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Old 03-21-2018, 10:17 PM   #57 (permalink)
Master EcoModder
Join Date: Dec 2010
Location: arkansas
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"Proper Shitbox" (SOLD) - '96 Honda Civic LX fully optioned no weight reduction
90 day: 41.9 mpg (US)

Plue Brius - '10 Toyota Prius II
90 day: 57.73 mpg (US)

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Title: kW/m^2, window tint, and HVAC in a Prius.

Was scrolling through looking for window tint for the Prius after all this time and it got me thinking! Whats the effect on fuel economy of the HVAC system? I've always noticed even at 50F if you have your AC off in the cabin on a nice sunny day it's like the greenhouse effect and it's still hot and I need to either hit recirc or open the windows to stay school. Now, unfortunately, I was pretty busy last year and didn't nerd out too much on the Prius and log some baseline fuel numbers, but I do remember getting a sharp drop in FE if I wanted to run the AC on 97F full sun days, and I remember its inability to cool the cabin effectively. I remember it took 2.7% usage of the AC to drop the interior temp 1 degree and calculated the AC would max out at a 37-degree temperature drop between the outside and the cabin in ECO mode in given environment. So I don't have a ton of useful numbers to compare, but I know for a fact my AC number is correct. And as we all know the ac required to cool the cabin is a function of the ambient temperature, total air flow rate of the ac system, percent recirculated air (not all vehicles have 100% recirc bc of humans emitting VOCs, breathing up the air, etc...) , humidity (cooling only), and the heat gain of the passenger compartment. (
But basically what I'm saying here is: Hybrids and especially EVs take a huge fricken hit on fuel economy when the devil tempts you to use the HVAC system as opposed to normal vehicles with mechanical ac compressors and the like... where the experience a small hit most normal people wont notice.
So to solve this? Well maybe my personal problem of frying like a fish in the AR summer: Window tint! Specifically 3m Crystalline.
Now this isn't a cost-effective method I can recommend just seeing it on paper, but since I plan to keep my vehicle for six more years, and I'm buying window tint anyways its pretty null and increasing comfort tremendously.
Also, I'm a cheapskate and will only be tinting the windshield, front windows, and quarter windows (omitting: rear windows, rear quarters, rear window). If it gets hot back there so be it until if I decided to tint them as well.
Stock Prius Glass numbers:
_________ UV Block __ Solar Energy Red. __ VLT __
Windshield 100 55 80
1/4 window 92 46 74
Front 91 49 75
3m CR 40 99.9 60 39
3m CR 70 99.9 34 86
Now onto calcs: I still need to be legal VLT wise so I have to stay around 70% up front to be DOT legal-ish and 25% on the side windows to be legal in AR.

UV cut: well given these numbers it's pretty close to 100% on all modded values so that's what I'll use.
Solar Energy reduction: This number is the inverse of the vlt's format, so you will need to take the 55% above and make it a 45%. 55% reduction in heat = 45% of heat transmitted.
Vlt: Same as above but in the correct format. You have a first layer that cuts x % then the second layer cuts x % of the remaining amount that passed through.
For my case:
UV cut: 100%
Solar energy reduction: 70.3%
0.45*0.66= 0.297% energy allowed through; 1-0.297 = 70.3% energy rejection.
VLT: 68.8%
0.8*0.86= 68.8% light transmitted.

1/4 window:
UV: 100% cut
SER: 78.4% energy transmittance reduction.
VLT: 28.86%

F. Window:
UV: 100% cut
SER: 79.6% energy transmittance reduction.
VLT: 29.85%

Now if I remember correctly AR is around 870w/m^2 of thermal transmissivity when the sun is out. We don't have the depleted ozone of Arizona just yet so its a little milder here. Too bad we have all that humidity we can't do anything about.
So if we run some guesstimate of the numbers using my handy dandy tape measure:
Front window (not counting black surround): 0.9855 m2
F. Window: 0.2695 m2
1/4: 0.02596 m2
So given how the sun shines it to me only really hits one side of the vehicle plus the top of the vehicle at a time. So adding those values up we get 1.5146 m2 of useful protection with our CR window tint at most times.
Doing the math individually:
Front windshield:
0.9855m2 x 870W/m2/h = 857.385W/h solar transmittance
Stock: 857.385 x 0.55 = 471.56 Watt/h reduction
Modded: 857.385 x 0.703 = 602.74 ...
Diff: 131.18 watts less needed per hour to maintain cabin temp.

F. Window: 0.2695m2 x 870W/m2/h = 234.465W/h
Stock: 234.465 x 0.49 = 114.8879W/h
Modded: ... x 0.796 = 186.63414W/h
Diff: 71.75 W/h less

Quarter window: 22.5852 W/h
S: 10.3892 W/h
M:17.7068 W/h
D: 7.32 W/h

So all in it looks like it will take the cooling load 210.25 Watts less per hour to maintain cabin temp, but the nonmeasurable effect is that this is all direct sunlight on the driver so it will feel as if more was reduced. Coming from a Civic with no AC and no window tint driving 150 mi a day in 100 degree sunny AR heat I completely understand this effect.

Polling it out at average speed my average speed over 9999 miles on the dash was 49 mph. Averaging the EPA 49 mpg combined thats a gallon per hour, using the EPAs gasoline energy content number of 33.7kWh/gal then converting the unit down by the thermal efficiency of 38.5% since 61.5% of our energy we put into the engine is lost to physics and not actually turned into any useful energy that means we are using 12,974.5W/h to travel. (12,974.5-210.25)/21,974.5 = 1.647% gain in FE on average.
Highway numbers at 69 mph and 48 mpg that converts out to
69/48 = 1.416... gal/h * 0.385 = 0.545416... gal * 33.7kWh/gal = 18,380.5 W/h to move down the interstate. That gives a 1.116% increase in the highway number of the vehicle.
City working out to 32.63 mph average (from our 49 mph number) if we do the same math as above given 50 mpg...
32.63/50 = x gal/h * 0.385 = x gal * 33.7kWh/gal = 8467.2 W/h to maintain my Prius road obstacle (slow Prius joke ha)... Divide it out = 2.546% gain in city mileage.

Well, I guess somehow after all after all my jokes about engineering school I've learned something useful this year in my third year. Thermoeconomics is starting to seem useful.

So if that's right and I do the same hocus-pocus math as the Prius ECO that would take my sticker from
50/49/48 to 51/50/49

Also, the interesting part to note here is running 50F weather around town I've been averaging 65 mpg. So when summer kicks in it will be interesting to note the increase!

So it looks like modifying my Aero by running a 4" race mirror on the drivers side, no passenger mirror, no passenger wiper, no antenna stick, no rear wiper or squirter, 50 pound weight reduction (Hitch kit added weight), 185mm LRR tires, smooth wheel covers, and running the solar rejection tint, I essentially have a new 2010 Prius II Eco (see: 2016 Prius II eco vs standard model).

Things to notice here:
The more windows you tint the more the window tint has an effect on fuel economy. So a 2010 Prius may gain 1.647% with just the front windows tinted and the windshield (theoretically on paper), but thinking about the effect the rear windows and hatch could also have you could have a much larger gain. Really only a nerd stat on gasoline-fueled vehicle bc will this ever ROI out? No probs not homies... But you can brag to your friends about your big dong.. i mean big mpg.
Secondly! The more efficient the vehicle, the higher the effect the window tint will make. Since you are consuming less fuel it becomes a larger piece of the pie (if you're needing to use the AC) and therefore a larger percentage. You insight guys I'm looking at you. We all know old Hondas have bad AC anyways. Imagine the gain on a fully tinted Insight getting 65mpg. I only did CR 90 on the windshield too, and depending on the area you could go to CR 70 and still be legal (Arizona, Texas, etc...)
Lastly, the big kicker here is the range anxiety on electric vehicles. Imagine on a new Hyundai Ionic EV (similar to a Prius in size). They are rated for 133 mpge combined, 150 city, 122 highway. I'm just gonna assume I can double my window tint number to simulate the entire vehicle being tinted you could end up gaining 6% city or 3.5% combined mileage.
Now I'm still in school and human so If you see anything wrong feel free to correct me.
So it looks like if you want every last little bit in a new fuel efficient vehicle buy the white model to reduces the solar gain on the skin of the body, and tint all the windows with some nice solar heat rejecting film.

Splittin hairs folks, splittin hairs...
"I feel like the bad decisions come into play when you trade too much of your time for money paying for things you can't really afford."

2010 Prius
1996 Ranger
1987 Bass Tracker 70hp Yamaha
1970s 15' Water Rover 30hp Yamaha
2009 Suzuki GS500F

Last edited by hayden55; 03-21-2018 at 10:37 PM..
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