Thermodynamic and economical analysis of the ORC module application to an existing combined heat and power unit with the backpressure turbine

Abstract

This paper presents the results of the thermodynamic and economic analyses of two variants of integration of a CHP unit with ORC modules. The first variant consists in an integration of a CHP unit with a single ORC module. The module is installed on one of the steam outlets from the asymmetric part of a back-pressure steam turbine. The second variant assumes installation of modules on two outlets of this part of the steam turbine. An argument for the integration of a unit with ORC modules is the ability to obtain in the summer period additional electric power based on the use of heat, which in the period of lower demand for heat is dissipated in the environment. For these two variants, the thermodynamic analysis was carried out, which allowed to determine the electrical power that can be generated by ORC modules. Analyses were performed for various low-temperature boiling fluids. Economic analyses were also performed, which allowed to determine the net present value and the internal rate of return for the two variants of integration. Assumptions required to perform economic analyzes were adopted. As a part of the economic analysis, sensitivity analysis of the selected economic effectiveness indicators on the relative change of the investment cost and electricity prices was performed. The influence of changes of working time of the integrated unit within a year and the discount rate was also examined. The obtained results indicate a high potential of the proposed integrations. Supplying of ORC modules by steam leaving the backpressure steam turbine can allow for economically justified extension of the period of operation of many CHP units. Appropriate modernizations in the case of switched off post-industrial CHP units can contribute to the restoration of their ability to generate profit.