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. The Stirling cycle heat engine can also be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction.
There are several design configurations for Stirling engines that can be built which can introduce difficult tradeoffs between frictional losses and refrigerant leakage. A free-piston variant of the Stirling engine can be built, which can be completely hermetically sealed, reducing friction losses and completely eliminating refrigerant leakage. For example, a free-piston Stirling cooler can convert an electrical energy input into a practical heat pump effect, used for high-efficiency portable refrigerators and freezers. Conversely, a free-piston electrical generator could be built, converting a heat flow into mechanical energy, and then into electricity. In both cases, energy is usually converted from/to electrical energy using magnetic fields in a way that avoids compromising the hermetic seal.
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 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. It could be possible to use these in co-generation systems that use waste heat from a conventional piston or gas turbine engine's exhaust and use this either to power the ancillaries or even as a turbo-compound system that adds power and torque to the crankshaft.
Automobiles exclusively powered by Stirling engines were developed in test projects by NASA, as well as earlier projects by the Ford Motor Company using engines provided by Philips, and by American Motors Corporation with several cars equipped with units from Sweden's United Stirling built under a license from Philips. The NASA vehicle test projects were designed by contractors and designated MOD I and MOD II.
NASA's Stirling MOD 1 powered engineering vehicles were built in partnership with the United States Department of Energy and NASA, under contract by AMC's AM General to develop and demonstrate practical alternatives for standard engines. The United Stirling AB's P-40 powered AMC Spirit was tested extensively for over and achieved average fuel efficiency up to. 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.
Tests conducted with the 1979 AMC Spirit, as well as a 1977 Opel and a 1980 AMC Concord, revealed the Stirling engines "could be developed into an automotive power train for passenger vehicles and that it could produce favorable results." However, progress was achieved with equal-power spark-ignition engines since 1977, and the Corporate Average Fuel Economy requirements that were to be achieved by automobiles sold in the U.S. were being increased. Moreover, the Stirling engine design continued to exhibit a shortfall in fuel efficiency. There were also two major drawbacks for consumers using the Stirling engines: first was the time needed to warm up – because most drivers do not like to wait to start driving; and second was the difficulty in changing the engine's speed – thus limiting driving flexibility on the road and traffic. The process of auto manufacturers converting their existing facilities and tooling for the mass production of a completely new design and type of powerplant was also questioned.
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 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 the results showed that highway gas mileage was increased from and achieved an urban range of 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.
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.
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.
The New Hampshire Union Leader reported that Dean Kamen developed a series plug-in hybrid car using a Ford Think. Called the DEKA Revolt, the car can reach approximately on a single charge of its lithium battery.
Aircraft engines
Robert McConaghy created the first flying Stirling engine-powered aircraft in August 1986. The Beta type engine weighed 360 grams, and produced only 20 watts of power. 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".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 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 allows for it to be smaller.Swedish shipbuilder Kockums has built eight successful Stirling-powered submarines since the late 1980s. They carry compressed oxygen to allow fuel combustion submerged, providing heat for the Stirling engine. They are currently used on submarines of the Gotland and Södermanland classes. They are the first submarines in the world to feature Stirling air-independent propulsion, which extends their underwater endurance from a few days to several weeks.
The Kockums engine also powers the Japanese Sōryū-class submarine.
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 that can run a 3 kW generator or a centrifugal water pump.Electrical power generation
Combined heat and power
In a combined heat and power 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.Thermal power stations on the electric grid use fuel to produce electricity. However, there are large quantities of waste heat produced which often go unused. In other situations, high-grade fuel is burned at high temperatures for a low-temperature application. According to the second law of thermodynamics, a heat engine can generate power from this temperature difference. In a CHP system, the high-temperature primary heat enters the Stirling engine heater, then some of the energy is converted to mechanical power in the engine, and the rest passes through to the cooler, where it exits at a low temperature. The "waste" heat actually comes from the engine's main cooler, and possibly from other sources such as the exhaust of the burner, if there is one.
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.
WhisperGen, a New Zealand firm with offices in Christchurch, has developed an "AC Micro Combined Heat and Power" Stirling cycle engine. These microCHP units are gas-fired central heating boilers that sell unused power back into the electricity grid. WhisperGen announced in 2004 that they were producing 80,000 units for the residential market in the United Kingdom. A 20 unit trial in Germany was conducted in 2006.