*4.5. Wankel Expanders*

Wankel expanders are positive displacement machines featuring rotational displacement movement. These machines are currently experimentally tested to be applied in small steam and ORC systems [191–196]. The principle of operation of the Wankel expander is similar to that of the Wankel engine, but unlike in the engine, the combustion of fueloxidant mixture is not proceeded in the expander. A Wankel expander use a triangular rotor which moves in an oval cross-section cylinder and the side edges of which are curveshaped. The cylinder is closed on both sides with covers in which inlet and outlet ports are placed. The crank shaft, which drives the rotor, is mounted on bearings embedded

in the side covers. The shaft is coupled to the rotor by means of a gear that synchronizes their mutual movement. The gear consists of a fixed rack, which is embedded in the side cover of the machine, and a ring rack assembled inside the rotor. Vane seals are placed in the tips of the triangular rotor. Seals limit gas leakage between the working chambers and separate the working chambers from each other. Seals are also placed on the rotor faces to limit gas leakage between the piston and side covers. The cross-section through the Wankel expander, with a description of the most important components of this expander, is presented in Figure 40a. The machine has two inlet and two outlet ports, thanks to which the gas can be expanded in two working chambers at the same time. The working fluid is supplied to the machine through two inlet ports (see Figure 40a) and the working fluid pressure exerted on the rotor causes its motion. Dosing of the working fluid to the working chambers is proceeded via the inlet and outlet valves which opening is controlled by timing belt driven by the rotating crank shaft (see, Figure 40b).

**Figure 40.** Design and assembly details of a Wankel expander: (**a**) cross section of Wankel expander; (**b**) a general view on Wankel expander [192].

Wankel expanders have a number of advantages when compared to the other positive displacement and turbine expanders. The main advantages of these machines are high power-to-weight ratio, compact design and small external dimensions, lack of reciprocating parts, high rotational speeds, lack of vibrations generated during operation and a small number of moving parts. The main disadvantages of Wankel expanders include piston face seals issues (piston face seals are stressed by temperature variation during the machine operation), piston apex seals issues (piston apex seals are receiving significant loads related to the difference in gas temperature and pressure in adjacent working chambers). At low rotational speeds or low expander load, it is possible that the seal does not fully adhere to the cylinder surface, which may result in an increase of internal gas leakages between adjacent working chambers.

Early works on the possibility of using a Wankel machine as a steam expander were started in 1970s [193–195]. In [195] the results of experimental tests carried out on a Wankel expander using steam as a working fluid were presented. During the tests the steam pressure at the inlet of the expander was varied between 2.76 and 6.5 MPa while the steam temperature was varied between 231 and 410 ◦C. For these experimental conditions, the obtained expander power was ranging between 12 and 17 kW and the rotational speed was ranging between 2196 and 2578 rpm. Further works on the application of these machines as steam expanders were carried out in the 1990s [193–195] and are continued currently. In [192] the results of experimental tests of a prototype of Wankel expander designed for steam expansion are reported. The authors developed an expander prototype using parts of a standard Wankel engine (i.e., bearings, shaft and seals were used) and a new specially designed cylinder that was adapted to the supply system consisting of control valves in order to increase expander compression. The view of this prototype is presented

in Figure 40b. The valves are controlled by means of a mechanical system based on a timing belt driven from the main expander shaft.

## *4.6. Gerotor Expanders*

The other type of volumetric expanders that are used in prototypes of small ORC systems are gerotor expanders [197–199]. The design of this type of volumetric expander is similar to the design of a gear pump. The basic components of gerotor expander are a cylinder and two rotors—internal and external. The internal rotor is assembled on the shaft. The cylinder is closed on both sides by side covers with inlet and outlet ports. The inner rotor is placed eccentrically to the outer rotor. Figure 41 shows the view of the components of the gerotor expander.

**Figure 41.** View of the components of the gerotor expander [199].

Gerotor expanders were experimentally tested for their applicability in micro-power ORC systems. In [197] the results of tests on an expander featuring a power output of 1 kW designed for application in the ORC system using the solar heat were presented. The tests were carried out with the use of the R134a. The pressure of the working fluid at the inlet to the expander was 3.28 MPa, while inlet temperatures were varied between 80 and 100 ◦C. The working fluid pressure at the outlet of the expander was 1.64 MPa and the rotational speed of the expander was 3000 rpm. For these experimental conditions, expander power output was ranging between 0.2 and 1 kW and efficiency was ranging between 35 and 75%. The optimal expansion ratio was found between 3.0 and 4.0. In [198] the results of experimental tests of three gerotor expanders, which were characterized by different geometrical parameters were presented. These tests were carried out using the ORC system utilizing R123 as a working fluid. The tests were carried out for various parameters of the working fluid at the inlet and outlet of the expander. The working fluid pressure at the inlet to the expanders was varied between 412 and 1878 kPa, the working fluid pressure at the outlet of the expanders was varied between 139 and 331 kPa, the working fluid temperature at the inlet to the expanders was varied between 84 and 160 ◦C, while the working fluid temperature at the outlet was varying between 61 and 129 ◦C. For these experimental conditions, the power output of these expanders was ranging between 0.28 and 2.07 kW and the achieved efficiency was ranging between 59 and 85%. It was also indicated that, compared to other positive displacement machines, gerotor expanders are characterized by a lower internal friction.

## **5. Fuel Cells**

In 1839, British physicist William R. Grove demonstrated that an electrochemical reaction of combining hydrogen with oxygen produces an electric current [200–204]. Such a cell has no moving parts, works noiselessly, and its only waste substance is water. However, fuel cells based on this phenomenon were merely a laboratory curiosity for over a century. It was not until the sixties of the last century that NASA started to install light and compact (though expensive) versions in spacecraft to supply them with electricity. Today, this technology, which is promising, ecologically clean, efficient and silent, is being used in many new earthly applications, including powering mobile phones, notebook computers, homes and apartments and electric car engines. Chemical energy is directly converted into electricity in a fuel cell. It is a cell in which the fuel—hydrogen in a pure state or in a mixture with other gases—is fed continuously to the anode, and the oxidant—pure oxygen or a mixture (air)—is fed continuously to the cathode. Electrochemical processes are accompanied by the flow of an electron from the anode to the cathode. The closure of the circuit is carried out by ions that are transferred through the electrolyte. As a result of the electrochemical reaction of hydrogen and oxygen, electricity, water and heat are generated. Reagents are fed continuously to the fuel cell and theoretically it will not discharge; in fact, degradation or component failure will limit the life of any fuel cell.

Most fuel cells use hydrogen to produce electricity and heat [205–209]. Nevertheless, high-temperature fuel cells can run on natural gas due to the possibility of using the socalled internal reforming. The electrical efficiency of modern fuel cells is ca. 40–60 percent.

Different types of fuel cells are developed and are generally classified according to the type of electrolyte used, as it determines the operating temperature of the system and the type of fuel that can be used. The comparison of the different types of fuel cells is presented in Table 1.


**Table 1.** Comparison of different types of fuel cells [117].

Fuel cells are used both in small domestic power and heat generating units or auxiliary power sources with a capacity of several dozen kilowatts, as well as in large power plants with a capacity of several megawatts. Small systems with a power of 1–10 kW with fuel cells are able to provide electricity and heat to residential houses, offices and public buildings.

Another advantage is the design based on a modular system, which allows for relatively quick and easy construction of the installation and its possible expansion. Fuel cells are characterized by a high power yield per unit volume of fuel, and at the same time the process of direct conversion of fuel chemical energy into electricity takes place without the emission of toxic components and while maintaining high efficiency of fuel energy use. If the waste heat from the cell installation is used in combined CHP systems, the total energy efficiency may increase even up to 95% [210–212]. Waste heat can be used for heating, domestic hot water heating, cooling or air conditioning. Hybrid fuel cell installations connected to the gas turbine cycle achieve efficiency of 70% and more. The dynamic development of fuel cells in recent years means that they are more and more often alternative sources of electricity and heat.

There are many types of fuel cells. They differ mainly in the type of electrolyte, and thus in the operating temperature (see, Table 1). This indirectly also affects the power density that can be taken from the surface of the fuel cell. A Polymer Electrolyte Membrane Fuel Cell (PEMFC) and Solid Oxide Fuel Cell (SOFC) are the most popular in small- and micro-generation. PEM fuel cells belong to the group of low temperature fuel cells. Their working temperature does not exceed 100 ◦C. Their power ranges from a few watts to hundreds of kilowatts. The unquestionable advantage of the operation of these fuel cells is a very quick response to load changes. The disadvantage, however, is the need to use clean fuel due to the platinum catalyst used in this type of fuel cell. An example of a small cogeneration source based on a PEM cell is shown in the Figure 42.

**Figure 42.** View of the small CHP system based on a Polymer Electrolyte Membrane Fuel Cell (PEM) fuel cell [213].

The system consists of a PEM fuel cell, a methane steam reformer, an inverter, a pump system and a heat exchanger. It produces 36 kW of electricity and 50 kW of heat. The system has been built in a container which is divided into mechanical and electrical parts. In the mechanical part there is a reformer and a fuel cell, while in the electrical part there is an inverter and automation and control systems. The view of the container can be seen in the Figure 43.

**Figure 43.** View of the container with 36 kW PEM fuel cell system [213,214].

The most important part of the system is the fuel cell. It is a cell produced by the Canadian company Ballard with a nominal power of 36 kW and a maximum power of 80 kW. The cell is connected with a fan in order to supply the appropriate amount of oxidant to the reaction process. The fan has high power in relation to the cell's power (3 kW), which is almost 10%. Figure 44 shows a fuel cell (four stacks at the top) and a fan (below the fuel cell at the bottom).

**Figure 44.** PEM fuel cell with the fan delivering air as an oxidant [213,214].

Another type of fuel cell used in small and micro cogeneration is an SOFC fuel cell. The systems based on this technology offer higher electrical efficiency than the systems based on PEM technology and are especially focused on the continuous operation mode. Contrary to what it was said in the case of PEM cells, which show grea<sup>t</sup> keeping up with the demand. In the case of SOFC fuel cells, the surplus electricity is sent to the grid or accumulated if the system is equipped with a battery.

As SOFC fuel cells are one of the most popular types of fuel cells, there have been many studies and publications on combining these fuel cells into systems for the simultaneous generation of electricity and heat. Basic information on such systems can be found in [215–220].

An example of micro cogeneration system based on an SOFC fuel cell is presented in Figure 45.

**Figure 45.** Micro cogeneration system based on a Solid Oxide Fuel Cell (SOFC) fuel cell made by Vaillant [164,221].

Due to the high operating temperature (800–1000 ◦C), SOFC fuel cells can also be combined into systems with other energy sources, such as gas turbines [222–229] and burners [230–235]. An example of an SOFC fuel cell coupled to a gas turbine can be seen in Figure 46, while an SOFC fuel cell coupled to an additional heat source in Figure 47.

**Figure 46.** Micro cogeneration system with a flame-assisted SOFC fuel cell [234,235].

**Figure 47.** Cogeneration system with an SOFC fuel cell and gas turbine (the so-called hybrid cycle) [229].

The use of the small and microcogeneration systems based on fuel cells in countries where the energy sector is characterized by low CO2 emissions or is largely based on renewable resources will not always bring the expected benefits. Sometimes it can even contribute to the deterioration of the current condition. In the above situation, it is necessary to carry out a detailed profitability analysis for various possible operating modes of the system:


For example, in Scandinavian countries, due to the significant share of renewable energy sources in the energy sector and thus the low emission of harmful compounds, the only economically viable solution is to follow the small and microcogeneration system with instantaneous heat demand. Electricity is a by-product of this operating mode of the system.
