Saturday, August 15, 2009

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.