In the OSTI Collections: the Stirling Cycle

Back to In the OSTI Collections Listing for 2015

View Past"In the OSTI Collections"Articles.

 

Article Acknowledgement:
Dr. William N. Watson, Physicist
DOE Office of Scientific and Technical Information

Sunlight

Cooling

Radioactivity and Fission

Noise vs. Sound

References

Patents available through DOepatents

Journal article available through DOE PAGESBeta

Reports available through SciTech

Video available through ScienceCinema

Further references

 

 

One way to produce mechanical energy from a heat source is to use a fluid—gaseous or liquid—that pushes something when heated.  Rocket engines propelled by heated fluid work this way.  When the rocket’s propellant is heated, the propellant molecules move much faster than before and push outward in all directions.  When the hot propellant’s molecules collide with the front of the rocket, part of the molecules’ momentum is transferred to the rocket and drives it forward.  (If the nozzle at the back of the rocket were sealed so that the propellant couldn’t escape, its molecules would collide with both the front and back within the rocket and transfer equal amounts of momentum in both the forward and backward directions, so the rocket wouldn’t go anywhere.)  While the rocket acquires more and more momentum from the propellant molecules, the propellant gets cooler as its molecules slow down. 

 

Such rocket engines are thus driven by a single exertion of pressure by a heated fluid.  But other engines powered by heating operate in cycles, in which one or more pistons are driven by repeated expansions of a fluid from a smaller to a larger volume, between which the pistons are reset to their starting positions for the next expansion phase. 

 

Cyclic engines powered by internal combustion are much like rockets whose propellants are heated by burning and emitted as exhaust, except that the cyclic engines are repeatedly supplied with new fluid to burn for each piston-driving cycle.  Other cyclic engines can use external heat sources to warm the same working fluid over and over.  If the pistons of these engines were to be reset after each expansion stroke while the fluid stayed at the heat source’s temperature, there’d be no net production of mechanical energy:  although the working fluid would perform work on the pistons as the fluid absorbed heat energy from the external source and, expanding, drove the pistons forward, pushing the pistons back to their initial positions at the same fluid temperature would simply transfer the same energy back through the fluid and into the heat source again.  To get any net output from such an engine, the pistons have to be repositioned while the fluid is cooler, so that it doesn’t take as much work to squeeze the fluid back down to its original smaller volume.  The cooler the fluid is during its compression, the less work already done by the engine will have to be put back into the engine to compress the fluid. 

 

To cool the engine’s working fluid, some of the energy of its molecules’ random motion has to go somewhere else.  If any of this energy is just dissipated to warm up the environment, it’s lost; it represents an amount of energy that the heat source has to put into the working fluid with each cycle of the engine without ever being turned into mechanical work.  In 1816, Robert Stirling[Wikipedia] patented an engine design that doesn’t lose all this energy.  A Stirling engine[Wikipedia] cools its working fluid by having the fluid warm up an internal heat exchanger as the fluid streams past on its way to a lower-temperature part of the engine, where it’s recompressed.  The compressed fluid is then run back over the internal exchanger and rewarmed to (ideally) the temperature of the external heat source, so the energy doesn’t get wasted.  Instead of a certain amount of energy being dissipated every cycle through the working fluid into the environment without doing any work, that amount of energy keeps recycling between the working fluid and the internal heat exchanger, so the external heat source doesn’t have to keep resupplying it.  The internal heat exchanger is now generally referred to as a “regenerator”; Stirling himself called it an “economiser”.  Whatever the name, it represents an advance over earlier heat engines and was the key feature of Stirling’s patent.[Note] 

 

All the various types of heat engine have relative advantages and disadvantages.  In systems with the appropriate operating conditions, Stirling engines are used to transform energy between the random thermal motions of heat sources and the coherent motion of machinery.  Unlike internal combustion engines, Stirling engines get their heat from outside, so they can absorb energy through a heat exchanger from any kind of external heat source, whether burning fuel or something else, and as described above they turn a portion of that energy into useful work, dumping the rest through another heat exchanger into cooler heat sinks.  And like any type of heat engine, a Stirling engine can operate with its cycle reversed as a refrigerator, having work done on it to extract heat from a cooler source and dump it into a hotter heat sink.[Wikipedia; OSTI]  Both Stirling engines and Stirling refrigerators are found in recent energy transformer designs, as shown by several reports accessible through DOepatents, DoE PAGESBeta, ScienceCinema, and SciTech Connect. 

 Top

Sunlight

 

A 2010 patent entitled “Self-pressurizing Stirling engine”[DOepatents] describes a type of Stirling engine designed specifically to power an aircraft using solar energy.  Other solar-powered aircraft use photovoltaic cells to directly convert sunlight into electricity, which can drive motors to turn propellers or charge batteries to do the same job when there’s not enough sunlight.  As the patent notes, though, photovoltaic power is problematic for aircraft:  batteries to store energy for night or other times when the sun is obscured are massive and add to the burden on the plane’s engines, photovoltaic cells aren’t very efficient at turning the energy of light into electrical energy to begin with, and photovoltaic cells mounted on a plane’s wings won’t always face the sun directly enough to produce sufficient power (hardly ever when flying at high latitudes near the north and south poles).  While the last problem can be addressed by putting the photovoltaic cells in a transparent section of the plane’s fuselage and mounting them on a device that keeps them facing the sun, they’d need to be cooled since they are much less efficient when hot, but a cooling system’s mass would burden the plane’s engines further. 

 

The patent describes a way to solve these problems by using a Stirling engine to transform heat from sunlight into propeller motion.  A solar tracking concentrator, mounted in a transparent section of the aircraft and steered by solar sensors that power its motion, would focus sunlight onto a heat collector, which would conduct heat to a heat-storage medium in thermal contact[Wikipedia] with the Stirling engine’s working fluid.  Such media in heat-engine systems serve much the same purpose as batteries in photovoltaic systems:  both store energy from the unsteady sunlight to make a reservoir that can steadily power machinery.   In this case, as the storage medium acted as the working fluid’s heat source, the fluid would expand to drive the engine’s piston and one or more aircraft propellers linked to it.  Afterwards, the fluid, now moved away from the hot storage medium, would be cooled for recompression before being put back in contact with the heat source begin the next cycle—by a portion of the propeller’s backwash air in some embodiments[Wikipedia] of the invention. 

 

 

Figure 1.  Drawings of a solar-powered aircraft design that incorporates Stirling engines to transform energy from sunlight into mechanical power for propulsion.  The aircraft (100) features a transparent section (112) to allow sunlight to reach a curved mirror (110) that concentrates it onto a heat collector (120), which conducts heat into a thermal storage device (130).  The Stirling engine (140) is heated by contact with the storage device and cooled, in this case, by backwash from the propeller (109) taken in through an inlet (108).  Details of the thermal storage device (131-136) and Stirling engine (141-149), including the storage device’s thermal insulation (10) and the engine’s linkage to the propeller (8), are also shown.  (From United States Patent 7,810,325 B2, “Self-pressurizing Stirling engine”[DOepatents], assigned to Lawrence Livermore National Security, LLC.) 

 

A particular type of thermal-energy battery to be added to an already proven ground-based solar-power plant is described in a report entitled “Metallic phase change material thermal storage for dish Stirling”[DoE PAGES], which Sandia National Laboratories researchers presented to the 2014 International Conference on Concentrating Solar Power and Chemical Energy Systems (SolarPACES 2014) and published in the journal Energy Procedia.  The thermal battery for this system is to store energy collected by the system’s reflecting dish so it can serve as a constant-temperature heat source for the system’s efficient Stirling engine.  A thermal battery can maintain a constant temperature, in spite of its absorbing and emitting heat at different times, if the constant temperature is the battery material’s melting or boiling point.  A melting solid, for instance, will liquefy as it absorbs energy without changing its temperature; a liquid will likewise emit energy at a constant temperature when it solidifies.  A Stirling engine whose working fluid is heated once each cycle by such a thermal battery will always absorb heat at the same high temperature.  The thermal battery is to be heated by a vapor duct to the battery from the dish’s focus, where it is produced when the dish-focused sunlight heats a liquid.  Once the vapor heats the battery it recondenses to a liquid, which is recirculated to the dish’s focus to be vaporized again. 

 

 

Figure 2.  Schematic layout (not to scale) of proposed latent energy storage system for dish Stirling power generation.  The dish focuses sunlight onto the heat pipe receiver, where fluid is vaporized; vapor flows through a duct to the thermal storage unit behind the dish, which contains a Phase Change Material (PCM) that melts when heated.  A heat pipe transfers energy from the thermal storage unit to a Stirling engine at the melting-point temperature of the storage unit’s Phase Change Material.  This energy drives the expansion of the Stirling engine’s working fluid, which pushes the engine’s piston so it can do mechanical work.  (From “Metallic phase change material thermal storage for dish Stirling”[DoE PAGES]; figure also seen in quarterly presentation charts “Dish Stirling High Performance Thermal Storage FY[x]Q[y] Quad Chart”[SciTech Connect; SciTech Connect; SciTech Connect; SciTech Connect; SciTech Connect].) 

 

The problems addressed in the presentation were to find suitable materials for (a) storing the thermal energy at the Stirling engine’s heat-absorption temperature, and (b) containing the repeatedly melted and solidified storage material without being corroded by the pipe that conducts heat to the Stirling engine from the thermal storage unit.  The researchers identified candidate materials for job (a) from published technical data, then narrowed the candidate list by identifying eutectic alloys whose melting points are near the desired temperature range for heating the Stirling engine.  Chemical elements’ melting points are lower when their atoms are mixed with those of other elements; in a eutectic[Wikipedia] mixture, all the elements’ melting points are lowered to the same temperature.  The researchers experimented with likely candidates, found ways to produce the mixtures, and after examining their properties settled on a eutectic alloy of copper, magnesium, and silicon for storing thermal energy.  Since the existing dish Stirling system used sodium in the heat pipe, the investigators examined materials known to be compatible with sodium that were reasonably strong around the Stirling engine’s heating temperature.  Initial experiments showed that all the materials examined tend to react with the eutectic alloy proposed for thermal energy storage, or with any material containing silicon, which seems to be necessary in any thermal storage material that could operate at the desired temperature.  Since looking for alternatives to either the containment materials or the eutectic alloy would be expensive, the researchers began looking for a coating material to keep the alloy and its container from reacting with each other.  Success in this could lead to making the dish Stirling power plant able to run cost-effectively. 

 

Progress with this power plant’s development is summarized in a series of quarterly one-page charts presenting the project objectives and approach, each quarter’s latest key results and outcomes, and plans for the next quarter.  Five charts for the period from the third quarter of fiscal year 2014 to the third quarter of fiscal year 2015[SciTech Connect, SciTech Connect, SciTech Connect, SciTech Connect, SciTech Connect] describe progress in finding a suitable coating and in the development of an advanced wick for the heat pipe.  Plans for the last quarter of fiscal year 2015 included the selection of 1 to 3 coatings for 20,000-hour tests and 1 coating for an integrated storage module already under construction. 

 

A detailed overview of the entire dish Stirling power system with thermal energy storage was given at the outset of Sandia National Laboratories’ project, in the 2012 report “Technical Feasibility of Storage on Large Dish Stirling Systems”[SciTech Connect] by authors at Sandia and at Bucknell University.  Sandia’s project built on findings from earlier work that was done by the Jet Propulsion Laboratory but “did not result in hardware or experimental demonstration of the approaches considered”.   Sandia’s report also cited different investigations by Infinia Corporation to store about an hour’s worth of dish-collected solar energy in a device whose small mass could be supported at the dish’s focus while the dish followed the sun, or to store several hours’ worth of energy in a more massive unit that would be less easy to accommodate at the dish focus.  At that time, Infinia researchers “[had] yet to determine a clear path to an appropriate configuration and transfer of heat to the larger storage unit.”  Infinia received support from the Department of Energy to test a different system of thermal energy storage that used, instead of a eutectic alloy, a eutectic salt (namely a mixture of sodium chloride and sodium fluoride).  Their experiments are described in the 2013 report “Innovative Phase Change Thermal Energy Storage Solution for Baseload Power Phase 1 Final Report”[SciTech Connect].  The report also discusses some of their ideas about how to position the storage unit in relation to the dish and Stirling engine.  One of their early ideas, as shown on page 39 of their report, was similar to the Sandia concept shown in Figure 2. 

 Top

Cooling

 

Infinia also pursued the development of a compact air conditioner based on running the Stirling cycle the other way.  As their fact sheet “Stirling Air Conditioner for Compact Cooling”[SciTech Connect] pointed out, Stirling refrigeration had been expensive and lacked heat exchangers of the kind that could cool air efficiently.  Infinia’s idea was to use computer-industry technology for cooling chips to improve Stirling devices’ heat exchangers, and thus improve the devices’ performance.  Their air conditioners also used nonpolluting, nonreactive/noncombustible helium gas as the refrigerant.  However, the experiments done over 3 years of Energy-Department funded research don’t seem to have improved Stirling air conditioners to the point of competitiveness with other air conditioners; the current product list of the company that acquired Infinia, Qnergy, includes Stirling-engine power sources but no air conditioners. 

 Top

Radioactivity and Fission

 

Running equipment on solar power is feasible when there’s enough solar power available.  Less solar power flows through a given area, though, the further that area is from the sun—too little to run many of the space probes sent into the outer solar system.  Earlier space probes directly converted the heat from the decay of a radioactive material carried onboard into electrical energy using arrays of junctions between different electrically conductive materials.[Wikipedia]  If different junctions in the same circuit are at different temperatures—for instance, if one junction is heated by radioactive decay while another is kept at the ambient temperature of outer space—a current will flow in the circuit.[Wikipedia]  However, the heat from radioactivity can also drive an onboard Stirling engine.  While thermoelectric junctions offer the advantage of no moving parts, they convert heat into electricity less efficiently than a Stirling engine can.  Stirling-based power systems thus require less radioactive material to supply the same electrical power. 

 

Since the performance of thermoelectric junctions over their operating temperature range in space could be calculated from their behavior in the rather different temperature ranges of lab experiments, it was possible to evaluate their output power without actually reducing the cooler junctions’ temperature below room temperature.  What matters most in these devices is the junctions’ temperature difference, not their actual temperatures.  However, because a Stirling engine’s efficiency is a function of the ratio of its hot and cold temperatures, the low-end temperature of a radioactivity-driven Stirling engine in a vacuum test does need to approach the ambient temperature of its intended environment to see whether the engine’s output will be adequate—a small change in the low temperature can make a big change in the high/low temperature ratio.  New testing equipment that can reduce the Stirling engine’s lower temperature to what it would reach in space is described in the Idaho National Laboratory report entitled “Enhanced Thermal Vacuum Test Capability for Radioisotope Power Systems at the Idaho National Laboratory Better Simulates Environmental Conditions of Space”[SciTech Connect], which was prepared for the 2012 conference on Nuclear and Emerging Technologies for Space (NETS).  The equipment can not only produce a vacuum to simulate outer space, but can simulate the atmospheric composition and temperature of various environments that a space-probe power system might encounter.  The vacuum pump can also limit overheating that would degrade the power system during testing. 

 

While the Idaho equipment could be used to test any system that used radioactive decay to power Stirling engines, further development of the particular power system that the equipment was originally built to test was cancelled after the development cost had risen to almost double the expected amount.[Wikipedia]  A different Stirling-engine system for powering spacecraft, whose heat source would be a small-scale controlled nuclear chain reaction[Wikipedia] instead of independent radioactive decays, is illustrated in the 2012 video “Small Reactor for Deep Space Exploration”[ScienceCinema] from Los Alamos National Laboratory.  The reactor for supplying heat at high temperature would consist of a hollow uranium cylinder in which fissioning atoms would emit neutrons that are absorbed by other uranium atoms, causing them to fission and emit more neutrons.  The uranium cylinder would be surrounded by a material that would reflect escaping neutrons back into the uranium to continue the chain reaction.  The hollow space in the uranium would admit a neutron-absorbing rod to control the reaction rate.  Equipment in the spacecraft would be driven by eight Stirling engines separated from the reactor by a radiation shield, with each engine connected to the reactor by a single heat pipe.  By using uranium to supply the heat, this space-probe power system would offer an advantage over existing types whose heat source is the alpha-particle decay of the less-readily available specially-synthesized isotope plutonium-238. 

 

 

Figure 3.  Left:  cutaway view of a space-probe power system based on Stirling engines (in middle, highlighted) and a nuclear-reactor heat source (far right) consisting of a hollow uranium core (shown as black) that can admit a control rod and surrounded by a material that can reflect escaping neutrons back into the core to more efficiently maintain the chain reaction.  Radiation shielding separates the reactor and Stirling engines; heat pipes at the periphery of the system conduct thermal energy across the shielding from the reactor to the engines.  Space probe components powered by the Stirling engines would be housed at the left.  Right:  the eight Stirling engines behind the radiation shielding, with the four engines at the rear shown in a cutaway view.  (From the 4’32” Los Alamos National Laboratory video “Small Reactor for Deep Space Exploration”[ScienceCinema].)

 

 

Figure 4.  A modified design for a Stirling-engine power source for space probes driven by heat from a small nuclear reactor.  Whereas the design illustrated in Figure 3 involves a reactor core surrounded by a fixed neutron reflector whose reaction rate is governed by the placement of a control rod, the modified design is controlled by repositioning a mobile reflector.  When the core (here shown red) is not surrounded by the reflector (left), neutrons escaping from the core aren’t reflected back into it, so the reaction rate decreases; when the reflector does surround the core (right), more neutrons stay in the core to produce further fission reactions, so the core is hotter.  In a different modification, the heat pipes of the new design are not individually coupled to single Stirling engines, but convey heat to a common plate.  Thus, even if some of the heat pipes malfunction, the single common plate will transfer heat to all eight Stirling engines, two of which are depicted here.  (From the Los Alamos National Laboratory research note “Kilowatt Reactor Using Stirling TechnologY (KRUSTY) Demonstration. CEDT Phase 1 Preliminary Design Documentation”[SciTech Connect].) 

 

Further development of this basic concept is evident in the 2015 description of component tests entitled “Kilowatt Reactor Using Stirling TechnologY (KRUSTY) Demonstration. CEDT Phase 1 Preliminary Design Documentation”[SciTech Connect].  This Los Alamos research note shows that the earlier design’s method of controlling the uranium core’s chain reaction, by the extraction or insertion of a control rod, has been replaced with a new control method:  placement of the reactor’s neutron reflector either around the core or away from it (as shown in Figure 4).  Another modification was made to ensure that all eight of the Stirling engines continue to generate electricity even if one of the heat pipes quits working, by connecting all the heat pipes to a flat plate, which all the Stirling engines are in thermal contact with, instead of connecting each heat pipe directly to just one Stirling engine. 

 Top

Noise vs. Sound

 

Monitoring the transport of materials used for nuclear power requires the ability to detect the materials’ presence.  The most effective way to do this is with devices made of very pure germanium[Wikipedia], a semiconductor that can react to the gamma rays emitted by some materials, such as nuclear fuels.  Each gamma ray absorbed by a semiconductor energizes one of its electrons, giving it a new state of motion and leaving a “hole” in its previous state of motion among other similarly-moving electrons, with the energy difference between the old and new states depending on the energy of the gamma ray.  If the germanium is cold enough, the only electron-hole pairs will be those produced by absorbing gamma-ray energy, but warming the germanium will energize even more electrons, as though the germanium had absorbed many gamma rays of various energies.  Thus if a germanium device is to be used to precisely distinguish gamma rays of different energies, and thus distinguish different radioactive materials by their characteristic gamma-ray energies, the germanium needs to be kept very cold—near the temperature at which nitrogen can be liquefied. 

 

Germanium gamma-ray detectors actually cooled by liquid nitrogen require frequent replenishment of the nitrogen, so they won’t stay cold if they have to be left unattended for a long time.  Detectors have been cooled with Stirling-cycle refrigerators, but existing refrigerators’ piston oscillations produce low-volume noise that sets detectors to vibrating.  These vibrations affect the arrangement of the germanium’s electric charges, so that the excited electrons’ energies don’t match those of the absorbed gamma rays.  Vibrations thus reduce even cold germanium detectors’ ability to distinguish gamma rays of different frequencies. 

 

As Lawrence Livermore National Lab researchers reported for a 1998 symposium[OCLC] in “Electromechanically-cooled germanium radiation detector system”[SciTech Connect], a counterbalance was used with a germanium detector to mechanically cancel its Stirling refrigerator’s piston noise in the 1990s; however, detectors so equipped weren’t able to adapt to changes in boundary conditions and mass loading through time.  Another research group, from Los Alamos National Lab, Brigham Young University, the University of Vermont, and Oklahoma State University, has made progress in solving this problem using an improved feedback mechanism.  They described their accomplishments at the time of the 2012 International Modal Analysis Conference[Springer] in a slide presentation entitled “Vibration cancellation in a Stirling-cooled HPGe detector”[SciTech Connect].  At that time, the group’s Stirling refrigerator had yet to cool the detector to liquid-nitrogen temperatures, and they had yet to optimize the feedback mechanism or test their system under field conditions. 

 

Sound waves from the Stirling device definitely interfere with the operation of a detector whose sensitivity is maximized by staying still.  But for some Stirling devices, their ability to generate or absorb sound is essential to their purpose. 

 

A Stirling engine’s working fluid expands to do work when it absorbs energy from a heat source, and then is cooled as it passes the regenerator, then compressed, and then rewarmed as it passes back over the regenerator.  In Stirling’s original design, and in many variations by others since, the fluid expands against a piston and gets compressed by a piston.  But when sound waves travel through a material, portions of the material also expand and get compressed as those portions interact with their neighbors.  With these expansion and compression cycles come cyclic changes in the material’s pressure and temperature.  Thus, sound waves traveling in a fluid that can exchange thermal energy with suitably spaced objects that it passes through can make the objects act as a series of Stirling devices.  In this case, sound is the form of mechanical energy that the devices either produce from the heat of a higher-temperature source or consume to remove heat from a lower-temperature source. 

 

Two of the latest designs for such devices are described in patents entitled “In-line stirling energy system”[DOepatents] and “Thermoacoustic refrigerators and engines comprising cascading stirling thermodynamic units”[DOepatents] and assigned to Los Alamos National Security, LLC.  The first invention, when run to produce sound waves from the energy of a heat source, can amplify a sound wave by heating the device’s working fluid.  When the device is coupled to an alternator, the heat input is ultimately converted to electricity.   Run in reverse mode, the device takes energy from a sound wave to act as a refrigerator or heat pump, removing heat from a low-temperature source to a higher-temperature sink.  The second patent specifically addresses a problem found in other high-power acoustic heat pumps.  Since maintaining appropriate conditions at the lower-temperature heat source keeps all of the input sound wave’s energy from being used directly to pump energy to the higher-temperature heat sink, more efficient use of the sound wave requires some way to recover the otherwise wasted portion of its energy.  One way is to use the leftover energy in a second acoustic heat pump, but earlier methods of doing this have been inefficient because of how the two heat pumps were connected.  The second patent shows how a connecting tube that’s narrower than either of the two heat pumps and approximately a fourth of the length of the sound wave can optimize the wave’s characteristics so that more of its energy left over from the first heat pump can go into moving heat at the second heat pump. 

 

All these devices demonstrate the power of Stirling’s thermal regenerator concept, exemplifying how new technologies continue to enable new uses for an idea conceived about two centuries ago.   

 

 

 

 

 

Figure 5.  Top:  Schematic illustrations of U.S. Patent 7,908,856 B2’s “In-line stirling energy system”[DOepatents].  Fig. 1 illustrates a series of Stirling devices in an acoustic shell (5) enclosed in a pressure vessel (3), which is connected to a heat source (18) and heat sink (20).  A closeup of the upper components in the series is shown in Fig. 2, with region 16 representing a single Stirling unit; Fig. 3 shows part of a Stirling core consisting of two heat exchangers (23 and 25), for rejecting and accepting heat respectively, on either side of a regenerator (24).  Bottom:  The method of U.S. Patent 8,468,838 B2 (“Thermoacoustic refrigerators and engines comprising cascading stirling thermodynamic units”[DOepatents]) for cascading two thermoacoustic refrigerators to efficiently use the energy of a sound wave.  Segment 7 represents the source of sound waves, segments 2 and 64 represent the Stirling devices, and segment 55 represents a connecting tube approximately one-quarter wavelength (?/4) long. 

 Top

 

References

 

^ [Note] In 1824, a few years after Stirling patented his engine design, Sadi Carnot[Wikipedia] published his theoretical analysis of heat engines’ maximum efficiency.  Instead of cooling its working fluid by warming an internal heat exchanger, Carnot’s idealized heat-engine model cooled its fluid by continuing to perform work on the piston after losing contact with the external heat source.  After the working fluid was recompressed at low temperature, the extra work went directly back into compressing the fluid and rewarming it to the external heat source’s temperature, just as energy transferred to the heat exchanger rewarms the fluid in a Stirling engine.  Thus in both the ideal Carnot engine and the ideal Stirling engine, the net work done equals the difference between the work done while the working fluid absorbs energy from the external heat source and the work done to compress the fluid at its cooler temperature, since the fluid’s other energy gains and losses cancel out.  Real engines also make some unwanted energy transfers, such as leaking thermal energy to places other than the heat source or sink, or losing mechanical energy to friction with moving parts, but a Carnot or Stirling engine that made no such unwanted transfers would transform the maximum possible fraction of thermal energy into useful work for a given source/sink temperature ratio.  Carnot’s analysis received little attention when it was first published, but was later recognized and became a standard way to explain heat engines’ maximum efficiency.  Although the Stirling cycle involves some different energy transfers, its ideal version has the same efficiency as the ideal Carnot cycle. 

 

 Top

 

Wikipedia

 Top

 

Research organizations

 Top

 

Patents available through DOepatents

  • “Self-pressurizing Stirling engine” [Metadata]
  • “In-line stirling energy system” [Metadata]
  • “Thermoacoustic refrigerators and engines comprising cascading stirling thermodynamic units” [Metadata]

 Top

Journal article available through DoE PAGESBeta

  • “Metallic phase change material thermal storage for dish Stirling” [Metadata]

 Top

 

Reports available through SciTech Connect

  • “Dish Stirling High Performance Thermal Storage FY15Q3 Quad Chart” [Metadata] and similar charts for earlier quarters [Metadata, Metadata, Metadata, Metadata]
  • “Technical feasibility of storage on large dish stirling systems.” [Metadata]
  • “Innovative Phase Change Thermal Energy Storage Solution for Baseload Power Phase 1 Final Report” [Metadata]
  • “Stirling Air Conditioner for Compact Cooling” [Metadata]
  • “Enhanced Thermal Vacuum Test Capability for Radioisotope Power Systems at the Idaho National Laboratory Better Simulates Environmental Conditions of Space” [Metadata]
  • “Kilowatt Reactor Using Stirling TechnologY (KRUSTY) Demonstration. CEDT Phase 1 Preliminary Design Documentation” [Metadata]
  • “Electromechanically-cooled germanium radiation detector system” [Metadata]
  • “Vibration cancellation in a Stirling-cooled HPGe detector” [Metadata]

 Top

Video available through ScienceCinema

  • “Small Reactor for Deep Space Exploration” [Metadata]

 Top

Further references

Last updated on Wednesday 27 July 2016