Monday, September 28, 2009

Nitrogen (N2) Gas Cooling For a Closed Cycle Nuclear Heated Gas Turbine

The exact date of my revelation that helium was not the ideal working fluid for closed cycle gas turbines is lost in a pile of papers that may or may not include an old calendar or two. It is not really important, except for the fact that it happened after I had left active duty in the Navy, moved my family to Tarpon Springs, Florida and established Adams Atomic Engines, Inc. as a Florida registered 'C' corporation. I think you can understand that it was a disappointing day when I found out that conventional gas turbines could not move helium. Instead of adapting existing machines to operate with a cooler and a reactor heater instead of open air suction and exhaust with combustion chambers, I was faced with the fact that I would need to find a machine that was designed to handle a much lighter gas, with much higher probability of leaking between stages and a much higher sound velocity.

I probably should have figured this out through my reading in preparation for the throttle valve patent application or my reading to learn and understand how nuclear gas turbines could work. Unfortunately, I got the impression from the technical papers that helium gas turbines were already available or could be easily manufactured. The researchers who wrote the papers I read were far more interested in doing the computations to show how the system could work. As far as I remember they did not talk much about the difficulties of actually making it work. The handful of projects that had used helium as opposed to air or nitrogen for their closed cycle gas turbines were completed by manufacturers who had the resources to build prototype machines.

My revelation about the difficulty of designing and manufacturing a completely different kind of Brayton cycle compressor and turbine came about ten years before Hans Ulrich Frutschi published his excellent reference titled Closed-Cycle Gas Turbines: Operating Experience and Future Potential". If that book had been available, I might have made a completely different career decision. None of the papers I read about the 50 MWe Oberhausen II helium turbine included anything like the following comment from someone with intimate knowledge of its operation:
Because GHH had no gas turbine development staff of its own, the task was outsourced to an institute of a technical university. Although they had been working on this topic for years, the helium turbine, which was designed to produce 50 MW, only just managed 30 MW. The efficiency only reached 23%, instead of 34.5% as planned. Since this large deficit was the result of many small ones, no successful reconditioning was possible. (It would be necessary to design and build a new turbo machine.)
. . .
This turbo set, which had a rather low output for a helium turbine, should have been designed for a much higher compressor and high pressure turbine rotational speed. The low speed of only 5500 rpm (adequate for air) resulted in very unfavorable hub to tip ratios for compressor and turbine, which led to poor polytropic efficiency levels in this machine. Also, the cycle pressure losses were excessive, especially the cooling and sealing mass flows, by a factor of 4.
Instead, I learned just how difficult, expensive and lengthy a process it would be to obtain a suitable helium turbine and compressor for the system I envisioned during an hour long discussion with a gas turbine expert at the University of South Florida. I cannot recall his name or how I found him, but he was a guy who had spent 20-30 years in an industrial gas turbine design and manufacturing career before he decided to spend the remainder of his career teaching.

I entered the meeting with the assumption that producing a helium cooled closed cycle gas turbine would be a fairly simple matter of assembling well proven, already manufactured "off the shelf" components. I left it with a much deeper understanding of the enormous differences in gas characteristics between helium and air, which was the working fluid that essentially all existing gas turbines use. I also learned just how much "art" and trial and error was involved in turbine and compressor design and construction, and how much money even experienced firms invest to develop a brand new design to the point where it could be manufactured to provide reliable service. I learned that nearly all "new" jet engines and industrial gas turbines are built by tweaking or modifying existing designs to make use of as much proven knowledge and as many proven parts as possible, but even then an engine manufacturer can spend hundreds of millions on relatively small machines and billions on larger ones designed for applications like passenger aircraft.

The only bright spot of the meeting came when I asked the professor what he would do if he wanted to build a closed cycle machine that operated on an inert gas to prevent corrosion and other unwanted reactions. He thought for just a moment and told me that it would be pretty simple to use compressors and turbines designed for air as the working fluid if the inert gas was nitrogen. After all, air is 80% nitrogen already and the thermodynamic characteristics of O2 and N2 are nearly identical. He told me there might need to be some small amount of O2 left in the system to prevent nitriding of the turbine blades, but he was not even sure that would be an issue with properly selected machinery.

I knew that N2 had been the cooling gas selected for at least one of the closed cycle gas turbine demonstration projects that I had researched - the Army's ML-1 - but I had shied away from that selection initially because there seemed to be such an overwhelming agreement in the papers that helium was a better choice. I also knew that the ML-1 had only operated for a few hundred hours, but I had not found any real details about why that was true. I left the meeting with a lot of chagrin - after all, I had taken a huge leap of faith based on my excitement in finding something "new" that others had overlooked. However, I also had found some hope that a different path could lead to a similar result.



    Reading your article about the ML-1, it seems that nitrogen wasn't the reason why the ML-1 was less than successful...rather, the ML-1, like the helium reactors that followed it, weren't *exactly* K.I.S.S. designs using reliable COTS equipment; instead, they set about trying to reinvent the wheel (whether with turbines or with recirculators) without enough resources to do a good enough job to make up for the challenges the design posed.

    How about some Argon? 1% of the atmosphere, so it's plentiful and relatively cheap, it's noble and completely nonreactive, and it has an atomic weight of ~40, ~1.4x air's weight of ~29? (Less than CO2, and none of the reactivity.) I read in Collier and Hewitt (Introduction to Nuclear Power, that an isotope of it produced by neutron activation (Ar-41, 1.8 hours) has both gamma and beta decay, though. They claim it precludes use as a coolant. What do you think?

  2. Dave:

    Argon's activation tendency is definitely an issue, but so is the fact that it is only 1% in air and not 79% like N2.

    N2 is not only cheap and abundant, but it is essentially the gas that all combustion gas turbines are already optimized to use. I am not interested in reinventing the wheel, just in coming up with a new application for the wheels that are already invented and refined.

  3. This is a link to the report for 1964 on the ML-1 test. It's interesting. It seems that they had way too much going on at once, and were trying to satisfy a whole bunch of criteria (low weight, compact size, turbine efficiency (first one has to get the turbine working, then work on efficiency - one has to build the Model T before one can build a Prius), implement an advanced core (why did they try the water-tube calandria, I wonder?))

    They had materials problems (metals not to specification), lubrication problems (and gas loop contamination with lubricant problems), vibration problems, machining not to tolerance (the turbine decided to machine its own blade in one case), sensor problems, nitrogen leaks, all sorts of side-channel problems, not problems with the basic design...the design still managed to produce power.

    Here's another earlier test report that had some of the issues covered, but not to the full extent that the first one did.