Returns the specific entropy.
The function is based on Modelica.Media.Water.WaterIF97_base.specificEntropy. However, for the typical range of temperatures and pressures encountered in building and district energy applications, an invertible polynomial fit sufficies. This implementation is therefore a polynomial fit (quadratic in pressure, linear in temperature) of the IF97 formulation s(p,T) in the ranges of 100°C ≤ T ≤ 160°C and 100 kPa ≤ p ≤ 550 kPa. The fit is scaled by the dataset's mean and standard deviation values to improve conditioning. The largest error of this approximation is 0.047 kJ/kg-K (0.70%), which occurs at 160°C and 550 kPa. The root mean square error (RMSE) is 12.56 J/kg-K.