5 Gallon Air Conditioner - Does It Work? - Solar Affect

5 Gallon Air Conditioner - Does It Work? - Solar Affect

Here is the 4th article in the series, that will help you answer the question, "does it work?"

So, the next concept that one needs to understand when it comes to Air Conditioning, or more appropriately, temperature control,I need to describe some concepts called "solar affect" and "greenhouse effect."  

So, in the most basic terms, solar affect is the phrase used to describe the heat that is absorbed in certain materials by the sun.  It's why the dashboard of your car is hotter than the outside temperature in the summer, or your vinyl seats are hot on your legs in your car when you get in.

So that is "solar affect."

The "greenhouse effect" is the phrase for the "trapping" of that heat that is being radiated from the hot surfaces.  So, the hot dashboard, seats, and carpet of your car absorb the heat from the sun. But, there is also air (oxygen, nitrogen, CO2, etc.) inside the car as well.  So, the heat radiates off the surfaces, and the air absorbs the heat energy.  That's why cars are hot and "muggy" in the summertime with the windows closed.

That is the "greenhouse effect."

So, in doing the BTU capacity calculations for an air conditioner, one also needs to take into account the solar affect and the subsequent greenhouse effect of the space needing to be cooled.

Let's use an example.

Let's say there you have a van, parked in a parking lot, with the windows rolled up, and it's 90 degF outside. There are multiple clear windows in the van, and no "heat shades" or shade or anything like that, just a van sitting in the middle of some empty parking lot somewhere.  For the purposes of this example we will disregard several factors like color of van, or color of interior, or humidity etc.

So, after 1 hour, what is the approximate value of the solar affect, and the resulting greenhouse effect, and bottom line, how hot is it inside the van after 1 hour?

Well, there are engineering calculations for this, that involve surface area calculations, emmissivitiy, thermal convection, solar reflectance, sun angle of incidence, and it also involves solving the root of a 4th order polynomial, pretty complicated stuff.

So, to spare you all of that headache, let's just say "hot."  For ROUND numbers, let's just say that it will get 40-50degF hotter.  So, the interior temperature after 1 hour will be approximately 130-140 degF.

Now, you also have to calculate, the continuing solar affect (heat energy that is continuing to be added due to the van still being parked outside in the sun) and the interior air volume of the van.  In order for the van to cool down, you must cool off all of the interior space, as well as overcome the additional heat that the sun is continuing to generate.

For ROUND numbers, we can use 100 btu/hr per sqft of exterior in the sun, as the continuing solar affect calculation.

So for ROUND numbers, let's assume the van is a cube, that is 15ft long, 7ft high, and 7 ft wide.  So, to calculate the square footage of the roof of the fan, simply multiply 15ft x 7ft = 105 sqft.  To calculate the square footage of the side of the van, 15ft x 7ft = 105 sqft. (sf)

Then of course the VOLUME of the van is 15ft x 7ft x 7ft = 735 cubic feet (cf)

Next, we need to make an assumption, that the van will be in direct sunlight, but that direct sunlight will heat the roof of the van and 1 side (the bottom of course not, and the other side will be shaded). Of course we know these are big assumptions, but we are working towards a ballpark estimate.

So, we are asking our air conditioner, to drop the temperature from 140 degF down to 70 degF (70 deg temperature drop), in a space of 735 cf, with the sun beating down on the van where the sun is adding 21000 BTUs/hr to the space due to the solar and greenhouse effects.

Also, keep in mind, this is a SEALED system, or at least "not rolling down the windows" situation.  Obviously, that will be the next subject of the next blog post, but for this example, we are glutton for punishment and keeping the windows rolled up and are assuming no airflow.

So, first off, we know we need AT LEAST the 21000 BTU/hr in cooling power to offset the solar affect. Then we need to calculate how much capacity to cool the space 70degF.

There are many online calcuators that will do this for you, but for the purposes of this article, I will just average them out at about 9000 BTU/hr in this example to cool 735 cf 70degF.

So, we will need an air conditioning unit capable of 30,000 BTU/hr to keep this van "comfortable" in this situation. Granted, this is a worst case condition, and round numbers, but it is "ballpark."

So, remembering from previous posts, a 10lbs block of ice has about 2000 BTU of cooling capacity. So, in this situation, utilizing an ice model air conditioner, one would use 15 blocks of ice per hour, or utilize 150 lbs of ice/hr.

It could be done of course with enough coolers, but for all practical purposes, a singe ice model 5 gallon air conditioner would NOT work in this situation.

But remember those nasty assumptions.. we will come back to those in some upcoming articles.