton thermodynamic cycle. It's motor cycle, but you can literally just reverse the arrows on this cycle, it becomes a reversed Brayton cycle, which corresponds to a heat pump (or a refrigeration machine, depending on which inlet of the machine you're interested in). In practice, the reversing of the arrow means the reversing of the fluid flow, which means compression processes are now expansion processes and vice versa.
Carnot theory does still apply. The COP (Coefficient Of Performance) of a Carnot heat pump is 1/n, where n is the usual Carnot efficiency of a heat engine.
With n=1/3, you get a COP of 1/1/3 = 3 = 300%
Carnot efficiencies consider ideal machines. So ideal Carnot batteries have a n * 1/n = 100% efficiencies. It means you can't increase the efficiency of a heat engine by putting a heat pump in front of it. In an ideal case (ignoring all losses and inefficiencies), you would get the same efficiency. In practice, you can only be worse.
Are you sure you aren't confusing the coefficient of performance with the efficiency? If a heat pump was >100% efficient, I could cool my house by leaving my refrigerator door open.
Correct, that's what refrigerated air conditioners do. But that's really just playing a trick with the system boundary in the problem; it doesn't mean the global efficiency has exceeded 100%.
To be clear, I don't think the term is imprecise. Efficiency is always (work out)/(work in). The numbers get wonky when we aren't fully accounting for the "work in". In the case of refrigeration, we have to account for the work it takes to compress the fluid and the resulting waste heat.
I'm not sure if there is any way to make heat pump operating in this scale of temperatures, and keep in mind that current heat pumps can only "pump" over about 50°C max until it's no longer profitable.
That’s not really true at all. Home heat pumps are stuck there because of the refrigerants used. Ammonia heat pumps can go way hotter (though still nowhere close to those temps - more like 150c iirc), it’s just that ammonia requires safety controls that are impractical on a small scale.
Combined cycle power plants take heat at 2000°C using a combination of gas and steam turbines. Their conversion efficiency to electric power is 52%, see Wikipedia. This is proven technology, at least 30 years old, and quite a few of those exist out there.
Steam turbines alone can be operated at higher temperatures by using mercury instead of water. Some plants using this approach were built in the 1920/30s.
Not a steam turbine, a combined gas turbine (for the high temperatures) - steam turbine (for the low temperatures).
The waste heat from the gas turbine would power the steam turbine. Something like a combined cycle gas turbine, but with a closed loop for the gas turbine (say, helium) rather than combustion.
High temperature, though I'm confused myself about how a 40% efficient conversion done twice can match a battery. Probably will need to read the actual paper, I thought batteries were on the order of 95%. Maybe it is more energy dense or something.
"Evaluated herein is one E-TES concept, called Firebrick Resistance-Heated Energy Storage (FIRES), that stores electricity as sensible high-temperature heat (1000–1700 °C) in ceramic firebrick, and discharges it as a hot airstream to either (1) heat industrial plants in place of fossil fuels, or (2) regenerate electricity in a power plant. … We report that systems of 100–1000 s MWh may be cycled daily, and discharged at a constant heat rate typically for 70–90% of the storage capacity. Traditional insulation can reasonably limit heat leakage to less than 3% per day. Preliminary cost estimates indicate a system cost near $10/kWh, substantially less expensive than batteries."
I think it's fair, if we're talking about capabilities of the designs. Since a turbine can't take it's steam at 2000 degrees in practice, that's a limitation of the system, and that limitation limits it's efficiency potential.
That said, we don't really know what this new design can take in practice, so it's probably a comparison drawn too soon.
Heat supplied at high temperature is more valuable than heat supplied at low temperature. Look up Carnot efficiency, and "Exergy". There are limits on how efficient a heat engine can be, defined by the "hot" and "cold" temperatures. A hotter "hot" side allows higher efficiency.
That sounds like room for innovation, rather than an unfair trick. If you can find ways to achieve high temperature cheaply and easily, you get better efficiency.
I think the point they're making is that 40% efficiency is amazing when you have a 1500° difference between the hot and cold side. But at 2000° the theoretical efficiency limit is higher, for all heat engines.
It's like if I compared the top speed of your Porsche on a perfectly straight road, clear weather, no wind, to the top speed of my sedan, but I've got it pointed toward a very high cliff and I'm claiming a top speed of 400 mph (straight down). It's not a fair comparison, because I am cheating by exploiting a greater 'height'.
If we actually compared the same thing, yours would still be at least 50mph faster than mine (at a slight angle from straight down)
Solar concentrators, yes. Or even cut out the steam completely and use the hot exhaust gas from the fire directly. Or combine the element into existing powerplants extracting high temperature and use the residual heat to run steam turbines.
A turbine system would have much better efficiency than 40% if its heat was available at that temperature. For example a closed Brayton cycle gas turbine + steam turbine system. Certainly complex and expensive, but could get much better than 40% efficiency.