Thursday, July 13, 2017

Date explanation

Note: These stories were published in 2009-2010. The year of publication for the first five entries was changed on July 13, 2017 to force Blogger to display the posts in true chronological order instead of reverse chronological order. 

Tuesday, June 21, 2016

The Adams Engine™ Story From the Beginning

The Adams Engine™ started off with a Boolean search on the following string - "nuclear AND gas turbine". Amazingly enough, there were dozens of hits with articles about various projects and conceptual designs that combined the benefits of nuclear reactors producing the heat and Brayton Cycle gas turbines turning that heat into useful power. I did that search in the fall of 1991, while auditing a course in power conversion (EM443) taught by Professor Mark Harper at the US Naval Academy. I was on shore duty after just completing a 40 month tour as the Engineer Officer of the USS Von Steuben, SSBN 632 (GOLD).

It is funny how little things can change your life. There I was, a thirty-one year old lieutenant commander with ten years of commissioned service, a wife, and two young daughters. Within a few months of performing that search, I had written a paper on nuclear gas turbine engines, had become addicted to spending as much free time as possible devouring everything I could find on the topic and I was trying to figure out how to turn the paper designs I was doodling into something real. Though I have no intention of dwelling on it during this tale, I have to admit here and now that there were several people in my life, including my lovely and loyal wife, who were not at all pleased with my new obsession.

This blog will not shy from the personal, but it is mainly aimed at trying to share the thoughts and technical decisions that have resulted in the concept I call the Adams Engine™. (By the way, that name itself has a story which I plan to share along the way.)

What's Wrong With Steam?

If you have ever done much reading about nuclear power plants or even received an introduction at a visitor's center or in a textbook, you will probably be familiar with the idea that nuclear reactors are a means of producing heat that is used to boil water to create steam that then turns a turbine and produces electricity. Some of the many people that fight against developing nuclear energy generators like to demean this process by saying something like "just another way to boil water". Of course, boiling water is a venerable and useful thing to do with large scale heat sources; technical historians have often pointed to the development of the steam engine as the main driver in the Industrial Revolution that enabled rapid travel and reliable, mass production of many wealth enabling commodities.

However, as a steam plant engineer officer, I had a pretty deep experience base that led me to understand that steam power has its disadvantages that had resulted in the development of several more advanced cycles used to convert heat into useful mechanical power. Steam was fine for locomotives, factories and ships, but steam automobiles were far too bulky and balky for mass consumption and the Wright Brothers would have never gotten off of the ground if they had to carry a boiler and feed water with them. As time moved on, the machines like diesels and jet engines used to power smaller, lighter applications had grown up to compete quite well with steam for the larger scale applications like ship propulsion and were even making large inroads in the electric power generation market.

There is always an advantage in using less material when making a product and in making products that take less effort to operate. Steam plants by their nature require lengthy, thick-walled piping systems, heavy water storage tanks, and very careful chemistry control to reduce corrosion. In their boilers, multi stage heat exchangers and condensers, they also use a multitude of thin walled, small diameter tubes that are in challenging chemical, thermal and physical environments that tend to lead to deterioration over time.

Because of the large temperature variations and high pressures used, there is always a need for great care to prevent material failures that can cause serious injury or death. Steam plant operators are a proud bunch that need excellent training and often extra pay to account for the difficulty and importance of their assigned work. Even with the extra pay, it is often difficult to attract enough people to the field; the nature of steam plants also leads to a rather uncomfortable work environment characterized by high ambient temperatures and elevated humidity.

These characteristics of steam plants can be mitigated with advanced designs and materials along with automated control systems, but they can never be eliminated. I am proud to be a steam engineer, but I recognize that steam is not for everyone, especially if they happen to be accountants. The only places where steam can still be competitive in power markets is where it enables the use of really low cost fuel - like lignite or coal - that has too much "stuff" in it to burn cleanly enough in an internal combustion engine or in Brayton cycle turbine engines. The need for reasonably clean fuels in those systems is not just because of concerns about pollution - the ash and other contaminants in coal, lignite and many biomass fuels would degrade the internal components of pistons and turbines so much that the machinery would fail.

Traditionally, steam has also been applied in nuclear fission power plants, even though the fuel releases its heat very cleanly. A major reason that nuclear power plants are considered more expensive than other alternatives like combustion gas turbines is actually associated with that assumed need for steam as the working fluid.

In the Navy, we had been taught that diesels and gas turbines had replaced steam in most applications because of the reduced complexity of operation, the reduced manning required, and the smaller machinery systems required. Our teachers acknowledge that the choice had required the use of higher quality, more expensive fuel, but the tradeoff was considered to be worth it except in cases like very large, high speed ships that consume enough heat to make fuel costs a large consideration or on submarines where oxygen and exhausts are strictly limited.

Is There An Alternative to Steam For Fission Heat?

Before I had served as Engineer Officer, I had attended the Navy Postgraduate School in Monterey, CA. While there, I had a running mate named Mike LeFever who had just finished serving as an engineer officer on a gas turbine ship; we often talked about the difference in his experiences and mine while running along the path between Monterey and Pacific Grove. While struggling with steam generator chemistry, leaky steam valves, and slow plant warm ups during my tour, I often thought back to those conversations. However, I also used to tease Mike about "that underway replenishment thing" and the need for large smokestacks to dump his plant's waste products. I liked having a plant that could run for decades between refuelings and that allowed us to breathe while submerged.

When on shore duty at the Naval Academy, I decided to take a few advanced engineering courses. You see, though the Navy allowed me to serve as an Engineer, my undergraduate degree was in English, so I felt a little disadvantaged at times in groups of my peers and wanted to fill in some gaps in my detailed knowledge. As a member of the staff/faculty, I could take the courses for free. (There are not many people that take advantage of that benefit; I have been stationed at the Academy for 4 academic years and have never seen any other officers in courses with me. I guess I really am kind of a geek - or maybe just a cheapskate who prefers free classes without credit to the same class at another school in the evening.)

The first course I took was Power Conversion and one of the assignments was a paper on an advanced power system. That was the assignment that took me to the library and initiated the search to determine if it was technically possible to combine the mechanical advantages of Brayton Cycle gas turbines with the fuel cleanliness and density of uranium fission reactors. I realized that not only was it possible, but that it had been recognized for years as an almost ideal way to capture and use fission heat.
"The "ultimate" nuclear plant for merchant ship propulsion appears to be some form of direct cycle reactor-turbine, eliminating steam and other forms of intermediate heat exchange. There are indications that such a cycle might be a pressurized gas reactor coupled directly to a gas turbine."
Source: Crouch, Holmes F., Nuclear Ship Propulsion, Cornell Maritime Press, Cambridge MD, 1960 p. 140
Once I figured out that atomic Brayton Cycles were possible, I decided to apply one of the important lessons I had learned as an Engineer in Rickover's part of the Navy. I began "pulling the string" to figure out why this idea had not been pursued to wide scale implementation.

Monday, June 22, 2015

Learning More About Energy Choices

If you have taken the time to read the paper that I wrote for EM443 in the fall of 1991, you will see that I had some pretty significant conceptual errors in my understanding of both turbo machinery and the politics and history of energy industry developments. I was definitely naive and had a lot to learn. Fortunately, I was in a great place to learn and had access to a world class university library.

I also began what remains a habit today - I started participating in on-line discussions using the handle of Atomic Rod. (Aside - just this past week I met someone for the first time who told me that she used to read my posts on USENET about atomic energy and that is part of the reason that she got interested in the field. That was a humbling experience and one that makes me realize that words do have meaning and impact.)

In the Naval Academy library there were several sections that focused on energy topics that included numerous shelves of books talking about nuclear energy from a variety of points of view. At one time, the Naval Academy had one of the largest nuclear focused engineering programs in the country under the title of Marine Engineering; by the time I was doing my research that program had nearly completely disappeared. It had been ten years since Rickover had retired, the 80/20 split (80 percent of the students in engineering and science majors, 20 percent in "bull" majors like History, English and Political Science) had gone the way of the dodo bird, and political science was the most popular major at the school.

For my purposes that turned out to be okay; there were plenty of good technical references from the days when the Marine Engineers demanded resources and there was a depth of coverage of the political debates that dominated the 1970s and 1980s.

Dr. Chih Wu - USNA's Alternative Energy Guru

I decided I needed some more depth to my understanding of energy topics in general, so I signed up for Dr. Chih (Bob) Wu's course in Alternative Energy. Dr. Wu was kind of a local celebrity; he had published a number of books including one on ocean thermal energy conversion. He was also quite a dynamic teacher.

One day, Dr. Wu and I had a conversation about his research interests. I knew he had started off in nuclear related fields and wondered why he had shifted to alternative energy systems. He explained that he never lost interest in nuclear, but that essentially all of the research money for nuclear power had disappeared by the mid 1970s and he had to do something to support his academic advancement. I knew that several of the professors did work for Naval Reactors, but Dr. Wu told me that was not an option for him because he wanted to publish his work.

After I had taken Dr. Wu's formal courses in Alternative Energy, I asked him to mentor me for an individual study research project. I am pretty sure that was an almost unique request - having a serving company officer as a research student at USNA - but Dr. Wu agreed. I learned a lot about conducting research and writing technical papers during that semester and owe a good deal to Bob's cheerful and questioning attitude, especially since I was working in an area that was somewhat outside of his usual academic interest areas.

As it turned out, Dr. Wu got interested in what I was doing and even worked with me on a published paper on the topic - my first ever published academic paper. (If you do not want to order the official published version, I have posted the advance copy with one of the reviewer's comments. I am still looking for the original article in the periodical - I have moved several times since 1993.)

(Update: June 27, 2009: Here is a link to a scanned PDF of the paper titled Nuclear Powered Gas Turbines: An Old Idea Whose Time Has Come that was published in the Proceedings of the IASTED International Conference August 5-7, 1992.)

Not Invented or Developed Here

During the same time that I was working with Dr. Wu, I made contact with several of the people who had published articles on high temperature reactors and gas turbine nuclear plants. Colin MacDonald from General Atomics was one of the most prolific authors on the topic with dozens, if not hundreds of papers published in various engineering journals. Richards T. Miller (CAPT, USN ret.) was another guy with whom I corresponded and whose papers I read with great interest.

Both of those gentlemen opened up a whole world of information to me about pretty well developed projects designed to take advantage of the steady heat that nuclear fission can produce by combining it with the steady, low cost flow that can be provided by modern gas compressors and used by modern turbines. In both cases, they warned me that my employer - the US Navy - did not have any interest in pursuing nuclear plants that were not steam plants. Both of them had received rather stern warnings from Naval Reactors about articles that they had published in various technical journals advocating the use of such machinery on board ships or submarines. I have included a copy of one of the notes that I received from Colin; I found it attached to a rather dusty paper.

During my recent digs through history files, I also found a letter to the editor from Richards Miller to the Naval Institute Proceedings written after I had left the Navy - the first time - and published my own article about using closed cycle nuclear gas turbines as the basis for future submarine propulsion plants. That, however, is getting a little ahead of the story.

My correspondence with those gentlemen and others like them helped me to answer one of the questions I had after writing my first paper - why was there such little information or development if the idea was such a good one? This is something that young, idealistic people often ask themselves when they find a great idea buried in a library or on a back shelf. Looking back through the lens of a bit more experience in the world, I have a bit more understanding of the real depth and logic behind a "not invented here" attitude.

Once an organization picks a path forward, they make a tremendous investment in that path, especially when it is something as complicated as not just one nuclear power plant but an entire fleet of them. Tooling, machinery, training programs, and supply chains are not easy to change. Attempting to change them can often result in what systems engineers call "hunting" which is a rather unstable condition that provides a lot of movement with little forward progress. Unlike a lot of people that have run afoul of NR's desire to keep control of their destiny, I get it and admire their continued success at refining the PWR steam plants that they invented rather than hunting for something better. That does not change my frustration with their efforts to slow down or halt others from thinking about new or different ways to capture and make use of fission heat.

So after learning that part of the challenge that had slowed closed cycle gas turbines was technical inertia, I continued digging to see if there were any technical challenges that had limited development. One of the other things that I have learned in researching successful and unsuccessful technology programs is that the unsuccessful ones often have a hard barrier to development that gets minimized in promotional materials prepared by boosters. The successful ones also have their challenges, but they have somehow managed an acceptable work around that could be implemented in time to allow commercial introduction and acceptance.

Areas for Technical Improvement

As I learned more about the closed cycle gas turbine proposals - and nearly all of the papers that I read were about proposed systems rather than real ones - I became convinced that the promoters just did not understand how difficult it was going to be to build and operate the machines that they drew on paper. Nearly all of them included multiple stages of heat exchangers for intercooling and recuperating, all of them proposed very high gas pressures, and nearly all of them proposed a control system that involved large pressurized storage tanks of gas that could alter the system pressure to alter its mass flow rate.

Based on what I had learned from my friends about combustion turbines, I realized that these proposed plants did not have the simplicity advantages shown by commercially successful gas turbines. Without some changes, it looked to me like closed cycle nuclear heated gas turbines were destined to remain on paper. I'll have to leave it here for now - time to go earn a living.

Friday, July 25, 2014

Seeking Simplicity, Finding Complexity

More than a month ago, I concluded the second installment of the Adams Engine™ story with a comment about needing to go earn a living. I had no intention of letting the story go dormant when I wrote that. Sometimes life gets in the way of a good project. I got pretty busy during the period leading up to the 4th of July and kept meaning to get around to telling you more about the Adams Engine™ evolution. Then, in the very early morning hours of July 3rd, I got hurt and have spent the past month recovering. That required taking some items off of the normal "to-do" list.

When I left off my tale back in late June, I had found out about the resistance to new ideas from the people building and operating light water reactors and I was curiously picking through various technical papers to find out what had been done to develop closed cycle gas turbines (CCGT) to solve real problems.

Energy dense, no pollution, low cost fuel

With detailed knowledge of light water reactor heated steam plants and a good academic and practical knowledge of simple cycle combustion gas turbines, I was enthusiastic about the potential of combining the best of both technologies. The pollution free, high energy density fuel that made it possible to power a submerged submarine for months at a time between maintenance and crew rest periods and for a decade or more between fuel stops told me that the nuclear heat source was as close to perfect as I could conceive with available technology. My main concern was what I had always been told was the reason that the Navy had limited its use of nuclear power - I thought that the fuel was prohibitively expensive.

In addition to the fuel availability and cost challenge that I thought existed, I knew that making use of that concentrated nuclear heat with the systems available in the 1950s and 1960s required high pressures, careful chemistry control, thick walled piping, phase changes, air tight cold water piping, and rather substantial fresh water storage tanks to make up for any plant losses. I had crawled the piping systems and signed enough work packages to know the plant almost by heart. I was pretty certain that I had a reasonable basis for knowing the source of much of the construction, design and operational cost.

I am not sure exactly when it happened or what information source opened my eyes, but I soon discovered that uranium based fuel, even when it is carefully engineered and manufactured, is far cheaper than the distillate oil burned in most combustion turbines. On the basis of heat content, commercial nuclear fuel cost less than 1/6 as much as distillate fuel - even in the early 1990s.

There were still sources that claimed that the highly enriched fuel used for naval propulsion systems was an exception to that rule, but then I found out that at least one marine propulsion reactor - the one that powered the NS Savannah - was fueled with low enriched uranium that was almost identical to commercial nuclear fuel. Even with that limitation, the Savannah's core was compact - especially compared to fuel tanks - and provided a reasonably long 5 year cycle between refueling.

Here is a quote from the Savannah's National Historic Landmark Nomination, a document that was not available to me at the time that I was learning about low enriched propulsion reactors. It provides similar information to the documents that I found buried in the USNA library:
Within the larger containment vessel, the reactor itself was housed within a "primary shield." This shield was a water-filled, 17' high, 2" to 4" thick lead tank. The reactor's active core was a circular right cylinder 62" in diameter and 66" high. The core was made up of 32 fuel elements. Each fuel element comprised 164 stainless steel fuel rods, .5" in diameter. The rods contained uranium oxide pellets, enriched to an average of 4.4 percent of U-235. The fuel rods in the centermost 16 fuel elements contained uranium oxide at an enrichment of 4.2 percent U-235, and in the outer 16 fuel elements the enrichment was 4.6 percent U-235. This compares to the longer lasting, 90 percent enriched uranium used in Navy reactors. Savannah's uranium oxide pellets, were .4255" in diameter, and the space between the pellets and the inner tube wall contained helium gas under pressure to assure good heat transfer across the fuel rod.
I also discovered that the US had already shut down one of its enrichment plants (Oak Ridge) and was considering shutting down one of the two remaining facilities because there was too much enrichment capacity in the world market. That situation did not make it sound like enriched uranium was a scarce, inherently expensive resource based on my understanding of Economics 101.

Simple, light-weight, series-produced heat engines

After identifying the main cost drivers for the nuclear steam plants that had fallen out of favor with both the general public and the Navy surface fleet - with the exception of the aircraft carriers, I turned to evaluate Brayton Cycle gas turbines, the heat engine that was all the rage in the electric power industry. Not only were the merchant power producers and the electric utilities excited about machines that were durable, lightweight (compared to steam turbine systems), easy to build, cheap to install and simple to replace, but the Navy surface fleet had made the same decision for many of the same reasons.

Jet Engine Diagram

(Source: Wikipedia Jet Engine Diagram under creative commons.)

Ship propulsion gas turbines look like the engines that hang under the wings of jet aircraft. That resemblance has a logical basis - technically speaking there is little difference. In fact, the LM-2500 workhorses that power frigates, cruisers, destroyers and now some amphibious ships are classified as aero-derivative (modified jet engines) gas turbines. Combustion gas turbines are simple machines that have just a major components - there is a compressor, a heat source (burners) and an expansion turbine that powers the compressor.

For jet engines, the energy left over after the compressor drive turbine exits at high velocity to produce thrust; in turbo props, generators and ship propulsion engines, hot gases for thrust are not particularly useful. Instead those systems need a slight modification in the form of an additional power turbine. Instead of shooting high energy gas out the back end of the engine to push a plane, ship engines and power generators use that high energy gas to spin a turbine connected to a propeller or a generator - through reduction gears, if necessary, to match the optimal rotational speed.

All heat engines need a heat sink; for steam plants the heat sink is a condenser cooled by water flow. For Brayton Cycle combustion gas turbines, the heat sink is the atmosphere that accepts the hot exhaust gases. After turning a power turbine or providing thrust, the hot exhaust gases are back to atmospheric pressure but they are far hotter - and more contaminated - than the air sucked in by the compressor. With the exception of the burners where fuel is injected at a regulated rate, I recognized that there were no heat exchangers in the vast majority of combustion turbines in commercial service.

Systematically, ejecting hot gases and ingesting colder air connects the bottom curve on an h-s diagram and matches the same function as the condenser in a Rankine Cycle steam plant. Since no one has to pay for the atmosphere, that heat sink costs the combustion turbine owners and operators a lot less than the multi-tube condensers and sea water systems used in steam plants. Of course, on board ships and in most power plants using gas turbines, the heat sink portion of the system is not free - intake and exhaust stacks require metal, construction and design work, and some amount of routine maintenance. On board ships, intake and exhaust stacks consume valuable midships space in multiple levels. For military use ships, the exhaust stack on a gas turbine ship is a detectable vulnerability due to the heat and contaminants contained in the gases.

As I thought about the basic characteristics of both uranium fuels and Brayton Cycle gas turbines, I kept getting disappointed by the papers and drawings that I was seeking and finding on the topic of high temperature gas reactors with closed cycle gas turbines. The idea is deceptively simple. Replace the burner portion of a combustion gas turbine with a reactor heat source and add piping and a cooler between the turbine exhaust and the compressor to close the cycle with a heat sink resembling a steam plant condenser.

Most of the paper authors writing about closed cycle gas turbines though it would be better to add "refinements" to the system. Their papers describe the ideas for adding components and changing cycle parameters to increase the thermal efficiency by adding stages of reheating and intercooling, reduce the cross-sectional area of the machinery by increasing system pressure, and flatten the efficiency versus power output curves by using system pressure changes to control power output. There was nearly unanimous agreement that helium, a lightweight, hard to contain gas was the best choice as the working fluid and heat transfer gas. The HTGR/CCGT advocates described designs to each other in ASME or ANS sessions that would allow construction of "commercial" plants with turbine power output in the several hundred to one thousand Megawatt electric range, even though the largest combustion gas turbines in commercial service were significantly smaller than 200 MWe.

Throughout the 45 year period I studied - from 1946-1990, there were less than a handful of actual machines using high temperature reactors with closed cycle gas turbines constructed, but there were hundreds of papers and dozens of special sessions held on the topic at technical gatherings like IAEA, ASME and ANS meetings. During that same period, nuclear plant vendors built hundreds of light water reactors while simple cycle combustion turbine manufacturers refined their systems through dozens of generations that resulted in an installed base of millions of machines in markets as diverse as corporate turboprops, high speed hovercraft, large military ships, land based power generation, and Army tank engines.

The first mystery worth solving was why had this situation happened? The second was to determine if there was a path forward that could alter the situation and provide a real opportunity to reach the ultimately simple, low capital cost machine that could run on high energy, low cost, abundant, pollution free fuel.

Keep it Simple, Stupid.

I have learned over time to favor systems designed under the principle of KISS. From the point of view of a former operating engineer, atomic gas turbine researchers had long since departed from this advice.

The complicating component additions offered by closed cycle gas turbine advocates like C. Keller and D. Schmidt in their 1967 ASME paper titled Industrial Closed-Cycle Gas Turbines for Conventional and Nuclear Fuel illustrates how the small community of people interested in the topic gradually made choices that took them away from a development path that would allow them to reach their goals. It is important to understand that C. Keller presented his first paper on closed-cycle gas turbines at an ASME meeting in 1946, just a year after WWII ended, so this paper indicates his continuing interest in the subject for more than 20 years along with his professional interest in producing something new and publication worthy.

A later paper presented by R. Calvo and R. E. Thompson titled Compact Closed Cycle Brayton System for Marine Propulsion again illustrates how the closed cycle gas turbine researchers had gradually moved to a point where most accepted that any system that would be built would use pressurized helium gas, an intercooler between the compressor stages, a recuperator (heat exchanger) between the outlet of the compressor and the inlet of the reactor (heated by turbine exhaust gases), and a system of pressure reservoirs that would allow operators to add and or subtract helium gas from the system in order to control helium mass flow rate and thus system power output.

Each one of those refinements disturbed me. Heat exchangers were one of the banes of my existence, especially when operating in the warm and ecologically abundant waters in King's Bay, Georgia. High pressure gas systems are a big part of submarine design and operation, but we learned to respect the hazards and the cost of operating such carefully designed systems. There is a good reason that as much high pressure piping as possible is located in places that are not normally occupied by people. I had also learned first hand just how hard it was to reliably keep low atomic number, monatomic gases in their place - they find their way through the tiniest pores and gaps in sealing systems.

One of the things that enabled the success of the light water reactors connected to steam plants is that they followed the KISS principle at the time that they were designed. They did not reach for ultimate efficiency, they did not try using multiple technologies without a track record, and they always seemed to be designed with the operators and maintainers in mind. My quest was to determine if there were any inherent reasons that the combination of high temperature reactors and closed cycle gas turbines led people to make things so darned complicated.

Thursday, August 15, 2013

Throttle Valve Control For Fission Heated Simple Closed Cycle Gas Turbines

The best way to lower construction and operating costs is copy the successes and avoid the failures of other projects. That is easy to say, hard to accomplish. As I dug into more details about combining atomic heat with Brayton cycle gas turbines, I opened my research aperture as wide as possible so that I could find out what made both simple cycle combustion gas turbines and light water reactors as successful as they were while also trying to learn where each technology ran into obstacles that limited its market dominance.

Jet Engine Development Summary

Both Brayton cycle gas turbines and light water reactors entered the energy options world at about the same time. Jet engines were the first truly successful gas turbines; they were developed and refined during World War II, but did not see widespread use until a few more years of testing and development were completed after the war. At first, they only lasted a few hundred hours before needing significant overhaul. The large amount of air flow and close tolerances required between the internal blades and nozzles required some creative solutions to the challenges of flying in heavily polluted areas, places where birds tend to flock, and fuel quality issues. The large piston engines with propellers that they were competing against had many years worth of head start in solving these challenges, but the gas turbine had some real advantages in terms of power to weight ratios, smooth operation, simple construction (compared to multi-piston engines) and maximum speeds.

Gas turbine developers could envision a large market for their product if they could solve the reliability issues, so they worked diligently, with significant government assistance to mitigate those issues. One major source of technical push and support was the desire for long range, high speed, reliable bombers to carry the growing arsenal on which the MAD defense philosophy rested. Commercial airlines would have never been able to make the technology investments required to improve jets on their own, but the partnership of government demand with commercial applications resulted in a very useful technology. One feature of jet engines that was quite an advantage over the piston/propeller engines they replaced was that they more readily fit under the wings or inside plane. They required a lot less cross-sectional area than a propeller that could produce equal thrust because the velocity of the exhaust was so much higher.

The smooth operation and low noise also made them well suited to passenger airliners. The high power to weight ratio available provided additional advantages for both long distance passenger aircraft and military applications. Even after many refinements, gas turbines still had some disadvantages in competition with piston engines in specific fuel consumption and fuel flexibility so gas turbines did not make much of an inroad in applications where these were decision drivers.

Light Water Reactors Married to Steam Turbine Heat Engines

Light water reactors with steam plant power conversion systems developed under somewhat similar circumstances. After World War II had ended, there were many scientists and engineers who clearly understood the energy density opportunity provided by fissioning heavy metal. There were many research paths to explore and many test reactors built. However, one application demanded a rather immediate finished product that could provide reliable power - submarines. More than any other proposed application, submarines capability was improved by access to a very compact, emission free heat source. For applications where the atmosphere is accessible and there is adequate storage space, the cost and difficulty of taming nuclear fission heat looked too hard to compete with readily accessible, well proven combustion power sources.

That was especially true in the years immediately after the war, when the fossil fuel industry had a very large excess capacity built to serve navy's with several thousand ships, air forces that could put a cloud of planes over distant cities every night and armies that could move hundreds of thousands of people from place to place on trucks and tanks. In the second half of the 1940s and through most of the 1950s, combustion fuel was cheap and easy to obtain. On submarines, however, hull sizes limited the amount of fuel that could be carried while operating underwater severely restricted access to the other necessary component of combustion - oxygen. Then Captain Rickover, who had been one of the most fuel conscious engineers in the Navy, recognized the potential advantages that fission could provide to submarines. He also wanted a success while still in the Navy, so he decided to minimize risk by using a well proven heat engine to convert fission heat into useful propulsion.

Rickover and his team kept the system very simple and avoided features in the steam plant that had been added over the years to make them more thermally efficient. That decision had two primary justifications; those features - mainly extra heat exchangers to reuse "waste" heat - took up valuable space, and complicating a design in order to conserve fuel when you have an almost infinite amount stored in the reactor seems like a waste of valuable engineering time and construction money.

One of the refined features of steam turbine power plants that Rickover and his team retained was the poppet type throttle valve. These multi-port valves had been developed over the years in order to overcome what had been a fairly major disadvantage of steam turbines used in applications like ship propulsion where they need to operate at part load much of the time. Unlike a single throttle valve where restricting flow leads to frictional energy losses and the potential for some high pitched noise, poppet throttle valves avoid frictional losses by popping open one at a time. When there are a lot of small valves in the same body, they can be set up to be either fully open or fully closed. As more valves are opened, more steam flows, putting more power to the turbine. Shutting valves to reduce flow does not cause significant throttling losses because there is not much area with high speed flow where friction can occur. These throttle valves make it very easy to rapidly change power in a steam turbine power plant and they also help to ensure that system efficiency does not suffer too much at partial loads.

Electric power utilities were encouraged to develop very large versions of the light water reactor-steam plants that had been developed for submarine and ship propulsion, but there was a lot of effort put into discouraging the development of commercially useful small versions of the machines. That can be traced to a trip taken by Admiral Rickover to visit Russian nuclear icebreakers. Following that trip, where he learned the state of his rival's technical development of nuclear propulsion, he pressed for a tight layer of security to be applied to the details of nuclear propulsion. That is a considerable contrast to the way that the military aviation project managers encouraged their contractors to seek commercial markets to help share the fixed costs of research, development and production.

Using the Best of Both Machines

The more I thought about gas turbines and nuclear heat sources, the more I realized that there was an untried opportunity available. With combustion gas turbines, power is normally controlled by varying fuel flow, but that was not an option for a nuclear system. Gas turbines can make good use of higher temperatures than steam plants because of the nature of the fluid being used, but the metal cladding that works well to prevent corrosion in high temperature water would not work very well at the temperatures needed for reasonably efficient gas turbine operation.

The graphite and silicon carbide coated fuels that had been developed for higher temperature operation seemed like they would work well with inert gas to move their heat from the source to a heat engine. Because of the potential political and technical issues associated with exhausting gases to the atmosphere that had been directly exposed to a neutron flux inside a reactor, people interested in gas turbines with nuclear heat determined that a sealed system with a cooler would be the preferred design alternative. Many of the researchers immediately made the mental leap from sealing the piping system to deciding that higher pressures would allow smaller components for the same power output.

One of the things I took away from my operating experience is that lower pressures make many things easier. One of the phrases I like to use to describe engineers is "a good engineer is a lazy cheapskate." Once you get past the idea that it sounds offensive, think about the implications. Engineers are always interested in finding a better way to do things, they do not like to waste time or effort. That trait qualifies them as "lazy". They are also generally interested in producing better products with less input; they dislike wasting physical or financial resources. That qualifies them as being "cheapskates". However, since they are also lazy, they do not take shortcuts that will result in reworking a job, so they understand the value of investing in quality workmanship and quality materials and they avoid things that add excess complication.

For all of those reasons, I decided that it would best to think about operating closed cycle gas turbines at relatively low pressures. Once I determined the effects on rotating machinery from operating with more dense, higher pressure fluid, I firmed up my interest in operating closed cycle gas turbines at temperatures and pressures that most closely resembled those already in use in combustion turbines.

I also determined that I would pursue a design where the power output of the turbine would be controlled by a throttle valve similar to the ones that had been used for many decades in steam plants. At first, there was some resistance to that idea by my mentors, but once we all determined that steam is just a gas anyway we determined that throttling a gas flow was already well proven; the concept had just not been applied to Brayton Cycle gas turbines. Part of the historical reason for that was that gas turbines already had fuel consumption rate issues compared to their competition; putting a throttle valve in the system would add to those challenges. In addition, poppet throttle valves tended to be quite large in comparison to jet engines; they would not be welcome in a tight space and weight constrained environment. For ship propulsion or land based power production, that disadvantage was not a decision driver.

Making a long story short, I met a terrific patent attorney named John Clarke who lived very close to the Naval Academy. He had served as a naval officer and engineer during World War II and later decided to attend law school. As a former engineer turned lawyer, he naturally gravitated towards patent law. We had some informative and educational sessions at his home overlooking Weems Creek where he taught me a lot about patent law. John was semi-retired; I think he enjoyed helping out an active duty naval officer with what he thought was an interesting project to provide a better power source. Of course, his time did not come for free, but I think he failed to bill quite a few of the hours we spent together in discussion.

After about 9 months of work, we filed an application for a control system for a closed cycle gas turbine. That was in April 1993. I left the Navy - for the first time - in September 1993 and moved with my young family (our girls were going into the 3rd and 5th grade) to Tarpon Springs, Florida to found Adams Atomic Engines, Inc. I rather naively thought I was ready to change the world.

There were plenty of people who warned me that I was crazy to leave a good job with a generous salary and benefits in order to start up a company to do something no one else seemed interested in doing. I was stubborn. One of my colleagues gave me a warning that I dismissed at the time, but still remember, even 16 years later. Dave LLewellyn told me "The oil guys will never let you succeed."

Patent number 5309492 titled "Control for a Closed Cycle Gas Turbine" was issued in May 1994. Unfortunately, I soon learned that having an issued patent and a dollar can still just buy you a cup of coffee if you shop carefully. I also found out that earnestly presenting the idea of adapting existing combustion gas turbine machinery to run on hot helium could make a rude gas turbine expert laugh in my face. Not very pleasant, but a good means of rapid learning.

Friday, September 28, 2012

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.

Sunday, February 28, 2010

One Reason for Choosing Nitrogen (N2) for Adams Engines is the Growing Scarcity of Helium

One of the reasons that I decided to use N2 gas as the coolant for Adams EnginesTM is the fact that helium supplies are limited enough so that a new large demand would drive up the price of the gas. I fully expect that someday, direct cycle gas turbines using nuclear heat sources will have the potential for rapid market expansion; I determined a long time ago that I did not want to limit the potential for that expansion based on a limit to the amount of available helium.

Since it is an element and a noble gas that is produced by a very slow process - decay of uranium, thorium and their daughter products - helium could be the bottleneck that would slow development and deployment of Adams EnginesTM. I can point to a lot of esoteric science papers that supported the decision to go in a completely different direction than the conventional gas cooled reactor wisdom as practiced by companies like PBMR and General Atomics, but this video does a much more entertaining job of sharing the reasons for my concern.