Applications of the Stirling engine
Applications of the Stirling engine range from mechanical propulsion to heating and cooling to electrical generation systems. A Stirling engine is a heat engine operating by cyclic compression and expansion of air or other gas, the "working fluid", at different temperature levels such that there is a net conversion of heat to mechanical work.[1][2] The Stirling cycle heat engine can also be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction (i.e. a heat pump, or refrigerator).[3]
There are several design configurations for Stirling engines that can be built (many of which require rotary or sliding seals) which can introduce difficult tradeoffs between
Mechanical output and propulsion
Automotive engines
It is often claimed that the Stirling engine has too low a power/weight ratio, too high a cost, and too long a starting time for automotive applications. They also have complex and expensive heat exchangers. A Stirling cooler must reject twice as much heat as an Otto engine or Diesel engine radiator. The heater must be made of stainless steel, exotic alloy, or ceramic to support high heating temperatures needed for high power density, and to contain hydrogen gas that is often used in automotive Stirlings to maximize power. The main difficulties involved in using the Stirling engine in an automotive application are startup time, acceleration response, shutdown time, and weight, not all of which have ready-made solutions.
However, a modified Stirling engine has been introduced that uses concepts taken from a patented internal-combustion engine with a sidewall combustion chamber (US patent 7,387,093) that promises to overcome the deficient power-density and specific-power problems, as well as the slow acceleration-response problem inherent in all Stirling engines.
Automobiles exclusively powered by Stirling engines were developed in test projects by
NASA's Stirling MOD 1 powered engineering vehicles were built in partnership with the United States Department of Energy (DOE) and NASA, under contract by AMC's AM General to develop and demonstrate practical alternatives for standard engines.[7] The United Stirling AB's P-40 powered AMC Spirit was tested extensively for over 50,000 miles (80,467 km) and achieved average fuel efficiency up to 28.5 mpg‑US (8.3 L/100 km; 34.2 mpg‑imp).[8] A 1980 4-door liftback VAM Lerma was also converted to United Stirling P-40 power to demonstrate the Stirling engine to the public and to promote the U.S. government's alternative engine program.[9]
Tests conducted with the 1979 AMC Spirit, as well as a 1977
The MOD II project in 1980 produced one of the most efficient automotive engines ever made. The engine reached a peak thermal efficiency of 38.5%, compared to a modern spark-ignition (gasoline) engine, which has a peak efficiency of 20-25%. The Mod II project replaced the normal spark-ignition engine in a 1985 4-door Chevrolet Celebrity notchback. In the 1986 MOD II Design Report (Appendix A) the results showed that highway gas mileage was increased from 40 to 58 mpg‑US (5.9 to 4.1 L/100 km; 48 to 70 mpg‑imp) and achieved an urban range of 26 to 33 mpg‑US (9.0–7.1 L/100 km; 31–40 mpg‑imp) with no change in vehicle gross weight. Startup time in the NASA vehicle was a maximum of 30 seconds, while Ford's research vehicle used an internal electric heater to quickly start the engine, giving a start time of only a few seconds. The high torque output of the Stirling engine at low speed eliminated the need for a torque converter in the transmission resulting in decreased weight and transmission drivetrain losses negating somewhat the weight disadvantage of the Stirling in auto use. This resulted in increased efficiencies being mentioned in the test results.[13][14]
The experiments indicated that the Stirling engine could improve vehicle operational efficiency by ideally detaching the Stirling from direct power demands, eliminating a direct mechanical linkage as used in most current vehicles. Its prime function used in an extended-range series electric hybrid vehicle would be as a generator providing electricity to drive the electric vehicle traction motors and charging a buffer battery set. In a petro-hydraulic hybrid the Stirling would perform a similar function as in a petro-electric series-hybrid turning a pump charging a hydraulic buffer tank. Although successful in the MOD 1 and MOD 2 phases of the experiments, cutbacks in funding further research and lack of interest by automakers ended possible commercialization of the Automotive Stirling Engine Program.[7]
Electric vehicles
Stirling engines as part of a hybrid electric drive system may be able to bypass the design challenges or disadvantages of a non-hybrid Stirling automobile.
In November 2007, a prototype hybrid car using solid biofuel and a Stirling engine was announced by the Precer project in Sweden.[15]
The
Aircraft engines
Robert McConaghy created the first flying Stirling engine-powered aircraft in August 1986.[17] The Beta type engine weighed 360 grams, and produced only 20 Watts of power.[18] The engine was attached to the front of a modified Super Malibu radio control glider with a gross takeoff weight of 1 kg. The best-published test flight lasted 6 minutes and exhibited "barely enough power to make the occasional gentle turn and maintain altitude".[18]
Marine engines
The Stirling engine could be well suited for underwater power systems where electrical work or mechanical power is required on an intermittent or continuous level. General Motors has undertaken work on advanced Stirling cycle engines which include thermal storage for underwater applications. United Stirling, in Malmö, Sweden, are developing an experimental four–cylinder engine using hydrogen peroxide as an oxidant in underwater power systems. The SAGA (Submarine Assistance Great Autonomy) submarine became operational in the 1990s and is driven by two Stirling engines supplied with diesel fuel and liquid oxygen. This system also has potential for surface-ship propulsion, as the engine's size is less of a concern, and placing the radiator section in seawater rather than open-air (as a land-based engine would be) allows for it to be smaller.
Swedish shipbuilder
The Kockums engine also powers the Japanese
This capability has previously only been available with nuclear-powered submarines.
Pump engines
Stirling engines can power pumps to move fluids like water, air and gasses. For instance the ST-5 from Stirling Technology Inc. power output of 5 horsepower (3.7 kW) that can run a 3 kW generator or a centrifugal water pump.[22]
Electrical power generation
Combined heat and power
In a combined heat and power (CHP) system, mechanical or electrical power is generated in the usual way, however, the waste heat given off by the engine is used to supply a secondary heating application. This can be virtually anything that uses low-temperature heat. It is often a pre-existing energy use, such as commercial space heating, residential water heating, or an industrial process.
The power produced by the engine can be used to run an industrial or agricultural process, which in turn creates biomass waste refuse that can be used as free fuel for the engine, thus reducing waste removal costs. The overall process can be efficient and cost-effective.
Inspirit Energy, a UK-based company have a gas fired CHP unit called the Inspirit Charger which is on sale in 2016. The floor standing unit generates 3 kW of electrical and 15 kW of thermal energy.[23][24]
WhisperGen, a
Solar power generation
Placed at the focus of a parabolic mirror, a Stirling engine can convert
Nuclear power
There is a potential for nuclear-powered Stirling engines in electric power generation plants. Replacing the steam turbines of nuclear power plants with Stirling engines might simplify the plant, yield greater efficiency, and reduce the radioactive byproducts. A number of breeder reactor designs use liquid sodium as a coolant. If the heat is to be employed in a steam plant, a water/sodium heat exchanger is required, which raises some concern if a leak were to occur, as sodium reacts violently with water. A Stirling engine eliminates the need for water anywhere in the cycle. This would have advantages for nuclear installations in dry regions.
United States government labs have developed a modern Stirling engine design known as the
Heating and cooling
If supplied with mechanical power, a Stirling engine can function in reverse as a heat pump for heating or cooling. In the late 1930s, the Philips Corporation of the Netherlands successfully utilized the Stirling cycle in cryogenic applications.[29] During the Space Shuttle program, NASA successfully lofted a Stirling cycle cooler in a form "similar in size and shape to the small domestic units often used in college dormitories" for use in the Life Science Laboratory.[30] Further research on this unit for domestic use led to a Carnot coefficient-of-performance gain by a factor of three and a weight reduction of 1kg for the unit.[31] Experiments have been performed using wind power driving a Stirling cycle heat pump for domestic heating and air conditioning.[citation needed]
Stirling cryocoolers
This section needs additional citations for verification. (February 2022) |
Any Stirling engine will also work in reverse as a heat pump: when mechanical energy is applied to the shaft, a temperature difference appears between the reservoirs. The essential mechanical components of a Stirling cryocooler are identical to a Stirling engine. In both the engine and the heat pump, heat flows from the expansion space to the compression space; however, input work is required in order for heat to flow "uphill" against a thermal gradient, specifically when the compression space is hotter than the expansion space. The external side of the expansion-space heat exchanger may be placed inside a thermally insulated compartment such as a vacuum flask. Heat is in effect pumped out of this compartment, through the working gas of the cryocooler and into the compression space. The compression space will be above ambient temperature, and so heat will flow out into the environment.
One of their modern uses is in
The first Stirling-cycle cryocooler was developed at Philips in the 1950s and commercialized in such places as liquid air production plants. The Philips Cryogenics business evolved until it was split off in 1990 to form the Stirling Cryogenics BV, The Netherlands. This company is still active in the development and manufacturing of Stirling cryocoolers and cryogenic cooling systems.
A wide variety of smaller Stirling cryocoolers are commercially available for tasks such as the cooling of electronic
Heat pumps
This section needs additional citations for verification. (February 2022) |
A Stirling heat pump is very similar to a Stirling cryocooler, the main difference being that it usually operates at room temperature. At present, its principal application is to pump heat from the outside of a building to the inside, thus heating it at lowered energy costs.
As with any other Stirling device, heat flow is from the expansion space to the compression space. However, in contrast to the Stirling engine, the expansion space is at a lower temperature than the compression space, so instead of producing work, an input of mechanical work is required by the system (in order to satisfy the
The expansion side of the heat pump is thermally coupled to the heat source, which is often the external environment. The compression side of the Stirling device is placed in the environment to be heated, for example, a building, and heat is "pumped" into it. Typically there will be thermal insulation between the two sides so there will be a temperature rise inside the insulated space.
Heat pumps are by far the most energy-efficient types of heating systems, since they "harvest" heat from the environment, rather than only turning their input energy into heat. In accordance with the Second Law of Thermodynamics, heat pumps always require the additional input of some external energy to "pump" the collected heat "uphill" against a temperature differential.
Compared to conventional heat pumps, Stirling heat pumps often have a higher coefficient of performance[citation needed]. Stirling systems have seen limited commercial use; however, use is expected to increase along with market demand for energy conservation, and adoption will likely be accelerated by technological refinements.
Portable refrigeration
The Free Piston Stirling Cooler (FPSC) is a completely sealed heat transfer system that has only two moving parts (a piston and a displacer), and which can use helium as the working fluid. The piston is typically driven by an oscillating magnetic field that is the source of the power needed to drive the refrigeration cycle. The magnetic drive allows the piston to be driven without requiring any seals, gaskets, O-rings, or other compromises to the hermetically sealed system.[33] Claimed advantages for the system include improved efficiency and cooling capacity, lighter weight, smaller size and better controllability.[34]
The FPSC was invented in 1964 by William Beale (1928-2016), a professor of Mechanical Engineering at
Other suppliers of FPSC technology include the
For several years starting around 2004, the
Low temperature difference engines
A low temperature difference (LTD, or Low Delta T (LDT)) Stirling engine will run on any low-temperature differential, for example, the difference between the palm of a hand and room temperature, or room temperature and an ice cube. A record of only 0.5 °C temperature differential was achieved in 1990.
However, larger (typically 1 m square) low temperature engines have been built for pumping water using direct sunlight with minimal or no magnification.[46]
Other applications
Acoustic Stirling Heat Engine
Los Alamos National Laboratory has developed an "Acoustic Stirling Heat Engine"[47] with no moving parts. It converts heat into intense acoustic power which (quoted from given source) "can be used directly in acoustic refrigerators or pulse-tube refrigerators to provide heat-driven refrigeration with no moving parts, or ... to generate electricity via a linear alternator or other electro-acoustic power transducer".
MicroCHP
WhisperGen, (bankruptcy 2012
Based on the companies' published performance specifications, the off-grid diesel-fueled unit produces combined heat (5.5 kW heat) and electric (800W electric) output, from a unit being fed 0.75 liters of automotive-grade diesel fuel per hour. Whispergen units are claimed to operate as a combined co-generation unit reaching as high as ~80% operating efficiency.
However the preliminary results of an Energy Saving Trust review of the performance of the WhisperGen microCHP units suggested that their advantages were marginal at best in most homes.[49] However another author shows that Stirling engine microgeneration is the most cost effective of various microgeneration technologies in terms of reducing CO2.[25]
Chip cooling
MSI (Taiwan) developed a miniature Stirling engine cooling system for personal computer chips that uses the waste heat from the chip to drive a fan.[50]
Desalination
In all thermal power plants there has to be an exhaust of waste heat. However, there's no reason that the waste heat cannot be diverted to run Stirling engines to pump seawater through reverse osmosis assemblies except that any additional use of the heat raises the effective heat sink temperature for the thermal power plant resulting in some loss of energy conversion efficiency. In a typical nuclear power plant, two-thirds of the thermal energy produced by the reactor is waste heat. In a Stirling assembly the waste heat has the potential to be used as an additional source of electricity.[citation needed]
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A Stirling engine is a mechanical device which operates on a *closed* regenerative thermodynamic cycle, with cyclic compression and expansion of the working fluid at different temperature levels.
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