**6. Conclusions**

The article presents an overview of currently used and developed technologies for the production of electricity and heat in the so-called cogeneration (CHP) in small and micro scale. These technologies are becoming more and more popular and needed in relation to the development trend of distributed generation and the so-called virtual power plants. The technologies presented in the article are inter alia, Stirling engines, gas and steam microturbines, various types of volumetric expanders (vane, lobe, screw, piston, Wankel, gerotor), and fuel cells. There is no definite answer as to which of these technologies is the best. Each of them has its advantages and disadvantages and can be adapted to the specific conditions in which it has to operate. These conditions are, for example, the type and availability of fuel, the ability and speed of reaction to load changes, reliability, noise, generated power, environmental and social conditions, availability of service personnel and many others, which should be taken into account when selecting a given technology to meet needs.

Stirling engines are mature technology. Nevertheless, over the years, research centers have been working on their improvement. Microgen company, previously mentioned in the text, has developed an advanced technology that is commercially available. The company does not produce ready-made systems containing a Stirling engine, but supplies engines to system manufacturers. They are used in systems powered by both solid and gaseous fuels. Stirling engines are most often featuring the power output ranging from hundreds of watts to hundreds of kilowatts. In addition to applications in small and micro-cogeneration, Stirling engines can be used in solar energy systems and in the refrigeration industry. These engines are also applied in submarines. Works on their application in cars are ongoing.

The review of the volumetric expanders that are currently applied in small and micro cogeneration systems gave an outlook on their operating conditions and technical details. The following conclusions can be drawn on volumetric expanders basing on the reviewed literature.


Both gas and steam microturbines are mature technology dating back several dozen years. Research on microturbines is carried out in scientific units around the world, but more often they result from attempts to use ready-made microturbines from well-known manufacturers for use in systems with their participation. There are few research centers that deal with the design of microturbines alone. As a mature technology, microturbines are used mainly in small and industrial microcogeneration. This technology is not used in households, mainly due to the costs and noise generated by microturbines. Due to the nature of their construction, they are high-speed machines. In addition to the noise generated, this has an impact on problems in the use of generators (a gear or a high-speed generator is necessary). The power range of microturbines is in the range of hundreds of watts to several hundred kilowatts. Their efficiency is 20–30% for gas microturbines (electrical efficiency) and 60–70% for steam microturbines (internal efficiency). It should be noted that the gas microturbine can work as an independent device for the supply of fuel, and in the case of a steam microturbine it must be part of the system (e.g., the Rankine cycle) and then the system's electrical efficiency should be taken as an output parameter of the system, which is ca. 20%.

Fuel cells are a commercialized technology. The most common types of cells used in microcogeneration are PEM fuel cells and SOFC fuel cells. The first one is a fully mature technology and commercially produced in the power range from a few watts to hundreds of kilowatts. They are characterized by a quick start-up and low operating temperature, and hence high flexibility in terms of load change. The problem is the fuel that should be clean (CO free) due to the platinum catalytic converter. It requires the use of pure hydrogen, e.g., from the electrolysis process, or, if it comes from other processes, such as methane steam reforming—purifying it. As for SOFC fuel cells, they are also a

relatively mature technology. Unfortunately, the problem is the complicated production process that involves sintering components together. Scientific centers conduct research on the improvement of components (electrodes and electrolyte) in order to achieve better performance, but the finished devices are manufactured and sold, especially in Asian countries. The disadvantage is the high operating temperature, which is associated with a longer start-up and the need for more stable operation when it comes to changing loads. In practice, devices with a power of several hundred watts to hundreds of kilowatts are the most popular. The authors of this article believe that the long-observed trend towards decentralizing the production of electricity and heat will contribute to even more dynamic development of small and micro technologies in the CHP sector.

It is also important that most of the technologies presented here can be used to use waste heat from industrial plants. As a result of electricity and heat generation processes or other technological processes taking place in industrial plants, some energy is irretrievably lost to the environment. Especially in small and medium-sized industrial plants, energy and environmental awareness are at a low level. These plants were often built a long time ago, have old machinery and are located in energy-inefficient halls and buildings. The application of the systems presented here can contribute to the improvement of energy efficiency thanks to the recovery of waste heat. Thanks to their positive features and utilization of high-quality fuels, domestic micro CHP systems can possibly contribute to the significant reduction of the amount of pollutants emitted into the environment from standard heating systems. Standard furnaces that are often used for heating the houses during winter are often low-efficient and fed by low-quality fuels. Therefore, worse and worse air quality is being observed in many countries. Many of the currently applied furnaces can be successfully replaced with domestic CHP units based on the technologies described in this article. In this way, their application may have a positive influence on air quality.

**Author Contributions:** Conceptualization, M.W.; writing—original draft preparation, M.W. and P.K.; writing—review and editing, P.K., M.W. and K.B.; visualization, M.W. and P.K.; project administration, K.B.; funding acquisition, K.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank the Scientific Council of the Discipline of Environmental Engineering, Mining and Power Engineering and the Dean of the Faculty of Power and Aeronautical Engineering of the Warsaw University of Technology for their support. The publication is the result of an Internal Grant for employees of the Warsaw University of Technology in 2020.

**Conflicts of Interest:** The authors declare no conflict of interest.
