Right now, Europe is experiencing a record-breaking heat wave, thanks in large part to global warming. Ask me some time about arctic amplification and the destabilization of the jet stream.
And sure, the U.S. Southwest is experiencing a mega-drought, something that has been one of the most consistently-predicted outcomes of global warming since climate models were first developed at the end of the last century. Ask me some time about the permanent dust-bowlification of the U.S. Midwest, likely occurring before 2100, part-and-parcel of that same set of predictions.
But here in Northern Virginia, we’ve got our own troubles. Thanks to consistently high nighttime temperatures, it looks like we may have nothing but green tomatoes for two whole weeks (Post G22-041). (Alternatively, you can check this post from the U. of Minnesota extension service).
Oh the humanity.
What can be done?
Now, I’m not talking about any crazy stuff, like driving fuel-efficient cars, maybe not flying everywhere at the drop of a hat, cutting down on beef consumption, and so on. None of those ludicrously difficult crypto-socialist sacrifices that exceed the fortitude of most Americans, and most of the world’s wealthy populations in general. (But those guys are still doing it, why can’t I, wah wah wah, says the author, from the comfort of his air-conditioned suburban home.)
Nope, I’m asking how I can consume even more fossil fuels, trying to solve my particular problem. That’s the American way.
Evaporative coolers
Other than unfocused rage at the likely trajectory of earth’s environment, the actual point of this post is to ask whether or not there is anything practical I can do to get tomatoes to ripen during a hot spell. And that, in turn, allows me to put down, in one place, everything I think I understand about evaporative coolers.
First, what do I need to do? I need to reduce nighttime temperatures in my tomato patch by a few degrees. If I can just get them below 70F, from predicted levels that are not much higher than that, in theory, my tomatoes will continue to ripen. I need to do that for a couple of weeks, at most. And I will no longer be stuck in green tomato hell.
Point 1: Traditional AC, 25¢ a night.
Let me first state the obvious: As weird as it may seem, it would be perfectly possible to do that with a standard window air conditioning unit or portable AC. It wouldn’t even cost much to run it.
Air-condition my garden, for the sake of a handful of ripe tomatoes? Why not? Is that fundamentally more wasteful than any of the other excesses of modern America?
Sure, the neighbors might look at me a little funny. But I’m used to that.
Let’s say you already owned a portable AC, like this one, from Home Depot. Conservatively rated at 6000 BTU (which actually means BTU per hour, in AC-speak), draws 9 amps at 120V running full blast. Call it 1000 watts. Capable of cooling a 250-square-foot room. Built-in timer, even.
So, toss a plastic tarp over my 24-square-foot tomato patch, place this unit under the tarp, lead the exhaust duct outside, and set it to reduce the temperature to the mid-60’s F in the middle of the night.
If I really wanted to nerd out, I’d do a detailed estimate of the electricity required. As it is, I’m just going to guess that it couldn’t possibly take as much energy as running this full blast for a couple of hours. Or two kilowatt-hours of electricity.
Or, at Virginia Power’s current rates, less than 25 cents a night. I could literally air-condition my tomato patch at night for less than two bits.
I wonder why we use so damn much fossil fuel?
Point 2: Bag of ice and a fan, 15¢ a night.
This one takes a bit of calculation to show it. But it would work.
It takes about 200 British Thermal Units (BTUs) to bring a pound of ice up to room temperature. Most of that is the energy required to melt the ice. So 10 pounds of ice can generate about 2000 BTUs of cooling.
My 24-square foot tarp-covered tomato bed would have about 72 square feet of surface area exposed to the air. A plastic tarp might be able to achieve R1 insulation. And I need to pull maybe as much as 10F temperature difference across that tarp surface. So that’s about (72 square feet x 10 degrees / R1 =) 720 BTUH of cooling capacity required.
With a fan to circulate the air (50 watts/hour), my 10-pount bag of ice should be able to achieve the required cooling, under that tarp, for three hours. Let me buy more, just in case. My local Safeway sells 16-pound bags for $4 each.
So, at full retail, $4 per night to cool my green tomatoes enough so that they would ripen.
But, in fact, if you did this at home, and only looked at operating costs, it would be vastly cheaper. Here, rather then calculate it, I looked around, and the default estimate from Energy.gov seems about right: 5.5 KWH per 100 pounds of ice. At my current rate, it would cost me 11 cents to make that $16 pounds of ice. Toss in another (0.05 KWH x 6 hours x 12 cents per KWH =) 4 cents for the fan. And the total cost comes to 15 cents a night.
I wonder why we use so damn much fossil fuel?
Point 3: Evaporative cooling, just don’t go there.
I actually started out this post trying to get a grip on how evaporative cooling works. But that took all of about 30 seconds.
In theory, to understand how evaporative cooling works, you have to understand the difference between sensible heat and latent heat. Allowing for a little sloppy language, sensible heat is the temperature of the air. Latent heat is the humidity in the air. If you inject a lot of water into the air, it absorbs heat as it evaporates. It cools the air, as it raises the humidity. Evaporative cooling converts sensible heat to latent heat. It trades heat for humidity.
In practice, I think everybody gets the idea that blowing air over a wet surface cools that surface off. You really don’t have to go much deeper than that. The only new idea about (e.g.) the mist coolers that you might see at a public fair is that if you put the water directly into the air — instead of on some surface — it cools the air off. At a cost of higher humidity.
In the eastern U.S., you don’t see evaporative coolers on homes. (That is, devices that literally directly cool the air with evaporative cooling.) That’s because it gets humid here in the summer, and if the air is already humid, evaporative coolers can’t achieve much cooling. Instead, those devices are mostly found in the arid West, because the generally low humidity makes them an energy-efficient way to cool your house.
(You do see evaporative cooling towers on commercial AC, and for (e.g.) cooling power plants and such. But those are operating with hot fluids at temperatures far in excess of air temperature.)
Weirdly, though, we do routinely take advantage of evaporative cooling, for our homes, here in the Eastern U.S. Or, at least, anybody with a relatively modern window AC unit does.
When I was a kid, all window ACs dripped water. Humidity would condense on the cold coils, and was typically routed to the exterior by a little pipe at the bottom of the unit. Window ACs don’t drip water any more. Some sharp engineer figured out that if you slung the water onto the hot coils, it would evaporate, and that heat removed that way would increase the efficiency of the unit. All that takes is a plastic “slinger” ring on the fan, to splash the water around. And so, modern window ACs no longer drip condensate, they use it to improve their efficiency.
Back on task, when the relative humidity is high, you can’t achieve much cooling via evaporation. Which is just a fancy way of saying what every U.S. Southerner knows: It’s not the heat, it’s the humidity.
Which brings me to yet another global warming fun fact, this time dealing with “wet bulb” temperature. That’s the temperature you can achieve by blowing huge volumes of air over a water-saturated piece of cloth. Per Wikipedia:
The wet-bulb temperature is the lowest temperature that may be achieved by evaporative cooling of a water-wetted, ventilated surface.
The fun fact is that if it gets hot and humid enough, no amount of breeze will remove your body heat, and if you stay in those conditions long enough, you end up slowly cooking yourself to death with your own body heat. It’s not even hard to grasp how that can happen. If the “wet bulb” temperature exceeds 35C (= 95F), you can’t shed enough heat to survive. An hour of that won’t kill you. But 24 hours of it will. Those conditions are rare with the current climate, but will become less rare as global warming progresses. See The Emergence of Heat and Humidity too Severe for Human Tolerance.
Just in case you think that’s worrying about high wet-bulb temperatures is something for lilly-livered liberals, ponder the chart below, from a U.S. Army website that uses wet bulb temperature to limit training exercises.
Source: Cite above.
In any case, all you really need to know is that the “wet bulb” temperature is the most you can hope to achieve with evaporative cooling. And that, if the air is already humid, the wet bulb temperature isn’t all that much lower than the dry bulb (i.e., ambient air) temperature.
The only other fun fact is that, for a given amount of moisture in the air, as the temperature falls, the relative humidity rises. So that, at night, in Virginia, in the summer, relative humidity is typically in the 80 percents.
Here’s the data from yesterday, at Dulles Airport, courtesy of Weather Underground:
Source: Weather Underground, cited above.
I think I’m belaboring something that everybody actually knows. When the relative humidity reaches 100%, dew forms. (Hence the term, dew point.) Whenever you wake up and there’s dew on the grass, some part of your area reached 100% relative humidity overnight.
In any case, at 80 percent relative humidity, you can’t achieve much evaporative cooling. The chart below is a bit hard to read, but it shows that if the air temperature is 72F, and the relative humidity is 82 percent (typical for a Virginia summer night), at best, you can achieve 4 degrees F of evaporative cooling.
Source: Cleveland.com
For the period last night when relative humidity was above 90%, I would have been lucky to achieve one degree of evaporative cooling.
And think about what you’d be doing. You’d be blowing absolutely water-saturated air, over plants that are subject to a vast number of fungal diseases.
So, in theory and in practice, trying to achieve cool nighttime tomatoes via evaporative cooling, in Virginia, is a fool’s errand.
Conclusion
I must confess that I’ve never heard of anybody air-conditioning their outdoor garden. But energy is so cheap, there’s really no cost-based reason not to. It’s somewhat unconventional, but for pennies a day, I could ensure that my tomatoes got cool enough to ripen fruit, either by literally running an AC in the garden (under a tarp), or bringing home-made ice to the garden (and putting it under a tarp).
At the end of the day, it’s no mystery why we use so damned much fossil fuel. Energy is ridiculously cheap, and we use it accordingly.
Sure, air-conditioning an outdoor garden bed highlights a certain spectacular wastefulness. And yet, I am sure that the savings from raising my house temperature by 1 degree F in the summer would dwarf the cost of air-conditioning my garden. If nothing else, this seemingly frivolous exercise had made me sit back and re-think my everyday baseline energy use.
