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ADVANCED EXTERNAL COMBUSTION ENGINES – PART II
In the previous entry I dealt with legacy external combustion engines, steam turbines and steam powered reciprocating engines, as well as with the perennial also-ran and favorite of environmentalists, the Stirling cycle engine. Here I will move on to more exotic types including Kalina cycle engines, organic Rankine engines, Ericsson cycle engines, alkali metal thermoelectric devices, solid state thermoelectric power sources, and magnetohydrodynamic systems.
First let me note an omission, a fact which should have been included in the last piece. Dean Kamen, the much celebrated inventor of the Segway personal transportion vehicle, has also done extensive research on Stirlings, and has a number of patents on improvements in the technology. Rumor had it that Kamen wanted to power the Segway with a Stirling but could not complete a unit that met with his own requirements.
Ericsson Cycle Engines
John Ericsson, sometimes known as the Swedish Edison, was an internationally renowned nineteenth century inventor best known for his patent on the screw propeller and his design of the Union Monitor, the forerunner of the modern battleship. Ericsson made numerous other inventions, however, and among the most interesting was his “caloric engine”, as he called it, which went through several iterations and upon which he was still hard at work at the time of his death.
Like the Stirling, the Ericsson caloric runs on heated air, but the principle of operation is somewhat different. The Ericsson is not necessarily a closed cycle system, and does not require a displacer to move air from the hot to the cold side of the engines. Rather, the operation is more akin to that of the internal combustion Brayton turbines used in all modern turbojet and turboprop aircraft and in fast turbine powered watercraft as well.
In the Ericsson engine’s first stage, air is simultaneously compressed and cooled by means of a heat exchanger, cooling making the compression process easier and reducing the amount of work required to achieve a given compression ratio. The air is then allowed to expand while being simultaneously heated to maintain a constant pressure. As the air leaves the expansion section at an elevated temperature heat is transferred back via a heat exchanger so that little thermal energy is lost. A high proportion of the thermal energy produced by combustion ends up performing mechanical work.
Ericsson engines represent a very interesting and sophisticated exercise in maximizing thermodynamic efficiency but they have seen little commercialization to date. Ericsson himself achieved little success with design until just before the end of his life when factory production models finally reached the market, mainly being used to operate water pumps on farms. Ericsson built one very large engine of thousands of horsepower that was used to power a merchant ship.
Currently, one small California startup calling itself PROE is attempting to re-introduce Ericssons into the marketplace. The PROE uses a jet engine type turbine compressor and an afterburner to control emissions. The expansion section uses a fairly conventional piston and crankshaft. The engine will operate on gas, diesel, kerosene, and biofuel.
Richard Proeschel, the inventor, is a retired aerospace engineer who spent most of his career developing fuel cells for satellites and spacecraft. His experience with the latter convinced him that the technology is unlikely to succeed in the stationary power and transportation markets which the fuel cell industry is attempting to penetrate.
Incidentally, Proeschel is not the only fuel cell design engineer we have met who has grown skeptical as to the market potential of these devices. The popular press, whose members generally confine themselves to interviewing spokepersons for fuel cell companies and avoid advocates of competing technologies, generally present a very favorable view of fuel cell prospects, a view which we regard as largely unwarranted at present.
Kalina Cycle Turbines
Named after Alexei Kalina, a Russian American who holds a patent on the design, these engines represent some very fresh thinking. In terms of its mechanical design the Kalina turbine is quite similar to the conventional Rankine turbine used in coal fired electrical power plants thoughout the world. The difference between the Kalina and Rankine has to do with the working fluid which in the Kalina normally consists of a combination of ammonia and water. The proportions of ammonia and water vapor are made to vary during the compression and expansion cycles in order to maximize thermodynamic efficiency. The basic principle of operation has been borrowed from the refrigeration industry.
Kalina turbines have been built by Siemens and by an Australian firm called Geodynamics but to date they have won little market acceptance. The Kalina is capable of yielding double digit increases in efficiency over Rankine turbines but only at low operating temperatures not exceeding a few hundred degrees. At higher operating temperatures Kalinas are only slightly more efficient than conventional Rankine turbines.
Most of the Kalina turbines built to date have been designed to run on factory waste heat or used in geothermal power plants where ordinarily the steam emanating from the underground heat source is at a fairly low temperature. The aforementioned Geodynamics has undertaken a project in Australia in which water will be injected into deep “heat mines” located more than 10,000 feet beneath the surface of the ground and the resulting steam will be recovered to operate Kalina turbines. Geodynamics principals calculate that in one small geothermally active region of Australia enough thermal energy can be extracted to provide sufficient electrical power for the entire nation.
The type of geothermal plant that Geodynamics is attempting to build has been constructed on an experimental basis in the U.S. by Los Alamos National Laboratory while a similar pilot project is underway in France. The plants are expensive to build, but they offer one singular advantage over any of the more commonplace renewable energy sources with the exception of large scale hydroelectric installation. These deep “hot dry rock” geothermal generation facilities are extremely energy dense and readily scale to enormous size. There’s no need to cover the landscape with wind turbines or solar collectors. Plants having equivalent outputs to those of large coal or nuclear facilities are entirely possible. Moreover, the size of the resource appears to be very vast, at least in certain parts of the world, among them the western United States.
To be sure, questions remain as to the feasibility of hot dry rock geothermal done on a large scale. We have interviewed geologists on the subject who believe that contractors attempting to build such plants might encounter a high failure rate. That’s because hot dry rock geothermal involves a good deal more than just drilling a hole in the ground and pumping in water. Before the heat resource can be effectively utilized, the contractor must perform a process known as water fraccing where high pressure jets of water are used to fracture rock at the bottom of the well and thus create a large reservoir that may be flooded with water and will serve as a sort a natural boiler for the turbine. At present water fraccing is an uncertain, hit or miss business, and a miss means that the hole has to be abandoned and a new excavation begun.
We believe that the combination of hot dry rock geothermal and Kalina turbines could provide at least some countries with a renewable resource that would require far less construction than the wind and solar plays which most Greens advocate, and would also result in much less environmental degradation because the plants would be far more localized. Our expectation, however, is that no company would undertake the risk of building such plants until the energy situation became desperate, and of course by that time the difficulty in amassing funding could be intensified due to a decline in overall national wealth.
Organic Rankine Turbines
Organic Rankine turbines substitute various organic working fluids for water, hence the name. Among those fluids are terphenyl, pyridine, and biphenyl ether. Such fluids provide greater efficiency than steam at low operating temperatures, and like Kalina turbines, the organics find application in geothermal installations where they are generally indirectly heated by low temperature steam. Most cannot be operated at high temperatures due to the corrosive effects of the working fluids and those fluids themselves present toxicity problems. Organic Rankines have never achieved wide acceptance in the marketplace.
This is an interesting technology that has chiefly been used in powering satellites. The principle of operation is based on the fact that plasmas are highly conductive and at the same time highly fluid. By causing plasma to flow at right angles to a magnetic field one induces electrical current in the plasma which can utilized to do work via magnetic coupling. Essentially one is substituting plasma for the ordinary copper wires used in a conventional generator. The plasma itself is derived by heating hydrogen or water vapor mixed with cesium.
Magnetohydrodynamic Systems are highly efficient but necessitate extremely high operating temperatures and are best suited for concentrating solar plants or high temperature nuclear reactors. Much work has been done in Israel toward developing solar magnetohydrodynamic generators, but so far no commercial products.
Commercial thermoelectric devices convert heat directly into electricity by means of the Seebeck effect whereby two dissimilar metal plates impinging on one another produce an electrical charge in the presence of a temperature differential between them. Unfortunately, conventional Seebeck generators are only a few percent efficient because metals are highly thermally conductive and thus heat differentials are difficult to maintain.
DARPA has sponsored quite a few research projects in recent years where various crystalline compounds were substituted for metals with higher conversion efficiencies as a result but practical generators remain an elusive goal.
A couple of other technologies show promise as well. Alkali metal thermoelectric devices, which may use hydrogen as well as alkaline materials, use a heat source and a catalytic membrane to strip electrons from the working fluid. These devices are somewhat analogous to fuel cells. Johnson Electromechanical Systems appears to be the sole manufacturer active in this field today and these devices have yet to find real markets outside of remote power for satellites—the market of first and last resort for exotic energy technologies. These devices require a motor in order to do mechanical work.
CoolChips, LLC, a British firm, has an interesting thermoelectric technology where electron flow takes place across a hard vacuum via quantum tunneling effects within a wafer construction resembling a semiconductor. Dissimilar metals are used and the insulating effects of the vacuum conspire to raise thermal efficiency. CoolChips claims efficiency of over 50% and has a patent on a propulsion system for vehicles where fuel is burned in a combustion chamber and the thermal energy is used to drive the coolchips. The electricity is then used to power an interesting multi-pole motor developed by a sister company called Chorus Motors.
CoolChips has received extensive funding from Rolls Royce and has received favorable attention in the scientific press. But whether the technology will ultimately achieve commercialization remains to be seen. We think CoolChips is most likely to succeed in the field of refrigeration, not in power generation.
What’s in the Future?
We think more of the same. Rankine turbines will continue to predominate in large scale electrical generation which will increasingly favor coal. In competitive markets clean coal technologies will fail on the basis of cost and the world’s air will grow dirtier yet and more laden with carbon dioxide
Although certain external combustion engines, including Stirlings, Ericssons, Kalina turbines, alkali metal thermoelectric devices, and magnetohydrodynamic systems may provide efficiencies rivaling those of fuel cells and superior to those diesel engines, we see the diesel predominating in small scale power generation for the indefinite future. Indeed, for a number of reasons, we think diesel engines are poised to replace spark ignition engines eventually in the key market of transportation with the United States probably being the last holdout for traditional gasoline engines. But all of these are subjects for future posts.