SOLARFAN

Solar Powered Helmet Ventilation

Project Driver:

The forced ventilation helmets have to be used in remote locations and often in third world countries where batteries are hard to acquire and expensive. Any system that is self-contained, generates its own energy and doesnít require the replacement of batteries is therefore an advantage.

Introduction:

Small low-cost, integrated fan and solar cell units were acquired from an Italian supplier (fig. 1). While being able to use an already developed product is desirable these fans were found to be unsuitable and in need of some modifications.

Progress:

Firstly the fans were found to provide insufficient airflow for the helmet ventilation requirements. The motors were working under capacity because the solar cells were unable to provide enough power. The solar cells only provided enough power for the motors to move when they were placed in direct sunlight.

The power output of a photovoltaic cell is dependent on the light it absorbs. The shorter the wavelength of the light, the more energy it contains and the higher the voltage it generates on the photovoltaic cell. The intensity of the light striking the cell affects the current produced by the cell. The more light striking the cell, the more current that is produced.

A single PV cell powering the fan was found to supply 1.2V and 15mA under the best available light conditions. Even under these optimum conditions the fan isnít very effective. To try and increase the current supplied to the motor two PV cells are wired in parallel to the motor. This caused a noticeable increase in fan speed, though not as much as expected. With two cells current draw was 20mA, which is the maximum that the motor will draw at 1.2V.

The fanís output in bright sunlight was acceptable, but whenever the PV cell was moved into shadow the fan would slow down or even stop. With the PV cell mounted on a helmet this shadow would occur whenever the fan was on the opposite side of the helmet to the sun. The fan needed enough energy to keep it spinning even when the solar cell is in shadow.

By wiring a rechargeable Ni-Cad (nickel-cadmium) battery in parallel to the PV cells (fig 2.), the battery could be recharged by the solar panels in bright light and then power the fan in dim light. The idea works well in bright light, with the battery being recharged at a rate of approximately 5mA. In dim light, energy is lost to the PV cells which place resistance on the circuit.

This can be stopped by the use of a diode to stop the reverse electricity flows (fig 3.). unfortunately there is approximately a 0.5V voltage drop across the diode, which with the PV cells wired in parallel would create an unusable voltage. By wiring the PV cells in serial (fig. 4), the diode ís voltage drop is less important. With the cells in serial they produce 2.4V under optimum conditions, too high for the battery and motor. When the 0.5V drop from the diode is taken into account the voltage supplied to the battery and solar cells peaks at 1.9V, which is also a high enough voltage to allow the battery to be fully charged by the PV cells.

The battery was now acting as a very usable backup to boost the fan speed when the PV cells were shaded. Concerns still existed as to how reliable the system would be and how fast the battery would be drained on a day with less than full sun.

Figure 5 shows the parallel circuit without the diode (top) and the circuit wired in serial with the diode (bottom).

It was at this stage that a single AA-size carbon cell battery was tested and found to provide approximately ten hours of battery life. With such long run-times available from a cheap and readily available source it was decided to forgo the solar power in favour of the cheaper and simpler alternative.

Conclusion:

The PV cell on its own is not a viable option because it becomes basically useless as soon as the PV cell is in shadow. The battery boosted version with two solar cells in serial works superbly and would be excellently suited to conditions of bright sunlight. Unfortunately the design is more expensive and difficult to manufacture. The battery will quickly run down in overcast or less sunny conditions when it is constantly boosting the motor, however this could be overcome by replacing the battery with a fully charged one when it runs down.

While the battery-only design isnít as versatile and does require the constant replacement of batteries it is far simpler and more reliable than other designs. The design demonstrates that simplicity is often the most practical approach.

References:

The information regarding Photovoltaic Cells was found on the World Wide Web at the U.S. Department of Energy Photovoltaics Program website (http://www.eren.doe.gov/pv/turning.html).

For more information on any of these send us E-mail at demining@mech.uwa.edu.au
All graphics by Demining Research Team. Aug 1998.
Last modified: 12:40:53 Wed 4^th Sept 1998 by Brian McLean