*2.1. Historical Background*

The Stirling engine was invented in 1816 by Robert Stirling in Scotland, some 80 years before the invention of the diesel engine, and enjoyed considerable commercial success until the early 20th century.

Before the Stirling engine was created, however, attempts were made to develop a hot air engine. The first hot air engine was the atmospheric fire-mill. It was an air engine in which heat was transformed in mechanical power. Its inventor was Guillaume Amontons [17–19], a Frenchman, who invented it in 1669. The engine proposed by Guillaume was a pistonless engine, and the air was heated and cooled in a closed circuit. Heat was supplied from external source (i.e., fossil fuel combustion chamber). The drawing of Guillaume engine is presented in Figure 1.

**Figure 1.** View on the Guillaume Amontons hot air engine [20].

Another example of a hot air engine is the engine proposed by Sir George Cayley [21,22]. It is considered to be the first hot air engine that worked successfully. Cayley has shown grea<sup>t</sup> ingenuity in overcoming the practical difficulties of high operating temperatures. This type of engine was one in which the fire is shielded and fed with air forced under the grate in an amount sufficient to maintain combustion, while by far the largest part of the air goes above the fire to be heated and expanded; the air with the combustion products then acts on the piston and passes through the operating cylinder, no metal heating surface is required, the heated air is brought into direct contact with the fire. One of these engines worked for many months for testing. It was better than any design of steam engine known at the time in terms of fuel economy compared to the power output. However, the joints were very troublesome, and the cylinder and piston seal were quickly destroyed by dust and gravel particles from the fuel, which acted as abrasive and prevented lubrication. An attempt was made to filter the air before entering the cylinder with sheets of wire mesh,

but these either subsided or were soon choked and rendered useless [20]. The drawing of the Cayley engine is presented in Figure 2.

**Figure 2.** View on the Cayley hot air engine [20,23].

The Stirling engine was invented and patented in 1816 by Robert Stirling. It was originally used in Scotland, Ayrshire, in 1818 to drive a water pump. Unfortunately, due to the shortcomings of the materials at that time, the engine only worked for two years and was then replaced by a steam engine. In later years, Robert Stirling and his brother James improved the design by, among other changes, adding a second piston. The end result of these works was that the engine obtained a higher efficiency than steam engine, but unfortunately there were still problems with the materials, which caused users to return to steam engines. A view of Stirling's engine patent is presented in Figure 3.

**Figure 3.** View of Stirling's engine patent drawing, 1816 [24].

Until the First World War, many other interesting concepts of using a hot air engine, e.g., for powering aircrafts, were created, but they were not widely used. These engines were mainly used in small workshops and for driving water pumps because, unlike a steam engine, they did not require a qualified engineer to operate them. Unfortunately, their power and efficiency remained low compared to their size [25–27].

The renaissance of interest in the Stirling engine took place in the interwar period thanks to the concern of the Dutch Philips, who was looking for a simple, light engine for powering a radio. Thanks to the invention of steel resistant to high temperatures after the First World War and its application in the Stirling engine, it was possible to reduce the failure rate. At that time, Stirling engines sometimes had an efficiency of less than 1% (while theoretically they could achieve an efficiency of 60%). Professor Holst believed in this possibility when he started his research on the Stirling engine. Unfortunately, the German occupation during the Second World War significantly inhibited this research. However, in 1947, after ten years of development, a small 30-horsepower engine was presented, featuring a rotational speed of 3000 rpm and efficiency similar to internal combustion engine. Another result of this work was a small engine that ran for over 2000 h without any visible damage. The important direction of research on Stirling machine design was also the Stirling cooler based on reversed engine cycle, which application gave the opportunity of obtaining of very low temperatures. Initially, the obtained temperature was around −190 ◦C, but in later years even the temperature of −260 ◦C was achieved. However, the main goal of Philips, who was creating a small power source, has not been achieved despite the use of solutions such as the diamond-shaped mechanism [28].

In late 1950s, Philips engines achieved an efficiency of about 38%, which was higher than the efficiency achieved by the gasoline and diesel engines. In 1958, the automotive concern General Motors was interested in these works, looking for a new type of propulsion in the automotive industry, as well as for powering generators and submarines. The result was a developed prototype of 150 hp Rinia engine [29]. However General Motors senior managemen<sup>t</sup> decided to abandon the program before the engine was put into production. The Stirling engine concept was revisited at the turn of the 1960s and 1970s, mainly due to the rising oil prices. The first prototypes of buses powered by Stirling engines were then created, but putting this type of bus into production after the end of the oil crisis was found unprofitable. As a curiosity, it is worthy to mention that Stirling engines have had a significant impact on the design of today's conventional submarines. Thanks to application of the Stirling engine, it was possible to extend the time they can be submerged. The first class of submarines in which the Stirling engine is applied for propulsion is the Swedish project A-19 Gotland, equipped with two Stirling engines. Thanks to their application, the ship can stay fully submerged for 2 weeks and travel at a speed of 5 knots. The efficiency of these engines is ca. 40%. Nowadays, in addition to the above-mentioned applications, the possibility of using Stirling engines in thermal energy and renewable energy for electricity production is also considered [30].

Some engine companies, such as MAN-MWM, United Stirling of Sweden, and Ford Motor Company of Detroit, have started research programs to develop Stirling engines for automotive applications. To date, few multi-cylinder Stirling engines featuring different power ranges have been prototyped by these companies. The thermal efficiency of Stirling engines designed for automotive applications is higher than 40%. The main design and application problem that needs to be solved in the future is the high weight and large size of such an engine [31–33].

## *2.2. Functional Description*

A Stirling engine [27,34,35] is an external combustion reciprocating engine that uses one or more pistons to achieve useful operation by supplying heat from an external source. They differ significantly from the internal combustion engines found in most vehicles. Stirling engines use the same gas during operation, as opposed to internal combustion engines which constantly take in and discharge gas (they take the air-fuel mixture and

discharge the exhaust gases). In addition, Stirling engines do not use the combustion effect inside the chamber as is the case of conventional internal combustion engines, which makes their operation very quiet.

A key unique feature of Stirling engines is that there is a constant amount of gas inside the cylinders. The gas pressure can be regulated by supplying or receiving heat, i.e., thanks to the changes in gas volume. Thanks to the supplied heat pressure and increasing temperature of the gas contained in the cylinders, on the contrary, removing heat form cylinders reduces pressure and temperature. By changing the way these two processes are performed engine deliver useful work. The engine operates according to the "Stirling cycle" described below [28,35]. The operation cycle is organized as follows:


The principle of the Stirling engine operation and its basic components are shown in Figure 4.

**Figure 4.** Example schematic diagram of a free-piston Stirling cycle engine with a linear alternator for energy extraction [36].

Heat transport in the Stirling engine is realized by a displacer piston. There is more gas in the working chamber on the hot side of the engine when the displacer piston is closer to the bottom dead center. When the heat is supplied to cylinder from the heat source the gas pressure inside the engine also increases. When the displacer is closer to the top dead center, the gas pressure drops and most of the gas is cooled. An additional effect is the movement of the power piston which interacts directly with the displacement element. By balancing the area and masses of the pistons, the dynamics of the pistons movement, and the restriction of mass flow from one side of piston to the other, a self-sustained cycle can be achieved to convert the heat absorbed by the engine into useful work [36].

To increase efficiency, most Stirling engines use a regenerative heat exchanger, simply referred as to "regenerator". A regenerator works like a thermal condenser in which heat is absorbed and released from the gas as it passes from one cylinder to another. This heat transfer takes place cyclically. This cyclicality corresponds to the operating frequency of the engine. The regenerator is clearly visible in Figure 5, which shows a block diagram of the operation of a Stirling engine [36–38].

**Figure 5.** Block diagram showing working fluid flow in the Stirling engine and its main components.

#### *2.3. Stirling Engine Designs*

Stirling engines can have many different designs. Three classic designs are described in the following. An alpha-type engine has two cylinders in which two pistons move. Pistons are loaded on the one hand with a variable pressure of the working gas, and on the other hand with a constant gas pressure prevailing in the so-called buffer space. The phase shift (between 85◦ and 120◦) of the pistons is required. It is important that when assembling this type of engine, the structural and kinematic elements of the working mechanism and engine block are so arranged that the piston working in the hot cylinder is shifted in phase ahead to the piston moving in the cold cylinder. More details about the alpha-type Stirling engine can be found in [37,39–46].

The beta-type design [47–54] has one cylinder in which two pistons move coaxially with the required phase shift. The upper piston moves in the cylinder, forcing the gas twice in circulation between the compression and expansion chamber through a set of heat exchangers. As a result, the engine is only loaded by the pressure difference resulting from the gas flow through the heat exchangers and through the resistances. The group of heat exchangers is connected to the compression and expansion chamber, and the buffer space is located under the piston. The movement of the lower piston, which is phase-lagged, compresses and decompresses the gas.

The gamma-type design is the simplest and easiest to manufacture of the Stirling engines. Similar to the beta design, the gamma design has two cylinders (sometimes the diameter of one cylinder is larger and diameter of the other is smaller). Cylinders are connected by channels to the built-in set of heat exchangers. A more detailed description of the gamma-type Stirling engine's application, as well as results of its operation modeling is presented in [55–64]. Starting this type of engine can be proceeded with smaller heat input, compared to the alpha and beta designs. Schemes of alfa, beta and gamma Stirling engines are presented in Figure 6.

**Figure 6.** Alpha, beta and gamma configuration of a Stirling engine. (**A**) alpha configuration with crank drive; (**B**) beta configuration with crank drive; (**C**) gamma configuration with crank drive; (**D**) beta configuration with a rhombic drive replacing the crank drive; (**E**) alpha configuration with Ross yoke instead of a crank drive [65].

#### *2.4. Stirling Engine Applications*

At the beginning of the 19th century, as a result of the rapid development of internal combustion engines and electrical machines, the further development of Stirling engines was severely impeded. However, due to the high thermal efficiency, quiet operation and the ability of Stirling engines to use multiple fuels, it meets today's requirements related to energy efficiency and environmental protection. Cogeneration units based on a Stirling engine are considered to be one of the best among the low power range electricity generation units.

Stirling engines are built in a fairly large power range. These units can have a power of a few watts to over 1 MW, but the most popular are those for single kW. Stirling engines are a viable alternative to currently used heat engines. The main barrier to the development of this technology is the presence on the market of competitive solutions in the form of well-developed classic technologies.

Microgen is a leader in the production of small Stirling engines. It offers engines with a capacity of several kilowatts. An example of a Stirling engine manufactured by Microgen is shown in Figure 7.

**Figure 7.** View on the Stirling engine made by Microgen [66,67].

Stirling engines are used in micro-cogeneration mainly due to the nature of their construction, i.e., their operation requires an upper and lower heat source [67–75]. However, the source from which this heat comes is not significant. This means that these engines can operate with virtually any fuel. The most popular microcogeneration systems found today are those based on gas fuel.

An example of such systems based on gas fuel are the systems of the German company Viessmann. These systems are known under trade names Vitotwin 350-F and Vitotwin 300-W. Their view is shown in Figure 8.

**Figure 8.** Combined Heat and Power (CHP) system based on a Stirling engine made by Viessmann [76,77]. 1—peak gas burner, 2—stainless steel heat exchanger, 3—valve dosing air supplied to the burner, 4—ring gas burner to feed the Stirling engine, 5—Stirling engine, 6—control panel, 7—hot water buffer tank, 8—vessel for pressure equalization.

Viessmann cogeneration modules can be used in almost any facility, e.g., in residential houses, office buildings, industrial plants and local district heating networks.

The Vitotwin 300-W micro cogeneration system with an electrical output of 1 kW and a thermal output of 26 kW is a real alternative to conventional heating systems in singleand two-family houses. It works particularly efficiently with an annual gas consumption of at least 20,000 kWh and an electricity consumption of over 3000 kWh. As heat is constantly generated during operation, a combination with a heating water buffer cylinder is required. The storage tank can be installed in devices that only need 0.36 m<sup>2</sup> of floor space to be installed, such as the Vitotwin 350-F.

Compact micro-cogeneration systems with an integrated condensing boiler can be a self-sufficient source of heat and electricity for a household. The Stirling engine in the Vitotwin can work with power modulation in the range of 0.3 to 1 kW of electrical power and requires virtually no maintenance. The electricity generated this way covers the basic demand of the building, correspondingly reducing the consumption of electricity from the power grid and contributing to savings.

Increased demand for electricity at home can be signaled to the micro-cogeneration system using the function of producing electricity on demand. This function is activated by the timer or by a button on the maintenance-free remote control or by means of a wireless socket. This way, electricity can be generated during greater demand for it, e.g., during washing and cooking. Lower the consumption of electricity from the grid will be, the more electricity will be produced by the micro-cogeneration device. An exemplary installation diagram is shown in Figure 9.

**Figure 9.** An exemplary installation diagram with CHP system Vitotwin [77]. 1—Vitotwnin 300-W microcogeneration unit, 2—heating water buffer cylinder with built-in domestic hot water heater-Vitocall 340-M with a capacity of 400 L, 3—heating installation, 4—wireless remote control module, 5—monitoring device-remote control, 6—bidirectional meter, 7—meter of produced electricity (installed in the Vitotwin system), 8—home electrical network, 9—external power grid.

Thanks to the aforementioned advantage of the Stirling engine, i.e., the possibility of cooperation with any heat source, microcogeneration systems powered by solid fuel were created. Such a system was implemented by Okofen, which offered a Pellematic biomass boiler with an integrated Stirling engine on the market. The view of such a set is shown in Figure 10. The Stirling engine is located partially outside, while the engine head is located inside the biomass boiler.

**Figure 10.** View of the Okofen biomass boiler coupled with a Microgen Stirling engine [77,78].

A major problem in microcogeneration systems with a Stirling engine is the transfer of heat generated in the combustion process to the engine head. Manufacturers of engines as well as boilers and burners are still carrying out optimization work in order to better receive heat by the Stirling engine head. Various methods are used, such as ribbing. This problem does not arise in the case of gaseous fuel systems, since the configuration of the burner can be adapted to the shape of the head of the engine. Stirling engine heads adapted to receive heat from solid fuel systems are intensively developed by means of advanced numerical analyses. The example of the concept Stirling engine with an additional heat exchanger located at an angle of 45◦, designed specifically for the system analyzed by the authors [79,80] is shown in Figure 11. The manufactured engine based on this design is shown in Figure 12.

**Figure 11.** Visualization of a Stirling engine with an additional heat exchanger attached to work with a solid fuel boiler [79,80].

**Figure 12.** Manufactured Stirling engine with an additional heat exchanger attached for operation with a solid fuel boiler [79,80].

The views on the heat exchanger which was designed specifically for this system, are presented in Figures 13 and 14.

**Figure 13.** Side view of a heat exchanger adapted to operate with a Stirling engine and a solid fuel system [80].

**Figure 14.** A view of a specially designed exchanger for the operation of a Stirling engine with a solid fuel system [80].

A scheme of the micro-cogeneration system with the previously presented heat exchanger collecting heat from the exhaust gases and transmitting it to the Stirling engine head is shown in Figure 15. Sample temperatures at characteristic points of the system are plotted. The heat exchanger of the Stirling engine located at an angle of 45◦ in the combustion chamber of the boiler can be also noticed.

**Figure 15.** View of the cogeneration system with a Stirling engine with a visible heat exchanger [80].

Another example of a heat exchanger cooperating with a Stirling engine head is presented in Figure 16. This heat exchanger was designed at West Virginia University, Morgantown, USA [81,82]. The concept of this exchanger differs from the previous one mainly in shape. It features toroidal design, which is the most common design of Stirling engine heads.

**Figure 16.** View of the Stirling engine head designed for operation with a solid fuel boiler [77,81].

The aforementioned Microgen company, as a leading manufacturer of small Stirling engines in the world, has developed different types of heads dedicated to Stirling engines. However, their design is kept confidential and not widely published. The example of 1 kW engine head implemented by Microgen company is presented in Figure 17.

**Figure 17.** View of the Stirling engine head designed by Microgen [66,77].

Stirling engines can be used in many industries and domestic applications. First of all, they can be used as small cogeneration systems for the needs of domestic [83–85] customers and for industrial purposes [86–91]. Due to the development of renewable energy, especially wind and solar, Stirling engines have found application in solar power plants [92–99]. Another sector where they are used is the refrigeration sector [100–108]. They are an alternative to the commonly used compressor systems. The helium used as a working medium is safe in terms of toxicity and flammability.
