It’s that time of year again–temperatures are rising and the non-fulltimers are out in force. Depending on where you’re camping, that may mean managing your power use on a limited hook-up, listening to the air conditioner run non-stop, and possibly still being hot inside in the afternoon sun. We can improve on that with a few misters.
At first glance, you might think I’m about to talk about swamp coolers, solar shades, or misters on the patio. Not quite…
What I’m talking about is using misting nozzles to improve the performance of the air conditioner. How’s that going to work?
At the most basic level, we’re going to be cooling the condenser coil. Under normal circumstances, the condenser coil will be significantly hotter than the ambient air, as air isn’t exactly the best heat transfer fluid. If we bring that temperature down, the temperature of the inside coil drops as well, increasing the amount of heat that the air conditioner can remove.
Just dropping the coil temperature to about the same as the outdoor air would be a big improvement, but we can do better. On a hot day, our source of water will be cooler than the outside air. So right off, we can reduce the coil temperature from 10s of degrees (F) above air temperature to perhaps 20-30 degrees below it.
But that’s not all. We can also evaporate the water, which has an additional cooling effect. The phase change allows the water to absorb energy in the form of latent heat, meaning that it can absorb energy without an increase in temperature. That means the temperature of the air falls, with an increase in its humidity. Going from completely dry air to saturated air can be as much as 40 degrees under the kinds of conditions where this might be taking place.
On the hottest of days, many of us are accustomed to an air conditioner that starts running constantly around midday, and doesn’t shut off until the evening. Depending on solar gains, it may also mean that the temperature inside can rise quite a bit above where we’d like it to be.
By improving the amount of heat the air conditioner can remove, we help it to keep up on those hot days, and keep us more comfortable inside, even if it still runs all the time.
So how does that save electricity?
In the case where the air conditioner is able to keep up, you’ve improved its performance, and consequently reduced the amount of time it has to run. That part is pretty obvious–less run time, less power consumption.
What if it still runs all afternoon? By reducing the saturation temperature of the refrigerant, we reduce the pressure (head) that the compressor is working against. That means the current flowing through the compressor motor windings is reduced, and the current drawn in from the pedestal, through a meter, or generator is less as well. I’ll be using the clamp-on current meter I’ve talked about before.
The second-order benefit of that is that the windings in the compressor will run cooler, which should prolong its life a little bit.
That’s Going to Waste a Lot of Water, isn’t it?
Not as much as you might think. Lets work through a little math, assuming we were in a dry climate, at 105ºF outside, and a 70ºF water source, and we’ve adjusted our water flow to be completely evaporated as it passes through the condenser.
First things first:
- An air conditioner removing 15,000Btu/h from the RV is providing 4400W of cooling. That’s a simple unit conversion, with 1 W=3.41214 Btu/h.
- The specific heat of water is approximately 4.18kJ/kgK. That means it takes 4.18kW, or 4,180W to raise the temperature of 1 kg (2.2 lbs) of water by one degree Celsius in one second, or about 1000W to raise the temperature of one pound of water by one degree Fahrenheit in one second.
- The latent heat of vaporization, that is, the property that describes how much energy it takes to evaporate water, is 2257kJ/kg.
In our example, we have a 35ºF temperature difference, and we’re evaporating the water completely. That means that a single pound of water could absorb:
If we ignore (for now) for the increased efficiency of the air conditioner, that means at 0.5 gal/hr (I’ll get to that number later) we’ve increased the heat removal of our air conditioner by 2650W, or more than 50%!
Here’s the set of misting nozzles I’ll talk about next time when we put the theory to use:
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The way I look at it, even if they don’t work as intended, I can hang them from my awning rail to keep cool while sitting outside. I know they work for that.