Abstract
Simulations on CO2 circuits require many hundred-thousands of calls to different thermodynamic functions. The CPU-time used for calculating these can reach almost 90% of total simulation time. To reduce the simulation time, a precompiled table of thermodynamic data replaces the use of full equation of state (EOS). The table is compiled from the reference EOS by Span and Wagner, which is considered to be very accurate but also causes high calculation times. The table is explicit in temperature and pressure, which has been regarded as efficient for steady state simulation purposes. Using this table of Gibb's derivates, high speed and good accuracy is achieved. For direct calls of relevant functions needed for simulation, this approach has proven to be 100-300 times faster (calculations per second). Deviation from the Span-Wagner EOS is typically in the range of 0.001% of absolute value. The Gibb table has also been tested in a real application, the in-house simulation software CSIM. The Gibb table implementation proved to be significantly faster, reducing simulation times from ~400 to 5 seconds (compared with the full Span-Wagner EOS). Simulation results were practically identical.
