Reply To: Mind, Spirit, Science connection?

[redsnapper]

Thanks for starting this thread, because it gets directly at the topic of most interest to me. I joined FFS about a month ago (i.e. signed up with an email address so I can do posts) and have been mulling things over. I’d love to engage in some serious engineering dialog. Just so you know where I’m coming from, my education (BSE and MSME degrees) is in Mech Engr, with specialization in heat transfer and energy systems. For the last twenty years, my career has revolved around the thermal management of semiconductor devices. To me, a *large* amount of power dissipation is anywhere from 5 to 200 W. (That’s W, not kW, or MW.) Nevertheless, the basic laws of heat transfer don’t change, you just slide up the scale (a few orders of magnitude :cheese: ).

Now in the first post, you suggest that waste heat might be something like 144% of the net usable electrical energy out. If I understood correctly, a portion of the energy produced went directly back into the capacitor bank. This implies, I think, that my own assumption is slightly more pessimistic – but at least it’s in the same ballpark, and a ballpark is the place to start. Based on what I’d heard/read elsewhere on the DPF topic, I simply assumed that we had an overall net useable energy production efficiency of 50%; thus if you were building a 5MW powerplant (net usable electrical output), you’d have 5MW of waste heat. That’s my starting point, and obviously everything has to be taken with that initial assumption in mind.

The main point is, getting 5MW thermal out of a breadbox is no mean feat. It’s not impossible (I think), but to put it in perspective, the flux density at the surface of the sun is about 60MW/m^2. If you were trying to pull 5MW thermal from a 6″ diameter, 6″ long DPF core (again, for simplicity, I assume the plasma loses 5MW thermally to the walls of the core – all other energy exits via Xrays or current), you’d have a surface area of roughly 0.073m^2, and the flux density would be 68MW/m^2. So what we’re trying to do is cool an object that emits thermal energy at approximately the flux density of the sun – only the sun’s surface runs at about 5000K, and we need to do it at 1000K. This is just so you can go “wow.” It’s not saying it can’t be done, it’s just setting the stage for the intuitive grasp that it ain’t gonna be a slam dunk.

With that perspective, what are some possibilities?

Let’s suppose we want to keep the maximum temperature of the DPF core at 700C. If you want to drop 500degC (leaving 200C at the “outside” to deal with carefully) while passing 5000000W, that requires a thermal resistance of about 0.0001degC/W. (These are the basic units of thermal resistance when we’re sizing cooling systems in the electronics world. But jeese – I think 0.1 is a small thermal resistance!) The best reasonably inexpensive material for conducting heat is copper. (Diamond is better, and some other exotic materials are somewhere in between, but we’re trying to think inside the box for the moment.) The thermal resistance of a solid (hollow) sphere of copper with a 6″ diameter spherical cavity in the center, and a 22″ outer diameter, is 0.002degC/W. At 20x larger than the target thermal budget, that’s a problem. (It means, those 5MW actually came at the expense of a 10000C temperature drop, so we kind of melted the core.) You might ask: why did I pick 22″ outer diameter? Just for convenience, but it happens to have a surface area of 1.0m^s, so at that distance from the heat source (again, we’ve wrapped the DPF core inside solid copper, with no contact resistance, etc. – i.e., best possible case for non-exotic materials) – we’ve still got a 5MW/m^2 flux to deal with at our hoped-for external surface temperature of 200C. But, as I say, we’ve already exceeded our thermal budget by a factor of 20. Diamond, in fact, isn’t even 20x better than copper, so this is saying that if you could mount the DPF core inside a solid diamond crystal 22″ in diameter, it would still melt in the middle and you’d still have 5MW/m^2 to deal with at the surface. Not good. In fact, things are going to be a lot worse than this, I fear, because that 6″ to 22″ space is probably where we want the X-ray absorber – and the X-ray absorber probably has an effective thermal conductivity somewhat worse than copper.

I think where this takes us is something like impingement cooling. The only way, with state-of-the-art heat transfer mechanisms, to remove heat that quickly is by using a phase change. Water, for instance, at a flow rate of a paltry 2kg/s, if you vaporize it, can remove 5MW of thermal energy. And as somebody in a previous post has suggested – if you’ve got that much thermal energy to remove, it seems a shame not to use it. And if the only way you can get it out of the small space is by turning water into steam, you’ve now at least got it in a form that ME’s have been dealing with for a couple of centuries. I’d say to consider heat pipes (known for their phenomenally high flux capability and low effective thermal resistance), except that we’ve still got the issue of rejecting that 5MW at the other end of the heat pipe. So whether with closed heat pipes or an open system, if we impinge the outer surface of the DPF core with water, we’ll have 2kg/s of high quality steam that can be carried a long way away fairly easily, and that’s where we actually do the main cooling job.

It was suggested in the first post as well, I think, that thermoelectric conversion might be a possibility. I think these above back-of-the-envelope calculations show that there are some serious issues that have to be considered before this heat even gets safely to the place you might consider thermoelectric conversion. But this post is about to run out of space, so I’d better close. The idea is to get people thinking seriously about this cooling problem.