**1. Introduction**

For several decades the continuous energy consumption growth has been observed globally. This trend is mainly caused by the growing energy needs of an increasing world population, life quality improvement and technological development. The social access to different energy receivers (e.g., household appliances, cars, electronic devices, etc.) and their availability is nowadays easier than before, which directly translates into increasing consumption of fuels, heat, electricity and chill. Research on the energy consumption forecasting and modelling its variations is proceeding by different agencies and researchers around the world [1]. Reported data show [1] that the global primary energy consumption (i.e., the energy contained in fuels and renewable energy sources which is then converted into electricity, heating and sanitary heat and chill) in 2020 was ca. 1.58 × 10<sup>8</sup> GWh. The forecast for 2050 [1] indicates that the global primary energy consumption will probably increase nearly by 50% up to ca. 2.64 × 10<sup>8</sup> GWh. It is expected that ca. 28% of this energy demand will be covered by renewable energy sources, 27% by petroleum products and other liquid fuels (including biofuels), 22% by natural gas, 29% by coal and 4% by nuclear energy. Reliable and highly efficient energy conversion devices and systems (additionally characterized by low emission of harmful substances into the environment) have to be applied to meet this constantly growing energy demand and at the same time fulfil the strict regulations related to the natural environment protection. Therefore, the research, design and optimization activities related to modern energy conversion systems should be focused on limiting the fossil fuels consumption and increasing the use of alternative energy sources or clean fuels (e.g., natural gas). Energy conversion systems can be classified by different criteria (e.g., by the operating principle, design, cost, etc.). One of the commonly applied classification criteria is the system power output. By this criterion, systems can be classified into large-power (1.5 MW and more), medium-power (500 kW–1.5 MW), smallpower (15 kW–500 kW) and micro power (up to 15 kW). Large- and medium-power energy conversion systems (such as e.g., large steam power plants) are usually highly efficient

and used for industrial energy generation (they are supplying cities, regions or countries). Small- and micro-power systems are mainly used in distributed energy systems or by individual prosumers. Currently, different research works are carried out on the development of modern energy conversion technologies. In the field of large-power energy conversion technologies new solutions (such as the International Thermonuclear Experimental Reactor, i.e., ITER [2]) are investigated. In addition to these emerging technologies, research is still proceeding on the design optimization and improving the conversion efficiency of traditionally used units (e.g., steam power plants [3] and combined cycle gas turbine, i.e., CCGT units [4]). In addition to activities aimed at large-power systems, important research works are proceeded on small- and micro-power units dedicated for application in distributed energy systems. For many years, one of the visible development trends in modern power sector has been pursuing the diversification of the energy systems [5–7] by supporting activities related to the implementation of energy clusters [8] and small energy conversion units. Therefore, much attention is now paid to research and development works on small and micro-power systems which can be used by individual energy recipients (e.g., apartments, houses, shops or small industrial plants) for covering their own energy needs. Nowadays special focus is paid on cogeneration systems. Cogeneration, i.e., Combined Heat and Power (CHP), is an important tool for achieving significant fuel savings and reduction of CO2 emissions. Compared to separate fossil fuel-based electricity and heat generation, fuel savings and corresponding CO2 reductions will often be in the order of 30%. The CO2 reductions can sometimes be higher as a fuel conversion, e.g., from coal or oil to natural gas or biomass, can be taken into account. These benefits have been recognized by the European Commission and several initiatives have been taken to increase the use of cogeneration in the generation of heat and electricity.

Cogeneration is commonly used in large generating units-combined heat and power plants. However, there is a noticeable trend towards the use of cogeneration in smaller systems, especially those designed for local and distributed applications. Currently, there are several technologies used in micro-cogeneration such as small gas turbines, small steam turbines, Stirling engines, organic Rankine cycle systems (ORC systems) and fuel cells. Technological progress [9–12], as well as the general trend towards smaller generating units, resulted in an increased interest in small cogeneration units, hoping that they would be able to efficiently supply electricity and heat to individual facilities [13,14]. Small heat and power plants are usually based on internal combustion gas engines and are fully adapted to the needs of customers. They are characterized by high efficiency, thus providing cheaper energy, and they also allow for long-term cost planning. It should also be noted that such systems meet the strictest European environmental standards. The EU Cogeneration Directive defines micro-cogeneration as a unit featuring a maximum power of less than 50 kWe, while in Germany micro-cogeneration systems are treated as those that feature a power below 15 kWe. This is due to the following reasons: these systems are clearly intended for use in single family homes, apartment buildings, small businesses or hotels [15,16]. In the following part of this paper modern small and microcogeneration systems are reviewed, with special focus on the technologies utilizing energy sources of high-quality. The features and operating parameters of these systems are discussed together with their possible applications.

## **2. Stirling Engines**

There has recently been an interest in using Stirling engines as electricity generators in domestic CHP systems. Many households, especially in Europe, use natural gas for heating. The natural gas burner is easily customizable to provide a heat source for a small 1 to 10 kW Stirling engine, sized to meet the electricity needs of a typical household or a small business unit. The engine then forms a part of a system in which the waste heat that first heats the Stirling cylinder head is then directed to domestic hot water distribution system and/or central heating system. Several commercial systems based on this type of engine are already commercially available.
