**3. Microturbines**

Research and implementation activities on microturbines are currently proceeding in different scientific units and companies. Microturbines are investigated to be applied as expansion machines in many technical applications, such as power generating systems and CHPs [109], unmanned airplanes [110] and hybrid cars [111]. The microturbine implementation was possible thanks to recent progress in different fields of science (especially materials science, thermodynamics, fluid mechanics and computer aided design). Advanced computer aided design (CAD) techniques and numerical flow modeling are widely used to design microturbines, and computer numerical control (CNC) machines are used for manufacturing microturbine parts. By the direction of the working fluid flow through the microturbine, they can be classified into radial and axial machines and by the type of the applied working fluid into gas microturbines and steam microturbines [112]. A characteristic feature of microturbines is their high rotational speed, ranging from several dozen to several hundred thousand revolutions per minute [113]. Therefore, very precise tools must be applied to manufacture microturbines. The microturbine shaft is coupled with the shaft of the generator in a different way than in the case of standard large-power turbines (i.e., small-scale, specially designed high-speed generators are applied and electronic systems are used to convert the generated current into a current of frequency and voltage that can be transferred directly to the grid). What is more, due to high rotational speeds of the microturbine shaft and large heat load, magnetic or foil bearings are often applied instead of classically used slide bearings [113]. Compared to volumetric expanders, microturbines are featuring smaller dimensions, a smaller number of moving parts, lower friction losses and higher efficiency [114]. They are also lighter. However, working fluid flow through microturbine and machine cost are much higher.

## *3.1. Gas Microturbines*

In recent years, gas microturbines have gained a reputation as a refined technology and are boldly entering a variety of municipal and industrial facilities where reliable, independent electricity and/or heat generation at competitive prices is required. The leaders of this technology are mainly British [115,116], Italian [117] and American [118–120] companies. Currently, their offer includes devices with a power of 50-several hundred kW, and in the future, it is planned to gradually expand this range. Recently, microturbines have had a number of original applications. One of the largest sports and recreation centers in London used a micro-turbine to generate 80 kW of electricity and 150 kW of heat for its own facilities and equipment, including a swimming pool, sports hall and other rooms using the Bowman Power TG80CG gas microturbine [115]. This original 80% efficient power plant has been supplemented with a conventional boiler to cover peak heat loads. The microturbine itself is only a slightly more complex design than a typical low power turbo generator. What distinguishes this device from classic machines of this type is a high-speed, four-pole self-excited generator and dedicated software controlling the operation of the unit. The alternator was made of rare earth metals of extremely high density, which allowed for such a significant reduction of elements that the turbine and generator rotors were placed on one shaft, thus eliminating the troublesome mechanical transmission. This single shaft assembly rotates at over 100,000 rpm producing an output voltage with a frequency in the range of 1000 to 3000 Hz. A special power electronic converter converts them into voltage with a mains frequency of 50 or 60 Hz and an ideal sinusoidal shape and value. Thus, the unit becomes a reliable power source with a quality that meets the most stringent requirements [121]. Manufacturers produce microturbines in two main types: with and without exhaust gas heat recovery. In microturbines without heat recovery, a compressed mixture of natural gas and air is burnt at constant pressure, and the resulting hot exhaust gas stream expands in the gas turbine, driving the generator. These systems follow a simple thermodynamic cycle and are cheaper and more reliable than microturbines with heat recovery. The latter devices have an exchanger in which part of the heat contained in the turbine exhaust gas stream is transferred to the inlet air. As a result, microturbines with heat recovery are characterized by higher efficiency (fuel savings up to 30–40%), comparable to diesel-based combined heat and power plants. In some implementations the exhaust/air heat exchanger has been replaced with a hot water boiler. In other applications, the microturbine exhaust stream without heat recovery is routed to the furnace, eliminating traditional gas burners. Currently, microturbines with a capacity of 25–250 kW and an electricity generation efficiency of 30% are offered in the world. With combined production of electricity and heat, this ratio can reach 80%. Gas microturbines have many advantages and offer a number of advantages, especially when used in small- and micro-power distributed energy. A small number of rotating and moving parts, compact design, small dimensions and weight-facilitate assembly and maintenance. At the same time, very low emission of pollutants and noise level allow their use in virtually every facility. Microturbines can be supplied by different types of fuel, e.g., dairy cattle biogas [122], syngas [123] and biofuels [124].

In [125] authors presented very small model of gas microturbine, featuring only 500 W of power. The turbine is called the Ultra Micro Gas Turbine (UMGT). The test bench of this turbine is presented in Figure 18.

**Figure 18.** The test bench of the Ultra Micro Gas Turbine (UMGT) power generator [125].

The elements of the tested gas turbine are shown in the Figure 19.

**Figure 19.** Elements of the first integrated test rig: (**a**) compressor, (**b**) turbine, (**c**) rotor shaft with compressor and turbine, (**d**) radial-thrust integral static air bearing, (**e**) graphite hot-bulb igniter and (**f**) annular-type combustor with 12 nozzles [125].

Furthermore, the technology of ultra-small gas turbines is presented in [126–130]. Ansaldo Energia [131–133] and Capstone [134] are one of the world's leading manufacturers of micro gas turbines. A view of the Capstone 30 kW gas turbine is shown in Figure 20.

**Figure 20.** View of the Capstone 30 kW gas turbine [134].

Ansaldo Energia is a global manufacturer in a distributed generation market with its AE-T100 Gas Microturbine, available in three different versions: natural gas-fired AE-T100NG, biogas-fired AE-T100B, and fed by heat from external combustion: An example ofEFGT technology is shown in Figure 21. The AE-T100 is producing 100 kWe of electrical power and about 200 kWth of thermal power. The efficiency of this system is up to 90%. A broad power range is achieved by good modularity of this systems and can spread by adding additional units.

**Figure 21.** View of the external combustion gas turbine schematic layout [135].

## *3.2. Steam Microturbines*

A characteristic feature of microturbines is their high durability and reliability, which results from their relatively simple design, as they have only one rotating element in the form of a shaft assembly with rotors and a generator [136–139]. High rotational speeds are also typical of this type of turbine machine, thanks to which, with small overall dimensions, the microturbines enable a high power output. However, high rotational speeds lead to complications in the design of the clutch connecting the turbine to the generator and bearings [140–144].

Many research centers around the world conduct research and development work on the continuous improvement of steam microturbine technology. Works on small steam turbines are carried out by, among others, The Institute of Fluid Flow Machinery of the Polish Academy of Sciences in Gda ´nsk and the Institute of Turbomachinery of the Łód´z University of Technology.

As part of the research tasks proceeded at Institute of Fluid Flow Machinery of the Polish Academy of Sciences, several alternative solutions for devices enabling the conversion of thermal energy into electricity were developed and tested. With the assumed power level and limitations resulting from the target place of operation of domestic conditions, steam microturbines turned out to be the optimal solution. Among the examined expansion devices, the most promising results were obtained for the variant of the four-stage radial microturbine and the single-stage radial microturbine. It was decided to couple the steam microturbine with the ORC system [145–147]. The working fluid in the ORC system is a low-boiling fluid. To drive the microturbine, it is firstly heated in a heat exchanger to the temperature at which the state changes from liquid to gas (evaporation takes place).

The gaseous medium at the appropriate pressure is fed to the microturbine blade system, causing its acceleration and then maintaining a constant rotational speed. The mechanical energy of the shaft rotation is then converted into electricity (by means of a generator) which, after appropriate preparation, can be used e.g., in a household. The lowboiling fluid used to drive the microturbine circulates in a closed system; after condensation, it flows through the pump and then it is reheated [148].

The four-stage microturbine developed at Institute of Fluid Flow Machinery of the Polish Academy of Sciences has two centripetal and two centrifugal stages. The shaft is supported by two radial-thrust gas bearings in which a low-boiling fluid is used as the lubricant. At the nominal rotational speed of approx. 24,000 rpm, the microturbine allows to obtain ca. 2 kW of electric power [148]. A cross-section through the turbine and the rotor disk is shown in Figure 22.

**Figure 22.** Cross-section and manufactured rotor disc of the four-stage radial microturbine made by Institute of Fluid Flow Machinery [148].

A single-stage microturbine has also been developed at the Institute of Fluid Flow Machinery (see, Figure 23). It is characterized by a centrifugal stage where the flow velocity is more than twice larger than the velocity of sound. It was built on the basis of the experience gained in the implementation of a four-stage microturbine. It uses the previously proven radial-thrust gas bearings, lubricated with a low-boiling fluid. The nominal rotational speed of this micro-turbine is ca. 30,000 rpm, which allows it to generate ca. 2.5 kW of electricity.

**Figure 23.** Radial, single-stage steam microturbine installed on the Institute of Fluid Flow Machinery test stand [148].

The developed steam microturbines were tested in the laboratory of Fluid Flow Machinery in conditions corresponding to their operation in the ORC system, using their own control system. Research has shown that prototype microturbine solutions have many advantages. Thanks to the use of high rotational speeds, machines with a compact structure and small dimensions were developed. Unlike other devices that enable the conversion of heat into electricity, steam microturbines are characterized by high durability and reliability because they do not have wear parts and parts that require periodic replacement or repair. Since the same low-boiling fluid is used in the flow system and the bearings, the risk of mixing the working medium, e.g., with oil, has been eliminated. The test results also confirmed the very low vibration level and quiet operation of the developed turbines. The mentioned advantages of microturbines mean that they can be successfully used in domestic ORC micro-cogeneration installations, as well as in other installations requiring a small and reliable device that enables electricity generation [148].

As mentioned earlier, work on small steam turbines is also carried out at the Institute of Turbomachinery of the Łód ´z University of Technology [136].

The experimental steam turbine with a nominal power of 50 kW was built on the basis of the Institute of Turbomachinery project. The turbine is powered by a steam generator that uses waste heat from a biogas combustion engine. Firstly, the following live steam parameters were considered: temperature equal to 613 K, pressure equal to 12 bar, while the mass flow rate was 0.075 kg/s. The turbine works in condensing mode. For these parameters, the isentropic drop in enthalpy is as high as 732 kJ/kg, which excludes a single-stage design, except for the Curtis two-ring (or three-ring) stage, operating in the range of very high Mach numbers. Therefore, after discussion, the live steam temperature was limited. Ultimately, the following turbine design parameters were established [136]:


The rotational speed was imposed due to the unavailability of a suitable gear.

A small backpressure turbine was also designed at Institute of Turbomachinery of the Łód ´z University of Technology. It generates 165 kW of power. The turbine was designed to cooperate with the existing technological installation; therefore, its operating parameters were strictly defined and were not subject to any discussion. These parameters were established as follows:


In this case, the isentropic enthalpy drop is only 119.7 kJ/kg, which greatly facilitates the adoption of the advantageous design solution. After preliminary calculations, it was assumed that the turbine would be implemented as a single action stage, powered on the entire circuit.

Experimental research on the application of a microturbine (featuring a maximum power of 1.9 kW) in domestic ORC CHP systems using ethanol as a working fluid was also proceeded at the Gda ´nsk University of Technology. The results of these experiments were reported in [149–152]. The experimental tests were proceeded for varied thermodynamic parameters of the working fluid at the inlet and at the outlet of the microturbine. The pressure at the inlet to the machine was varied between 0.36 and 0.6 MPa, while the working fluid flow was varied between 15 and 20 g/s. The maximum temperature of the working medium at the inlet to the machine was equal to 143 ◦C. For these experimental conditions, the obtained electric power of the ORC system was ranging between 0.66 and 0.76 kW, electrical efficiency was ranging between 6.40 and 6.65% and the total efficiency of the ORC CHP system was ranging between 22.53 and 23.54% [150].

There are also suppliers on the world market that offer microturbine technology. One of them is Spirax Sarco. This technology is described in more detail in [153]. An example of a Spirax Sarco steam microturbine in a container version is presented in Figure 24.

**Figure 24.** View of the Spirax Sarco steam microturbine [153].

Other companies that have in their portfolio steam microturbines are e.g., Siemens Dressel-Rand and General Electric.

#### **4. Volumetric Expanders (Vane, Lobe, Screw, Piston, Wankel, Gerotor)**

Volumetric expanders can be applied in small CHP steam and ORC systems as an alternative to the earlier described microturbines. The principle of operation of volumetric expander differs from that of turbine. In the case of volumetric machine, the working fluid expansion proceeds in a working chamber which volume is limited by the cylinder and the displacer. The operation of volumetric expander is cyclical and working chamber volume changes during machine operation. For this reason, gas expansion processes are proceeding periodically. Compared to microturbines, volumetric expanders are featuring simpler design and lower investment costs. What is more, they are also featuring lower rotational speeds, higher pressure drops that can be obtained in one stage, lower mass flows of the working fluid and the possibility of wet-gas expansion. In selected cases, it is possible to design and implement oil-free volumetric expanders.
