Using parabolic trough technology to collect solar thermal energy and preheat feedwater, is one of the most practical integration between solar and coal-fired power generation system. This study designs and compares two thermodynamic calculation methods related to solar aided power generation (SAPG) system. Method I simplifies oil/water heat exchanger and boiler as "black boxes", while Method II is a new calculation method in which the heat transfer processes in the boiler and oil/water heat exchanger are considered. The results show that if the extraction steam to be replaced is located before the inlet side of reheated steam, using Method I would bring much calculation error in the main parameters of solar aided power generation (SAPG) system. Therefore, the calculation of thermal-to-electricity efficiency, solar-to-electricity efficiency (SEE) and solar power generation would be incorrect. Besides, Method I could not optimize the oil/water heat exchanger. As an improvement, the results derived by Method II demonstrated that totally replacing the extraction steam in the third stage is preferable to doing so in the first or the second stage. Such integration mode would not result in temperature fluctuations of the main steam or reheated steam without affecting the safety or boiler's operation. In addition, the design SEE for this integration mode is the highest at 28.114%, whereas the other two modes are 18.188% and 20.575%, respectively. Furthermore, such integration mode requires lowest initial investment in the solar field and oil/water heat exchanger. (C) 2016 Elsevier Ltd. All rights reserved.