> 100-megawatt reactor measuring seven feet by 10 feet, which could fit on the back of a large truck
A typical thermal power station has an efficiency below 50% for electricity generation, so the plant dissipates at least as much heat as it generates electrical power.
I wonder how you could get rid of 100MW of waste heat from a volume small enough to fit on a truck. That's a heat flux of more than a megawatt per square meter of surface area.
FWIW, a single Boeing 747 engine does about 100MW. Such an engine can be considered to "fit on a truck" for some definition of truck.
Also consider that much of the size is due to the fan on the front (which is not part of the actual engine power plant), which makes the engine's area in the plane transverse to the axis of rotation seem larger.
However, jet engines have the advantage of burning their fuel directly in the air that is flowing through the engine, which results in an extremely rapid transfer of heat to the air compared to other (this is why 10 Boeing 747 engines == 1 GW coal/nuclear plant which is "buildings" in size).
That's an excellent point! Perhaps they intend the truck to be sitting at the bottom of a fast moving river.
But what gets me is that they are burying the lead. No fusion reactor of any size has reached sustainability...so why is the story here talking about size at all? Small size would be a very nice added bonus; fusion reactors that don't consume more energy than they produce are earth-changing.
What the commenter is pointing out is that it may not matter how you cool the reactor. It could be in interstellar space and it might not matter because the material the reactor itself is made of may not be able to transmit the heat away from the reactor. And no such material may exist.
I agree that the announcement is a bit burying the lead here, though maybe the size is actually important to making it work. From Aviation Week[0]:
"But on the physics side, it still has to work, and one of the reasons we think our physics will work is that we’ve been able to make an inherently stable configuration.” One of the main reasons for this stability is the positioning of the superconductor coils and shape of the magnetic field lines. “In our case, it is always in balance. So if you have less pressure, the plasma will be smaller and will always sit in this magnetic well,” he notes."
As far as I understand this (IANAP), they're saying that smaller size = less pressure, and that helps their design to work (in theory).
I think it is more a matter of tolerances than of size what makes ITER take so long. Also, it is experimental. That means that it is not just a matter of ordering stuff and putting it together.
Note that it converts ~100kW thermal into ~37-47 kW electric. It also has high air flow. I suspect that means that they are aiming to make their fusion reactor small enough that it can replace the burner section of the turbine.
Thermal transfer is still going to be a challenge, unless (Idea!) they are planning on spraying water into the airflow and using heat from neutron moderation in the water to provide primary heat transfer. Hmm, I wonder how much water you would need for 14 MeV neutrons? Let's see, half-value layer is around 10 cm, so if you threw a 40 cm thickness of water across a neutron flux, you'd absorb most of the heat (you could probably catch the rest in the duct casing). That's a lot of water in a pretty small space, but physically possible.
Note that the above is a seat-of-pants calculation and should not be taken as accurate by any means.
The Bugatti Veyron 16.4 has 1200 hp and therefore probably produces around 2.5 MW waste heat at full power. A factor of 40 is still quite a difference but it does not sound like an unsolvable problem.
That's peak power, though. Basically no street car is designed to be able to produce peak power for more than a minute or two at a time, and probably very few can do it for more than a few seconds. I'm pretty sure that car doesn't have the cooling capacity to cool itself at full power like this plant would have to.
You can drive your average care at peak power for an hour without problems. The problem with the Veyron is that at top speed the tires will wear off in 15 minutes and you will run out of fuel after online 12 minutes but I think it is not really limited by its cooling capacity - at 400 km/h a LOT of air passes through its 10 radiators.
temperature difference between air entering and leaving the radiator ~ 10degC
==> 1MW
So handling 2.5MW waste heat doesn't seem out of the question from this analysis.
As another ballpark "upper bound" analysis, consider that copper is one of the most thermally conductive materials, with thermal conductivity ~ 400 Wm/(m^2 degC), let's round that up to 1kWm/(m^2degC).
Let's assume that there is a thermal conducting surface of 1m^2 (i.e. the surfaces of the pipes that interface with the radiators). Let's assume that the copper is 1mm thick (= 1m/1000). Let's see what temperature difference would be needed to transfer 1MW of heat across:
As another ballpark "upper bound", the convective heat transfer coefficient for forced air is ~ 100W/(m^2degC), which means that assuming that the radiator fins are 100 degC above the air temperature, in order for the fins to transfer 1MW of power to the air, you would need an area of
1MW == 100W/(m^2degC) * 100 degC * area m^2
==> area ~ 100m^2 of surface area in the radiator, which doesn't seem unreasonable (a radiator 1m^2 area * 10cm deep, made up of thin plates spaced 1mm apart has this total surface area).
So they Veyron dissipating ~ 1MW in waste heat at 400km/h doesn't fail any of these basic sanity tests.
More complicated though is the interaction between the stages considered here. For example, how do we interface our 1m^2 of copper with our 100m^2 of radiator? Making the radiator fins thin lets us pack more surface area into the same volume, but also makes it difficult to keep the "edges" of the fins at a sufficiently high temperature so that they pull their weight transferring heat to the air: since the "copper tubing" has significantly less surface area, it only contacts (and thus transfers heat to) the radiator fins "sparsely".
My point may not have been as strong as I thought, but that tends to support it - airplane and boat engines do need to operate at max power nonstop, so they're designed for it. Consumer automobiles usually just accelerate for a few seconds and cruise at relatively modest speed, and I'm pretty sure their cooling and other related systems are designed around that.
That's because you usually have a speed limit. If you have a car with say about 100 hp your top speed will be about 200 km/h and that is a speed you can drive at for extended period on an Autobahn in low traffic. A more powerful car will of course make it harder to keep the pedal at the metal.
In addition, in the step before where the heat is transferred away from the reactor walls to the turbines, a heat flux is needed of 100MJ/s. Assuming a contact surface of 100m2, that is 1MJ/s per m2. That sounds like a huge flux and I am not sure if there is any medium that could do this.
Normal cooling (cooling towers or similar) requires a huge structure to dissipate 100MW. The cooling infrastructure can't fit in a truck. What you can to is transport the plant to somewhere where the environment can help with the cooling, such as where there is a good supply of cold water.
One can imagine these to be simple drop in replacements for existing coal plants with existing cooling infrastructure,which would be a huge environmental win in places where those are still built.
A typical thermal power station has an efficiency below 50% for electricity generation, so the plant dissipates at least as much heat as it generates electrical power.
I wonder how you could get rid of 100MW of waste heat from a volume small enough to fit on a truck. That's a heat flux of more than a megawatt per square meter of surface area.