The potential of natural driving forces for ventilation in buildings is the possibility for providing sufficient outdoor air by only natural ventilation. Based on the early work of Fracastoro et al. (Fourth international conference on indoor air quality, ventilation and energy conservation in buildings-IAQVEC, vol. III, Hong Kong: The City University of Hong Kong; 2001. p. 1421-9.), we develop a simple prediction model for this natural ventilation potential applicable to Chinese residential buildings, using a simple analytical model of natural ventilation considering the combined effect of wind and thermal buoyancy forces. Comparing with the existing method developed by Fracastoro et al. (2001), the present prediction does not need sophisticated multi-zone modeling calculations and the constants in the model are no longer variables. Using the weather data from International Weather for Energy Calculations (IWEC) into our simple prediction model, the natural ventilation potentials for low-rise residential buildings in four representative cities of China including Beijing in the north, Shanghai in the east, Guangzhou in the south and Urumqi in the northwest were analyzed. We introduced the concept of the pressure difference Pascal hours (PDPH) for natural ventilation, and PDPH was calculated and compared for four cities. A high PDPH value means a great potential for application of natural ventilation. In addition, hourly effective pressure differences can be obtained and analyzed statistically. This information can help the designers to determine the building opening size, or to assess whether or when mechanical ventilation is necessary. The application of the model can be a simple design tool at preliminary design stage.

The application of thermoeconomic optimization design in an air-conditioning system is important in achieving economical life cycle cost. Previous work on thermoeconomic optimization mainly focused on directly calculating exergy input into the system. However, it is usually difficult to do so because of the uncertainty of input power of fan on the air side of the heat-exchanger and that of pump in the system. This paper introduces a new concept that exergy input into the system can be substituted for the sum of exergy destruction and exergy output from the system according to conservation of exergy. Although it is also dicult for a large-scale system to calculate exergy destruction, it is feasible to do so for a small-scale system, for instance, villa air conditioner (VAC). In order to perform thermoeconomic optimization, a program is firstly developed to evaluate the thermodynamic property of HFC134a on the basis of Martin-Hou state equation. Authors develop thermodynamic and thermoeconomic objective functions based on second law and thermoeconomic analysis of VAC system. Two optimization results are obtained. The design of VAC only aimed at decreasing the energy consumption is not comprehensive. Life cycle cost at thermoeconomic optimization is lower than that at thermodynamic optimization.