Zhou et al. [16] designed the solar collector of a SSUPP case SC1 and two SUPP cases HC1 and HC2 based on the 5 MW SSUPP proposed in Ottawa [2]. A theoretical model is developed based on essential definition of buoyancy to study the performance of SSUPP by regarding the air as a compressible fluid, as compared to conventional SUPPs. The parameters for SC1, HC1 and HC2 using essential expression of pressure potential are respectively compared with the expressions containing no integral for SC1, HC1 and HC2. The sloped collector height and width changes are carefully chosen to produce an ideal condition under which the airflow is gradually accelerated from the collector inlet to outlet. Results show that the expression containing no integral for the pressure potential of the conventional SUPP ,was developed by Kröger and Blaine [17], is accurate for conventional SUPP based on a compressible fluid model, and that the expression containing no integral of pressure potential for SSUPP based on an incompressible fluid model , was developed by Bilgen and Rheault [2], is not accurate for predicting the driving force of SSUPP, because it is neglecting the change of the atmosphere density with heights, and the change of difference of the atmospheric density and the density of the air current inside the short chimney with heights