DIY A/C More Observations

Today I took more measurements after my cooling tower had stabilized.  The ambient conditions were 33.4C ( 94.6F) and 41.4% RH.  My psyrometric chart indicates the wet bulb temperature should be 22.8C (73F).  Measurement of the exit air showed it was 30.35C (86F), substantially higher than the wet bulb temperature.  The temperature of the (recirculated) water was about 74.5F so it IS close to the wet bulb temperature.

The reason the exit air is much warmer than wet bulb may be due to excessively high air flow through the evaporation pad; or perhaps the air never is going to get all that close to wet bulb.  After all, the wet bulb measurement doesn't measure the temperature of the air after evaporation, it basically measures the residual water surrounding the thermometer bulb.  Videos done by Desertsun02 suggest that the exit air temperature should be much lower, but his setup isn't exactly like mine.

I'd like to get the temperature of the exit air lower, because I can use it (via a second heat exchanger) to cool the return water from the interior heat exchanger.  That would increase the system efficiency, perhaps by a significant amount.  To work on this, I bought a couple of PWM motor speed controllers to experiment with air flows through the evaporation tower and inside heat exchanger.

I did try throttling down the water pump to see how that would affect the system, but reducing the flow rate by about 50% didn't have a noticeable impact on the measurements.

Examination of my psyrometric chart did suggest a way to improve the system performance by a small amount.  Although it sounds counterintuitive, if the air entering the cooling tower is pre-cooled by passing it through a heat exchanger, the wet bulb temperature of the cooler-but-more-humid air is lower.  To check this out, you need to look at what happens when the air is cooled by a heat exchanger.  It doesn't pick up any more moisture so the "humidity ratio", which is the ratio of dissolved water vs air masses, remains constant.  So the heat exchanger just moves the air straight to the left (i.e., it just moves along a constant humidity ratio line).  Then you look at what the resultant wet bulb temperature would be.  Here's an example.  Looking at the ambient conditions I get a humidity ratio of 14.  Cooling the air down to 25C before it enters the tower should produce a wet bulb temperature of 21.5C, which is about 1C lower.  Not a huge improvement, so I don't think it's worth the added cost and complexity.  It's more worthwhile to get the exit air temperature closer to wet bulb so I can reduce the heat load on the recirculated-water loop.  This will ONLY work if the exit air temperature is lower than the return water from the interior heat exchanger....which, in turn, can't be any higher than the ambient temperature in the house.  Clearly, we want to cool the house down so ideally there will be a substantial change in coolant temperature from inlet to outlet.

I have to say that my measurements don't look all that promising for cooling our house with this setup.  There is a final way to (potentially) greatly improve the performance of an evaporative cooler, but it comes with quite an increased bit of complexity.  It would reduce the humidity of the air going into the cooling tower using something called "liquid desiccant", which will improve operation of the system in regions where the relative humidity is  moderately high to high (and our region seems to fall into that category) .  The liquid desiccant absorbs moisture but must be regenerated on a continuous basis in order to keep working.  This requires some sort of heat source and another "tower" to help extract the absorbed water from the desiccant.  Sounds complicated?  Yes, but fortunately it appears that it can be done using a fairly low-tech approach.  I will leave it there for now.

DIY A/C, A Brief Follow-up

 I went back through my spreadsheet and found a couple of errors in the math, in both the evaporated-water and fan CFM calculations.  After correcting them, both numbers are pretty close, within .07%.  That's scary-close.  I would have been pleased to have them within 5%.  If nothing else, I would have expected the (cheap) fan I bought not to really deliver 1700CFM.

The corrections also showed that the amount of heat power being transferred is amazingly high, but water does require a lot of energy to change it from its liquid to gas phase.   The fact that a lot of heat energy is being moved around suggests that my indirect-cooled A/C setup has a chance of really working, but we shall see.

Homebrew A/C testing

 I have moved on to making some measurements on my first version of a DIY A/C system.  My main interest at this point is to get a rough indicator of how much heat energy the unit is capable of absorbing.  I currently don't have two temperature sensors to measure the air's temperature drop (nor do I have an anemometer to measure the air flow through the fan), so I tried doing it by measuring the amount of water consumed by the cooling tower.

Heat energy can be derived from water consumption using the fact that it takes 4184  2,451, 824 joules to convert 1 gram of water to vapor (at the same temperature).  To get the volume change of water, I measured the dimensions of my water reservoir so I can calculate the volume as a function of height change of water.  The test setup looks like this:

The metal ruler that's at an angle is used to measure the change in water level (in the photo, it's about parallel to the house's shadow line).  It's at about a 50 degree angle, which helps to improve the effective resolution of the measurement.  To convert the distance on the ruler to actual height change, I multiply the measurement (in decimal numbers, not fractions) by sin(50), about .776.  If I laid the ruler along the long axis of my reservoir I could get even better resolution, with a multiplier of .545.  Multiplying the volume change (in mililiters) by 4184 results in the number of joules absorbed by that amount of evaporated water.  Dividing that by the elapsed time gives me the watts of heat energy involved

So far I have two data points, starting earlier in the day when the ambient temperature was relatively low (76F) and the relative humidity was relatively high (61%).  In about 34 minutes the system moved approximately 2.2 megaJoules (!), which works out to an average heat power close to 1KW.  A second measurement taken 2 hours 10 minutes later showed an average heat power around 1.9KW.  Since the second number was calculated for the start of the experiment, as the outside air warmed up the effective power went a bit higher than 1.9KW (2.15KW, to be precise).

At the same time, the fan and pump consumed about 90 watts, so it can be seen that the system is capable of absorbing at least 10 times the amount of heat than the system needs to operate.  Not bad!

I need to point out that, as it is now, the system is not suitable for cooling our house.  The exit air is pretty humid so any improvement in comfort due to lower temperature is more than offset by the increased humidity.  I have gotten a 12x12 heat exchanger, a second fan and higher-volume water pump so I can try the indirect-cooled approach, where the evaporatively-cooled water in the reservoir will be pumped through the heat exchanger.  I still need to get some bulkhead fittings to make a clean system for pumping water out of the reservoir.  Desertsun02 has an indirect-cooled setup he shows on a youtube video, but he just snaked the added tubes up and over the side of the reservoir, which permits uncooled outside air to get into the tower.  While I don't really care what the exit air temperature is, that air leakage also reduces the evaporation rate, which is a concern.  Using two bulkhead fittings will get around this problem, as long as the hoses never have to go above the surface of the water.

It also will be necessary to make an enclosure for the heat exchanger so I can pull air through it with my second 12 volt fan.  So I likely will miss the opportunity to try the whole thing during our current heat wave (100F predicted today, 104F or higher tomorrow).

Update:  I let the system run until almost 5PM (a total runtime of about 60,000 seconds).  The overall heat transfer rate came to 2.125KW, for a total of around 12KWH. (51 mega joules).  

I need to think about this some.  The actual heat transfer may be substantially more because the cooling tower cooled a whole lot of air, in addition to evaporating a bunch of water (about 12 liters).  I could calculate what this is, assuming that the fan really moved 1700CFM and (based on ambient vs exit air temp) the temperature drop.  But I believe I can't add the two results -- the heat absorbed by the evaporating water was pulled out of the air (and the residual water in the cooling pads).  So using just the latent heat in the evaporated water probably is correct.  But my calculation produces a result that is about an order of magnitude higher than the water-evaporation calculation.  I definitely need to think about this.....

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.