Abstract
This paper provides an overview of a one-dimensional modelling methodology for equipment and systems for heat to power conversion based on a staggered grid space discretization and implemented in the commercial software GT-SUITE®. Particular attention is given to a newly developed modelling procedure for twin-screw machines that is based on a chamber modelling approach and considers leakage paths between cells and with the casing. This methodology is then applied to a low-grade heat to power conversion system based on a Trilateral Flash Cycle (TFC) equipped with two parallel two-phase twin-screw expanders and a control valve upstream of the machines to adapt the fluid quality for an optimal expander operation. The standalone expander model is used to generate performance maps of the machine, which serve as inputs for the TFC system model. Parametric analyses are eventually carried out to assess the impact of several operating parameters of the TFC unit on the recovered power and cycle thermal efficiency. The study shows that the most influencing factors on the TFC system’s performance are the inlet temperature of the heat source and the expander speed. While the first depends on the topping industrial process, the expander speed can be used to optimize and control the TFC system operation also in transient or off-design operating conditions.
Highlights
The use of numerical simulations to support the design activities of engineers and researchers has been constantly increasing over the years
In the field of Computational Fluid Dynamics (CFD), modelling techniques for turbulent flows have evolved from empirical correlations to a direct solution of the Navier-Stokes equations (DNS)
A typical energy conversion system based on thermodynamic cycles includes, in its simplest form, two heat exchangers, to reject and adsorb thermal power to and from external sources; two machines, a pump to pressurize the working fluid and an expansion device to convert its thermal energy into mechanical power; tanks, to prevent pump cavitation and the working fluid thermal expansion during transients; and a series of pipes and valves to connect all the system components
Summary
The use of numerical simulations to support the design activities of engineers and researchers has been constantly increasing over the years. Designs 2019, 3, 41 of the physical structure due to the flow, as well as a change of flow topology due to the change of structure Another trend noticed with the increase of computing power is the greater complexity of the simulations. Notwithstanding the increase of cheap computational power, model developers and simulation engineers still have to balance the complexity, time, and cost of their calculations. For these reasons, the lower order models that have been developed in the past are still highly beneficial for today’s simulations. These integration issues are highly significant to ensure that the design specifics for the expander are the ones provided by the TFC loop
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