DIY A/C experiments

 As is often the case, when the weather turns hot I start thinking about making some kind of home-brew A/C system for our house.  In the northern Willamette Valley of Oregon, for the most part summers are fairly mild so in some ways it doesn't make monetary sense to install a whole-house A/C system.  This, of course, assumes that the house in question has a gas, electric or wood heating system.  Heat pumps have A/C as a "freebie", but we went with forced-air gas.

So earlier this season I was once again doing online searches for DIY A/C systems that don't require exotic stuff like compressors, refrigerant etc. -- in other words, something that could be built using commonly available materials and tools (like a saw, drill, screwdriver and so on).  I came across a series of youtube videos produced by Desertsun02, and this one looked interesting.  He provides a lot of build information so, even though his emphasis was on using solar power to run the thing, it looked like it could be adapted for a test setup.

I built most of Desertsun02's evaporative cooler -- I omitted the 90 degree elbow duct on the output side of the fan.  Here's a photo of it (minus the fan):

 I attached two of the blue evaporation pads using his approach, using copper wire pushed through the pad and wrapping the ends around the PVC pipe, but didn't like the gap between the wires -- any path for air to enter without going through the pad will reduce the cooling capacity of the unit -- so for the remaining pads I used carpet thread, threading it through the pad and around the pipes in a corkscrew fashion.  This worked much better, but I don't think the thread will hold up very long being exposed to the sun's UV.  It also is clear that it will be a pain to replace pads using either of these approaches.  So that part of the design needs some work.

I found a similar problem with the way the fan is attached to the top of the cooler.  The PVC 3-way fittings on the top of the cooler raise the board so there is a ~1/8" tall gap all the way around the board, also permitting uncooled outside air to enter the exit air stream (and it also reduces the quantity of air that _does_ flow through the wet evaporation pad).  This latter problem could be addressed with the judicious use of adhesive-backed foam weather stripping.  However, just to compound the problem, I found that my piece of scrap plywood I'd used for the board was warped.  Since this thing could potentially be exposed to rain, there's no guarantee this wouldn't become a problem even if I started with a perfectly-flat board.

Alright, so despite these problems, how well does my setup perform?  I have to say, so-so; but mostly because the outside air's relative humidity can be pretty high, even in an Oregon summer.  Example:  right now my (homebrew) remote-reading temperature and RH sensor is reporting 72F and 42% relative humidity.  When I tested my evaporative cooler, the ambient temperature was 80F and the relative humidity was about 58% -- not the best when it comes to getting a lot of cooling out of a swamp cooler.  According to my psyrometric chart, the wet bulb temperature was 68F, so that's about the best I could hope for.  My measurements showed the exit air temp was about 71F, and the recirculating water in the cooler had cooled down to about 70F.  If I eliminated the gaps around the edges of the cooling pads and between the fan board and cooling tower, I probably will get the exit air and recirc water close to the same temperature.

Oh, BTW, here's a photo of my remote-reading sensor:

OK, it's a little rustic, shall we say :).  But I just got it working.  It uses a couple of items I bought from Adafruit -- a Feather M0 with an RF69 radio transceiver, and an SHT40 temp/humidity sensor.  I'm powering it with a spare cell phone power bank, which has much higher capacity than the LiPo batteries Adafruit sells for these things.  The Feather boards were designed to be battery powered, so supplying power some other way can be a little tricky -- but, since they also are designed to be powered off a USB cord, the power bank scheme works a treat.

One big issue with evaporative coolers is that the cooled exit air also is much higher in humidity, which is a problem if you're starting out with a relatively high RH (as in, where I live).  So my long-term solution is to add another cooling loop to the swamp cooler.  It will circulate the evaporatively-cooled water through a water-to-air heat exchanger that is inside the house.  A fan will pull warm interior air through the HX and cool it down.  If the interior dew point is relatively high, it also may condense some water and lower the interior relative humidity, too -- but I really don't expect that to have a significant impact on the interior RH.  But I will make sure to design the HX enclosure so condensation that DOES occur is directed back outside, rather than dripping all over the floor.

What would an improved version of the cooling tower look like?  I'm considering the use of U-channel aluminum extrusions to capture the edges of the cooling pads.  To prevent the pads from being sucked into the cooler, I will attach support panels made from fencing mesh.  The U-channel will be screwed to square aluminum tubing, so replacing a pad would be easy -- pull the old one out and install the new one by tucking its edges into the U channel.  An aluminum sheet would be used for the fan mount.

Switching over to aluminum extrusions would still be compatible with hand tools -- a hack saw for cutting the aluminum and a drill for making screw holes would just about do it.  It may be necessary to make corner brackets to assemble the parts into the tower shape, but I haven't gotten that far yet.