ENERGY EFFICIENCY OF COMBINED HEAT AND POWER SUPPLY IN THE CONTEXT OF INCREASING STEAM COMPRESSION HEAT GENERATION
Abstract
Abstract. This study addresses the pressing issue of improving the energy efficiency of combined heat supply systems with two-stage heat generation and expanding vapour-compression heat generation through the utilisation of subcooling energy from spent heat carrier in the district heating network, cold water of general-purpose supply, and exhaust gases. The objective of this research is to advance the previously proposed combined heat supply system by integrating the low-temperature energy potential of such sources as subcooled spent heat carrier from the network, general-purpose cold water, and exhaust gases. The system's development aligns with current trends in increasing the share of vapour-compression heat generation, driven by the need to enhance overall energy efficiency. To achieve this objective, the study conducts an analytical investigation into the conditions under which the energy efficiency of the system’s structural subsystems can be improved, depending on their operational parameters and thermal conditions during different periods of the heating season. This approach makes it possible to identify the most rational modes of integrating these low-temperature sources into the heat supply system under varying shares of vapour-compression heat generation. The research methodology is based on the further development of a structural-functional diagram of the system, supported by theoretical justification. As a result of the analysis, relationships have been derived for the actual coefficient of performance (COP) and the share of thermal power generated by the heat pump subsystem relative to the total thermal output of the system. These relationships are presented as functions of the operational parameters and temperature conditions of the structural subsystems throughout the heating season. The established dependencies enable a comprehensive assessment of the conditions necessary to improve energy efficiency and increase the share of generated heat, taking into account variations in the temperatures of low-temperature sources, as well as the ratios and distribution of water flows, including cold water used for municipal and industrial purposes. Based on a generalised formula for calculating the actual COP of the heat pump unit, it has been determined that this value significantly depends on the temperature of the spent heat carrier at the outlet of the condenser, prior to its entry into the heat-generating unit. The analysis of the share of thermal power produced by the heat pump subsystem has shown that, under rational values of the intermediate temperature of the spent heat carrier and with decreasing outdoor air temperature, it is possible to significantly increase the proportion of heat generated by the heat pump subsystem. The obtained analytical results provide a foundation for the engineering design of a combined heat supply system as a component of energy-saving technologies for buildings in both municipal and industrial sectors.
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