Henderson D., Odeh N., Muneer T., Grassie T.

School of Engineering, Napier University, Edinburgh, EH10 5DT, UK

A photovoltaic-driven fan-duct system is being optimised as part of a solar air heater. The fan is used to draw warm air through roof slates into a duct system. The optimisation of this flow system is based on maximising the daily volume of air delivered for a given irradiance profile. This partially necessitates the accurate prediction of the speed and head-flow characteristic of the fan. This paper is concerned with analysing the performance of this solar electrical system composed of a PV module, a permanent-magnet brushless DC motor and an axial flow fan. The fan’s operating speed was predicted as a function of irradiance and module temperature. The resulting model predicts the speed of the fan to within 10 % of the measured values. The dependence of the head-flow characteristic of the fan on its speed is simplified by an easily manipulated relationship. The performance of two fans with different ratings (9.5 W with 69 l/s capacity and 20.3 W with 110 l/s capacity) was studied. The model predicts that the smaller fan delivers more volume of air than the 20.3 W fan. It is concluded that the start-up point of a motor/fan is an important factor in maximising the volume of air delivered.

1. Introduction

Roof slates are generally warmer than the ambient by a few degrees [1] and so mechanical ventilation systems can be designed to both ventilate the house underneath and harness this extra heat and supply it to the house by means of a small fan. This supplied heat can contribute to the heating demand thus reducing the need for auxiliary heating as well as reducing heating bills. A mechanical ventilation system has the disadvantage of requiring an auxiliary electric source to operate the fan in order to deliver warm air from the slates through the duct and into the house. This drawback of the system can be overcome by using a photovoltaic (PV) module to power the fan.

The use of a PV module has another advantage. The PV-driven system acts as a fast — response sensor to solar radiation and so the fan will deliver air only when the slates are also receiving radiation [2]. However, due to the comparatively high cost of PV modules, for such a system to be viable, its performance must be optimised. Optimum performance may be considered so that which delivers the greatest proportion of the incident energy to the house. The energy yield will be maximised when the daily volume of air delivered is maximised. A model, relating system flow rate to irradiation and PV module temperature has therefore been developed to determine the optimum configuration of system components to meet the above criterion.

For a given length and properties of delivery duct, the flow rate of air in the system depends on the head-flow (H-Q) characteristic of the fan. The head developed across the fan is a function of the flow rate and the speed of the motor of the fan. Furthermore, the speed of the motor is a function of irradiance and the PV module temperature. The current work is concerned with the estimation of the motor’s speed and the H-Q characteristic of the fan as a function of irradiance and module temperature.