DIY A/C, some measurements

Optimizing any complex system requires good metrology -- making measurements to evaluate how things are working.  A model is a good start:  but without real data you don't know if your model is accurate or not.  So I have been working on that.  Significant measures of performance for input and output air would be the air temperature (of course), and the relative humidity of the air.  To that end, I bought two SHT40 temperature/humidity sensor boards from Adafruit so I can measure ambient conditions and air coming out of the evaporative cooler.  I'd like to include measurements of the water temperature in the recirculated-water loop portion of the evap cooler but the humidity sensor is directly exposed to the environment so that's not a good idea.  As a workaround, I used a cheap thermocouple temperature measurement unit purchased from Harbor Freight.

One suspicion I had was that the air flow through the evaporative cooler is too high, so the relatively-warm air entering the evaporation pad doesn't spend enough time to aborb as much water as it could -- therefore reducing the exit air's temperature drop.  The temperature delta depends on the temperature of the entrance air, so it's necessary to measure the temperature and RH of the entrance air -- as the day goes on, the ambient temperature and RH changes so my setup's operating condition changes along with that.

Anyway, here are a couple of photos showing my current setup and one measurement of the recirculated water temperature:

The firs photo shows my Adafruit Feather NRF69 RF transceiver board with two sensors -- one in the fan's exit air stream and the other is there to measure ambient conditions.  It's powered using a LiPo battery.  This particular board only has one native I2C port so (because the sensor chip's I2C address is fixed) I had to implement a second I2C port using the old-fashioned bit-bang approach.  That took a week or so to get working OK.

The second photo shows the temperature of the water being recirculated through the evaporative cooler.  At the time the photo was taken, the ambient temperature was about 87F so I was getting about a 16 degree temperature drop.  71F isn't too bad for house air, depending on what the interior RH is, so that's sort of encouraging.  But I'd expect the water temperature to increase if it's used (via a water-air heat exchanger) to actually cool the house.  The one saving grace here is that water's specific heat is MUCH higher than air, so a little water should be able to cool a lot of air.  But that part of the system isn't built yet so I can't report any measurements.  Metrology.

I also added a PWM motor speed controller to adjust the fan's speed (and hence the volume of air flowing through the evaporator).  As I suspected, running the fan at 100% of its maximum speed was not optimal with regard to delivering the lowest exit air temperature and highest RH.  Setting the speed at about half its maximum seemed to deliver the best performance for lowest exit air temperature.

However.....I'm not really interested in the lowest-possible exit AIR temperature.  My plan is to circulate the evaporatively-cooled water through a heat exchanger that is in the house, so it is desirable to optimize the system so the water temperature is as low as possible.  In this regard, I observed two things.  First, the temperature of the water that is recirculated through the evaporative cooler doesn't have a strong dependency on the fan speed.  That's a good thing to know.  But the second observation is that the water temperature seems to have some variation that can't be totally explained by the ambient air temperature and its RH.  I suspect it is due to solar heat input, so further experiments (using insulation and light shading measures) are needed.  This summer's temperature extremes probably are at an end -- today's high was only in the high 80's -- so more progress may not happen until next summer.

One main result is that our region's relatively high humidity substantially reduces the effectiveness of an evaporative cooler when it uses un-processed ambient air.  By "un-processed", I mean air that has not been dehumidified before being fed into the evaporation unit.

The Tech Ingredients youtube channel has demonstrated two different approaches to evaporative cooling.  Their first system used liquid desiccant to dry air before it entered an evaporative cooler.  On first examination this would seem to a viable approach for more-humid environments:  but they also have to use an ambient-air "swamp cooler" to cool the liquid desiccant after it's been regenerated.

Their second approach uses liquid desiccant to directly cool room air (and remove humidity), but depends on an ambient-air swamp cooler to get the liquid desiccant cold enough to cool the house air.  I think the second approach is less useful (in a broad sense), since evaporative coolers in regions with higher humidity won't be able to deliver liquid desiccant that's much below ambient.  However, reducing the humidity in a house, independently of the temperature, will make the house feel cooler (because of the evaporatively-cooled inhabitants).

A swamp cooler that uses pre-dried air can output coolant water that is below the ambient dew point, which will remove moisture -- thus improving living conditions in two ways.  In addition, such a system can be used in a "bootstrap" mode*, using the coolant water to also chill the liquid desiccant before it drys the evap-cooler's entrance air.  This IS an additional heat load so pre-cooling the liquid desiccant, using an approach similar to Tech Ingredient's second LD cooling scheme, would be desirable.  For these reasons, I believe a hybrid scheme using parts of their first and second systems would be more useful for most who are interested in a DIY approach.  It utilizes the cooled exit air, which otherwise is unused.

*I call this a "bootstrap" mode because at first the effectiveness of the liquid desiccant (LD) will be relatively poor because it's not cool enough to pull a lot of moisture out of the air.  But as the system starts to work, the temperature of the coolant water should drop and thus help the LD dry the entrance air more -- thus further improving the performance of evaporative-cooling step, dropping the temperature of the coolant water.  And so on. 

A hybrid scheme probably won't deliver dry, cool air the instant it's turned on.  It might actually take a day or more to really get up to speed, depending on the amount of water being recirculated through the chiller and the house's heat load.  Then there's the question of how to deal with night-time conditions, where temperatures can drop to a reasonable level but the ambient relative humidity still is uncomfortably high.  For that, it might be necessary to add some auxiliary heat (rather than solar) to regenerate the LD.  But now I'm getting 'way ahead of myself -- currently being far from any kind of real, practical home-brew A/C system.  

Metrology.