Optimisation of a solar-photovoltaic-driven, roof slate-based ventilation preheating system

Condensation related dampness and the subsequent mould formation, resulting from the lack of ventilation, in airtight buildings lead to significant health problems inside dwellings. Delivering sufficient ventilation rates for energy efficient modem buildings is a necessity. In cold climates, in order to avoid energy losses, this ventilation air must be preheated before being admitted to the house. Due to the continuous depletion of oil resources and the escalating impact of the greenhouse effects on global climate it is necessary to economise energy consumption. Solar energy offers itself as a means for preheating ventilation air. Solar ventilation preheating is an emerging technology not only in countries with cold climates but also in warm climates where it is used for drying crops. The newly developed roof-slate based solar ventilation preheater makes use of the existing roof slates as a solar collector. A photovoltaic-driven fan draws warm air through the spaces between the slates and delivers it via a flexible duct into the house underneath for ventilation. The photovoltaic module acts as a quick response sensor, which allows the fan to operate only when the slates are warm. A model for predicting the flow rate of air in the system as a function of irradiance, ambient temperature and wind speed in addition to the photovoltaic, fan and duct specifications was developed.The model predicts the fan's speed and the system's flow rate with an accuracy of 92% and 88% respectively. The model was employed for system optimisation with respect to the maximum volume of air delivered. The optimum system comprises a 24 V DC, 9.5 W fan with a free flow capacity of 69 1/s.This system delivers 9.6 x 104M3of warm air (a constant9.2 I/s on a continuous basis which meets the ASHRAE recommendations for a 100-m3 room with one occupant). The optimum system can also potentially deliver 161 kWh energy with a solar fraction of 11% during the heating season.The optimum system was found to be optimum in several respects including maximising volume, efficiency, solar savings,and minimising payback period (13 years). This system can contribute to C02 savings by as much as 100 kg annually. Recommendations to further maximise the utilisation of the maximum power of the PV module included using an optimum motor constant so that the fan operates in the maximum power vicinity for most of the year. A new method for obtaining the photovoltaic current-voltage characteristic was developed.This method predicts the maximum power of the photovoltaic module with 92% accuracy. The optimum motor constant was expressed directly as a function of irradiance and module temperature. The yearly output of the PV module employing the optimum system is 10.4 kWh. Using a fan with the optimum motor constant would increase the energy utilised by 14 %. Furthermore,an optimum length of duct (8 m) would maximise pneumatic output and deliver higher overall flow efficiencies. Upon comparative testing, it was found that a cooling effect was associated with an early start of the optimum system.This can be overcome by delaying the start of the fan until the slates are warm enough or by enhancing the properties of the slates(using dark paint or anti-reflective coating) so that they warm up synchronously with the fan starting time.