Abstract
Despite the increasing interest in organic Rankine cycle (ORC) systems and the large number of cycle models proposed in the literature, charge-based ORC models are still almost absent. In this paper, a detailed overall ORC simulation model is presented based on two solution strategies: condenser subcooling and total working fluid charge of the system. The latter allows the subcooling level to be predicted rather than specified as an input. The overall cycle model is composed of independent models for pump, expander, line sets, liquid receiver and heat exchangers. Empirical and semi-empirical models are adopted for the pump and expander, respectively. A generalized steady-state moving boundary method is used to model the heat exchangers. The line sets and liquid receiver are used to better estimate the total charge of the system and pressure drops. Finally, the individual components are connected to form a cycle model in an object-oriented fashion. The solution algorithm includes a preconditioner to guess reasonable values for the evaporating and condensing temperatures and a main cycle solver loop which drives to zero a set of residuals to ensure the convergence of the solution. The model has been developed in the Python programming language. A thorough validation is then carried out against experimental data obtained from two test setups having different nominal size, working fluids and individual components: (i) a regenerative ORC with a 5 kW scroll expander and an oil flooding loop; (ii) a regenerative ORC with a 11 kW single-screw expander. The computer code is made available through open-source dissemination.
Highlights
The investigation of organic Rankine cycles (ORCs) both numerically and experimentally [1,2,3,4,5]has seen a rapid growth in the last years due to increasing concerns about the environment and theEarth’s limited fossil fuel resources
Numerical simulations play an essential role in the analysis of ORC systems because of the variety of boundary conditions imposed by heat sources, cold sinks, and different applications
The working fluid mass flow rate is imposed by the pump; the evaporating temperature is related to the pump outlet pressure and the pressure drop between pump and the evaporator; the superheating is imposed by the evaporator for a fixed expander rotational speed; the condensing pressure is imposed by the condenser; the subcooling level at the condenser outlet can be specified as an input or predicted by introducing a refrigerant charge model
Summary
The investigation of organic Rankine cycles (ORCs) both numerically and experimentally [1,2,3,4,5]. It is worth mentioning that one of the first detailed vapor compression cycle models was published by Hiller and Glicksman in 1976 [18] This comprehensive model based on several subroutines included a moving boundary model for the heat exchangers, single and two-phase pressure drops, oil entrainment in suction and discharge risers, liquid line flashing, moisture removal on the evaporator side, refrigerant-oil solubility, oil circulation, compressor load effect on motor efficiency, motor cooling effect on compressor, compressor valve dynamics and friction losses. The option to solve based on the subcooling level, which represents the more common modeling approach, is included Such a model is useful for cycle performance prediction of real units and as a starting point for further fault detection and diagnostic analyses applied to ORCs. the simulation code is made available open-source. Additional results are proposed regarding the charge-based solver
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