This paper presents the results of analysis of energy and economic efficiency of the hierarchical gas-gas engine, with a note that a trigeneration system was analyzed, in which the production of electricity, heat and cold are combined. This solution significantly increases the energy efficiency of the gas and gas system compared to a system without cold production. The analysis includes a system comprising a compressor chiller which is driven by an electric motor in the system, as well as a system applying the mechanical work that is carried out via a rotating shaft of rotor-based machines, i.e., a gas turbine and a turboexpander. The comfort of the regulation of the refrigerating power rather promotes the use of a solution including an electric motor. Analysis contains also a schematic diagram of the system with a absorption chiller, which is driven by low-temperature enthalpy of exhaust gases extracted from a hierarchical gas-gas engine. Application of turboexpander with heat regeneration in the trigeneration system is also analyzed. Based on the multi-variant economic and thermodynamic calculations, the most favorable system variant was determined using, among others, the specific cost of cold production.

An analysis is performed with regards to technologically outdated heating plants operating in many areas where fossil fuels such as coal and gas are utilized, in order to consider the alternatives of their modernization. By application of a chart using a variety of alternatives, the economic feasibility of executing two types of modernization of heating plants are explored: a single-fuel gas–steam CHP plant and a coal-fired heating plant to a coal-fired CHP plant with a condensing turbine. This study demonstrates how the selection of modernization technology is affected, in terms of profitability, by the value and variability in time of the price relationships between energy carriers, rapidly growing charges related to CO₂ emission allowances, and costs depending on other pollutant emissions that originate from the operation of electricity and heat sources powered by fossil fuels. In both technical cases of modernization, lower prices of energy carriers coupled with CO₂ emissions allowances lead to higher prices of electricity that can be sold as additional products following this modernization, and consequently, the specific cost of heat production in the repowered heat sources is lowered. The calculations were performed by the application of models of heating plant modernization applying continuous time notations, which offer the determination of the most suitable time of initiation of this modernization. Such relationships would be difficult to describe in the case of the use of traditional discrete models. In the case of a simultaneous increase in the prices of all main factors affecting the cost of heat generation, such as the price of gas, electricity and CO₂ emissions, the fastest modernization of the heating plant to single-fuel gas–steam CHP provides the possibility of the best economic performance.