*2.5. Summary*

The Stirling engine has the unquestionable advantage of being able to work with any heat source. It can be powered by heat obtained, for example, from the combustion of biomass or other solid fuel source. However, as mentioned in this article, there are technological problems, primarily in the process of efficient heat transfer from its source to the engine head. For this reason, among microcogeneration systems utilizing Stirling engines those fed by natural gas are much more popular. The challenge of better designing of solid fuel-based systems may be perspective. There are many areas of world (such as the countries of far Asia) where there is a shortage of gaseous fuel, while solid fuels are widely and cheaply available. Today's Stirling engines are refined and virtually maintenance-free units. They do not generate vibrations or noise.

The application of a Stirling engine as part of a small CHP system also has some disadvantages. The production of electricity is closely related to the production of heat, while the demand for electricity and the demand for heat often do not coincide. Therefore, there may be an overproduction of electricity or an insufficient amount of it. It is therefore necessary to verify contracts with electricity suppliers in terms of contracted power as well as to enable the opportunity for sale of excess, not consumed, electricity generated by the Stirling engine. This can be done through two-way electricity meters and appropriate contracts with the operator of the distribution grid. For example, Polish regulations allow electricity to be fed into the grid and treated as an energy storage. Unfortunately, for such a service there is a fee in the form of 20% of the delivered energy. This means that the excess of produced electricity can be "pumped" to the grid at any time and 80% of it can be taken back at any time. It should be noted that this regulation applies to installations featuring a maximum power of 10 kW (for larger installations the percentage increases up to 40%) and utilizing renewable energy sources. Thus, in order to take advantage of this possibility, the Stirling engine would have to be powered by heat obtained from biomass combustion.

Other disadvantages include, for example, long start-up times in the case of solid fuel systems and low flexibility in the case of load changes.
