About the Fan and Filter...
Breathless
The ventilation system was by far the most challenging feature to incorporate into the solar powered webcam. It presented challenge after challenge and consumed the greatest amount of time to figure out and construct. The funny thing is that I'm not even sure it's really necessary! It's necessity was intuitive to me having helped build a solar powered race car whose battery box was ventilated. The theory behind ventilating the car's battery box was that hydrogen gas, which is highly flammable, would be generated as a by-product of charging the lead acid batteries and we couldn't risk letting this gas accumulate and create a hazard to the team members. But the car contained more than one battery, its batteries were larger and they weren't of the same type of construction as the one I opted to use for this project. Furthermore, I read that the recharge chemistry of the type of battery I'm using is different than that of car batteries and production of hydrogen is less of a cause for concern. But I got the idea stuck in my head and I ran with it. Looking back at all the hassle this feature caused me, and in light of the fact that it's probably not even necessary I like to characterize it as optional. The pros for this feature are that it will certainly blow out any gases that do accumulate and which might pose a danger, and it will help cool the inside of the chassis on very hot days. The cons for this feature are that it contributed significantly to the amount of time and money needed, it compromised the weather tightness of the electronics enclosure, and it added a maintenance requirement to the assembly. So the decision is left to you whether or not your chassis should be ventilated. If you decide to include this feature in your project then please read on and let me show you how I went about it.▼
My Fans
The ventilation fan was one of the first pieces I obtained for this project. And I got really lucky finding the type and size of fan that I did. Of course, the fan had to be small so as to be able to fit inside the electronics enclosure and to also not require too large of an opening for the intake and relief air. Pancake fans are often used in electronic applications and is in fact the type of fan that was used to ventilate the solar car's battery box. But this type of fan would have required a much larger box than I wanted to use so I hunted for a different type and found it. I won an online auction for a box of surplus fans of the "squirrel cage" type which use a centrifugal, backward inclinded fan wheel instead of a propeller type fan blade like the pancake fans. Thankfully the auction was for about a dozen of these fans so I have some in reserve should the one I'm using now ever fail. The trouble is I can't imagine where one would go other than auction websites to find a similar fan for sale. The fan wheel and housing are made entirely of what appears to be nylon so it's light and weather resistance. Finally, it operates on 12 volt power.▼▲
Fan Power
Throughout the course of this project I worried about the battery being able to power the camera in a worse case scenario such as successive days of stormy weather with little sunlight to recharge the battery during the day. I could never be sure how long the battery would last in these conditions. So the best I could do was make every attemp to maximize the battery's life by minimizing the load on it. Powering a fan off the battery totally contradicted this philosophy and wouldn't be acceptable. The fan had to have a power source independent of the battery. But the solar panel I had already purchased generated more current than this fan was designed to handle. Thus the idea to add a secondary solar panel was born. Click over to the solar panels page to read more about the panel that was used and the reasoning behind its selection.▼▲
Filtered
For the fan to work properly the air it sucks out of the box has to be replenished somehow. This is easily accomplished by opening up a second hole to allow make-up air in. But you can imagine that the make-up air that enters the box also brings dirt and dust with it. So, to minimize the amount of dust that gets in an air filter was needed. An internet search lead me to discover filter pads which were a near perfect solution. The real challenge in adding the filter was in figuring out how big the filter needed to be. If sized too small the air coming in would be subjected to a large pressure drop which would, in turn, cause the fan to become inefficient. To determine the size of the filter and opening that was needed I first had to determine the amount of air produced by the fan. Using a wind speed meter to measure the speed of the wind produced by the fan and using a scale to measure the size of the fan discharge opening I was able to figure out the speed and volume of fan wind in cubic feet per minute, or CFM. The filter pads give a rating for the amount of pressure drop across them for a given air face velocity. So, knowing the CFM that was needed I was able to work backwards to calculate the size of the opening I would need to keep the wind speed across the filter within the velocity the filter ratings were based on. This is how the size of the make-up air cut-out in the chassis wall was determined. The fan cut-out and the filter cut-out were strategically located at opposite ends of the enclosure to maximize the renewal of air inside the box.▼▲
In a Pinch
So the next challenge was to figure out how to place and keep the filter in the air stream. At the same time the holding mechanism had to also allow the filter to be replaced on occasion. I agonized over ways to do this without cutting even more holes into the electronics enclosure but in the end I gave in to convenience and embraced the idea of drilling four tiny screw holes into the box. These screws pass through the chassis wall and grab on to a rigid rectangular piece of acrylic on the inside of the box. When tightened these screws pull the acrylic ring tight against the inside wall of the box. This creates the clamping force used to pinch the filter in place between the acrylic and the chassis wall. Of course, only the edges of the filter pad are in contact with the clamp and wall. The center of the pad is exposed across the cut-out where the air passes through. I placed teflon washers under the screw heads to add some degree of weather resistance back into the enclosure.▲
