only just up to the job
presumably they could use several of them to enable firing longer bursts?
Raygun fans will be overjoyed today to hear that a major problem of energy weapons - the fact that they tend to be hugely more devastating to themselves and their users than they are to their targets - may soon be solved. No, really. Even the very latest laser and high-powered microwave weapons have beam efficiencies well …
If someone points out to you that your pet theory of the universe is in disagreement with Maxwell's equations — then so much the worse for Maxwell's equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
Sir Arthur Stanley Eddington, The Nature of the Physical World (1915), chapter 4
To turn heat into electricity you need a source of heat and a source of cold. The more extreme the better because the best possible efficiency is:
1 - Tcold/Thot [temperatures in Kelvin, which is centigrade+273]
At present, politicians like killing people in hot countries, so pretend Tcold 27°C (300K). I am not sure what the operating temperature of a solid state laser is, but mil spec is 125°C (398K). That gives a theoretical best possible figure of 25% efficient. Getting 12% of the power needed to run a 50% efficient laser is not worth lugging around something bigger and more complicated than the generator providing the other 88%. (You cannot have the heat from the generator - it must be at a low temperature already because the generator is near the limit of efficiency.)
Remember the entire point of the article was that it is difficult to dump 100kW into air. Computers dump heat into air by raising its temperature. You want a large difference in temperature to reduce the amount air whistling through the fans. For a generator running on waste heat, you want to maximise the amount of air screaming through your cooling turbines to avoid increasing Tcold. You also have the same problem on the hot side - you have to run the lasers just short of melting point and pump coolant through them very hard so the coolant is not much colder than the laser and you get a useful Thot at the generator.
The wax stores what is referred to as 'low grade' heat energy, which, by definition, isn't very useful, and won't give you a large enough difference in temperature to run an efficient thermal cycle, which is how you to convert it to mechanical or electrical energy. The best you'd probably end up with is a few percent conversion efficiency at the cost of large, heavy, and expensive equipment.
It could, however, be used for heating or possibly cooking. So now at least the shark tank water is warm, to avoid your captors going into shock before they get torn limb from limb by teeth and high energy weaponry.
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The reason why sharks are the carrier of choice for lasers is they normally swim in the sea - which is a much easier place to dump heat than the air. Dumping 100kW into sea water requires about 1L/s of water going through the heat exchanger. (You could use less, but then you risk scolding a shark armed with a big laser.) To fire your 100kW laser continuously, you need to pump over 600kW of fuel into an engine to get 200kW of electricity to the laser. As well as dumping 100kW of heat from your laser, you also have to dump 400kW from the generator.
Even so, the real challenge is fitting your shark with a generator:
Fitting a 3 ton generator onto a 2 ton great white shark is going to sink the shark. Perhaps a 21 ton whale shark could carry the kit, but they are filter feeders. They lack the 'I am going to bite off your torso' image that people expect from a shark guarding an evil super-villain's base.
They've just invented a big lava lamp!
I'd buy a lava lamp that could also kill intruders at 20 paces. I'm not so sure about the 40 seconds running limit until it explodes, showering your disco lounge with 30 kilos of boiling wax though...
There wouldn't even be time to finish your Campari and soda - and the only music quick enough to finish before your lava-lamp-of-death goes nuclear is going to be punk, which rather puts the dampeners on the romantic mood - almost as much as being showered in molten wax and flying glass.
Oh well, back to the drawing board.
Can anyone explain why this is difficult?
Consider a (largish) 130bhp motorcycle - this has a peak power output of 100kW. It also has an efficiency somewhat south of 25%, so about 300kW thermal output. On a full-bore launch, the rider is dumping the power into the oil-cooled clutch, starting at the full 100kW, and declining to zero as the bike starts rolling and the rider stops slipping the clutch. Additionally a proportion of the thermal output has to be dissipated through the engine's liquid or air cooling system (the remainder leaving via the exhaust gas). Such a bike can usually sustain a steady state cruise at max power for several minutes: the clutch isn't a consideration here, but it still has to dissipate 300kW (thermal load, via cooling system and exhaust) plus 100kW (aerodynamic drag, and thermal loss from transmission).
Am I missing something obvious?
The key thing is that a large proportion of the heat generated in an internal combustion engine is going to be lost via its exhaust. A car engine might only need to shift ~20kw of heat via its cooling systems. It is possible to have an open cycle laser, but a device that chucks out very hot, very toxic clouds of gas is commonly known as a 'chemical weapon' and would be frowned upon.
Next, vehicle design takes advantage of forward movement through air to provide a fair bit of cooling. Static or slow-moving laser emplacements don't have this luxury. Try running an engine hard on a stationary vehicle and see what happens.
Engines can also run quite warm and are generally made of quite robust components, whereas lasers like to run cool to prevent degradation of the beam or damage to delicate optical bits.
There are probably other issues regarding energy densities making it harder to cool smaller components generating the same amount of heat energy before they toast themselves, but I don't know enough about the workings of a chemical laser to comment on that. I shall leave it as an exercise to the reader ;-)
"[A 130bph] bike can usually sustain a steady state cruise at max power for several minutes"
Nah, most sports bikes will run out of fuel well before that, and in any case the distance to journey termination is strictly limited by the next bend, tree, pothole or the natural predator of bikes, a BMW X5 helmed by a peroxide blonde on her mobile.
Many sharks are only found within certain temperature ranges and that the pool is like to be pr-warmed anyway to suit the shark's metabolism. As for the solution to this issue, the article mentions that heat build up only becomes a major problem is the super-villan waffles on at length about his plans. The simple remedy is to have said shark carved up the super-villan with aforementioned skull-ray, thus allowing the plot to move forward and scoring points with the whole "ironic death" thing.
..have you ever heard of the term "evaporation" ??
Water is particularly good at absorbing heat while evaporating: 2.2 MegaJoule per Liter (yeah, that's a French-Euro-Communist unit of volume, ya know).
I admit that you might need a refrigerator-type heat-pump to cool your laser, because it might not allow for 373 Kelvins (or 100 degrees in French-Euro-Communist units of temperature).
Early engines (steam or internal combustion) use(d) this principles very effectively. Also machine guns.
You could also collect soldiers/airmen/seamen piss and excrement and use the H20 of that as a coolant.
Forget the sharks, mount all the crap you need on some monster tank or robot, can anyone say mechwarrior?
but seriously, how long does it take to actual course 9mm like damage, mobile foot soldiers are unlikely to stand around too long whilst getting shot at by regular weapons. I have no idea of the technical side to these things but if you had to choose between smaller laser based weapons that may be slower than convential weapons or some form of weapons platform that can dump a shed load more energy in to something for those "quick" shots i think id rather the later.
Hook it up to a sterling engine to recycle some of the heat energy and help power the laser after you've fired it a couple of times. Granted I'm in IT and completely unfettered with any engineering/physics know-how but sounds like it would work...
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